33 IGC excursion No 10, August 15 – 20, 2008

Palaeozoic impact craters Organizers: Maurits Lindström, Stockholm University, Sweden Birger Schmitz, Lund University, Sweden Erik Sturkell, Technical University of Tallinn, Estonia Jens Ormö, Centro de Astrobiología, Instituto Nacional de Técnica Aeroespacial,

Madrid, Spain

TABLE OF CONTENTS

Abstract ...................................................................................................................................... 4

Logistics ..................................................................................................................................... 4 ............................................................................................................................................ 4 Dates and location

Travel arrangements......................................................................................................................................... 4 Fig. 2. Travel route of the excursion. ....................................................................................... 5

............................................................................................................................................... 5 Accommodation................................................................................................................................................... 5 Field logistics

General Introduction................................................................................................................. 6 ......................................... 6 Mid-Ordovician fossil meteorites and the breakup of the L chondrite parent body

Regional Geology ...................................................................................................................... 8 ....................................................................................................................................... 8 Proterozoic basement

............................................................................................................................................. 9 Lower Palaeozoic........................................................................................................... 10 Lockne crater (early Late Ordovician)

............................................................................................................................ 10 Siljan structure (Devonian)................................................................................................................................. 12 Late Palaeozoic dolerite

Excursion Route and Road Log.............................................................................................. 12

Excursion Stops....................................................................................................................... 13 .............................................................................................................................................................. 13 Day 1

................................................................................................................................................ 13 Introduction................................................................ 13 Stop No 1 : Kinnekulle, Thorsberg quarry at Österplana

Introduction .................................................................................................. 13 Description ................................................................................................... 13

.............................................................................................................................................................. 15 Day 2................................................................................................................................................ 15 Introduction

.................................................................................................... 16 Stop No 2: Sjurberg, Siljan shoreIntroduction .................................................................................................. 16 Description ................................................................................................... 16

.......................................................................................... 16 Stop No 3: Kallholen (Kallholn) quarryIntroduction .................................................................................................. 16 Description ................................................................................................... 16

........................................................................................................................ 17 Stop No 4: KårgärdeIntroduction .................................................................................................. 17 Description ................................................................................................... 18

.............................................................................................................................................................. 18 Day 3................................................................................................................................................... 18 Introduction

................................................................................................................. 19 Stop No 5: GesundbergetIntroduction .................................................................................................. 19 Description ................................................................................................... 19

..................................................................................................................... 20 Stop No 6: TrollbergetIntroduction .................................................................................................. 20 Description ................................................................................................... 20

................................................................................................................ 22 Stop No 7: StyggforsenIntroduction .................................................................................................. 22 Description ................................................................................................... 22

....................................................................................................................... 24 Stop No 8: DalhallaIntroduction .................................................................................................. 24 Description ................................................................................................... 24

....................................................................................................................... 24 Stop No 9: AmtjärnIntroduction .................................................................................................. 24 Description ................................................................................................... 24

.............................................................................................................................................................. 25 Day 4................................................................................................................................................... 25 Introduction

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................................................................... 30 Stop No 10: Nordanbergsberget, Skanska quarryIntroduction .................................................................................................. 30 Description ................................................................................................... 30

................................................................................................................ 33 Stop No 11: Crater trailIntroduction .................................................................................................. 33 Description ................................................................................................... 34

.............................................................................................................................................................. 38 Day 5................................................................................................................................................... 38 Introduction

.................................................................................................. 38 Stop No 12: Rödberget, BrunfloIntroduction .................................................................................................. 38 Description ................................................................................................... 39

....................................................................................... 41 Stop No 13: Hälle at LundbomsbergetDescription ................................................................................................... 41

..................................................................... 41 Stop No 14: Church Community Center, BrunfloIntroduction .................................................................................................. 41 Description ................................................................................................... 41

.............................................................................................. 42 Stop No 15: Geocenter at ÅngstaDescription ................................................................................................... 42

........................................................................................................... 43 Stop No 16: Handsjö roadIntroduction .................................................................................................. 43 Description ................................................................................................... 43

.................................................................................................................. 44 Stop No 17: HällnäsetIntroduction .................................................................................................. 44 Description ................................................................................................... 45

.............................................................................................................................................................. 46 Day 6................................................................................................................................................... 46 Introduction

................................................................................................................ 46 Stop No 18: YnntjärnenIntroduction .................................................................................................. 46 Description ................................................................................................... 46

............................................................................................................... 49 Stop No 19; KlocksåsenIntroduction .................................................................................................. 49 Description ................................................................................................... 49

......................................................................................................................... 50 Stop No 20: KajanIntroduction .................................................................................................. 50 Description ................................................................................................... 51

Acknowledgments.................................................................................................................... 52

References................................................................................................................................ 52 Fig. 1 (Front page). The fossil meteorite Österplana 035 (syn. name Sex 001) from the Sextummen bed in the Thorsberg quarry at Kinnekulle. The meteorite is ca. 8 x 6 cm in cross section and has well preserved chondrule textures. Based on chondrule diameters, chromite chemistry and chromite maximum diameters Sex 001 is identified as a pseudomorphosed L4 or L5 ordinary chondrite. The rock is Middle Ordovician Holen Limestone that is typical of the orthoceratite limestone facies in large parts of Sweden. Note that a small conch has been telescoped into the siphonal tube of a larger one. Note also that the different fillings of the chambers of the conchs record a complex and prolonged history of sea-bed diagenesis. Photo by Birger Schmitz.

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Abstract Growing evidence suggests that the disruption of the L chondrite parent body about 470 Ma ago led to two orders of magnitude increase in the flux of meteoritic material to Earth during ca. 10-30 Ma. The most compelling evidence for this flux of extraterrestrial material is the high abundance of fossil meteorites (1-20 cm in diameter) in mid-Ordovician limestone at Kinnekulle (Stop 1) that we are going to visit at first. This limestone is peculiar also for its uniformity of facies through many million years and over a large area, as well as for the slowness of its deposition (1-3 mm/ky) and its predominant composition of skeletal detritus from a depauperate fauna of arthropods, that is, scavengers and mobile predators. A relatively deep environment is indicated. Next, we are visiting the Siljan impact structure. With a diameter of at least 45 km it is largest in western Europe. It is younger than mid-Silurian, and radiometric datings of assumed impact melts suggest a late Devonian age. Its deep erosion has revealed a 30 km wide central uplift of Proterozoic crystalline target rock, the sub-circular contour of which is surrounded by a synclinorial structure of excellently preserved, fossiliferous Ordovician and Silurian rocks with original, sedimentary contact on the Proterozoic. Though the sum of these features is somewhat unexpected in an impact of this size, the impact is ascertained through shocked quartz and other reliable indicators. The impact and its subsequent erosion have created one of the most appreciated sceneries of Sweden. The Lockne crater, situated at 63°00’ N. 14°50’W in the lakeland scenery of central Sweden. is the ultimate goal of the excursion. Its age is early Late Ordovician, only a few million years younger than the meteorites of Stop 1. It consists of a bowl-shaped inner crater, 7.5 km wide and blasted into Proterozoic crystalline bedrock, surrounded by a brim of ejected crystalline rock. Whereas the impact left very little of the Cambrian and Ordovician cover in the zone where the brim was emplaced, an increasing thickness of this part of the target bedrock is preserved with increasing distance from the inner crater until about 9 km from the point of impact, the full thickness of about 80 m of the pre-impact sediment cover is present. The Ordovician facies is the same as at Kinnekulle and Siljan. The Lockne crater formed at sea, and an enormous backrush (“resurge”) of sea-water took place immediately after cratering. The resurge deposits are one of the spectacular features of this crater.

Logistics Dates and location Timing: 2008, August 15, 08.30 hrs – August 20, 14.30 hrs. Start location: The start is at the Oslo Bus Terminal in central Oslo. This is from

where the costs for transportation, lodging, and meals, covered by the prepaid excursion fee, are applying.

End location: The excursion ends at Best Western Hotel Gamla Teatern, in Östersund, at 14.30 hrs on Aug. 20. This is from where the participants must make their own travel arrangements.

Travel arrangements The Bus Terminal is a unique location in central Oslo that can be reached either by taxi or by public transportation. The bus, belonging to a Swedish company located in Östersund, will be plainly and visibly marked for easy identification. It will take us comfortably through a small part of Norway and a large part of Sweden (Fig. 2)

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Fig. 2. Travel route of the excursion. The hotel at which we stay in Östersund, and at which the excursion ends, is 600 m from Östersund Central Station. According to a timetable that is unlikely to change a great deal, there is a train at 17.00, arriving at Stockholm Arlanda Airport at 22.16. Another train leaves for Trondheim (Norway) at 16.15, arriving at 20.13, with a plane leaving for Oslo on 06.10 the following morning and several flights following later on the same day. If you want to leave via Östersund(Frösön) Airport the only flight available after 2 p.m. on Aug. 20 is at 16.00, arriving at Stockholm Arlanda at 17.00. There is an airport bus departing outside our hotel in Östersund.

Accommodation We will be lodged in reasonably comfortable hotels, with shower and toilet to each room. The total cost for the excursion depends on whether you want a single room (EUR 725 for transport, lodging and meals) or share a two-bed room (600 EUR). Breakfast and, mostly, supper will be at the hotel, luncheon will be en route. Because we start from the convention city, there appears to be no reason to recommend any accommodation for the night before the excursion. Those who want to stay on in Östersund after the excursion could use the Best Western Hotel where we have been staying (info@gamlateatern.se), or contact the local Tourist Bureau (kongress@ostersund.se).

Field logistics The excursion takes us north to within 360 km of the Arctic Circle, but summers compare with for instance Maine and New Hampshire, being fairly warm and with many sunny days. We will go with the same touring bus all the time. Walking is comfortable along roads, tracks, and laid-out paths, and nowhere longer than about 1.5 km. The track to the panoramic view at

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Stop 5 is a walk, but a physically rather breathtaking one, and the view is fine even where the bus stops, 200 m lower. We are not walking through any permanently wet ground. If it so happens that July was wet and we have a windless day in August, there might be insects about, but they are not really bad and do not transmit diseases.

General Introduction The influence from outer space on the surface of the Earth might have been particularly intense during certain parts of the Palaeozoic. Sweden is privileged by having some striking geological evidence of this cosmic influence within its national boundaries. The excursion will show a collection of the oldest known fossil meteorites in Middle Ordovician limestones and the properties of these limestones that caused the meteorites to be preserved. It will also show the Siljan impact structure that is the largest known in western Europe. It formed in the second half of the Devonian Period, within the range of direct geological influence from the Caledonian mountain building that culminated at that stage. It helped decisively to lay the ground for one of the most beautiful landscapes of Sweden. The same can be said for the over 100 million years older Lockne impact that formed a crater in the open sea of the early Late Ordovician. Cratering at sea is a highly complex and dynamic process that is spectacularly illustrated by the well-preserved Lockne crater. In the history of geological thinking in northern Europe Lockne has played a very important role because the area formed the basis for Per Thorslund’s (1940) reconstruction of Ordovician palaeogeography in this part of Europe. Mid-Ordovician fossil meteorites and the breakup of the L chondrite parent body More than 60 fossil meteorites (1-21 cm in diameter) with a total original mass of ca. 10 kg have been recovered in the first systematic search for fossil meteorites, pursued in the active Thorsberg quarry in Middle Ordovician marine limestone in southern Sweden (Schmitz et al. 2001). The meteorites represent at least 15 different falls over a seafloor area of ca. 10,000 m2 during less than 2 Myr, making the quarry one of the most meteorite dense areas known in the world. Comparisons with recent meteorite influx rates determined from meteorite search programmes in wind-eroded deserts and in sky-watch camera networks indicate that the flux of meteorites in the Mid-Ordovician was enhanced by up to two orders of magnitude. Geochemical analyses of relict chromite grains indicate that all or most of the fossil meteorites are ordinary L chondrites (Schmitz et al. 2001). Subsequent oxygen isotopic analyses of chromite grains, and chondrule size measurements have confirmed an L chondrite origin (Bridges et al. 2007, Greenwood et al. 2007). Most likely the abundant relict meteorites in the Mid-Ordovician strata at Kinnekulle reflect the disruption of the L chondrite parent body in the asteroid belt at about this time (Schmitz et al. 2001). This is the largest documented asteroid break-op event during the past few billion years (Keil et al. 1992). Evidence for an early Palaeozoic major asteroid brek-up was already established in the 1960s when recent ordinary chondrites of the L type were shown to commonly have K-Ar gas retention or shock ages of about 450-500 Myr. About 20% of the meteorites reaching Earth today are shocked L chondrites from this event. The systematic search for fossil meteorites in the Thorsberg quarry was initiated after two earlier stray finds of fossil meteorites, the Brunflo and the Österplana meteorites. The Brunflo meteorite was found in 5-10 million year younger rock in the Rödberget quarry in Jämtland (Stop 12) (Thorslund et al. 1984). The Österplana meteorite was found on a dump pile in the Thorsberg quarry (Nyström et al. 1988).

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The Ordovician fossil meteorites are almost completely pseudomorphosed by secondary minerals such as calcite, phyllosilicates and barite. The only relict phases are chromite and platinum group element carrying minerals. In the limestone beds containing common fossil meteorites, abundant chromite grains from decomposed meteorites are also common (Schmitz et al. 2003, Schmitz and Häggström 2006). This has made it possible to study the meteoritic flux related to the break-up of the L chondrite parent body also in other sections worldwide (Schmitz et al. 2008). The extraterrestrial chromite can be readily distinguished from terrestrial chromite by its distinct element composition (Nyström et al. 1988). In the Hällekis section at Kinnekulle, in 379 kg of limestone from 14 levels across 9 m of strata below the Lenodus variabilis conodont Zone, only 5 extraterrestrial chromite grains were found (Fig. 3). The values then increase dramatically to typically 1-3 extraterrestrial chromite grains per kilogram of rock in the L. variabilis, Yangtzeplacognathus crassus and Microzarkodina hagetiana Zones. In this interval, a total of 332 extraterrestrial chromite grains were found in 174 kg of rock. In southern Scania, 350 km to the south, and in sections studied near Yichanf in central China, the distribution trends of extraterrestrial chromite grains are very similar to that at Kinnekulle (Fig. 3). In southern Scania some beds in the L. variabilis Zone contain up to 6 extraterrestrial chromite grains per kilogram rock, whereas only 2 grains were found in 125 kg in the beds spanning the 7 m below. In the Chinese sections, 89 kg of limestone below the L. variabilis Zone yielded only 1 extraterrestrial chromite grain compared with 117 such grains in 89 kg in the overlying beds. The first appearance of common extraterrestrial chromite grains in the lower L. variabilis Zone in the three sections is a strong indication of the precise timing of the disruption of the L chondrite parent body. That the disruption event occurred at this time is consistent with cosmic-ray induced 21Ne ages of chromite grains from the fossil meteorites (Heck et al. 2004). These ages increase upward in the strata, supporting a common origin from an asteroid break-up event. High precision 40Ar-39Ar dating of recent L chondrites has constrained the timing of their parent-body disruption to 470±6 Myr., which is identical within uncertainties to the age of 467.3±1.6 Myr for the beds with fossil meteorites according to the latest geologic time scale (Korochantseva et al. 2007). Modelling studies suggest an enhanced flux of extraterrestrial matter, including large asteroids, during 10-30 million years after major asteroid disruption events (Zappala et al. 1998). The L chondrite parent-body breakup at 470 Myr is thought to have created the Flora family of asteroids (Nesvorny et al. 2002). These asteroids were particularly prone to enter Earth-crossing orbits because of their position relative to an important orbital resonance. There are about 170 known impact craters on Earth, and their record shows that impacts may have been more common by a factor of 5-10 during the Middle Ordovician compared with other periods of the Phanoerozoic (Schmitz et al. 2001). There are four confirmed impact craters in Baltoscandia (Granby, Lockne, Kärdla and Tvären craters) of Middle to Late Ordovician age. For only very few of Earth’s craters has it been possible to determine the impactor type, but for at least the Lockne crater (458 Myr) in central Sweden, abundant chromite in resurge deposits has implicated an L-chondritic impactor (Alwmark and Schmitz 2007). This gives strong support for the assumption that both the Lockne impactor and the fossil meteorites originate from the same parent body and were ejected following the disruption of this body after a major collision, possibly with a comet.

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Fig. 3. Chromite frequencies throughout the Kinnekulle (Stop 1) section, compared with equivalent sections in southernmost Sweden and Yangtze Platform. The diagram for the Kinnekulle section is supplemented with corroborating 187Os/188Os data. Regional Geology Proterozoic basement The excursion deals with geological structures formed by cosmic events and located on the western part of Baltica, This name stands for northern Europe that formed a separate continental plate in the Early Palaeozoic. Western Baltica is divided into four crustal units that will be visited by the excursion (Koistinen et al. 2001). The roughly 1.95-1.80 Ga old Svecofennian Province occupies vast areas in the east. It is a complex of near-continental and oceanic sediments and volcanics in different states of deformation and metamorphosis, and major bodies of deep intrusives. It has several important ore districts. The Trans-Scandinavian Igneous Belt (TIB) runs north-south through south Sweden, with a northern continuation through Norway. Aged about 1.83-1.65 Ga, it is dominated by granitoid intrusives but also contains continental sediments and volcanics. It separates the Svecofennian Province from the Sveconorwegian Province (roughly 1.7-0.9 Ga; the Sveconorwegian orogeny was toward the end of this interval) that forms the south of Norway and a wide belt along the southwest coast of Sweden. It contains thick sedimentary and volcanic successions in which both marine turbidites and continental deposits are represented. The pre-orogenic basement is represented by i. a. gneisses and granitoid intrusives. The very large Bohus-

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Iddefjord Granite, shared between Sweden and Norway, transects both pre-orogenic basement and sediments. The fourth province is the Caledonides that form most of Norway and the northwest coast of Scandinavia. The age is about 0.50-0.37 Ga. The overthrusting was mainly southeastwards and thus at an angle to the structural grain of the three Proterozoic provinces. Lower Palaeozoic Deep-reaching erosion levelled the landscape before the Sveonorwegian orogeny and between the Sveconorwegian and Caledonian orogenies. At the beginning of the Caledonian plate-tectonic cycle the Early Cambrian sea trangressed over the peneplain created by this erosion. Much of the Sub-Cambrian peneplain was as flat as we shall see it in Västergötland on the first day of the excursion. An extensive sheet of mainly quartz sand was spread over the shallow sea-bed. The resulting Cambrian sandstone is over 100 m thick in some areas but is missing in others, such as the Siljan area that we shall be visiting. There, the Lower Ordovician rests directly on Proterozoic basement. The Middle and Upper Cambrian are characterized by mudstone and black, bituminous shale that takes over altogether in the Upper Cambrian (Bergström and Gee 1985). The deposits are relatively thin, as a rule only few tens of metres. The Early Palaeozoic advance of the Caledonian orogen onto the (present) northwestern margin of Baltica led to northwestward depression of the Baltican lithosphere (Greiling and Garfunkel 2006) and deepening of the Ordovician sea in that direction (Heuwinkel and Lindström 2007). Consequently, we find calcareous deposits mainly in the east and graptolite-carrying mudstones mostly in the west (Bruton et al. 1985). The eastern shore of this sea could have been in southernmost Finland. It was in this sea that the meteorites were preserved and the Lockne impact formed. Although the Ordovician carbonate facies of Baltica generally formed at less depth than the graptolitic shales, most of it was deposited deeper than, for instance, the shelf margin of modern seas (Lindström 1963, Heuwinkel and Lindström 2007). The dominant sediment at the levels where meteorites have been found is thin to medium bedded, either red or grey, fine-grained limestones that were deposited at average net rates of very few millimetres per thousand years. Because the non-carbonate fraction of the sediment is fine mud that was deposited even slower, any sediment-yielding land area that was being significantly eroded must indeed have been remote. Cosmic dust alone could have been a significant component of this sedimentation. This generalization is not contradicted by the chemical composition of the sediment (Lindström 1974). The successive carbonate beds were lithified on the sea-bed and are most frequently separated by bedding-planes that functioned as hardgrounds (Lindström 1963, 1979). Sessile organisms were either scarce or absent on these surfaces. The detrital fragments that form the limestone mostly came from trilobites and ostracodes. The dominant role of these mobile scavengers, detritus feeders and predators characterizes an environment with scarce and sought-for nourishment. The red colour of much of the carbonate indicates an oxygenated sea-bed, a circumstance that signals depletion of organics in the sediment. In the Middle Ordovician Kunda Age there was a temporary change to a more diverse and demanding, bottom-living fauna. The limestones are grey and relatively rich in phosphate. This was also the time of increased influx of meteorites (see above). The Late Ordovician was initiated by an episode of cataclysmic volcanism (Kolata et al. 1996), at the same time as the influx of major meteorites was still going on, At least three noteworthy impacts occurred in Baltica: there formed the 2 km wide Tvären crater (Lindström et al. 1994), the 4 km wide Kärdla crater (Puura and Suuroja 1992), and the 7.5 km wide Lockne crater that we shall be visiting. All three craters formed at sea, although at different

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depths. The water was shallowest at Kärdla that formed offshore a probable coast in southern Finland. Lockne formed at about 500 m (Shuvalov et al. 2005), in the marginal zone of the Caledonian foretrough. The Late Ordovician sea of Baltica was characterized by diversity of sedimentary and biological environments and relatively rich faunas. There even formed thick, reef-like volumes of limestone, for instance in the Siljan area. In the Silurian the proximity to the accreting Caledonian orogen led to an increase in sandy sedimentation that reached the Siljan area relatively early but later on spread southwards and eastwards. Autochthonous Silurian beds are missing north of the Siljan area and thus do not exist at Lockne. Lockne crater (early Late Ordovician) The Lockne structure (63°00’ 20”N, 14°49’30”E) is dated as early Late Ordovician (about 455 Ma) by conodonts of the Subzone of Baltoniodus gerdae occurring below and above impact-related deposits. It consists of a 7.5 km wide central (or inner) crater surrounded by a brim of crushed and ejected rocks from the crystalline basement. This brim is 1-2.5 km wide to the west but less than 1 km to the east (Lindström et al. 2005). The bolide has been mathematically modeled to have struck obliquely from the east into a 500-700 m deep sea (Shuvalov et al. 2005). The approximately 80 m thick sedimentary cover rocks, consisting of about 30 m of Cambrian bituminous mud (“alum shale”) and 50 m Ordovician, mainly lithified limestone, was almost totally swept away within 2-3 km from the central crater and strongly affected by stirring and partial removal as much as 3 km still farther away. The most characteristic feature of a maritime crater such as Lockne is the sedimentary megaclast breccias and graded gravel and sand sediments that are collectively called resurge deposits. They represent the phase of violently returning water at the end of cratering at sea. Siljan structure (Devonian) The Siljan impact structure consists of a 25-30 km wide central uplift that is surrounded by a 5-10 km wide, annular trough (Fig. 4). The uplift consists of granitoid and, to minor extent, sedimentary and volcanic rocks belonging to the Transscandinavian Igneous Belt (TIB). The trough contains Ordovician and Silurian sedimentary rocks. The major lakes of the area formed in the annular trough through erosion of these sediments. Comparison with other large impact structures (Turtle et al. 2005) suggests that the transient crater was about 45 km wide, whereas the final crater might have been as large as 90 km before erosion. The annular trough contains a well preserved Ordovician layered succession of mainly limestone that rests with sedimentary contact on Proterozoic basement rocks without intervening Cambrian beds. This Ordovician is about 100 m thick, except where the Upper Ordovician contains bodies of reef-like limestone that increase the thickness. Owing to their good preservation, some Ordovician outcrops are type localities in Swedish Ordovician stratigraphy. The Silurian includes bituminous mudstones with graptolites. The Lower Silurian is overlain by fine-grained and poorly cemented Orsa Sandstone that heralds the continental Old Red Sandstone facies of the Devonian of northern and northwestern Europe. The Siljan impact struck at about 377 Ma (Reimold et al. 2005), which was late Middle Devonian. The area belonged to the foreland of the Caledonides. It seems to be generally agreed that thick foreland sediments existed in the area (Rondot 1976, Grieve 1984). The maximum estimate, based on sedimentary petrology (Collini 1988), is 2 km. However, the preservation of the exposed rocks suggests that an even thicker cover, significantly weakerthan the preserved bedrock, was present at the time of impact. Considerations on oil

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maturity and early diagenetic goethite are at present the only clues that constrain the thickness of this cover. The Lower Palaeozoic of the Siljan district contains oil, the maturity of which is low (Pedersen et al. 2007), but nevertheless enough to have required temperatures stably over 60°C. Geothermal gradients in the Baltic lithospere being normally about 15°C/km a burial depth of as much as 4 km does not appear exaggerated. On the other hand the Lower to Middle Ordovician limestones of the district, like those of other parts of Sweden (Lindström 1979), contain early diagenetic goethite that would have dehydrated to hematite at stable temperatures above 75°, which suggests a thickness not over 5 km for the Old Red Sandstone cover of the area in mid-Devonian time. Because these quite young, fluvial siliciclastics are unlikely to have been well lithified, they could have provided enough soft cover to explain the unexpected features of the structure (Fig. 5). Impact melt, of which a large volume would normally occur in a structure of this size, has only been identified in small pockets in the crystalline basement. The formation of a large crater through impact into basement with a significant overburden of much weaker material has been modeled by Collins and Melosh (2004) and Crawford and in-Jha (2004). The resultant structure is very similar to the Siljan case.

Fig.4. Distribution of the principal bedrock units of the Siljan structure. The numbers refer to Sops in the area: 2 Sjurberg, 3 Kallholn, 4 Kårgärde, 5 Gesundberget, 6 Trollberget, 7 Styggforsen, 8 Dalhalla, 9 Amtjärn.

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30 km 30 km

crater center

100 km Fig. 5. Hypothetical section through the Siljan structure, assuming impact in a Caledonian foretrough with essentially non-consolidated fluviatile sediments of the Devonian Old Red Sandstone facies. RED is for crystalline basement rocks.BLUE represents Ordovician and Silurian, most of which formed after the Finnmarkian (Late Cambrian to earliest Ordovician) orogenic phase. BROWN represents mainly Cambrian that pre-dates the Finnmarkian phase.ORANGE shows rocks overthrust in the Finnmarkian phase. YELLOW shows rocks oveerthrust in the Scandian (Devonian) phase. GREEN is Silurian to Devonian sediments in the Caledonian foretrough. – Impact products are not shown in this sketch. They would have appeared as a thin streak of colour that thickened somewhat toward the crater. – Erosion has removed the whole structure down to just below the sub-Cambrian peneplain that is symbolized by the upper boundary of the red (basement) field. Late Palaeozoic dolerite No Palaeozoic sedimentary rocks younger than Silurian are exposed in Sweden. However, the thick dolerite sheets that cap the Palaeozoic sedimentary successions of the table mountains in Västergötland (including the one that we shall see at Stop 1) were intruded as sills in the latest Carboniferous or earliest Permian. They are thus coeval with the volcanism of the Oslo area, from where the excursion starts.

Excursion Route and Road Log Oslo is situated in the Sveconorwegian province, and the first day’s tour crosses different segments of the Sveconorwegian orogen, worn down to rounded granitic hills and fertile valleys. After luncheon at Vänersborg on the south shore of the large lake Vänern, we cross one of the dolerite-covered table mountains and then come down on the sub-Cambrian peneplain. Our goal is the large hill Kinnekulle, also situated on the Vänern shore. Its Cambrian to Lower Silurian was described already by Linnaeus. Stop 1 shows the by now classic meteorite locality at the Torsberg quarry. After overnight stay at Mariestad (Fig. 2) still at Lake Vänern, we travel north mainly through the Transscandinavian Igneous belt but for some 100 km also through the westernmost part of the Svecofennian. This is the oldest mining district of Sweden, active since the late 13th Century, for a while largely abandoned, but now revitalized. Kopparberg, where we have our luncheon, is one of those old mining sites. Dalarna, where the Siljan structure is central, and where we spend two nights at the old and picturesquely situated town of Rättvik, is a province of much Swedish folk lore, literature, and art. The road from Rättvik to Östersund takes us through a hilly landscape with much forest and many lakes but few people. The bedrock at first is volcanics, granite, and dolerite of the TIB belt, then more granite that is Svecofennian. When passing Svenstavik we get the first view of Storsjön, principal lake of central Sweden. Hoverberget, outstanding hill in the field of view, is a Caledonian nappe outlier. The lake is famous for being haunted by a monstrous serpent-like animal, Storsjöodjuret. We should arrive at the day’s goal, the Lockne crater, in time for

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luncheon. Some 20 km farther along the road is our overnight stay, Östersund, a neat, modern city in a romantic setting at the lake. Its bedrock is all Palaeozoic sediments, folded and overthrust in the Caledonian orogeny.

Excursion Stops The UTM references by which the excursion Stops are located are taken from a nationally adopted version of UTM that is used on Swedish topographic maps.

Day 1

Introduction We cross the eastern part of the deeply eroded Sveconorwegian orogen, to reach Kinnekulle, where we spend the afternoon. Kinnekulle is one of the Palaeozoic table mountains of the Swedish province of Västergötland. The Lower Palaeozoic succession, beginning with Lower Cambrian and ending in the Lower Silurian, rests on the sub-Cambrian peneplain that surrounds the hill, and is capped by dolerite that was intruded in latest Carboniferous to earliest Permian. The succession is classic for large parts of Scandinavia. The Lower Cambrian is sandstone, with basal conglomerate on Proterozoic gneisses. The Middle and Upper Cambrian is shale that is everywhere dark grey to black and highly bituminous in the Upper Cambrian (“alum shale”). It has significant contents of uranium. The Ordovician is predominantly limestone (commonly referred to as “orthoceratite limestone”, a name referring to the occurrence of conchs of orthocone endoceroid cephalopods in some beds), the lower part of which is red, except for a 1.2-1.4 m thick, light grey, phosphatic and richly fossiliferous member (“expansus limestone”) in the lower Middle Ordovician. The reddish limestones immediately below and above this grey member are the principal sources of meteorites. The Silurian is represented by graptolite-carrying, dark grey shale near the top of the hill, which is capped by dolerite. It has a fine view towards the next dolerite plateau east of Kinnekulle, the Billingen hill, which is quite extensive.

Stop No 1 : Kinnekulle, Thorsberg quarry at Österplana

Location National UTM coordinates 649770/1361940. Introduction The locality shows the Middle Ordovician part of the orthoceratite limestone in which meteorites occur in several beds. Description The Thorsberg quarry is a small family enterprise that provides all of Sweden with beautifully polished slabs and other objects for indoors decoration. It is the only remaining of a local industry that kept numerous stonemasons active all round Kinnekulle. The quarry is accessed by a small road the leaves the north side of the main road at a sharp angle just north of the church of Österplana (where there is a convenient parking if you do not prefer to take your vehicle into the quarry).

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Fig. 6. Layering in the orthoceratite limestone quarry Thorsberg at Österplana. The stonemason’s names for the beds are included. Names in capitals are those used by Schmitz et al. 1997. Lower case names are based on supplementary information from the local expert on quarrying tradition, Tage Claesson, on Oct. 10, 1987. The colour distinguishes between red and grey limestone. The V-shaped depressions in the Arkeologen bed are typical of this level all round Kinnekulle. They represent spots of less efficient cementation of carbonate on the sea-bed.

3.2

1.6

ARKEOLOGEN

GOLVSTENEN

BOTTEN Stålranden

GRÅKART

BLYMÅKKA

FJÄLLBOTT Stålbotten

LIKHALLEN Likhallsmåkka

Likhallen

1.5

1.4

1.3

1.2

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.4

0.3

0.2

0.1

0.0 m

3.1

3.0

2.9

2.2

2.3

2.4

2.5

2.6

2.7

2.8

TREDJE KARTEN

SEXTUMMEN

2.1

RÖDKARTEN 2.0

1.9 MUMMA

1.8 Likhallsflåra FLORA

1.7 Hallemaren

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The limestone beds are well exposed and accessible throughout the quarry. They can be identified throughout the area and are named in accordance with the old, local stone-mason’s tradition (Schmitz et al. 1997). The grey “expansus limestone” (locally referred to as “täljstenen”, i. e., “the carvingstone”) comprises the beds Botten (“the bottom”) through Flåra (Fig. 6). Most of the meteorites were found in the reddish Arkeologen bed. Internal cavities in the fossils of the grey limestone are largely filled with calcite cement spar with the later addition of sub-mature oil. In connection with the visit to this quarry Mario Tassinari has promised to show meteorites from this quarry, recovered by the Thorsbergs during the process of handling slabs from the different limestone layers for sawing and polishing. If time permits we will also visit the large, abandoned Hällekis quarry on the northwest side of Kinnekulle. This quarry gives a magnificent view of the entire orthoceratite limestone, as well as of the large lake Vänern that spreads at the foot of the hill. The quarry cannot be sampled without special permission.

Day 2

Introduction On our way from Mariestad to the Siljan area we first leave Lake Vänern behind us, going northeast to Örebro. We first cross the TIB seeing little of its gneisses and igneous rocks. At Hova, site of a medieval monastery and traveller’s refuge but also of a famous battle in 1275, we enter Tiveden Forest, following the old trail along which outlaws were a very serious threat in the Middle Ages. The smooth agricultural landscape along the southern approach to Örebro has Cambrian and Ordovician bedrock that rests on the downfaulted sub-Cambrian peneplain. Between Örebro and Borlänge we travel almost due north through the first major ore province in pre-Cambrian crystalline bedrock to be exploited on a large scale in Europe. Its name is Bergslagen, and it goes back to the 13th Century. The bedrock is mixed volcanic, sedimentary, and plutonic magmatic. Much of the volcanism is Si-rich. The whole region is part of the 1.95-1.80 Ga old Svecofennian province. From Borlänge north-westwards to Leksand we follow the valley of Dalälven one of the larger rivers of Sweden (it has less than 1/4 of the drainage area, and 1/7 of the discharge of the Rhine). It is an agricultural scenery, with forest beginning at the valley margins. Between Leksand and Rättvik it is the shore of Lake Siljan that we follow. Rättvik is a quite small, modern town with old traditions, and with villages in the old style all around it. The local bedrock is Palaeozoic sediments of the annular trough (Figs. 4, 5). The higher ground east of the town is mainly granite of the Svecofennian province. For the rest, the crystalline basement that we shall see belongs to the Transscandinavian Igneous Belt (TIB). The TIB rocks at Siljan are cut by 1.45 Ga and 1.25 Ga old dolerite dikes. In its north-western sector the Siljan structure developed in 1.8 Ga and 1.7 Ga old, well-preserved Dala volcanics with associated Digerberg sediments. Much of the central uplift consists of 1.8 Ga old Värmland-Järna granite and 1.7 Ga old Dala granites that were probably intruded at a quite shallow level (Karin Högdahl, written communication). The afternoon will be spent at outcrops in the southern and western sectors of the annular trough of the Siljan impact structure (see the chapter on Regional geology and the Introduction to Day 3).

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Stop No 2: Sjurberg, Siljan shore

Location UTM 6754650/1458855 Introduction The locality shows predominantly red, mid-Ordovician orthoceratite limestone in the excellent preservation typical for this particular large impact structure. Description The locality can be reached from the old road between Rättvik and Vikarbyn. About halfway between Sjurberg and Jonesgården, there is one small road, negotiable even by a bus, that takes off south-westwards and downhill, across the railway and to the shore, where, at the end of the road, there is a loop that allows you to turn the vehicle. From there you walk for about 250 m along the shore to the outcrop. The well-known railway section at Sjurberg (Thorslund & Jaanusson 1960), where the orthoceratite limestone rests with sedimentary contact on the crystalline basement, can no longer be visited (too many, too fast, and too unpredictable trains, and, furthermore, forbidden access). A few metres of limestone are exposed with a west-northwest dip, in agreement with the circumstance that it belongs to the southeast limb of a major, local syncline that strikes north- northeast. The orthoceratite limestone formed through sea-floor lithification of successive beds of biodetrital mud (Lindström 1979, 1984). Discontinuities in this process are marked by surfaces that are very rough on a small scale. Much of the roughness was created by organisms that were either burrowing (in firmgrounds) or boring (in hardgrounds). In the case of this limestone member at Siljan the discontinuity surfaces (Jaanusson 1961) contrast from the red limestone through a strong yellow stain that is caused by goethite. The parallel bedding planes, marked by discontinuity surfaces, and the bioturbation are the only sediment structures observed in this facies of limestone.

Stop No 3: Kallholen (Kallholn) quarry

Location UTM 6741100/1484000 Introduction The body of the quarry is Boda Limestone, i.e., a major, Upper Ordovician reef-like mound structure. It is overlain with perfectly preserved, sedimentary unconformity, by Lower Silurian bituminous shale with spheroidal limestone concretions and fine graptolites. Description The access to the quarry is from the south and clearly visible from the main road between Orsa and Skattungbyn. The high walls of the quarry should generally not be approached. The limestone occurrence and its fauna were described by Thorslund (1936).The rock quarried is a whitish, massive limestone with some steeply inclined bedding. It is transected by a few thin dikes of blackish, argillaceous material. Silurian graptolites found in some of them indicate that some cracking up of the mound limestone occurred in the Silurian, probably owing to differential settling of the large limestone body, Most of the limestone is relatively poor in fossils, but where fossils, for instance trilobite remains, do occur, their preservation is excellent. The hanging-wall stratigraphy is exposed to the south, near the entrance. The top surface of the mound shows a karst-like surface with rounded knobs and pits on the 0.1 m scale.

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Together with the overlying, Lower Silurian (Aeronian; Loydell 1991) it dips rather steeply westward, which agrees with a general younging of the stratigraphic succession in that direction; the annular trough of the Siljan structure can be described as synclinorial, in parts of the structure even synclinal. The Lower Silurian shale of the outcrop has abundant concretions of dark grey, fine-grained limestone of variable size, commonly about that of a football (Fig. 7). Their shape is regularly spherical to spheroidal. Some of these concretions are slightly deformed along fractures that subsequently healed without spar filling, i.e., quickly and without opening. One may attribute this deformation to the impact.

Fig. 7. Spheroidal concretions at Kallholn. Photo by Agneta Arnesson-Westerdahl / Länsstyrelsen Dalarna. Stop No 4: Kårgärde

Location UTM 6774850/1436600 Introduction Western margin of the central uplift. The locality is a trench cut through a virtually complete section through the Lower and Middle Ordovician orthoceratite limestone, resting with sedimentary contact on Upper Proterozoic supracrustal rocks. The bedding is vertical, younging westwards away from the central uplift. The absence of brecciation is typical, and therefore noteworthy.

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Description It takes some attention to locate the outcrop. There is a smaller road departing uphill at a rather acute angle from the main road between Orsa and Mora, 2.50 km south of the circle at the exit from Orsa. After 1 km this minor road begins curving to the left. At this point there is a “Folkets Hus” (cooperative social center) with parking and a ground marked out with a fence. Along the north side of this ground there is a marked-out trail leading to the outcrop. The signpost is placed at the road and is unlikely to be missed. On the 1:50000 topographic (“green”) map the site is marked with an angular B for objects of biological interest. This should probably have been an angular G for geology. The outcrop is protected by law and not to be collected without a permit. This protection has not prevented the site from being to some extent filled with leaves, branches, soil, and overgrowth. We hope to find this situation remedied, however. The entire section was described by Jaanusson (1982). Tjernvik (1956) described the Lower Ordovician part in some detail. The Lower Ordovician is 8.7 m thick. At its base there is a thin, polymictic conglomerate resting on reddish, Upper Proterozoic Dala porphyry. It is followed upwards by greenish sand and clay that are rich in glauconite. This weak rock may have survived the impact in good shape but is quickly destroyed in contact with the atmospheric forces. The rest of the Lower Ordovician is mainly reddish, fine-grained limestone The Middle Ordovician begins with the 7.1 m thick Holen limestone, a topostratigraphic (informal) unit for which this is the type locality. Its lower 1.2 m, which may correspond to the Arkeologen bed of Stop 1, is grey and fairly fine-grained, and contains goethitic ooids. The next higher 1.1 m is characterized by greyish, richly fossiliferous biocalcarenite and evidently corresponds to the grey limestone above Arkeologen. The top 4.8 m is mainly reddish, more or less fine-grained limestone and would correspond to Rödkarten and higher beds at Kinnekulle. The rest of the Middle Ordovician is about 20 m thick, the lower 8 m of which are reddish, whereas the upper 12 m are grey. The Upper Ordovician is represented by grey limestone with fossils of i. a. spherical echinoderms (cystoids of the genus Echinosphaerites).

Day 3

Introduction The objective of the day is to get a general overview of the Siljan structure. For this reason we begin at one of the highest points of the area. We then enter the central uplift to see basement rock at one of the very scarce outcrops near the center of the structure and end up in Ordovician and Silurian rocks in the eastern sector of the annular trough. Underway we shall pass through some of the finest landscape in this part of Sweden. The first descriptions to deal specifically with the Siljan structure as astrobleme were by Rondot (1976) and Wickman (1988). Wickman referred to chaotic structures occurring in the annular trough, a feature to which later authors have also referred. However, there are no described observations to support this statement. As stratigraphers and palaeontologists working in the area are well aware, there is a lot of tectonic reorientation of bedding but no chaos and generally good continuity in the exposed successions. The summary by Wickman (1988) shows that the “Siljan ring” was a geological mystery, although the hypothesis that it formed by a truly great impact was the only that could explain its broader features. The discovery of quartz grains with Planar Deformation Features (“shocked quartz”) by Svensson (1967) proved the impact to be a fact. Through the size of the

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structure this impact ranks as a giant, but this giant was surprisingly gentle with the Palaeozoic succession and much of the Proterozoic basement preserved in the area. This circumstance, which will be emphasized again and again, puts constraints on the model of the impact situation. The first discussion of the Siljan Ring to be based on structural data was published by Kenkmann & von Dalwigk (2000) They pointed out that the Palaeozoic succession of the annular trough could not have been inside the transient crater formed by impact excavation. Because the transient crater would have been larger than the central uplift but the central uplift extends to the very margin of the well-preserved Palaeozoic, the authors concluded that the Palaeozoics had slumped inward by as much as 6-7 km during the modification stage that came immediately after the excavation stage. A converging movement of such dimensions would have created impressive radial deformation structures, termed radial transpression structures by the authors. This argument is geometrically compelling, but structures that would qualify as being the postulated compression ridges have not been identified within the Palaeozoic, or in such a manner that the deformation expected within them can be documented. Kenkmann & von Dalwigk (2000) reported that Proterozoic basement rocks as far as about 32-33 km from the center of the impact structure show impact damage. The diameter of the final crater would therefore have been about 65 km. The distance that the Palaeozoic moved towards the crater center during modification would have been about 20% of the radius. The principal detachment plane would have been at the contact between the basal Palaeozoic sediments and the crystalline basement. However, this contact is primary and sedimentary at all outcrops where it has been observed. Furthermore, a comparison with the excellently exposed and thoroughly investigated Wells Creek crater (Wilson & Stearns 1968) suggests that a 20% radial shortening of the cover rock could be an exaggerated estimate. The tangential shortening that must accompany an inward radial movement of rock has indeed occurred at Wells Creek, as demonstrated by the map and structural data of Wilson & Stearns, but, according to their calculation, it amounted to only 2-4% of the radial distance. The impact process at Siljan has been discussed by Grieve (1988), who gave particular attention to the formation of the central uplift. However, the process reconstructed by Grieve does not explain the structures along the margin of the uplift, or the good preservation of rocks found in the annular trough. Also, Grieve discusses the case of a vertically homogeneous target rock, whereas the actual case almost certainly was that there was a considerable thickness of weak sediments that formed the upper part of the target (Fig. 5). The day will be spent at outcrops that illustrate different aspects of the problems referred to. Stop No 5: Gesundberget

Location UTM 6751000/1430250 Introduction The hill, consisting of granitic basement rock, rises to 514 m above sea-level and 354 m above the lake Siljan. Being situated just outside the southwest margin of the annular trough, it gives a fine view of the southwest half of the impact structure. Description Transport vehicles must stop at about 300 m level, either at the ski lift or at a track that begins 1.5 km northwest of the top of the hill. The former alternative provides a hike that is about 1

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km shorter but about twice as steep as the north-western option (the ski lift will not be working in summer time). The view is dominated by Lake Siljan that occupies the position of the annular trough (Fig. 8). The far side of the lake is mainly Järna Granite belonging to the central uplift that forms a vast and relatively flat, wooded landscape. Palaeozoic rocks belonging to the annular trough form the shores at the eastern end of the lake (right, with reference to the observer). The lower ground northwest of the lake (to the observer’s left) is likewise underlain by the Palaeozoic sedimentary rocks of the annular trough. It is occupied by the small but regionally important town of Mora and north of that town, the lake Orsasjön. In the foreground, Lake Siljan has the major island Sollerön with several Palaeozoic outcrops.

Fig. 8. Gesundberget (300 m level). View towards the NNE part of the annular trough with Lake Siljan (Lake Orsasjön behind the horizon to the left), Sollerön (the stretch of land to the right), and the very low central uplift that forms the horizon behind Sollerön. Photo Jens Ormö. Stop No 6: Trollberget

Location UTM 6768150/1448800 Introduction In the midst of the flat, forested landscape of the central uplift, this outcrop, situated near the center, is one of the scarce localities at which the Proterozoic basement can be observed. The locality has been noted for an alleged pseudotachylite formed in connection with the impact. Description

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As a rule it is possible to drive up to the locality. Two kilometres east of the weekend hamlet of Brunvasslebodarna there is a private road taking off to the south. In principle this road can be locked but no case has been reported of its having been so, and access is not formally prohibited. After 800 m the main track takes off sharply to the left. Another right-angle turn comes a little more than 300 m farter on, and after 500 m more one can park at the outcrop that is a rock just to the left (east) of the track. From this it follows that the unforeseen event that the road is indeed locked means that there is a leisurely forest walk of 1.6 km (just about a mile) and back. The outcrop shows Proterozoic (790-1015 Ma K-Ar and Ar-Ar ages; Collini 1991) dolerite cutting into Järna Granite. The assumed pseudotachylite, of monzonitic composition and containing xenoliths of Järna Granite, cuts into both host lithologies (Fig. 9). It has been K-Ar dated (Collini 1991) to 349±2 Ma, which comes relatively close to current datings of the Siljan structure. However, as pointed out in a Swedish publication by Lundqvist et al. (2004) this date may be the strongest argument for interpreting the monzonitic rock as impact-related. Rocks of this kind and composition may form easily at dolerite-granite contacts because the granite-enriched basaltic melt will remain liquid even after solidification of the dolerite and can therefore intrude the latter. Moreover, the groundmass of the assumed pseudotachylite is microcrystalline, not glassy, which it should normally be if the interpretation were correct.

Fig. 9. Trollberget. Detail of the Trollberget pseudotachylitic dike in granite (contact at lower left). Photo Jens Ormö.

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Stop No 7: Styggforsen Location UTM 6765820/1466800 Introduction The locality shows steeply west-dipping but otherwise well-preserved Lower Silurian carbonate-banded mudstones (Fig. 10) with a western, tectonic contact against Proterozoic granite. We are on the east flank of the central uplift. The major structure of the locality can be understood as the eastern limb of an east-vergent syncline with sheared-out western limb and Proterozoic granite at the core of the anticline that follows to the west. Description Turn west for the church of Boda from road 301. Turn right at the church and then the next to the left (after 250 m). Take the fork to the left after 300 m, and you are on your way to the fairly spacious parking of Styggforsen natural reserve. A comfortable and secure footpath is laid out to the waterfall that gives the locality its name. The flow volume at the waterfall is mostly modest, but the site is high, steep, and dramatically craggy.

Fig. 10. Styggforsen. Silurian “cement limestone” dipping steeply westwards. The beds are younger in the left direction. Photo by Agneta Arnesson-Westerdahl / Länsstyrelsen Dalarna.

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The Silurian succession dips westward, away from a rise of Ordovician limestone on which the church and village of Boda are situated. It begins with the same lithologic unit that overlies the Boda Limestone mound of Stop 3. This part of the succession is very poorly exposed at present. The rhythmic alternation of thin limestone bands and somewhat thicker beds of grey mudstone that follows above it is all the better exposed (Fig. 10). Its traditional name is “cement limestone”, an informal name that, for want of a modern investigation, can still be used. At the waterfall the “cement limestone” abuts against a dike of brecciated vein quartz that is several metres thick (Fig. 11). On the west side of this dike is Proterozoic Dala Granite. The quartz dike could scarcely have formed in connection with the impact. The circumstance that it is brecciated indicates that it formed earlier, in which case its principal function in connection with the cratering event would have been to provide a structural discontinuity along which movement would have occurred when the central uplift originated.

Fig. 11. Quartzitic breccia along boundary between Proterozoic basement of central uplift (left of picture) and Palaeozoic sediments of the annular trough. The Silurian limestone-shale alternation of the preceding figure is adjacent to the right of this picture. Photo Jens Ormö.

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Stop No 8: Dalhalla Location UTM 6759350/1462075 Introduction The site comprises two Upper Ordovician reef-like mounds the younger and larger one being overlain by Silurian beds. The succession is raised to approximately vertical orientation without loss of stratigraphic continuity. It belongs to the western limb of a major, north-south striking syncline. Description After a long time of deep and extensive quarrying the site has been successfully transformed to a modern, large open-air stage for major musical performances including opera. For this reason you can scarcely miss finding it when touring through the Rättvik area. The old and not quite as catchy name of the quarry, under which it is known in the geological literature, is Unskarsheden (Jaanusson 1982). The abundant fossils of diverse kinds (coquinas of either trilobites or brachiopods, echinoderms, different groups of molluscs, etc) are regrettably not there for collecting. The older, Kullsberg Limestone mound occurs to the west. It is barely separated from the younger and much larger Boda Limestone mound by a thin, greyish unit of argillaceous limestone. Like Stop 3, Dalhalla has major fracture fillings of Lower Silurian black shale in the Boda Limestone, The relatively good preservation of graptolites in these fillings speaks against injection in connection with the impact; synsedimentary injections of this kind are rather to be expected in major isolated bodies of limestone set in a weaker sedimentary environment. Stop No 9: Amtjärn Location UTM 6758028/1460475 Introduction The outcrop shows the western flank of the same syncline, the eastern flank of which we visited 4 km farther south at Stop 2. The dip of the bedding is much steeper than at that Stop, because the syncline is east-vergent. A complete section through about 75 m of the Upper Ordovician is exposed apparently without significant internal deformation in spite of some relatively weak lithologies. Description The locality is an abandoned quarry that is open for the public as a protected geological site. It is located 6 km north-west of Rättvik and can be reached through either of the two villages Västberg and Nittsjö. A brief but adequate stratigraphic description was given by Ebbestad and Högström (2007). Bergström (2007) deals in some detail with the stratigraphy of the central part of the succession. Whereas the Stops 2 and 8 are dominated by the thick and partly massive Boda Limestone that is the upper of the two generations of Upper Ordovician limestone mounds of the region, Amtjärn shows the underlying generation that carries the name Kullsberg Limestone. It is partly marly and contains a profusion of crinoid fragmens. Above a sedimentary disconformity there follows the Skålberg Limestone that includes a sedimentary breccia with clasts slumped from the Kullsberg Limestone mound. There follows a fine-grained, grey

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limestone with nodular and irregular bedding (Slandrom Limestone), about 3 m thick and overlain in its turn by black, bituminous Fjäcka Shale. Above 8 m of this shale there follows the shaly-calcareous Jonstorp Formation. The last-mentioned unit is, however, poorly exposed at present. Its age corresponds to the lower part of the Boda Limestone. The conclusion is that no Boda Limestone mound was built at this locality. The described succession is best exposed at the south entrance to the quarry.

Day 4

Introduction Leaving Rättvik at 08.30 hrs, we go to Orsa and from there route E45 toward Östersund (Fig. 2). The scenery is hills, forest, and lakes, and the visible geology is dominated by more or less boulder-rich, sometimes also sandy tills. Not much bedrock is visible, but what is there to be seen belongs to the Trans.Scandinavian Igneous Belt (TIB). Initially it is mainly volcanics, but major granitic bodies dominate to the north. Between Sveg and Ytterhogdal the route follows the valley of the river Ljusnan. This is a narrow strip of land with fertile soil and old agricultural centers. The last days of the excursion will be spent in the country about Lake Storsjön in the province of Jämtland. We will come close enough to the lake to see it as we travel along the hillslopes east of Svenstavik. From there we can also see the large hill Hoverberget. This is an outlier of the Caledonian overthrust terrain that extends all through the country to the west of here. Between Svenstavik and Hackås we cross a major ductile shear zone that controls the NNW-SSE architecture of the Proterozoic basement; it was active mainly between 1.82 and 1.79 Ga (Högdahl et al. 2001). The basement of the area that we shall see is characterized by gneisses with the fabric of this shear zone, as well as by extensive occurrences of 1.86-1.85 Ga old granite that is known as Revsund Granite. These rocks are intruded by major sills of the roughly 1.25 Ga old Åsby Dolerite. At Hackås northwards along the road the basement is overlain by Cambrian shale and Ordovician limestone that were folded by the Caledonian orogeny although otherwise, at least at their base, they are essentially autochthonous. Passing Lake Näkten that extends east of the road we are at the margin of the Lockne crater (Fig. 12) about 12 km north of Hackås. Route E45 does not enter the inner crater but presents a panoramic view of it after another 8 km. Very shortly thereafter, just after passing the church of Lockne, we turn left for the golf course and its agreeably situated restaurant, where luncheon is waiting. The diameter of the Lockne impactor is calculated to have been about 600 m and its velocity about 15 km/s (Shuvalov et al. 2005). It may have come from a north-easterly direction at an inclination of about 45° to the Earth’s surface. Its target most likely was a 500-700 m deep sea. The sea-bed consisted of about 50 m thick Lower and Middle Ordovician orthoceratite limestone of the kind studied at Stops 1 and 2, which was underlain by about 30 m of Cambrian “alum shale”, a dark, bituminous mudstone that was compacted but probably still in a relatively weak state. Beneath the Cambrian there was crystalline basement of mostly granitic composition. The impact blasted away the sea within about 6 km from the touchdown point (Fig. 13). Even the Palaeozoic sediments were stripped from most of this area. A 7.5 km wide inner (or central) crater formed in the granitic basement inside the transient water crater. The margin of the inner crater became surrounded by a brim of ejected basement rock that was turned upside down in the process of ejection. The part of this ejected flap nearest to the margin of the inner

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crater had the least kinetic energy and was least damaged (Stop 10). The brecciated basement ejecta are called Tandsbyn Breccia (Lindström & Sturkell 1992) (Fig. 15). The initial velocity of water leaving the crater was many tens of metres per second. The top of the orthoceratite limestone that remained in situ was strongly stirred by the impact of this water, as well as by pelting by granitic hypervelocity ejecta. This stirring created a monomictic limestone breccia that has been named Ynntjärnen Breccia (Lindström et al. 2005) (Figs. 14, 16).

Fig. 12. Bedrock of the Lockne crater and surrounding area.

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Fig. 13. Schematic illustration of the excavation and early modification of an impact crater of the Lockne type. RED is basement, YELLOW is sediments, and BLUE is water. White arrow indicates the resurge flow. Crater center is at the right.

Fig. 14. Schematic illustration of the formation of Ynntjärnen Breccia (4). The diagram shows the situation at the wall of the water cavity at the outer reach of the outer crater that develops in the water column (1) and the target sediments (2 and 3). The breccciation is further

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assisted by the pelting of crystalline ejecta (5) from the inner, deeper basement crater located far to the right outside the view. Adopted from Lindström et al. (2008).

Fig. 15. Tandsbyn Breccia with fine-grained matrix at the southern rim of the inner crater. Photo Jens Ormö.

Fig. 16. Ynntjärnen Breccia at Stop 14. Photo Jens Ormö.

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accompanied by water velocities that were capable of eroding and carrying into suspension even very large limestone blocks. During the complex process of decreasing current activity in alternating directions the boulders, cobbles, pebbles, gravel, and sand carried by the resurge were successively deposited. The coarsest sediments were deposited first. They have been called Lockne Breccia (Lindström et al. 1983, 2005). This unit is polymictic and carries much crystalline material among the limestone clasts (Fig. 17). When the deposited grain-size had been reduced to sand, a greywacke-like arenite formed for which the old, local term loftarsten (anglicized: Loftarstone) has been adopted (Fig. 18).

Fig. 17. Lockne Breccia at Stop 18. This polymic breccia contains material from both the sedimentary and the crystalline parts of the target succession. Photo Jens Ormö.

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Fig. 18. Loftarstone resurge arenite on top of Lockne Breccia at Stop 17. Photo Jens Ormö. Stop No 10: Nordanbergsberget, Skanska quarry Location UTM 6993300/1449000 Introduction Section through an overturned flap of granite 1 km north of the margin of the inner crater from which it was thrown out (Fig. 19). At the bottom of the grantic quarry wall is the weathering surface that was the top surface of the Proterozoic basement, on which the Palaeozoic sediments were laid down, as well as the thin sandstone and black, bituminous shale that were at the base of the Palaeozoic sediments. This inverted succession is in its turn underlain by remains of the basal Palaeozoic sandstone, and by autochthonous granite that forms the floor of the quarry (Fig. 20). Description The entrance to the quarry is by a road across the lower part of the golf course. The road is closed weekends and holidays, so you must use the parking of the golf course for your vehicle. During working hours you can drive into the quarry. If you have not done so before you must ask for permission at the quarry office that you must pass in order to get to the inner quarry where the section is. Hard hats are mandatory inside the quarry, though they do not help much against half a cubic metre of rock, this being the reason why you must keep some respectful distance to most quarry walls.

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The floor of the entire inner quarry represents the top of the Proterozoic basement on which the Lower Palaeozoic sediments were laid down. This surface can be identified in large parts of Sweden and is commonly referred to as the sub-Cambrian peneplain. In the area visited it is exceptionally flat (Sturkell and Lindström 2004), although with a general inclination of just over one degree toward the north-west. Most parts of the quarry floor are not clean enough for seeing the thin patches of basal Cambrian conglomerate to coarse sandstone that have been observed on it. The Cambrian “alum shale” that overlies the basement varies greatly in thickness. It can be as much as about 3 m thick and quite well-preserved (Simon 1987 a), but as a rule it is very strongly deformed, and the thickness dwindles to zero within short distances (Fig. 21). It is absent in the major part of the quarry. The “alum shale” is overlain by a coarse sandstone to fine-gravel conglomerate that may be 0,1-0.2 m thick. It is strongly folded along with the relief of the overlying granite, on which it was originally deposited (Fig. 21). The lowermost part of the granite is more or less strongly weathered. This granite forms the walls of the inner quarry. It shows various degrees of impact brecciation, including a porphyry-like variety in which granitic fragments occur scattered in a dark, very fine-grained matrix.

Fig. 19. The overturned ejecta flap at the Skanska quarry (Stop 10). The relatively little brecciated crystalline masses rest on top of Cambrian sediments best visible to the left of the person. The floor of the quarry corresponds to the Sub-Cambrian peneplain. Photo Jens Ormö.

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Fig. 20. Schematic stratigraphy of the south-eastern part of the Skanska quarry at Nordanbergsberget, Lockne. The sketched rock units vary greatly in thickness. This is especially true for the “alum shale” that is missing in large parts of the quarry.

Proterozoic crystalline basement, inverted during ejection Cambrian basal conglomerate Cambrian “alum shale” Cambrian basal conglomerate Sub-Cambrian peneplain Proterozoic crystalline basement

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Fig. 21. Detail of the contact between the overturned crystalline flap and the dark Cambrian sediments that are strongly folded under overburden rocks. Note the strong weathering of the crystalline rocks just above the dark shales. The stick is one meter long. Photo Jens Ormö. Stop No 11: Crater trail

Location UTM 6787125/1445250 – 6987375/1445680 Introduction The trail (Fig. 22) is laid out on the south flank of the so-called Tandsbyn Gully that is an east-west directed, small and relatively shallow valley through the western part of the brim of crystalline ejecta that surrounds the inner crater. The circumstance that the Gully contains thick resurge deposits indicates that it existed already tens of seconds after the impact. The fragmented crystalline ejecta are visible in several places. The resurge deposits that form a thick cover on the ejecta at the base of the flank of the Gully wedge out completely upwards, and the ejecta become instead covered by richly fossiliferous limestone of the lowermost Upper Ordovician. This limestone (Dalby Limestone) occurs as several lithologic variants, all of which that are exposed at the Lockne crater postdate the impact event. However, at the locality Hallen, 45 km south of the crater beds of lowermost Dalby predates the impact layer, thus constraining the age of the event (Sturkell et al. 2000).

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Fig. 22. Bedrock map of the crater trail area. Description Take the road from Tandsbyn direction Klocksåsen (Kloxåsen). The parking of the Crater trail is 1.2 km south-west of the T-crossroads in central Tandsbyn. At the parking there is an outcrop of a shaly facies of Dalby Limestone that is rich in small lenses of light gray, clay-rich carbonate. Some lenses contain an occasional fragment of trilobite or other fossil, such as a chambered conch of orthocone nautiloid. This facies characterizes the Dalby Limestone at the bottom level of the Tandsbyn Gully (Fig. 23). The trail runs upslope from the parking and almost immediately crosses a railway with very little traffic, but look out nonetheless. Arrows indicate the direction you are supposed to go. The outcrop where the trail forks just after crossing the railway is Dalby Limestone of the same facies as at the parking. It contains folded carbonate beds with the folds verging downslope. The very plastic mode of deformation, without consistency of axial plane orientation, suggests gravity-driven folding in weak sediment, Take the right fork that goes uphill, at about right angles to the railway. The first outcrop with an information post is Tandsbyn Breccia. This is the ejected and crushed crystalline rock on which the sediments along the trail were deposited. At this outcrop the trail starts out westwards into the wood. It was well laid out and is being well used, so there are neither uncertainties nor inconveniences in following it.

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Fig. 23 (previous page). Stratigraphy of different parts of the Lockne crater (not strictly to scale). PCB = Proterozoic crystalline basement. ALU = Cambrian “alum shale”. ORT = Ordovician of the orthoceratite limestone facies. YB = Ynntjärnen Breccia. TBB = Tandsbyn Breccia (crystalline clasts only). LBmxs = matrix-supported Lockne Breccia. LBcls = clast-supported Lockne Breccia. LOF = Loftarstone. DBY = Dalby Limestone. The next outcrop is Loftarstone. It is a coarse, greywacke-like arenite that consists of sand-sized limestone fragments, Quartz sand, fragments of basement-derived silicate rocks, and more or less modified shards of melt rock (Simon 1987b). The last-mentioned grains represent impact melt. The Loftarstone is enriched in meteorite-carried elements, such as iridium, chromium, and nickel (Sturkell 1998; Alwmark & Schmitz 2007; Tagle et al. 2008). The Loftarstone sand was deposited in the final stages of resurge of the sea into the crater. At this and many other localities it appears massive, with only very weak and barely observable lamination. The circumstance that the laminae are horizontal is one of several criteria against the hypothesis that Caledonian folding is important in the structure of the Gully (Kenkmann et al. 2008). The next outcrop along the trail is at the railway, where granitic Tandsbyn Breccia is exposed (Fig. 24). This outcrop was lucidly described and figured by Thorslund (1940, Figs. 18-21, and 24-27). At present only the Tandsbyn Breccia is exposed, and much of what Thorslund documented has been blasted away. However, it was documented that a succession consisting, from below, of very thin Lockne Breccia, Loftarstone, and shaly Dalby Limestone dips away from upwards protruding Tandsbyn Breccia at angles of 10°-20°. In the Dalby Limestone are interlayered two seams of Lockne Breccia that evidently (not Thorslund’s interpretation) interfered with the sedimentation of Dalby Limestone through gravity flow from an original site of deposition somewhat higher upslope.

Fig. 24. Railroad-cut through hillock of Tandsbyn Breccia next to the crater trail. Photo Jens Ormö. One outcrop further westwards, showing the same aspect of horizontally layered Loftarstone as the outcrop before the last one, the trail turns south and upslope.

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We pass an outcrop of Tandsbyn Breccia beside the trail before we get into north-west dipping Dalby Limestone that represents the predominant lithology of this unit on the higher part of the slope, thereby confirming the existence of the Gully as an original feature after the impact (Fig. 23). It is a thick-bedded, relatively pure biocalcarenite. An outcrop of this lithology at the top of the slope has got an information post. The biocalcarenite rests on granitic Tandsby Breccia that is well exposed a few tens of metres to the north. A few metres above the base of the limestone there is a bed of brachiopod coquina that dips at 35°-40° to the north-west (Fig. 25).

Fig. 25. Outcrop of brachiopod coquina at the crater trail. Close-up to the right. Photos by Jens Ormö. Because this is a too steep dip for normal foreset lamination the occurrence belongs to the evidence for a Caledonian syncline that hypothetically explains the Tandsbyn Gully (Kenkmann et al.2008). However, the argument from megaripple foresets is inadequate, because the deposition of the coquina did not take place on the leeward slope of a megaripple but on a very rough surface that was based on the Tandsbyn Breccia and most likely in a sheltered position where the shells came to rest after having been swept over the edge of a minor precipice of ejecta. From here the trail follows the top of the slope towards north-east. There are several good outcrops of Tandsbyn Breccia and a good view of the Tandsbyn Gully and the north-west sector of the margin of the inner crater After about 550 m the trail turns eastwards for a short distance, following the boundary between the Tandsbyn Breccia and the overlying calcarenitic facies of the Dalby Limestone. As recorded by Thorslund (1940) the dip is almost due northerly. In general it follows the local orientation of the rough upper surface of the Tandsbyn Breccia. The trail then turns northwards, down into the valley, where Dalby Limestone of the biocalcarenitic facies is the dominant lithology. Two of the outcrops show surfaces that are very rich in straight cephalopod conchs with parallel orientation. The chambers of the conchs are filled partly with calcareous mudstone, partly with calcite spar. The boundaries between the two kinds of filling tend to strike parallel to the strike of the bedding, which indicates that the direction of dip of the bedding is original rather than being caused by later tectonics. The trail now takes us back to where we started.

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Day 5

Introduction The theme of the day is an overview of the Lockne crater. We begin with a Middle Ordovician target lithology, outside the reach of the water crater and not appreciably influenced by the impact. After a view from the north over practically the whole crater we study an outcrop of Middle Ordovician target lithology that got a severe blow and was strongly stirred by water thrown out by the impact. The Geocenter at Ångsta is situated just inside the inner crater. It is a modern museum with a strong visitor’s appeal and shows among other things a 3D model of the crater, typical crater lithologies, and a vivid animation of the impact. A north-south road transect will then take us from the inner crater (that is partly occupied by an erosional outlier of a Caledonian overthrust nappe, traveled at least 10 km from the west) through its margin, and into the brim of ejected Tandsbyn Breccia. A further, similarly situated section shows the great variation of impact-related rocks near the margin of the inner crater.

Fig. 26. View over the central parts of the Lockne crater. The photo is taken from the northern rim towards the south. The hills at the horizon form the southern and western parts of the inner crater rim. Photo Jens Ormö. Stop No 12: Rödberget, Brunflo Location UTM 6997900/1453000 Introduction The Brunflo area contains the most complete and best preserved and exposed, autochthonous Lower Palaeozoic succession of the region. The Middle Ordovician red orthoceratite limestone of the old Rödberget quarry can be considered typical of the area. Rocks of the same age and lithology are abundantly represented among the material dislocated by impact, in and around the Lockne crater.

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Description The quarry is hidden from the road by trees. A minor truck road leads into it on the left side of the road toward Rissna (signpost at E 45 at the south entrance to Brunflo), 2.2 km north-west of E 45 and 0.7 km north-west of the Gärde and Lunne cross-roads. Different sections in the quarry show different aspects of the rock. All sections show the upper part of the Middle Ordovician Holen Limestone in which the Brunflo meteorite was found (Thorslund et al. 1984). The rock is brownish red, distinctly bedded, and dominated by silt-sized carbonate particles. Parabola-shaped pygidia of the large trilobite Neoasaphus platyurus are fairly common in a roughly 1 m thick sequence of beds at the top of the quarry that also contains an abundance of straight, orthocone cephalopod conchs. These beds are typical of what has been named Lunne facies (Larsson 1973) (Figs. 27, 28). They are characterized by a profusion of low, flat stromatolites that have been interpreted as formed by photosynthesizing algae. The stromatolites are separated by furrows that have been referred to as shrinkage cracks (Karis 1998). These interpretations lead to the conclusion that the Lunne facies formed in an intertidal environment. Because other known features of the orthoceratite limestone cannot easily be reconciled with this conclusion (Lindström 1963, 1971, 1984), the interpretation of the stromatolites as algal structures separated by shrinkage cracks has to be reconsidered.

Fig. 27. Stromatolite structures in the Lunne facies at Rödberget quarry. Photo Jens Ormö.

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Fig. 28. Lunne facies. The bedding of the red limestone is accompanied by small stromatolitic structures of unknown biological origin. Note that orthocone conchs and trilobite pygidia are perched on stromatolite-coated supports consisting of early-cemented limestone. Many conchs were truncated upwards before being covered by the next calcareous bed. The chambers in those conchs that were not opened by truncation have fillings that bear evidence of a prolonged history of sea-bed diagenesis. The environment is thought to have been relatively deep rather than nearshore as was believed for several decades.

10 cm

In the first place, the organisms responsible for the origin of stromatolites are not necessarily photosynthesizers (George et al. 1997). In the face of evidence to the contrary, the presence of stromatolites alone is therefore no proof of deposition in well-lighted water. Secondly, the furrows that adorn the stromatolite beds are meandering, which means that the figures defined by them are interlocking like the pieces of a jigsaw puzzle. Karis (1998) illustrates this feature that geometrically eliminates the possibility of interpreting the furrows as shrinkage cracks. Instead, they are integral parts of the stromatolite structures. Because the question of the depth of deposition of the Lunne facies and the depth of sea-water modeled for the Lockne crater are to some extent relevant to one another, the Lunne facies is more than a mere curiosity in the present context.

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Stop No 13: Hälle at Lundbomsberget Location UTM 6996535/1453365 Description The locality is situated at the road on the south side of the hill Lundbomsberget. The rock of the orthoceratite limestone at this locality is grey calcarenite at the middle of the Holen Limestone; it is stratigraphically equivalent to the very similar grey limestone at Stop 1 (Kinnekulle).

Fig. 29. Thorslund’s sketch of the view from Hälle, showing the margin of the inner crater of Lockne (double line). This feature was believed to be an Ordovician shoreline. This locality offers a view of the Lockne crater that is fully exposed to the south and south-west. The view was sketched by Per Thorslund in his field notes for September 12, 1935 (Fig. 29). He drew a double line for what he interpreted as the shore of the Middle Ordovician sea visible on the flank of the hills that surround the northern part of the lake. However, what he saw is the margin of the inner crater at Lockne. Interpretations come and go, but accurate observations remain. The Tandsbyn Gully (yesterday’s Stop 11) is marked with Tandsbyn-sänkan. Stop No 14: Church Community Center, Brunflo Location UTM 6998225/1450190 Introduction The outcrop shows how the blows from a mass of water, ejected at high velocity, broke up and stirred the uppermost part of the orthoceratite limestone just outside the water crater (Fig. 14). Description The outcrop occurs in the lawn behind the community center a red house on the other side of the road, viewed from the church and its parking. The rock, identified as Ynntjärnen Breccia (Lindström et al. 2005), is grey Holen Limestone with nodular as well as less argillaceous, bedded members that have been disrupted, dislocated, and strongly stirred (Fig. 30). The nodular members are partly disintegrated into a chaotic mixture of nodules and mud. However, such disintegrated portions pass into volumes in which the original layering is preserved. The bedded members are broken up into

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megaclasts that are more or less rotated. The breccia apparently does not contain any admixture of clasts from other parts of the orthoceratite limestone, or from the basement.

Fig. 30. Ynntjärnen Breccia with preserved bedding of the original limestone at Stop 14. See also Fig. 16. Photo Jens Ormö. Stop No 15: Geocenter at Ångsta Location UTM 6991120/1448725 Description There are adequate signposts indicating where to find the museum. It is situated 1.5 km due south-west of the church of Lockne. The museum was opened in 2006. Its modern design is based on the idea that the visitors should realize right away what it is all about, and that this should be fun and at the same time very serious. Rock samples and 3D models are related to the cratering process and the structure of the crater. An advanced and acoustically very perceptible animation creates a reality impression of the impact into the open sea. The museum is situated in a peripheral part of the inner crater.

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Stop No 16: Handsjö road Location UTM 6987500/1447115-69859960/1447185 Introduction The section runs southward from the inner crater across the crystalline ejecta that surround it. It includes resurge deposits that were laid down on the ejecta. Description The private road to the lakes Stor-Handsjön and Lill-Handsjön begins at the main road from Tandsbyn to Bodsjö, 1.0 km from the railway crossing at the exit from Tandsbyn. The gate 300 m down the road has not been observed to be locked the last few years. The outcrop is along the sides of the road. It begins in the inner crater. The inner crater was filled (Fig. 23), from below upward, with monomictic crystalline breccia, polymictic resurge deposits with a general coarsening-upwards tendency, and Dalby Limestone that formed in the course of the million or so years after the impact. The facies of Dalby Limestone in the inner crater is almost exclusively shale-rich, with nodules of fine-grained limestone that are several centimetres large. In the south-western sector of the inner crater The Dalby Limestone is overlain by an erosional outlier of a Caledonian overthrust nappe with Lower and Middle Ordovician orthoceratite limestone on a sole of Lower Ordovician Töyen Shale. The Stop 16 section begins in this Töyen Shale that is strongly sheared, as may be expected at the base of a nappe. It contains nodules of grey, fine-grained limestone like the Dalby Limestone that underlies it on the south side of a small brook (Blekabäcken) that is crossed by the road. The Dalby Limestone contains poorly preserved cystoids, trilobites, and cephalopods. A thin bed of bentonite has been observed. The dip is north-eastward into the crater. Following under the Dalby Limestone upslope along the road there is resurge-deposited Loftarstone that hides the margin of the inner crater.

Fig. 31. Two varieties of fine-crushed rock in the same outcrop at the Handsjö road. A fine-grained, dark occurs in contact with, and within, a coarser. Photo Jens Ormö.

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South of the margin of the inner crater the road keeps on a level, which is that of the brim of crystalline ejecta. The largest outcrop of ejecta is about 0.8 km south of the outcrop of Dalby Limestone at the crater margin. To the west of the road there is a small cliff of Tandsbyn Breccia consisting of angular granitic fragments of gravel size and larger. The eastern roadside has Tandsbyn Breccia that is dominated by dark grey, very fine-grained matrix in which most of the granitic fragments are small and have hazy contours (Fig. 31). About 75 m farther to the south the Tandsbyn Breccia is overlain by a few metres of resurge deposits, consisting of granite-rich Lockne Breccia that is gradually transitional, although occasionally repeated, upwards to coarse-grained Loftarstone (Fig. 32).

Fig. 32. Upwards transition from Lockne Breccia to Loftarstone at the Handsjö road. Beds of Lockne Breccia at this locality may be repeated but are often discontinuous (center right) indicating that fluctuations in the flow may have occurred over the crater rim. Photo Jens Ormö. Stop No 17: Hällnäset Location UTM 6985600/1451220 – 6984640/1450360 Introduction The southernmost exposures of the margin of the inner crater of Lockne. Represented in the outcrops are: the autochthonous basement just outside the inner crater, varieties of locally derived ejecta from just inside the inner crater, resurge deposits from megaclast-carrying

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Lockne Breccia to Loftarstone, with and without gradual transitions between the two, Dalby Limestone, and an overlying, graptolite-carrying siltstone. Description Hällnäset is a small promontory on the shore of Locknesjön, and is situated 5 km south-east of Tandsbyn. There are tracks taking off on both sides of the road from Tandsbyn to Bodsjö. We will use both of them. The track to the right takes you uphill and will carry almost any vehicle. There are no places for meeting (which scarcely takes place, luckily, for reversing is tough). The track to the left is hardly, or only at a risk, negotiable with a car. It takes you down to the lake, 500 m easy walk, and is where we will begin. Halfway to the shore outcrop one passes a couple of weekend huts, beyond which the path can be difficult to follow; however, the shore offers orientation, and the wood is relatively open and not shrubby. The outcrop consists of flaggy Loftarstone that dips to the north, into the crater. A few tens of metres to the south there is Proterozoic granite that belongs to the autochthonous basement. Going back to the road the same way we came, we pass more or less overgrown outcrops of Lockne Breccia. The bedrock at the road is massive, relatively coarse-grained Loftarstone with an admixture of gravel-sized clasts. Because the outcrop is continuous we do the track on the other side of the road on foot. The first stretch is mainly light grey Dalby Limestone of the shaly, nodular variety. Fossils occur in a few places. There is a low, northerly dip, approximately parallel to the slope of the ground. After about 350 m, at an electricity line and a smaller track towards the left, the Dalby Limestone overlies coarsely sandy Loftarstone with dewatering marks (Simon 1987b). Among the shrubs a few tens of metres east of the track there is a small cliff in which this Loftarstone rests on Lockne Breccia with gravel-size clasts (Fig 18). The contact between the two lithologies is sharp and somewhat discordant in a part of the outcrop, but in another part it is a gradual transition. The situation is typical for deposition by a strongly sediment-charged water current. Walking further upslope along the main track we find Lockne Breccia with large blocks of orthoceratite limestone under the Loftarstone. The Lockne Breccia is in its turn underlain by Tandsbyn Breccia, of which there are three varieties along the track: monomictic with clasts of Åsby dolerite, monomictic with clasts of Proterozoic metavolcanics, and monomictic with clasts of Proterozoic granite. We stop and return after reaching a steepening of the slope that is caused by a north-west striking fault that has off set this part of the marginal zone of the inner crater in a vertical sense, the local throw of the fault being somewhat over 50 m.

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Day 6

Introduction The focus will be on the effects of the impact west of the central crater and its brim of ejected crystalline rock. There will still be major masses of crystalline ejecta, but they are isolated from the more or less continuous occurrence of such ejecta in the immediate vicinity of the central crater, and they are underlain by Cambrian and Ordovician sedimentary rocks the top of which is, however, more or less strongly affected both by the water-blow and by being hit by masses of crystalline ejecta. This pelting began in the water-blow phase and continued until immediately afterwards. Stop No 18: Ynntjärnen Location UTM 6988060(1443700 Introduction A small tarn in the forest. On its west side, and along the adjacent railway section to the west of it, there is a stratigraphic succession through, from below upwards, orthoceratite limestone, Ynntjärnen (water-blow) Breccia, masses of granitic ejecta, Lockne Breccia, Loftarstone, and different facies of Dalby Limestone. This succession has been laid into open, roughly north-south striking folds in the Caledonian orogeny (Fig. 33). Description The locality can be reached through walking the railroad about 1.7 km from the road crossing at Grubban. This is worthwhile for the excellent outcrops of Ynntjärnen and Lockne Breccias in different places, but it can also be a bit tiresome and time-consuming. We prefer to make a short cut via the Tandsbyn to Skute road. At somewhat over two kilometres from Tandsbyn, where the road turns rather sharply right, there is a private forest road taking off to the left (southwards). Deviously, because one cannot see it from the main road and might be tempted to drive into what will be a difficult situation after 100 m, it is locked (or worse, isn’t, in which case it will be when you want to drive back after seeing the outcrops). A walk of 250 m takes you to the first outcrop. After another 250 m you are at the railway section, for which you turn right, along the railway. The outcrops on the west bank of the small lake are mapped as Dalby Limestone (basal Upper Ordovician; sedimentation beginning after the impact) There are two varieties of it. To the north and west there are small quarries in which shaly-nodular Dalby Limestone is exposed with sub-vertical bedding (Fig. 34). It is difficult to ascertain its direction of younging from bedding criteria, but mapping, and the circumstance that the vergence of tectonic folds in the area is easterly, make younging to the east the only probable alternative. Because the other facies assigned to the Dalby Limestone – a very hard, non-layered, micritic and more or less bituminous rock – occurs to the east of the shaly facies, it must be assumed to represent the higher stratigraphic level of the two. The massive, micritic limestone has a patchy, reef-like mode of occurrence in the Lockne area. No acceptable explanation of its origin has been proposed so far. Because no fossils have been identified from it, even its assignment to the Dalby Limestone is admittedly uncertain. It might instead belong to a distinctly younger stratigraphic unit.

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A

0 100 200 300 400 500 m

B

Fig. 33. A. Section along the north side of the railroad west of Ynntjärnen. For color codes see Fig. B. B. Reconstructed section of the folds cut by section A. The reconstruction is based on known thicknesses within the local orthoceratite limestone facies as well as on the average thickness of the highly plastic Cambrian shale in the area. The rigid basement on which the Palaeozoic rocks are resting will be flat on the scale of the section. The reconstructed section of the folds is based on known thicknesses within the orthoceratite limestone facies and the average thickness of the highly plastic Cambrian shale in the area. The rigid basement on which the Palaeozoic rests can be assumed to be flat on the scale of the section.

basement

alum shale sub-Cambrian peneplain

Loftarstone

orthoceratite limestone

Ynntjärnen syncline (exposed along track to the railway)Tandsbyn Breccia

Ynntjärnen Breccia Lockne Breccia

Dalby

railway

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Fig. 34. Folded Dalby Limestone as part of the Ynntjärnen syncline. View towards the south. Photo Jens Ormö. Along the track to the railway the hanging-wall of the Dalby Limestone consists of relatively thin and tectonically squeezed Lockne Breccia with steep cleavage. To the west, and therefore stratigraphically below this unit, there is granitic Tandsbyn Breccia. At the railway we then find that this breccia must be underlain by sub-horizontal orthoceratite limestone, on which it thumped down during the cratering process. The change from vertical to horizontal bedding is due to the circumstance that we have left a synclinal fold, which can be called the Ynntjärnen syncline (Figs. 33, 34), and crossed the hinge of the east-vergent anticline that adjoins it to the west. The sequel of the railway section had better be followed with the aid of the two sections in Fig. 33. There are, however, some particular features to point out. The circumstance that the orthoceratite limestone could be folded as well as subjected to minor thrust-faulting in most of the section is due to the fact that it rests on 30 m thick alum shale. This tectonically non-resistant formation occurs beneath the orthoceratite limestone throughout the region. It is to be seen at Stop 19. The size and shape of the folds is determined by the presence of two separate masses of granitic Tandsbyn Breccia just above the limestone. Only the westerly occurrence of this breccia is exposed at the railway. The one to the east is hidden from the railway by trees, but its eastern margin could be observed as we approached the railway. The ortoceratite limestone is represented by marly-nodular Ynntjärnen Breccia in much of the section. This monomictic breccia also separates the orthoceratite limestone from the overlying Tandsbyn Breccia. It formed through stirring of the orthoceratite limestone in connection with the expansion of the water crater. It can both difficult and, at least in some cases, little meaningful to draw a sharp distinction between the Ynntjärnen Breccia and the limestone on which it rests.

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Resurge sediments (Lockne Breccia and Loftarstone, the former dominated by gravel at this outcrop) are present in the section at 200 m, 255 m, 420 m, and 490 m. The Tandsbyn Breccia was missing at where they are preserved in contact with the Ynntjärnen Breccia. They are followed upwards by the Dalby Limestone at 200 m and 255 m. In this case the Dalby Limestone begins with a few beds of relatively pure biocalcarenite. Stop No 19; Klocksåsen Location UTM 6984285/1444425 Introduction The Stop shows the autochthonous Palaeozoic near the front of the continuous ejecta sheet that surrounds the inner crater. In this position only the basal part of the orthoceratite limestone remained after the cratering process. On its top there rest isolated masses of Tandsbyn Breccia with clasts of either granite or Åsby Dolerite.The Stop offers a magnificent view of a vast land surface that is practically identical with the sub-Cambrian peneplain. Description Klocksåsen is at the end of a road that is indicated by signposts, for instance at Tandsbyn. The attention will focus in particular on outcrops near the first farmhouse to the left as one reaches the place. The foot of the slope on which the houses are situated coincides with the sub-Cambrian peneplain. Thus, the lower part of the slope consists of Cambrian alum shale that is about 30 m thick here, as in much of the rest of the region. A major exposure of this shale occurs at the side of the road and merits the visit for, among other things, its large spheroidal lenses of bituminous limestone (often called “stinkstone” and, in Swedish, “orsten”) that contain Middle Cambrian trilobites and inarticulate brachiopods. The shale is strongly sheared tectonically, but the spheroids are intact. About 1 m of Upper Cambrian alum shale is exposed at the foot of the driveway up to the farm. Its stinkstone layers are crammed with trilobite remains, It is overlain by basal Ordovician limestone that is grey, partly rich in glauconite (Tjernvik 1956), and shaly-nodular as well as bedded. Like the rest of the Ordovician in the area about Klocksåsen it has suffered Caledonian folding about north-west to south-east striking axes. It was the underlying alum shale that made the folding possible through serving as detachment level. The horizontal shortening in connection with the folding probably did not exceed 200 m (Lindström et al. 2005). Just behind the farmhouses there is a small wooded hillock that exposes granitic Tandsbyn Breccia (Fig. 35). The houses stand on Ordovician limestone. The boundary between a nodular member of this limestone and the overlying Tandsbyn Breccia is exposed just north and north-east of the houses. - Be careful not to tread on the flowers. At the driveway that continues northwards from the farm there is an exposure of doleritic Tandsbyn Breccia 150 m north-west of the houses. The view from Klocksåsen reaches thirty kilometres southwards across the wood-covered sub-Cambrian peneplain. The low hills that limit the field of vision to the east are the brim of crystalline ejecta that borders the inner crater. The hill on the other side of the valley to the west is Ordovician limestone resting on Cambrian alum shale. The limestone is folded about axes that generally strike north-south. The facies is, however, the same as at Brunflo and in the Lockne area, including some impact deformation, so there is no question of major horizontal movement connected with the Caledonian orogeny.

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Relatively far-travelled overthrust nappes are, however, visible as erosional remnants that form major hills in the far distance to the south. The most prominent of these hills is Hoverberget that was visible at closer range when we passed Svenstavik on our way from Rättvik to Östersund.

Fig. 35. Crystalline ejecta resting on orthoceratite limestone. Maurits is pointing at the contact. Inset diagram shows the location relative to the crater (center at far right). Photo Jens Ormö. Stop No 20: Kajan Location UTM 6985615/1442040 Introduction The locality shows the local top of the undisturbed Middle Ordovician Holen Limestone, a Formation that belongs to the orthoceratite limestone facies. The locality is 8.5 km south-west of the center of the impact structure. The undisturbed limestone is overlain by Ynntjärnen Breccia with clasts derived from the uppermost part of the same Holen Limestone (Fig. 36). The driving force of brecciation probably came from the high velocity flow of water that was expelled from the growing crater (Fig. 14).

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Fig. 36. Ynntjärnen Breccia on top of more resistant beds of orthoceratite limestone at Kajan. Description The outcrop dominates a south-facing slope on the north side of the road from Klocksåsen to Fåker, 2.3 km north-west of Klocksåsen. Because it is the only one of the kind described one can hardly miss it. From the road to the near the top of the slope there is about 10 m of sub-horizontally bedded Holen Limestone with colours between light brownish red (base of slope) and light grey (top). The top of the slope consists of Ynntjärnen Breccia that is essentially monomictic with clasts of light greyish limestone in a marly matrix. The aspect is rather like coarse and poorly mixed mortar. At its base the breccia has a few clasts that have got stuck in the process of becoming disengaged from the underlying limestone. The monomictic character of the breccia is disturbed only by the presence of very few, isolated clasts of granitic ejecta. These exotic elements are interpreted as the result of pelting of the Ynntjärnen Breccia with high-velocity crystalline ejecta in its nascent state (Fig. 14). The outcrop offers a fine view of the Oviken Hills (Oviksfjällen) far to the west. Depending on the year’s spring and summer temperatures, there may even be a few snow patches visible at the upper parts of the hills. The bedrock of Oviksfjällen is mainly Late Proterozoic to earliest Cambrian sandstones, slightly metamorphic and overthrust at least about 100 km from the west. Most of this thrusting may have taken place in an early (Finnmarkian) phase of the Caledonian orogeny (Heuwinkel & Lindström 2007).

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Acknowledgments We are indebted to Karin Högdahl for summaries on the Proterozoic basement geology of the Siljan and Lockne structures.

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