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The Lower and Middle Devonian of the south-easternBaltic SeaMonica Bjerkéus aa Department of Geology and Geochemistry , Stockholm University , SE-106 91,Stockholm, Sweden E-mail:Published online: 06 Aug 2009.

To cite this article: Monica Bjerkéus (2001) The Lower and Middle Devonian of the south-eastern Baltic Sea, GFF, 123:2,97-105, DOI: 10.1080/11035890101232097

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GFF volume 123 (2001), pp. 97–105. Article

The Lower and Middle Devonian of the south-eastern Baltic Sea

MONICA BJERKÉUS

Bjerkéus, M., 2001: The Lower and Middle Devonian of the south-east-ern Baltic Sea. GFF, Vol. 123 (Pt. 2, June), pp. 97–105. Stockholm.ISSN 1103-5897.

Abstract: In the Baltic region, Devonian sedimentary bedrock is ex-posed in a wide area extending from north-eastern Estonia throughLatvia to the south-eastern Baltic Sea. Westwards in the Baltic Sea, theoutcrop terminates against a presumably Permian fault, which extendssouthwards from Gotland. In the southernmost part of the Baltic Sea, theDevonian sedimentary bedrock was eroded during the Permian-Carbon-iferous uplift event. The Devonian sedimentary bedrock of the investi-gated area southeast of Gotland, consists of near-shore marine deposits,namely sandstone, siltstone, clay, dolomite, dolomitic marl, and lime-stone. Gypsum and anhydrite deposits occur frequently in the lowerparts. The Silurian–Devonian boundary is marked by a stratigraphicbreak across the presently investigated offshore area. All regional stagesof the Lower and Middle Devonian are present in exposures in the area.In the north-eastern part, offshore Latvia, the basal Devonian Gargzdaideposits follow ontop of the Silurian sedimentary bedrock. Further tothe south-west, the Gargzdai beds are missing, and deposits of the nextyounger Kemeri Stage rest directly on the Silurian. In the Middle Devo-nian, the Pärnu Stage contains widespread sandbars, which are locallydeeply truncated. This may indicate a shallow nearshore environment.Within the Old Red Sandstone facies of the late Middle DevonianAruküla-Burtnieki complex, several discontinuity surfaces are present.The reflectors exhibit a mound-like pattern, which again may indicate ashallow nearshore environment with rapid deposition, erosion, and re-deposition.Keywords: Baltic Sea, Baltic Basin, seismic reflection profiling, LowerDevonian, Middle Devonian, Old Red facies.

M. Bjerkéus, Department of Geology and Geochemistry, StockholmUniversity, SE-106 91 Stockholm, Sweden, monica.bjerkeus@geo.su.se.Manuscript received 15 September 1999. Revised manuscript accepted21 March 2001.

The Baltic Basin is a prominent morphologic feature within theSvecofennian Domain (Puura & Flodén 1998). Its southern part,which forms the present-day Baltic Sea, is located between theBaltic Shield in the west and the East European Platform in theeast. In the early Palaeozoic, a large epicontinental sea occupiedthe Baltic Basin. Within the investigated offshore area south-eastof Gotland, the Palaeozoic sedimentary bedrock reaches some1500 m in thickness, thinning out stepwise towards the north andnorthwest where the crystalline basement is exposed along theSwedish and Finnish coasts (Winterhalter et al. 1980). In theSilurian, the sea was tectonically constricted to form a wide baycovering much of the Baltic Sea and extending into the presentBaltic countries. The bay opened into the Tornquist Sea in thesouth-west. In the Devonian, the bay became largely filled bysediments and thus extremely shallow. In the early Carbonifer-ous it ceased to exist.

Today, sedimentary bedrock is distributed beneath the majorpart of the Baltic Sea. Silurian sedimentary bedrock, reaching

uppermost Ludlow, is exposed on the island of Gotland where itrests on Cambrian and Ordovician sequences. The uppermostSilurian, Pridoli, is exposed on the south-western part of theEstonian island of Saaremaa. Exposed Devonian rocks make upa large part of eastern Estonia and of Latvia, extending west-wards into the Baltic Sea. Devonian sedimentary rocks form thebedrock surface across a large part of the offshore area south-eastof Gotland. Eastwards this Devonian succession continues intothe onland exposures in Latvia. Towards the west and north-westthe outcrop area is erosionally limited by a line, which extendsfrom north-eastern Estonia to the south-eastern slope of the Got-land Deep and onwards to the area south of Gotland. South ofGotland the outcrop area is terminated westwards by a fault thatextends southwards from Gotland to Poland.

The Devonian sedimentary bedrock of the present area con-sists entirely of shallow marine deposits. The Lower Devoniandeposits only rarely consist of limestone and dolomite and aredeposited in the central parts of the basin. In the Middle andlowermost Upper Devonian, the terrigenous content increasesand sedimentary rocks representing the sandy-silty Old RedSandstone facies are present.

The present paper deals with the seismic stratigraphy of theDevonian outcrop area in the Baltic Sea south-east of Gotland.The stratigraphic interpretations are based on structural featuresand well data from offshore Latvia.

Materals and methodsThe shallow seismic recordings used for the present investiga-tion were made in 1993–1995 (Fig. 1). In 1993 continuous seis-mic reflections profiles of a total length of 2,630 km were re-corded, forming the beginning of a base net of profiles across thearea east and south-east of Gotland. In 1994 another 2,950 km ofseismic profiles were recorded in the area. In 1995, the base netwas extended to the coasts of the Baltic countries. During thiscampaign about 2,900 km of profiles were recorded (Grigelis &Flodén 1995).

The seismic reflection profiling was performed using analoguesingle-channel seismic reflection equipment based on a PAR-1600 air gun (Bolt Tech. Inc., USA). The reflection data wererecorded on an EPC graphic recorder at the frequency band 250–500 Hz. Hersey (1963) described the basics of the continuousseismic reflection profiling method. The performance and use ofthe present equipment is described by Flodén (1981).

Water depth and bottom topography were recorded by echo-sounders. A SKIPPER 607 echosounder was used during thefirst year and during the following years a FURUNO FE-881MK-II echosounder was used.

During the first year a Raytheon Boatnav GPS and during thetwo following years a Trimble NavTracXL GPS were used forthe positioning. The accuracy in the positioning was roughly 50m. The positioning of the vessel was noted at 10-minute inter-vals, as well as at the points where the course of the vesselchanged. The geophysical measurements were performed on-

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98 Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea GFF 123 (2001)

board the Lithuanian vessel R/V VEJAS.The interpretation of particularly the Lower Devonian was

hampered by the circumstance that the seismic reflectors disap-pear in the recordings as the overload thickens. This effect con-ceals the reflectors below depths of 300–350 m in the sedimen-tary bedrock.

One offshore well in the south-eastern part of the Baltic Sea,namely P6, has been used for stratigraphic control of the seismicreflectors. The descriptions of the well was made at the Lithua-nian Institute of Geology in Vilnius. The P6 well is located in thenorthern part of the investigated area (Fig. 1) and placed in theoutcrop of the Middle Devonian.

The locations of seismic profiles and geologic sections pre-sented in the text are shown in Fig. 2.

General geologyThe early Palaeozoic development in the Baltic Sea region issummarised by Winterhalter et al. (1980) and Grigelis et al.(1991). As a result of extensive transgressions in the Cambrianand Ordovician, Palaeozoic rocks once covered the entire area ofthe Baltic Sea, the Bothnian Sea, and the major part of the Swed-

ish mainland. Towards the end of the Ordovician the sea becamemore restricted and by the Middle Silurian there remained only alimited bay extending from the vanishing Tornquist Sea in thesouth to Gotland and the Baltic countries in the north. The tec-tonic and sedimentary development of the basin in the late Siluri-an and earliest Devonian was largely controlled by the Early Cal-edonian orogenic tectonism in the west and south. The orogenyled to a general shallowing and shift of deposition from carbon-ate to terrigenous matter. A major tectonic rearrangement withupheaval and erosion in a wide zone along the south-westernborder of the Baltica Plate ended the long lasting basin develop-ment in the early Carboniferous. The thickness of the Palaeozoicsequence increases towards the south-southeast in the Baltic Seaand is c. 3,000 m in the Gulf of Gdansk.

Regional tectonic developmentIn the early Palaeozoic, until the end of the Silurian, Balticaformed an independent plate that was delimited by the Iapetusand Tornquist oceans to the west and south, respectively. In Mid-dle to Late Cambrian times, Baltica began to converge with theLaurentia-Greenland plate. At the end of the Silurian the colli-sion of the continents was completed and Baltica was incorpo-rated into a large megacontinent called Laurussia (Ziegler 1989).The suture extended along the present Norwegian west coastfrom the Arctic in the north to present-day England in the south.This Caledonian orogeny culminated in Early Devonian timeswith foreland basins developing along the south-eastern marginof the Baltic Shield. In Poland a foreland basin developed as ashallow marine basin that later developed into a continental ba-sin in middle Early Devonian times (Nikishin et al. 1996). Later,the environment again changed back to shallow marine.

Fig. 1. Location of the seismic profiles shot in the period 1993–1995and the drilled P6 deep well in the south-eastern part of the Baltic Sea.

Fig. 2. Location of seismic profiles and geologic sections mentioned inthe text.

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GFF 123 (2001) Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea 99

The collision between Baltica and Laurentia-Greenlandcaused a re-organisation of the plates. The Arctic-North Atlanticsubduction zone was abandoned and a new one developed in theUral Ocean.

On Baltica, the sedimentary cycle was terminated in the LowerDevonian with a regional disconformity that coincides with aglobal low water stand (House 1983; Johnson et al. 1985). Thisseems to be connected with a lithospheric deformation and inver-sion of structures in the Mid-Russian aulacogen and the BalticBasin. These activities started in the latest Silurian and reachedto the mid-Devonian (Chaikin 1986; Milanovsky 1987; Kuz-menko et al. 1991). The main uplift of the area took place in theMiddle Lochkovian to Middle Emsian (Alekseev et al. 1996).Cyclic rising of the sea-level led to Middle and Late Devoniantransgressions on Baltica and the development of carbonateshelves on the Moscow Platform and in the eastern Barents Sea(Nikishin et al. 1996).

ResultsWell descriptionsA large number of deep wells have been drilled through theDevonian sequence in the Baltic area, and many of them are de-scribed in the literature (Lyarskaya et al. 1975; Grigelis et al.1991; Kanev & Lyarskaya 1992). However, the majority of thedescriptions are published in the Russian language. The offshorewell used in this paper for stratigraphic correlation of reflectorsis the P6 well.

The following brief description of the Lower and MiddleDevonian of the above-mentioned well is based on well loggingsmade available to the author by the Lithuanian Institute of Geol-ogy in Vilnius and also published by Grigelis et al. (1991).

The P6 well (Fig. 1). – The offshore well P6 was drilled in theoutcrop area of the Old Red Sandstone facies Aruküla-BurtniekiFormations (Fig. 3). In this well the Lower Devonian GargzdaiGroup is present on top of 228 m of Upper Silurian Pridoliandeposits. The Gargzdai deposits consist of clayey siltstone, clay,siltstone, and sandstone. The thickness of the beds is 43.5 m.Ontop of the Gargzdai Group follows the Kemeri Formationconsisting of sandstone with gypsum and ferruginous cement,siltstone cemented with dolomite, and clay. These beds are 50 min thickness.

The Middle Devonian Pärnu Regional Stage is represented bytwo separate units. The lower unit consists of clay, siltstone, andsandstone with dolomitic and gypsic cement. The upper unitcontains sandstone with ferruginous cement, dolomitic cement,and inclusions of gypsum, calcitic cement, intercalations ofsiltstone, silty dolomite, and silty dolomitic marl. The total thick-ness of the Pärnu Formation is 81 m. The overlying Narva For-mation is divided into three different Members. The lower VadjaMember contains clay with intercalations of siltstone, dolomiticmarl, and clayey dolomitic marl. The middle unit, the LeivuMember consists of clay, siltstone with gypsum inclusions,siltstone with calcitic cement, and clayey dolomitic marl. Theuppermost unit, the Kernave Member, consists of siltstone andsilty dolomitic marl, and sandy siltstone with plant remains. TheNarva Formation is 74.5 m thick altogether. The well terminatesupwards with the Aruküla-Burtnieki formations which consistsof sand and loose sandstone, siltstone, sandy siltstone with inter-layers of clay and clayey siltstone. These beds are 60 m thick andare terminated by an erosion surface.

The Quaternary deposits in the P6 well are only 10 m thick andare represented by loam, silt, and clay.

Seismic interpretationThe main goal of the seismic interpretation was to characterisethe depositional and post-depositional history of the area, forwhich purpose it was necessary to distinguish the stratigraphicunits; to interpret their boundaries, lithological changes, erosionsurfaces, etc.; to follow their thickness distributions and litho-logical variability, i.e. the changes of the units in vertical and lat-eral direction and to interpret the tectonic disturbances againstthe background of the marginal position of the Devonian distri-bution area in the Baltic onshore and offshore area.

In the shallow seismic profiles used for this investigation, fiveseismic units were distinguished in the Lower and Middle Devo-nian. The units are bounded either by discontinuity surfaces orby strong reflectors that mark either shifts in lithology or erosion.

Fig. 3. Stratigraphic correlation of the Devonian in the Baltic Sea area.The boundary between the Lower and Middle Devonian is for Lithuaniabased on Paskevicius (1997), for Latvia on Stiebrins (1995), and forEstonia on Kleesment & Mark-Kurik (1997).

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100 Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea GFF 123 (2001)

The Silurian–Devonian boundary. – Thisboundary is marked by a distinct disconti-nuity surface in the seismic profiles. Thesurface conforms to the top of a unit con-taining a reef-like barrier, interpreted asbeing of Late Silurian Pridolian age.

In the northern part of the investigatedarea, close to where the Silurian–Devo-nian boundary reflector becomes exposedin the seabed (Fig. 4), the boundary levelfollows the top of a large reef-like barrier(Fig. 5) which seems to be of the sametype as the reef barriers earlier describedby Flodén (1980). The barrier has a north-east–southwest extension and has beentraced across large parts of the investi-gated area. In a small area it is exposed ina cuesta-like wall in the sea floor (Fig.6A). The cliff rises 30–40 m above thesurrounding sea floor as demonstratedalong one of the seismic profiles shot in1993 (Fig. 6A). The cliff is a very localfeature and is only seen in four shallowseismic recordings, three from 1972 andone from 1993. It diminishes in height toboth sides away from the 1993 recording.To the west of the 1993 profile the cliffcontinues as a distinct morphological fea-ture for some distance, whereas on theeastern side of the profile it is rather flat.The cliff is, however, discernible belowthe Quaternary deposits in the surround-ing area but nowhere as prominent as inthe above-mentioned area. It is rapidly re-duced to only a few metres in height. Thereef-like barrier below the D1 reflectormarking the Silurian–Devonian boundaryis flanked on the seaward side by smallerstructures found at successive deeper lev-els (Fig. 6B).

The Silurian–Devonian boundary ismarked by a stratigraphic break of vari-able size across the present area of inves-tigation. In the northern part, the Pridoliansedimentary rocks are overlain by theLower Devonian Gargzdai Group. To-wards the south-west the Gargzdai Grouphas been removed by erosion and theKemeri Formation is the lowermost De-vonian sedimentary bedrock.

Seismic unit 1. – This unit forms thelowermost Devonian deposit in the north-ern part of the investigated area (Fig. 4).Its lower boundary reflector (D1 in Fig.6B) marks the erosional top Pridolianlevel. The upper boundary reflector of theseismic unit 1 (D2 in Fig. 6B) is a ratherstrong discontinuity surface. The unitwedges out towards the north.

The internal reflectors within seismicunit 1 are short and irregular (Fig. 6B) al-

Fig. 4. Revised map showing the Lower and Middle Devonian sedimentary bedrock exposure be-low the Quaternary deposits in the south-eastern part of the Baltic Sea. Compare with Fig. 9 forcorrelation with seismic units.

Fig. 5. Seismic profile 9309 from the south-eastern Baltic Sea. The section shows the Silurian–Devonian boundary sequence with barrier development in the uppermost Silurian. D1 reflectormarking the Silurian–Devonian boundary; B6 - reef-like barrier; B - biohermal structures; F -fault. For location, see Fig. 2.

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though strong. They are cut at an angle bythe D2 surface. The unit wedges out to-wards the north (Fig. 6C), but is stillpresent at the bedrock surface.

In some of the seismic profiles, the unitnever reaches the bedrock surface. Ero-sion has removed the northernmost part ofthe unit and it is thereby found further tothe south, and further down in the seismicrecord. The D1 surface is thus overlain bya unit terminated by the D3 reflector. Thisdevelopment is seen in the western part ofthe investigated area.

Seismic unit 1 is 43 m thick in seismicline 9804 in the vicinity of the P6 well.

Seismic unit 2. – Towards the south-westalong the Silurian–Devonian boundaryexposure (Fig. 4), seismic unit 1 wedgesout and seismic unit 2 rests directly on thePridolian. The lower (D2) as well as theupper (D3) boundary reflectors of seismicunit 2 are of erosional character in the re-cordings.

The reflectors present within the unitare generally short and irregular as dem-onstrated in seismic profile 9306 (Fig.6B). In this profile, there are also indica-tions of onlap structures in the lower partof the unit.

The thickness of seismic unit 2 in-creases towards the south. In the northernpart of the area, the thickness of the unit is76 m along seismic line 9804 close to theP6 well.

Seismic unit 3. – This unit is exposed be-low the Quaternary deposits in the north-ern and western parts of the investigatedarea (Fig. 4). The upper boundary of theunit (D4 in Fig. 7A) is marked by a reflec-tor, which is by far the most prominent

GFF 123 (2001) Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea 101

Fig. 6. A. The Upper Pridolian cliff. This isone of the locations where the cliff forms asteep cuesta-like wall in the sea floor. It isabout 30 m high and formed in Upper SilurianPridolian beds. For location, see Fig. 2. B.Seismic profile of the Upper Silurian–LowerDevonian sequence. D1–D2 corresponds toUnit 1 and D2–D3 denotes Unit 2 described inthe text. Unit 2 shows onlap structures in thebasal parts. BR - reef-like barrier in the UpperSilurian Pridoli sequence; R - biohermal struc-tures on the seaward slope of the barrier; S -structure interpreted as sand bar. For location,see Fig. 2. C. Seismic profile 9308 showingthe erosional character of the D2 surface wherethe underlying reflectors of Unit 1 are cut atand angle. The overlying unit, Unit 2 that isbounded by the D2 and D3 surfaces, exhibitsdistinct and strong reflections compared to theunderlying unit. For location, see Fig. 2.

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within the seismic records from the area.It is present throughout the entire investi-gated area. Compared to the underlyingseismic units, seismic unit 3 revealsstrong and regular reflectors (Fig. 6B).

Near its exposure area, seismic unit 3contains structures interpreted as sandbars (Fig. 6B). The thickness of the unitincreases at these locations. In some casesthe structures have a smooth relief butthey may be eroded at the top. The smoothrelief structures are mainly found in theexposure area of the unit.

Moving further to the south, and down-wards in the seismic records, the struc-

The thickness of seismic unit 3 is calcu-lated to be 41 m in seismic line 9804 closeto the location of the P6 well.

Seismic unit 4. – Seismic unit 4 is exposedat the bedrock surface south of seismicunit 3 (Fig. 4). Its exposure area is muchwider than that of unit 3. Seismic unit 4 isbounded by strong reflectors (D4 and D5in Fig. 7A) that mark erosional surfaces.The internal reflectors are however, ratherweak and of rather limited extensionwhere the unit is buried below youngersedimentary rocks. However, in the expo-sure area, the reflectors are strong and dis-play a rather regular pattern, which mayreflect a more thinly bedded structure ofthe sedimentary rocks.

The unit is quite homogenous in theseismic records. The reflection pattern issimilar in all records close to the exposurearea. However, the unit is frequently cutby deep glacial channel structures, whichmostly reach down to the D4 reflector.Only in some cases do the channels breakthrough the D4 reflector (Fig. 7B).

The thickness of unit 4 is rather uniformalong all the seismic profiles in the inves-tigated area although the wells in the areaindicate an increase in thickness towardsthe south-west. Unit 4 is 95 m thick alongseismic line 9804 close to the P6 well.

Seismic unit 5. – This unit is exposed atthe bedrock surface further to the souththan the preceding unit 4 (Fig. 4). The in-ternal reflectors in the unit are more disor-dered than in any of the underlying units(Fig. 7A). Occasionally the unit exhibitsmound-shaped seismic reflection patternswhich may indicate cross-bedding. Sev-eral discontinuity surfaces are present in-side of the unit. The bounding reflectors(D5 and D6 in Fig. 7A) are, however,rather strong and towards the top the in-ternal reflectors becomes stronger andmore parallel in their appearance.

Like the previous unit, this unit is alsodissected by deep channel structures filledwith Quaternary deposits. These aremainly found in the northern area, veryclose to the outcrop area.

The unit is 53 m thick along seismic line9804 close to the location of the P6 well.

DiscussionStratigraphic correlationTo correlate the interpreted seismic units1–5, with the stratigraphy in the main ref-erence well, P6, some aspects had to beconsidered.

tures show a more truncated upper sur-face. Nearly all these structures are moreor less cut at right angle by the seismiclines as they show a rather short north-south extension. This is, however, not thecase in seismic profile 9804. Here the areawith the bars is much wider in a north–south perspective. This would indicatethat the bars are cut at a low angle by theseismic survey line. The seismic recordsalso reveal a heavy truncation of the bars,more so than in any of the other seismicprofiles. The upper boundary of the unit,the D4 reflector, has been placed at the topof the bar structures.

Fig. 7. A. Seismic profile of the Middle Devonian sequence. D3–D4 corresponds to Unit 3, D4–D5 denotes Unit 4, and D5–D6 correlates with seismic unit 5 described in the text. M - moundshaped reflection. For location, see Fig. 2. B. Seismic section showing a deep channel that cutthrough parts of seismic units 4 and 5. The erosion has cut through the D4 reflector which marksthe lower boundary of Unit 4. For location, see Fig. 2.

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GFF 123 (2001) Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea 103

No refraction soundings have been done in the area. Thereforethe sound propagation velocities had to be calculated. To do this,velocity modelling was used. The technique is described byFlodén (1980, p. 204) and will not be further discussed here.However, the velocities given by the modelling are the follow-ing: seismic units 1, 2, and 3 - 2600 m/s; seismic unit 4 - 2000m/s; seismic unit 5 - 2400 m/s. The mean sound velocity used forthe Quaternary is 1800 m/s.

Two seismic profiles were then selected for the correlation,seismic profile 9311 that almost passes over the P6 well and seis-mic profile 9804 that passes the well slightly to the west (Fig. 1).The stratigraphic column of the P6 well was superimposed onthe interpreted seismic profile 9311. The result of this is shownin Fig. 8. The Quaternary deposits are thinner in the P6 well thanin the 9311 profile. The stratigraphy in the well comprise all theLower and Middle Devonian Beds, no units are missing. The‘zero-level’ for the correlation is the bedrock surface. The lowerreflector of seismic unit 5, correlates rather well with the lowerboundary of the Aruküla-Burtnieki complex in the well (Fig. 8).The bounding surfaces of the underlying unit, seismic unit 4 alsocorrelates well with the boundaries of the successive sequence,the Narva Beds. The bounding reflectors of the lowermost threeseismic units had to be extrapolated in their general dip to thelocation of the P6 well. Despite this the correlation fits ratherwell, as may be seen in Fig. 8.

The Silurian–Devonian boundaryMany authors (Lyarskaya et al. 1975; Kanev & Lyarskaya 1992;Seredenko 1997; Paskevicius 1997) have discussed the Silurian–Devonian boundary layers in the Baltic Sea and in the Balticstates. According to the investigations by Kurss (1992) extensiveerosion started in late Early Devonian times and in places theerosion reached as deep as into the Silurian sedimentary bed-rock. Flodén (1980) stated that the sedimentation was more orless continuous across the Silurian–Devonian boundary in thecentral part of the Baltic Sea basin but that breaks are present inthe area east of Gotland and in north-western Latvia. This state-ment is in accordance with the previous investigations by Kurss(1975) and Lyarskaya et al. (1975). Flodén, as well as Grigelis etal. (1991), also stated that the indistinct lithological boundarybetween the Silurian and the Devonian, as it is developed innorth-western Latvia, is probably not represented by any markedreflector in the Baltic Sea.

Flodén’s (1980) interpretation of the Silurian–Devonianboundary level in the seismic profiles available at that time wasbased on the presumption that the thickness as well as the lithol-

ogies of the boundary units were similar between the Latvianmainland and the offshore area south-east of Gotland. No sup-porting well data from the offshore area were available at thattime.

Another aspect that has to be considered when correlating re-flectors is earlier investigations. Kumpas (1977) investigated thecuesta like wall (Fig. 6A) off the coast of Latvia which he calledthe Lower Devonian Clint. In addition to interpretations of theseismic profiles he sampled the Clint at five different places. Thesamples contained two types of sedimentary rock, a fossiliferouslimestone and a fossil-devoid limestone. The fossils in theformer limestone were brachiopods and ostracodes of pre-Devo-nian age. The latter limestone was both grey and reddish andcompletely free of fossils. He made his tentative correlationpurely on similarities in lithology of the fossil-devoid limestonewith the Narva siltstone in the Gargzhde-1 well on the Latvianmainland, some 90 km to the east. Over such a long distance, fa-cies changes and sedimentation variations may change consider-ably and thus a reliable correlation is very difficult. No firm evi-dence is presented by Kumpas (1977) for correlating the lime-stone with the Narva Formation. On the other hand, the fossilif-erous limestone present at all locations, reveal pre-Devonian fos-sils. This limestone is, however, regarded by Kumpas as ice-driftmaterial but no evidences are presented for this assumption. It ishere postulated that both limestone types do in fact come fromthe cuesta-like wall. The barrier is therefore considered to be ofpre-Devonian age and not of Narva age.

The last aspect that needed to be considered was the structurebelow the lowermost reflector of seismic unit 1, the large reef-like barrier described above. The reflection pattern of the sedi-mentary sequence just below the D1 surface is similar in all ofthe seismic profiles, they disappear rather suddenly towards thesouth (Fig. 5). It seems as they are ‘cut off’ by some seismicallyharder surface above. This reflection pattern is consistent withwhat can be seen in connection to the reef barriers in the north,earlier described by Flodén (1980). Together with the large reef-like barrier, several smaller structures are found on the slopingseaward surface. These smaller structures are very similar tobioherms on a biohermal slope connected with reef barriers.

The present work conclusively indicates that the Silurian–Devonian boundary is located several tens of metres above thelevel suggested by Flodén (1980). Thus, the Silurian–Devonianboundary of Flodén (1980) is here re-interpreted as an internalsurface within the Pridolian. The, Silurian–Devonian boundaryis in this paper placed at the pronounced erosion surface whichmarks the top of the large reef-like barrier south-east of Gotland,and which is also the top of a prominent cliff structure (Fig. 6A).

Fig. 8. Interpretedsection of seismicline 9311, which hasbeen used forcorrelating thereflectors to thedrilled P6 well.Seismic units 1–5are described in thetext. For location,see Fig. 2.

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104 Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea GFF 123 (2001)

As mentioned, this level is coincident with the top of the NarvaFormation proposed by Kumpas (1977). Seredenko (1997)placed the Silurian–Devonian boundary somewhere in betweenthe boundary of Flodén (1980) and the boundary of the presentinvestigation.

The present investigation also shows that there was a signifi-cant period of erosion either at the end of the Pridolian or at thebeginning of the Lower Devonian Gargzdai Epoch. This isshown by the fact that the Silurian–Devonian boundary is ero-sional throughout the investigated area. The Gargzdai deposits,where preserved, rest on this surface.

The Lower DevonianIn the Baltic Sea basin, the Lower Devonian deposits have gener-ally been regarded as incomplete (Kurss 1992; Grigelis et al.1991; Flodén 1980; Kumpas 1977) or even missing (Sorokin1981). The investigations by Grigelis et al. (1991) and Kurss(1992) showed that the Lower Devonian Gargzdai-Kemeri for-mations, and also the Middle Devonian Pärnu Formation, arediscordantly overlain by the Middle Devonian Narva Formation.The Narva Formation is interpreted as formed during the MiddleDevonian transgression maximum (Kurss 1992; Grigelis 1991).

Seismic unit 1 (Fig. 6B) forms the basal part of the Devonianin the present area. It is located directly on top of the discontinu-ity surface that forms the top of the Pridolian as described above.The unit outcrops along a narrow strip in he northern part of theinvestigated area (Fig. 4). It can be followed for only a short dis-tance towards the south below the increasing thickness of overly-ing sedimentary bedrock. As the overload thickens, the reflectorsbecome weaker in the present seismic records. It has not beenpossible to follow the Lower and Middle Devonian beds furtherto the south. It is, however, reasonable to suggest that the sedi-mentary rocks of unit 1 increase in thickness towards the south.

Unit 1 has been removed by erosion in the western part of thearea where seismic unit 2 rests directly on the Pridolian.

Seismic unit 1 is correlated with the level of the Lower Devo-nian Gargzdai Group in the P6 well. This correlation is based onan extrapolation of the boundary reflectors in the direction of theP6 well (Fig. 8), using an assumed sound propagation velocity of2600 m/s for the unit. However, one can calculate the thicknessof the Gargzdai Group along its outcrop area in the north. In theseismic profile 9311, which runs close to the P6 well, the thick-ness of the Gargzdai Group is about 46 m, which is not far fromthe 43.5 m in the P6 well.

Seismic unit 2 (Fig. 6B) was deposited on the upper erosionalsurface of the Gargzdai Group, seismic unit 1. Unit 2 is exposedin the northern and western parts of the Devonian outcrop area(Fig. 4). Similarly to seismic unit 1, the thickness of seismic unit2 seems to increase southwards. It is not possible to trace the re-flectors any longer distance towards the south, however. Corre-lation with the P6 well shows that seismic unit 2 coincides withthe Lower Devonian Kemeri Formation (Figs. 8, 9). This corre-lation is based on a sound propagation velocity of 2600 m/s in theunit. The bounding reflectors are extrapolated in their generaldip direction, like those of the underlying Gargzdai Group.

The Middle DevonianSeismic unit 3 (Fig. 7A), which follows on top of the KemeriFormation, unit 2, is also bounded by erosional surfaces. Seismicunit 3 contains structures interpreted as sand bars in the areaclose to the outcrop (Fig. 6B). Sand bars tend to develop in ashallow water environment.

In our seismic material it has been possible to follow the in-ferred bar structures across parts of the offshore Devonian area.They show a north-east to south-west direction. The presence ofthese bars indicates that the depositional environment was shal-low, and that the actual shoreline was not very far away.

Seismic unit 3 correlates with the Pärnu Formation in the P6well (Figs. 8, 9). The internal sound velocity of the Pärnu Forma-tion has been estimated to 2600 m/s. Only the lower reflector ofthe Pärnu Formation has been extrapolated to the location of theP6 well. The upper reflector is by far the most prominent in theseismic records of the present area. Thus, it is possible to followthis reflector across the entire Devonian outcrop area. In this pa-per the Pärnu Regional Stage also includes the Rezekne RegionalStage. It has not been possible to separate this unit into the twoformal units, the Rezekne and the Pärnu Regional Stages.

Many previous authors have advocated that exposures of theKemeri and Pärnu Formations at the bedrock surface are missingin the Baltic Sea area (Grigelis 1991; Kurss 1992; Flodén 1980).The main reason for their assumption was that the evidence forextensive erosion preceding the Narva transgression was so im-pressive that major stratigraphic intervals were suspected to beremoved. This investigation shows, however, that both theKemeri Formation and the Pärnu Formation are in fact present inexposures in the northern and western part of the offshore Devo-nian distribution area (Fig. 4).

The next upwards following unit, seismic unit 4 (Fig. 7A), wasdeposited on top of the Pärnu Formation, unit 3. A velocity testrevealed an internal sound velocity of only 2000 m/s in unit 4.This is significantly lower than in the underlying units. The out-crop area of seismic unit 4 is also wider than that of the underly-ing units. The unit is correlated with the Middle Devonian NarvaFormation in the P6 well (Figs. 8, 9). The Narva Formation isconsidered by many authors as having been laid down during the

Fig. 9. Correlation ofreflectors betweenthe drilled P6 welland the adjoiningseismic profiles9311 and 9804. W -water; Q - Quater-nary; B - BurtniekiFormation; A - Aru-küla Formation; N -Narva Formation; P -Pärnu Formation; K- Kemeri Formation;G - Gargzdai Group.Numbers in circlesrefer to seismic unitsdescribed in the text.D1–D6 denotereflectors describedin the text. Note thatthe three lowermostreflectors in theseismic profiles arebased on extrapola-tion.

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GFF 123 (2001) Bjerkéus: The Lower and Middle Devonian of the south-eastern Baltic Sea 105

maximum expansion of the Devonian sea. Thus, its lateral extentwas larger than those of the subjacent deposits (Grigelis 1991;Flodén 1980). The basal part of the Narva Formation does notreveal any interruption in the deposition, at least no such featureis evident in the present recordings. Nor has it been possible todistinguish any onlap features or any other evidences of a trans-gressional depositional environment at the base of this unit. Fur-thermore, the dip of the basal boundary reflector does not sup-port an interpretation of a discordant depositional environment.

The topmost sequence, seismic unit 5 (Fig. 7A), occurs in ex-posures south of those of the Narva Formation. The belt of expo-sure is more or less oriented in a north-east to south-west direc-tion (Fig. 4). Unit 5 is correlated with the Aruküla-Burtnieki for-mations of the Middle Devonian (Figs. 8, 9). The sound propaga-tion velocity used for this unit is calculated to 2400 m/s, which isslightly higher than in the underlying Narva Formation. TheAruküla and Burtnieki formations are the lowermost part of theOld Red Sandstone facies in the Baltic Sea basin. According toKurss (1992) and Kleesment (1993, 1995) the Old Red Sand-stone deposits in this area were laid down in a shallow marinenearshore environment. They base their assumption on finds offish remains in the sequence onland in Latvia. A marine near-shore environment is also supported by the reflection pattern ofthe Aruküla-Burtnieki formations in our seismic recordings fromthe area. The reflectors are very disordered, short and exhibit amound-like reflection pattern in places (Fig. 7A). This would in-dicate a near-shore environment with frequent changes betweensedimentation and erosion.

ConclusionsThe Silurian–Devonian boundary in the Baltic offshore area isrevised in this paper, and is placed at a higher level in the seismicrecords than previously assumed. It is here placed at the level ofan erosion surface that follows the top of several biohermalstructures.

A prominent cliff in the area, which was previously describedas sculptured in the Middle Devonian Narva Formation is nowplaced in the uppermost Pridolian. This cliff, may be followedacross the entire part of the present area, either forming a steepcuesta-like wall on the sea floor or forming a minor morphologi-cal feature buried below the Quaternary deposits.

The Silurian–Devonian boundary reflector, located at the topof the morphological cliff, constitutes the top of a large reef-likebarrier of the same size and general arrangement as the reef-likebarriers previously described from the area east of Gotland.

The boundary between the Silurian and the Devonian ismarked by a stratigraphic gap of variable size in the present area.In the north, the Pridolian is overlain by Lower DevonianGargzdai Group. Further towards the south-west the GargzdaiGroup has been eroded, and the Kemeri Formation forms thelowermost Devonian sedimentary bedrock.

It is shown in the present work that both the Lower DevonianKemeri sedimentary rocks and the Middle Devonian Pärnu rocksare present in outcrops in the northern and western part of theDevonian area.

Extensive sand bar structures have been found in the MiddleDevonian Pärnu Formation. They exhibit a north-east to south-west direction and indicate a nearness to a shoreline.

No evidence for a major discontinuity between the Middle De-vonian Pärnu Formation and the overlying Narva Formation hasbeen found.

The reflection pattern of the Old Red Sandstone faciesAraküla-Burtnieki complex supports a marine shallow near-shore deposition environment.

Acknowledgements. – The author thanks Doc. Tom Flodén, Prof. Maurits Lindström, andProf. Väino Puura for reading the manuscript and suggesting improvements. Their com-ments have been invaluable. The reviewers Dr. Anne Kleesment and Prof. AlgimantasGrigelis are acknowledged for their work. The study was made possible by grants from theStockholm University, the Royal Swedish Academy of Sciences, the Swedish Natural Sci-ence Research Council (NFR) and the Geological Institute in Vilnius, Lithuania. Field workwas performed from the R/V Vejas and the R/V Skagerrak. The crews of these vessels arethanked for their cooperation.

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