Genesis of crystal-rich volcaniclastic facies in the Permian red beds of the Central Pyrenees (NE Spain)

ELSEVIER Sedimentary Geology 106 (1996) 1-19

Sedlmen*_= Geology

Genesis of crystal-rich volcaniclastic facies in the Permian red beds of the Central Pyrenees (NE Spain)

Joan Martf

Institute of Earth Sciences 'Jaume Almera', CSIC, Sold i Sabarfs s/n, 08028 Barcelona, Spain

Received 20 September 1994; accepted 31 October 1995

Abstract

The Permo-Carboniferous volcanism of the Central Pyrenees developed in subaerial conditions, and is mainly represented by the products of explosive eruptions of calc-alkaline silicic magmas occasionally associated with caldera- forming events. The sedimentological record of the Permo-Carboniferous basins of the Central Pyrenees is characterized by the abundance of crystal-rich volcaniclastic deposits reflecting a high crystal content in the erupting magmas (18-40 vol.%). Sedimentation during the Early Permian was dominated by massflows, stream floods and braided channels, organised in poorly developed alluvial fans at basin margins. A wide spectrum of crystal-rich deposits can be identified in the Permian terrigenous red beds. The grouping of these deposits into facies and facies associations, allows the recognition of several volcaniclastic marker horizons extending through the Central Pyrenees. These can be used to establish stratigraphical correlations between the different Permo-Carboniferous basins. The study of these crystal-rich volcaniclastic deposits has revealed that the Lower Permian crystal-rich volcaniclastic deposits of the Central Pyrenees originated from pyroclastic and reworking processes that sometimes acted simultaneously. Pyroclastic deposits derived from high intensity explosive magmatic eruptions of rhyodacitic and rhyolitic crystal-rich magmas. These eruptions generated high eruption columns that occasionally collapsed, giving rise to the formation of widespread pyroclastic flows and associated pyroclastic surges. The entrance of the eruption columns into the atmosphere had a significant effect on the local weather, provoking sporadic storms that led to reworking of pyroclasts by rainfall run-off during eruption. Different crystal concentration processes acted in the eruption columns and in the subsequent pyroclastic flows and overriding ash clouds, producing the accumulation of a high volume of crystals in the resulting deposits. Reworking and redeposition of pyroclastic deposits by fluvial processes further enhanced crystal concentration.

1. Introduction

One main feature of Permo-Carboniferous successions in the Central Pyrenees is the occurrence of thick volcanic sequences interlayered with non- volcanic sedimentary rocks, deposited in different sectors of the Pyrenees in separate basins of relatively small dimensions. Traditionally, the Permo- Carboniferous basins of the Central Pyrenees have been considered to have originated from strike-slip

dynamics that developed during a compressional episode at the end of the Variscan orogeny. This has been established from facies analysis (Gisbert, 1981, 1983; Bixel and Lucas, 1983; Speksnijder, 1985; Martf, 1986, 1991; Besly and Collinson, 1991), and from palinspastic reconstruction of the Alpine thrust units where these late-Variscan sequences were in- corporated (Mufioz, 1985, 1992; Casas et al., 1989; Soriano et al., in press).

The existence of late-Variscan volcanic rocks in

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2 J. Marff/Sedimentary Geology 106 (1996) 1-19

the Pyrenees has been known for a long time (Dal- loni, 1910, 1930; Viennot, 1929; San Miguel de la C~imara, 1935, 1936; Mirouse, 1959; Morre and Thiebault, 1964, 1966; Nagtegaal, 1969), but most of the references concern the Upper Carboniferous volcanics which are represented mainly by thick sequences of lavas and ignimbrites. However, the presence of primary pyroclastic rocks in the Permian has been controversial. Crystal-rich deposits in the Permian red beds have usually been considered as resulting from epiclastic reworking of the Upper Car- boniferous volcanic terrains (Mirouse, 1959; Morre and Thiebault, 1964, 1966; Nagtegaal, 1969; Lucas, 1977; Speksnijder, 1985) rather than being the products of Permian volcanic events. However, recent studies (Puga and Fontbot6, 1979; Gisbert, 1981, Martf, 1986, 1991; Martf and Barrachina, 1987) demonstrated the existence of primary pyroclastic deposits in the Permian red beds along the Central Pyrenees.

In this paper, the characteristics of these crystal- rich deposits are documented and their volcanolog- ical significance established. A clear distinction between pyroclastic (primary volcaniclastic deposits originating from explosive eruptions) and secondary volcaniclastic (Fisher, 1966) (secondary deposits re- suiting from reworking and redeposition of pre- existing volcaniclastic deposits) deposits is made on the basis of facies analysis of the deposits. This has also allowed the recognition of pyroclastic marker horizons that can be followed along the entire Central Pyrenees and which provide important constraints for stratigraphic and palinspastic recon- structions of the Pyrenean Permo-Carboniferous terrains. Finally, many well documented crystal-rich deposits are submarine (Roobol, 1976; Cas et al., 1981, 1990; Cas, 1983; Cas and Wright, 1991; Gimeno, 1994), whereas in contrast, this paper documents crystal-rich pyroclastic and secondary volcaniclastic deposits formed in a subaerial environment.

2. Geological setting

The Pyrenees is an Alpine fold and thrust belt, trending WNW-ESE, that involves Variscan basement rocks, Cambrian to Lower Carboniferous in age, and a Late Carboniferous to Oligocene cover (Fig. 1). The Pyrenean belt has been described as an imbri-

cate thrust system dipping south in the northern part, an antiformal stack in the central part, and an imbri- cate thrust system dipping north in the southern part (Mufioz, 1992) (Fig. 1). The Permo-Carboniferous rocks are located in the lower structural units of the antiforrnal stack and, in plan view, represent a nar- row WNW-ESE trending belt, roughly following the shape of the Alpine structural units and defining a nearly continuous band which stretches from Pont de Suert to Camprodon (Figs. 1 and 2).

A detailed stratigraphy of the Permo- Carboniferous rocks of the Central Pyrenees has been proposed by Gisbert (1981, 1983). Integrating lithological, sedimentological and structural data, he divided the Permo-Carboniferous rocks of the Cen- tral Pyrenees into five major units: the Grey Unit (Middle Stephanian), the Transition Unit (Upper Stephanian-Lower Permian), the Lower Red Unit (Lower Permian), the Upper Red Unit (Upper Per- mian), and the Bunter Sandstone Facies Unit (Lower Triassic) (Fig. 3).

According to Gisbert (1981, 1983), the Grey Unit (400-1000 m) was deposited under humid climatic conditions and comprises pyroclastic deposits of dacitic to rhyolitic composition, andesitic and occasionally rhyolitic lavas, with some polymictic sedimentary breccia and conglomerate located at the base of this unit at the margins of the original basins. The Transition Unit (120-400 m) conformably overlies the Grey Unit and comprises rare lacustrine lime- stone, detrital sedimentary deposits, coal beds, and volcanic rocks (rhyodacitic ignimbrites and dacitic lavas). The Lower Red Unit (500-1500 m) lies conformably above the Transition Unit and was deposited under arid climatic conditions. It is represented mainly by fluvial deposits, and contains voluminous rhyolitic lava flows and pyroclastic deposits. The Upper Red Unit (100-350 m) lies un- conformably over the Lower Red Unit and is represented entirely by fluvial deposits rich in volcanic fragments. The Bunter Sandstone Facies Unit (60- 630 m), separated from the Permo-Carboniferous units by a major angular unconformity, contains no volcanics and is composed of fluvial sediments.

Exposures of Permo-Carboniferous rocks are discontinuous, and sometimes Triassic beds lie directly on the Variscan basement. Each area of continuous exposure of Permo-Carboniferous rocks is thought to

J, Marff/Sedimentary Geology 106 (1996) 1-19 3

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Fig. 1. Schematic geological map of the Pyrenean mountain belt (after Puigdefabregues and Souquet, 1986) and location of the studied area. Variscan basement, Permo-Carboniferous and Mesozoic were involved in Alpine deformation. Triassic includes: Lower Triassic sandstones, Middle Triassic limestones and Upper Triassic evaporites that acted as an upper detachment surface during Alpine deformation. Oligocene conglomerates are post-orogenic Alpine deposits.

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248 Ma

285 Ma

300 Ma

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AGE UNITS

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UNIT

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Fig. 3. Age of Permo-Carboniferous units based on the Upper Carboniferous time scale of Hess and Lippolt (1986). The age of the Permian-Triassic boundary has not been calibrated by these authors, but a minimum age of 248 Ma is considered by Harland et al. (1982).

belong to an independent basin. On the basis of facies analysis of volcanic and sedimentary rocks and structural geology data, five Permo-Carboniferous basins have been identified in the Central Pyre- nees (Gisbert, 1983; Mufioz, 1985; Marti, 1986, 1991; Soriano et al., in press): the Erill CasteU, Estac, Gram6s-Prats d'Aguil6 (the Cadf basin in Marti, 1991), Castellar de N'Hug and Campelles basins (Fig. 2). The palinspastic reconstruction of several Alpine thrust units which include Permo- Carboniferous terrains (Mufioz, 1985; Casas et al., 1989; Soriano et al., in press) constrains the original position and characteristics of these basins (Fig. 4).

3. Characteristics of Permo-Carboniferous volcanism

Permo-Carboniferous volcanic rocks of the Cen- tral Pyrenees are represented by a suite of basaltic

andesite to rhyolite of calc-alkaline orogenic type (MartL 1986), with a predominance of rhyolitic and rhyodacitic volcaniclastic deposits. Volcanic activity was essentially continuous from the deposition of the first Middle Stephanian (Upper Carbonifer- ous) sediments to the beginning of Upper Permian sedimentation, a duration of about 18 Ma (Martf, 1986) (Fig. 3). No significant change in the nature of the Permo-Carboniferous volcanism is apparent despite the considerable time period encompassed. The Permo-Carboniferous basins of the Central Pyre- nees can be considered as volcano-tectonic depres- sions in which caldera-forming episodes occurred (Martf, 1991). A striking characteristic of Permo- Carboniferous volcaniclastic deposits is their high crystal content. High magmatic crystal contents (18- 40 vol.%), inferred from contents in pumice, show that these silicic magmas commonly erupted near their solidus temperature (Mart/, 1986).

During the Late Carboniferous, volcanism is represented by lavas and pyroclastic deposits covering a wide range of compositions. Pyroclastic deposits were mainly associated with caldera-forming events that affected entire basins. Most of these deposits appear to be remnants of intra-caldera fill. Calderas were filled by thick (sometimes more than 1000 m) sequences of volcanic rocks, which locally make up the only exposures of Upper Carboniferous rocks, for example at the Erill Castell and Estac basins. More- over, during this period climatic conditions were humid (Gisbert, 1981, 1983). A large amount of me- teoric water was available, both as groundwater and surface water, resulting in phreatomagmatic activity sometimes preceding caldera-forming eruptions.

During the Early Permian, only rhyodacitic and rhyolitic magmas were erupted as relatively thin crystal-rich deposits interbedded within the sedimentary red beds. The only outcrop of massive lavas of Early Permian age is the Greixer Rhyolitic Complex, located at the western margin of the Castellar de N'Hug basin. The Greixer Complex is characterised by the continuous exposure (wider than 12 km) of a 500-m-thick sequence of intra-caldera lava flows with some associated pyroclastics (Mart/and Bar- rachina, 1987). Climate during this period changed to arid conditions (Gisbert, 1981, 1983). No hy- drovolcanic eruptions have been identified (Martf, 1986).

J. Martf / Sedimentary Geology 106 (1996) 1-19 5

~i,_ N Greixer Rhyolitic iEstac basinl Complex . ~

Erill Castell basin I ]Grambs-Prats d'Aguilo Ibasin

Castellar de N'Hug basin

20km

10 Icampellesl

0 10 20 km ib.,.. I Fig. 4. Schematic representation of the original relative position of the Permo-Carboniferous basins of the Central Pyrenees from the palinspastic reconstruction of the Alpine thrust units (based on data of Mufioz, 1985; Casas et al., 1989; Soriano et al., in press).

During the Late Carboniferous each basin seems to have had an independent volcanic history, with several volcanic centres located around the basin margins. In contrast, during the Early Permian most of the volcanic activity seems to have been restricted to the Castellar de N'Hug basin, and particularly to the Greixer Rhyolitic Complex. This is known from facies analysis and stratigraphical correlations of the volcanic units (Gisbert, 1981; Martf, 1986). Most of the pyroclastic deposits appear in the Lower Red Unit in the other basins as distal facies which, according to their variations in thickness and grain size, can be traced back to a plausible vent area that coincides with the original location of the Greixer Rhyolitic Complex. This is confirmed by the grad- ual disappearance of primary pyroclastic deposits away from that area. However, the possibility that other volcanic centres were active during the Early Permian in the Central Pyrenees cannot be ruled out.

4. Characteristics of the Lower Permian red beds

The Lower Red Unit appears in all the basins as thick (500--1500 m) sequences of continental sedimentary rocks with some interbedded pyroclastic deposits and calcareous palaeosols (caliches). The restricted character of the basins implies that the Lower Red Unit, as the rest of the Permo- Carboniferous units, may show different character-

istics in each particular basin depending on their dimensions, rate of subsidence and nature of the associated volcanic rocks. However, the Lower Red Unit always contain components of volcanic origin, although these diminish progressively towards the top of the sequence. Detrital sedimentation during the Lower Permian was dominated by mass flows, stream floods and small ephemeral braided channels, organised in poorly developed alluvial fans at the base of the basin margins (Nagtegaal, 1969; Gisbert, 1981; Speksnijder, 1985).

Volcaniclastic deposits are mainly concentrated in the lower third of the Lower Red Unit. In the Erill Castell, Estac and Prats d'Aguil6 basins, these deposits mainly appear as distal facies forming relatively thin units. In the Castellar de N'Hug area there is a sequence of crystal-rich volcaniclastic deposits (the Castellar de N'Hug Ignimbritic Member; Martf and Barrachina, 1987) consisting of primary pyroclastic deposits interbedded with secondary volcaniclastic deposits. The sequence is 100 m thick near the village of Castellar de N'Hug close to the Greixer Rhyolitic Complex and thins progressively towards the eastern margin of the basin (Fig. 5). As indicated above, some deposits of this sequence can also be traced in the Prats d'Aguilr, Estac and Erill Castell basins. Martf and Barrachina (1987) related the emplacement of the Castellar N'Hug Ig- nimbritic Member to the long-lived activity of the

J. Marff /Sedimentary Geology 106 (1996) 1-19

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The Castellar de N'Hug ignimbrite

Lower Permian crystal-rich volcaniclastlc facies

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Volcaniclastic sandstone facies (VSF) (channelised VSF sub-facies)

Volcaniclastic sandstone facies (VSF) (interfingered VSF sub-facies)

Crystal-rich massive facies (MF)

~ Pumice-rich facies (PF) (thin PF sub-facies)

~ umice-rich facies (PF) (thick PF sub-facies)

~ Laminated to low angle cross-bedded facies (CBF) (top CBF sub-facies)

~ Laminated to low angle cross-bedded facies (CBF) (base CBF sub-facies)

~ Caliche facies (C~

Fig. 5. Representative stratigraphic column of the Lower Permian volcano-sedimentary sequence at Castellar de N'Hug (see Fig. 2 for location). The intervals 5-68 m and 120-810 m are not represented due to scarce pyroclastic material.

J. Marff / Sedimentary Geology 106 (1996) 1-19 7

Greixer Rhyolitic Complex which culminated in a large-volume caldera-forming eruption and the emplacement of the intra-caldera rhyolite lavas.

5. Facies of the Lower Red Unit crystal-rich deposits

On the basis of geometry, lithology and sedimentary structures, several different facies and sub- facies have been defined among the Lower Red Unit crystal-rich volcaniclastic deposits. The lateral continuity of some of these deposits for distances on the order of several tens of kilometres has allowed precise stratigraphic correlations between the different basins, permitting a comparison between the different Lower Red Unit sequences.

In general, the original lithology and sedimentary structures in the facies can be easily identified. The original vitric components have devitrified to crystalline aggregates during diagenesis, but in most cases the original morphology of these components has been preserved. However, the complex nature of the outcrops due to Alpine deformation sometimes makes the identification of facies geometries difficult. Non-volcanic sedimentary facies have been described extensively in previous contributions (Nagte- gaal, 1969; Lucas, 1977; Gisbert, 1983; Martf, 1986; Speksnijder, 1985) and are not considered here.

5.1. Crystal-rich massive facies (MF)

This facies appears in the Castellar de N'Hug and the Gram6s-Prats d'Aguil6 basins, distributed radially around the Greixer Rhyolitic Complex. Only two units have been recognised in the Lower Red Unit of both basins. These occur close to the base, forming a single succession but clearly separated by a sharp contact. Beds of the MF are internally massive and composed of crystals and crystal fragments of plagioclase (35% rock volume), quartz (10%) and biotite (8%) (Fig. 6a) less than 1.5 mm in diameter, in a cryptocrystalline matrix (47%). Pla- gioclase and biotite crystals are generally idiomorphic or sub-idiomorphic while the quartz crystals are mainly fragments, many of them displaying embayed morphologies. Plagioclase crystals are occasionally zoned. Very few vent-derived lithic clasts of Or- dovician and Devonian rocks have been found in

this facies. The matrix has been completely altered and no original vitric components remain. However, some original morphologies of shards have been preserved despite diagenetic devitrification (Fig. 6b). No pumice clasts have been observed. The MF has a maximum thickness of 2 m in areas close to the Greixer rhyolites, diminishing progressively with distance away from the rhyolites. The MF is laterally continuous along the basin length over a maximum distance of 60 km except where they have been eroded out by epiclastic processes. The MF is thicker (0.7 to 2 m) where it infills pre-existing channels and thinner (0.25 to 0.6 m) on pre-existing topographic highs.

The laterally continuous (for the entire length of the Castellar de N'Hug and Gram6s-Prats d'Aguil6 basins) sheet-geometry, the thickening in topographic valleys and lithology (especially the juvenile vitroclastic nature) of the MF suggest that they are the deposits of crystal-rich pyroclastic flows that probably originated by the collapse of an eruption column from the Greixer vent area (Martf and Bar- rachina, 1987). The absence of pumice clasts makes the crystal content of the original magma difficult to estimate, but the comparison with the Lower Red Unit pumice-rich deposits suggests that some crystal concentration processes acted during the eruption and subsequent emplacement of the ME No signs of reworking and redeposition have been observed in these facies, again suggesting this facies is a primary pyroclastic facies. A flow origin rather than a fall origin is supported by the thickening in topographic lows.

5.2. Pumice-rich facies (PF)

The PF is similar to the MF facies but includes abundant crystal-rich pumice fragments. The PF is included in the Castellar de N'Hug Ignimbritic Mem- ber (Fig. 5) which is about 100 m stratigraphically above the MF units. The PF can be subdivided into two sub-facies. First, the thick PF sub-facies includes two units, This facies is present in the Castellar de N'Hug basin. The crystal volume, made up by free crystals and crystal fragments in the matrix and by phenocrysts in the pumice fragments, represents about 25--40% of the total rock volume. Crystals are of plagioclase, alkali feldspar, quartz and biotite;

8 J. Martt’/Sedimentary Geology 106 (1996) 1-19

J. Martf/Sedimentary Geology 106 (1996) 1-19 9

Fig. 6. Photomicrographs of the Lower Permian crystal-rich volcaniclastic facies (scale bars 500 /~m). (a) Crystal-rich massive facies (MF). (b) Crystal-rich massive facies (MF) (vitroclasts). (c) Pumice-rich facies (PF) (thick sub-facies). (d) Pumice-rich facies (PF) (crystal-rich pumice fragment in the thin sub-facies). (e) Laminated to low angle cross-bedded facies (CBF) (base sub-facies). (f) Laminated to low angle cross-bedded facies (CBF) (top sub-facies). (g) Parallel laminated facies. (h) Caliche facies (CF) (light, carbonate matrix; dark, clay matrix). (i) Volcaniclastic sandstone facies (VSF) (interfingered sub-facies). (j) Volcaniclastic sandstone facies (VSF) (channelised sub-facies).

they are idiomorphic to subidiomorphic. Centime- tre to tens of centimetre sized crystal-rich flattened pumice fragments (Fig. 6c) are abundant. The phenocryst content (18-40%) of these juvenile clasts is close to the total crystal content of the facies. Pumice clasts have been transformed into cryptocrystalline quartz but the original morphology of the fragments has been preserved. This suggests that flattening of pumice clasts probably resulted from diagenetic compaction (cf. Branney and Sparks, 1990) rather than from a welding process. The matrix of the facies is presumably vitroclastic, and has been silicified, as have the pumice clasts, probably due to diagenesis. This facies is internally massive and thickens in topographic lows. It has a total thickness of 17 m near the village of Castellar de N'Hug and decreases toward the eastern margin of the basin. It extends laterally over a distance of 30 km (Fig. 5). No signs of reworking were observed in this facies. The two units which form this facies are separated by a sharp planar contact and show at their bases and tops thin crystal-rich, cross-bedded primary pyroclastic deposits (laminated to low-angle cross-bedded facies) (Fig. 5). No internal textural variations have been observed in these two units.

Second, the thin PF sub-facies comprises several units. It is characterised by the presence of abundant crystal-rich pumice fragments. The pumice fragments of the thin PF sub-facies are centimetre- sized, sometimes flattened and occur preferentially

towards the top of each unit. They are set in a completely devitrified matrix transformed into chlorite, clay minerals and Fe-oxides. The pumice fragments, however, are transformed into cryptocrystalline quartz (Fig. 6d). This facies is also interpreted as ignimbritic. The content of crystals of this facies is relatively lower than in the thick PF sub-facies, the crystals being concentrated in the pumice fragments. A distinctive characteristic of the thin PF sub-facies is that the pumice fragments have been preferentially weathered, giving the rocks a 'Swiss cheese' aspect (Fig. 7). The most representative unit of this sub-facies corresponds to the topmost unit of the Castellar de N'Hug Ignimbritic Member (named the Castellar de N'Hug ignimbrite in Martf and Barrachina, 1987). This unit forms one of the best marker horizons of the Lower Permian red beds in the Central Pyrenees. It is laterally continuous along the entire Castellar de N'Hug, Gram6s-Prats d'Aguilo, and Erill Castell basins, involving a distance of more than 100 km. This unit is thickest at Castellar de N'Hug (10 m) and decreases radially from that area (Fig. 8). However, the other units of the thin PF sub-facies, with each unit having a maximum thickness of only a few tens of centimetres, were completely eroded out a few kilometres away from Castellar de N'Hug. The presence at its base and top of thin crystal-rich, cross-bedded primary pyroclastic deposits is also characteristic of the PF.

This facies can be interpreted as ignimbrites that

10 J. Mart[/Sedimentary Geology 106 (1996) 1-19

Fig. 7. Close-up view of the Castellar de N'Hug ignimbrite (pumice-rich facies, PF) characterised by the presence of abundant flattened pumice fragments (light). Flattening is probably due to diagenetic processes rather than welding. Note also the presence of pumice fragments that have been preferentially weathered giving the rock a characteristic 'Swiss cheese' aspect.

basin I 2.3m I

0 km

~ distribution of the Greixer fallout deposits Icampelles I

I ha'in I I thickness of the Castellar de N'Hug ignimbrite

Fig. 8. Variation in thickness of the Castellar de N'Hug ignimbrite and distribution of associated fallout deposits.

J. Martf/Sedimentar3, Geology 106 (1996) 1-19 I 1

originated from the eruption of crystal-rich magmas. This interpretation is based on the following fea- tures: lateral continuity over 100 km; no evidence of internal reworking; presence of primary crystal-rich pumice fragments increasing in importance toward the top of the sequence. The absence of any dis- continuity between the two units of the thick PF sub-facies suggests they were emplaced in rapid succession. However, the interbedding of reworked sediments with the thin PF sub-facies units indicates that this sub-facies includes the products of several distinct eruptions.

The top CBF sub-facies (Fig. 6f) always occurs at the top of the PF facies and shows both erosional and transitional contacts with it (Fig. 9). This sub-facies is always less than 30 cm thick. It is generally similar to the base CBF sub-facies but shows smaller crystal sizes and lower crystal contents than the base CBF sub-facies. The top CBF sub-facies is laminated and occasionally shows low-angle cross-stratification. No lithic clasts have been observed. The top CBF sub- facies has been interpreted as turbulent ash cloud surge deposits generated above the pyroclastic flows that produced the PF (Martf and Barrachina, 1987).

5.3. Laminated to low-angle cross-bedded facies (CBF)

The CBF is invariably associated with both PF sub-facies but never appears as single units. This facies is composed of abundant angular to sub-rounded crystal fragments, devitrified vitroclasts and small lithic fragments in a relatively sparse fine-grained matrix. The distinctive characteristic of the CBF is the presence of internal low-angle cross-bedding. From their lithological characteristics and to their position with respect to the associated PF, two sub- facies are recognised: the top CBF sub-facies and the base CBF sub-facies.

The base CBF sub-facies occurs at the base of both PF sub-facies and is much richer in crystals (45-65% of the total rock volume) of the same nature than those in the associated overlying PF facies (Fig. 6e). Occasionally, crystals and crystal fragments are slightly rounded. The base CBF sub-facies is everywhere less than 25 cm thick, is laminated and in places shows low-angle cross-stratification. This sub-facies displays a discontinuous erosional base and at the top shows gradational transition into the PF facies above. The matrix has been completely altered to cryptocrystalline quartz or clay minerals and chlorite, depending on the characteristics of the associated PF facies. Another distinctive feature of the base CBF sub-facies is the presence of small (mm) lithic fragments whereas the PF facies typically lacks lithic fragments. Martf and Barrachina (1987) interpreted the base CBF sub-facies as pyroclastic ground surge deposits formed in advance of a moving pyroclastic flow (now represented by the associated PF).

5.4. Parallel laminated facies (PLF)

A single unit of PLF has been recognised and, together with the Castellar de N'Hug ignimbrite, constitutes one of the most extensive beds of the Central Pyrenees. This facies is exposed close to the western margin of the Gramrs-Prats d'Aguil6 basin and in the Erill Castell basin, areas thought to be relatively distal from the source. The PLF appears at the top of the Castellar de N'Hug ignimbrite, clearly separated by a planar contact. The PLF mantles topography and shows poorly defined planar stratification. The thickness of the PLF ranges from 70 cm in the Gram6s-Prats d'Aguil6 basin to 30 cm at the western margin of the Erill Castell basin. The PLF is composed of sub-millimetre size idiomorphic and subidiomorphic crystals and angular fragments of crystals of zoned plagioclase, quartz and biotite (Fig. 6g). The volume of crystals represents more than 60% of the total rock volume. The matrix is composed of chlorite and clay minerals probably produced by devitrification of an original vitric matrix, as is indicated by the presence of pseudomorphs of the original vitroclasts. Martf and Barrachina (1987) interpreted this facies as a primary pyroclastic fall deposit resulting from a high eruption column generated in the same eruptive episode responsible for the formation of the Castellar de N'Hug ignimbrite, and possibly the formation of the Greixer caldera. However, the characteristics of this facies (located at the top of the Castellar de N'Hug ignimbrite; crystal-rich, internally finely stratified) and its close association with the Castellar de N'Hug ignimbrite, suggest instead that the deposit could be co-ignimbrite ash generated by that ignimbrite, or

12 J. Marff / Sedimentary Geology 106 (1996) 1-19

Fig. 9. Parallel to low angle cross-lamination in top CBF sub-facies

else the result of a secondary explosion resulting from the entry of the pyroclastic flow into water (cf. Rotoiti Breccia and Rotoiti Ash, New Zealand; Walker, 1979).

5.5. Caliche facies (CF)

The CF is characterised by both well stratified and internally massive units from a few to a few tens of centimetres thick which are laterally continuous and mantle the palaeotopography (Fig. 10). This facies can be recognised in the Lower Permian red beds in all the basins and some of the units can be correlated across different basins. The CF consists of variable proportions of fine-grained angular crystal fragments in a microcrystalline carbonate (calcite) matrix. The carbonate clearly replaces an original vitroclastic matrix primarily altered to clay minerals (Martf, 1986). The degree of replacement of the matrix by carbonate varies from an incipient stage in which the volume of carbonate is less than the 10% of the total rock volume, to a final stage in which the original matrix has been completely replaced by car-

at the top of thin pumice-rich sub-facies at Castellar de N'Hug.

bonate (Fig. 6h). The CF occurs either at the top of PF and CBF or as isolated units characterised by the presence of root and mud crack moulds. The presence of relict pyroclastic textures in some deposits and the order of replacement of the original matrix suggest that the CF represents caliche palaeosols developed on primary fine ash fall deposits. These were probably generated by both plinian eruptions and by co-ignimbrite ash clouds developed during the emplacement of pyroclastic flows and associated surges.

5.6. Volcaniclastic sandstone facies (VSF)

The VSF is the most abundant crystal-rich facies of the Lower Permian red beds of the Central Pyre- nees. This facies comprises multiple units associated with any of the previous facies or interbedded with sedimentary deposits. Beds of the VSF show well developed internal planar or cross-stratification and have channelised geometry of a limited (from a few metres to a few tens of metres) lateral extent. The bases of the beds typically display sharp erosive con-

J. Marff / Sedimentary Geology 106 (1996) 1-19 13

Fig. 10. Distal caliche facies (CF) overlying pumice-rich facies (PF) at Gram6s.

tacts (channelised VSF sub-facies), but sometimes when associated with PF, this contact is irregular and interfingers with the underlying facies (interfingered VSF sub-facies) (Fig. 1 l). A distinctive characteristic of the interfingered sub-facies is the presence of abundant fragments of PE The crystal content of the VSF varies significantly from one unit to another but is always higher (55-82% of the total rock volume) than that of the other crystal-rich facies. The matrix is composed of clay minerals and Fe-oxides. Crys- tals are quartz, biotite and plagioclase, the relative proportions of which vary throughout the entire sequence of Lower Permian red beds (Figs. 6i and 6j). Plagioclase is more abundant at the lowest part of the sequence, especially when the VSF is associated with the PF (interfingered VSF sub-facies), and dis- appears progressively towards the top of the Lower Permian sequence. The proportion of matrix and the size of crystal fragments also decrease in the same sense, whereas the roundness of crystals increases.

The VSF can be interpreted as the result of reworking and redeposition by fluvial processes of crystal-rich pyroctastic deposits or previous volcani-

clastic sandstones. From Fig. 12 it is possible to interpret the different significance of the two geometries of the VSF in terms of their association or the eruptive process. The well-defined channelised geometry of the channelised VSF sub-facies may be the result of erosion and redeposition of volcaniclastic deposits by fluvial processes when volcanic activity temporarily ceased. The decrease in the proportion of this facies towards the top of the sequence and its textural variations suggest that the influence of volcanism progressively decreased during the last episodes in the formation of the Lower Per- mian red beds. However, the presence of irregular and narrower channels showing interfingering with PF (interfingered VSF sub-facies) suggests an oc- casional synchronicity in the emplacement of both facies. This is also confirmed by the immaturity of the interfingered VSF sub-facies, by the existence of erosion at the top of this sub-facies due to the emplacement of CBF, and by the presence in some cases of the same unit of caliche (CF) covering both PF and VSE

14 J. Mart£/Sedimentary Geology 106 (1996) 1-19

Fig. 11. Interfingered volcaniclastic sandstone sub-facies (VSF) forming an erosional contact with a thin pumice-rich (PF) sub-facies at Castellar de N'Hug. Note the irregularity of the contact and the presence of thin PF sub-facies fragments (light and dark parts) in the interfingered VSF sub-facies.

6. Discussion

The facies analysis presented in the previous section combined with the structural data concerning the reconstruction of the original position of these basins (Mufioz, 1985, 1992; Casas et al., 1989; Soriano et al., in press), constitute the basis for interpreting the origin of the Lower Permian crystal-rich deposits. Accordingly, the following discussion is addressed to three main aspects: (i) origin of crystal-rich volcaniclastic deposits, (ii) crystal concentration processes, and (iii) characteristics of eruptions responsible for the formation of these deposits.

6.1. Origin of crystal-rich volcaniclastic deposits

This study strongly suggests that pyroclastic, reworking and redeposition processes were important in the formation of the crystal-rich volcaniclastic

deposits. Most of the crystal-rich volcaniclastic deposits resulted directly or indirectly from volcanism that was coeval with the deposition of the continental Lower Permian red beds of the Central Pyrenees. Pyroclastic processes were predominant during the formation of the lower part of the Lower Permian deposits, whereas reworking and redeposition processes are represented throughout the entire sequence. As has been previously described, volcaniclastic sedimentation in the lower part of the Lower Permian sequence, especially in the Castel- lar de N'Hug Ignimbritic Member, was dominated by pyroclastic and reworking processes that acted simultaneously. Most pyroclastic deposits were derived from the explosive activity of the Greixer Rhy- olitic Complex, mainly characterised by rhyolitic and rhyodacitic eruptions (Martf and Barrachina, 1987). The existence of reworking and redeposition processes acting simultaneously with the emplacement

J. Martf / Sedimentat3" Geology 106 (1996) 1-19

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

15

;i~i~ ~ i ~ i ~ i ~ i ~ i ~ ~i~i~i~i~ ~i~ii ! i ~ i ~ { ~ ! ~ ~i~J~ ~H~HiI~ ~i~ ~iii~i~ {i~!~H~i~ ~i~ ~i~ ~ J ~ i ~ . . . . . . . . . .

~i~i!i~i~ii~ii~i~ ~i~ i~ i i~i i i!i!i~i i~i!i~ i~ i~i~i~ ~i!i!~ ~ i~!!i~ ~ i!~i! i ~i i~ ~ ~i ~! i Zi;~Z~Z~'! - ~i | ' ~ - i - i - i ' i - i " - ~ - E ' I ' I - I - i - ~ - i - i - ~ - - i - ~ ' i " - I Z I - I ' I ' I ' I - I * ~ - ~ ' I ' I I - E - I - I - I - I - I - I - I - I - - - i - - * i . . . . i - ~ - i . . . . . . i - - •

i ~ i i~ !~ I~ i i i i i i ~ i~ i~ .~ !~ i I i i ~ i i [~ I i~ !~°T~ i i i i ! °~° i i~ ' i~ i i~ i~ i i ! i i ~ `~ ! i~ i i ! i !~ i i !~ i i i ....... i l i ' i ...... i ° i ...... i ' " " i ..... :!:i:|:i:} :i:i: i: i : :i |: : : : :|: : : i: i:i :i: : i:i i i: | :~:~:~:~:~:~:i:i:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:i:~:~:~:i:~:~:~:~:|:~:~:E:i:~:~:~:~:~:~:~:|:~:~:~:i:~:~:~:~:~:~:!:~:~:~:~:~:~:!:~:~:~:~:~:~:~:~:i:~ ~i~i~i~i ~i~ !i i~ i~ i~i ~ i; i~! ~ii~i ~ I ~ i:i ~!~i~! i i ~ ~ :i ~i~i:i~ i~i~i

:i:i:i:|:!: I:!:i:i:i:i ! | : i : I | i: :~:~: I l:I:l:|: :~:~:!:i: :~:~:~ I :~:~:Ei :!:!:! :~:~:|: :i :~:E ~: : : iEi i:i:

ilillliililiilliililiiiiiilliiiii:iiiiiiiililililliiiiiliiiiijliiliiiiliili : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

~ Volcaniclastic sandstone facies (VSF) (channelised sub-facies)

~ Volcaniclastic sandstone facies (VSF) (inteffingered sub-facies)

I T ~ Crystal-rich massive facies (MF)

~ 1 Caliche facies (CF)

~ Siltstones and claystones

0 2m I I

Pumice-rich facies (PF) (thick sub-facies)

I umice-rich facies (PF) (thin sub-facies)

~ Laminated to low angle cross-bedded facies (CBF) (top sub-facies)

~ Laminated to low angle cross-bedded facies (CBF) (base sub-facies)

Fig. 12. Schemat ic representation of the characteristic facies associat ions of the Lower Permian crystal-rich volcaniclast ic deposits in the Castel lar de N ' H u g area,

16 J. MartNSedimentary Geology 106 (1996) 1-19

W

simultaneous emplacement of reworking pyroclastic flows of pyroclasts and associated by rain fall surge and fall run-off deposits

simultaneous reworking of pyroclasts by rain fall run-off

Fig. 13. Interpretation of the processes responsible for the deposition of the main Lower Permian crystal-rich volcaniclastic deposits of the Central Pyrenees.

of the main pyroclastic units would suggest the presence of abundant water, perhaps in the form of a crater lake, or ice or snow during volcanic activity. However, there is no evidence for the existence of crater lakes in the vent area, and the arid climate that characterised the Lower Permian considerably restricted the amount of surface water (Gisbert, 1981, 1983), despite episodic storms that occurred during the deposition of the continental red beds. It is clear that dry explosive eruptions can inject significant amounts of water vapour in the upper troposphere (Woods, 1993), thus modifying the weather locally (e.g. Robock and Mass, 1982). Explosive eruptions typically create their own electric field (e.g. Gilbert et al., 1991; Lane and Gilbert, 1992) resulting in lightening and locally in violent thunderstorms and heavy rainfall. The intense storms and rainfall run- off can cause reworking of pyroclasts during eruption (Fig. 13). This can satisfactorily account for the facies association observed in the crystal-rich volcaniclastic deposits and the characteristics of secondary volcaniclastic deposits (VSF) that are intimately associated with the pyroclastic deposits.

6.2. Crystal concentration processes

The characteristics of the studied deposits suggest the existence of different crystal concentration processes during their formation. Cas (1983) and Cas and Wright (1987) distinguish three main factors that can produce a high crystal content in volcaniclastic aggregates: eruption of phenocryst-rich magmas, physical fractionation and sorting processes associated with pyroclastic eruption and transporta- tion processes, and reworking and redeposition. All these factors can be identified in the Lower Permian crystal-rich deposits of the Central Pyrenees.

The magmatic crystal content can be estimated from the crystal content in compacted pumice clasts. Well preserved pumice clasts are abundant in the PE The volume of crystals in pumice fragments in different pumice-rich deposits ranges from 18 to 40% (Fig. 14). These values point to relatively high crystal contents in the source magmas of Lower Permian pyroclastic eruptions. However, these values are significantly lower than the volume of free crystals and crystal fragments observed in many of the studied facies (Fig. 14). This implies the existence of very effective eruption-related and post-eruptive crystal concentration. Eruption-related crystal concentration

,L Marff/Sedimentary Geology 106 (1996) 1-19 17

volcaniclastic sandstone facies (VSF)--

parallel laminated facies (PLF) --

laminated to low angle cross-bedded facies (CBF)

crystal-rich massive facies (MF) --

pumice-rich facies (PF)--

caliche facies ( C F ) -

pumice fragments --

0

(32)

I I I I I I I I 0 0 0 0 0 0 0 0 ~-- 04 CO ~ LD £0 I'-.- (30

% in volume of crystals

30 F

~ 20

~ 5

0

% in volume of crystals

,I o

Fig. 14. Crystal content range of the different Lower Permian crystal-rich volcaniclastic facies of the Central Pyrenees. The total number of samples analysed in each case is indicated. Modal analysis of the crystal content in pumice fragments of the pumice-rich facies (PF) is also indicated.

processes probably operated in the eruption column, in the pyroclastic flows and in the associated ash clouds, similar to those inferred for recent pyroclastic deposits (e.g. Walker, 1972; Sparks and Walker, 1977). From the modal analyses of these rocks (Fig. 14) it is clear that the degree of crystal concentration varies significantly according to the nature of the deposit. In the pyroclastic deposits the maximum crystal content is observed in the MF (47- 55% of the total rock volume) and PLF (65-75%), the crystal content in the CBF also being relatively high (45-65%). The lowest contents appear in the matrix of the PF (25-40%) and in the CF (17-34%), but are nearly always higher than the crystal content of the erupting magma.

The crystal content is maximum (50-82% of the total rock volume) in those deposits formed by reworking and redeposition of pre-existent crystal-rich volcaniclastic deposits. However, there is a differ- ence in the concentration of crystals according to

the textural maturity of the deposits. The crystal-rich volcaniclastic sandstone (VSF) formed simultaneously with pyroclastic deposits shows a variable proportion of matrix composed of altered juvenile components (shards and pumice fragments) incorpo- rated into the reworked deposits. In upper parts of the Lower Permian sequence, where the influence of volcanism is minor, volcaniclastic sandstones are more mature and essentially composed of rounded crystal fragments and some rock fragments, with sparse interstitial matrix.

6.3. Characteristics of eruptions

On the basis of the reconstructed original distribution of the volcanic deposits and their lateral facies variations, it is possible to define the main characteristics of the explosive eruptions responsible for the formation of the crystal-rich deposits. The Castellar de N'Hug ignimbrite (PF) has a radial dis-


tribution from the vent area (the Greixer Rhyolitic Complex), a total extent of more than 100 km, and has an original maximum thickness of 10 m near Castellar de N'Hug and a minimum thickness of 35 cm at the western margin of the Erill Castell basin. The associated fallout deposit (PLF) is widely dispersed. The single unit that represents the PLF always overlies the Castellar de N'Hug ignimbrite in the intermediate and distal areas, but does not appear in the proximal areas. It displays a thickness of 27 cm in the most distal outcrop, containing fragments of crystals of less that 1 mm in size. Comparison with some well documented young fallout deposits with similar characteristics (e.g. Self and Sparks, 1978; Ninkovich et al., 1978; Walker, 1980) suggests that the explosive eruptions responsible for the formation of the Lower Permian crystal-rich deposits of the Central Pyrenees involved plinian eruption columns that generated widespread pyroclastic flows and associated pyroclastic surge and fall deposits (Fig. 13). The extent of Lower Permian ignimbrites and their low thickness over a large area with low re- gional palaeo-gradient suggests that the ignimbrites were ignimbrites of a low aspect ratio, indicating high energy pyroclastic flows. It is also probable that the eruption of some of the widespread pyroclastic units, such as the Castellar de N'Hug ignimbrite, was related to caldera-forming events (Martf and Barrachina, 1987)

7. Conclusions

The Lower Permian crystal-rich deposits of the Central Pyrenees originated from pyroclastic and reworking processes that sometimes acted simultaneously. Pyroclastic deposits were generated by high-intensity explosive magmatic eruptions of rhyodacitic and rhyolitic crystal-rich magmas. These eruptions involved high eruption columns that occasionally collapsed, giving rise to the formation of widespread pyroclastic flows and associated pyroclastic surges. Sporadic storms triggered by the eruptions led to reworking of pyroclasts by rainfall run-off during and immediately following eruptions. Different crystal concentration processes acted in the eruption columns and in the subsequent pyroclastic flows and overriding ash clouds, producing the accumulation of a high volume of crystals in the resulting

deposits. Reworking and redeposition of pyroclastic deposits by fluvial processes further enhanced crystal concentration.

Acknowledgements

I particularly wish to express my gratitude to J. Gisbert for introducing me to the geology of Permo- Carboniferous terrains of the Catalan Pyrenees. I am grateful for an EC contract (ERBCHBICT93052) of the Human Capital and Mobility Programme. Ray Cas, Gerald Ernst and Jocelyn McPhie are gratefully acknowledged for their constructive reviews of the manuscript.

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Documents

Genesis of crystal-rich volcaniclastic facies in the Permian red beds of the Central Pyrenees (NE Spain)