The drowning of a carbonate platform: an example from the Devonian-Carboniferous of the southwestern Holy Cross Mountains, Poland

Sedimentary Geology Sedimentary Geology 106 (1996) 21-49

ELSEVIER

The drowning of a carbonate platform: an example from the Devonian-Carboniferous of the southwestern Holy Cross Mountains, Poland M. Szulczewski a, Z. Belka b,*, S. Skompski a " Institute of Geology, University of Warsaw, Zwirki i Wigurv 93 02-089 Warszawa, Poland ~' Geologisch-Paliiontologisches Institut der Universitiit, Sigwartstr 10. 72076 Tiibingen. Germany

Received 16 December 1994; accepted 1 November 1995

Abstract

The broad Middle Devonian carbonate platform of the Holy Cross Mountains was fragmented during the Frasnian and afterwards submerged as isolated blocks and transformed into a topographic depression. The section exposed in the Ostrowka quarry illustrates a well-documented drowning of the last small fragment (seamount) of this carbonate platform which submerged during Late Devonian and Early Carboniferous times. The recognized facies succession reflects a progressive, stepwise lowering of the sediment surface through time from a peritidal, lagoonal environment to a deep-water, anoxic basin. The Frasnian shallow-water carbonate platform is represented by lagoonal facies with peritidal sediments. Small-scale deepening-upward cycles punctuated with emersion surfaces are a feature of this stage. They resulted from a combination and continuous balance between carbonate production, subsidence and low-order, eustatic sea-level fluctuations. The carbonate platform aggraded at rates around 100-125 m/Ma. The shallow-water peritidal carbonates are separated from the post-platform deposits by an unconformity surface developed during subaerial exposure. The flooding of the faulted block of Galezice in the pre-late marginifera time and the subsequent initial drowning resulted primarily from a rapid sea-level rise. Small dimension of the seamount and its cemented upper surface were the most important factors that facilitated the sediment removal and thus suppressed the sediment accumulation. A condensed section was deposited on the top of the seamount. Its stratigraphic succession and rare cephalopod storm beds account for a model of redistribution of sediment over the swell, disintegrated by small-scale faulting. During drowning, the seamount traversed down through the photic zone at an average rate not higher than 20-25 m/Ma. Transformation of the swell into a basin accelerated in the late Tournaisian anchoralis Zone. It is manifested by an increase of deposition rate and by a lithology of alternating limestones and clays, with participation of fine-grained calciturbidites. The associated tephra deposits and syndepositional ferruginous hydrothermal mineralization were the result of extensional tectonics on the facies evolution. The drowning was completed in the Visean with deposition of black siliceous shales in the starved basin. The 'death' of the Devonian carbonate platform in the Galezice area illustrates well the famed 'paradox of drowned platforms' as the subsidence rate was much slower during the drowning than during the phase of the platform aggradation.

* Corresponding author. 0037-0738/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 0 3 7 - 0 7 3 8 ( 9 5 ) 0 0 1 4 5 - X

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M. Szulezewski et al./ Sedimentary Geology 106 (1996) 21-49

1. Introduction Small carbonate platforms are relatively shortlived depositional systems, known throughout the entire Phanerozoic (see the reviews of Kendall and Schlager, 1981, Bosscher and Schlager, 1992), which developed in a number of tectonic settings, but especially on passive continental margins. Their facies architectures and evolution were mainly controlled by relative sea-level changes due to tectonics and eustasy (Kendall and Schlager, 1981). Since most platform sediments are biogenic, the dynamics of biological ecosystems, dependent on favourable ecological conditions (i.e. on depth, temperature, light, salinity, currents, etc.) seem to be the crucial factor in determining the response of the carbonate platform to rapid relative rises of sea-level. Although the growth potential of carbonate platforms was generally more than sufficient to match the relative sea-level rise, drowning was most frequently the reason for the death of carbonate platforms in the past (Schlager, 1981). The estimated accumulation rates of the ancient platforms are generally much lower than their growth potential (Bosscher and Schlager, 1992), which in turn is one to two orders higher than the rate of eustatic sea-level rises. Drowned platforms, which became submerged below the euphotic zone, typically exhibit a stratigraphic succession consisting of neritic deposits followed by non-deposition or pelagic deposition (Kendall and Schlager, 1981, Schlager, 1981). The deep-water sediments are commonly separated from the shallow-water carbonates by an unconformity surface, locally associated with hardgrounds or karst and soil development. The drowning unconformities have a similar geometric appearance irrespective of whether they were affected by subaerial exposure or if they are related to sea-level rise (Schlager, 1989). The stratigraphic record of drowned platforms clearly reflects the increasing water depth. Several processes, however, that caused the condensation and sediment starvation or contributed to the formation of internal stratigraphic gaps in the pelagic portion of the succession, are still a matter of debate. In this paper, we describe the 'death' of a small carbonate platform that submerged on the shelf of the Fennosarmatia during Late Devonian and Early Carboniferous times. The section we have analysed

occurs in the southwestern part of the Holy Cross Mountains (central Poland), and is perfectly exposed in the Ostrowka quarry (Figs. 1, 2). It displays the widest stratigraphic range, at least from the upper Frasnian to the upper Visean, available in a single exposure in this region and offers a unique possibility to study the Frasnian/Famennian unconformity. Moreover, the succession of Ostrowka documents the fragmentation of the broad Devonian carbonate platform in the Holy Cross area, which as isolated blocks have been submerged, buried and finally transformed into a topographic depression during the Early Carboniferous. Compared with numerous examples of drowned Mesozoic platforms (e.g. Bernoulli and Jenkyns, 1974; Bosellini, 1989; Bice and Stewart, 1990; Santantonio, 1994), the succession has an advantage in accessing the roles played by tectonic factors in the drowning because of the high-resolution stratigraphy based on the conodonts. The study illustrates well that stratigraphic successions may be controlled locally at scales of even hundreds of metres by individual tectonic features. The conodonts used here to enhance the biostratigraphy provide not only the greatest resolution as yet possible for the Late Palaeozoic, allowing a detailed reconstruction of the drowning scenario to be formulated, but also, as small particles in the sedimentary environment they reveal precisely some events of limited redeposition and the age of the transported fine-detrital material. Conodont zonation applied in this paper follows schemes proposed for the Upper Devonian by Ziegler and Sandberg (1984) and for the Toumaisian by Sandberg et al. (1978) and Belka (1985). On the correlation diagrams (Figs. 7, 14), it is simplified only for purpose of the graphic resolution. Numerical values of the ages of stage boundaries used for calculating accumulation rates were taken from Odin (1985), Claou6-Long et al. (1993), and Roberts et al. (1993).

2. Geological setting The Ostrowka quarry is situated in the southwestem comer of the Palaeozoic Holy Cross Mountains, near the village Galezice, at the completely exploited Ostrowka Hill, belonging to the chain of the Galezice Hills (Fig. 1). The quarry is excavated in the Devonian limestones, capped by the Lower Carboniferous

23

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21-49

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founded on the Early Palaeozoic basement, significantly deformed by the late Caledonian folding. Carbonate deposition was established in the southern region of the Holy Cross Mountains during the early Eifelian. The carbonate platform exhibits a progressive evolution expressed in the increasing impact of biogenic deposition and facies differentiation. Its main Givetian to Frasnian portion is composed of the 330-800 m thick Kowala Formation, composed predominantly of stromatoporoid-coral limestones (Narkiewicz et al., 1990; Racki, 1993). The development of the carbonate platform was strongly influenced by relative sea-level changes and at least four deepening pulses have been recognized (Racki, 1993). They resulted in an intermittent and incipient drowning, as well as in a stepwise shrinkage of the platform. The stratigraphic record available in the Ostrowka quarry illustrates the late phase of the platform development. During this phase the carbonate system became restricted to an 'isolated carbonate platform', as defined by Tucker and Wright (1990), with

M. Szulczewski et al./ Sedimentary Geology 106 (1996) 21-49

24

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coarse-grained carbonates deposited over its flanks. The studied section represents an inner, probably back-reef, lagoonal setting, situated near the southern margin of the carbonate platform and characterized primarily by peritidal deposits (Racki, 1993). A significant stratigraphic break associated locally with an angular unconformity (Szulczewski, 1978) separates these shallow-marine carbonates (Fig. 2, unit A) from their pelagic cover (Fig. 2, units B, C). The post-platform Famennian-Lower Carboniferous succession records a stepwise drowning in several distinct stages. The following chapters deal with successive phases of platform development, destruction and drowning.

the uppermost part of the Kowala Formation, extensively described in numerous papers, mainly by Kazmierczak (1971), Narkiewicz et al. (1990) and Racki (1993). In most of the descriptions this portion of the Formation was colloquially termed 'Amphipora Limestone', due to the typical faunal components. The investigated unit is about 80 m thick and is composed of a monotonous series of light coloured, thick-bedded, fine-grained or micritic limestone beds, randomly interlayered with laminites (Figs. 3, 4). The repetitive rhythm of these two lithotypes is intermittently disturbed by horizons providing evidences of palaeokarstic alteration.

3.1. Lithology 3. Carbonate platform development The lower part of the section outcropping in the Ostrowka quarry (Fig. 2, unit A) comprises

Amphiporoid limestones are predominant in the unit and commonly range in texture from amphiporoid floatstones to the calcispherid-gastropod

M. Szulczewski et al./ Sedimentary Geology 106 (1996) 21-49

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Fig. 3. Lithology of the Frasnian peritidal sediments. (a) Angular unconformity between the Frasnian and Famennian/Carboniferous complexes (Dfr = Frasnian, Dja = Famennian, Ct = Tournaisian) in the northwestern part of the Ostrowka quarry. Arrow indicates palaeokarstic horizon schematized in Fig. 5. (b) Thin section of intertidal laminite showing microbial lamination. (c) Palaeokarstic horizon at the top of the subtidal unit. Note the gradual downward transition (brecciation) of the rubble zone to the substrate. (d) Fine-laminated intertidal unit affected by weathering and cementation in the vadose zone; polished slab. (e) Erosional contact of the intertidal laminite and subtidal knobbly, calcispherid-amphiporoid limestone. (f) Typical microfacies of subtidat units-peloidal t)ack~tone with calcispheres, multilocular foraminifers and tubular algae; thin section.

26

M. Szulczewski et al. / S e d i m e n t a r y Geology 106 (1996) 2 1 ~ 1 9

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packstones/wackestones with rare amphiporoid sticks (Fig. 3f). Subordinate elements include peloids, tubularcalcareousalgae (palaeoberesellids),

ostracods, multilocular foraminifers, rare crinoids and oncoids. Typical are large massive stromatoporoids scattered throughout the layers, generally

27

M. Szulczewski et aL /Sedimentary Geology 106 (1996) 21-49

Marly-shale succession Cephalopod-crinoidal pacldgrainstone

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Fig. 5. Sketch illustratingdetails of lithology and general stratigraphyof the Frasnian/Famennianunconformityin the northwestern corner of the Ostrowkaquarry. preserved in growth position, although completely overturned and crushed specimens were also found. The limestone beds are usually 30-100 cm thick. Laminites are considerably thinner than amphiporoid limestones and display fiat, regular lamination, locally crumpled into small stromatolites. In a few layers fiat-pebble conglomerates and desiccation cracks occur (Fig. 4). Diverse fenestral structures, common especially in the topmost part of the section, complete the appearance of the lithotype, strongly reminiscent of loferites. The microfacies is composed of peloidal laminae with rare calcispheres, alternating with spar-filled fenestrae or rarely with fibrous and continuous laminae, probably of microbial origin (Fig. 3b). Locally, the laminites are underlain and separated from the amphiporoid limestones by palaeokarstic horizons (Fig. 4). There are at least two different types of surface which reveal palaeokarstic features: rubble zones (regoliths?) and levels of dissolution pipes. The rubble zones are usually up to 15 cm thick, only locally reaching 40 cm (Figs. 4, 5). While the upper boundaries of rubble zones are sharp, their transition to the substrate is gradual, similar to the typical regolith (Figs. 3c, 3d). The rubble consists of clasts of host-rock, mainly calcispherid wackestones or packstones. Its matrix is a red to green marly clay.

Densely packed clasts generally have irregular, but rounded margins and range in size up to several centimetres (Fig. 6b). No preferred orientation of large fragments is observed. Bent fragments are relatively numerous suggesting reworking of semi-lithified carbonate mud. Cements, commonly developed on the underside of clasts, mostly exhibit typical stalactitic internal structure (Figs. 6c, 6d), whereas others are recrystallized but easy distinguishable due to their characteristic pendant form (Fig. 6e). Drusy cements with crystals coated with ferruginous rims occur sporadically (Figs. 6a, 6f). The second type of emersion surface is characterized by dissolution pipes. The only level revealing this feature is developed in the topmost part of a knobbly limestone bed and is covered by a laminite (Fig. 4, uppermost inset). The pipes are small, usually about 15 cm deep and 3-14 cm wide. Their walls are approximately vertical, while the bottoms are fiat to concave. The infill contains clasts of laminite similar to the overlying one, embedded with green clay. Only in one case is the pipe partly infilled with laminite, being primarily deposited within it. 3.2. Stratigraphy

Czarnocki (1928) attributed the 'Amphipora Limestone' to the Givetian, but later investigations

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M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

Fig. 6. Vadose zone cements of the Frasnian palaeokarstic horizons. All scale bars are 1 mm, except in (b). (a) Dissolution cavity in the rubble horizon lined by acicular cement and infilled with equant crystal mosaic. (b) Rubble zone with a typical brecciation texture. Clasts of Frasnian limestone host rock are encased in dark, iron-stained matrix formed largely by dissolution and subsequent vadose cementation; polished slab. (c, d) Microstalactitic pendant cements lining the underside of pebbles; lower part of cement shown in (c) is partially dissolved due to stylolitization. (e) Completely recrystallized dripstone cement growing from the roof of a cavity infilled by silt. (f) Small cavity in the rubble horizon with initial lining of drusy cement crystals covered with ferruginous rims.

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21-49

suggested rather a Frasnian age. However, the evidence is not direct, but based on the lithological correlation with the better documented succession outcropping in the 10 km distant Panek quarry (see for discussion Szulczewski, 1978). Recently, Malec and Racki (1993) found in the Ostrowka section a scarce ostracod assemblage of probable Frasnian affinity. This age is here confirmed by foraminifers ubiquitous in the topmost part of the unit (Fig. 4). The assemblage is composed of Tikhinella fringa, Eonodosaria stalinogorski, Eogeinitzina rara, E. alta, Nanicella ex gr. galloway and (?)Multiseptida sp. Similar microfauna was identified by Racki and Sobon-Podgorska (1993) in a few other outcrops of the highest part of the Kowala Formation and was regarded as the late Frasnian invasion event of multilocular foraminifers (cf. Kalvoda, 1986; Zadorozhnyj, 1987).

3.3. Depositional environment The amphiporoid limestones and laminites are interpreted as lagoonal-peritidal sediments, and such facies are common in the Devonian (e.g. Read, 1973; Wong and Oldershaw, 1980; Racki, 1993; Skompski and Szulczewski, 1994). Usually, the Amphiporarich members with an extremely impoverished biotic assemblage are interpreted as the deposits of a lowenergy subtidal lagoon with poor circulation (cf. Jansa and Fischbuch, 1974; Larsen et al., 1988). Dramatic changes of salinity and temperature as well as bathymetry may have been of major importance in the control of this biotope (e.g. Kazmierczak, 1971; Preat and Racki, 1993). Laminated members are typical for intertidal environments (comprehensive discussion in Boulvain and Preat, 1987) and usually they are overprinted by early diagenetic features indicating the supratidal, vadose zone. Both members are usually found together in the shallowing-upward cycles, extensively reported from different Devonian platforms (see for review Tucker and Wright, 1990). The palaeokarstic features present show that the development of the rubble zones evidently resembles the result of weathering of bedrock and cementation in the vadose zone. The investigated rubble is closely related to the numerous examples of subsoil palaeokarst, known from the Carboniferous (see Wright, 1988; Meyers, 1988; Davies, 1991). Also

29

the pipe level shows several similarities to the dissolution pipes, described by Wright (1988), who compared them to the kavornossen karren which are distinctive tropical karst characteristics.

3.4. Cyclicity pattern The shallowing-upward cycles have been described from the Middle Devonian part of the Kowala Formation and its time equivalents (Preat and Racki, 1993; Skompski and Szulczewski, 1994). Racki (1993) assumed that this type of metre-scale cyclicity is also predominant in the upper part of the Kowala Formation, and explained its nature by autocyclic rather than eustatic mechanisms, whereas he interpreted the higher-order cyclicity as a eustatic effect. However, detailed observations in the Ostrowka quarry surprisingly showed another type of cycle. The position of emergence horizons in the cycles is of crucial importance to this interpretation. The palaeokarstic phenomena described here are usually situated in the topmost part of the subtidal unit, below the intertidal laminites (rarely they are developed at the top of laminites). Both, the lower and upper boundaries of laminated beds are sharp, locally clearly erosional. This suggests that frequently the uppermost parts of the palaeokarstic horizons were truncated during a transgressive episode (see lowermost inset on Fig. 4). The cyclicity pattern, however, is different from that of the typical shallowing-upward cycle. It is composed in vertical succession of a palaeokarstic horizon (or only erosional surface), then a laminitic intertidal member, and finally a calcispherid-amphiporoid subtidal member. There are few descriptions of similar cycles and practically all of them occur within the Triassic carbonate successions in the Alps. Similar to the Devonian succession here discussed is particularly the Norian Dachstein Limestone from the Northern Calcareous Alps in Austria, containing Lofer cycles described by Fischer (1964) and Zankl (1971). A similarity of facies succession in both the Devonian and Triassic cycles is obvious. In particular, palaeokarstic intervals of the Devonian cycles correspond with Fischer's member A: a red or green argillaceous unit representing reworked residue of weathered material (compared to a modified soil horizon). Although the

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M. Szulczewskiet al. / Sedimentary Geology 106 (1996) 21-49

Triassic cycles are relatively thick (5 m), they are aggraded in a similar back-reef lagoonal setting with a comparably high rate of deposition. As shown by Strasser (1991), the overprinting of deeper subtidal deposits by intertidal, supratidal or terrestrial facies, and the presence of erosional features at the top of subtidal members in the peritidal cycles, indicate an allocyclic rather than an autocyclic mechanism for cycle repetition. Occurrences of erosional and palaeokarstic surfaces in the Devonian cycles thus indicate the allocyclic control of deposition. According to the interpretation of Triassic successions in the Alps (Fischer, 1964; Goldhammer et al., 1987) we can suggest that smallscale fluctuations of sea-level are the most important factor controlling this type of the cyclicity. This scenario is consistent with the general hypothesis of an essential influence of eustasy on Devonian sedimentation in this region (cf. Narkiewicz, 1988; Racki, 1993). However, typical results of eustatic small-scale sea-level movements appeared constantly in long sections, as for example in the Triassic successions cited above. The cycles can only be seen in a few parts of the section of the Ostrowka quarry (Fig. 4). It seems, therefore, that the cyclicity was also controlled by local subsidence, although allocyclic mechanisms of sedimentation seem to be most important.

4. Emergence-flooding unconformity The relationship of the lagoonal section of the Frasnian carbonate platform to the overlying deeperwater deposits is unconformable. The long face of the Ostrowka quarry gives a unique opportunity to detect bedding relationships across the unconformity (Fig. 7). An angular discordance with obvious stratal truncation can be locally seen across this surface. It has a low angle, no more than 12 degrees (Figs. 2, 3a, 5), but nevertheless indicates removal of at least 10 m of the underlying strata (cf. Szulczewski, 1978). Over the remaining parts of the quarry, the contact between well-bedded limestones of the carbonate platform and the overlying strata is apparently conform. In these areas, it can be either paraconformable, or the oblique orientation of the wall with respect to the inclination of strata does not make an angular unconformity apparent.

The unconformity surface is generally planar, flat and clean-cut (Figs. 8c, 8d). Locally it has a small-scale relief. It is limited to the low (25 cm high) sharp angular escarpments, suggesting removal of substrate clasts ripped up by physical processes (Fig. 8b). Associated with the unconformity are sparse and small-scale dissolution fissures. They are irregular, millimetres to centimetres thick and penetrate down no deeper than about 50 cm below the unconformity surface. The fissures are filled with green marly limestone related to those of the overlying unit. The unconformity surface is usually impregnated with iron oxides and coated with a red-coloured ferric crust, only where it is immediately overlain by the Tournaisian. No other features associated with unconformities, like substrate lithoclasts incorporated within the overlying deposits, borings, encrustations etc., have been found here. 4.1. Origin of the unconformity The features of the unconformity suggest a complex origin (cf. Szulczewski, 1978). Development of the angular discordance, associated with significant truncation, implies a prolonged period of subaerial exposure, longer than those earlier episodic phases of emergence punctuating the small-scale Frasnian cycles. The angular discordance appears to be related to the tectonic tilting. The generally planar and fiat unconformity surface suggests that after emergence the surface was finally shaped by physical abrasion. Shallow dissolution features seem to be caused by faint post-abrasion subsolution, which would be the last stage of the development of the unconformity surface. 4.2. Timing of formation of the unconformity The unconformity represents a significant stratigraphic gap of highly variable range along its surface (Fig. 7). The age differences of the beds truncated by the unconformity probably span less than a single conodont zone. This is suggested by the assumed high rates of deposition and the truncation which encompasses no more than several small-scale peritidal cycles. The age of the unconformity is not well constrained. According to Narkiewicz (1988),

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21-49

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FRASNIAN Fig. 7. Conodont-based stratigraphic correlation of selected sections of the post-platform Famennian/Lower Carboniferous cover from the northern wall of the Ostrowka quarry. See Fig. 1 for location of sections. Note the different scale of sections from the western (A, B, C, D) and eastern (E, F, G, H) parts of the quarry. Black triangles indicate main stratigraphic gaps within the condensed sequence. Thin volcanogenic intercalations are indicated with asterisks.

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M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

Fig. 8. (a) Contact (arrowed) between the Frasnian limestones and Famennian-Lower Carboniferous succession below Todowa Hill (section F). (b) Characteristic step-like morphology of the contact surface. (c) Sharp contact between Frasnian (Dfr) stromatoporoid massive limestone and the overlying Famennian (Dfa) pelagic condensed unit (section G). Hammer (arrowed) is 30 cm long. (d) Photomicrograph showing the contact between Frasnian fenestral laminites and the Famennian packstone. (e) Small angular unconformity between Frasnian and Famennian carbonates and the cross-bedding visible at the top of the Famennian crinoidal limestone; polished slab. Black arrow indicates neptunian dyke within laminites containing conodont fauna identical to that of the base of the Famennian (marginifera Zone). Some deposits from neptunian dykes were injected during early compaction into the voids after dissolved shells (white arrows). (f) Bottom surface of the Famennian condensed limestone with numerous fragments of large fish bones.

M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

the unconformity was caused solely by erosional processes during the postulated regression at the beginning of the Lower gigas Zone. The regression in the Lower gigas Zone, however, is nowhere soundly proved in the Holy Cross Mountains including the Ostrowka quarry (Szulczewski, 1990). The age of the deposits resting immediately on the unconformity differs substantially from place to place (Fig. 7). The oldest deposit capping the unconformity belongs to the middle Famennian Upper marginifera Zone, while the youngest represents the upper Tournaisian anchoralis Zone (cf. Szulczewski, 1978).

5. Condensed carbonate unit (initial drowning) The oldest deposit directly covering the unconformity surface is a relatively thin and stratigraphically condensed carbonate unit (Fig. 2, unit B). It clearly differs from the underlying Frasnian carbonates in its very low rate of deposition and high fossil content, including numerous open marine representatives, mostly cephalopods. The unit represents a pelagic facies and belongs to the Famennian. The condensed unit is laterally discontinuous and exhibits marked variations in thickness; about 20 cm or less in the western part of the quarry (Fig. 7, sections A, B), complete absence in the middle (Fig. 7, sections C, D), and about 50-150 cm in the eastern part (Fig. 7, sections F, G?). The thickest carbonate unit of 235 cm, described by Nasilowski (1975), occurred in the more southern part of the quarry then the existing wall, but now completely removed.

5.1. Lithology The condensed deposits are remarkably fossiliferous with predominantly crinoidal to cephalopod wackestones and packstones. The matrix is micritic or microsparitic, and contains comminuted skeletal debris. The limestones are distinctly bedded and rarely intercalated with thin clay partings (Fig. 8c). Locally individual beds display crossbedding (Fig. 8e) or they contain several graded units (Figs. 9, 10). Such units exhibit not only the upward-fining gradation, but also the distinct vertical succession of different biotic constituents. Generally,

33

the lower parts are crinoidal and the upper parts rich in cephalopod shells (Figs. 9a, 9b). The basal boundaries are sharp, more or less undulating, although not conspicuously erosive, while the internal transition from the crinoidal to the cephalopod-rich part is gradational. Clymeniids are most common among cephalopods (cf. Czarnocki, 1989), but goniatites and orthocone nautiloids are also present. Other associated organisms include trilobites and thin-shelled bivalves (posidonid Guerichia), while gastropods, entomozoid ostracods, brachiopods, solitary rugose corals, heterocorallids, and fish remains (Fig. 8t3 are clearly subordinate. Locally, redeposited fragments of solenoporacean algae have been found. Hence, a relatively wide range of nektonic, planktonic as well as benthic taxa is represented. Disarticulated pelmatozoan elements are mainly completely preserved, only their small fraction being fragmented, but not abraded. On the other hand, cephalopod and bivalve shells, and trilobite skeletons are usually fragmented.

5.2. Stratigraphy The stratigraphic range of the condensed unit correlates with its variable thickness: the thinner the unit, the shorter is its stratigraphic range, and the later the onset of deposition (Fig. 7). Where the unit is relatively thick, its deposition commenced in the Upper marginifera Zone (sections E, F). In the eastern part of the quarry (section G), where the unit is much thinner, it began in the Lower trachytera Zone, and in the western part in the Upper trachytera Zone (sections A, B; see also Szulczewski, 1978). The top of the condensed unit is usually sharp. It occupies a different stratigraphic position within the upper Famennian: the Lower expansa Zone (sections B, G) or Middle expansa Zone (section A), but locally also above the Lower praesulcata Zone (sections E and F). The stratigraphic relationships within the condensed unit reveal the standard succession of conodont zones, interrupted by several internal paraconformities (cf. Szulczewski, 1978), which are evidenced by the local absence of the Lower trachytera Zone (section E) and the Upper expansa Zone (sections A, B). The exception is an omission of the postera Zones, observed all over the quarry, although their conodonts were encountered as stratigraphical

34

M. Szulczewski et al. / Sedimentary Geology 106 (1996) 21--49

Fig. 9. Lithology of the Famennian condensed unit and neptunian dykes. (a) Tempestitic succession in the upper part of the crinoidal/cephalopod limestone: A = crinoidal packstone with normal grading, B = goniatite shell-hash, C = fine-grained to micritic sediment; polished slab. (b) Normal grading in the upper part of layer presented in (a). (c, d) Thin sections showing characteristic faunal components of the Famennian pelagic limestonesltrilobites, crinoids and rugose corals. (e-g) Infill of the largest neptunian dyke in eastern part of the quarry; polished slab; hydrothermal dolomite crystals (f, cathodoluminesce photomicrograph) and weathered amphiporoid sticks in the fine-grained matrix (g).

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21-49 , ~ oOo o o o o Io~oo~\~,~ Oo. . . . . o o~.z-'[°oc=oo o o ° o oo o o ° o o o o

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admixtures in higher zones, and the zones themselves are present in the nearby easternmost part of the Galezice Hills (Szulczewski, 1978). The rare occurrence of the postera Zones in the stratigraphical record of the condensed Famennian is a regional feature in the Holy Cross Mountains (Zakowa et al., 1984). Mixed conodont faunas are very rare. They contain mainly conodonts derived from the adjacent zones, so they were concentrated due to the extreme stratigraphical condensation (cf. Szulczewski, 1978).

5.3. Depositional environment Although the above described sequence is condensed and contains abundant cephalopod remains it does not match the typical cephalopod limestone widely distributed in the Devonian of the Variscan Europe and also known from other localities in the Holy Cross Mountains. In contrast to the typical cephalopod limestone, which is poor in benthic fossils (e.g. Tucker, 1974; Franke and Walliser, 1983), the Ostrowka example represents an intermediate link of mixed lithology between the cephalopod and crinoidal limestones. Both facies types in their typical development are the most

35

characteristic lithologies in the Devonian carbonate condensed sequences (e.g. Franke, 1973; Wendt and Aigner, 1985) and in their Jurassic counterparts (e.g. Bernoulli and Jenkyns, 1974; Santantonio, 1993). The crinoidal limestones usually characterise an early shallow-water phase of carbonate platform drowning, whereas the cephalopod limestones represent a later, pelagic deposition (e.g. Bernoulli and Jenkyns, 1974). The facies evolution within the Upper Devonian condensed succession of the Holy Cross Mountains generally follows this pattern (Szulczewski, 1981), whereas its development at the Ostrowka quarry is unusual on the regional scale. The distribution of skeletal material throughout the unit and the recognized fabrics show that the coexistence of the pelmatozoan and cephalopod debris results from the physical mixing of biotic constituents, derived from at least two different habitats. Disarticulated crinoidal debris probably originally accumulated over the relatively elevated areas of the truncated and subsequently drowned carbonate platform, whereas accumulations of cephalopod shells dominated its deeper parts. Crinoidal sand was swept by storms and washed into the sites of cephalopod accumulation. The storm wave base was then deep, so that cephalopod shells and the associated fine-grained sediment were stirred and heavily fragmented. The different floating properties caused the fragmented cephalopod and pelecypod shells to settle-out of suspension after the more rapid fallout of pelmatozoan debris. The micritic and finegrained material was largely produced by the physical breakdown of carbonate macroskeletons, most probably of the relatively fragile and abundant shells of cephalopods and thin-shelled bivalves. A similar origin is suggested for the fine-grained matrix of the Devonian Cephalopodenkalk (Tucker, 1974; Franke and Walliser, 1983; Tucker in: Tucker and Wright, 1990). The proposed sedimentary model assumes blockfaulted swell palaeotopography, prior to the deposition of the condensed unit. The data from the Ostrowka quarry, which comprises one tilted block only, do not illustrate this sufficiently, but observations in other parts of the western Holy Cross Mountains (Szulczewski, 1989) thoroughly confirm this supposition, and delimit the relief as enough to crinoidal and cephalopod facies differentiation.

36

M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

6. Swell to basin transition (accelerated drowning) A limestone-clay unit, about 350 cm in thickness, follows the condensed sequence. It reflects a strong influx of clay sediment and simultaneously a decline of carbonate production. The unit is often locally tectonically disturbed due to its more ductile properties in comparison to the underlying limestones. Over a substantial part of the outcrop the unit rests directly upon the post-Frasnian unconformity surface which is usually impregnated with ferruginous oxides or covered with the hematitic crust. The crust is also locally developed at the base of the unit where it rests on the condensed Famennian. It is several centimetres thick and composed of ferruginous oxides, but also sulphides and their carbonate relics are present. The crust is regarded as the result of syndepositional, submarine hydrothermal mineralization developed during the prolonged interval of prevailing non-deposition after sedimentation of the Famennian.

1928; Czarnecki, 1992). The benthic fauna is represented by large, usually disarticulated crinoid ossicles, trilobites, corals and very rare brachiopods (Figs. l l c - l l e ) . The rugose corals and heterocorallids are represented by reworked Famennian forms (e.g. Syringiacson sp., Oligophylloides pachythecus, B. Berkowski, pers. commun.). The monotonous mudstone interbeds, however, contain only rare agglutinated foraminifers, conodonts and sponge spicules. Whereas the foraminifers are restricted to the lower portion of the unit, the sponge spicules become more abundant in its upper portion. Moderate burrowing is common particularly in those limestone interbeds in which also benthic foraminifers were found. Although the reworking of biotic components is evident at least in three interbeds, the limestone beds do not reveal grading. The only exception constitutes the topmost limestone layer, in which crinoid ossicles are unusually concentrated above the ironstained erosive surface, and their frequency diminishes upward (Figs. 1 lc, 1 le).

6.1. Lithology

6.2. Stratigraphy

The unit basically consists of clays intercalated with numerous thin limestones. Subordinate to these two prevailing lithologies are accumulations of tephra. The clay coloration is red to cherry-red, green and rarely yellow. Its patchy pattern strongly suggests a post-depositional origin. The limestones are up to 15 cm thick, marly wackestones and mudstones, yellow, brown or pink coloured. They do not form continuous beds being commonly disrupted into lenses and nodules. The number of limestone intercalations in individual sections and the pattern of their vertical distribution against the clay background are variable. Their irregular distribution is of depositional origin, although in some places it is also caused tectonically. The unit is moderately fossiliferous, yielding a scarce assemblage of trilobites (Osmolska, 1962) associated with tabulate and rugose corals within the clay layers. The rare representatives of rugose corals correspond to the well known relatively deepwater 'Cyathaxonia' fauna. More abundant fossil accumulations are to be found in the first two limestone horizons. Nektonic macrofossils are goniatites, orthocone nautiloids, and fish remains (Czarnocki,

The basal part of the unit belongs mostly to the upper Tournaisian anchoralis Zone over most of the quarry. Only where the underlying Famennian sequence is the most completely developed (section E), the limestone-clay unit starts with the sandbergi Zone. Thus, there is everywhere a significant stratigraphic gap comprising at least five conodont zones between the Famennian and the Lower Carboniferous. Szulczewski (1978) explained the stratigraphical relationships at the Devonian/Carboniferous boundary with a regional subaqueous non-deposition. The top of the limestone-clay unit reaches the texanus Zone of the lower Visean. A similar stratigraphic position within the lower Visean is suggested by Czarnecki (1992) on the base of goniatites. In the currently best exposed section F (Fig. 7) the Tournaisian/Visean boundary can be traced using conodonts at the base of the third limestone horizon.

6.3. Depositional environment The origin of limestone interbeds is not quite clear, since they are fine-grained, poor in sedimentary structures and obliterated by diagenetic pro-

M. Szulczewski et aL /Sedimentary Geology 106 (1996) 21-49

cesses. Nevertheless, they bear several of the following features indicating that they might be low-density muddy turbidites (e.g. Piper and Stow, 1991): (i) They contain fine, partly reworked skeletal debris of fossils. High degree of debris fragmentation suggests lateral resedimentation from environment of far more energy than that of their final deposition. (ii) Although they are homogeneous, faint lamination is rarely discernible. (iii) If present, bioturbations are usually concentrated in their topmost parts. They are oblique to vertical burrows. The striking difference in thickness between conodont zones of the condensed unit and those of the overlying limestone-clay package reflects conspicuous acceleration in sediment deposition of the later unit. The fragmented swell covered with the condensed unit foundered below the critical depth of storm wave base, above which effective deposition was previously hampered by frequent winnowing of sediments. At the beginning of the transformation of the swell into a basin the remaining block-faulted palaeotopography still influenced variability of the thickness and completeness of the stratigraphic record. The pre-existing morphology was finally levelled-off and buried during the anchoralis Zone, when uniform hemipelagic deposition accelerated over the entire area, mostly due to the contribution of high amounts of clay sediments. The depositional environment of the limestone-clay unit was, however, still situated in the aerated zone as evidenced by the presence of bioturbation, indigenous epibenthic fossils and red to green coloration of sediment. The composition of the limestone interbeds shows that palaeohighs with carbonate production still persisted somewhere in the nearby area of the Holy Cross Mountains, at least until earliest Visean time.

37

the surrounding Frasnian host-rocks are sharp-edged and only locally slightly corroded (Fig. 9e). Other large particles in breccia are remains of amphiporoid sticks, perfectly exhumed from the Frasnian substrate (Fig. 9g). A greenish matrix consists of clay material, rounded quartz grains up to 0.3 mm in diameter, scarce crinoids, fish remains, and numerous conodonts (several hundreds of specimens&g). The most remarkable feature of the matrix is abundance of idiomorphic dolomite crystals (Fig. 9t). The isotopic analysis showed for this dolomite 313C = - 0.64 to -1.23%o PDB and 6180 = - 7.45 to -9.89%0 PDB. This indicates a rather high temperature of crystallization (80°C to 100°C) according to Land's equations (cf. Tucker and Wright, 1990). On the other hand the CAI analysis of conodonts (Belka, 1990) showed that Devonian rocks from the Ostrowka area have never been buried enough to attain this temperature. Therefore the origin of the dolomite is apparently hydrothermal. The matrix of the fissure infill does not exhibit stratification and conodonts from many biozones are mixed together. They are badly preserved and represent an extremely long time interval from the Upper marginifera to delicatus Zone. The initial fissure was probably created during the pre-Famennian faulting and was infilled with material swept from the unconformity surface (weathered Amphipora sticks). Later on hydrothermal fluids moving along the fissure caused dolomitic mineralization. There are close similarities of the fissure to the structures recently originating on the edges of submarine blocks. The best example is described by Vachard et al. (1987) from the scarps of the Messina Strait (southern Italy), where a dense network of neptunian dykes and internal breccias was identified and related to the tectonic history of the area.

7. Starved basin deposits (terminal drowning) 6.4. Neptunian dyke with breccia infilling 7.1. Black siliceous shales Another feature restricted to the blocks of Frasnian carbonates directly covered by a limestoneclay unit is a fissure zone with breccia infilling, found in the eastern part of the quarry (Fig. 7, section H). Its width attains several tens of centimetres. The fissure infill is composed of clasts floating in detrital matrix. The clasts derived from

The unit considered here (Fig. 2, unit D) known as Zareby Beds is widespread in the Lower Carboniferous of the Holy Cross Mountains (Zakowa, 1981; Zakowa and Paszkowski, 1989). In the study area, it is up to 25 m thick and contains mainly black siliceous shales. Near the Todowa Hill (Fig. 7,

38

M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

section F), where the basal portion of the unit is well exposed, a gradual transition from the underlying greenish-grey shales into black facies can be observed. 7.1.1. Lithology The Zareby Beds start with an 2 m thick interval of interbedding of black and green shales. Organic-rich and non-calcareous shale and mudstone make up most of the unit; these rocks are black, laminated, monotonous, and contain abundant radiolarians (Figs. 1la, l lb), chiefly spumellarians and albaillerians, which are accompanied by subordinate sponge spicules and rare conodonts. Locally small fragments of poorly preserved wood and plant remains are also present. The shelly calcareous fauna and bioturbations are totally absent. Phosphate nodules are distributed vertically throughout the unit. In the whole section there are only a few thin (1-2 cm thick) distal turbidites with sandy material consisting of silicified carbonate grains and tephra. The limited transport of allochthonous material from the adjacent carbonate platform, which was situated south of the Galezice area (Belka and Skompski, 1988) is documented additionally by reworked conodont elements found in the middle part of the unit. Except for the basal part of the unit, most of the mudstone and shale layers of the Zareby Beds have been locally silicified. The silification is moderate and displays an irregular pattern. In other Lower Carboniferous sections of the Holy Cross Mountains, outside the Galezice area, the Zareby Beds are much thicker and contain more diversified biota. Moreover, this unit spans a generally much longer stratigraphic interval as the black radiolarian shales in the Ostrowka quarry (Zakowa and Paszkowski, 1989). The macrofauna includes bivalves (most probably epiplanktonic), crinoids, trilobites, and rare brachiopods. The trilobites found in the black shales are usually extremely small (only a few mm in length), and are represented primarily by young ontogenetic stages (Osmolska, 1962). 7.1.2. Stratigraphy The conodont fauna found on the bedding-planes of the black shales includes only ramiform elements, which belong to the various Gnathodus apparatuses. Because the multielement composition of the major-

ity of the Carboniferous polygnathacean apparatuses is rather poorly understood, the age of the unit could not be determined precisely. The position of the unit within the succession indicates the lower-middle Visean, comprising the interval in the conodont zonal scheme from the texanus Zone to bilineatus Zone. This conclusion corresponds well to radiolarian data (Zakowa and Paszkowski, 1989). 7.1.3. Depositional environment Black, laminated shales, rich in organic carbon, are characteristic of deposition in an anoxic (anaerobic) environment with a stratified water column (e.g. Byers, 1977). Anoxic conditions may have developed in two different palaeobathymetric settings. Many black shales formed in deep, stratified and stagnant basins (e.g. the Late Devonian-Mississippian black shales in the Appalachian Basin; Ettensohn and Elam, 1985; Gutschick and Sandberg, 1991; and the Early Carboniferous Kieselschiefer Formation of the Rhenohercynian Zone; Dehmer et al., 1989; Gursky, 1992). On the other hand, there are also records of black shales deposited during rapid transgressions, which yield evidence for relatively shallow- water origin (e.g. McCollum, 1988; Leckie et al., 1990). The general absence of benthic organisms, the lack of any erosional features, and the preservation of primary non-bioturbated depositional laminae in the Zareby Beds suggests a deep-water environment. The upward transition from greenish-grey to black shales is typical for the vertical shift of dysaerobic/anaerobic boundary in the water column (Cluff, 1980) and suggests deposition in a stratified anoxic basin. The change of the depositional regime recognised at the base of the unit seems to have been the result of the subsidence of the area and not from the eustatic rise of sea-level, since the onset of the carbon-rich facies in the Lower Carboniferous of the Holy Cross Mountains is clearly diachronous over a distance of several kilometres (see Zakowa, 1981). The rarity of redeposited material indicates that most muds were hemipelagic and settled from the overlying water column. However, the general aspect of the Zareby Beds was not exclusively one of quietwater deposition. The conodont elements, which occur isolated on the bedding-planes and do not constitute assemblages, indicate the existence of some bottom currents. Recent studies provided evidence of

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21-49

current activity in the ancient, anoxic, deep-water depositional environments (e.g. Baird and Brett, 1986). Water depth was surely an important factor in the development of water-column stratification and consequent deposition of black shale in the Holy Cross area. In modern large euxinic basins, such as the Black Sea, the stagnant, anaerobic zone extends from 150 m depth down to the basin floor, whereas the pycnocline is approximately about 100 m thick (Casper, 1957). However, in the California Continental Borderland basins, which are rather small and situated on the continental slope, anoxic depositional conditions prevail below 500-800 m water depth (Savrda et al., 1984; Christensen et al., 1994). It is not unlikely, that in the basinal environment of the Holy Cross Mountains, which was located on the craton at least 200 km away from its margin, oxygen deficiency may have been achieved at depths much shallower than 500 m but certainly deeper than 100 m. 7.2. Carbonate gravity flow deposits

Black siliceous shales represent already the basinal deposition over the drowned carbonate platform of Ostrowka. At the top of this complex, however, there is a coarse-grained carbonate unit (Fig. 2, unit E), which gives evidence for a close connection with another, adjacent carbonate platform. The latter, most probably situated only about 10 km south of Ostrowka, developed at least until middle Visean time (Belka et al., 1996). The Visean carbonates have been comprehensively described by Belka and Skompski (1988) so that here only a brief outline, emphasizing data critical for recognition of the depositional environment, will suffice. The deposition of the carbonate unit, during latest Visean time, coincides with a radical change of sodimentary regime in the basin, which has afterwards been filled with a thick clastic series (Fig. 2, unit F). The latter unit known as Lechowek Beds consists of sandy shales with intercalations of sandstone and volcanogenic deposits (Zakowa, 1981). It terminates the Variscan sedimentary sequence in the Holy Cross Mountains. 7.2.1. Lithology The carbonate unit forms three lenticular bodies, several hundred metres long and maximum 25 m in

39

thickness (Fig. 1). It starts with a polymictic, poorly sorted breccia bed. The spectrum of clasts ranges from the Frasnian to the Visean, being stratigraphically similar to the section exposed in the Ostrowka quarry but different in lithology and microfacies (Belka et al., 1996). The breccia bed is overlain by a package of poorly sorted, coarse-grained crinoidal packstone beds. Allochems include bioclasts, micritic lithoclasts, and many relatively well-preserved skeletons. Their distribution is usually random. In fact, all beds contain an extremely abundant shallowwater benthic fauna dominated by crinoids, corals, and brachiopods. In the numerous palaeontological reports and investigations of the rich fauna (see for review Belka and Skompski, 1988) most of the authors stressed the opinion that organisms representing different ecological niches were here mixed together. In the topmost part of the carbonate lenses there occur thin cephalopod wackestone layers containing juvenile goniatites, conodonts, trilobites, and numerous globochaetids. This algal form is well known primarily from pelagic deposits (Skompski, 1982). 7.2.2. Stratigraphy Although the allochthonous character of the most of the carbonate components is apparent, the unit can be dated using conodonts which were found in the pelagic cephalopod wackestones. The conodont assemblage is indicative of the bilineatus Zone of the middle/upper Visean (Belka and Skompski, 1988). 7.2.3. Depositional environment The position of the carbonate unit within the basinal shale succession and the shallow-water character of the abundant biota seem to be in conflict. The unit represents, however, gravity flow deposits forming three distinct submarine deep-water aprons (Belka and Skompski, 1988). The detrital material along with the shallow-water fauna derived from an adjacent carbonate platform, were deposited in the deep lower-slope environments. This is indicated by a mixing of fauna coming from different ecological niches and by many sedimentary features such as the nature of the carbonate lenses, grain-supported texture, chaotic arrangement of clasts, preferred orientation of longer particles, and the presence of lithoclasts from the substrate. Generally, it appears

40

M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49

that grain-flow mechanisms were predominant but cohesive debris-flow also played a significant role in the transport and deposition (Belka and Skompski, 1988). 8. Volcanic activity Accumulations of pyroclastic material are common within the Famennian/Lower Carboniferous succession of the Holy Cross Mountains (Migaszewski, 1995). In the section of the Ostrowka quarry, the volcanogenic intercalations are usually only 1 to 10 cm thick, but locally the thickest one attains 50 cm. The lateral continuity of individual tephra layers is very limited. Therefore, their vertical distribution does not reveal any constant pattern, and there is no individual tephra layer traceable along the entire outcrop (Fig. 7). The most common are thin bentonites, more or less amalgamated with the pelagic sediment, which originated from ashfall directly on the sea floor. The thicker interbeds, however, are lenses of vitroclastic and crystaloclastic tufts resulting from pyroclastic flows, which travelled down the palaeoslope into the basin. The oldest tephra accumulation has been found in the upper part of the Lower expansa Zone (Fig. 7, section F). The volcanic ash is mixed here with carbonate mud, and additionally the limestone bed reveals intraformational reworking (Fig. l lf). Just above, at the top of the Famennian condensed unit in the expansa and praesulcata Zones, a strong input of pyroclastic material can be observed across the whole study area, although lateral facies changes from section to section occur. Similar variations, i.e., the occurrence of individual pyroclastic flows deposited next to the thin bentonite layers (see Fig. 7, section C), are noted also within the limestone--clay unit, in the upper part of the anchoralis Zone. Another distinct peak in volcanic activity coincides with the transition from the limestone-clay unit to the overlying black siliceous shales in the texanus Zone. It is primarily represented in section H (Fig. 7), where three crystaloclastic tuff layers are sandwiched within an interval of 150 cm, encompassing the boundary of the units. The layers exhibit fiat, horizontal lamination accentuated by the presence of thin black laminae of the pelagic sediment. At the base of a tuft layer large, slightly sinuous

tubes produced by intrastratal burrowers have been found (Fig. 1 lg). The laminated tufts resemble submarine pyroclastic ash-cloud surge deposits (Fischer, 1979), which developed in response to elutriation of fine-grained material from fluidized pyroclastic flows. In the remaining part of the Lower Carboniferous succession the pyroclastic flow deposits are lacking. There are only several thin to very thin bentonite intercalations most probably representing subaerial fallout deposits. Although the locations of eruption centres are uncertain, the textural characteristics of many tephra deposits indicate sources that may have existed not farther than some tens of kilometres from the Galezice area. Rapid onset of volcanism and its most intensive episodes coincide well with the acceleration phase of the carbonate platform drowning. It therefore seems that the tectonic extension and faulting significantly influenced both foundering of the palaeohigh and the volcanic activity. 9. Accumulation rates Although the estimation of rates of sediment accumulation during the drowning of carbonate platforms is burdened with numerous approximations, the calculated absolute values constitute a basis from which the ancient crustal subsidence and/or eustatic sea-level rise can be estimated. Because non-depositional hiatuses are especially difficult to recognize and define in the sedimentary record, the shorter the time interval over which the accumulation rate is measured, the more accurate the value. An other basic problem in calculating the accumulation rates lies in the determination of the timespan in which sediments were deposited. Conodont data provide a high stratigraphic resolution and allow calculation with a precision of conodont zones to reveal the completeness (or rather incompleteness) of the stratigraphic record. On the other hand, however, the calibration of the numerical time scale with the biostratigraphic time scale is still poor for the Devonian and Carboniferous (see Claour-Long et al., 1993). In the succession exposed in the Ostrowka quarry, very low accumulation rates during the drowning are already apparent when the thickness of particular units is compared. Unfortunately, we do not have any precise stratigraphic data to determine directly

M. Szulczewski et al./ Sedimentary Geology 106 (1996) 21-49

41

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Fig. 11. (a) Visean black siliceous shales with pyroclastic interbeds and phosphatic nodules in the western part of the quarry. (b) Thin section view of the black siliceous shales with well-preserved radiolarians (spumellarians). (c) Allodapic character of the limestone intercalation from the upper Tournaisian carbonate-clay unit; polished slab. (d, e) Details of specimen presented in (c); strongly bioturbated wackestone with rare fragments of trilobites, ostracods and conodonts in the topmost part of the allodapic layer (d); amalgamation of crinoidal material with soft, not-lithified wackestone sediment below in the basal part (e). (f) Strongly reworked bioclastic packstone with high admixture of crystaloclastic (volcanogenic) material; Famennian, Lower praesulcata Zone (section F); polished slab. (g) Large tubes of intrastratal burrowers on the sole of the Visean tuff layer (section H).

42

M. Szulczewski et al./Sedimentary Geology 106 (1996) 21--49

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-10

-1

L non-deposition 0

! j

~

average sedimentation rate relative subsidence rate

Fig. 12. Schematic graph depicting changes in subsidence and sedimentation rates during the growth, emergence, and drowning of the carbonate platform of Galezice.

the vertical aggradation rate of the thick Frasnian lagoonal succession. It can be estimated to be in order of 100-125 m/Ma based on data obtained from other sections of the Kowala Formation in the Holy Cross area, which were described by Racki (1993) and have generally better biostratigraphic control. For the phase of initial drowning the accumulation rates can be calculated very precisely (Fig. 12). If we decompact the condensed Famennian unit, following the procedure of Bond and Kominz (1984), and assume that the interval of deposition from the Upper marginifera Zone to the end of the Lower praesulcata Zone was not longer than 4 -4- 0.4 Ma; the estimated accumulation rates vary laterally from 0.2 to 0.7 m/Ma in different sections. The Famennian condensed unit contains some tempestites, which were deposited extremely quickly, but the average frequency of the storms affecting and rearranging sediments was approximately one per 250 ka. Thus, in fact, the phase of incipient

drowning was characterized by extremely slow sedimentation or even by non-depositional conditions, interrupted by rare episodic storm events, which produced, however, much of the sediment column. With the acceleration of the drowning the contribution of redeposited material decreased, whereas simultaneously the accumulation rate of clays increased to the value of about 2 m/Ma, as calculated for the decompacted Tournaisian portion of the limestone--clay unit (Fig. 12). The deposition of the black siliceous shales was also very slow. Although the resolution of the stratigraphy and sedimentology of the Zareby Beds is difficult because of sparse biostratigraphic information, it appears that the average sedimentation rate must have attained values similar to that of the eastern part of the Rheinisches Schiefergebirge, where it was estimated to be about 4 m/Ma (Gursky, 1992). The direct calculation of the accumulation rate using the decompacted thickness of the black shales in the Ostrowka quarry (about 70 m) and the

M. Szulczewski et aL /Sedimentary Geology 106 (1996) 21-49

new radiometric data on duration of the early Visean (Roberts et al., 1993) provides values of about 5-6 m/Ma. 10. Causes of the drowning Drowning of a reef or carbonate platform occurs when the relative rise of sea-level exceeds the accumulation rate (Kendall and Schlager, 1981; Schlager, 1981). This is usually a result of a combination of various factors, such as crustal subsidence, eustasy, and ecological conditions. Because the carbonate ecosystem has a growth potential which is higher than rates of relative rises of sea-level, the drowning of reefs and carbonate platforms requires a significant change in one of the relevant factors. The principal problem in the analysis of the development of drowned platforms, however, is the difficulty in separating eustatic signals from tectonic processes, i.e., factors that are a common prime cause of drowning. The vertical facies succession in the Ostrowka quarry reflects a progressive deepening (Figs. 13, 14). The facies evolution, however, was clearly stepwise as emphasized by abrupt changes in lithology and biota, in certain cases associated with stratigraphic breaks. Hence, the particular phases of drowning, the initiation, the acceleration and the termination can easily be recognized. The progressive rise of sea-level interpreted for this particular succession does not have much in common with the Devonian and the Lower Carboniferous eustatic fluctuations identified in the western United States (Ross and Ross, 1987; Johnson and Sandberg, 1988). In particular, the long-lasting sealevel fall recorded for Euramerica during most of Famennian time, interrupted with repeated subordinate transgressive pulses, is not recorded here. Some features, however, correlate well with global eustatic events (Fig. 14). These are the lack of the postera Zones as well as the discontinuous stratigraphic record in the Devonian/Carboniferous boundary interval, both corresponding with global regressive episodes (see Johnson and Sandberg, 1988). They are recorded as paraconformities and explained by non-deposition caused mainly by sediment winnowing off from the relative highs into the adjacent lows (Szulczewski, 1978). The stratigraphic gap near the Devonian/Carboniferous boundary is known world-

43

wide or, in more complete sections, this sea-level fall is indicated by the occurrence of shallow-water sediments within pelagic units (e.g. Ziegler and Sandberg, 1984; Van Steenwinkel, 1993). The succession of events observed in the Lower Carboniferous portion of the Ostrowka section does not coincide with postulated global sea-level changes (Ross and Ross, 1987). In particular, the transgression in the crenulata Zone and the regression during the early Visean, commonly recognized in Europe (e.g. Krebs, 1979; Belka, 1985) are not recorded here (cf. Fig. 14). On the contrary, a deepening trend is strongly marked in the Ostrowka section during the early Visean. The central parts of the Frasnian carbonate platform in the western Holy Cross Mountains were probably widely covered with a condensed unit, comprising the Famennian and reaching up to the Tournaisian. In contrast to the Ostrowka section, the condensed post-platform carbonates commence in other places early in the Famennian (Szulczewski, 1981; Zakowa et al., 1984) or even in the uppermost Frasnian (Szulczewski, 1989). The Galezice Hills are the only area where the condensed unit begins not earlier than in the middle Famennian Upper marginifera Zone. This zone coincides with a sealevel highstand initiated with a transgression in the Lower marginifera Zone. It was eustatic in nature since its expansion can be observed in different regions of the world (e.g. Johnson and Sandberg, 1988; Talent, 1988; Wendt and Belka, 1991). Of course the onset of the condensed deposition in the Galezice area can not be equated with the flooding event. There is no doubt that the platform was flooded earlier, but the neritic phase is not represented in the sedimentary record of the Ostrowka quarry. The small size of this seamount, its cemented upper surface, and the lack of elevated rims along its margins (reef-building communities were extinct at this time) could probably have facilitated the removal of sediment after flooding. Unfortunately, the flooding event itself can not be directly dated, so that also the rates of subsidence and/or eustatic sea-level rise during the incipient drowning remain unknown. We can only speculate that if the flooding was initiated by the marginifera transgression, a rapid subsidence and/or eustatic sealevel rise (with rates of about 100-150 m/Ma) would be necessary to lower the sediment surface during

44

M. Szulczewski et al. / Sedimentary Geology 106 (1996) 21-49

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tephra accumulation Fig. 13. General scheme of facies and sedimentary evolution of the Famennian to Visean cover of the Frasnian carbonate platform (thickness not to scale).

45

M. Szulczewski et al. /Sedimentary Geology 106 (1996) 21-49 322

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Fig. 14. Chronostratigraphic diagram showing evolutionary stages of the carbonate platform of Galezice and time relations of deposition and stratigraphic gaps to eustatic sea-level changes. The timing of sedimentation during the platform drowning reflects no significant eustatic control, except of two events in the late Famennian (black arrows). The eustatic pulses during the Devonian are adopted from Johnson and Sandberg (1988) and during the Early Carboniferous from Ross and Ross (1987). Numerical ages of stage boundaries are from Odin (1985), ClaouE-Long et al. (1993), and Roberts et al. (1993).

46

M. Szulczewski et aL /Sedimentary Geology 106 (1996) 21-49

one conodont zone below the wave base, as it was positioned during the deposition of the condensed unit. The next phases of drowning are much more precisely time-constrained. As mentioned above, the Visean black siliceous shales were deposited at depths not greater than 500 m, thus indicating the maximum subsidence value for the basement during the late Famennian and the Tournaisian. The average rates of crustal subsidence can be roughly estimated to be not higher than 20-25 m/Ma. These rates are thus more or less one order of magnitude higher than the accumulation rates during late Famennian and Tournaisian times (Fig. 12). Moreover, surprisingly, the subsidence during drowning was slower compared with the subsidence rates that prevailed during the aggradation of the carbonate platform in the Frasnian and approximated the rates of sedimentation (100-125 m/Ma). This paradox points to a eustatic factor as the primary cause of flooding and incipient drowning. Tectonic subsidence possibly began to control the drowning during the acceleration phase when the depositional surface passed to greater water depths and the carbonate production was terminated. A distinct peak in volcanic activity at that time also suggests a significant impact of tectonic extension on the final drowning of the seamount in the lower Visean texanus Zone. The next response to the tectonic impulse seems to be the deposition of the Upper Visean calci-debrites (Fig. 13).

platform aggraded vertically keeping pace with the crustal subsidence, the rate of which was presumably in the order of 100-125 m/Ma. (2) The unconformity that separates the shallowwater carbonates from the post-platform deep-water deposits developed during subaerial exposure and was finally shaped by physical abrasion. (3) The flooding of the faulted block of Galezice in pre-late marginifera time and the subsequent initial drowning resulted primarily from a rapid sealevel rise. Small dimension of the seamount and its cemented upper surface were the most important factors that facilitated the sediment removal and thus suppressed sediment accumulation. The seamount traversed down the photic zone with an average rate not higher than 20-25 m/Ma. (4) Tectonic extension was a primary control during the accelerated and terminal drowning; it governed the subsidence and the volcanic activity. The drowning was complete in the early Visean with the deposition of deep-water, anoxic black shales. (5) The history of the seamount of Galezice resembles the demise of other fragments of the broad Devonian carbonate platform in the Holy Cross Mountains, but in particular, it clearly emphasizes the 'paradox of drowned platforms' since the subsidence during drowning was slower compared with the subsidence that prevailed during the growth of the carbonate platform.

11. Conclusions

Acknowledgements

The Upper Devonian/Lower Carboniferous succession exposed in the Ostrowka quarry documents the late developmental stage of a shallow-water carbonate platform and the successive phases of its drowning: the initiation, the acceleration, and the termination. The recognized facies succession reveals a progressive lowering of the sediment surface through time from a peritidal environment during the Frasnian to basinal conditions in the Visean. In summary, the study has led to the following conclusions: (1) The Frasnian shallow-water carbonate platform is represented by peritidal deposits with a distinct cyclicity. The metre-scale cycles display an upward-deepening pattern resulting from the cumulative effects of regular low-order (eustatic) sea-level fluctuations and local subsidence. The carbonate

Financial support from the Polish State Committee for Scientific Researches (grant KBN No. 6 6211 92 03) is gratefully acknowledged. We thank J. Nebelsick (Ttibingen) for reviewing earlier versions of the manuscript. Special thanks are for M. Joachimski (Erlangen) for the analysis of stable isotopes. The final version was greatly improved following the constructive reviews by M.E. Tucker, T.P. Burchette, B. Sellwood, and M. Santantonio.

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