Facies (2008) 54:277–296

DOI 10.1007/s10347-007-0133-1

ORIGINAL ARTICLE

High-energy sedimentary events in lagoonal successions of the Upper Silurian of Podolia, Ukraine

S. Skompski · P. GLuczyjski · D. Drygant · W. Koziowski

Received: 14 June 2007 / Accepted: 11 December 2007 / Published online: 3 January 2008© Springer-Verlag 2007

Abstract Three Late Silurian carbonate proWles of theMalynivtsy and Skala Formations from Podolia (westernUkraine) are discussed in terms of sedimentation dynamics.Their common feature is the appearance of thick, stroma-toporoid-rich beds within Wne-grained peritidal deposits.These intercalations are composed of fossils typical ofoVshore sedimentary environments. In the most spectacularcase, a channel, several tens of metres wide and inWlledwith stromatoporoids, is incised in a peritidal cyclic com-plex. The successions investigated exhibit sedimentary fea-tures that are diagnostic of onshore redeposition.Independently of the scale of the recorded sedimentaryevents, the onshore redeposition was caused by factors withenergy levels exceeding those of average storms, probablyby hurricanes or even tsunami waves. The dynamic natureof some of the stromatoporoid beds has to be taken intoaccount when constructing the curves illustrating bathymet-rical and facies development of the Silurian succession ofPodolia. The genesis of lens-shaped stromatoporoid beds,elongated depending on their origin, either parallel or per-pendicular to the facies belts, should be considered animportant factor in reconstructions of the depositionalarchitecture of sedimentary hydrocarbon collectors.

Keywords Peritidal facies · Stromatoporoids · Upper Silurian · Podolia · Ukraine

Introduction

The most typical and commonly observed eVect of stormson carbonate shores with no sedimentary barriers is theredeposition of deposits from shallower zones into deeperones and the formation of tempestite successions below orclose to the storm-wave base. The most diagnostic featureof such deposits is hummocky cross stratiWcation, whichresults in a characteristic undulating bedding. In the case ofshores that are protected by reef-type barriers, the stormsoften destroy these barriers and not only transport surpris-ingly large quantities of oVshore and reefal detritus into thelagoon but also wash out the sediments that were originallydeposited in the lagoon (KalbXeisch and Jones 1998; Liet al. 1998). Both situations are described in detail in mod-ern handbooks on carbonate sedimentology, e.g. Tuckerand Wright (1990; Chap. 4.1.3.) or Flügel (2004; Chap.12.1.2.).

These two highly simpliWed facies models describe mod-ern examples fairly well, while the situation in fossil suc-cessions seems to be more complex. In the case ofPalaeozoic complexes, the main obstacles to interpretationare the nature of the barrier structures, which diVer frommodern ones, and a general lack (or rarity) of buildups rep-resenting reefs in the ecological sense of the term (e.g.,Copper 1989; Flügel and Flügel-Kahler 1992; Flügel andKiessling 2002). Shores dominated by stromatoporoid–coral buildups, common in carbonate complexes of theSilurian and the Devonian (Riding 1981; Burchette 1981),are a speciWc example of this phenomenon. Stromatopor-oids, colonial rugose and tabulate corals are the mainframework builders of these buildups; however, their palae-oecological relationships commonly remain unclear. Bio-hermal forms of these biogenic complexes are relativelyrare; the predominant forms are biostromes of an undetermined

S. Skompski (&) · P. Guczyjski · W. KoziowskiFaculty of Geology, Warsaw University, Al. ôwirki i Wigury 93, 02-089 Warsaw, Polande-mail: skompski@uw.edu.pl

D. DrygantState Museum of Natural History, National Academy of Sciences of Ukraine, Teatralna 18, Lviv 79008, Ukraine

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origin, which are treated as less important surrounding ofthe “reefs” (Burchette and Wright 1992; Kershaw 1994;Kershaw and Brunton 1999). The topic has been studied indetail in the classic outcrops of Gotland (Riding 1981; Ker-shaw 1990, 1994, 1998; Riding and Watts 1991; Sandström1998; Sandström and Kershaw 2002). The work of Ker-shaw, who proposed a classiWcation of biostromes andshowed their modern analogues, seems to be the most sig-niWcant in this Weld. The problem of how stromatoporoidbuildups acted as energetic barriers in shallow-shelf searemains, however, unresolved.

The studies of the Upper Silurian carbonate successionof Podolia (southwestern Ukraine) carried out in an areabetween Skala Podil’ska and Kam’janec’ Podil’skyj(Fig. 1), have revealed a surprisingly common occurrenceof biostromes and organodetritic limestone beds with fre-quent graded bedding and chaotically arranged biogenicmaterial composed mainly of redeposited massive stroma-toporoids. These beds form intercalations in a succession ofextremely shallow-water limestones, marls and dolomites,with numerous surfaces showing desiccation polygons. Theopen-sea material building the biostromes and organodet-ritic limestones was supposedly periodically redeposited inan onshore direction from an oVshore zone of shoals and

barriers and Wnally deposited in shallow lagoonal or evennear-shore settings. This phenomenon, belonging to thewider category of “storm-induced event deposits”, ratherthan simply to “tempestites” (sensu Seilacher 1991; Einseleand Seilacher 1991; Einsele et al. 1996), may occur in bothmacroscale (when the thickness of the deposited beds ismeasured in metres) and microscale (when the thickness ismeasured in centimetres). This paper presents descriptionsand interpretations of events belonging to both categories.Only very few examples of this phenomenon (backXowdeposits within shallow marine settings) are known fromthe fossil record (see Discussion and References in Cantala-messa and Di Celma 2005). It seems that the depositionalmechanism proposed here could also be identiWed in otherareas with Silurian and Devonian deposition (Fig. 2).

General stratigraphic and sedimentary framework

In the Middle and Late Silurian, the Podolia region waslocated between the equator and the southern tropic (Tropicof Capricorn) (Tait et al. 2000; Baarli et al. 2003; Nawrockiand Poprawa 2006). The sedimentary development andfacies patterns of the Podolian Silurian were governed by

Fig. 1 Upper Silurian of Podolia. a Distribution of the Upper Silurianfacies on the margin of the East European Platform (after Einasto et al.1986, simpliWed). b Recent distribution of the Silurian “formations” in

the Podolia and Dnister regions: Kt Kytaigorod Formation, Bg Bago-vytsya Formation, Ml Malynivtsy Formation, Sk Skala Fm

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the location of the region close to the edge of the East Euro-pean Platform in the proximity of the mobile Teisseyre-Tornquist Zone (TTZ, Figs. 3 and 4). The Silurian successionis 370- to 430-m thick and reXects the palaeotectonicprocesses that took place along the TTZ.

Today, the region oVers the best exposures of a wide-spread facies belt belonging to the marginal sea of the EastEuropean Craton (Fig. 1). In its northwestern part, this beltrims a slightly deeper (but still epicontinental) East BalticBasin, recorded from borings into the bed of the Baltic Sea,and in southeastern Lithuania and northern Poland (Einastoet al. 1986; Samtleben et al. 2000; Calner 2005a). Over itswhole length of almost 2,000 km, the belt of carbonate sed-imentation was approximately 150- to 200-km wide andhad a constant facies pattern, the central position in whichwas occupied by stromatoporoid–coral shoals and barriers(Fig. 1). This central zone separated the inner shelf facies—usually dolomitized lagoonal facies, with a characteristicassociation of laminites and dolomicrites with eurypteridsand ostracods—from outer shelf and slope facies domi-nated by limestones and marls with tabulate corals, brachio-pods and other abundant and diversiWed benthic fauna,

which pass into basinal graptolitic shales. The inner shelfdeposits are represented mainly by algal, ostracod and, spo-radically, stromatoporoid facies. The zone of shoals andbarriers is dominated by crinoidal and stromatoporoid–coral facies forming bioherms and biostromes (Kaljo et al.1983; Einasto et al. 1986; Bjerkeus and Eriksson 2001;Sandström and Kershaw 2002).

The sedimentary history of the Dnister succession can becharacterised by its cyclic pattern (Fig. 2). Cyclothemsrecorded in detail were hierarchically arranged into at leastthree orders of units: elementary cyclothems, mesocyclo-thems and macrocyclothems (Tsegelnjuk et al. 1983;Predtechenskij et al. 1983). On a regional scale, the

Fig. 2 SimpliWed stratigraphic scheme of the Silurian of Podolia(Tsegelnjuk et al. 1983; Nikiforova et al. 1972; Drygant 1984; Koren’et al. 1989) and the stratigraphic position of the studied sections inSkala Podil’ska and Kam’janec Podil’skyj (indicated by two greybars)

Fig. 3 Location of stromatoporoid bioherms and shoals in particularSilurian time intervals on a simpliWed map of the Palaeozoic depositson the southeastern margin of the East European Platform. For legend,see Fig. 4

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smallest detectable units are the shallowing-upward meso-cyclothems (Fig. 2), which can be grouped into three mac-rocyclothems, the two older representing a regressivesuccession, with the youngest, terminating in the Devonian,showing a trangressive character.

The continuous Silurian succession of Podolia is dividedinto four so-called “formations”: Kytaigorod, Bagovytsya,Malynivtsy and Skala (Fig. 2). This stratigraphic subdivi-sion is based on detailed lithological and palaeontologicalinvestigations (Tsegelnjuk et al. 1983; Nikiforova et al.1972; Drygant 1984; Koren’ et al. 1989), and the recogni-sed “formations” and “members” are deWned by a mixtureof lithostratigraphical and biostratigraphical criteria, simi-larly to the classiWcation presented by HuV et al. (2000).The sea-level changes, most probably of a eustatic nature,caused landward and seaward shifting of the facies zones(Fig. 3); however, the facies pattern persisted in every chro-nostratigraphic unit. The relatively deeper water facies ofthe Kytaigorod and Malynivtsy formations are separated bythe Bagovytsya Formation, which is dominated by shallow-water facies. The succession terminates with shallow-waterdeposits of the Skala Formation, the uppermost part ofwhich is represented by the deeper shelf facies of theDzvenygorod Member. The sections studied represent theMalynivtsy Formation (Kubatchivka Quarry nearKam’janec’ Podil’skyj) and the Skala Formation (SkalaPodil’ska Quarry and the Zbrub River escarpment). Particu-larly suitable for detailed sedimentological studies is theSkala Formation, which shows the widest and most rapidfacies changes and abounds in numerous sections thatenable tracing all the diVerent facies represented. In

addition, the slightly less variable Malynivtsy Formationcan be divided into outer and inner shelf zones separated bybiogenic shoals.

Malynivtsy Formation

The Malynivtsy Formation is approximately 100-m thickand represents the middle part of the Podolian Silurian. Thedeposits of this unit cover the whole area of the sedimen-tary palaeobasin—from its eastern limit up to the westernmargin of the TTZ. In the Podolian outcrops, the formationis represented mainly by the upper slope and fore-reeffacies. In the proWles exposed on the banks of the Dnisterand its tributaries—the Smotryb and Mukna—the unit isdeveloped as nodular limestones, which pass gradually intodolomites and dolomitic marls north of Kam’janec’Podil’skyj. The formation is divided into three “members”:Konivka, Sokol and Grintchuk (Nikiforova et al. 1972;Drygant 1984,2000).

The Konivka Member, with a total thickness of 20–25 m, is represented by nodular limestones and marls withthin intercalations of organodetritic limestones, in someplaces dolomitised and with a thin (10–30 cm) metabenton-ite layer on top. The unit yields a rich and taxonomicallydiverse faunal assemblage (Tsegelnjuk et al. 1983) contain-ing rugose corals: Kodonophyllum truncatum (Wahl.), Pilo-phyllum weissermeli Wdkd., Acervularia sokolensis Kadl.,Tryplasma loveni (Edw. and H.), Microplasma gotlandicaDyb.; stromatoporoids: Densastroma astroides (Rosen),Desmostroma yakovlevi (Riab.), Clathrodiction mohica-num Nest.; tabulates: Paleofavosites alveolaris (Gold.),

Fig. 4 SimpliWed cross-section through the southeastern margin of theEast European Platform (compiled on the basis of geophysical datafrom boreholes). Note major inXuence of the tectonic activity of the

Teisseyre-Tornquist (TT) Line on the fast deposition rate and facieschange in the TT zone area

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Mesofavosites alveolitoides Tes., M. multiporus Sok., Lac-eripora cribrosa Eichw., Favosites eVusus Kl., Parastria-topora coreaniformis (Sok.); heliolitids: Stelliporellaintricata (Ldm.), S. lamellata Wenzel, Heliolites decipiensMcCoy; calcareous algae: Sokolella polymorpha Istch.,Solenopora Wliformis Nich.; and brachiopods: Gypidulamagna Ryhn., Kirkidium knighti (Sow.), Atrypa sowerbyiAlex., Janius pyramidalis (Wen.), Delthyris elevata Dalm.et al. North of Kam’janec’ Podil’skyj the Konivka Memberis represented by lagoonal complexes of dolomites withgypsum and anhydrite.

The Sokol Member is composed of a monotonous set(approximately 55-m thick) of nodular limestones andmarls with a number of thin (1–10 cm) metabentonite lay-ers (Tsegelnjuk et al. 1983). The deposits contain abundanttabulate and rugose corals: Favosites pseudoforbesi (Soko-lov), Squameofavosites incredibilis Chekh., Barrandeoliteslichaenaroides (Sokolov.), Syringopora multifaria Kl.,Spongophylloides perfekta Wdkd, Tryplasma loveni (Edw.and H.), Cantrilia minima Bulv., Rhizophyllum gotlandicusRoemer, Holmophyllum holmi Wdkd.; stromatoporoids:Lophiostroma schmidti Nich., Parallelostroma lamellosum(Riab.); and brachiopods: Levenea canaliculata (Lindst-roem), Schaleriella delicata Harp. and Boub., Protochone-tes striatella (Dalm. et al.).

The Grintchuk Member, which terminates the proWle ofthe Malynivtsy Formation, is 18-m thick and is representedby partly dolomitised nodular limestones and marls withintercalations of organodetritic limestones and with threethin (2–4 cm) horizons of metabentonites. Similar to thelower members of the formation, the unit abounds in tabu-late and rugose corals: Squameofavosites incredibilisChekh., Stereoxyloides pseudodiodontus (Weis.), Cantriliaminima Bulv., Rhizophyllum gotlandicus (Roem.); heliolit-ids: Syringoheliolites contrarius Bond.; brachiopods: Leve-nea canaliculata (Lindstr.), Janius barrandi (Vern.),Didymothyris didyma (Dalm.), Protochonetes striatellus(Dalm.); and trilobites: Proetus signatus Lindstr.

Skala Formation

The boundary interval of the Malynivtsy and Skala Forma-tions is marked by a rapid increase in subsidence rate in theTTZ (Fig. 4). The thickness of open-shelf sediments,related to the time of deposition of both formations, is rela-tively large (approximately700–800 m of graptolitic shalesin some of the boreholes). This process corresponded todevelopment of the Caledonian foreland basin, the mostdistal parts of which extended to the southwestern edge ofthe East European Craton (Poprawa et al. 1999). At thesame time, the Podolian epiplatform basin, located outsidethe area of accelerated subsidence, became shallower anddominated by facies ranging from open marine through

shoal domain up to lagoonal. The Skala Formation succes-sions, reaching a thickness of 170–190 m, often show acyclical facies arrangement. There is a distinct vertical suc-cession of diVerent lithologies, and the following litho-stratigraphical “members” have been distinguished(Koziowski 1929; Nikiforova et al. 1972; Tsegelnjuk et al.1983): Isakivtsi, Prygorodok, Varnytsya, Trubchyn andDzvenygorod.

The Isakivtsi Member is approximately12-m thick and isrepresented by grey, Wne-grained dolomites with thin inter-calations of limestones and dolomitic marls and with twohorizons of metabentonites. The limestones contain scarcebrachiopods: Levenea canaliculata (Lindstr.), Schalerielladelicata Harp. and Boub., Dayia navicula Sow.; trilobites:Proetus signatus Lindstr., Acaste podolica Balashova; andconularians. In addition, Nikiforova and Predtechenskij(1968) reported from the lower part of the unit the follow-ing brachiopods: Isorthis ex gr. orbicularis (Sow.), Ferga-nella borealis diodonta Eichw., Sphaerirhynchia wilsoniSow., Leptagonia sp., Delthyris elevatus Dalm., Rhynchos-pirina baylei (Dav.), Protochonetes hotinensis (Nikif.), P.striatellus Muir-Wood; and numerous ostracods: Herr-mannina isakovtsyensis (Abush.), creating lenticular accu-mulations. A remarkable feature of the unit is the presenceof abundant dasycladacean algae (Nikiforova and Predte-chenskij 1968). The brachiopods, although diverse, areextremely rare, except for Dayia navicula, which, togetherwith P. striatellus, constitutes a distinct community (60%and 40%, respectively; data from Gritsenko et al. 1999).The appearance of the brachiopod Dayia navicula and therapid shallowing of the depositional environment in theuppermost part of the Ludlow suggest that the base of theIsakivtsi Member might correspond to the beginning of theLau Event. This event is recorded in numerous Late Silu-rian sections (cf. Lehnert et al. 2007) as a major positivecarbon isotope excursion associated with extinctions andchanges in community structures (Jeppsson and Aldridge2000; Calner 2005a,b). However, in the present state ofknowledge, the occurrence of this event in the Podoliansuccession cannot be conWrmed either by conodonts or byisotope analysis. In general, conodonts have not been foundin the Isakivtsi and Prygorodok Members, while in theunderlying and overlying beds, only the long-ranging taxaOzarkodina conXuens, O. excavata and Panderodus unico-status occur.

The Prygorodok Member is approximately 23-m thickand is represented mainly by lagoonal facies—alternationsof laminated dolomitic marls, claystones and dolomites.The deposits are devoid of any fauna. The dolomitic marlsand claystones contain numerous surfaces with desiccationpolygons.

The Varnytsya Member, with a thickness of approxi-mately 60 m, corresponds to the lower part of the so-called

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Skaia Limestones of Koziowski (1929) and to the RankivBeds of Nikiforova et al. (1972). The unit is composed of awhole range of dolomites, dolomitic marls and micritic,detritic and nodular limestones. The member contains a 35-cm-thick layer of metabentonites and is characterised by arich and taxonomically diverse assemblage of tabulate andrugose corals: Mesosolenia reliqua (Sok.), Favosites forb-esi Edv. and H., F. eichwaldi Sok., Syringopora unicaTchud., Spongophylloides nikiforovae (Bulv.), Tryplasmaformosa (Prantl.), Holmophyllum holmi Wdkd.; heliolitids:Okopites okopiensis Bond., Heliolites pachycanaliculoidesBars; brachiopods: Protochonetes dniestrensis (Kozi.),Dalejina staszici (Kozi.), Atrypa dzwinogrodensis Kozi.,Delthyris magnus Kozi.; trilobites: Proetus scalicus Bal., P.kuresaaremensis Männ.; and algae and Xoristic fragments:Algites enteromorphoides Basson and Wood, Eorhyniaprimigenia Istch., Primochara calwata Istch. and Said.,Cooksonia pertonii Lang., Zosterophyllum myretonianumPenh. The lower part of the Varnytsya Member yielded theconodont Ozarkodina crispa (Drygant 1971,1984), a spe-cies that in the Barrandien does not pass the Ludlow/Pridoliboundary (Chlupab at al. 1980). On this basis, the Ludlow/Pridoli boundary is drawn approximately 20 m above thebase of the Varnytsya Member (which is 55 m above thebottom of the Skala Formation).

The Trubchyn Member is about 37-m thick and showsfacies similarity with the underlying deposits, from which itdiVers by the domination of stromatoporoid and stromatop-oroid–coral limestones with numerous bioherms, and alesser amount of dolomites and dolomitic marls. The unitcontains a rich and diverse fauna (Tsegelnjuk et al. 1983) oftabulate and rugose corals: Mesosolenia reliqua (Sok.),Mesofavosites pinnatoides Tes., Favosites eichwaldi Sok.,Squameofavosites intericatus (Pobta), Syringopora fascicu-laris (L.), Mucophyllum crateroides Ether., WeissermeliadeWnita Syt., Tryplasma formosa (Prantl); heliolitids:Pseudoplasmopora karaespensis (Kov.), Heliolites pachy-canaliculoides Bars., Dnestrites transitus Bond.; stroma-toporoids: Vikingia podolica Bog., Desmostroma stratosumBog.; trilobites: Proetus scalicus Bal., Calymene dnestrovi-ana Bal.; and brachiopods: Salopina crassiformis (Kozi.),Protochonetes dniestrensis (Kozi.), Delthyris magnus Kozi.

The Dzvenygorod Member is represented by a set ofopen-marine deposits approximately 20-m thick composedof alternating nodular limestones and marls with intercala-tions of bioclastic limestones. Bioherms occur in the lowerpart of the unit and a thin metabentonite layer in the upper.The member contains a rich and diverse fauna (Tsegelnjuket al. 1983) of tabulate and rugose corals: Squameofavositesbohemicus (Pobta), Syringopora fascicularis (L.), Mesole-nia reliqua (Sok.), Mesofavosites pinnatoides Tes., Muco-phyllum crateroides Ether., Weissermelia deWnita Syt.,Tryplasma formosa (Prantl), Holmophyllum holmi Wdkd.;

trilobites: Proetus socialis Bal., P. volcovcianus Bal., Encr-inurus stubbleWeldi Tripp; and brachiopods: Salopinacrassiformis (Kozi.), Atrypa dzwinogrodensis Kozi.,Pseudoprotathyris infantilis (Kozi.). The conodontsOzarkodina eosteinhornensis (Walliser) and Ligonodinaelegans (Walliser) and trilobites Acaste dayiana (Richterand Richter) and Acastella spinosa podolica (Balysheva)indicate a Pridolian age for the Dzvenygorod Member. TheSilurian/Devonian boundary, corresponding to the appear-ance of Monograptus uniformis angustidens, is located3.2 m above the lithological change from nodular lime-stones, typical of the Dzvenygorod Member, into dark clay-stones with carbonate intercalations. This graptolite taxonis associated with the conodonts Icriodus woschmidti andOzarkodina remscheidensis and the trilobite Acastellaheberti, which appeared slightly earlier (Tsegelnjuk et al.1983; Drygant 1984; Abushik et al. 1985).

The Sokol Member in the Kubatchivka Quarry

The Kubatchivka Quarry is located about 1 km south ofKam’janec’ Podil’skyj on the western bank of the Smotryb

River, which is a tributary of the Dnister (Fig. 1). Thequarry exposes strata of the Sokol Member of the Mal-ynivtsy Formation, dated as Ludlow (Fig. 2; Nikoforovaand Predtechenskij 1968; Nikiforowa et al. 1972; Koren’

Fig. 5 Part of the section exposed in the Kubachivka Quarry (comparephoto in Fig. 6a): for descriptions of the a, b and c complexes, see text.Inset shows large bioclasts (stromatoporoids, corals) incorporated intofenestral-type deposits

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et al. 1989). The quarried walls are 30-m high, but only thelowest 10 m are accessible for study. The clearest section isexposed on a rock spur closing the entry to the quarry fromthe riverside (Fig. 6a). The rocks are almost horizontal,with a regional dip of a few degrees to the west.

The exposed succession comprises three distinct com-plexes. The lower complex, with a thickness exceeding10 m in the quarry, but with only its upper approximately2 m accessible for detailed study on the spur (Fig. 5, com-plex a), is developed mainly as massive and weakly beddedstromatoporoid and stromatoporoid–coral limestones. Thedominating massive stromatoporoids, sometimes attaininglarge dimensions (up to 0.5 m in diameter), are accompa-nied by branching stromatoporoids (Amphipora), tabulates(?Syringopora, Favosites), solitary corals, and crinoid frag-ments (with long unsegmented columns). Bryozoans arealso relatively common. The surfaces of the bioclasts arecommonly covered by Girvanella coatings (Fig. 6f). Thebenthic fauna remains mainly in growth position or showsevidence of only minor redeposition. Branching coloniesare overturned but not broken and occur in distinct accumu-lations; the massive stromatoporoids show no evidence ofdamage resulting from extensive transport (cf. Kershaw1990; Guczyjski 2003). Such a situation Wts the descriptionof an autobiostrome or a paraautobiostrome according tothe classiWcation of Kershaw (1994). The uppermost(approximately 0.5 m) part of the complex shows faintwavy bedding and contains very common branchingstromatoporoids (Amphipora), tabulates (?Syringopora)and abundant crinoidal detritus. Microfacially, the matrixof the stromatoporoid framestones is developed as stroma-toporoid–coral–crinoidal grainstones/packstones. Theupper surface of the complex is erosional, with visibleintrabiohermal depressions.

The upper complex, with its total thickness in the quarryreaching a dozen or so metres but with only its lowermostapproximately 1.5–2 m accessible for study on the rockspur (Fig. 5, complex c; Fig. 6a) contains a larger propor-tion of distinctly bedded rocks. The stromatoporoid faciesare more commonly accompanied by crinoidal facies. Thelower boundary of the complex is clearly erosional. Thelowermost 20–30 cm are developed as bioclastic lime-stones, with numerous laminar stromatoporoids colonisingthe bottom (Fig. 6d), these being usually interpreted asindicators of soft muddy substrate (Kasmierczak 1971;Guczyjski 2005). Microfacially, these rocks are developedas rudstones, with stromatoporoid-crinoidal-peloidal pack-stones/grainstones as matrix. The stromatoporoid complexchanges its character upwards and passes into dark, nodu-lar, marly, wavy-bedded limestones with redeposited andpartially fragmented massive stromatoporoids (parabios-trome sensu Kershaw 1994). Microfacially, the matrix ofthese rocks is developed as mudstones, with rare peloids

and scattered crinoidal-shell detritus. Laterally, the faciespasses into Wne- and coarse-grained crinoidal grainstonesand rudstones.

The two thick stromatoporoid complexes are separatedby complex b, with a thickness of 60–80 cm (Figs. 5,6a). Itis developed as pale-coloured, partially laminated micriticlimestones containing ostracods belonging to the familiesLeperditicopida and Promitiopsida, passing upwards into alayer with numerous and conspicuous fenestral structures.Microfacially, the rocks are developed as mudstones withpeloids and as ostracod wackestones. In the fenestral part ofthe complex (Fig. 6e), a distinct erosional surface is devel-oped, emphasised by later stylolitisation (Fig. 6b, g). A layera few centimetres thick directly capping the erosional sur-face is composed of small lithoclasts of micritic or fenestrallimestones and contains numerous large (a few centimetresin diameter) bioclasts of benthic fauna: fragments of mas-sive stromatoporoids, tabulates and solitary corals (Fig. 6g,h). The overlying fenestral limestone also contains isolatedfragments of tabulates and stromatoporoids (Fig. 6c). Theupper part of the complex shows the eVects of distinct ero-sion, which in some places completely removed the fenes-tral part of the complex that elsewhere reaches a thickness of20 cm. The erosion within the laminite–micritic–fenestralcomplex recognised on the rock spur is also visible on thesouthern wall of the quarry (Fig. 6b).

Interpretation

In the section described, facies co-occur that in palaeonvi-ronmental reconstructions are usually placed in zoneslocated rather far away from each other. The stromatopo-roid–coral biostromes of complex a and the crinoidal–stromatoporoid biostromes of complex c most probablyrepresent the zone of shoals and barriers. The biogenic mate-rial building the biostromes is either mainly in situ (complexa) or a substantial part of it has been redeposited (crinoidsand stromatoporoids of complex c) from shallow zones of theouter shelf located seaward of the barrier zone. On the otherhand, complex b, composed of laminated micritic ostracod-bearing limestones that pass upwards into fenestral lime-stones is typical of an intermediate environment betweenintra- and supratidal zones (discussion in Flügel 2004). Suchan interpretation is also supported by its low-diversity fossilcommunity consisting of numerous ostracods belonging tothe families Leperditicopida and Primitiopsida, which arecharacteristic of the Silurian marginal-marine zones (Pred-techenskij et al. 1983; Abushik and Evdokimova 1999;Vannier et al. 2001). Such deposits formed as a result of atemporary fall in sea level. They usually occur either in thelittoral part of the lagoonal zone or surround temporarilysubaerially exposed stromatoporoid barriers. The erosionallower boundaries of complexes b and c and other evidence

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of substantial erosion within complex b (Figs. 5, 6b), aswell as the close vertical proximity of the stromatoporoidcomplexes to each other, tend to support the second possi-bility. The deposits of complex b probably formed on smalltidal Xats, perhaps located on the lee sides of stromatopor-oid–crinoidal shoals/islands. This interpretation corre-sponds to the tidal-Xat model of Pratt and James (1986). Itis diYcult to Wnd any literature data listing diVerencesbetween the developments of tidal-Xat successions locatedin littoral zones and those surrounding islands. It seems,however, that areas around islands were much more sub-jected to the activity of erosional factors than the mainlandshores that were separated from the islands by a lagoonzone (see comments on the variability of the Devonian tidalzones in Skompski and Szulczewski 1994, p. 263).

In this context, the occurrence of relatively commonlarge fragments of complete specimens of stromatoporoids,rugose corals and tabulates in the deposits of complex b issurprising. Their co-occurrence in fenestral facies can onlybe explained by rapid episodes of storm-induced onshoretransport. The fragments of benthic faunal elements, tornout from the barrier zone or even from shallow areas of theouter shelf, were carried by sediment-laden Xow and left inthe splash zone above the high tide, where they becameincorporated into fenestral type deposits. The high-energyevents were recorded as both depositional and erosionaleVects. Judging from the rarity of their occurrences, theseevents belonged to extraordinary phenomena; however,they could also happen more commonly but not be pre-served in fossil succession (cf. Einsele et al. 1996; Cantala-messa and Di Celma 2005). The triggering mechanismresponsible for this type of Xow could have been a storm,hurricane or even a tsunami, but the precise determinationof the event involved demands more widely based studiesof the depositional architecture of the succession.

The Trubchyn Member in the Skala Podil’ska Quarry

The Skala Podil’ska region abounds in exposures that allowobservations of a more than 30-metre-thick Silurian succession

belonging to the uppermost part of the Trubchyn Memberof the Skala Formation. The best natural exposures occur inthe historical centre of the town on steep escarpments of theZbrub River, near the ruins of the fourteenth century castle(Zbrub River escarpment locality described below). Thelower part of the succession is exposed in an unnamedquarry located in the southern part of the town, near theroad to Kam’janec’ Podil’skyj (here referred to as SkalaPodil’ska Quarry). The stratigraphic position of the succes-sion exposed in the quarry and its relationship to otherexposures of the Skala Formation has been illustrated byAbushik et al. (1985, Fig. 2, Sect. 219).

The quarry exposes two diVerent sedimentary complexesseparated by greenish clays nearly 1-m thick. Only thelower complex was studied in detail (Fig. 7). It is generallycomposed of dolomitic facies developed as dolomicritesdevoid of fauna (lacking even the leperditiids that are com-mon in shallow-water successions of Podolia), with spo-radic intervals showing Xat and crinkled lamination. Thelaminated intervals coincide with the occurrences ofnumerous desiccation polygons of diVerent sizes (Fig. 8h).Some of the beds show evidence of subaerial weathering intheir top parts, which are developed as pedogenic brecciasresembling initial regoliths (Fig. 8c–e) and are sporadically

Fig. 6 Details of the section exposed in the Kubachivka Quarry.a General view of the section (a, b, c complexes described in the textand drawn schematically in Fig. 5); black arrow indicates erosionalboundary of complexes b and c. b Erosional surface within complexb (southern wall of the quarry). c Large fragment of stromatoporoid(8 cm, arrow) within the layer with fenestral structures (complex b).d Contact of complex b with complex c (laminar stromatoporoidscolonising the bottom after erosional event). e Details of fenestralfacies b (thin-section photo). f Girvanella coatings on the bioclasts(complex a, thin-section photo, e, f length of scale bar = 1 mm).g Erosional surface within fenestral facies b (arrowed). h Largefragments of benthic fauna, indicated by black arrows, deposited onthe erosional surface within fenestral facies b (compare inset in Fig. 5)

Fig. 7 Schematic section exposed in the Skala Podil’ska Quarry; en-larged part of the section illustrates the lagoonal interval with interca-lation of redeposited oVshore benthic fauna. Note that the distinctionbetween the limestone and dolomite legends applies only to the left-hand Wgure. a–h refers to samples shown in Fig. 8

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accompanied by structures resembling rhizoliths. Thethickness of these regolithic beds does not usually exceed10 cm. In the most general sense, their development is anal-ogous to the palaeosols known from the Devonian and Silu-rian deposits. (e.g., Wright 1988; Davies 1991; Skompskiand Szulczewski 2000). The upper part of the lower com-plex contains two beds of brown dolomicrites (x1, x2) withlarge cavities (Figs. 7,8a). The lower bed (x1) starts with a12-cm-thick bed of highly bioturbated dolomitic marls withthin laminae of black anoxic shales at the base. This bedcontains numerous ichnofossils (Planolites) as well as rareGigantostraca (Pterygotus?) and millipedes of the genusEoarthropleura? (Fig. 8f, g; cf. Shear and Selden 1995).The overlying dolomitic bed is practically devoid of fossils,but the shapes of the cavities, which sometimes reachdimensions of a dozen or so centimetres, reveal their bio-genic nature: they developed mainly by dissolution ofbranching tabulate and rugose corals (Fig. 8a, i). The lowerdolomite is capped by a coarse-grained limestone com-posed of crinoids, fragmented stromatoporoids and solitaryand colonial corals. The lower part of the unit comprisingthe coarse-grained limestone and the brown dolomicritewith cavities is uneven; its shape indicates sinking of largerbiogenic elements into the muddy bottom. The upperboundary of the coarse-grained limestone with the overly-ing bed of laminated dolomites is similarly uneven. Thehigher of the two brown dolomicrite beds with biogeniccavities (x2) is overlain by a bed of pure dolomicrite with-out any biogenic or sedimentary features and capped by aninitial regolith that forms the top of the dolomitic succes-sion.

The dolomitic succession then passes upwards intogreenish clay deposits. The colour of the clays suggests theoccurrence of bentonites, which are indicated on the log ofAbushik et al. (1985). The present state of the exposuredoes not, however, enable recognition of bentonites in astrict sense. In any case, such a bed would have littlechance of preservation in a relatively shallow-water deposi-tional setting. The upper complex is dominated by repeatedbeds of stromatoporoid limestones and most probably rep-resents the lower part of the unit exposed in the BridokQuarry and in the Zbrub River escarpment (see descriptionof the Trubchyn Member below).

Interpretation

The sedimentary environment of the lower complexbelongs to the shallowest of those occurring in the Silurianof Podolia. Numerous desiccation polygons, the prevalenceof dolomites devoid of fauna and the recurring initial rego-liths indicate a shallow lagoon that periodically dried outand became subaerially exposed. The lack or rarity oflaminites, stromatolites and pseudomorphs of calcite after

evaporitic minerals, which would be expected in such anenvironment, was probably caused by the lowered salinityof a near-shore zone that was periodically supplied by rainwater. This interpretation is supported by the occurrence ofarthropleurids, which lived in the intermediate zonebetween land and marine environments (Shear and Selden1995). During the periods of sea-level highstands, arestricted lagoon developed in the area, with a poor brack-ish water biocoenosis and the deposition of poorly oxidizedclay-marly sediments. In the external parts of the lagoon,rapid syndepositional dolomitisation took place. Rare epi-sodes of better communication with an open-sea basinresulted in the development of burrowing benthic organ-isms. Sporadically, the biogenic material was delivered intothe lagoon from zones further oVshore, these being thesource areas for stromatoporoid fragments, tabulates andcrinoidal sands carried by the sediment-laden sheet Xowsduring storm swells. Larger and heavier clasts thrown ontothe muddy bottom of the lagoon sank gradually into thesediment, while the lighter elements formed a coarse-grained layer. Only one such layer was found in the section.It shows a vast lateral spread (despite its small thickness),as in the quarry alone it can be traced over a distance of fewhundred metres. These deposits Wt fairly well with the deW-nition of washover Xats or aprons (sensu Jackson 1997),which, during particularly strong storms, reached the exter-nal near-shore parts of the lagoon.

The Trubchyn Member along the Zbrub River escarpment

The deposits exposed above the greenish clays in the SkalaPodil’ska Quarry (Fig. 7) are also accessible further north-east in the Zbrub River escarpment as well as in the BridokQuarry, which is located northeast of Skala Podil’ska. AspeciWc feature of this succession constituting the upper-most part of the Trubchyn Member is the regular occur-rence of beds packed with stromatoporoids, whichintercalate with thicker complexes of light-coloured, Wne-grained and micritic limestones. The thickness of thestromatoporoid beds varies between a dozen or so centime-tres and 2 m. An exceptional feature in the succession is adark grey bioclastic–stromatoporoid complex, which is6–9 m thick and which is exposed in the Bridok Quarry (for

Fig. 8 Details of the section exposed in the Skala Podil’ska Quarry(location of samples indicated in Fig. 7). a Bioturbated muddy dolom-icirite (enlarged in b), covered by dolomicrite with redeposited benthicmacrofauna. c–e Initial regoliths. f Fragments of eurypterid Pterygo-tus?. g Fragment of millipede Eoarthropleura? h Surface with desicca-tion polygons. i Uppermost part of dolomicrite from a, withredeposited benthic fauna (stromatoporoids, tabulates); length of scalebars = 10 cm

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details of the microfacies, see Skompski et al. 2006). In theother stromatoporoid beds, the fossils are densely and com-monly regularly packed, but beds with both normal andreversed graded bedding are equally common. Despite thefact that particular parts of this succession are visible inclosely spaced exposures, a bed-by-bed correlation is diY-cult due to the discontinuity of the stromatoporoid bodies,which are developed mainly in the form of lenses (earlierusually interpreted as biostromal beds). The origin of theseforms and their microfacies patterns are best revealed in anatural exposure in the Zbrub River escarpment »500 mnorth of the ruins of the castle in Skala Podil’ska.

At least two stromatoporoid beds are visible in the expo-sure, the upper of which is clearly an inWlling of an ero-sional channel (Fig. 9a, c), about 40 m wide and with amaximum height of approximately 2.5 m. The lower

stromatoporoid bed, which appears to thin laterally, is alsomost probably a channel inWlling.

The material forming the “debritic” inWlling of the upperchannel is composed mainly of stromatoporoids accompa-nied by much less abundant colonial rugose corals, tabu-lates and lithoclasts. The stromatoporoids, with a diameterusually of a dozen or so centimetres, are chaoticallyarranged, generally not in growth position, and with com-mon collision contacts of neighbouring specimens. Theyshow numerous indications of redeposition, such as over-turning and breakage. The variety of their shapes and ofother macroscopic morphometric features, such as thearrangement of the latilaminae (internal growth bands),suggests that they were derived from various areas, diVer-ing in sedimentary conditions (cf. Kershaw 1990,1998;Guczyjski 1998,2003). The stromatoporoid deposits Wll the

Fig. 9 Zbrub River escarpment section with stromatoporoid-Wlledchannels, incised in peritidal succession. a General view of the section;red arrows/dark grey arrows point to the bottom surface of the young-er channel, green arrow/light grey arrow points to the bottom surfaceof the older channel, detailed in b, which shows the bottom surface ofthe older channel with large stromatoporoids (green arrow/light greyarrow) pressed into muddy basement. c Sketch of the section; redtriangles/dark grey triangles indicate shallowing-upward peritidal

cyclothems in the intertidal part of the succession (green arrow/lightgrey arrow shows the bottom surface of the older channel). d Verticalcalcite-Wlled tubes and shrinkage pores in the peritidal part of the suc-cession. e Erosional base of channel in section adjacent to that shownin a. f Erosional contact (arrowed) of intertidal deposits and stromatop-oroid-Wlled channel. g Detail of the contact-surface from f: 1 intertidalhost rock and 2 inWlling of channel, in marginal part composed ofintraclasts. h Polished slab of inWlling of channel

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whole space of the channel and show no distinct directionalvertical variability. Particularly large stromatoporoidsoccur, however, in the bottom part of the bed, where thediameter of individual forms reaches 50–60 cm. Microfa-cially, the lithoclasts are usually represented by mudstonesand ostracod–gastropod wackestones with dominatingleperditiids. The walls of the erosional channel are usuallysmooth, but in some places, a characteristic small-scalerelief can be seen (Fig. 9g) embracing depressions a fewcentimetres long (cf. comparison of incisions and channelerosion features presented by Grélaud et al. 2006). Thedepressions are usually Wlled with small lithoclasts that alsooccur in the marginal parts of the channel inWlling (Fig. 9g,h). The lithoclasts have distinctive “bioerosional coats”(Fig. 10d, e, j) preserved as a network of microchannels andmicrospherules, sometimes forming horizons within thelithoclast. The occurrence of microchannels and spherulesresembles the fungal hyphae and associated reproductivestructures that are well known from modern shallow-marine deposits. In fossil material, such structures wereinterpreted as eVects of fungal activity and described undervarious names, e.g. Peronosporites and Palaeachlya (see:Johnson and Konishi 1959; Edwards and Perkins 1974;Golubib et al. 2005). Among the bioclasts, sporadic frag-ments of the cyanobacterian microproblematicum Hedstro-emia can also be found (Fig. 10a, b).

The matrix of the stromatoporoid inWlling of the channelis mainly sparitic or detritic, with a speciWc association ofbioclasts dominated by gastropods and leperditiids, but itsmost characteristic element consists of microproblematicalalgal-like forms, most probably representing TuxecanellaRiding and Soja and Sphaerinia Riding and Soja (Ridingand Soja 1993). Both forms have so far only been describedfrom the Alexander Terrane in Alaska. Tuxecanella frag-ments are particularly abundant and well preserved(Fig. 10c, k).

The beds, in which the channel is incised, are muchpaler, more marly and represented by three main microfa-cies. The Wrst of these (MF-1) consists of Wne-grainedcrinoidal–brachiopod–ostracod calcarenites, sometimesaccompanied by accumulations of small rhynchonellid bra-chiopods belonging to the genus Camarotoechia (presentname Microsphaeridiorhynchus) and much larger formsbelonging to the genus Delthyris. The second microfacies(MF-2) consists of Wne-grained ostracod–bioclastic wacke-stones. The ostracods are represented either by small,smooth forms or by beyrichiids (Fig. 10g) and large leper-ditiids, sometimes occurring in mass accumulations andforming umbrella structures (Fig. 10h). The third microfa-cies (MF-3) consists of mudstones with fenestral structures(dismicrites) and is generally devoid of fossils, as well assporadic laminites, with both Xat and wrinkled lamination.

The fenestral limestones contain characteristic verticallarge-scale fenestrae resembling Skolithos-type burrows,some of them Wlled by calcite and some by slightlyreworked surrounding sediments (Figs. 9d, 10l). In theillustrated example, the fenestrum is Wlled by alternatingblocky calcite and microsparite, most probably formed byrecrystallisation of peloidal sediment. The fenestrae areaccompanied by irregular shrinkage pores, with theirshapes resembling stylolites (cf. Shinn 1983, Fig. 20).

The fenestrae resemble structures described from thedeposits of the Canning Basin by Read (1975) as “subverti-cal tubular fenestrae” and later interpreted by Shinn (1983),who analyzed modern, Ordovician and Cretaceous speci-mens as worm burrows or gas escape structures (Shinn1983: Fig. 11). Modern specimens, almost identical tothose found in the Silurian of Podolia, were collected byShinn (1983) from supratidal zone deposits of a mud islandin Florida Bay. Barwis (1985) described similar structuresformed by the polychaete Diopatra from present-day inter-tidal environments of South Carolina shores.

An important feature of the interstromatoporoid facies isthe occurrence of desiccation polygons. They have not beenrecognised in the Zbrub River escarpment section, butnumerous surfaces covered by such features can beobserved in coeval and slightly younger beds exposed inthe Bridok Quarry (Skompski et al. 2006, Fig. 9). This partof the succession also yields relatively common calcitepseudomorphs after dolomite (Fig. 10f,i).

The variability of the three microfacies under discussiondisplays a cyclic pattern. In the case of the deposits, inwhich the channel is cut, the deposition of fenestral facieswas preceded by sedimentation of ostracod–gastropodlimestones and terminated together with the formation andbiodetritic inWlling of the small-scale channel structure that,during the following sedimentary cycle, has again been cutby the larger-scale channel (Fig. 9c).

Interpretation

Due to the distinct facies contrast between the stromatopor-oid beds and the beds that separate them, it is crucial to beable to interpret the processes involved in the formation ofthe channels and the conditions under which the channelsbecame inWlled. The common occurrence of “corroded”clasts not only in the inWlling of the channel but also inmost of the stromatoporoid beds higher in the successionindicates that the times of channel formation and of itsinWlling were separated by a period at least long enough toallow formation of the corrosion aureoles on the clasts. Ittherefore seems highly probable that the formation of thechannel and its subsequent inWlling involved diVerentprocesses.

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In the literature, there are numerous examples of chan-nels Wlled by carbonate deposits, but the development of thechannels is usually linked to sea-level Xuctuations (cf.papers in Burchette and Wright 1992; Rankey 2003; Gré-laud et al. 2006). The erosional form of the channels devel-ops during sea-level falls, whereas the inWlling sedimentsusually represent shallower facies or the bathymetric equiv-alents of the interchannel deposits (cf. Cloyd et al. 1990). Inthe case of the present succession, however, such an inter-pretation is diYcult to accept, mainly because of the lack ofdistinct evidence of emersion in the interchannel (interstro-matoporoid) complexes. If the channel was formed as aresult of a radical sea-level fall of at least several metres,the interchannel sequences would show signs of karstiWca-tion or even pedogenesis similar to those described fromthe Skala Podil’ska Quarry. In this situation, the processesgoverning the channel development and its later inWllinghave to be perceived as integrally connected with the depo-sitional environment of the interchannel succession.

The set of features of the interstromatoporoid sediments(the in situ sedimentation) points unequivocally to a

peritidal sedimentary environment. Microfacies MF-1 andMF-2 represent subtidal deposits analogous to the amphip-oroid or gastropod beds described from the intertidal suc-cessions of the Middle Devonian (e.g. Read 1975;Skompski and Szulczewski 1994). The MF-1 microfacieswas deposited under conditions characterised by normalsalinity, most probably at some distance from the shore.The speciWc, numerically abundant, but low-diversity bra-chiopod association, consisting mainly of rhynchonellidsand small spiriferids, is similar to the association describedfrom the Tofta Formation on Gotland, which, according toWatkins (1992), occupied back-reef areas with a normalsalinity, inferred from the occurrence of crinoids. Such anassociation was similarly interpreted by Samtleben et al.(2000). Salinity changes, accompanying the occurrence ofmore near-shore facies, caused distinct impoverishment ofthe faunal assemblages and a domination of ostracods,mainly leperditiids. The lateral relationships between thefacies described are clearly visible in the dark-grey horizonexposed in the Bridok Quarry, where sediments of normalmarine salinity (MF-1) rich in brachiopods, crinoids, trilo-bites belonging to the family Proetidae, and even fragmentsof nautiloids, pass laterally into high-salinity leperditiidlimestones (MF2). According to Predtechenskij et al.(1983), the leperditiid–polocopid ostracod assemblages aretypical of intrashelf environments, and its impoverishedversion also occurs in the dolomitic lithofacies characteris-tic of lagoonal sedimentation. In the Ludlow deposits ofGotland, ostracods with predominantly smooth-shelledforms appeared in great abundance in back-reef areas(Samtleben et al. 2000). Nonetheless, the problem of univo-cal interpretation of the ostracod assemblages seems to bequite complicated, as the environmental demands of evenparticular species belonging to the same genus may be verydiVerent: Kaljo et al. (1983) gave an example of Frostiellalebiensis, characteristic of deep-water deposits, and Frosti-ella groenwalliana, typical of the shallow-water parts ofthe basin. The upper part of the cyclothems, containing theMF-3 microfacies with fenestral structures, mudstones withSkolithos-like bioturbation, desiccation polygons and tracesof dedolomitisation, shows in turn a set of features typicalof the intertidal facies. The vertical sequence of the micro-facies indicates that the investigated interval contains atleast two shallowing-upwards successions (Fig. 9c). Thisfeature, together with the characteristics of particularmicrofacies, indicates unequivocally that the sedimentationof the interchannel sediments was modelled by tidal pro-cesses.

In this context, the simplest explanation of how the ero-sional channels developed is to treat them as tidal channels.The dimensions and shapes of the channels generally corre-spond to those of modern tidal channels (cf. e.g. Shinn1983; Tucker and Wright 1990); however, there are no

Fig. 10 Details of the section exposed in the Zbrub River escarpment.a, b Cyanobacterial microproblematicum Hedstroemia sp. c Fragmentof leperditiid shell in matrix dominated by calcareous algae Tuxecanel-la. d, e, j Intraclasts with external “coats” corroded by fungi (Pero-nosporites sp.). f, i Calcite pseudomorphs after dolomite. gWackestone with Beyrichia sp. h Umbrella structure in peloidal–ostra-code wackestone. k Ostracod shell in Tuxecanella wackestone. l Ver-tical fenestrae similar to Skolithos-type burrows, partly Wlled bycalcite, typical of intertidal facies, associated with shrinkage pores.Length of scale bars = 1 mm. Derivation of thin sections: a, b, g lowerpart of peritidal cyclothems (red triangles in Fig. 9c); c, k matrix ofstromatoporoid bed (inWlling of channel); d, e, j sample illustrated inFig. 9g; f, h, i upper part of peritidal cyclothems, compare Fig. 9c; lsample illustrated in Fig. 9d

Fig. 11 Model of sedimentary environments typical of shallow-waterparts of the Upper Silurian basin of Podolia—palaeofacies position ofsections described in the text is indicated by arrows

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clear indications that would allow determination of theiroverall geometry. A very rough estimate of the erosionalincisions seen on vertical faces indicates a northeast tosouthwest azimuth for the channel axes, which is more orless perpendicular to the general direction of a hypotheticalshore line, deduced from the general pattern of facieszones. The channels were cut in Wrmly cemented deposits,indicated by the sharp character of the boundary surfacesand by their irregular shape. The inWlling of the channels is,however, diVerent from those in modern examples. Themodern channels are usually Wlled with a basal lag contain-ing intraformational clasts, frequently building Xat-pebbleconglomerates, commonly accompanied by skeletal debris,composed mainly of gastropod and bivalve shells. Thebasal lags are covered by strongly bioturbated and poroussediments generally derived from the surrounding tidal Xat,with their fraction roughly corresponding to that of thetidal-Xat deposits. The migration of the channels results inthe formation of lateral accretion beds and in consequence,leads to shallowing upwards of the interchannel succession.

In the case of the channels Wlled by stromatoporoids, theabove conditions generally remain unfulWlled; however,some analogies do occur. The material derived from thechannel banks is represented mainly by the “corroded” lith-oclasts that are abundant in most stromatoporoid horizons.Microfacially, the clasts usually correspond to microfaciesMF-2. Borings occurring on all sides of the clasts indicatethat these clasts remained loose on the bottom of the chan-nel for quite a long time and were moved repeatedly duringtides. Tuxecanella algae, which are very common in thematrix of the stromatoporoid debrites, probably representanother element of a generally local origin, which is indi-cated by their excellent preservation and the lack ofattached older sediments. It seems probable that the tidalchannel and its margins represented a very favourable envi-ronment for the development of these forms. On the otherhand, Soja and Riding (1993) suggested that the “Girva-nella-Tuxecanella” association inhabited quiet, back-reefenvironments in protected areas of the shelf. In such a situ-ation, the Tuxecanella specimens would have been redepos-ited from inner lagoonal areas, in a similar manner to theHedstroemia specimens that occur as bioclasts torn outfrom bigger forms.

Stromatoporoids, on the other hand, seem to be an ele-ment that is totally exotic to the environment of tidal chan-nels. They are practically absent in the peritidal facies, inwhich the channel is incised, being found there onlyoccasionally. It is therefore diYcult to imagine that theirenvironmental demands were so narrow they could liveonly within the tidal channel. It is more probable that thestromatoporoids were brought from other settings. Thesame conclusion is also supported by analyses of the tapho-nomical and morphometrical features of the stromatoporoid

assemblage. Looking at the facies pattern in the Late Silu-rian sea sketched in the “Introduction”, the stromatoporoidswere most probably derived from more oVshore areas ofbarriers and shoals. In modern environments, a similar typeof transport takes place over relatively long distancesreaching several kilometres; Barwis (1978) describedonshore transport of open marine fauna over a distance of5 km. An obvious interpretational problem arising here isthe extremely high energy of Xow needed to carry (drag onthe bottom) large (but more or less round) stromatoporoids,particularly the largest forms with a diameter of severaltens of centimetres. The channel probably focused theenergy of Xow, which must have been particularly high butat the same time low enough not to destroy the originalshape of the tidal channel. This problem becomes easier toovercome if we accept that stromatoporoids were relativelylight during transport, because prior to crystallization ofsparry calcite, the internal voids of their skeletons, follow-ing the destruction of the soft tissue, were probably Wlledwith water (Stearn and Pickett 1994). In this case, gravita-tional selection during transport would also explain theabsence of other open-sea faunal elements, such as rugoseand tabulate corals and crinoids, which commonly accom-panied the stromatoporoids in their biohermal in situ occur-rences.

Similar accumulations of stromatoporoids occurring inthe form of lenticular units were described by Laporte(1967) from the Lower Devonian Manlius Formation(USA). Laporte considered that stromatoporoids were par-ticularly abundant in tidal creeks, thereby opposing theinterpretation of stromatoporoid accumulations as small,low-relief ecological reefs. He stated that massive develop-ment of stromatoporoids in tidal creeks was favoured by“continuous circulation of water bringing suspended foodand dissolved nutrients and oxygen”. These factorscould indeed have inXuenced the development of thestromatoporoids; nonetheless, in the light of the lack ofstromatoporoids in the interchannel deposits and with sharperosional boundaries between the stromatoporoid andnonstromatoporoid beds, treating stromatoporoids as alienelements that drifted into the channel seems to be a morelogical and simple interpretation. Following Laporte’sargument, the channel interpretation of stromatoporoidbeds can also be applied to many other reef-like beds with arelatively well-washed sediment matrix (e.g. thosedescribed by Klovan 1964, p. 48), as well as some thin bedsfrom the Pillara Formation (Australia) described by Read(1975) or some Silurian biostromes from Gotland (seeSandström and Kershaw 2002).

The overwash currents destroying the zone of stromato-poroid–coral barriers and shoals occurred periodically dur-ing high-energy events—storms, hurricanes or tsunamiwaves. However, more precise identiWcation of the driving

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factor is not possible in the current state of understanding ofthe sedimentary features of the whole Upper Silurian suc-cession.

Conclusions and discussion

1. High-energy events punctuated the prevailing shallow,but quiet, water sedimentation represented by theUpper Silurian succession of Podolia.

The three examples presented in this paper illustrate diVer-ent scales of such high-energy events. All of them are con-nected with peritidal settings; however, each represents aslightly diVerent sedimentary subenvironment. The Wrstexample (section in the Kubatchivka Quarry) probably rep-resents a zone of tidal Xats that developed in the vicinity ofstromatoporoid shoals located at a considerable distancefrom shore (Fig. 11). The formation and inWlling of an ero-sional channel (Zbrub River escarpment section) is in turnlinked with a near-shore tidal Xat. The Skala Podil’skaQuarry succession represents the shallowest of settings—atransitional zone between the intertidal and supratidal envi-ronments.

2. All the described events were related to onshore rede-position caused by factors with energy levels exceed-ing those of average storms, probably by hurricanes oreven tsunami waves.

The events described, particularly megascale inWllings oferosional channels, are infrequently but quite regularlyrecorded in the successions. This may indicate that theonshore redeposition responsible for their formation wascaused by factors with energy levels exceeding those ofaverage storms. Such events are recorded from Silurian suc-cessions fringing the Laurussia continent; however, theirenergy and frequency is a subject of controversies. Johnson(1989), based on a sedimentological analysis of Pentameruslayers from the Oslo region, suggested that “storms stirredthe sea bed in deeper waters on an 8–10 year cycle. In shal-lower waters storms more regularly disturbed the sea bedevery 2–3 years”. On the other hand, Brett et al. (1993),based on the lack of storm disturbances below the zoneinhabited by benthic fauna BA-3 (according to Boucot1975), presented an opinion that “Silurian storms may havebeen far weaker than those of the present, possibly owing toa diVerent climate regime”. The interpretation is furthercomplicated by the observations of the sedimentary eVectsof modern hurricanes that in tidal Xats areas—contrary tocommon expectations—are surprisingly small (Rankey et al.2004). If, therefore, fossil tidal-Xat successions reveal dis-tinct records of storm-type events, these must havedeveloped as a result of the activity of extremely strongmega storms, which presently do not occur in such intensity.

In the case of the Silurian of Podolia, a compromisesolution to this problem can be proposed: the mega stormswere obviously not so common as postulated by Johnson(1989), but at the same time, their energy was not muchhigher than in modern storms, as has been suggested byBrett et al. (1993). In the Podolian successions, the recordof storm events seems to have been governed mainly by theextent of the barriers/shoals zone, on the one hand, and thatof the inner shelf/lagoon zone, on the other, and by theposition of the section within the pattern of facies zones.During storm swells, the main momentum of the wavescaused outwashing and destruction of the area covered bystromatoporoid banks and buildups. More or less concen-trated accumulations of stromatoporoids were spread overlarger areas and adopted the sedimentary form of para-biostromes, which replaced the original autobiostromes andbioherms [compare the remarks of Yeltyschewa et al.(1971) on the relatively small participation of biohermalforms in the 300-metre-thick Silurian succession of Podo-lia]. In periods of maximal sea-level falls, peritidal zonedeposits formed in the study area (examples from theKubatchivka and Skala Podil’ska quarries), and thus onlythe most peripheral parts of the washover Xoods becamerecorded in the sedimentary succession (Fig. 11). The bedscomposed of redeposited stromatoporoids formed in subse-quent periods of slight deepening of the depositional area assea level rose. These beds, typical of the Skala Formation,undoubtedly represent the more internal (near-shore) partsof the currents transporting the material derived from barri-ers and utilising the previously (prior to the storms) formedchannels. In areas located closer to the barrier zone, thereorganisation of the original architecture induced bystorms was probably markedly stronger (Fig. 11).

3. The dynamic interpretation of a substantial part of thestromatoporoid beds implies changes to the cyclicitycurves of the Silurian succession in Podolia.

The bathymetrical interpretations of the succession contain-ing stromatoporoid beds needs revising, as it seems not beso simple as presented on earlier curves that illustrate thedepositional rhythms (compare Predtechenskij et al. 1983).One of the speciWc features of these curves is the presenceof shallowing-upwards cyclothems, with intertidal mem-bers at the top. In accordance with traditional interpreta-tions, which explain the occurrence of stromatoporoid bedsby shifting of facies zones, their appearance in cyclothemsuccession marks the deepest (or at least the most open-marine) sedimentary environment. When accepting theinterpretation presented in this paper, the occurrence ofchannels and their stromatoporoid inWllings is connectedrather with a relatively shallow water sedimentary environ-ment. It thus seems that determination of the origin ofevery single stromatoporoid bed is of crucial importance in

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bathymetrical analyses of the sedimentary rhythms. Suchdetermination is particularly complicated in the case ofborehole successions, when the architecture of the sedimen-tary beds cannot be observed.

4. The genesis of the stromatoporoid beds inXuenced theirorientation in relation to facies pattern.

In the case of structures of a biohermal or biostromal ori-gin, it can be expected that their elongation would beroughly parallel to the overall elongation of facies zones. Inthe case of bodies of a channel origin, this elongationwould instead be perpendicular to that direction. Awarenessof this fact is of crucial importance in all investigationsinvolving the interpretation of the architecture of deposi-tional beds that are recognised solely in boreholes and bygeophysical methods, as in eastern Poland, Lithuania, Lat-via and Estonia. This aspect may be of great importance tothe identiWcation and localising of sedimentary hydrocar-bon collectors.

Acknowledgements The studies were Wnanced by the Polish Minis-try of Education and Science, Grant No. 2 P04D 075 29 and partiallyby the Foundation for Polish Science (WK, Domestic Grant for YoungScientists for 2007). The microscopic photographs were taken in theScanning Electron Microscope and Microanalysis Laboratory of theFaculty of Geology, Warsaw University, on Nikon ECLIPSE E600WPOL with LUCIA software. Mr. Grzegorz Widlicki is thanked for pre-paring the thin-section plates. We thank Mikael Calner (Lund Univer-sity, Sweden) and an anonymous reviewer for their useful comments,which improved the paper considerably. We are obliged to ChristoferWood (Somerset, UK) for linguistic advice and constructive commentsthat improved the paper.

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