Depositional environment of Upper Carboniferous – Lower Permian beds in the Karavanke Mountains (Southern Alps, Slovenia)

Late Paleozoic rocks were studied in detail in the Dovžanova soteska section. The Upper Carboniferous sedimentary succession, correlated with upper part of Auernig and Schulterkofel Fm. in the Carnic Alps, indicates cyclic clastic-carbonate deposition in a coastal to shallow marine ramp setting with strong influence of coarse-grained fluvial-deltaic siliciclastics from the hinterland, storm dominated regime of nearshore sediments, and offshore algal buildups. The Lower Permian sequence is developed differently from its time equivalent Grenzland Fm. and is subdivided into Dovžanova soteska Fm., Born Fm., and Rigelj beds. It is marked by the formation of a reef mound on the platform margin. Open-marine inner platform close to the marginal shoals represented the depositional environment of the mixed carbonate-siliciclastic sediments. Thus, a platform evolution from a ramp into a rimmed shelf is suggested.


Auernig beds, Upper Pseudoschwagerina
Limestone of the Rattendorf beds and Trogkofel beds were recognised, and within the latter clastic and carbonate units were distinguished (S c h e l l w i e n, 1898a, b, 1900Teller, 1903;H e r i t s c h, 1933, 1938, 1939Ramo v {, 1963Ramo v {, , 1966Ramo v {, , 1968; K o c h a n s k y - Devidé , 1965Devidé , , 1969Devidé , , 1970Devidé , , 1971; K o c h a nsky-D e v i dé & R a m o v {, 1966; B u s e r, 1974, 1980; J u r k o v { e k, 1987). Earlier works are discussed later in the text.
In this paper a part of the doctoral thesis on the biostratigraphy of Late Paleozoic beds in the Dovžanova soteska (Dovžan's gorge), NE of the town of Trži~, is summarized. In the Dovžanova soteska, the northsouth trending valley of the Trži{ka Bistrica river cuts deep into the southern slopes of the Karavanke Mountains and exposes the most complete section of marine fossilrich Late Carboniferous and Permian beds in Slovenia (Fig. 1). The main focus of the paper is on the facies characteristics of the succession, biostratigraphic correlation, and the interpretation of the depositional environment. The interpretation of the Late Paleozoic succession refers also to similar, but better exposed deposits in Carnic Alps, which were studied in detail in the last decades (Venturini, 1990;Schönlaub, 1992;K rainer, 1992;Samankassou, 1997;F orke et al., 1998, 2006Forke, 2002).
In the Dovžanova soteska, the Carboniferous/Permian boundary is not exposed due to a tectonic contact. Limestones above the contact were erroneously correlated with the younger Trogkofel Limestone in Carnic Alps for decades. Recently , 1974, 1980). They probably represent a time-equivalent to predominantly clastic and fossil-barren beds of the Grenzland Formation (Forke, 2002). Therefore, these fusulinoidean forms have an important role in filling the gaps in the knowledge of the phylogenetic evolution and stratigraphic range of the present genera. The uppermost part of the section below the Tarvis breccia is poorly exposed. Fusulinoideans from this interval, informally named Rigelj beds (N o v a k, 2007), indicate early Sakmarian age due to the presence of Dutkevitchia cf. splendida, Sphaeroschwagerina cf. asiatica, Quasifusulina tenuissima, and Pseudochusenella sp. (Fig. 2). A similar faunal assemblage is present in the uppermost Grenzland Formation of the Carnic Alps (N o v a k & F o r k e, 2005).

Lithology and facies interpretation
The sedimentary succession in both, the upper part of the Auernig and the Schulterkofel Formation shows a clear cyclic siliciclastic-carbonate depositional pattern, known as Auernig cyclothems in the Carnic Alps (Austria/Italy) (K a h l e r, F. , 1955; 1962; Butter s a c k & B o e c k e l m a n n, 1984; B oeckelm a n n, 1985; M a s s a r i & Ve n t u r i n i, 1990; K r a i n e r, 1992; Samankassou, 1997Samankassou, , 2003. Because of tectonic deformations and thick cover of weathering residue outcrops are isolated and complete sections are rarely exposed. That makes it impossible to trace cyclothems or even reference horizons over longer distances. However, repeated occu-rences of every facies or facies association composing the idealised model of Auernig cyclothem, drawn by Krainer (1992) andK rainer &Davydov (1998) (Fig. 3), can be recognised in the stratigraphic succession.
The base of the typical cyclothem is represented by conglomerates above the diastem. Based on only a few sedimentary structures that can be observed, a fluvial to coarse-grained deltaic or coastal depositional setting can be proposed for these conglomerate sequences. The overlying trough cross-bedded coarse-grained sandstones of the foreshore and upper shoreface settings mark the beginning of the transgressive systems tract (TST). With further sea-level rise the deposition of finer-grained sandstones follows. Hummocky cross-stratification (HCS) is the result of wave or the combination of wave and current oscillation during storms between the fair-weather and storm wave base on the lower shoreface (Tucker, 2001;F lügel, 2004). Upwards gradually bioturbated siltstones with scarce marine fauna prevail, interbedded with HCS storm sandstone beds. However, besides the structures, characteristic of tempestites (sharp basal erosion contact with groove casts, HCS, brachiopod shell lag at the base of event beds, and dwelling burrows of the Skolithos-type ichnofacies in the upper part of event beds (Frey, 1990;Pemberton & MacEach ern, 1997)), also many typical turbiditic features (Bouma-like sequences, vortex and load structures) and normal water current structures (parallel-laminated sandstones and lense-shapped concentrations of sandstones within bioturbated siltstones) can be observed. The multitude of the described features most probably reflect various depositional mechanisms, complex nature of storm-generated currents and amalgamation of storm beds (Reading, 1996). The peak of the TST is represented by intensely bioturbated siltstones with highly diverse elements of a Cruziana ichnofacies suggesting stable conditions in an offshore setting.
The following carbonate complexes mark the maximum relative sea-level. They are represented by algal mounds in which basal, core, flanking and capping beds can be distinguished. Basal beds are usually very rich in fusulinoideans, smaller foraminifera, ostracodes, gastropods, brachiopods, and bryozoans. Almost unbroken thalli of Anthracoporella spectabilis and Archaeolithophyllum missouriense in growth position build the delicate framework of the massive micritic mound core. In flanking beds, they are accompanied by fragments of phylloid algae Epimastopora and Eugonophyllum, and binded with Tubiphytes or small sessile foraminifera. Capping beds are composed predominantly of crinoid debris.
Optimal conditions for growth of calcareous algae is the upper part of the photic zone, i.e. few tenth of metres. Since their delicate thalli were unable to resist agitated water, they point to restricted environments with moderate current action, most probably just below the storm wave base zone (Toom e y et al., 1977). The bedded wacke-to packstones underlying and overlying massive algal mound core were deposited in a shallower zone of higher energy. Predomination of algal biostromes, composed of fragmented algal thalli, over bioherms indicates that most of the buildups grew within the wave action zone. Accumulation of calcareous algae generally started during transgression, while the micritic core facies marks the sea-level highstand. Upwards, in the highstand systems tract (HST), the mirror image of clastic sequences complete the idealised cyclothem (Krainer & Davydov, 1998) (Fig. 3).
The described facies associations and microfacies characteristics suggest that the deposition of Auernig and Schulterkofel Formations took place on a platform of the mixed carbonate-siliciclastic ramp type at the margin of a shallow intracratonic basin (Read, 1985). Similar conclusions were reached by Massari & Venturini (1990) in the Carnic Alps. They pointed out, that the only plausible explanation for substantial shifts of facies belts in relatively short time periods is a flat topography of gently steeping ramp, where even the slightest change of sea-level was causing considerable shifts of the coastal line. However, worldwide recorded Late Paleozoic cyclic deposits exhibit rapid facies changes that reflect both high frequency and high amplitudes of sealevel fluctuations due to glacio-eustatic control associated with Gondwanan glaciation (Soreg h a n & G i l e s, 1999; J o a c h i m s k i et al., 2006). An up to 200 m thick unit of thick-bedded coarse-grained and poorly-sorted quartz conglomerates most probably represents a lateral fan-deltaic depositional environment within the upper part of the Schulterkofel Formation. A rapid sea-level rise following the deposition of conglomerates resulted in the transgressive lag that caps a flooding surface. It is characterised by high content of bioclastic and quartz pebble components from the eroded surface (R e a d i n g, 1996).
The Dovžanova soteska Formation also shows clear transgressive-regressive trends. Black wavy to nodular-bedded limestones with marlstone intercalations begin the TST. The following black bioturbated fossil-rich siltstones and claystones pass gradually into the Dovžanova soteska Limestone through increasing carbonate/clay ratio. Since the massive limestone body is grossly built of postmortally segmented skeletal fragments of crinoids, bryozoans, green calcareous algae and brachiopods in a micritic matrix, bounded only with encrusting Tubiphytes, algae, bryozoans and small sessile foraminifers, while the true reef-building metazoans play only a subordinate role, we can refer to it as a reef (or skeletal) mound (F lüg e l, 2004). The bioclastic packstone to microbreccia composed of fragmented allochthonous reef mound derived debris in the upper part of Dovžanova soteska Limestone, corresponds to SMF 5 (sensu Wi l s o n, 1975; Flüge l, 1982) of the reef-flank facies. It was deposited in the forereef facies belt and suggests a substantial topographic relief and the rigidity of the reef mound body. Further evidence of this are neptunian dikes and the brecciated horizon in the uppermost part of the complex. Since there is no evidence of regional tectonism at that time, this can not be regarded as the triggering factor of dike formation. Taking into consideration the inherited instability of poorly cemented reef mounds, we can explain fissure opening as a result of high local depositional relief that leads to passive gravitational movements and fracturing (R e a d i n g, 1996; F lüg e l, 2004; S t a n t o n & P r a y, 2004). However, seismic events and the loss of hydrostatic support during short-termed relative sealevel falls can not be excluded. Most dikes exhibit multiple phases of fissure opening and filling with marine sediment.
The following horizon, composed of deeper-water calcareous siltstones, marlstones and thin-bedded marly limestones speak for the short-term drowning of the reef complex prior to the deposition of red bedded crinoidal limestones with a rich and diverse shallow-water biotic association. Red stained silty crusts capping almost every limestone bed represent omission surfaces of the hardground type. They were formed during periods in which lowering of the effective wave base reached the sea-floor and there constant water agitation resulted in submarine cementation of calcareous ground and an impregnation with Fe and Mn oxides (Brett, 1998). Similar lithologies have been found in the upper slope sediments of a completely preserved carbonate platform to basin configuration in the Cantabrian Mts. (Bahamonde et al., 2004). The uppermost part of the Dovžanova soteska Formation is marked by a reestablishment of reef growth with strong marine cementation, suggesting a steep slope inclination.
The described development of the Dovžanova soteska Formation with drowning event, restored reefal sedimentation and intermediate tongue of upper-slope facies fits the description of a back-stepping reef with the landwards shift of carbonate production during the episode of relative sea-level rise (Reading, 1996).
Basal quartz conglomerates of the Born Formation cut into brecciated uppermost beds of red limestone with erosional unconformity. A clear erosional surface and features like calcareous pisoids and infillings of vadose silt in the topmost limestones of the Dovžanova soteska Formation suggest that the reef sedimentation was terminated as a result of subaerial exposure. During the following transgression, sedimentary depocentre migrated towards the open-marine inner platform. The alternation of black bedded bioclastic grain-to packstones, biocalcarenites, oolites, sandy limestones and quartz sandstones with shallow-water benthos in the lower part of the Born Formation indicates deposition in an open lagoonal setting repeatedly affected by the sedimentary influx from platform-margin oolitic and sand shoals. Some of the mixed carbonate-siliciclastic rocks (e.g. paraconglomerates) have characters of the debris flow deposits (Nov ak, 2007). One of the rocky pyramids is built of massive light grey micritic limestone with the rugose coral Carinthiaphyllum kahleri (H o l z e r & R a m o v {, 1979) forming an isolated patch-reef.
In the upper part of Born Formation often folded beds of dark limestones with clay admixture, concetrated as irregular interbeds prevail. They contain numerous thalli of phylloid algae, many genera of smaller foraminifera, fusulinoideans, and in some places large planispiral euomphalid gastropods. The original depositional structures and textures are modified by the intense burrowing, differential early diagenetic cementation, and the differential solubility of clay-rich and carbonate-rich sediments during late diagenetic processes connected with pressure-solution. Evenlybedded sediments were transformed into wavy or nodular limestones (M c I l re a t h & Jame s, 1984).
The lower retrogradational succession of the Rigelj beds indicates gradual shift of the facies belts from high energy coast through open-marine lagoon towards the shallowmarine, and shelf edge. In the transitional coastal belt, conglomerates, sandstones and oolitic limestones were deposited. Sedimentation of black bedded algal limestones with clayshale intercalations took place in the inner-shelf environment. There, in the restricted marine shoals limestones with low diversity algal association were deposited, while in the open lagoon with normal water circulation near to platform edge, sedimentation of limestones with high diversity algal association took place (F lüg e l, 1977). Reef limestones and limestone breccias mark shelf edge setting. Development of the upper part of Rigelj beds suggests a shift of facies belts back into the open-marine lagoon, where black limestones with high-diversity biota and Osagia-type oncoids were formed (Flüg e l, 1977). Substantial content of fine-grained, well-rounded quartz pebbles in several limestone beds indicates periodical terrigenous influx from a distant hinterland. Regressive trend continues with the deposition of sandstones and calcitic siltstones in high-energy shoreface setting.
With the deposition of the Tarvis breccia, a new tectono-sedimentary cycle started in Southern Alps in the Middle Permian. It has been interpreted as the deposits of alluvial fans and/or delta fans with periodic lacustrine pans and sabkhas (R o t a r, 1999).

Conclusion
Based on facies relationships in the section of Upper Paleozoic rocks in the Dovžanova soteska, a change in platform relief can be suggested. A gently steeping ramp morphology without both, the marginal barrier and the shelf break in the basinward direction evolved into a rimmed shelf with steeper slope as a result of lateral and vertical accretion in response to numerous relative sea-level changes. During periods of sea-level stillstands or slow rises the reef mound on the platform margin rapidly prograded, while as a response to periods of rapid sea-level rises the initial drowning and back-stepping events caused vertical accretion and steeper slope angle (Reading, 1996). Similar platform evolution had been suggested in many sedimentary basins in different geologic periods. However, the closest parallel to the platform evolution in the Dovžanova soteska can be found in the evolution of the Permian Capitan Reef in the Delaware Basin in West Texas (Babcock, 1977;Read, 1985;Tinker, 1998;Pomar, 2001;Stanton & Pray, 2004 Zaporedje zgornjekarbonskih plasti v Dovžanovi soteski zaznamuje cikli~na sili-ciklasti~no-karbonatna sedimentacija na položni platformi s konfiguracijo obalne klan~ine (rampe). Debelej{i apnen~evi kompleksi predstavljajo razli~ne oblike algnih kop, ki so nastajale v foti~ni coni pod bazo u~inkovanja nevihtnih valov. V nekoliko globljih delih je bil odložen bioturbiran meljevec, v plitvej{em priobrežnem in obrežnem pasu pa pe{~enjaki in konglomerati fluvialno-deltnega okolja (sl. 3).