Equatorial Shelf of the Palaeozoic Supercontinent – Cradle of the Adriatic Carbonate Platform

Carbonate sediments have been deposited in oceans and seas from the Proterozoic up to the present day. Their palaeogeographic distribution, extent and composition changed substantially through time. Recent carbonate platforms are closely linked with reef constructions, but, during the past, non-reefal carbonate production played an important role in the periods of the depressed reef-growth (KIESSLING et al., 2003). Under the term “carbonate platform” in Croatia we usually consider the sedimentary environment in the Karst Dinarides from the Late Triassic up to the end of Cretaceous, or even to the Middle Eocene. Nevertheless, the first evidence of shallow-water carbonate platform sedimentation in the western part of equatorial Palaeotethys is related to the Late Viséan (Fig. 1A), with ooids, crinoids and small foraminifera as carbonate producers (VELIĆ et al., 2002; KIESSLING et al., 2003). Equatorial Shelf of the Palaeozoic Supercontinent – Cradle of the Adriatic Carbonate Platform


INTRODUCTION
Carbonate sediments have been deposited in oceans and seas from the Proterozoic up to the present day.Their palaeogeographic distribution, extent and composition changed substantially through time.Recent carbonate platforms are closely linked with reef constructions, but, during the past, non-reefal carbonate production played an important role in the periods of the depressed reef-growth (KIESSLING et al., 2003).
Under the term "carbonate platform" in Croatia we usually consider the sedimentary environment in the Karst Dinarides from the Late Triassic up to the end of Cretaceous, or even to the Middle Eocene.Nevertheless, the first evidence of shallow-water carbonate platform sedimentation in the western part of equatorial Palaeotethys is related to the Late Viséan (Fig. 1A), with ooids, crinoids and small foraminifera as carbonate producers (VELIĆ et al., 2002;KIESSLING et al., 2003).

Equatorial Shelf of the Palaeozoic Supercontinent -Cradle of the Adriatic Carbonate Platform
Jasenka SREMAC The Late Carboniferous-Early Permian period is defined as the lower cycle of Late-to Post-Variscan sedimentation (KRAINER, 1993;ALJINOVIĆ & TIŠLJAR, 2001).Transgressive-regressive clastic-carbonate sediments from this period reflect the eustatic sea-level fluctuations caused by the Gondwana glaciation.
During the upper cycle (Middle-Late Permian to Early Triassic) sedimentation patterns were more uniform, with similar sedimentary sequences developed in the Southern Alps, Eastern Alps and Dinarides (Fig. 3) (KRAINER, 1993).Carbonate platforms existed in the western part of equatorial and subequatorial Palaeotethys (Fig. 1B) (KIESSLING et al., 2003).Terminal Permian catastrophic event, although killing a high percentage of living taxa on land and in the oceans, did not cause a significant reduction of the carbonate platforms, but Lower Triassic platforms were formed without the contribution of reefal debris.Grains and micrite on the platform were formed by the activity of disaster biota -microbes (cyanobacteria).

CARBONIFEROUS AND PERMIAN PALAEOGEOGRAPHY AND CLIMATE
During the Late Carboniferous, and especially in the Permian, continental masses on Earth were united in the largest continent ever existing -Pangaea (Figs.1A, B, 4).This supercontinent, extending almost from pole to pole, was positioned between the giant Panthalassa Ocean to the west, and the smaller Palaeotethys Ocean to the east.At the end of the Lower Permian a new ocean -Tethys (Neotethys) was born within marginal Gondwana, south of Palaeotethys, extending from Timor to Sicily (Fig. 4) (SCOTESE & LANGFORD, 1995;STAMPFLI et al., 2001;SCOTESE, 2002).
Formation of such a large continental mass resulted in the domination of an arid continental climate and deposition of evaporites in shallow water basins.Carboniferous rain forests were replaced with xerophytic vegetation, and the accumulation of organic matter significantly decreased.The low rate of precipitation diminished the erosion of land.The increased amount of CO 2 in the atmosphere caused the "greenhouse effect" with global warming.Ice caps were melted, causing the additional evaporation of CO 2 into the atmosphere.Cold climate conditions were restricted to the northeastern part of the Siberian land, and southern parts of Australia and Antarctica (BARRON & FAWCETT, 1995).This hot climate favoured carbonate sedimentation, and enabled the formation of large carbonate platforms, not only in equatorial and subequatorial regions, but also in high subtropical latitudes (Fig. 1) (KIESSLING et al., 2003;ZIEGLER et al., 2003).Platform sedimentation took place over the wide area of the Alps and Dinarides (HERAK, 1991;KIESSLING, 2003), often associated with the formation of reefs, which can be found today in Italy, Hungary, Slovenia, Croatia, Serbia, Montenegro and Greece (FLÜGEL et al., 1984;RAMOVŠ & SREMAC, 1986;RAMOVŠ et al., 1987RAMOVŠ et al., , 1990;;PEŠIĆ et al., 1988;SREMAC, 2001).Porous reef structures at the margin of Palaeotethys and Panthalassa entrapped significant amounts of oil, thus forming world famous oil-fields (Texas, Iran, China) (KLEMME & ULMISHEK, 2001).

THE PALAEOZOIC IN CROATIA (FROM THE DEEP OCEAN TO THE CARBONATE PLATFORM)
Palaeozoic sediments in Croatia outcrop sporadically, often in tectonically limited belts (Fig. 2).Although geographically restricted, some of these localities are the treasuries of data from ancient geological history.Lower Palaeozoic, Pre-Variscan sediments of Croatia were deposited in a deep marine basin, which was part of the Rheic Ocean.They contain fossils of pelagic biota (graptolites, conodonts) and spores, preserved despite subsequent metamorphic processes (ĐURĐANOVIĆ, 1968(ĐURĐANOVIĆ, , 1973;;SREMAC & MIHAJLOVIĆ-PAVLOVIĆ, 1981;JERINIĆ et al., 1994).Opening of the Palaeotethys Ocean in the Late Silurian and Devonian positioned this sedimentary basin between these two oceans.
Lower Permian limestones of the Rattendorf group have been found in the Southern Alps, Velebit Mt., Gorski Kotar region, and in clasts of younger sediments on Medvednica Mt. (Pl. 1, Figs. 3-5) (KOCHANSKY-DEVIDÉ, 1959;SALOPEK, 1960;SOKAČ, 1973;GRIMANI et al., 1973;SOKAČ et al., 1976;SREMAC & KOCHANSKY-DEVIDÉ, 1982;ALJINOVIĆ, 1997;SREMAC & ALJINOVIĆ, 1997;KRAINER, 1993).These limestones are relatively thin in the whole area of the Alps and Dinarides, sometimes forming lenseshaped structures within the clastic sediments.At some localities on Velebit Mt. continuous sedimentation from the Upper Carboniferous occurred.In the Gorski Kotar region large blocks of Rattendorf limestones can sometimes be found isolated within shales (SALOPEK, 1960).ALJINOVIĆ (1997) explains these phenomena by intrabasinal erosion of the partly uplifted carbonate sediments and their transportation by fan deltas down toward the steep basin slopes.Numerous clasts of pelagic Gzhelian to Lower Permian limestones with deep water radiolarians (Albaillellacea), indicating water depths of over 500 m are common in the vicinity of Mrzle vodice in Gorski Kotar.These sediments were deposited at the base of slopes along the northern active margin of Palaeotethys (ALJINOVIĆ & KOZUR, 2003).In the Julian Alps, continuous long-lasting carbonate sedimentation began at this time (RAMOVŠ et al., 1990).
Carbonate sediments of the Trogkofel group have been found in the Gorski Kotar region, within the clastic Košna-conglomerates on Velebit Mt., and in clasts of calcareous breccias on Ravna gora Mt.Oil exploratory-wells in Zebanec (Hrvatsko Zagorje) and Újfalu-I (Hungary) also contain carbonate sediments of this age (SALOPEK, 1960;KOCHANSKY-DEVI-DÉ, 1973;SOKAČ et al., 1976;BERCZI-MAKK & KOCHANSKY-DEVIDÉ, 1981;SAVIĆ & DOZET, 1985).At the same time, in some neighbouring areas, clastic sedimentation was dominant throughout the whole lower cycle of late to post-Variscan sedimentation (KRAINER, 1993) (Fig. 3).During the Middle and/or Upper Permian, carbonate platform conditions were common in Velebit Mt., the Julian Alps, Southern Karavanke Mts., Carnic Alps, and, finally, the Dolomites.A transgression of Palaeotethys from the southeast towards the northwest can be traced (RAMOVŠ et al., 1990;KRAINER, 1993).Dolomite deposits predominate, with intercalations of limestones and evaporites.The longest period of continuous platform sedimentation can be traced in the Julian Alps (during the whole Permian and the lowermost Triassic).On Velebit Mt. carbonate sedimentation took place for approximately 20 million years, producing a sequence of shallow-marine carbonate sediments more than 900 m thick (KOCHANSKY-DEVIDÉ, 1965;SOKAČ et al., 1976).
Fig. 3 Global eustacy curve and schematic stratigraphic framework of Post-Variscan sediments of the Southern Alps and Dinarides.
The upper part of the Middle Permian on Velebit Mt. shows no major changes in the composition of sediments or fossil communities.Calcareous algae and foraminifera are still the dominant skeletal carbonate producers.According to the abundance of Yabeina, the Yabeina syrtalis (DOUVILLÉ) zone was proposed by KOCHANSKY-DEVIDÉ (1965) for these limestones.The brachiopod fauna shows more distinct changes than the microfossil assemblages.New brachiopod genera (Derbya, Streptorhynchus, Orthotetes), as well as some cephalopods (Temnocheilus), occur sporadically.Early diagenetic dolomites and dolomitic fossiliferous grainstones-packstones with intercalations of black mudstones and shales are the prevailing sediments (TIŠLJAR et al., 1991).
Middle/Upper Permian carbonate sediments from Velebit Mt. are not entirely compatible in age with the fossils and sedimentary features of the "Bellerophon formation" of the Southern Alps.The majority of macrofossils (e.g.brachiopods) belong to the Indo-Armenian type of fauna, while several taxa are endemic (SREMAC, 1986).Continental barriers between these two sedimentary basins were presumed by Simić (SALOPEK, 1942).FLÜGEL (1977 -p.315) even proposed the category "Velebit-formation" for these sediments (Fig. 3).A similar development of the Upper Permian, named the "Žažar-formation", can be observed in Slovenia (RAMOVŠ, 1958;BUSER et al., 1988;SKA-BERNE & OGORELEC, 2003).
In central and northern Dalmatia and the southeastern Lika regions evaporitic sediments (gypsum and anhydrite), associated with clastic and carbonate rocks and some volcanics, outcrop at several localities (Fig. 2).Palynological analysis suggest an Upper Permian age for these sediments (ŠUŠNJARA et al., 1992), and they are interpreted as regressive cycles deposited in an arid climate through the progradation of sabkha environments over shallow subtidal lagoons (TIŠLJAR, 1992).According to global palaeogeographical reconstructions (KRAINER, 1993;STAMPFLI et al., 2001;SCOTESE, 2002;IZART et al., 2003;KIESSLING et al., 2003;ZIEGLER et al., 2003) (Figs. 1 and 4) evap-orites are partly related to the coastal areas of the western inactive margin of the Palaeotethys ocean (Dolomites, Carnic Alps, Southern Karavanke Mts.), but they also occur within the shallow parts of the inner carbonate platform (e.g.Central Dinarides).

PERMO-TRIASSIC BOUNDARY IN CROATIA
In the area of the Alps and Dinarides, continuous carbonate sedimentation from the uppermost Permian to the Triassic has been observed at several localities in the Dolomites, throughout the Carnic Alps, Southern Karavanke Mts., Julian Alps, Velebit Mt. and Central Dinarides (Fig. 3).
A similar "continuity" of sedimentation at the Permo-Triassic boundary has been observed in the Karavanke Mts. and in the vicinity of Žažar and Idrija in Slovenia (see DOLENEC et al., 1999, andSKABERNE &OGORELEC, 2003 for older references).Anomalies in δ 13 C and δ 18 O represent evidence of an abrupt change in organic production and palaeotemperature (DOLENEC et al., 1999(DOLENEC et al., , 2001)), thus suggesting the possibility of a sudden catastrophe (impact of an extraterrestrial object?).
There are different opinions as to the age of the Palaeozoic sediments and the Permo-Triassic boundary in the Gorski Kotar region.Some authors (e.g.ŠĆAVNIČAR & ŠUŠNJARA, 1967;RAFFAELLI & ŠĆAVNIČAR, 1968) propose the continuity of sedimentation from the "Upper Permian" clastic sediments to the Lower Triassic dolomites.Careful reviewing of the published papers and field investigations did not confirm this continuity, because the uppermost Permian fossils have not been found in clasts (ALJINOVIĆ & SREMAC, 1997;SREMAC & ALJINOVIĆ, 1997).It is possible that this area was uplifted in the Late Permian, and again submerged during the Early Triassic transgression (Pl.5, Fig. 2).A similar situation can be observed in neighbouring parts of Slovenia (RAMOVŠ & KOCHANSKY-DEVIDÉ, 1965;RAMOVŠ, 1968).
Apart from the Karst region, "continuous"(?) sedimentation at the Permo-Triassic boundary has been observed on the Samoborska gora Mt. and in the Banovina region.The uppermost Permian in these areas is represented by clastic sediments with scattered lenses of carbonate or evaporite sediments (Pl.4, Fig. 3) (HERAK, 1956;RAMOVŠ et al., 1990).
Upper Permian sediments in Croatia show a general shallowing upward trend, in harmony with the global eustatic curve (Fig. 3), but the extreme ideas of more than 200 metres of sea-level falls could not be confirmed in this area.Nevertheless, evidence of occasional emersions, such as laminar structures, stromatolites, vadose cements and highly porous carbonate breccias ("rauchwacke"), can be found in Croatia from the Middle Permian, and, particularly in the Upper Permian sediments (SREMAC, 1991;TIŠLJAR et al., 1991;TIŠLJAR, 1992;IBRAHIMPAŠIĆ & SREMAC, 2002;ALJINOVIĆ et al., 2003).Fullerenes and shocked quartz grains, the typical impact markers, have not been found in neighbouring regions, but carbon, sulphur and strontium excursions have been confirmed at many places, including Slovenia (DOLENEC et al., 1999(DOLENEC et al., , 2001)).Therefore sediments at the P/T boundary in Croatia should be subject to isotope and microelement analysis.
Permian platform sediments in Croatia were less affected by the P/T catastrophic event, than by tectonic movements and uplift, which caused emersion in some parts of the platform.Input of terrestrial material, tectonic instability and sporadic volcanic activity were the main features during the Lower and Middle Triassic.This area was stabilized in the Late Triassic, and platform sedimentation in the Tethyan realm took place up to the Middle Eocene.

CONCLUSIONS
The first carbonate platforms in equatorial Palaeotethys existed during the Variscan cycle.In Croatia, Viséan shallow-marine fossils have been found in clasts of younger sediments.
The lower part of the Late to Post-Variscan cycle (sensu KRAINER, 1993) KRAINER, 1993).Their lateral and vertical distribution was controlled by transgression of the Palaeotethys from the southeast towards the adjacent shelves.Platform carbonate sediments and evaporites of this age crop out in the Outer Dinarides of Croatia (Velebit Mt., Croatian-Bosnian border), and were also found in clasts of younger sediments (Gorski Kotar region), and in bore-holes (Rovinj).Sporadic occurrences of Palaeozoic shallow water carbonate and evaporite sediments were also found in the Inner Dinarides (Bregana, Medvednica Mt.).The main carbonate producers were calcareous and non-calcareous algae and foraminifera, accompanied by calcisponges, corals, molluscs, bryozoans, brachiopods and echinoderms.
Palaeozoic rocks of Croatia are in most cases in tectonic contact with the surrounding deposits, and it is impossible to estimate their original horizontal and vertical distribution.We can presume that they were deposited either on one large carbonate platform with occasionally emerged areas and intraplatform basins, or, more likely, on several smaller platforms, separated by continental or basinal barriers.
During the Upper Palaeozoic and the Lower Triassic, carbonate sedimentation was interrupted by episodes of uplift, erosion and deposition of molasse sediments.Phases of regression are marked by laminar structures (stromatolites), vadose cements and highly porous carbonate breccias ("rauchwacke").
An End-Permian catastrophic event did not cause the collapse of the carbonate platforms, but it strongly affected the carbonate-producing biota.Therefore no visible discontinuity of sedimentation can be traced at the Permo-Triassic boundary, and this event could only be traced through carbon, oxygen, sulphur and strontium anomalies.Cyanobacteria, as the disaster taxa, are blooming around the extinction event.They are the first lithogenetic biota after the cataclysm, soon accompanied by simple, opportunistic, predominantly molluscan communities.
After the phase of instability marked by clastic-carbonate sedimentation and sporadic volcanic activity, platform conditions were stabilized in the Late Triassic and lasted till the Middle Eocene.
During 200 million years of existence, the Palaeotethyan-Tethyan "carbonate factory" produced several thousand metres of fossiliferous carbonate sediments, thus representing a palaeontological and sedimentological treasure for many generations of Croatian geologists and palaeontologists.
tals and rare microfossils (gymnocodiaceans, foraminifera; Pl. 4, Fig.4)(RAMOVŠ & KOCHANSKY- DEVIDÉ, 1981;IBRAHIMPAŠIĆ & SREMAC, 2002).WIGNALL et al. (2002) describe similar sediments from China and relate them to the global anoxic event at the end of the Permian.Near the Permo-Triassic boundary, intercalations of red shale became common, and Permian carbonates gradually pass into the sandy yellow-coloured dolomites of the Lower Triassic is dominantly represented by molasse sediments.Clastic-carbonate sedimentation was influenced by phases of Gondwanian glaciations.Shallow water carbonate sediments were deposited in the Late Carboniferous (Moscovian-Kasimovian) and Early Permian (Asselian-Sakmarian).The dominant carbonate producers were calcareous algae, foraminifera and crinoids.In Croatia these sediments are the most completely developed on Velebit Mt.Middle/Upper Permian to Lower Triassic carbonate sediments of the Southern Alps and Dinarides belong to the upper part of the Late to Post-Variscan cycle (sensu