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Introduction to the Stratigraphy of the Karst (Outer) Dinarides

Dinarides represent a relatively long (more than 600 km) mountain chain stretching along the NE Adriatic coast, connecting the Southern Alps with Albanides and Hellenides. They were formed as a consequence of the collision between Adria and Europe, which started already during the Late Cretaceous, but reached its peak in Oligocene and Miocene with the final uplift of the mountain chain. Dinarides can be divided into two genetically different parts: the Outer (Karst or External) Dinarides along the Adriatic Sea, composed mostly of relics of the Adriatic Carbonate Platform and its basement and overlying deposits, and the Inner (or Internal) Dinarides, situated between the Outer Dinarides and the Pannonian Basin, composed of passive and active continental margin rocks including ophiolites (e.g. Pamić et al., 1998). Here we will discuss only Karst (Outer or External) Dinarides. When discussing Karst Dinarides most people consider this term as a synonym to the Mesozoic Adriatic Carbonate Platform (AdCP ; also referred to as Adriatic– Dinaric Carbonate Platform, or Dinaric Carbonate Platform), which is not completely true, although the relics of the AdCP compose the major part of the outcrops in the Karst Dinarides and submarine area along the NE Adriatic coast. However, Karst Dinarides are composed of deposits of a much wider stratigraphic range, which can be divided into 4 major sequences, deposited from the Middle Carboniferous to Eocene/Oligocene within significantly different palaeogeographic framework. During our field trip we will see and discuss some examples from all 4 major sequences. 1. Middle Carboniferous– Middle Triassic During more than 80 MY the study area represented an epeiric carbonate platform as a part of the NE margin of the Gondwanian part of Pangea, resulting in mixed siliciclastic– carbonate deposition. Older deposits (Carboniferous to Middle Permian) are mostly siliciclastic, as a result of abundant import of terrigenous material from the uplifted Hercynian massifs, resulting in an important, laterally variable stratigraphic hiatus during Early Permian and earliest Middle Permian (see schematic geological column – Fig. 1 in Velić et al., this Vol.). From Middle Permian to the end of Middle Triassic/beginning of Late Triassic shallow marine carbonate deposition prevailed (including regionally important Fusulina– Mizzia deposits in Middle– Late Permian, and Diplopora limestones in Middle Triassic), except during Early Triassic (with significant influence of terrigenous clastic deposits) and Carnian/Early Norian (which is on the major part of the Karst Dinarides represented by continental deposits). 2. Upper Triassic– Toarcian Forty-five MY of shallow-marine carbonate deposition from the Late Triassic to Toarcian took place on a very wide carbonate platform area formed on the Adria basement, partially separated from Gondwana, and therefore lacking any continental influence. Therefore, the area of the future Karst Dinarides represented only a small part of this huge depositional area (Southern Tethyan Megaplatform of Vlahović et al., 2005) characterised by very similar depositional sequences. Probably the best example is so-called Main Dolomite (Dolomia principale, Hauptdolomit), alternation of early- and late-diagenetic dolomites covering area from Italy, Austria, Slovenia, Croatia, Hungary, Bosnia and Herzegovina, Montenegro, etc. ; similar regional correlation can be made with Lower Jurassic Palaeodasycladus– Orbitopsella– Lithiotis limestones. 3. Toarcian– end Cretaceous This major sequence comprise almost 120 MY characterised by existence of a specific palaeogeographic entity – the Adriatic Carbonate Platform (AdCP). This platform became a separated entity during the Toarcian Oceanic Anoxic Event, when former megaplatform became disintegrated into several smaller “ isolated” platforms (including Apenninic, Apulian and Adriatic Carbonate Platforms) surrounded by deeper marine basins (including Adriatic Basin and Bosnian and Slovenian troughs – see review in Vlahović et al., 2005). Question marks regarding the term “ isolated” point out to the fact that these platforms were separated from the continents by deeper marine areas, but were, at least temporarily, connected to the mainland by palaeogeographic bridges, as witnessed by frequent occurrences of dinosaurs over the huge stratigraphic span – approximately 80 MY at the AdCP, from Upper Tithonian (Mezga et al., 2003) to Campanian/Maastrichtian (Debeljak et al., 2002). During almost 120 MY a several thousand metres thick pile of pure, almost completely shallow-marine carbonates was deposited on a relatively constantly subsiding basement. Some stratigraphic levels can be recognized throughout the Perimediterranean area, e.g. Clypeina– Campbelliella limestones in Upper Jurassic, Palorbitolina– Mesorbitolina– Orbitolina limestones from Upper Barremian to Middle Cenomanian, and Rudist limestones from Upper Barremian/Lower Aptian to Maastrichtian. Several important platform-wide events of drowning and emergence were recorded (see review in Vlahović et al., 2005), and progressively stronger synsedimentary tectonics caused gradual disintegration of the platform in the Late Cretaceous. Gradually larger parts of the platform became emerged, so during the Maastrichtian only small parts of what will become NE Italy/W Slovenia, and parts of southern Dalmatia were still characterised by shallow-marine deposition, but they also emerged close to the K/T boundary. Probably continuous K/T transition within the AdCP area can only be found within deeper-marine slope deposits of separated intraplatform troughs formed by synsedimentary tectonics in southern Dalmatia (Jelaska et al., 2003). 4. Palaeocene– Oligocene The final major sequence comprise approximately 40 MY of deposition within specific environments, mostly controlled by intense synsedimentary tectonics resulting in significant compression of the area and formation of asymmetrical flysch basins within the former platform. Deepest parts of newly formed basins were gradually transgressed during the Palaeocene (usually by bluehole successions from fresh-water to brackish to marine environments, in some places with coal deposits), while in most places oldest Palaeogene deposits were deposited during the Early Eocene. Continuous deformation caused gradual retreat of the carbonate factory, resulting in generally clear transgressive trend of so-called Foraminifera Limestones, from restricted Miliolida Lms., inner ramp Alveolina Lms., open marine Nummulites Lms. to deeper ramp facies of Discocyclina Lms., followed by Transitional Beds (marls with crabs and glauconite) and, finally, thinner or thicker flysch deposits. Palaeogene deposits are of very variable thickness and age, because deposition was controlled mainly by synsedimentary tectonics, resulting in laterally and vertically different successions. In some places, like northern Dalmatia or Herzegovina, flysch basin was gradually infilled by a thick regressive sequence of Promina beds (succession from deeper marine, shelf, coastal and alluvial deposits), while during the final uplift of the Dinarides in Oligocene/Miocene major fault zones were in some areas covered by coarse Jelar breccia. The youngest deposits are Oligocene/Miocene deposits found within local very restricted lacustrine basins.

Sequence stratigraphy; Middle Carboniferous-Middle Triassic; Upper Triassic-Toarcian; Toarcian-Maastrichtian; Palaeocene-Oligocene; Karst (Outer) Dinnarids

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