Carbonate Platform Megafacies of the Jurassic and Cretaceous Deposits of the Karst Dinarides

Abstract

Platform carbonate deposits of the Karst Dinarides area have a stratigraphic range from the Middle Triassic (or even Carboniferous in some places) to the Middle Eocene, forming a belt nearly 700 km long and, (after reduction by younger tectonics) 80–210 km wide. Besides their significant thickness (4500 to 8000 m) they are characterised by frequent lateral and vertical alternations of different facies, mostly associated with shallow marine environments. Environments ranging from peritidal through low-energy shallow subtidal–lagoons, restricted inner platform shallows, high-energy tidal bars, beach and shoreface to reefal–perireefal predominate, but there are also carbonate slope deposits and those representing temporarily drowned platform facies and intraplatform troughs.The Jurassic to Cretaceous part of this carbonate succession has been subdivided into 19 megafacies units (9 for the Jurassic and 10 for the Cretaceous), the majority of which represent an inner part ofthe ancient Adriatic Carbonate Platform. Marginal parts of the platform are mostly buried, either by the recent Adriatic Sea along theSW margin, or younger deposits along the NE margin; at some localities such Jurassic and Cretaceous deposits are represented by debrite sand/or carbonate turbidites. An additional short review of the overlying Uppermost Cretaceous and Palaeogene deposits (4 megafacies units) enabled a better insight into the post-platform evolution.The very complex vertical and lateral alternation of different megafacies units, including emerged areas which were observed throughout the studied sequence in different parts of the Karst Dinarides, indicate the significant palaeogeographic dynamics of the region. This variability resulted from interaction of the global eustatic signal and local factors, including extensive organic production on the carbonate platform and synsedimentary tectonics controlled by the specific palaeogeographic position of the platform during its geological history.

Assessing Milankovitch forcing in disconformity‐prone cyclic shallow‐water carbonates, Upper Jurassic (Kimmeridgian), Adriatic Platform, Croatia

“…Tectonics may have played a role in influencing the timing of the sequence boundaries in the Kimmeridgian succession. Tectonic influences are evidenced by the rapid thickness variation of the Upper Jurassic units along strike (Tišljar et al ., 2002; Bucković et al ., 2004) related to differential movement on basement fault blocks (cf. Bosellini et al ., 1981).…”

“…These barriers prevent wave and tidal‐current sweeping of the platform, so that the near‐shore muds in protected areas can continue to build to intertidal–supratidal levels (Kaczmarek & Hasiuk, 2008; Harris et al ., 2015). The BG–BK section location on the leeward side of the pelagic re‐entrant into the Adriatic Platform suggests that there might have been some sort of island system or energy barrier on the windward (eastern) reef‐fringed margin of the platform (Tišljar et al ., 2002). The small Milankovitch sea‐level oscillations during greenhouse times allowed carbonate tidal flats to keep pace with regression of the shoreline across the gently sloping platform surface.…”

“…The Middle and Upper Jurassic of the region contains four supersequences JSS1–JSS4 (Fig. 3; Tišljar et al ., 2002; Čadjenović et al ., 2008). The studied section makes up the highstand of JSS3, whose transgressive tract consists of poorly cyclic Oxfordian subtidal carbonates and highstand is the Kimmeridgian study interval.…”

“…The dominantly Mesozoic Adriatic Platform (Figs 1 and 2) was a large intra‐oceanic carbonate platform 5 to 7 km thick, over 600 km long, with a present width of ca 200 km in the external Dinarides fold‐thrust belt (Tišljar et al ., 2002; Vlahović et al ., 2005; Husinec & Jelaska, 2006). The platform was bordered on the west and east by steep margins (Dragičević & Velić, 2002; Grandić et al ., 2010) and the south‐eastern platform margin has a long‐lived, pelagic re‐entrant (Budva Basin; Vlahović et al ., 2005; Črne & Goričan, 2008; Bega, 2015).…”

“…The thickness of the Upper Jurassic units varies considerably along strike on the platform, due to differential movement on basement blocks (Tišljar et al ., 2002; cf. Bosellini et al ., 1981).…”