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The Early Triassic epeiric ramp depositional sequence in the Dinarides

The Early Triassic depositional sequence at many localities investigated along the Dinarides (Slovenia, Croatia and Bosnia and Herzegovina) represent deposition in an overall shallow, flat, extensive environment with a developement of only slightly deeper depositional conditions during the Spathian. Sedimentary features reflect a deposition of the epeiric ramp. The Early Triassic sedimentary sequence has been studied at several sections (Studorski Preval and Lukač in Slovenia) but the interpretation of depositional environment is based mostly on the section recently investigated at locality Plavno (Croatia). The Plavno section represent the almost complete Early Triassic ca. 1.000 m thick depositional sequence and can be correlated to the well known carbonate ramp successions in Italy (Southern Alps) and Hungary (Bükk Mts). It has a characteristic threefold division: i) carbonates representing the oldest Early Triassic strata (40 m thick pale yellow macrocrystalline subhedral dolomites with conodonts Isarcicela Isarcica and small foraminifers Earlandia and Cornuspira) ; ii) dominantly red clastics (shale, siltstones and sandstones) with intercalation of ooid/oncoid or bioclastic grainstones (uppermost Griesbachian, Dienerian and Smithian) and iii) dominantly grey carbonaceous lime mudstones, marls and calcisiltites with ammonoids representing Spathian-Anisian strata. The beginning of Griesbachian is represented by carbonates deposited after the initial transgression, drowning of the extensive Permian evaporitic complexes and establishing of overall shallow marine conditions. A deposition. in the broad shallow sea lasted up to the end of Spathian. In the uppermost Griesbachian, Dienerian and Smithian there was a strong input of sliliciclastics and the deposition was influenced periodically by waves, storm waves and currents. There is no barrier shoals separating the subtidal mid-ramp from the back-barrier lagoon and tidal flat regions of the inner ramp (as it is interpreted for Early Triassic in Southern Alps e.g. Brandner et al., 2012 Haas et al., 2007). In the Spathian slightly deeper conditions were established but even in this deeper environment storms play a significant role assuming deposition above storm weather wave base. Therefore the nature and distribution of facies represent deposition on an extensive area above storm weather wave base that is best described as epeiric ramp model (sensu Lukasik et al., (2000). An epeiric ramp is defined here as having a very low bathymetric slope (negligible in its inner regions), no grainy shoreface facies, water depths of tens of meters, a width of many hundreds of kilometers and depositional processes dominated by storms. References: Brandner, R., Horacek, M., Keim, L., 2013. Permian-Triassic-Boundary and Lower Triassic in the Dolomites, Southern Alps (Italy). Field trip guide 29th IAS meeting of Sedimentology Schladming/Austria. Journal of Alpine Geology 54, 379-404 Haas, J., Demény, A., Hips, K., Zajzon, N., Weiszborg, T., Sudar, M., Pálfy, J., 2007. Biotic and environmental changes in the Permian-Triassic boundary interval recorded on a western Tethyan ramp in the Bükk Mountains, Hungary. Global and Planetary Change 55, 136-154 Lukasik, J.J., James, N.P., McGowran, B., Bone, Y., 2000. An epeiric ramp: low-energy, cool-water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia. Sedimentology 47 (4), 851-881

Early Triassic; Dinarides; epeiric ramp; C isotope curve

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