Characteristics of a Triassic regional unconformity between the second and third shallow-marine depositional megasequences of the Karst Dinarides (Croatia)

Abstract

Two depositional megasequences of the Karst Dinarides that record two different, emersion-separated, depositional periods are presented; the older lasted from the upper part of the Middle Permian to the Middle Triassic, and the younger one from the Late Triassic to the Late Cretaceous. Both megasequences are typified by shallow-water platform deposits; the earlier megasequence formed under epeiric carbonate/clastic platform conditions and the later one under isolated carbonate platform conditions. Significant geodynamic movements within the southern Tethys realm led to Middle Triassic tectonic uplift, i.e. to the emersion of the huge platform area where a regional unconformity between two platform megasequences was formed. Four types of terrestrial phase horizons are identified, each representing different subaerial conditions that existed during the long-lasting emersion. These are: a significant disconformity and related stratigraphic gap between the Upper Scythian and Lower Norian; a tuffaceous horizon between the Upper Anisian and Lower Norian; an Upper Ladinian-Upper Norian brecciated horizon; and an Upper Ladinian-Lower Norian claystone/bauxitic horizon. Due to the widespread diachronous transgression during the Norian, shallow-water platform sedimentation was restored over the entire emergent area, initiating a long-lasting isolated carbonate platform regime.