Original paper

Lithology of the Upper Jurassic Lower Cretaceous (Tithonian-Lower Berriasian) Aj-Petri reef complex (southern Ukraine, the Crimea Mountains)

Krajewski, Marcin

Abstract

The following paper presents a lithological characterization of Tithonian-Lower Berriasian limestones from the southern Crimea Mountains. Both the massive and bedded limestones forming the Aj-Petri reef-complex comprise four main lithofacies groups: stromatoporoid-microbial framestone-bindstone, bioclastic wackestone-floatstone, oncoidal packstone and oolitic and intraclastic grainstone-rudstone. Due to strong cementation, the specific lithofacies groups appear as macroscopically homogenic, massive Aj-Petri limestones. The rocks were deposited in shallow, subtidal and intertidal environments related to the shallowing sea level trends. The principal reef-builders were stromatoporoids (demosponges), microbialites and microencrusters, which produced numerous stromatoporoid-microbial patch-reefs and biostromes. These rocks, along with detrital sediments form the main part of the Aj-Petri reef. A dominance of stromatoporoid bioconstructions may reflect the restricted environmental conditions, which limited or even precluded the growth of other reef-building organisms, particularly the corals. Among the most important factors facilitating the vigorous growth of stromatoporoid bioconstructions in such environments could have been their high tolerance for changing depositional conditions resulting in dense packing of skeletons. Such packing, in turn, might have facilitated the intensive growth of microencrusters in the interskeletal spaces, particularly Bacinella and Thaumatoporella, enabling rapid lithification of the initial, stromatoporoidal reef frame supporting the development of a rigid framework, resistive to intensive erosion. During periods of more stable environmental conditions, intensive microbialites growth took place which enabled the binding of specific patch-reefs and peri-reefal deposits, resulting in the formation of the massive Aj-Petri limestone monolith as it appears today.

Keywords

microbialitesmicroencrustersstromatoporoidsreef faciesupper jurassic-lower cretaceouscrimea mountains