UPPER JURASSIC LAGOON-EVAPORITE FORMATION IN THE WESTERN UKRAINE

Abstract

In this paper, the Jurassic lagoon-evaporite formations (Rava-Ruska suite), distributed in the territory of the Ukrainian Precarpathians, are characterized at the macroand microscopic levels. Drilling materials from more than 30 areas are analyzed, reinterpreted and summarized. Along with our research results, all stratigraphic, lithological-petrographic, biostratigraphic data obtained by other specialists is taken into account. Throughout the history of studying lagoon-evaporite deposits, researchers have attributed them to different levels of Middle and Upper Jurassic, to the Nyzhniv or Rava-Ruska suites. V.G. Dulub (1964) determined the age of these formations as Early Kimmeridgian based on foraminifera. Later, in stratigraphic schemes, carbonate deposits with evaporites on periphery were identified as the Rava-Russka suite of Kimmeridgian, which replaces the Opary Reef to the east. Thus, its composition combined the reef and lagoon-evaporite facies of the Kimmeridgian. Our micropaleontological, microfacies, lithological-sedimentological and paleoecological studies of these deposits have shown that the Jurassic lagoon-evaporite sediments belong to the Lower Kimmeridgian. They replace the back-reef facies to the east. We are considering the Rava-Ruska suite as lagoon-evaporite formation in the composition of two subsuits. The variegated terrigenous rocks dominate in the lower subsuite, grey carbonate rocks in the upper one. A detailed description of lithological, paleontological and microfacies composition of the Rava-Ruska suite is given, the conditions of its sedimentation are determined, and the typical section is characterized. The distribution of the Rava-Ruska suite is outlined. It extends submeridianally in a strip up to 40 km wide. Its thickness reaches 300 m and decreases to the east. In the west, it lies on Oxfordian sediments, in the east – on Paleozoic rocks, partly on the Middle Jurassic Sokal suite. It is overlapped by Nyzhniv suite, or by Cretaceous formations with erosion. A lithofacies profile, map, and microphotographs of some typical microfacies are presented.