Facies of the Upper Jurassic–Lower Cretaceous sediments in the basement of the Carpathian Foredeep (western Ukraine)

The Upper Jurassic–Lower Cretaceous carbonate sediments developed in a narrow, Ukrainian part of the basement of the Carpathian Foredeep show high facies diversity. Based upon thin section studies, the authors identified eleven principal microfacies varieties. Three main stages of development of carbonate platform were distinguished: (i) Oxfordian–Early Kimmeridgian, (ii) Kimmeridgian–Tithonian, and (iii) Berriasian–Valanginian. The Oxfordian sediments are rather thin and represent both the outer and inner, distally steepened ramp facies. In the Late Kimmeridgian and, mainly, in the Tithonian, the intensive growth of rimmed platform took place with distinct zones of peritidal, margin barrier and platform slope, bearing calciturbidite facies. Development of the rimmed platform was controlled by synsedimentary tectonic movements along faults rejuvenated southwest of the Holy Cross Mts. Fault Zone. In the Berriasian–Valanginian, the dominant process was sedimentation onto not-rimmed platform controlled by small sea-level changes. Both the facies development and literature data indicate that the Late Jurassic sedimentation in the Ukrainian part of the Carpathian Foredeep basement shows considerable differences in comparison to that of the Polish part. In the studied successions, large Oxfordian microbial-siliceous sponge reef complexes, known from the Polish part of the Carpathian Foredeep basement and other areas in Europe, were rare. In the study area carbonate buildups were encountered mainly in the intervals representing the Upper Kimmeridgian–Tithonian where small, microbial-sponge and microbial-coral biostromes or patch-reefs were formed. Their growth was presumably restricted to a narrow zone of the upper slope, close to ooidal-bioclastic margin platform facies. In the Polish part of the Carpathian Foredeep basement, the Late Jurassic sedimentation took place on a vast, homoclinal ramp while in the Ukrainian part it proceeded on a narrow, distally steepened ramp and rimmed platform with distinct marginal platform barrier. Similar platform facies distribution in both regions appeared mainly in the Early Cretaceous, although with some stratigraphical differences. The facies distribution of the Upper Jurassic sediments was closely controlled by the block structure of the basement and by orientation of the main, transcontinental Holy Cross Mts. Fault Zone, which supports the opinion on its activity in the Mesozoic era. The Ukrainian part of the Carpathian Foredeep basement located over the Palaeozoic Kokhanivka Block, between the Krakovets and Holy Cross fault zones, includes predominantly the slope, marginal and inner platform facies. Facies observed over the Palaeozoic Rava Ruska Block (south-western part of the East-European Platform), between the Holy Cross and Rava Ruska fault zones, represents mainly the inner platform and the peri-shore deposits.