Jurassic system stratigraphy at the Kamianka village. Part 2. The Cherkas’ka formation

Abstract

Introduction. The paper is devoted to the study of the Cherkaska formation of the Jurassic system of the North-Western outskirts of the Donbas and is a continuation of a series of publications highlighting the stratigraphy of Jurassic in key areas near the town of Izyum and the village of Kamyanka. Material and methods. The scientific search was based on the analysis of literature, results of previous drilling and mapping works, as well as own field, laboratory and analytical studies. Chemical and X-ray structural analyzes of clays, mineralogical analysis of sands were carried out. Micropaleontological analysis and generalized data on the distribution of fossil remains were carried out. The palynological analysis of these deposits was also carried out, including, for the first time, according to the orthostratigraphic group – dinocysts. Results and discussion. The stratotype of the Cherkaska Formation was not specified. These were probably outcrops of ferruginous sandstones in the area of the village of Cherkaske, Slavic district, Donetsk region. Unfortunately, the deposits of this formation are exposed very fragmentarily, which makes it impossible to indicate a single typical section. As a number of such sections, it is possible to indicate Topalska balka - a clay stratum, and Pidluzhnyi farm - the upper stratum, stratigraphically important Vitcheliian layers were noted only in the village of Sukha Kamianka. Lithology. In this section of the paper, we present the detailed lithological composition of the deposits of the Cherkaska Formation from the three studied outcrops. Paleontological remains. Based on the findings of macrofauna in the sandstones, the dated levels of Witchellia rossica and Stephanoceras humphriesianum can be distinguished, which correspond to the two upper zones of the Lower Bajocian of the International Stratigraphic Scale. Upon the appearance of dinocysts of Pareodinia sp. and a small percentage of Clasopollis pollen (remains of gymnosperms) for the first time dated clays of the Cherkaska sub-formation. Stratigraphy. Within the studied territory, the Cherkaska formation is clearly divided into two parts: the lower one, composed of clays, and the upper one, composed of sandstones, which makes it possible to divide it into sub-formation. Witchellia limestone may be present if the upper sub-formation is eroded. Paleogeography. At the beginning of the Cherkaskyi period, the territory was a flat lowland covered by an extremely shallow sea basin. In the Late Cherkaskyi period, the supply of detrital material increases significantly, but the depth of the basin also increases, the salinity becomes normal, and a normal marine fauna appears. In some places, on the slopes of paleo-elevations, accumulation does not occur, which leads to the formation of deposits of a facies similar to Ammonitico rosso - limestones from Witchellia. Conclusions. The Cherkaska formation represents an important stage in the geological history of the development of northwestern Donbas. The formation marks the beginning of the Middle-Late Jurassic transgression, the beginning of the formation of normal marine sediments, and the activation of differentiated tectonic movements of local structures.