Identification of the Stratigraphic Position of a Bioclastic Limestone Unit in the Aalenian–Bajocian Boundary Deposits of the Laba-Malka Zone (North Caucasus)

Abstract

Relevance. The marker horizon of bioclastic limestones, located in the boundary zone of deposits of the Aalenian and Bajocian stages, has been the subject of a long-standing debate among geologists studying the region. Some authors considered it to be the uppermost layer of the Dzhigiat Formation of the Upper Aalenian, while others viewed the crinoid limestones as the basal horizon of a sequence of Bajocian clays and mudstones, corresponding to the underlying Bajocian-Bathonian deposits of the Dzhangur Formation of the Lower Bajocian. The formation patterns and composition of the lithogenic base of the marker member are very important when refining boundary deposits of any age, since the obtained results can significantly change the understanding of sedimentation in this time interval. Elevated contents of rare earth metals were found in the iron ores and phosphorites located below the crinoid limestones. If the crinoid limestone bed belongs to the upper Dzhigiat Formation (Aalenian), it can be considered a protective layer for the closed Aalenian deposits in adjacent areas. This may serve as a search criterion for identifying rare earth metal deposits and will minimize exploration costs. The aim of this study was to determine the stratigraphic affiliation of a bioclastic limestone member in the Aalenian-Bajocian boundary deposits of the Labino-Malka zone of the Northern Caucasus. Research methods included a petrographic description of the rocks, determination of their chemical composition using a TESCAN VEGA 3 SBU scanning electron microscope (SEM), and the collection and identification of marine fossils. Results. Patterns in the formation of the properties and composition of the marker lithogenic member, which in this outcrop is attributed to the upper Dzhigiat Formation of the Aalenian Stage, were established.