Late cretaceous–early Paleogene sandy-to-gravelly debris flows and their sediments in the Silesian Basin of the Alpine Tethys (Western Outer Carpathians, Istebna Formation)

Abstract

The study focuses on Upper Cretaceous-Paleocene deposits from the Beskid Śląski mountain range in southern Poland constituting the Istebna Beds. The Istebna Beds, also referred to as the Istebna Formation, are part of the Silesian tectonic unit, which forms the Outer Carpathian fold-and-thrust belt (part of the Alpine-Carpathian system). The results of qualitative and quantitative lithological-sedimentological studies were the basis for the interpretation of lithofacies types, sedimentary processes and palaeoenvironment as well as for the reconstruction of the architecture of the depositional system. The analysis conducted on the basis of field description of the de posits shows the prevalence (nearly 70%) of siliciclastic strata representing a sand stone-conglomer ate association (S-C), which is the main subject of this work. The S-C lithofacies: sand stones, gravelly sand stones, sandy conglomerates and conglomerates constitute the deposits formed mostly by mass gravity-flows such as sandy-to-gravelly debris flows. The distribution of the coarse-clastic material indicates a sediment supply from southerly directions and implies the presence of an active source area in the rear part of the Silesian Basin. A succession of the sand stone-to-conglomerate deposits with the secondary participation of other lithofacies, with a thickness of approximately two thousand metres, indicatestem porary increased diastrophic activity in the Silesian Ridge (source area) and the intense denudation of this area. The uplift of the alimentation area and its destruction coinciding with enforced relative regression and the uncovering of the proximal depositional zone of the basin led to resedimentation of the older intrabasinal material and repeated mass deposition to gether with delivery of extraclasts of pre-existing rocks and minerals. The lithofacies development of the sandstone-to-conglomerate debrites and the related sedimentary palaeotransport directions suggest an accumulation domain in the form of alinear apron depositional system developed in a deep-water setting. Experimental modelling of subaqueous sandy flows has contributed to a better understanding of the complex genesis of deep-water sediment gravity flows developing indepositional systems rich in sand material.