Subglacial conglomerate as a sensitive recorder of the ice-sheet base conditions – An example from eastern Poland

This study reports a unique example of an exceptionally compact and hard calcified conglomerate identified in eastern Poland for which we indicate a subglacial formation conditions. To infer its origin, we employed macro- and micro- studies (fieldwork, thin-section analysis), and determined the isotopic composition of the carbonate cements. The conglomerate, ranging in thickness from a few cm up to 70 cm, occurs between the MIS6 age till and underlying glaciofluvial deposits. Its formation is attributed to the advance of an ice sheet over glaciotectonically deformed and permafrost-affected glaciofluvial deposits within a glaciomarginal fan. These frozen deposits formed mega-scale obstacles (folds and rafts) that impeded ice movement, leading to increased subglacial water pressure and the development of a thin water film at the ice-sheet base. The studied conglomerate represents a valuable archive for reconstructing subglacial conditions under active ice sheet, as evidenced by the following features: (1) calcite precipitation on the down-glacial (lee) sides of the ground obstacles; (2) precipitation in phreatic conditions as evidence by continuous cements around mineral grains and the lacking grain-to-grain contact; (3) bipartite structure comprising sparitic and overlaying micritic cement layers; sparite formed under open-drainage subglacial conditions, whereas micritic thin layer developed during the final stage of water film freezing under closed-system; (4) carbon and oxygen isotope compositions confirming the sequential precipitation of sparite followed by micrite. Calcified conglomerates formed during the advance of the ice sheet front onto the substrate covered by permafrost, have not been described in the literature so far.