SURFACE TEXTURES OF SAND GRAINS FROM QUARTZ ARENITE STRATOTYPE SECTIONS OF SABLINKA (CAMBRIAN, SERIES 3; SAINT PETERSBURG VICINITY, NORTHWEST RUSSIA): THE KEY TO UNDERSTANDING OF CONTINENTAL PROCESSES AT THE GUZHANGIAN-FURONGIAN BOUNDARY

摘要

Poorly cemented quartz arenites of Sablinka Formation form a thin sand cover which was formed during the Cambrian time at the shallow fringe of epicontinental Moscow Paleobasin. Surface textures of sand and silt grains from stratotype sections of Sablinka Formation were studied using scanning electronic microscope. Etching and crystalline overgrowth textures were found as predominant on the surface of quartz grains. Since these textures may only form in continental environment, modern continental environments with chemical processes of silica dissolution and deposition were considered. The search for analogs was based on the presence of a system of polygonal subvertical fractures in the top of Sablinka Formation, traditionally considered “desiccation cracks” and the pebbles of densely cemented sedimentary quartzite in the basal layers of the sandstones, overlaying Sablinka Formation. The recent analogs were found in South Australia and west New South Wales: the mature pedogenic silcretes, where the zone of silica concretions (pseudo-breccia and pseudo-conglomerates) is located over the zone with polygonal prismatic blocks. It was concluded that during the continental hiatus similar profile began to form at the top of Sablinka Formation, but maturing of pedogenic silcretes was interrupted by Furongian transgression. The pseudo-breccia and pseudo-conglomerate zone, and the upper portion of the prismatic zone were eroded by the transgressing sea. The time interval of silcrete formation was correlated with the interval of deposition and leaching of Kakeled LimestoneBed within the Alum Shale of the Baltic Paleobasin. Refs. 20. Figs. 10.