Lacustrine successions in fault bounded basins: 1. Upper Anthracosia Shale (Lower Permian) of the North Sudetic Basin, SW Poland

Abstract

Osady jeziorne w basenach sedymentacyjnych ogranizonych uskokami: 1. Górne łupki antrakozjowe (dolny perm) niecki północnosudeckiej The Upper Anthracosia Shale is a 40-m thick succession of lacustrine deposits, terminating the Świerzawa Formation (lower Autunian) in the North Sudetic Basin. This succession contains a number of distinct lithofacies which represent the result of sedimentation in different lacustrine subenvironments. These lithofacies include: onshore flat, nearshore, offshore, deltaic, swamp/moor, carbonate, and mass-gravity deposits. The Upper Anthracosia Shale forms a transgressive-regressive succession which is subdivided into four segments connected with the four major phases of the Anthracosia Lake evolution. The onshore flat fades mark the initial phase of the lake development, when the basin floor originally occupied by fluvial channel belt evolved into the fluviallyinfluenced onshore zone of a lake. The phase of transgression was characterized by the development of widespread nearshore zone. The lake expansion was interrupted by several episodes of slope failures and subaqueous mass redeposition of sediments, which were generated along the tectonically active, NNE margin of the basin. Renewed nearshore conditions resulted in the formation of nearshore shoal bars. Organic-rich black shales representing the lacustrine offshore facies reflect a phase of the maximum lake extent. Lacustrine deltaic deposits forming coarscning-up sequence at the top of the lacustrine succession mark the regressive phase. The transgressive, thick portion of the lacustrine succession resulted from long-lasting transgression, while the thin regressive portion is related to rapid regression induced by rejuvenation of the southern basin margin. Water column in Anthracosia Lake was probably permanently stratified. Bottom conditions changed from oxic-to-suboxic in the nearshore zone to anoxic, sulphidic in the offshore one. The lake level fluctuated frequently. Stromatolitic buildups and swamps/moors appeared locally in the nearshore/offshore transitional zone and along the lake shore, respectively. Wave reworking played an important role in the nearshore zone. Synsedimentary tectonic activity and basin floor subsidence, climate, sediment supply, water feeding system, and the activity of living organisms were the most important factors controlling sedimentation in Anthracosia Lake.