Evidence for a very low-energy fluvial system: a case study from the dinosaur-bearing Upper Triassic rocks of Southern Poland

Abstract

The Upper Triassic succession in S Poland in which dinosaur bones have been found consists predominantly of siltstones and claystones. Three units are distinguished. The lowermost and the uppermost units reflect an alluvial environment, whereas the middle one represents lacustrine facies. The lower alluvial unit is interpreted as a record of ephemeral, sinuous, suspended-load channels with rapid vertical accretion. Channel barforms are lacking. The environment is interpreted as a low-energy anastomosing fluvial system. The clayey middle unit is interpreted as having formed in a wide long-lived lake. The top of the lacustrine deposits shows signs of vertisol-type pedogenesis, most probably under subtropical conditions, with seasonally-induced wet and dry intervals. The upper unit reflects a low-energy meandering river system. Silty point bars were abundant and the channels migrated freely. The energy level of this fluvial system was slightly higher than that of the earlier one, which is interpreted as an effect of base-level lowering in combination with an increasingly humid climate. The almost exclusively silty/clayey alluvial deposits represent an exceptionally rare facies. The drainage basin must have been an extremely flat lowland. The presence of vertebrate bones within the anastomosing and meandering river deposits indicates that low-energy alluvial plains were apparently favourable habitats for both reptiles and amphibians during the Late Triassic: under the subtropical, seasonally dry conditions, the animals must have preferred moist low areas, i.e. the flood basins and abandoned channels on the flat valley floors.