Development of a Gilbert-type delta in the kame environment: An example from NE Poland

Abstract

Pleistocene kame deltas are known mainly from glaciomarginal environments, whereas there are fewer examples of deltas where kames formed within decaying ice sheets. In this article, we describe deposits of a Gilbert-type delta formed inside a glacial system. The delta succession was identified in the upper part of the hill, a glacial landform developed within the southern sector of the Scandinavian ice sheet in NE Poland during the Warthian glaciation. The landform is interpreted as a kame, the sediments of which were accumulated in an elongated ice crevasse oriented perpendicularly to the front of the ice sheet. The kame was formed in several stages. In the main stage of development, the crevasse was filled with a lake into which the delta prograded. The lake subsequently underwent a few phases of shallowing, and even an episode of complete drainage.The main delta unit was identified in the outcrop. It is composed of a delta slope and prodelta sediments. A partly preserved older deltaic unit of the initial phase of delta sedimentation also was found. The steep (22–42°) slope of the delta is dominated by gravel and sand. The gravels were deposited from high-energy grain flows and grain falls. Sands were deposited mainly from traction currents during periods when the delta-supplying system was less active. Aggradation of the prodelta was mainly due to sand deposition from traction underflows. Deposition from turbidity currents occurred sporadically there. We explain this unusual situation by the fact that the gravelly sediments feeding the delta slope were relatively well-sorted and contained only small admixtures of fine sand and silt. The sediment sorting process took place in a system of englacial or supraglacial conduits, where the gravels were depleted of fine-grained particles. The development of the Gilbert-type delta was possible because the glacial lake, at least 6–8 m deep, periodically maintained a relatively constant water level, which allowed the delta to prograde. This process required the existence of a steady outflow of water from the lake, which was then an open hydrological system. In the final stage of kame formation, when the lake was completely drained, gravel accumulated from shallow sheet flows.