Morphology and depositional architecture of supercritical alluvial fans: control by autogenic processes or high-frequency climatic oscillations?

The depositional architecture and geomorphology of alluvial fans that have evolved in response to similar regional environmental conditions can differ strongly, implying that autogenic processes may play an important role and lead to similar cycles of fan aggradation and incision that may be difficult to be distinguished from the effects of tectonics or climate change. Here we present new data from two different Late Pleistocene (33-18 ka) alluvial fan systems in northern Germany. These fans formed under similar climatic and tectonic conditions, but differ in size, type, and drainage area allowing to estimate the role of climate and autogenic controls on flow processes, facies architecture, and fan-stacking patterns.

Sand-rich, sheetflood-dominated fans are related to larger, low-gradient fan catchments. Steep depositional fan slopes (5°-17°) favored supercritical flow conditions. Steep, dip-slope catchments enhanced stream gradients and promoted the transport of coarser-grained sediments. These fans have lower gradient slopes (2-6°) and are dominated by channelized flows, alternating with periods of unconfined sheetfloods. Fan onset and aggradation occurred in response to climate change at the end of MIS 3. Meter-scale coarsening-upward successions are characterized by sandy sheetflood deposits at the base and overlain by multilateral or smaller single-story gravelly channel fills are related to high-frequency climatic fluctuations or seasonal fluctuations in water and sediment supply. In contrast, the recurrent pattern of multistorey, multilateral, and single-storey channel bodies with a lateral offset to vertical stacking pattern most probably was controlled by autogenic switch in an avulsion-dominated system.