Sequence analysis of early Saalian glacial lake deposits (NW Germany): evidence of local ice margin retreat and associated calving processes

Abstract

This paper presents a detailed analysis of high-resolution facies architecture of glaciolacustrine ice-margin deposits, which formed at the southern margin of the Scandinavian ice shield. The ice margin depositional systems are characterised by coarse-grained deltas and subaqueous fans, which are formed by stacked transgressive–regressive sequences, recording an overall lake transgression, interrupted by minor short-term lake-level falls. The delta complexes at the northern margin of glacial Lake Rinteln are thought to reflect a relatively stable position of the ice margin in front of the Weser Chains mountain ridges. The onset of delta progradation probably represents a halt in ice advance and a related high sediment supply period. The sedimentary facies and depositional architecture resemble those of nonglacial Gilbert-type deltas, except for the deposition of glacial debris. The main delta progradation is recorded from the highstand systems tract, when high meltwater and sediment discharge occurred during the melt season. A forced regression during opening of outlets led to the formation of subaerially exposed sequence boundaries and the erosion of the highstand systems tract. Deeply incised channels (incised valleys) were filled during the subsequent transgression. At the eastern lake margin, subaqueous fan deposits reflect an unstable ice-front that was rapidly retreating and subject to periodic calving, which resulted in the generation of floating icebergs, dumping ice-rafted debris and ploughing into subaqueous fan deposits. The rapid retreat of this eastern ice margin is interpreted to result from the overall lake-level rise. The fan-stacking pattern and bulk geometry were determined by the ice-front fluctuations, the shifting of meltwater outlets and the short-term lake-level fluctuations. Depositional processes reflect the character of sediment supply and distance from the ice margin. Ice-proximal upper-fan deposits are characterised by coarse-grained gravel, deposited by debris flows that document continuous discharge. Slump and slide deposits are related to steep depositional slopes. The mid-fan contains basinward fining and thinning deposits of quasi-steady and surge-type high- and low-density turbidity currents, indicating more ice-distal and periodic deposition. The outer-fan deposits mainly consist of surge-type low-density turbidites and glacial debris dumped by icebergs. The forcing parameters governing the development of depositional sequences in both delta and fan settings were lake-level fluctuations, sediment yield rates and physiography. The depositional sequences were deposited on a time scale of seventh- and eighth-order high-frequency cycles (10 1–10 2 years). In this short time span, lake-level fluctuations were on the order of 120 m; therefore, accommodation space was largely controlled by lake level and subsidence can be ignored.