High-resolution seismic geomorphology and stratigraphy of a tunnel valley confined ice-margin fan (Elsterian glaciation, Southern North Sea)

Abstract

The deep Middle Pleistocene subglacial incisions of the Southern North Sea (SNS) are commonly infilled by northward gently dipping clinoforms oriented toward the former ice-sheet core. We have focused on the terminal sector of the largest tunnel valley (TV) of the SNS and offer the first high-resolution reconstruction of the geomorphology and stratigraphy of the infill of the glacial incision, as well as the detailed geomorphology of the incision to better understand the genetic mechanisms of these uncommon but peculiar large-scale (up to 13 km in length) bedforms. For this study, high-resolution 3D seismic data, the grain-size distribution from ditch cutting samples, and the gamma-ray wireline log of borehole K14-12 are used. The TV formed in the subglacial environment by steady state flows in a time-transgressive fashion. Meltwater flow transports the eroded material southward and eventually deposits it at the ice-sheet margin, in the lightly grounded ice-sheet environment where the adverse slope forces the flow to wane. The process results in an elongated ice-margin fan made of clinoforms, whose grain-size distribution fines progressively southward, in the downstream direction. The formation and filling of the TV occurred during the retreat of the ice-sheet margin and cyclic fluctuations of the meltwater mass flow rate, which affected the internal stratigraphy and created an undulated top of the clinoforms’ unit. Sparsely distributed, horizontally layered units interpreted as distal proglacial lacustrine deposits filled depressions on the top of the clinoforms-bearing unit. The sequence was then sealed by a chaotic seismic-stratigraphic unit that probably belongs to postglacial times. The ice-sheet-oriented clinoforms were thus formed by sustained meltwater flow resulting from large-size Scandinavian and British convergent ice sheet flows in the freshwater environment of SNS developed between the complex ice margins and the topographic highs inherited from continental Europe.