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.

Illite-Smectite-Rich Clay Parageneses from Quaternary Tunnel Valley Sediments of the Dutch Southern North Sea — Mineral Origin and Paleoenvironment Implications

“…The continuity of the fresh water environment beyond the ice-margin in a proglacial lake that 520 covered the whole clinoform succession is furthermore suggested by seismic reflection data 521 (Benvenuti and Moscariello, 2016). Although very little sedimentological and mineralogical evidence 522 for a proglacial lake in the central part of the SNS is provided in the literature, this contribution 523 supports the studies of Gibbard (2007) and Gupta et al (2007) who were first to hypothesize the 524 existence of a large proglacial lake at glacial maximum in the area between the SNS and the Benelux 525 countries.…”

Tunnel valley deposits from the southern North Sea – material provenance and depositional processes

“…A lithological characterization of the material that provides the infill of a British offshore TV suggested that the offshore TVs must have been filled with reworked material that was locally mobilized by glacial processes (Praeg ; Kristensen et al . ; Benvenuti & Moscariello ). However, based on the inferred position of the north European Elsterian ice sheet, the specific internal architecture of the offshore TV infill successions is in apparent contrast with the expected southward displacement of glacial sediments (Figs ).…”

“…, ; Passchier et al . ; Benvenuti & Moscariello ). Accordingly, the sediment deposited in TVs must have been sourced either locally from the SNS substrate (subglacially mobilized by meltwater flow erosion and ice‐sheet abrasion) or regionally from Scandinavia and the British Isles (subglacially abraded by the ice sheet at a greater distance), or a mixture of both regions.…”

“…; Moreau et al . ; Moreau & Huuse ; Benvenuti & Moscariello ). A lithological characterization of the material that provides the infill of a British offshore TV suggested that the offshore TVs must have been filled with reworked material that was locally mobilized by glacial processes (Praeg ; Kristensen et al .…”

“…The figure shows the location of the seismic section (I‐I’) and the borehole K14‐12 (O). B. Stratigraphy and grain‐size composition of the studied succession from the K14‐12 borehole (Benvenuti & Moscariello ). The locations of the investigated samples are shown.…”