Evidence of iceberg‐ploughing in a subaqueous ice‐contact fan, glacial Lake Rinteln, NW Germany

Abstract

The Coppenbrügge subaqueous ice-contact fan complex of early Saalian age is about 10 km long and up to 10 km wide and is composed of offset-stacked fan clinothems that are transgressive-regressive sequences formed during an overall lake level rise. The individual fan bodies consist of coarse gravel in the ice-proximal part, passing distally into sandy facies and showing large-scale foreset bedding. The iceberg scour recognized in an open-pit outcrop is up to 1.5 m deep, up to 2 m wide and cut in undisturbed mid-fan deposits. The scour-fill can be traced laterally for about 15 m and consists of sheared sand and, in the frontal zone, of downbent overlying strata surrounded by a zone of deformed sediments. The deformed sediment produced by the iceberg keel's shearing of the trough walls is a sand mass containing angular soft-sediment clasts that show internal folds and fractures. The basal surface of the deformed sediment is a nearly horizontal shear plane, steepening up laterally as a discrete thrust and showing a flat-ramp-flat geometry. The scour was formed by the iceberg keel's ploughing the substrate and pushing the sediment sideways and frontally, forming a ridge of deformed sediments at the trough end. This ridge was concurrently eroded by an accompanying meltwater underflow which apparently developed a horseshoe system of scouring vortices around the grounded iceberg. The current's scour was filled with massive, nonstratified sand deposited rapidly from turbulent suspension. The iceberg eventually broke up and its keel part was buried. As these ice fragments gradually melted, the space was closed by normal faulting and downbending of overlying strata. The collapsing scour-fill became partly liquified, and the resulting water-escape structures cut the normal faults and the overlying deposits. Though produced chiefly by tangential shear strain, iceberg-ploughing features are readily distinguishable from other glaciotectonic deformations. They can serve as a diagnostic criterion for glaciolacustrine or glaciomarine environments and the distinguishing of ice-contact subaqueous fans from ice-contact deltas in the stratigraphic record.Glaciolacustrine ice-marginal deposits are important and well-studied palaeogeographic and palaeoclimatic archives recording the history and dynamics of glacier termini in lacustrine basins. Glaciolacustrine ice-contact systems are complex and there are major problems with the sedimentological recognition of such depositional settings (

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…A preferential orientation of the furrows is usually documented, which reflects driving wind or current directions, but more distributed orientation patterns also exist. Scour structures from floating ice are preserved in the Quaternary (Delage and Gangloff, 1993;Winsemann et al, 2003) and older rock record (Savage, 1972;Lang et al, 1991;Rocha-Campos et al, 1994;Woodworth-Lynas and Dowdeswell, 1994;Eyles et al, 1997;Ghienne, 2003) although their distinction from subglacial, softsediment, striated surfaces is not straightforward (see discussion in Deynoux and Ghienne, 2004).…”

Clastic injection dynamics during ice front oscillations: A case example from Sólheimajökull (Iceland)

“…Deposition of F5 occurred in topographic lows above a low-permeability till layer (Till 2) which promoted the storage of water. Localised collapsed structures suggest subsidence of sediments during buried ice melting (Winsemann et al, 2003).…”

Sedimentary Signatures of the Waterloo Moraine, Ontario, Canada

“…Cheel and Rust, 1982), erosion beneath a hydraulic jump and iceberg scour (Winsemann et al, 2003). The nested and stacked arrangement of scours, the diverse suite of sediment structures within the fill, and the lack of deformation associated with scour margins, indicate erosion and rapid fill beneath turbulent flows.…”