Geological models of ice keel scours, with an early sinian example

Abstract

Abstract Ice-keel turbate is widespread on recent continental shelves. The surface morphology of modern ice keel scours is well-known but their subsurface structures are not well studied. In the geological record ice keel scours are rare and no data on their subsurface structures from pre-Pleistocene glacial sediments exist. A late Proterozoic ice-keel scour was found in the Lower Sinian Xieshuihe Formation in the Yangijaping area, Shimen County, Hunan Province, South China. This comprises a 35 cm high dome-shaped sedimentary structure, composed of five different layers of silty and muddy sandstone. The semi-circular basal layer A, with a radius of about 20 cm, is structureless, and covered by pebbly layer B. This is the thinnest layer, thickening from one side to the other. The 4–8 cm thick pebbly layer C contains pebbles with axes parallel to the lower boundary of the layer. Layer D, at the top of the structure, is more laterally continuous with the adjacent horizontal beds. Layer E comprises the pinching-out part of thin post-scouring horizontal laminae. The characteristic features of these layers can be explained by a unique ”bulldozing“ force, and the bulldozing agent is believed to have been an ice-keel. This glacigenic interpretation of the structure is consistent with its stratigraphic position and paleogeographic location. In contrast to an existing fault model of ice-keel scour derived using an example in overconsolidated clay, our roll-and-mix model was developed in much less consolidated, soft, fine sediment and seems to give a more plausible explanation of the generation of ice keel turbate. Since the stratigraphic level of the scour is about fifty meters below the well defined lower boundary of the Dongshanfeng Glacial Epoch, it is reasonable to conclude that the onset of the glacial epoch would have been much earlier.