Mechanisms and environments of deposition of late precambrian geosynclinal tillites: Scotland and East Greenland
Abstract The late Precambrian tillites in Scotland and East Greenland lie in marine,, geosynclinal sequences and extend for hundreds of kilometers. Both tillite formations are hundreds of meters thick and contain largely conformable internal stratigraphies with many individual mixtite beds separated by sandstone, siltstone, conglomerate and dolomite interbeds. Direct evidence for deposition of the mixtures from grounded ice sheets is provided by the angular unconformities beneath the tillite formations, by the huge rafts of deformed sediment in the Great Breccia in Scotland, and by the bedded horizons within mixtites, which are often lenticular as a result of deposition in sub- or englacial tunnels. Indirect evidence that the surface of deposition was mostly too shallow for icebergs, or was even above sea level comes from: algal stromatolites, mudcracks, tidal palaeocurrent patterns and coarse (beach?) conglomerates; from the several internal, marine erosion surfaces in Scotland; and from the sandstone wedges, inferred to be of subaerial permafrost origin. Both tillite formations thus record complex glacial histories, with 17 main glacial advances in Scotland and 2 in Greenland. The tillites are compared with the thick, but little known Quaternary sequences of the North Sea Basin: they may have accumulated on continental shelves that were more gentle and extensive than the latter.