Abstract

Glacial rocks of Late Proterozoic age are widely distributed in South China and two glacial epochs are recognized. The epochs are the earlier Dongshanfeng and the later Nantuo, separated by the Datangpo interglacial epoch. The Dongshanfeng epoch is named after the Dongshanfeng Formation found in the Sinian section in the Yangjiaping Area, Shimen County, Hunan province. It was previously thought that the sequence below the lower boundary of the Dongshanfeng was preglacial and was deposited under warm climatic conditions. Recently several possible peri- and proglacial environmental indicators along the lower boundary of the Dongshanfeng Formation and in the top 60 m segment of the underlying Xieshuihe Formation have been discovered. These are ice-keel scoured ridges, dropstones with compressive concave morphology, ice-wedge casts, glacier meltwater-cut grooves, and glaciotectonic thrust structures. The first three are indicative of periglacial environments, and the rest record proglacial conditions.

Based on the stratigraphic order of the above mentioned structures, a depositional model for the Yangjiaping Area during the preceding stage or the early stage of the Dongshanfeng Epoch is proposed. The major controls on deposition were the gradual lowering of temperature and sea-level, the thickening and expanding of an ice-sheet, and the change from a submarine to subaerial setting. Later, due to crustal loading below the ice-sheet, the Yangjiaping Area once again became submarine and accommodated the glaciomarine Dongshanfeng Formation.

The structures described herein are of particular significance because most glacial stratigraphic records fail to show indications of the climatic transition from warm to cold as a result of glacial erosion. The sparse records of the preceding stage of Nanhua Ice Age found in Yangjiaping are, therefore, of special significance.