Ordovician carbonate contourite drifts in Hunan and Gansu Provinces, China

Abstract

abstract The Early Ordovician continental margin at the southern edge of the Yangtze Platform, China, is represented by a succession of deep-water carbonate sediments near Jiuxi. A distinctive elongate mound-like form, some 120 km long, 25 km wide and 350-450 m thick, has been identified between shallow-water platform carbonates and deeper-water mudstones, and interpreted as the Jiuxi drift. The principal facies include calcilutites, calcisiltites, calcarenites, calcirudites, and bioclas-tic contourites, interpreted as contourites on the basis of their slope location, alongslope palaeocurrent indicators, features of traction flow processes coupled with intense bioturbation, and distinctive contourite sequences. During the Middle Ordovician, along the western slope of the Ordos Platform, Gansu Province, a narrow deep gateway (here called the Pingliang gateway ) opened up between two deep-water basins. An elongate, mounded, carbonate body (70 km X 15 km X 150 m thick), developed along the eastern slope of this gateway, has been interpreted as the Pingliang contourite drift. A similar suite of contourite facies and sequences is found as at Jiuxi, although there is a greater abundance of calcarenitic contourites in the Pingliang drift, as well as other evidence that suggests a more active bottom current regime in this Ordovician gateway region