A fossil carbonate contourite drift on the Lower Ordovician palaeocontinental margin of the middle Yangtze Terrane, Jiuxi, northern Hunan, southern China

Abstract

The Early Ordovician (Tremadocian) in part of Jiuxi, northern Hunan Province, comprises a succession of deep-water carbonate sediments deposited on a palaeocontinental margin at the southern edge of the Yangtze Platform. A distinctive elongate mound-like form, some 350–450 m thick, can be recognized lying between shallow-water platform carbonates and deeper-water mudstones. This elongate body, here called the Jiuxi drift, is made up dominantly of sediments interpreted as contourites on the basis of their mid to base-of-slope location, alongslope current indicators, features of traction flow processes coupled with intense bioturbation, and distinctive contourite sequences (typically 30–80 cm thick). Five contourite facies are recognized: calcilutites, calcisiltites, calcarenites, calcirudites (possibly contourite lag deposits), and bioclastic contourites. Hemipelagites and turbidites make up only a small proportion of the drift, which accumulated at an average sedimentation rate of 38 m/m.y. Large-scale cross-stratified units found in parts of the calcilutitic contourite section are believed to result from seafloor development of mudwaves and/or erosional furrows under the influence of a semipermanent bottom-current regime.