286 / 2019-08-18 16:44:11

Cannibalism of contourites by gravity flows: explanation of the facies distribution of the Ordovician Pingliang Formation along the southern margin of the Ordos basin, China

contourites, gravity-flow deposits, Ordos basin, Ordovician, Pingliang Formation

The Late Ordovician Pingliang Formation accumulated along the southern margin of the Ordos basin (China). The convergence of the Yangtze Plate and the Sino-Korean Plate led to a trench–arc–basin system during the Middle Ordovician, with a platform- and slope-dominated setting in the eastern part where a graben complicated the overall simple paleogeographical picture, consisting of more or less parallel zones of a platform and a slope setting in the middle part, changing into zones consisting of a platform, a slope, a deep-marine setting to a trench in the west. This setting resulted in various types of gravity-flow deposits and contourites, with different compositions and pathways. The present study focuses on the typical characteristics of contourites in the geological record and the relationships between a contour current and gravity flows.
The Pingliang Formation contains eleven lithofacies, grouped in five facies associations. These facies associations represent deep-sea autochthonous deposits, several types of debrites, turbidites and contourites, but also turbidites of which the fine-grained top part was reworked by a contour current. The various lithofacies are concentrated in different parts of the study area: micritic contourites and debrites are concentrated in the eastern part, whereas debrites consisting of conglomerates as well as sandstone and siltstone turbidites are concentrated in the middle part; calcarenitic turbidites, contourites and reworked turbidites occur in the western part.
The main contour current ran parallel to the contour lines from east to west. Whereas most of the contour current kept moving westwards in the eastern part of the study area, a minor part split off and followed a half-circular pathway through the Fuping graben; its velocity became reduced here, so that micritic contourites were deposited. The velocity of the contour current was increased locally when it entered the confined environment of the Qilianshan-Qiling oceanic trough in the western part of the study area. The relatively high energy of the contour current here resulted in calcarenitic contourites. The velocity of the contour current was low where it ran through an open environment, resulting in fine-grained, thin contourites in the middle part of the study area. Large turbidity currents and debris flows occurred here; their energy was very high, so that they destroyed almost all earlier deposited contourites. This explains why traces of contour currents in the middle part of the study are very scarce, although the E-W running contourite must have passed this area.