DEPOSITIONAL MODEL OF DEEP WATER CARBONATE GRAVITY FLOWS IN THE MIDDLE-UPPER ORDOVICIAN SERIES IN FUPING, SHANXI PRVINCE
摘要: 陕西富平地区,在中—晚奥陶世位于华北地台南侧的弧后盆地的大陆边缘。其中完好地保存了一套以半远洋的泥晶灰岩夹重力流的角砾灰岩和砂屑灰岩为特征的深水沉积岩系。一般认为,与半远洋的石灰岩共生的碳酸盐重力流沉积的复杂岩相,是大陆坡的标志,而相应的陆源碎屑沉积,则被作为海底扇的产物。然而,富平地区的碳酸盐重力流沉积,却是在弧后盆地伸进浅水台地之间的深海前槽中,沿海槽轴向呈席状流搬运、沉积的。Abstract: The Fuping area in Shaanxi lies on the Weibei uplift along the southern margin of the Eerdousi Basin. The Zhaolaoyu Formation of the Middle-Upper Ordovician series in this area is characterized chiefly by hemipelagic thin-bedded lime mudstones interbedded with limestone breccias and calca-renites derived from gravity flow, In addition, there is a small amount of tuff, radiolarian chert and well-sorted thin-bedded calcarenite and calcisi-ltite which represent the equivalent of contourite. Body fossils are poor, mainly pelagic ostracod, radiolaria and graptolite. Benthonic organisms are rare, but deep-water trace fossils of the Nereites facies are abundant. Gravity flow deposits of the Zhaolaoyu Formation can be subdivided into five rock types: ( 1 ) homogeneous wackstone; ( 2 ) thin-bedded graded calcarenite; ( 3 ) graded rubble calcarenite; ( 4 ) limestone breccia with oriented fabric; and ( 5 ) chaotic limestone breccia. In general, the thin-bedded graded calcarenite and homogeneous wack-stones form turbidite with divisions BCE or CE of the Bouma sequence while the graded rubble calcarenite and homogeneous wackstones form turid-ite with divisions AE of the Bouma sequence.Limestone breccia with oriented fabric exhibits normal or inverse-to-normal grading. Tabular clasts are frequently sub-parallel to the bedding or imbricated. The features suggest that they were deposited from a gravally high-density turbidity current. Chaotic limestone breccias might have been debris flow sediments. The above lithofacies commonly form two specific associated sequences in the section. From bottom to top, the two sequences go as follows: ( 1 ) chaotic limestone breccia-→graded rubble calcarenite-→homogeneous wack- stone; ( 2 ) ungraded limestone breccia with oriented fabric-→inverse-to- normal grading limestone breccia-→graded rubble calcarenite-→calarenite with devisions BC of the Bauma sequence-→homogeneous wackstone. These sequences indicate that they are successive deposits derived from individual flow. However, the distinct lithofacies in the above sequences were probably deposited through several sedimention surges. Based on integrated sedimentological, paleontological and tectonic setting analyses, we suggest that the Weibei region, together with North China, be a united shallow-platform prior to the later period of the Middle Ordovician. Then it was subsided and developed into a continental margin of the back-arc basin owing to the subduction of the Qinling plate beneath the North China plate. The Fuping area might have been a N-E trending deep sea front trough that stretched from this back-arc basin into the North China carbonate platform.The gravity flow sediments were mainly transported in great sheet flows along the axis of the trough basin. These flows were probably resulted from the confluence of several minor gravity flows around the downslope margin of platform. The model presented here differs from either the submarine fan model or the carbonate slope model, but it is similar to the pattern of modern carbonate debris sheets of the Exuma Sound in the Bahamas.
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