一种低海平面时期的沉积岩相——深水盆地相碳酸盐砾岩层序

作者认为北美阿巴拉契亚地区和我国贺兰山地区奥陶系的一套碳酸盐砾岩层序,就是低海平面时期的一种特殊沉积岩相。这种岩相不仅是古海洋条件变化的一种标志,而且也是良好的油气储集层。文中主要从沉积学的角度解释了它们的形成机制。

A SPECIFIC SEDIMENTARY FACIES RESPONSIBLE FOR LOW-SEA LEVEL──BASINAL CARBONATE-CLAST CONGLOMERATE

A hemipelagic interbeds of carbonate-cfast conglomerate, micrite. dark shale, andcarbonate and siliciclastic turbidites were developed in the Taconic sequence of Lower Pale-ozoic in Appalachian of United States and, Ordovician of Helan Mountains in China.These sequences resulted from platform--basin interactions during sea-level oscillations.Sea level changes controlled the phases of productivity and exporting capability of platformand consequently, the amount and type of sediment delivered to the basin.We interpret the carbonate-clast conglomerates in these hemipelagic sequences as aspecial sedimentary facies.During the flooding period,amount of carbonate productivity is low in the platform,and is not enough to compensate for the sea level rise due both to subsidence and eustasy,thus export of carbonate toward the basin is very low. Sedimentation in the basins is domi-nated by settling of extra--basinal terrigenous muds.During the high--sea level period, carbonate production increases in platform,leadingto a shallowing-up evolution. Two main transport mechanisms are responsible for the ex-port of carbonate sediment: (1)carbonate currents; they bypass the platform slope and aredeposited on the basin floor: (2)the carbonate and terrigenous muds are taken into suspen-sion;they are transported along surface layers into the basin.During low-sea level period,due to subaerial exposure the platform undergoes erosionwhich supplies a large number of carbonate-clast. These materials are transported by high-density gravity flows and /or rock falls into basin. In keep-up low-sea Ievel stage, terrige-nous turbidites are deposited in the basin.