摘要:
沿台地边缘斜坡沉积的古代碳酸盐再沉积物是最近十年来才引起沉积学者广泛注意的一种深水碳酸盐类型,包括碳酸盐重力流,重力滑动(滑塌)沉积及塌磊裙。重力流及由于重力作用引起的沉积物与浅水碳酸盐或深水原地碳酸盐沉积相比,有很大的差异。它们的主要岩石类型是异地碳酸盐,为一套在重力作用下顺坡而下流动的碳酸盐沉积物。前礁塌磊是台地边缘礁坠落到陡崖下的礁块堆积,也称为礁崖塌积。
Abstract:
Ancient resedimented carbonates along platform-foreslope are considered as a deep-water type of carbonates, including gravity flows, gravity slide ( slump ) deposits, and talus wedges ( aprons ) . A series of deposits belonging to this kind of carbonates found from the intraplatform basin developed in Middle Devonian Qiziqiao Formation,north of Daoxing and east of Ningyuan, Hunan.They are around or adjacent to the restricted platform ( RC ) or open platform ( P ) (Fig.1 ) .The following four kinds of allochthonous carbonate sediments basically belonging to foreslope can be recongnized:( 1 )the slump psephitic limestone of fore-platform(Platel-l); (2)the allochthonous calcarenite Plate 1-2,3 ,Fig. 2 ); ( 3 ) the allochthonous calcisiltite ( Plate 2 -2 , 3 ); and ( 4 ) the talus limestone of reef-scarp (Plat 2-1 , Fig. 3 ) . These types of sedimentary rocks represent a transitional series which can be respectively regarded by the author as the product of gravity slide, secondary grain flows, foreset bed of turbidity flows (Fig. 2, below ) and talus reef-scarp aprons. Several different types of deposits, such as gravity slide and gravity flow, can be found on the same vertically geological section. The debris slides may synchronously bring about extensive debris flows, secon-dary grain flows and turbidity flows.According to their occurrence, petrological cha- racters, single bed thickness, inversely graded bedding,and massive non-graded unit, etc., it seems that the calcarenites of the study area may be compared with the modern secondary grain flows at the base of Bahama Escarpment 400m below reported by H.T.Mullins, and to have a similar sequence to allochthnous turbidity limestone ( Eder, 1970) .However, compared with mass calcarenites newly interpreted by I.M. Hurst,et al. ( 1983 ) ,the most likely mechanism for deposition of the calcarenites is the transition from high-density turbidity flows to liquefied flows.The gravity flows were quite developed from the last Middle to early Upper Devonian in South China. In addition to gravity flows and gravity slides of that age,synsedimentary faults at sea floor, a series of crinkled bedding of contemporaneous deformation, as well as submarine fans in northern Guangdong Province next to this area, and submarine erupted basalts in Guangxi Province are all the best evidences for the powerfully submarine tectonic movements. The gravitation on foreslope. especially the associated paleoearthquake, is probably the major factor of the mass movement of sediments at the sea-floor.