A Study On The Sedimentary Paleogeography Of The Late Jurassic And Cretaceous In The Mid-Yangtze Area

Based on the theories and methods of sedimentology, stratigraphy and palaeogeography, combining with the data of outcrop observing, logging, drilling and seismic, the sedimentary palaeogeographic of late Jurassic and Cretaceous were studied systematically in the mid-Yangtze area by using stratigraphy ,sedimentary and structure methods.In regard to stratigraphic division and correlation, the researched area was divided into three blocks including Youyang, Jianghan and Edong on the basis of forerunner's data and our research in this thesis. Finally, the paper make analysis of stratigraphy correlation from the Upper Jurassic to the Upper Cretaceous.By referring to the regional geotectonic setting and sedimentary setting data, based mainly on outcrop sections, combined with drilling, logging and seismic profiles, five sedimentary facies were identified in the mid-Yangtze area, which included alluvial fan, braided river, meandering river, delta and lacustrine facies, and deliberately discussed the characteristics of facies. The conclusions were made that Upper Jurassic was mainly developed meandering river; the Lower Cretaceous was mainly developed alluvial fan and braided river; the Upper Cretaceous was developed alluvial fan-fan delta- lacustrine facies.Based on the stratigraphic and sedimentary system, the paper also determined 3 structural sequence and 13 third sequence by using the Principles of Sequence Stratigraphy. Based the sequence characters and comparison , the paper revealed that Upper Jurassic was in the higher base level of meandering river depositional system; the Lower Cretaceous was in the lower base level of alluvial fan and braided river; the Upper Cretaceous was in the higher base level, which developed fan delta- lacustrine facies until late Oligocene.According to the result of previous studies, combining the investigation of sedimentary system, stratigraphic and Source, the structure-sequence sedimentary palaogeographical map of late Jurassic and Cretaceous of the mid-Yangtze area was compiled by adopting sttuctural sequence system tract as basic mapping unit. The sedimentary palaeogeographic characteristics and evolution of different structural sequence and system tracts was demonstrated in details, and make comparison with the upper Yangtze area. Then it concluded that the depositional setting of the Upper Jurassic in Zigui basin is foreland basin because of impaction of Qinling–Dabieshan orogenic belt and Jiangnan fold-thrust belt; the palaeogeographic patterns for the lower Cretaceous and the upper Cretaceous is significantly different, Tectonic setting was converted from compression into extension, at the same time, the jianghan basin and yuanma basin began to develop. The peak of extension was in the end of upper Cretaceous, scattered groups of small pull-apart basin continue to develop in the Southeastern and the jianghan basin began to appear during the period.