SEQUENCE-STRATIGRAPHIC FRAMEWORKS FOR THE CAMBRIAN OF THE UPPER-YANGTZE REGION:PONDER ON THE SEQUENCE STRATIGRAPHICBACKGROUND OF THE CAMBRIAN BIOLOGICAL DIVERSITY EVENTS
MEI Ming-xiang, MA Yong-sheng, ZHANG Hai, MENG Xiao-qing, and CHEN Yong-hong State Key Laboratory of Geological Processes and Mineral Resources, School of Earth-Sciences and Resources,China University of Geosciences, Beijing, 100083
In the Upper-Yangtze region, especially in the Guizhou Province and its adjacent areas, the Cambrian is well developed and is marked by a succession from black shale to carbonate rocks. This stratigraphic succession represents a complex sequence-stratigraphic framework. The Cambrian System in the study area constitutes two second-order sequences that can further be subdivided into twelve third-order sequences, and forms a regularly cyclic transgression-regression succession. There are regularly vertical stacking patterns,representing third-order sequences in the second-order sequence.From the Diandongian and the Qiandongian Series, third-order sequences characterized by “CS (condensed section)+HST (high-stand system tract) successions shift to those characterized by “TST (transgressive system tract)+CS+HST” successions. Correspondingly, drowning-type sequence boundaries are changed into exposure-type ones. Therefore, both the second-order and the third-order sequences have a similar sedimentary-facies pattern. In the middle-upper Cambrian second-order sequence is characterized by a generally upward shoaling successions with several features showing strong differences from the lower Cambrian second-order sequence.Firstly, massive dolostones poor in fossils, i.e. the Loushanguan Group, are transgressively pinched out from northwest to southeast in the study area. Secondly, there is no condensed-section unit in sequences in the Loushanguan Group. In contrast, the early Cambrian Diandongian and the Qiandongian Series contain five third-order sequences with a thickness of over 1000 m but grade into a condensed succession where third-order sequences cannot be identified toward the deeper water southeast Yangize reigion.In the Wulingian and the Furongian Series, a set of dolomites deposited in shallow-water platform facies in the northwest is changed into a set of shales and marls of shelf facies fundamental southeast. Therefore, the Cambrian sequence-stratigraphic framework in the study area can demonstrate both in the features of third-order sequences, i.e. the regularity of sedimentary-facies successions in space and the synchroneity of sedimentary-environment changes in time.Futhermore, two kinds of facies-changing surface and two kinds of diachrononeity can be recognized in the Cambrian sequences in the upper Yangtze region.Under this sequence-stratigraphic framework, it is possible to observe several interesting patterns of Cambrian biological evolution. During the period of the first second-order transgression in the early Cambrian, several biotas such as the Small-Shelly Biota, the Songlin Biota, and the Zunyi Biota that is coeval to the Chengjiang Biota,record spectacular biological diversity-events.During the second second-order transgression in the early-middle Cambrian, the Taijiang and Kaili biotas record other spectacular biological diversity-events. Whether regressive events are genetically related to biological extinction events remains uncertain. To better understand the complex relationship between transgressive-regressive events and biological diversity-events, many sequence-stratigraphic problems need to be clarified.