+Chapter 32 Neoproterozoic glacial records in the Yangtze Region,China
【摘要】：正The Nanhua rift occurred at c.820 Ma.The last rifting stage was likely associated with the Neoprotcrozoic glaciations.The glacially influenced sequence,in ascending order,comprises the Jiangkou Group(c.720-663 Ma),the interglacial Datangpo Formation (Fm.)(c.663 Ma) and the Nantuo Fm.(656-635 Ma).In deep-water environments the glacially influenced units exhibit conformable contacts with a transitional lower boundary and a sharp upper boundary.In shallow marine environments,the lower boundary is always unconformable and erosive and the upper boundary is sharp but conformable. The Jiangkou glaciogenic deposits likely comprise the waxing,the maximum and the waning stages of glaciation.The lower part of the Chang'an Fm.corresponds to the waxing stage.It initially inherited the pre-glacial deep-water environment,and later,the thickening and coarsening upward trend likely indicates a lowering of sea level and ice-sheet advance.The predominant,massive diamictite in the upper Chang'an Fm.probably highlights the maximum glacial stage,and likely resulted from an amalgamation of debris flow deposits,suspension and ice-rafted debris.The Fulu Fm.records the waning stage;its lower member is characterized by arkosic sandstone and greywacke indicative of an intcrstadial between glacial events,while the upper member represents the final glacial event of the Jiangkou glaciation. The carbonaceous black shale and Mn carbonate at the base of the interglacial Datangpo Fm.were likely indicative of sedimentary starvation associated with Jiangkou deglaciation.The Nantuo glaciation is characterized by an abrupt onset and termination with negligible waxing and waning stages.Tine sediments likely resulted from subglacial deposition,sedimentary gravity flows and ice-rafted debris with reworking by currents. The onset of Jiangkou glaciation took place around c.720 Ma and terminated at c.663 Ma.The Nantuo glaciation started atc.656 Ma and terminated at 635 Ma.The palaeolatitude during the Jiangkou glaciation is unconstrained and was 37±7°during the Nantuo glaciation.The palaeogeographic reconstruction for the Yangtze Block is controversial:it was either located between Australia and Laurentia or to the NW of Australia and to the north of India. Values ofδ~(34)S_(pyrite) from the top of the Fulu and the base of Datangpo formations exhibit extraordinarily high values up to | 60‰.Values ofδC_(carb)b from the base of the Datangpo Fm.range from -7.4‰to -13.0‰and from the cap carbonate of the Doushantuo Fm.are generally lower than -5‰;however,methanogenic values as low as -48‰have been reported.From the Datangpo Fm.,dinosterane has been detected,andphytane,triterpanes and hopane-typepentacyclic triterpanes are also abundant.Biomarker data from glacially influenced rocks indicate that the photosynthetic process never ceased and that the marine environment was probably normal during glaciations. The banded iron formation(BIF) at the base of the Fulu Fm.is either basaltic,tuffaceous,siliciclastic or carbonate-hosted.Rhodochrosite at the base of the Datangpo Fm.was probably associated with submarine volcanic and hydrothermal activity,but a detrital oxide origin cannot be excluded. The difference between the Jiangkou and the Nantuo glaciations is significant.Tectonics and associated topography are speculated to be +the main controlling factors on the Neoproterozoic glaciations in the Yangtze region.