Slump sheets as a record of regional tectonics and paleogeographic changes in South China

Abstract

Slumps of various scales are recognized in the late Katian (late Ordovician) mixed carbonate and siliciclastic slope successions in the Changshan area, western Zhejiang Province, South China. In order to understand their deformational processes, triggering mechanisms, paleo-slope orientation and geological implications, a detailed sedimentological study was carried out on four outcrop sections. In the southern Changshan (the Putangkou, Zhoutang and Shikeng sections), we recognized three slumping episodes (ca. 3.5 m, 11 m, and 73 m in thickness in stratigraphically ascending order) and co-occurred debris flows. At the Sanqushan section (~11.5 km north of Changshan), thirteen slump units, each being tens of cm to 6 m in thickness, are recognized. Facies analyses and diagenetic characteristics of these slumps and their internal deformation structures suggest that they formed as a result of plastic and brittle deformation, and liquefaction and fluidization during slumping. The rip-up blocks, cracks and ruptured beds in the slumps of southern Changshan indicate extensional features, whereas recumbent and disharmonic folds in the slumps of northern Changshan represent compressional features. These spatially different features suggest that the slumps may have been generated on a paleo-slope dipping generally to the north. The case study of slumps and debris flows from the Putangkou section indicates a significant mass flow transition from a slumped mass into debris flow deposits. Autogenic triggering mechanisms of these slumps, e.g., rapid deposition, storm-wave loading, and tidal shear, are ruled out by the results of facies analysis. Instead, the slumps might have been triggered by relative sea-level changes or tectonically-induced, progressively steepening of the slope, which is likely related to the contemporaneous Kwangsian Orogeny.