UPPER ORDOVICIAN SLUMP STRUCTURES IN WESTERN ZHEJIANG PROVINCE, SOUTH CHINA AND THEIR GEOLOGICAL IMPLICATIONS

Slumps of various scales were recognized in the late Katian (Late Ordovician) carbonate slope successions in western Zhejiang Province, South China. In order to understand their formative processes, triggering mechanisms, and geological implications, a detailed sedimentological work was carried out in 4 sections (the Putangkou, Zhoutang, Shikeng, and Sanqushan sections). Three slump units occur in the Putangkou section, 3.5 m, 11 m, and 73 m in thickness in stratigraphically ascending order. These slump units are separated by undeformed nodular lime mudstones with similar microfacies to those in deformed intervals. The slump units comprise sliding surfaces (with carbonate breccias sitting above) and dm-scale blocks with various soft-sediment deformation structures (SSDS) such as convolute laminations, asymmetric folds and faults complexes, and water-escape structures. The slump units in the Zhoutang and Shikeng sections (~3 km SW of the Putangkou section) are similar to those in the Putangkou section and are characterized by abundant convoluted blocks. Thirteen slump units (from 10s dm to 6 m in thickness) occur in the Sanqushan section (~10 km N of the Putangkou section) and contain SSDS that are mainly characterized by recumbent folds. The SSDS in slump units formed as a result of variable degrees of plastic and brittle deformation, as well as liquefaction and fluidization occurred during slumping. Autogenic triggering mechanisms (e.g., rapid deposition, storm-wave loading, tidal shear) are ruled out by facies analysis and consideration of local tectonic background. Instead, the slumps might have been triggered by allogenic processes, i.e., either the tectonically induced steepening of slopes or the possibly weakening of sediment by seismic activity, which was related to the contemporary Kwangsian Orogeny (Caledonian Orogeny), which led to the collision of Cathaysian Block with Yangtze Block.