Huanan Ocean remained active from Neoproterozoic to Cambrian: Evidence from stratigraphic sequences and paleoocean-related remnants in South China

Abstract

The timing of the assembly of the Yangtze Block with the Cathaysia Block has been disputed for a long time. Tectonics partly controlled deposition and stratigraphy, and consequently observable changes in the lithologic features and sequences provide insights into the evolution of the paleogeographic and tectonic settings. This study analyzes stratigraphic sequences from the Neoproterozoic (from ca. 820 Ma or late Tonian) to the Cambrian from six areas/sites across the Yangtze and Cathaysia blocks, three from each block. The contrasts in the depositional features and stratigraphic sequences reveal that the paleogeographic settings were different in the two blocks. The Yangtze Block evolved from a rift basin to a carbonate platform whereas deposition occurred in a pelagic to hemipelagic setting in Cathaysia in the late Tonian to the Cambrian. During the Cryogenian, sediments typical of a glacial setting were commonly recorded in the Yangtze Block but were absent in Cathaysia. In the upper Tonian to the Cryogenian tuffaceous sediments were occasionally discovered in the Yangtze Block, but in Cathaysia, igneous rocks (from acidic to intermediate, and basic or even ultrabasic) were commonly found. These contrasts indicate that the Yangtze and Cathaysia blocks were separate, and located far apart, leading to differences in the depositional and climate settings. Also, the difference in magmatism records suggests that the Yangtze Block was situated along a passive continental margin, whereas Cathaysia was in an active tectonic setting (subduction of paleoocean) in the Tethys Tectonic Domain.

The records of spatio-temporal distribution of paleoocean-related remnants further infer that at least two basins of the Huanan Ocean (= South China Ocean) could have been developed in the late Tonian to the early Paleozoic, the Jiang-Shao oceanic basin probably subducted eastward, and the Zhenghe-Dabu oceanic basin subducted westward.