Paleogeomorphology and shale distribution of Late Ordovician-Early Silurian Yangtze platform, South China: Implication for shale mineralogy and TOC content

Abstract

The distribution and mineral composition of the Wufeng-Longmaxi shale on the Yangtze platform are directly influenced by the geomorphology of the Late Ordovician-Early Silurian period. This research aimed to clarify the geomorphology of the Yangtze platform during this time period and its implication on the Wufeng-Longmaxi shale. Geophysical interpretation, chronostratigraphic division, and correlation, isometric map compilation, and mineral composition analysis were utilized to achieve this objective. The results of the study showed the following findings: (1) The Wufeng-Longmaxi shale was deposited on the southeastern slope of the Leshan-Longnüsi paleo-uplift on the Yangtze platform; (2) The southeastern slope exhibited three significant slope breaks, which allowed for the division of the slope into four geomorphology units: subaqueous high, subaqueous slope, subaqueous plain, and subaqueous sag; (3) The overlying Wufeng–Longmaxi shale was fully developed in the subaqueous plain and subaqueous sag, but lacked graptolite zones LM1-4 in the subaqueous high and subaqueous slope, with the shale onlapping the southeastern slope from southeast to northwest; (4) The southeastern slope significantly affects the grain size, mineral composition, and TOC content of the overlying shale. Specifically, as the slope transitioned from the subaqueous high to the subaqueous sag, the grain size becomes finer, the contents of TOC and silica increased, and the contents of carbonate and clay minerals decreased.