Clay and shaly claystones in the Upper Triassic Siliceous sponge-microbe reef mounds and oolitic bank complex in NW Sichuan, China

Abstract

The Upper Triassic Carnian siliceous sponge-microbe reef mounds and oolitic bank complex is located in the northwestern part of Sichuan province, China. The Carnian hexactinellid-thrombolite reef mound tract and its underlying oolitic bank were developed in a deeper marine gulf. The oolitic bank overlain by a bioclastic bank, corresponding to the Lower Carnian served as the foundation for growth of the reef mounds. The reef mounds, inter-mound strata and caprocks correspond to the Upper Carnian. There are some occurrences of clay and claystones in the reef mound–bank complex. The reef mounds contain a certain ratio of clay content. The clay content in reef mounds is originated from the turbid and muddy water in the Carnian gulf. It is conjectured that alive siliceous sponges preferred to grow in marine water with more silica content to be supplied to their skeletons, and that thrombolite preferentially dwelt in marine water containing a certain content of clayey mud with more organic matter, for cyanobacteria were heterotrophs in Mesozoic. The inter-mound and caprock facies consist mostly of claystones. There exist clay-filling neptunian dikes, karstic clay, and fault clay. Most of the clay is originated from claystone coating of the reef mounds, and fault clay is caused by earthquake.