Triassic (Carnian) hexactinellid-thrombolite reef mounds and oolitic bank complex in NW Sichuan, China

Abstract

The Upper Triassic Carnian hexactinellid-thrombolite reef mound tract was located in a gulf of NW Sichuan, China. To the northwest of the gulf, there was the Songpan–Ganzi deep marine trough, which was connected with Tethys. The foundation for growth of the reef mounds is a thick oolitic bank overlain by a thin bioclastic bank corresponding to the Lower Member of Hanwang Formation (the Formation approximately corresponds to Maantang Formation). The oolitic bank is composed mainly of oolitic grainstone and packstone, and the bioclastic bank is composed mainly of lithoclast-bearing bioclastic packstone and wackestone. Six representative microfacies are differentiated in the bioclastic and oolitic banks, which indicates that the banks were developed in a subtidal shallow-marine environment. Such an environment created a suitable foundation for the growth of siliceous sponge and calci-microbe reef mounds. The reef mounds, inter-mound facies and capping facies correspond to the Upper Member of the Formation. A reef mound is 1.5–80 m in height with a general dome shape, consisting of reef mound core, reef mound flanks and reef mound top which are differentiated by 11 microfacies. The reef mounds are mainly composed of massive hexactinellid-thrombolite boundstone. The inter-mound facies is composed mainly of bioclastic and lithoclastic packstone with one microfacies. The top part of the Upper Member is the capping facies composed of quartz-silt-bearing claystone, which overlies the whole reef mound and bank complex in the study area. Different occurrences of clay and quartz silt in the reef mound and bank complex are characteristic. The oolitic and bioclastic banks and the reef mound core contain <3 % clay, and the outer-flanks and inter-mound facies contain 8–20 % of clay and quartz silt. The capping facies is composed of quartz-silt-bearing claystone. Most of neptunian dikes penetrating the mounds also contain clay and quartz silt. The clay content of reef mounds may have originated from the terrigenous clay mud and quartz silt input of the paleo-lands surrounding the Carnian gulf. The paper has discussed the influence of terrigenous clay mud and quartz silt input to the development of the hexactinellid sponge mounds in the study area, in the light of a comparison of the Carnian siliceous sponge mounds in Sichuan, China with the Upper Jurassic siliceous sponge mounds in Europe and with the recent reef mounds of hexactinosidan sponges in Canada. Additionally the paper has made a comparison of the Carnian siliceous sponge mounds in Sichuan, China with the Upper Jurassic siliceous sponge mounds in Europe, in the field of distribution of siliceous sponge mounds in time and space, reef mound types and microbial crusts contained in siliceous reef mounds. Concerning the extinction of the Carnian siliceous sponge mounds in Sichuan, China the paper has discussed the response of the studied reef mound tract to the Carnian “Reingraben Turnover” or “Black Shale Events” which occurred at the end of the Early Carnian.