Carnian (upper triassic) sponge mounds of the Northwestern Sichuan Basin, China: Stratigraphy, facies and paleoecology

Abstract

Carnian reef mounds with abundant and highly diverse siliceous sponges (Lyssacinosa and “Lithistida”) occur at the northwestern margin of the Sichuan Basin (Anxian and Mianzhu counties), Southern China. Triassic sediments comprise Ladinian to early Norian deposits. Sponge mounds are confined to the Upper Member of the Carnian Hanwang Formation, being late Carnian in age. The mounds correspond to elliptical carbonate bodies with a longer axis up to 100 m and a shorter axis up to 60 m. The dip is 25–30 degrees (but may be accentuated by weathering processes). Mound formation followed a bank phase, represented by the underlying Oolitic Bank Facies and the Bioclastic Bank Facies, both of which were formed in medium to high energy environments. Deepening of the basin, perhaps caused by synsedimentary block-faulting at the shelf-margin, is paralleled by the formation of the sponge mounds. The mound core, proximal and distal mound flanks as well as the intermound sediments differ with regard to frequency, type (colonial, non-colonial, fasciculate etc.) and preservation of siliceous sponges and other biotic elements (especially brachiopods, bivalves, echinoderms). Colonial siliceous sponges dominated during the last mound phase. Termination of mound growth seems to have been caused by the increasing influx of siliciclastic sediment. Triassic siliceous sponge mounds differ from late Jurassic siliceous sponge reefs in the paleogeographical setting, lack of microbial crusts (and rapid synsedimentary cementation), different relief and size of the mounds, and, probably, also in different guild structures.