中国科学院南京地质古生物研究所机构知识库

The compositional characteristics of Changhsingian (Late Permian) sponge reefs have different magnitudes, and their growth and death could be considered in light of paleoenvironmental and paleozoogeographical changes across a wide range. Well-documented outcrops in South China that encompass Changhsingian sponge reefs have been systematically summarized based on the paleontology and sedimentology. The Changhsingian sponge reefs are of two types: platform-margin algae-sponge reefs and intraplatform sponge skeletal mounds at subtidal to intertidal zones. The two types of sponge reef both contain higher biodiversity of reef-building and reef-dwelling organisms, and they vertically exhibit multiple cyclic developments of alternating reef core and reef flat facies. However, there are significant differences between the platform-margin algae-sponge reef and intraplatform sponge skeletal mound in terms of the composition of the reef-building biota and the reef-related sedimentological characteristics. The contents of the frame-building components in the platform-margin algae-sponge reefs are considerably higher than those in the intraplatform sponge skeletal mounds, and the microencruster Archaeolithoporella is highly abundant in the former. The reef core deposits in the platform-margin algae-sponge reefs are dominated by sponge framestone with the encruster Archaeolithoporella, Archaeolithoporella, and cyanobacteria bindstone, and synsedimentary calcite cements, and they are subjected to strong diagenesis processes, such as dolomitization, cementation, recrystallization, and paleokarstification under the middle to high hydrodynamic conditions. The reef core deposits in the intraplatform sponge skeletal mounds are primarily characterized by sponge bafflestone and sponge-Tubiphytes-cyanobacteria bindstone with high percentages of micritic matrix and bioclastic debris without or less sponge framestone and calcite cements, which are affected by compaction-dissolution, micritization, recrystallization, and weak dolomitization under the low to middle hydrodynamic conditions. For the platform-margin algae-sponge reefs, a well-developed horizontal depositional sequence from a deeper-water carbonate-siliciclastic basin and fore-reef slope to a back-reef carbonate platform is evident, but no horizontal facies differentiation occurs in the intraplatform sponge skeletal mounds. The sedimentary environmental changes, driven by relative sea level fluctuations, are the key reason for these differences in sponge reefs. A global end-Permian regression with superimposed small-scale sea level changes can cause a cyclic development of alternating reef core and reef flat deposits of Changhsingian sponge reefs.