Early middle triassic stromatolites from the Luoping area, Yunnan province, southwest China: Geobiologic features and environmental implications

Abstract

Early Middle Triassic stromatolites are documented for the first time from the Guanling Formation of the Luoping area, eastern Yunnan Province, SW China. The Luoping stromatolites show six types of constructional microbial forms: ?1) typical stratified columnar structures with crinkled laminae. Dark colored laminae are composed of filamentous cyanobacteria, showing a vertical growth fabric. ?2) Laminoid fenestrae and ?3) clotted structures are also commonly present; these clotted fabrics also show an interweaved pattern of ?4) prostrate filaments, which are reflected by strong fluorescence in sharp contrast to dolomite cement in fluorescent images. ?5) Rod-like aggregates, filled with minute dolomite rhombs, are very common in laminae; they resemble present-day cyanobacterial trichome, and thus may represent fossilized cyanobacteria. ?6) Moreover, small pits, coccoid spheroids, calcified biofilms, and fibrous structures are also common in stromatolite laminae. The last two may represent calcified extra-cellular polymer secretions (EPS) that contribute to the development and lithification of stromatolites. Authigenic quartz grains are also common and may have involved biological processes in their formation. These six functional-groups driving accretion and lithification processes of stromatolite documented in literature, both the lithified cyanobacteria/oxygenic phototrophs and sulphate-reducing bacteria (SRB) which induced microbial formation of dolomite are evident in the Luoping stromatolites, suggestive of biogenic origin. The Luoping stromatolites differ from the Early Triassic counterparts in having a great amount of biomass in filamentous cyanobacteria and SRB, whereas both anoxygenic phototrophic bacteria and SRB characterize the Early Triassic stromatolites. Abundant filamentous cyanobacteria may indicate proliferation of oxygenic phototrophs in a normal, oxic habitat. However, abundant SRB indicate sulfate reduction in a stressed habitat. Accordingly, the Luoping stromatolites, coupled with coeval unusual biosedimentary structures, indicate that the post-extinction devastated oceanic conditions may not only have prevailed in the Early Triassic but also have extended to the early Anisian (Middle Triassic) in South China, just before the full recovery of marine ecosystems in middle-late Anisian.