Early Triassic (Griesbachian) spheroids in Beibei, Chongqing, Southwestern China: Characteristics, cause and implications for palaeo-oceanic conditions

Following the latest Permian mass extinction (LPME), abundant unusual sedimentary features and fabrics were widely distributed in Early Triassic carbonate platforms. As a unique type of carbonate grain, spheroids from the Lower Triassic are infrequently reported, yet differ significantly from giant ooids and oncoids. Griesbachian micritic spheroids are well preserved in the Lower Triassic Feixianguan Formation at the Baimiaozi (BMZ) outcrop in Beibei, Chongqing, southwestern China. These spheroids (diameter: 3.9–10.8 mm) were in a marl layer interbedded with massive oolites. They were deposited in the wave troughs of ripple marks comprising micritic clots, sparry calcite, clay minerals, organic matter and pyrite. A microbial spheroid origin is implied based on the coccoidal microbes (coccoid-like microspherules and bacterial clump-like microspherules), numerous pyrite framboids (indications suggest the presence of many sulphate-reducing bacteria and the high iron content in seawater can promote nitrogen-fixing cyanobacteria proliferation) and microbial-derived microspherules. The dark-coloured matrix between the spheroids primarily included microcrystalline calcite, clay minerals, organic matter and metazoan fossils. The spherical and ellipsoidal shapes of the spheroids suggested rapid accretion and lithification. As a unique carbonate depositional mode, Greisbachian spheroids may have been recorded for a palaeo-ocean with dysoxic and calcium carbonate supersaturation shortly after the LPME.