Lowermost Triassic (Griesbachian) microbial bindstone-cementstone facies, southwest Japan

Summary On the basis of the lithostratigraphy and microscopic characters, the paper describes the facies interpretation of the upper Upper Permian (Changhsingian) and Lower Triassic (Griesbachian to Spathian) carbonates of southwest Japan, with a focus upon the lowermost Triassic (Griesbachian) microbial bindstone-cementstone. We emphasize the significant sediment-binding and stabilizing agencies of microbes chiefly of cyanobacteria along with the syndepositional cementation for the carbonate deposition on a Panthalassan buildup in a period of the Scythian reef gap. Cyanobacteria flourished as postmass extinction disaster forms in the beginning of the Triassic. The Griesbachian microbial bindstone-cementstone we describe comprises the oldest known Triassic microbial facies. Examined were the Changhsingian Mitai Formation and the Triassic Kamura Formation (Griesbachian to Norian) in southwest Japan. These units consist entirely of carbonates and are reconstructed as relict of a shallowmarine buildup upon a seamount in the Panthalassa. The Changhsingian Mitai carbonates (ca. 35 m thick) consist mainly of grainstone and packstone with a small amount of lime-mudstone. The topmost part is intensely dolomitized. The carbonate succession is characterized by an upward-decrease in number and taxonomic diversity of shallow-marine skeletal debris and an increase up-section in an amount of peloidal particles. The lower Mitai rocks are interpreted to have accumulated as skeletal sand in an oxygenated subtidal environment and the upper Mitai carbonates are considered to have been formed in a quiet intertidal environment where peloidal particles predominantly accumulated. The facies interpretation suggests the late Changhsingian regression, which led to an increase of an inhospitable condition for shallow-marine benthic communities and to an intensive dolomitization. The Kamura Formation (ca. 38 m thick) disconformably rests upon the Mitai Formation with a drastic lithologic change. The Lower Triassic rocks we focused reach 15.5 m thick and comprise the Griesbachian and Dienerian to Spathian sections. The lower part (ca. 5.5 m) of the Griesbachian section consists of dark gray carbonaceous limestone composed of thinly layered triplets of a gastropod-bearing peloidal grainstone layer, a spar-cemented frame of clotted peloids, and a thin-laminated and occasionally stromatolitic cover of cryptomicrobial micrite in ascending order. The upper two members of a triplet often form a bindstone-cementstone layer characterized by a low-relief domed structure, or a broad hump. The upper part (ca. 2 m thick) of the Griesbachian section is composed of oncolitic limestone that contains laminae packed with gastropods. The Dienerian to Spathian section (ca. 8 m thick) consists of coquinites comprising an explosive flourish and accumulation of pectinacean bivalves. We interpret the Griesbachian rocks to have accumulated in a stagnant, ecologically rigorous tidal flat, where microbes, of possible cyanobacteria, flourished. The flourish of gastropods reflects an intermittent inundation by spring tide into the peritidal environment. The deposition of gastropods was followed by a dominant cyanobacterial activity that formed a microbial bindstone-cementstone layer along with the syndepositional cementation in an intertidal zone. The cyanobacterial activity contributed to the formation of gently undulated, sediment-binding and stabilizing mats. The oncolitic limestone in the upper part of the Griesbachian section also suggests the cyanobacterial, or algal activity. The Griesbachian microbial-controlled sedimentation was followed by the mass accumulation of bivalves that most possibly reflects a rapid transgression in Dienerian time. All the results permit us to conclude that possible cyanobacteria were the significant rock-forming organisms as post-mass extinction disaster forms on a panthalassan buildup in the beginning of the Scythian reef gap. The Griesbachian carbonates here described are similar in having the important microbial control on the sedimentation to the Lower Triassic stromatolitic and thrombolitic carbonates previously known in the Tethyan platform.