Carbonate Microfacies Analysis On The Horizon Bearing Luoping Biota And Its Paleogeographic Environment

The Nanpanjiang foreland basin began to shrink in the Middle Triassic and close in the Late Triassic as converging with the Yangtze craton platform under the function of Indo-China Orogeny. In the carbonate platform, there were a number of different scale scattered restricted basin within the platform(or intra-platform basin), due to biological action, sea-level changes, tectonic movements as well as other factors. In the process of geological mapping the regional distribution of Luoping Biota in Yunnan, by lateral stratigraphic correlation and measuring stratigraphic profile, the author and colleagues identifed that intra-platform basin in Luoping area is northwest-southeast direction. The main factors control the development of basin are variance growth among the algal limestone and the sea-level changes.Through measureing the stratigraphic section of Guanling Formation in the Middele Triassic (T2gl) in Jiangsuotian, Banqiao town, Luoping county, Yunnan province, as combining with auxiliary section and other achievements in scientific research, Guanling Formation in Middle Triassic was divided and correlated. In order to identify the distribution of Luoping Biota and its regional distribution characteristics, based on the achievements of 1:200000 and 1:50000 regional survey, we successfully completes lithological and biological division and correlation of the horizon bearing Luoping Biota in the Second Member of Guanling Formation.This biota is characterized by diverse fishes and associated with reptiles, amphibians (?), echinoderms, arthropods (crustaceans and possible insects), bivalves, gastropods, as well as plants, and is unique in its peculiar preservation, wide distribution, huge abundance and high diversity of the fossils. By observing the measured stratigraphic section, the occurrence position containing the Luoping Biota is characterized by gray black thin bedded micrite as well as a large number of sedimentary structrues, such as horizontal laminae, wavy bedding, deformation structures (slump structures, convolute bedding) and trace fossils etc. Through detailed field observation and microscopic examination, as well as reference to Wilson (1975) standard microfacies and depositional models of carbonate ramp, eight types of microfacies(RMF 1-8) are recognized: calcicutite with a few bioclasts, laminated calcicutite, pelletoidal calcicutite, calcisiltite, rudaceous micrite, erratic micrite with bioclasts, dolomite with gypsum pseudosparite and stromatolitic micrite. At the same time, combination of macro-structures and the preservation characteristics of the Luoping Biota, it reveals that the Second Member of the Guanling Formation is mainly deposited in the intra-platform basin which is similar to carbonate ramp environment, and its vertical sequence shows regressive sedimentary sequence. The paleotemperature in the Luoping area was restored usingδ18O data in the study, our results shows that the temperature changes between 27-35℃and its average temperature of seawater in the Pelsonian of Luoping was about 30.92℃, indicating a warm paleoenvironment during that period.The environment buried fossils are deep-water basin, distally steepened ramp.