Sedimentary facies, paleontological characteristics and stratigraphic delineation of the Ordovician strata through the CYY1 well in the Yingjing depression,southwestern Sichuan

A systematic core observation and analysis are carried out for the Ordovician strata through three wells, especially through the CYY1 well in the Yingjing depression,southwestern Sichuan. Below the Permian Liangshan Formation through the three wells,a succession of greyish black mudstones is developed,with a thickness of more than 10 m (16 m thick in the CYY1 well),and assigned to the shallow shelf facies. Then the nearshore sandstones are found below the greyish black mudstones in the shallow shelf facies,with some siltstone strips or lens interbedded,displaying an overall deepening-upward sequence. In the sandstones and siltstones of nearshore facies,there occur abundant sedimentary structures including wavy cross bedding,wave-built cross bedding, parallel bedding,oblique bedding,as well as plenty of soft-sediment deformational structures. In the upper greyish black mudstones through the CYY1 well,there occur numerous fossils and fossil fragments of trilobite,graptolite and brachiopods. In addition,there are also the Middle-Lower Ordovician members such as Didymograotus sp. and Taihungshania sp. The above-mentioned sandstone-mudstone successions are highly different in sedimentary facies and fossil from those in the Ordovician-Silurian Wufeng Formation and Longmaxi Formation around the Yingjing depression,but correspond with the stratigraphic horizons of the Middle-Lower Ordovician Hongshiya Formation, Meitan Formation,Dawan Formation and the middle member of the Daguanshan Formation.