High-resolution sequence analysis on Limestone Member,Upper Taiyuan Formation,northeastern Ordos

According to the analysis from outcrops,well logs and cores,in conjunction with the analysis on palaeontologic fossil assemblages,the Limestone Member of Lower Permian of northeastem Ordos is subdivided into 5 sequences based on the identification of key boundaries.The Limestone Member developed in the maximum transgression of Upper Paleozonic,is of carbonate ramp mixed with terrestrial clastics,of typical epiric sea depositional characteristics.The distribution of depositional facies is strictly controlled by sea level eustacy.Each sequence is composed of incised valley sandstone of low system tract,limestone,marl,coal and mudstone of transgressive system tract and clastics and coal of high system tract.It is recognized that the stacked thick sand stones are developed in incised valleys from three different sequences.The contour maps of limestone display that the maximum transgression is during Sq³ and Sq⁴,and the mainly transgression directions are from SW and SE of the study area.