Abstract:

The sequence stratigraphy and palaeogeography of the Late Permian Longtan and Changxing formations in Chongqing area of southwestern China have been studied based on the data of outcrops and borehole sections. A total of two thirdorder sequences are subdivided in the Late Permian; sequence Ⅰ is from the bottom of Longtan Formation to the bottom of C16 coal seam in Longtan Formation, representing a transgressive to regressive cycle; sequence Ⅱ is from the bottom of C16 coal seam in Longtan Formation to the top of Changxing Formation, representing a second regional transgressive to regressive cycle.The palaeogeographic maps of two sequences have been reconstructed based on the contour maps of a variety of palaeogeographic parameters including the  total thickness of strata, the thickness of sandstone, the thickness of limestones, the thickness ratio of siliciclastic rocks to carbonates and siliceous rocks, and the thickness ratio of sandstones to mudstones, as well as the thickness of the total coal seams.Both of the two sequences reflect a vertical and lateral pattern of palaeogeography, which shows that, from the bottom to the top and from the southwest to the northeast, there is a variation in facies units from the tidal flatdelta combination, through the bay lagoon tidal flat combination, to the shallow marine combination.In the two sequences, depositional environments are all deeper in the northeast, and shallower in the southwest, with the transgressive pulse of advancing from northeast to southwest.The sediment provenances were mainly from the Kangdian oldland to the west.The favorable coal forming facies units are mainly the bay tidal flat facies, followed by residual plain facies, and in the two sequences these facies units controlled coal distribution and they moved towards southwest with a continuous sea level rise.The thick coal zone in sequence Ⅰ is distributed in the area bordered by the Liangping Fuling Wansheng line to the east, and by the Tongliang Jiangjin Xishui line to the west, and the thick coal zone in sequence Ⅱ is distributed in the southwestern area restricted by Guang an Changshou Nanchuan line to the east with the coal accumulation center located in the Hechuan Chongqing Qijiang zone.

Key words: sequence stratigraphy；lithofacies palaeogeography, Chongqing area；Late Permian