Graptolite zone calibrated stratigraphy and topography of the late Ordovician-early Silurian Wufeng-Lungmachi shale in Upper Yangtze area, South China

摘要

Stratigraphic architecture and topography of the Late Ordovician-Early Silurian Wufeng-Lungmachi shale in the Upper Yangtze area, South China, are analyzed based on calibration of graptolite zones and integrated interpretation of data sets of 3D seismic, drilling wells, and outcrops. The lithostratigraphic architecture is well correlated with graptolite zones. Going upwards, the Wufeng Formation is equivalent to graptolite zone WF1-4, layer 1 to LM1, layer 2 to LM2-3, layer 3 to LM4, layer 4 to LM5, sub-member 2 to LM6-8, and member 2 to LM9 of the Lungmachi Formation. During this depositional period, the Upper Yangtze area was bordered by Chuangzhong thickness zero area (CZZA) in the northwest and Qianzhong-Xuefeng thickness zero area (QXZA) in the southeast and the zero lines migrated southeastwards progressively. The Upper Yangtze area had southwest (SDA) and northeast (NDA) depositional areas separated along Zuo3-FL-TH1. The SDA has fairly smaller shale thickness and distribution area relative to the NDA. In addition, both the SDA and the NDA had depocenters which migrated away from the QXZA progressively. The locations of the CZZA and QXZA may show the possible positions of the Chuanzhong Paleo-uplift and the Qianzhong-Xuefeng Old Land. During this period, the topography of the Upper Yangtze area was fairly rough and a submerged high might occur along Zuo3-FL-TH1. Because of sustained compression from the Cathaysia block, the depocenters migrated northwestwards and the sedimentation rate increased continuously.