Paleogeographic Evolution of the Dengying Formation in Hannan-Northeastern Sichuan Basin: Sedimentary evidence of the extensional tectonic setting for the northwest margin of the Yangtze Block in the Late Sinian

Key words: 
• Dengying Formation /
• paleogeographic evolution /
• extensional tectonic setting /
• Hannan-northeastern Sichuan /

Abstract: The appearance of paleogeography is the result of the joint action of tectonic evolution and sedimentary filling. In this study, which was based on previous research and exploration results, systematic observations and analyses of the field outcrops in Hannan-Northeastern Sichuan Basin were conducted. The results reveal that there are four types of paleogeographic units in Dengying Formation in the Late Sinian: ancient Hannan paleo-uplift， restricted platforms open platforms and slopes. The terrain in the northeastern part of the Hannan-Northeastern Sichuan Basin is lowest in the south and highest in the north; the depositional elements are distributed in the north-south and northeasterly directions. The paleogeographic appearance in the study area was of “one uplift, two depressions”. It is suggested that the study area was in an extensional setting, which is reflected by the characteristics of co-sedimentary faults identified from outcrops and drillings. In addition, the location of the rift/depression identified in the geophysical data is consistent with the location of the ancient land on the paleogeographic map. This confirms the existence of the Mianzhu-Lezhi-Longchang-Changning pull/rift and Wanyuan-Dazhou rift in Sichuan Basin, and also indicates an extensional setting in the study area during the Dengying period. The paleogeographic background was transformed from one of stable deposition to active extension, and experienced stable deposition, prelude and main extension. Thus, this study further indicates the coupling relationship between sedimentary evolution and tectonic setting.