4th International Conference of Palaeogeography

268 / 2019-07-01 22:42:02

Analysis of hybrid facies, hybrid sequence and its controlling factors of the Middle Devonian Yangmaba Formation in Longmenshan area, Sichuan Basin

Hybrid deposition, Hybrid sequence, Yangmaba Formation, Longmenshan area

There are abundant hybrid sediments composed of clastic rocks and carbonate rocks in the Middle Devonian Yangmaba Formation in the Longmenshan area. Based on the field measurements of the Yangmaba Formation, the hybrid sedimentation mechanism and its controlling factors in the sequence system tracts of the Yangmaba Formation are analyzed based on the hybrid facies and hybrid sequence studies. Research shows that: the Yangmaba Formation consists of the lower type-I hybrid type with mudstone background by bottom and the middle-up type-II hybrid type with sand and carbonate background. The hybrid mechanism includes punctuated mixing, facies mixing and in situ mixing, the first two are the main types. Three types of hybrid facies were developed, including hybrid shore, clastic hybrid shelf deposit, and carbonate hybrid shelf, established a sedimentary model for the deposition of hybrid shore-hybrid shelf deposit. The Yangmaba Formation develops the type-II hybrid sequence, which can be divided into four sedimentary system tracts: the shelf-margin systems tract (SMST), the transgressive systems tract (TST), the early highstand systems tract (EHST) and the late highstand systems tract (LHST). All system tracts have hybrid deposition phenomena to different degrees, and there are differences in the mechanism of hybrid deposition: SMST and LHST are mainly punctuated mixing, and the facies mixing is secondary, TST and EHST are mainly facies mixing. Storm action is the most important driving factor of punctuated mixing of Yangmaba Formation. Relative sea level change, carbonate productivity or the rate of terrestrial clastic supply, and climate change are factors that control and affect hybrid sedimentation. SMST and HST are mainly related to changes in relative sea level, while TST is controlled by sedimentary source recharge, and climate change affects the hybrid action of system tracts to varying degrees.