Sedimentological and Geochemical Responses of Black Shale to Late Ordovician-Early Silurian Geological Events in the Upper Yangtze Area

Abstract

The Upper Ordovician Wufeng-Lower Silurian Lungmachi (Longmaxi) black shales are of vertical heterogeneity in the Upper Yangtze area. Based on detailed sedimentological observations and geochemical analyses, four geological events have been restored and genetic depositional model has been established within high-resolution sequence stratigraphic framework. 10 parasequence sets develop from 1 at the bottom of Wufeng shale to 10 at the top of Lower Lungmachi shale. The siliceous shale interbedded with intense K-bentonite layers of high TOC (Total Organic Carbon) in the transgressive systems tract 1 (TST1; including parasequence set 1) is attributed to the first stage of volcanic eruption (started at ~447.62 Ma) caused by collision of the Cathaysia and Yangtze blocks. The appearance of Hirnantia Fauna, a sea-level fall and positive δ¹³C_org excursion provide evidences for Hirnantian glaciation equivalent event (begun at ~445.16Ma), which resulted in low TOC content in the highstand systems tract (HST; including parasequence set 2) in second stage. Siliceous shale interbedded with scarce K-bentonite layers of high TOC in the transgressive systems tract 2 (TST2; including parasequence set 3 and 4) is due to the second stage of volcanic eruption (started at ~443.83 Ma) and postglacial effects. Micro-offsite graptolite rich shale with silty laminaes and moderate TOC content relate to bottom current intrusion in the early highstand systems tract (EHST; including parasequence set 5, 6 and 7). Argillaceous rich shale and silty mudstone of the lowest TOC is due to terrigenous input in the late highstand systems tract (LHST; including parasequence set 8, 9 and 10). Our research also shows that volcanic ash caused by moderate volcanic activity and anoxic depositional setting benefit the enrichment of organic matter and siliceous minerals in the TST1 and parasequence set 3. Favorable preservation conditions of organic matter and biotic recovery during the postglacial age play significant roles in the formation of organic-rich shale in the TST2.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018