New evidence for compaction-driven vertical fluid migration into the Upper Ordovician (Hirnantian) Guanyinqiao bed of south China

Abstract

The Late Ordovician was a time of major climatic and sea-level changes linked to the Hirnantian Glaciation, marked by large positive excursions of δ13Ccarb and δ13Corg globally known as the Hirnantian carbon isotopic excursion (HICE). However, some Upper Ordovician sections in South China exhibit negative δ13Ccarb shifts within the Hirnantian-age Guanyinqiao Bed. We hypothesize that these unusual carbon-isotope patterns were due to influx of 13C-depleted dissolved inorganic carbon (DIC) from the underlying Wufeng and overlying Longmaxi black shales as a result of greater compaction of these fine-grained siliciclastic units compared to the coarse-grained carbonate Guanyinqiao Bed. The isotopically light DIC was generated through oxidation of organic matter by either microbial sulfate reduction (MSR) or methanogenesis (with subsequent anaerobic oxidation of methane, AOM) in the black shales. This hypothesis is supported by petrographic and geochemical evidence of vertical fluid migration within the Guanyinqiao Bed and across its contacts. In the Shiqiao drillcore, the Guanyinqiao Bed exhibits a V-shaped depression at its top that is connected to vertical channels filled with secondary carbonate cements having low δ13C values. In the Huanggexi section, both the base and top of the Guanyinqiao Bed are characterized by strong U enrichments, despite unambiguously oxic depositional conditions as inferred from other proxies (Fe speciation, Mo and V concentrations). These anomalous U enrichments resulted from oxidative remobilization of U along the Guanyinqiao-black shale contacts and subsequent transport into the Guanyinqiao Bed through compactional flow. This finding demonstrates the importance of recognizing remobilized trace metals in sedimentary successions in order to avoid potential errors in redox interpretations.