Connecting the marine red beds with the onset of the Great Ordovician Biodiversification Event: A case study from the Laojianshan Formation of western Yunnan, Sibumasu Massif

Abstract

Details of the onset of the Great Ordovician Biodiversification Event (GOBE) remain unclear, and an emerging picture indicates that this process may have been linked to oceanic oxygenation. To address this question, we carried out a case study involving facies analysis and sea-level reconstruction on the Early to earliest-Middle Ordovician marine red beds (MRBs) of the Laojianshan Formation, western Yunnan, Sibumasu Massif. The Laojianshan Formation, formed prior to the main pulse of the GOBE, was deposited in four sedimentary belts including the basal lag, nearshore, inner shelf, and outer shelf. Sea-level curves reconstructed based on facies analysis are comparable to those of other major paleo-plates, indicating a eustatic control. The MRBs are widely distributed from the nearshore to the outer shelf belts, in which shelf deposition constitutes the major part. No explicit relationships between nearshore MRBs and sea level are found; however, shelf MRBs consistently coincided with transgressive intervals. The shelf MRBs generally indicate a very low organic carbon burial rate, which could be the result of a higher oxygen content in the ocean water and lower primary productivity. Comparative studies show that widespread shelf MRBs characterize the study interval while black shales were deposited in deeper settings, indicating widespread oxygenation in the pre-Darriwilian shelf, beyond which anoxia was largely restricted to the deeper ocean. Oxygenation may have ended the biomere-associated extinctions since the late Cambrian and promoted the onset of the GOBE.