Benthic Foraminiferal Response to Pliensbachian - Toarcian (Lower Jurassic) Sea-Level Change and Oceanic Anoxia in NW Europe

摘要

The Early Jurassic was a time of rapidly rising sea level associated with the extensive spread of anoxic bottom waters. Sea-level rise across the Pliensbachian -Toarcian boundary culminated in the falciferum zone of the Toarcian and is one of the best authenticated eustatic events in the Jurassic. This major transgressive pulse was marked by deeper water marine sequences and was associated with the deposition of organic-rich shales. The development of anoxia coincides with a notable period of mass extinction of the marine fauna. High resolution sampling and study of the microfaunas through several sequences in the United Kingdom confirm that benthic foraminiferal faunas were similarly affected by an Early Toarcian falciferum zone event. Samples were analysed from Pliensbachian -Toarcian mudstone, clay and shale sequences of the South Dorset Coast, the Midlands and the Yorkshire Coast. The sections studied show distinct changes in assemblages across the Pliensbachian -Toarcian boundary and in the basal zones of the Toarcian. Evidence for a foraminiferal extinction event in the Pliensbachian -Toarcian includes the elimination of the important Lower Jurassic Lingulina tenera, Frondicularia terquemi and Marginulina prima plexus groups, initiating a significant period of turnover of the microfauna. A marked change also occurred in the character of associated nodosariid assemblages: the uniserial forms of Nodosaria, Frondicularia and Lingulina, dominating the Pliensbachian assemblages, were largely replaced by coiled Lenticulina in the Early Toarcian. A reduction in test size and a decline in species diversity, compared with Hettangian to Sinemurian foraminiferal assemblages, reflect the development of low oxygen conditions followed by a subsequent renewal of the microfauna in the Middle Toarcian. The foraminiferal data from the UK sections we have studied correlates with microfaunal responses reported from NW European Pliensbachian -Toarcian black shale sequences in SW Germany.