Diachronism of Lower Toarcian Calcareous Nannofossils events

Abstract

A micropaleontology study was effected using the Calcareous Nannofossils in Pliensbachian/ Toarcian interval for Alcabideque section situated in Lusitanian basin (Portugal).

The last section is correlated with other studied sections  founded in different basins.

 Four index spices events are identified, the FO of Lotharingius velatus, Discorhabdus ignotus, Carinolithus superbus and LO of Mitrolithus jansae in Upper Pliensbachian - Early Toarcian.

The bioevents identified in Alcabideque is compared with the data of Peniche section SSGP of Toarcian.

The same order of bioevents is noticed in Alcabideque and Peniche sections belonging to Lusitanian basin.

 Biostratigraphical correlation of Nannofossils events present a diacronism between the different basins.

A quantitative analysis of Nannofossils assemblage is calculated in Alcabideque section and studied with the curve of carbon isotope.

The curve of absolute abundance decrease in Lower Toarcian correspond a negative excursion of carbon isotope. This diminution noticed in the period dominated  by marl- clay sediments presenting anoxic conditions.