Abstract:

The Santonian-Campanian (S/C) transition in the Kalaat Senan area is well exposed in the Assila Wadi (OA) section and consists of marls with indurate glauconitic levels towards the base. Standard Mediterranean ammonite biozonations could not be applied to this section because the biomarkers are absent. However, planktonic foraminiferal biozonation is more reliable for indicating the S/C boundary, and here we propose to use the last appearance datum of Dicarinella asymetrica as the index marker of this boundary. A high-resolution quantitative analysis of the planktonic foraminifera of the OA section allows us to define the main bioevents across the S/C transition interval and to specify the S/C boundary. The section reveals that the extinctions across the transition interval occurred among species with a trochospiral keeled test and free portici of the genera Dicarinella and Marginotruncana. In fact, the dicarinellids eventually became extinct and the last appearance of the index species Dicarinella asymetrica defines the S/C boundary, whereas the marginotruncanids suffered a gradual extinction and several species crossed the boundary. Representative taxa of the genera Globotruncanita and Globotruncana first occurred in the uppermost part of the Santonian. These bioevents indicate a major but gradual planktonic foraminiferal turnover during the S/C transition, and may be related to adaptive changes and intraspecific competition. Because dicarinellids and marginotruncanids lived in tropical-subtropical oceanic realms, they seem to have been less able to adapt during the environmental changes associated with the transition; they were progressively replaced by more evolved globotruncanids belonging to the genera Globotruncana and Globotruncanita. This major turnover may have been initiated by an increase in temperatures succeeding a maximum flooding stage of a transgressive period. These oceanic conditions appear to have been favorable for radiation of the globotruncanids and heterohelicids.

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