Late Tithonian nannofossils from Dejen area, the Blue Nile Basin, central western Ethiopia

Abstract

The Jurassic rocks exposed in the Blue Nile Basin (central western Ethiopia), particularly the thick, fossiliferous and widely distributed Antalo Limestone Formation (ALF) has recently been dated between early Callovian and late Tithonian based on the occurrence of calcareous nannofossils from the Mugher section near Dejen, Ethiopia. However, the top part of the ALF is not well exposed at Mugher; in this study, the top part of the ALF was sampled at the Kurar section (150 km north of Mugher). It yielded a nannofossil assemblage containing Helenea chiastia, whose first occurrence (FO) marks the base of the Tithonian NJT16a calcareous nannofossil subzone, and Faviconus multicolumnatus, whose FO is near the base of the NJ15b subzone and corresponds with the FO of the Tithonian zonal marker Polycostella beckmannii recorded at the Mugher section. This age is also corroborated by the presence of recently recorded calpionellids (Bonetinae subzone) and the presence of late Tithonian ammonites, Djurjuriceras cf. sinuosum Tavera and Oxylenticeras Spath from the Kurar section. Hence, the upper part of the ALF exposed at the Kurar and Mugher sections are of late Tithonian age, although at the Mertule Mariam section (60 km northwest of the Kurar section), late Berriasian calpionellids were recently found from the top part of the ALF.