Discovery of chitinoidellids and calpionellids from the Blue Nile Basin and the Jurassic-Cretaceous boundary in Ethiopia

Abstract

The deposition of Mesozoic carbonates in the Horn of Africa is pervasive and forms an important marker for inter- and intra-basinal correlation. In central Ethiopia, this time interval is represented by transgressive deposits of the Antalo Limestone Formation in the Blue Nile and the Mekelle basins, and the Hamanlei Limestone Formation in the Ogaden Basin. These carbonate formations have traditionally been assigned to range from ?Lower to Upper Jurassic. Recently, based on calcareous nannofossils from the Blue Nile Basin, the Antalo Limestone Formation was constrained to between the lower Callovian and upper Tithonian with a suggestion of the presence of Cretaceous strata in the Blue Nile Basin, based on the record of Uniplanarius sissinghii in the Mugher section. Here, based on the discovery of chitinoidellids and calpionellids from two sections of the Blue Nile Basin, we confirm the presence of Cretaceous deposits and conclusively extend the age of the Antalo Limestone Formation to late Berriasian. Three calpionellid zones are recorded from the Blue Nile Basin: Bonetinae and Chitinoidellids/Primitive Calpionellids subzones, both assigned to the upper Tithonian Microcanthum ammonite Zone and the Calpionellopsis Zone (?Simplex‒Oblonga/Oblonga-Praecalpionellites Subzone) corresponding to the upper Berriasian ammonites Boissieri Zone. Two stratigraphic hiatuses: Kimmeridgian‒lower Tithonian and uppermost Tithonian‒lowermost upper Berriasian are noted in both sections. This discovery of calpionellids has a profound effect not only on the biostratigraphy of Ethiopia but also has allowed the integration of the study region of eastern Africa, for the first time, within the wide calpionellid domain of the South Tethysian margin.