Benthic foraminiferal assemblages and biostratigraphy of the Antalo Limestone, Mekelle Basin, Northern Ethiopia

Abstract

This study focused on the identification, detailed systematic descriptions and stratigraphical distributions of different foraminifera and other species to make the first systematic paleontological analysis, interpret paleoenvironment and determine the age of Antalo Limestone of the Mekelle Basin. Fieldwork and section logging, with sample collection at seven outcrop locations, followed by petrography and standard micropaleontological analysis were carried out. Abundant foraminifera fossils of 17 species were identified from 42 thin sections and 5 washed samples of marl and shale. The faunal assemblage also comprised dasyclad green algae, calpionellid, brachiopods and stromatoporoids (Cladocoropsis mirabilis). The benthic foraminiferal assemblage is represented by 3 species of the order Lituolida (Freixialina planispiralis, Nautiloculina circularis and N. oolithica), 11 species of the order Loftusiida (Alveosepta jaccardi, Choffatella tingitana, Conicokurnubia orbitoliniformis, Everticyclammina virguliana, Kurnubia jurassica, K. morrisi, K. palastiniensis, K. wellingsi, Praekurnubia crusei, Pseudocyclammina lituus and Siphovalvulina variabilis), 2 species of the order Textulariida (Redmondoides lugeoni and Valvulina lugeoni) and 1 species of the order Miliolida (Miliolinella quinquangula). The green algae comprised 3 species of the order Dasycladales (Clypeina jurassica, Salpingoporella annulata and Salpingoporella gr. pygmaea). The calpionellid is represented by 1 species of the order Calpionellidae (Calpionella alpina). The brachiopods comprised 2 species of the orders Rhynchonellida (Somalirhynchia africana) and Terebratulida (Somalithyris bihendulensis). Based on the stratigraphical distributions of these benthic foraminifera, dasyclad green algae, calpionellid and brachiopods, three zones namely; Kurnubia palastiniensis, Somalirhynchia africana/Somalithyris bihendulensis and Alveosepta jaccardi/Pseudocyclammina lituus have been established, signifying a Callovian – Oxfordian, a Callovian – Early Kimmeridgian and a Kimmeridgian – Tithonian ages respectively. Therefore, a Callovian – Tithonian age has been assigned to the Antalo Limestone. This faunal assemblage indicated a shallow – deep marine environment with normal marine salinity which has a strong affinity with the faunas of the Middle – Upper Jurassic Peri-Tethyan carbonates, indicating closely related geological evolution. Six benthic foraminiferal morphogroups have been identified, indicating epifaunal – infaunal microhabitats and active deposit-feeding, detritivorous, herbivorous, omnivorous, and bacterial scavenging feeding strategies.