Stratigraphy and facies analysis of the Antalo limestone (Callovian-Tithonian), Mekele Basin, northern Ethiopia

Antalo Limestone in the Mekele Basin, northern Ethiopia was studied to interpret depositional environment through facies analyses and provide insights on its Jurassic sedimentological evolution and paleogeographic implications. The full thickness of the unit was investigated through outcrop description, section logging and sample collection at eight outcrop locations. Two of the sections represent the lower to middle parts of the unit with about 200 m and 270 m thickness. Five of the sections represent the middle to upper parts of the unit with thicknesses ranging from 175 to 285 m and one section represents about 72 m-thick uppermost part of the unit. The field descriptions of the unit, in terms of lithology, texture, composition, bedding and sedimentary structures, show eighteen lithofacies types made up of limestone, sandstone, shale and marl. The petrographic analyses of the samples also identify seventeen limestone microfacies ranging from mudstone to grainstones including boundstone, one dolomitic limestone and two sandstone facies (quartzwacke and quartzarenite). The facies analysis indicates that the studied unit was deposited within shallow- to deep-marine environments. There are six facies associations that were correlated with the standard microfacies types and facies zones, indicating a typical homoclinal carbonate ramp with tidal flat, lagoon, sand shoal, mid-ramp, outer-ramp and deep basin depositional systems from the shore to open sea environments, respectively. The lack of reef facies, in the studied sections, has also been considered as evidence to deduce a homoclinal carbonate ramp for the unit. These sub-environments were juxtaposed and, the lateral shifting of their position through time caused the development of vertical facies successions of different environments. It is also deduced that marine water from the Tethys Sea covered the Mekele area since the early Callovian and persisted until the Tithonian, creating favorable conditions for carbonate production and sedimentation. Therefore, the Jurassic sedimentation history of Antalo Limestone was synchronous with that of equivalent carbonate platform sequences of the Horn of Africa and other peri-Tethyan regions, confirming a related paleogeographical setting.