A detailed study on the facies and paleoenvironment of the 330m thick carbonate unit of the Mertule Mariam section has been conducted. Field work and laboratory analysis were implemented inorder to accomplish the main objectives of this study. A total of 34 rocks and 17 dry marl samples are collected from the study area and have been examined under petrographic and binocular microscopes respectively. rnLithostratigraphically, the carbonate unit of the section is characterized by bedded and bioturbated limestone, fossiliferous limestone, mudstone, oolitic limestone and marl unit. rnBased on field investigations and based on paleontological and petrographical studies, 11 microfacies types have been recognized and are grouped into four microfacies associations related to paleoenvironments of tidal flats, lagoon, high energy shoal and open marine. Consequently, the carbonate units of the study area represent a shallow marine setting depositional system. rnBased on paleontological studies various macro and micro fossis are identified including foraminifera and ostracod microfossils; and Bivalve, Brachiopod and gastropod. Additionally algal fossils also recognized rnThe age of the carbonate unit is indicated as Callovian to early Kimmerdigian due to the presence of the following index fossils: pfenderina sp., Kurnubia palastiniensis and Alveosepta jaccardi. rnThe present carbonate unit is correlated with different measured sections in the Blue Nile basin and regionally with the Antalo limestone of the Mekele basin and Urandab Formation of the Ogaden basin depending on biostratigraphy and lithostratigraphy.