Petrology, Petrography and Stratigraphical Study of the Evaporite-Carbonate Rocks in the Korrahie Formation (Korrahie Locality), Ogaden Basin, Southeastern Ethiopia
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2020-06-06
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Addis Ababa University
Abstract
This thesis utilizes petrology, petrography and stratigraphical techniques to study the evaporite-carbonate rocks in Korrahie Formation, Ogaden Basin, southeastern Ethiopia, at Korrahie locality. Apart from detailed field observations and lithofacies descriptions as well as graphic logging made on the various outcrops in the studied section at Korrahie locality. A total of 29 representative samples composed of evaporite and carbonate rocks were described and analyzed in detail under petrographic microscope. The results obtained from these outcrop lithofacies and petrographic investigations permitted the interpretation of depositional paleoenvironment. Accordingly, the deposition of this sequence took place under three types of setting namely: shallow subtidal, intertidal and supratidal sabkha. The evaporite (gypsum) rocks were deposited on the intertidal and supratidal zones, mainly under subaerial conditions, and locally under subaqueous conditions along the tidal flats. Microfacies analysis of the carbonate rocks allowed the detection of six microfacies types that are associated with a shallowing upward pattern, and characterizes shallow subtidal through beach barrier/tidal bar to intertidal-supratidal setting. The presence of the bioturbated peliodal-bioclastic wackstone/packstone indicates a shallow subtidal setting and the overlying ooid-bioclastic grainstone represents a relatively high energy environment on the island beach barrier. Passing up on the section we have an interlaminated micrite-packstone which represents back barrier/wash-over setting, and in turn the overlaid by dark grey dolomudstone of upper restricted lagoon to lower intertidal. Laterally changing into microbial laminated boundstones indicates intertidal to supratidal environments along with non laminated and non fossilliferous micritic limestones on the sabkha. The lithological logs of this sequence indicate shoaling up stacked meter scale cycles where the lower part of each cycle is carbonate and the upper part is evaporite with thin layers of marl, shale and dolomite. Microscopic investigation shows that the gypsum in the outcropped part of Korrahie Formation is generally of secondary origin that formed from rehydration of precursor anhydrite during the evaporite return to the cooler and low salinity phreatic zone due to exhumation and uplift. Accordingly much of the gypsum textures observed under the microscope are secondary with minor anhydrite relics, and based on their fabrics four microfacies types of secondary gypsum were recognized which are: porphyroblastic gypsum, granoblastic gypsum, alabastrine gypsum and fibrous satin spar and selenitic gypsum.
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Korrahie Formation, Ogaden Basin, Evaporite-Carbonate, Depositional Environment, Secondary Gypsum