University of Khartoum

Sedimentological Characteristics and Depositional Environments of The Upper Cretaceous Shendi Formation, Umm Ali Area, Northern Sudan

Sedimentological Characteristics and Depositional Environments of The Upper Cretaceous Shendi Formation, Umm Ali Area, Northern Sudan

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Title: Sedimentological Characteristics and Depositional Environments of The Upper Cretaceous Shendi Formation, Umm Ali Area, Northern Sudan
Author: Ahmed, Elbashir Oshi Mohamed
Abstract: This study investigates the paleodepostional environment, facies types, architectures and paleogeograhy of the Upper Cretaceous sediments of Shendi Formation in northern central Sudan. The methodology included surface and subsurface lithofacies analysis, petrography, grain size, heavy minerals, clay minerals and geochemical analyses. The depositional environment of Shendi Formation is subdivided into lower lacustrine and upper fluvial members.These members are composed of three lithofacies associations, deep lacustrine lithofacies association C, fluvio-lacustrine association B and fluvial lithofacies association A. The deep lacustrine lithfacies association C is characterized by massive mudstone facies (Fm). The transitional lithofacies association B is dominated by fine laminated sandstone, siltstone and mudstone facies (Fl) and laminated sandstone, siltstone and mudstone facies (Fsm). Such lake environment dominated the subsurface area comprising almost the facies types with part of minor fluvial sediments recorded from the basin prephery boreholes. Moreover, the lakes were moderately large, broad and shallow are characterized by alkaline and slightly saline chemical nature near shore.This water type is subsequently become fresh water during hydrologically open lake that was controlled by fluvial input. The upper fluvial member is characterized by erosional channel surface and trough cross-bedded sandstone facies (St), planar cross-bedded sandstone facies (Sp), horizontally –bedded sandstone facies (Sh), ripple cross-bedded sandstone facies (Sr) and massive sandstone facies (Sm) with some overband and floodplain sediments. This fluvial member may represent different channel types such as low sinuosity braided channel at Bagrawyia with some difference in details to meandering around Umm Ali village. Northerly it is almost braided of platte type. Five architectural elements were identified within the Formation.These are sand channel element (CHs) and sandy bedform (SB) characterizing Fadnyia and Bagrawyia areas respectively. Lateral accretion element (LA), crevasse channel element (CR) and fine deposit element (FF) occur partially at Bagrawyia and concentrate around Umm Ali area. The facies depositional model of Shendi Formation represents coarsening and thickening-upward cycles and zonal distribution of depositional systems representing allocyclic and autocyclic control on sedimentation. Base level change is controlled by gradual decrease of tectonic activity leading eventually to the dominance of an open or external drainage system during sedimentation of upper member. The iron sediments are strongly leached within intensive warm humid climate from the source areas, transported and deposited within oxic and shallow environments such as overbank, floodplain and lake shore. These environments provided suitable physico-chemical conditions for the precipitation. The iron sedimentation is controlled by alloucyclic and autocyclic processes. The centripetal paleocurrent direction indicates that a circular high lands were surrounding the area. This leads to suggest that Butana, Sabaloka and Bayuda desert are source areas for Shendi Formation. Such source areas were dominated by medium to high-grade metamorphic rocks with contribution from older sedimentary and igneous rocks. The heavy minerals and petrography reflects the mineralogical maturation of the upper member sediments and indicates long transporting distance, reworking processes and intensive weathering under warm humid climate, peneplanation and tectonic stability. The clay mineralogy and geochemistry reveals the overall in- consistency and the variation in accordance to the depositional systems. However, the smectite might probably formed authogenically within alkaline and confined lake environment during the sedimentation of the lower member whereas the kaolinite and illite might represent detrital supplies with fluvial input
Description: 126 Pages
URI: http://khartoumspace.uofk.edu/handle/123456789/11405
Date: 2015-05-25


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