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

By Elbashir Oshi Mohamed Ahmed University of Khartoum (٢٠٠٠) M.Sc. qualifying

A thesis submitted in fulfillment of the Requirements

of Master Degree of Science in Geology

Geology Department Faculty of Science University of Khartoum

٢٠٠٤ April

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.

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ﺘﻨﺎﻭﻟﺕ ﻫﺫﻩ ﺍﻟﺩﺭﺍﺴﺔ ﺍﻟﺒﻴﺌﺎﺕ ﺍﻟﺘﺭﺴﻴﺒﻴﺔ ﺍﻟﻘﺩﻴﻤﺔ ، ﺃﻨﻭﺍﻉ ﻭﻤﻌﻤﺎﺭﻴﺔ ﺍﻟﺴﺤﻨﺎﺕ ﻭﺍﻟﺠﻐﺭﺍﻓﻴﺎ ﺍﻟﻘﺩﻴﻤﺔ ﻟﺘﻜﻭﻴﻥ ﺸﻨﺩﻱ (ﺍﻟﻁﺒﺎﺸﻴﺭﻱ ﺍﻟﻌﻠﻭﻱ) ﺒﺸﻤﺎل ﺃﻭﺍﺴﻁ ﺍﻟﺴﻭﺩﺍﻥ. ﺍﻋﺘﻤﺩﺕ ﺍﻟﺩﺭﺍﺴﺔ ﻋﻠﻰ ﺘﺤﻠﻴل ﺍﻟﺴﺤﻨﺎﺕ ﺍﻟﺼﺨﺭﻴﺔ ﺍﻟﺴﻁﺤﻴﺔ ﻭﺘﺤﺕ ﺍﻟـﺴﻁﺤﻴﺔ ، ﺘﺤﻠﻴـل ﺤﺠﻡ ﺍﻟﺤﺒﻴﺒﺎﺕ ، ﺘﺤﻠﻴل ﺍﻟﻤﻌﺎﺩﻥ ﺍﻟ ﺜﻘﻴﻠﺔ ، ﺘﺤﻠﻴل ﺍﻟﺘﻜﻭﻴﻥ ﺍﻟﻤﻌﺩﻨﻲ ﻟﻠـﺼﺨﻭﺭ ، ﺘﺤﻠﻴـل ﻜﻴﻤﻴـﺎﺀ ﻭﻤﻌﺎﺩﻥ ﺍﻟﻁﻴﻥ. ﺍﻟﺒﻴﺌﺔ ﺍﻟﺘﺭﺴﻴﺒﻴﺔ ﻟﺘﻜﻭﻴﻥ ﺸﻨﺩﻱ ﻗﺴﻤﺕ ﺇﻟﻰ ﻋﻀﻭ ﺃﺴﻔل ﺒﺤﻴﺭﻱ ﻭﻋﻀﻭ ﺃﻋﻠـﻰ ﻨﻬـﺭﻱ (B) ، ﻨﻬﺭﻴﺔ - ﺒﺤﻴﺭﻴﺔ (C)ﻴﺤﺘﻭﻴﺎﻥ ﻋﻠﻰ ﺜﻼﺙ ﻤﺠﻤﻭﻋﺎﺕ ﺴﺤﻨﻴﺔ ﻫﻲ ﺴﺤﻨﺎﺕ ﺒﺤﻴﺭﻴﺔ ﻋﻤﻴﻘﺔ . (A)ﻭﻤﺠﻤﻭﻋﺔ ﺴﺤﻨﺎﺕ ﻨﻬﺭﻴﺔ ﻤﻥ ﺍﻷﺤﺠـﺎﺭ ﺍﻟﻁﻴﻨﻴـﺔ ﺍﻟﻤـﺼﻤﺘﺔ . (C)ﺘﺘﻜﻭﻥ ﻤﺠﻤﻭﻋﺔ ﺍﻟﺴﺤ ﻨﺎﺕ ﺍﻟﺒﺤﻴﺭﻴﺔ ﺍﻟﻌﻤﻴﻘﺔ ﺘﺘﻤﻴﺯ ﺒﺎﻷﺤﺠﺎﺭ ﺍﻟﻁﻴﻨﻴﺔ ، ﺍﻟﻐﺭﻴﻨﻴﺔ ﻭﺍﻟﺭﻤﻠﻴﺔ ﺍﻟﻨﺎﻋﻤﺔ ﺍﻟﻤﺘﺭﻗﻘﺔ . (B)ﻤﺠﻤﻭﻋﺔ ﺍﻟﺴﺤﻨﺎﺕ ﺍﻻﻨﺘﻘﺎﻟﻴﺔ ﻫﺫﻩ ﺍﻟﺭﻭﺍﺴﺏ ﺍﻟﺒﺤﻴﺭﻴﺔ ﺘﺸﻜل ﻤﻌﻅﻡ ﺍﻟﺴﺤﻨﺎﺕ ﺍﻟﺘﺤﺕ ﺍﻟﺴﻁﺤﻴﺔ ﻤﻊ ﻗﻠﻴل ﻤﻥ ﺍﻟﺭﻭﺍﺴﺏ ﺍﻟﻨﻬﺭﻴـﺔ ﺍﻟﺘﻲ ﺴﺠﻠﺕ ﻤﻥ ﺍﻵﺒﺎﺭ ﻋﻠﻰ ﻫﻭﺍﻤﺵ ﺍﻟ ﺒﺤﻴﺭﺓ. ﺇﻀﺎﻓﺔ ﺇﻟﻰ ﺫﻟﻙ ﺘﻌﺘﺒﺭ ﻫﺫﻩ ﺍﻟﺒﺤﻴﺭﺓ ﻜﺒﻴﺭﺓ ﻨ ـ ﺴ ﺒ ﻴ ﺎﹰ ﻋﺭﻴﻀﺔ ﻭﻀﺤﻠﺔ ﺒﻴﻨﻤﺎ ﺍﻟﻜﻴﻤﻴﺎﺀ ﺘﺸﻴﺭ ﺇﻟﻰ ﻗﻠﻭﻴﺔ ﻭﻗﻠﺔ ﺍﻟﻤﻠﻭﺤﺔ ﺘﺤﻜﻤﺕ ﻓﻴﻬﺎ ﻻﺤﻘﺎ ﺍﻟﻤﻴﺎﻩ ﺍﻟﻌﺫﺒـﺔ ﺃﺜﻨﺎﺀ ﺘﺭﺴﻴﺏ ﺍﻟﻌﻀﻭ ﺍﻷﻋﻠﻰ. ﺍﻟﻌﻀﻭ ﺍﻟﻨﻬﺭﻱ ﺍﻷﻋﻠﻰ ﻴﺘﻤﻴﺯ ﺒﺴﻁﺢ ﺘﻌﺭﻴﺔ ﻗﻨﺎﻟﻴﺔ ﻭﺴﺤﻨﺔ ﺍﻟﺤﺠﺭ ﺍﻟﺭﻤﻠـﻲ ﺫﻭ ﺍﻟﺘﻁﺒـﻕ ﺍﻟﻤﺘﻘﺎﻁﻊ ﺍﻟﻤﻘﻌﺭ ، ﺴﺤﻨﺔ ﺍﻟﺤﺠﺭ ﺍﻟﺭﻤﻠﻲ ﺫﻭ ﺍﻟﺘﻁﺒﻕ ﺍﻟﻤﺘﻘﺎﻁﻊ ﺍﻟﻤﺴﺘﻭﻱ ، ﺴﺤﻨﺔ ﺍﻟﺤﺠﺭ ﺍﻟﺭﻤﻠـﻲ ﺫﻭ ﺍﻟﺘﻁﺒﻕ ﺍﻷﻓﻘﻲ ، ﺴﺤﻨﺔ ﺍﻟﺤﺠﺭ ﺍﻟﺭﻤﻠﻲ ﺫﻭ ﺍﻟﺘﻁﺒﻕ ﺍﻟﻤﺘﻤﻭﺝ ﻭﺴﺤﻨﺔ ﺍﻟﺤﺠﺭ ﺍﻟﺭﻤﻠﻲ ﺍﻟﻤـﺼﻤﺕ ﺇﻀﺎﻓﺔ ﻟﺒﻌﺽ ﺭﻭﺍﺴﺏ ﻓﻭﻕ ﺍﻟﻀﻔﺔ ﻭﺴﻬﻭل ﺍﻟﻔﻴﻀﺎﻥ ﺍﻟﻨﺎﻋﻤﺔ . ﻫﺫﻩ ﺍﻟﺭﻭﺍﺴـﺏ ﺭﺒﻤـﺎ ﺘﺭﺴـﺒﺕ ﺒﺄﻨﻅﻤﺔ ﻨﻬﺭﻴﺔ ﻤﺨﺘﻠﻔﺔ ﻤﻥ ﻨﻅﺎﻡ ﻨﻬﺭﻱ ﻤﺘﻘﺴ ﻡ ﻭﻗﻠﻴل ﺍﻟﺘﻌﺭﺝ ﺒﻤﻨﻁﻘﺔ ﺍﻟﺒﺠﺭﺍﻭﻴﺔ ﺇﻟﻰ ﻨﻅﺎﻡ ﻨﻬـﺭﻱ Platteﻤﺘﻌﺭﺝ ﺤﻭل ﻗﺭﻴﺔ ﺃﻡ ﻋﻠﻲ ﺒﻴﻨﻤﺎ ﻓﻲ ﺸﻤﺎل ﺍﻟﻤﻨﻁﻘﺔ ﺘﻤﺜﻠﺕ ﺒﻨﻅﺎﻡ ﻨﻬﺭﻱ ﻤﺘﻘﺴﻡ ﻀﺤل ﺃﻭ type . ﺘﻡ ﺘﻤﻴﻴﺯ ﺨﻤﺴﺔ ﻤﻥ ﺍﻟﻌﻨﺎﺼﺭ ﺍﻟﻤﻌﻤﺎﺭﻴﺔ ﻭﺍﻟﺘﻲ ﺘﻀﻡ ﻋﻨﺼﺭ ﺍﻟﻘﻨﺎﺓ ﺍﻟﺭﻤﻠﻴـﺔ ﻭﻋﻨـﺼﺭ ﺭﺴﻭﺒﻴﺎﺕ ﺍﻷﺠﺴﺎﻡ ﺍﻟﺭﻤﻠﻴﺔ ﻴﻐﻠﺏ ﻭﺠﻭﺩﻫﺎ ﻓﻲ ﻤﻨﻁﻘﺘﻲ ﺍﻟﻔﺎ ﺩﻨﻴـﺔ ﻭﺍﻟﺒﺠﺭﺍﻭﻴـﺔ ﻋﻠـﻰ ﺍﻟﺘﺭﺘﻴـﺏ .

ﻋﻨﺎﺼﺭ ﺍﻟﺘﺭﺍﻜﻡ ﺍﻟﺠﺎﻨﺒﻲ ، ﺸﻕ ﺍﻟﻘﻨﺎﺓ ﻭﺍﻟﺭﻭﺍﺴﺏ ﺍﻟﻨﺎﻋﻤﺔ ﺘﺘﻭﻀﻊ ﺠﺯﺌﻴـﺎ ﺒﻤﻨﻁﻘـﺔ ﺍﻟﺒﺠﺭﺍﻭﻴـﺔ ﻭﺘﺘﺭﻜﺯ ﺤﻭل ﻤﻨﻁﻘﺔ ﺃﻡ ﻋﻠﻲ.

ﺍﻟﻨﻤﻭﺫﺝ ﺍﻟﺴﺤﻨﻲ ﻟﻤﺘﻜﻭﻥ ﺸﻨﺩﻱ ﻴﻤﺜل ﺩﻭﺭﻩ ﺘﺭﺴﻴﺒﺔ ﺨﺸﻨﺔ ﻭﺴﻤﻴﻜﺔ ﺇﻟﻰ ﺃﻋﻠﻰ ﻭﺘﻭﺯﻴـﻊ ﺤﻠﻘﻲ ﻟﻸﻨﻅﻤﺔ ﺍﻟﺘﺭﺴﻴﺒﻴﺔ ﻤﻅﻬﺭﺍ ﺘﺤﻜﻡ ﻋﻭﺍﻤل ﺨﺎﺭﺠﻴﺔ ﻭﺩ ﺍﺨﻠﻴﺔ. ﺘﻐﻴﺭ ﻤﺴﺘﻭﻯ ﺍﻟﻘﺎﻋﺩﺓ ﺍﻟﻤـﺭﺘﺒﻁ ﺒﺎﻟﺘﻨﺎﻗﺹ ﺍﻟﺘﺩﺭﻴﺠﻲ ﻓﻲ ﺍﻟﻨﺸﺎﻁ ﺍﻟﺘﻜﺘﻭﻨﻲ ﺃﺩﻯ ﺇﻟﻰ ﺸﻴﻭﻉ ﻨﻅﺎﻡ ﺘﺼﺭﻴﻑ ﻤﻔﺘﻭﺡ ﺃﻭ ﺨﺎﺭﺠﻲ ﺨﻼل ﻓﺘﺭﺓ ﺘﺭﺴﻴﺏ ﺍﻟﻌﻀﻭ ﺍﻷﻋﻠﻰ. ﺭﻭﺍﺴﺏ ﺍﻟﺤﺩﻴﺩ ﺘﻡ ﻏﺴﻠﻬﺎ ﻤﻥ ﺼﺨﻭﺭ ﺍﻟﻤﺼﺩﺭ ﺨﻼل ﻅﺭﻭﻑ ﻤﻨﺎﺨﻴﺔ ﺩﺍﻓﺌﺔ ﻭﺭﻁﺒﺔ ﺜﻡ ﻨﻘﻠﺕ ﻭﺘﺭﺴﺒﺕ ﻓﻲ ﺒﻴﺌﺎﺕ ﻀﺤﻠﺔ ﻭﻤﺅﻜﺴﺩﺓ ﻜﺒﻴﺌﺎﺕ ﻓﻭﻕ ﺍﻟﻀﻔﺔ ، ﺴﻬﻭل ﺍﻟﻔﻴـﻀﺎﻥ ﻭﻫـﻭﺍﻤﺵ ﺍﻟﺒﺤﻴﺭﺓ. ﻫﺫﻩ ﺍﻟﺒﻴﺌﺎﺕ ﺭﺒﻤﺎ ﻭﻓﺭﺕ ﺍﻟﻅﺭﻭﻑ ﺍﻟﻔﻴﺯﻴﻭﻜﻴﻤﺎﺌﻴﺔ ﺍﻟﻤﻨﺎﺴﺒﺔ ﻟﺘﺭﺴﻴﺏ ﺍﻟﺤﺩﻴﺩ . ﻋﻠﻴﻪ ﻴﻌﺘﺒﺭ ﺍﻟﺘﺭﺴﻴﺏ ﻤﺤﻜﻭﻡ ﺒﻌﻭﺍﻤل ﺩﺍﺨﻠﻴﺔ ﻭﺨﺎﺭﺠﻴﺔ. ﺍﺘﺠﺎﻫﺎﺕ ﺍﻟﺘﻴﺎﺭ ﺇﻟﻰ ﺩﺍﺨل ﻤﺭﻜﺯ ﺍﻟﺤﻭﺽ ﺘﺸﻴﺭ ﺇﻟﻰ ﻤﺭﺘﻔﻌﺎﺕ ﺩﺍﺌﺭﻴﺔ ﺍﻟـﺸﻜل ﺘﺤـﻴﻁ ﺒﺎﻟﻤﻨﻁﻘﺔ ﻤ ﻘ ﺘ ﺭ ﺤ ﺎﹰ ﻤﺼﺎﺩﺭ ﺭﺴﻭﺒﻴﺎﺕ ﺭﺒﻤﺎ ﻤﻥ ﻤﻨﺎﻁﻕ ﺍﻟﺒﻁﺎﻨﺔ ، ﺍﻟﺴﺒﻠﻭﻜﺔ ﺃﻭ ﺼﺤﺭﺍﺀ ﺍﻟﺒﻴﻭﻀـﺔ . ﻫﺫﻩ ﺍﻟﺭﻭﺍﺴﺏ ﺘﻐﻠﺏ ﻋﻠﻴﻬﺎ ﻤﺼﺎﺩﺭ ﺼﺨﻭﺭ ﻤﺘﻭﺴﻁﺔ ﺇﻟﻰ ﻋﺎﻟﻴﺔ ﺍﻟﺘﺤﻭل ﺇﻀﺎﻓﺔ ﺇﻟـﻰ ﺼـﺨﻭﺭ ﻨﺎﺭﻴﺔ ﻭﺭﺴﻭﺒﻴﺔ ﻗﺩﻴﻤﺔ. ﺍﻟﻤﻌﺎﺩﻥ ﺍﻟﺜﻘﻴﻠﺔ ﻭﺍﻟﺘﻜﻭﻴﻥ ﺍﻟﻤﻌﺩﻨﻲ ﻟﻠﺼﺨﻭﺭ ﺃﺸﺎﺭﺕ ﺇﻟﻰ ﺭﻭﺍﺴﺏ ﺍﻟﻌﻀﻭ ﺍﻷﻋﻠﻰ ﺒﺄﻨﻬـﺎ ﺘﻌﺘﺒﺭ ﺭﻭﺍﺴﺏ ﻨﺎﻀﺠﺔ ﻤﻤﺎ ﻴﺸﻴﺭ ﺇﻟﻰ ﻤﺴﺎﻓ ﺔ ﺍﻟﻨﻘل ﺍﻟﻁﻭﻴﻠﺔ ، ﺇﻋﺎﺩﺓ ﺍﻟﺘﺸﻐﻴل ﻭﺍﻟﺘﺠﻭﻴـﺔ ﺍﻟﻌﺎﻟﻴـﺔ ﺘﺤﺕ ﻅﺭﻭﻑ ﻤﻨﺎﺨﻴﺔ ﺩﺍﻓﺌﺔ ﻭﺭﻁﺒﺔ ﻭﺘﻜﺘﻭﻨﻴﺔ ﻤﺴﺘﻘﺭﺓ. ﻤﻌﺎﺩﻥ ﻭﻜﻴﻤﻴﺎﺀ ﺍﻟﻁﻴﻥ ﺃﻅﻬﺭﺕ ﻋﺩﻡ ﺘﺠﺎﻨﺱ ﻋﺎﻡ ﻭﺍﻻﺨـﺘﻼﻑ ﻴﺘﻭﺍﻓـﻕ ﻤـﻊ ﺃﻨﻅﻤـﺔ ﺍﻟﺘﺭﺴﻴﺏ ﻟﺫﻟﻙ ﺭﺒﻤﺎ ﻴﻌﺘﺒﺭ ﻤﻌﺩﻥ ﺍﻷﺴﻤﻜﺘﺎﻴﺕ ﻤﺤﻠﻲ ﺍﻟﺘﻜﻭﻴﻥ ﻓﻲ ﺒﻴﺌﺔ ﺒﺤﻴﺭﻴﺔ ﻗﻠﻭﻴـﺔ ﻭﻤﻐﻠﻘـﺔ ﺨﻼل ﺘﺭﺴﻴﺏ ﺍﻟ ﺠﺯﺀ ﺍﻷﺴﻔل ﺒﻴﻨﻤﺎ ﺍﻟﻜﺎﻭﻟﻴﻥ ﻭﺍﻻﻟﻴﺕ ﺘﻌﺘﺒﺭ ﻓﺘﺎﺘﻴﺔ ﻨﻘﻠﺕ ﺒﻭﺍﺴﻁﺔ ﺍﻟﻨﻅﺎﻡ ﺍﻟﻨﻬـﺭﻱ ﺨﻼل ﻓﺘﺭﺍﺕ ﺍﻟﺘﺼﺭﻴﻑ ﺍﻟﻤﻔﺘﻭﺡ ﻓﻲ ﺍﻟﺒﺤﻴﺭﺓ.

Acknowledgements

I wish to express my gratitude and sincere appreciation to my supervisor Dr. Omar EL Badri Ali for his supervision, aid, guidance and encouragement with acumen and keen scientific in sight. The research is financed by the International University of Africa, which is greatly acknowledged. Thanks are also due to Geological Research Authority of Sudan (GRAS) and Wadies and Groundwater Coorporation for providing part of the data. Special gratitude and thanks to Prof. Farouk Ahmed the previous Head of the Geology Department (I.U. of A) who suggested the topic, guiding and encouraging. All thanks are due to Head of the Geology Department (Mr. Nur Alla) and my colleagues. Great thanks also to Prof. Ali Ahmed the Dean Faculty of Science (I.U. of A). Thanks to Dr. Dawoud the Head of Department, Dr. A. Sadig, Dr. O. Abdullatif for his reading the manuscript. Thanks are to Dr. Fathi El Bireir, Dr. Salah, Dr. Yasin and Dr. Ali for their help and kind support. Great thanks due to O. Mahjoub and Dr. Hassan the clinical assistant and all the Umm Ali village people due to their hospitality and cooperation during the field work. Thanks also extended to Mr. Ayad, Elgaili, Nur Eldeen, Ali Issawi, Abdelaziz, Zahir, Ibrahim, Salah, Ahmed Kamil, Hamadabi, Mogheera, Saba, Hana, (CPL geologists), Mutasim, Muhanad, Ali, and Hadbaa due to their help and encouragement. Last but not least, thanks are due to my family, whom has made much of this possible and whom have always supporting and helping.

List of Contents

Page Abstract ……………………………………………………. I Arabic Abstract ……………………………………………. IV Acknowledgments ………………………………………… VI List of Contents …………………………………………… VII List of Figures ……………………………………………... XI List of Tables ……………………………………………… XIII List of Plates ……………………………………………….. XIV Chapter One ١ ………………………………………………… Introduction

١ .……………………………………………… Location ١٫٢ ١ …………………… Physiography and drainage pattern ١٫٣ ٣ ………………………………………………… Climate ١٫٤ ٣ ………………………………………… Previous work ١٫٥ ٦ …………………………………Objective of the study ١٫٦ ٦ ……………………………… Methods of investigation ١٫٧ ٦ ……………………………………… Field methods ١٫٧٫١ ٦ ………………………………… Subsurface data ١٫٧٫٢ ٧ ………………………………… Laboratory methods ١٫٧٫٣ Chapter Two Regional geology and Tectonic setting ٢٫١

٩ ………………………………………… Introduction ٢٫١٫١ ٩ ………………………………… Basement Complex ٢٫١٫٢ ١٠ ……………………… Nubian Sandstone Formation ٢٫١٫٣

١٣ HudiChert Formation ٢٫١٫٤ ………………………………... ١٤ …………………………… Umm Ruwaba Formation ٢٫١٫٥ ١٤ Basaltic Lavas ٢٫١٫٦ ……………………………………….. ١٦ Superficial Deposits ٢٫١٫٧ …………………………………. ١٨ ..……………………………………… Tectonic Setting ٢٫٢ Chapter Three Surfaces and Subsurface Lithofacies Analysis ٢٠ Surface lithofacies analysis ٣٫١ ……………………………. ٢٠ Introduction ٣٫١٫١ ………………………………………….. ٢٠ ……………………………Description of the profiles ٣٫١٫٢ ٥٣ Facies type description ٣٫١٫٣ ………………………………. ٦٨ Bounding surfaces ٣٫١٫٤ …………………………………… ٦٩ Architectural elements ٣٫١٫٥ ………………………………. ٧٠ …………………………… Lithofacies associations ٣٫١٫٦ Discussion and Interpretation of Depositional ٣٫١٫٧ Environments ………………………………………. ٧٦ ٨٢ ………………………………………… Paleoclimate ٣٫١٫٨ ٨٣ …………………………………… Paleotopography ٣٫١٫٩ ٨٣ Sedimentary slump ٣٫١٫١٠ folds……………………………. ٨٦ Ferricrete ٣٫١٫١١ bands……………………………………... ٨٨ Channel architecture and facies depositional model ٣٫١٫١٢ ٩٤ Subsurface Lithofacies Analysis ٣٫٢ ………………………. ٩٤ Introduction ٣٫٢٫١ ………………………………………….. ٩٤ Profile description ٣٫٢٫٢ …………………………………… ١١٠ ...……… Interpretation of Depositional Environment ٣ .٣٫٢

Chapter Four Grain size analysis ١١٦ Introduction ٤٫١ ……………………………………………. ١١٦ ……………………………… Methods and procedures ٤٫٢ ١١٧ …………………………………… Analytical methods ٤٫٣ ١١٧ ..………………………………… Fine fraction analysis ٤٫٤ ١١٩ .…… Results and Interpretation of Grain-size Analysis ٤٫٥ ١١٩ Statistical parameters of the results ٤٫٦ ……………………. ١٢٥ Scatter plot diagrams ٤٫٧ …………………………………... ١٢٥ ..…………… Histograms and smooth frequency curves ٤٫٨ ١٢٥ Classification of the treated samples ٤٫٩ …………………... Chapter Five Heavy Minerals Analysis ١٢٩ ..………………………………………… Introduction ٥٫١ ١٢٩ .…………………………………………… Procedure ٥٫٢ ١٣٠ …………………………………… Slide preparation ٥٫٣ ١٣١ ……………………… Heavy Mineral Identification ٥٫٤ ١٣١ .……………………………………………… Results ٥٫٥ ١٣٥ .……………………… Discussion and Interpretation ٥٫٦ Chapter Six Sandstone Petrography ١٤٠ ..………………………………………… Introduction ٦٫١ ١٤٠ ..…………………………… Thin section preparation ٦٫٢ ١٤١ ..…………………………… Thin section description ٦٫٣ ١٤٢ .……………………………………………… Results ٦٫٤ ١٤٣ ……………………… Discussion and Interpretation ٦٫٥ Chapter Seven Clay Minerals Analysis ١٥٣ ………………………………………… Introduction ٧٫١ ١٥٣ .…………………………………………… Procedure ٧٫٢

١٥٥ ……………………… Identification of clay minerals ٧٫٣ ١٥٥ ..……………………………………………… Results ٧٫٤ ١٥٦ ……………………… Discussion and Interpretation ٧٫٥ Chapter Eight Clay Geochemistry ١٦٢ .………………………………………… Introduction ٨٫١ ١٦٢ .………………………………… Geochemical analysis ٨٫٢ ١٦٢ Spectrophotometric method for determination of ٨٫٢٫١ ..…………………………………………… and AlO٢ SiO٢ ١٦٥ Atomic absorption spectroscopy for detection of ٨٫٢٫٢ other major oxides ………………………………………… ١٦٦ Determination of loss on ٨٫٢٫٣ ignition…………………….. ١٦٧ .……………………………… Results and Discussion ٨٫٣ Chapter Nine Summary and Conclusion ……………………………….. ١٧٢ ١٧٨ .………………………………………………… References ١٩٢ ………………………………………………… Appendices

List of Figures

Figure Page ٢ ………………………Location map of the study area :(١٫١) ٨ ...………………………………General plan of work :(١٫٢) ١٥ ……………………Geological map of the study area :(٢٫١) Geological sketch map showing Cretaceous rift basin :(٢٫٢) ١٩ ...…………(١٩٩٨ ,and subbasins in Sudan (Bussert ٢٢ ..…………Profiles location map from the study area :(٣٫١) ٢٤ .………Lateral profiles collected from the study area :(٣٫٢) ٣٣ .……Vertical profiles collected from Bagrawyia area :(٣٫٣) ٤٨ ..… ..……Vertical profiles from around Umm Ali village :(٣٫٤) ٥٦ ..……………………Vertical profiles from Fadnyia area :(٣٫٥) Histogram showing relative facies abundance from :(٣٫٦) ٦٧ the study area…………………………………………….. Lateral correlation and variation in depositional :(٣٫٧) ٨١ ………………………………………………… .systems Lateral correlation of vertical profiles from the both :(٣٫٨) ٨٤ .....…………(sides of the road cut (slumped strates Lateral profile along the road cut showing part of the :(٣٫٩) ٨٥ .………………(slumped stratas (near by Umm Ali village Eh-PH diagram of physco-chemical condition for :(٣٫١٠) the stability fields of sedimentary ions. After ٨٩ ..…………………………………(١٩٧٩) Krauskopf Typical fluvial lithofacies assemblage and vertical :(٣٫١١) ١٩٧٨b) showing ,١٩٧٧ profiles (adapted from Miall proposed channel styles for the Upper Shendi ٩١ ..…………………………………………Formation

Generalized facies depositional and model for the :(٣٫١٢) ٩٣ .…….……………………………………study area ٩٥ …………………………Borehole profiles location map :(٣٫١٣) ٩٦ ..… ...…………………Umm Ali borehole location map :(٣٫١٤) ٩٧ ...... ………………Borehole profiles from the study area :(٣٫١٥) Histogram showing relative facies abundance of the :(٣٫١٦) ١١١ ……… .…………………………lower Shendi Formation Histogram showing relative facies abundance within :(٣٫١٧) ١١١ ...…………subsurface sediments around Umm Ali village Fence diagram showing coarsening –upward of :(٣٫١٨) ١١٥ …………………………………… .lacustrine sequence Cumulative frequency curves for the Bagrawyia :(٤٫١) ١٢٠ ...…………………………………………………samples Cumulative frequency curves for the Umm Ali :(٤٫٢) ١٢٢ …….…………………………………………… .samples Cumulative frequency curves for the Fadnyia :(٤٫٣) ١٢٣ .………………………………………………samples ١٢٦ .……………(١٩٦٧ ,Scatter plot diagram (After Friedman :(٤٫٤) Triangular diagrams showing the Nomenclature :(٤٫٥) ١٢٨ ...... classification of the sample in the studied Formation

Histogram showing heavy mineral percentage of the :(٥٫١) ١٣٢ ..……………………………………Shendi Formation Histogram showing percentage of ZTR index in the :(٥٫٢) ١٣٢ ..……………………………………………study area ١٥٢ ,Estimated provenance of quartz (Folk :(٦٫١) ..……………(١٩٨٠

List of Tables

Table Page Stratigraphic table for central northern and eastern :(٢٫١)

١٧ ...………(١٩٩٨ ,Sudan since upper Jurassic (Bussert ٧١ .Architectural element analysis of Shendi Formation :(٣٫١) Surface lithofacies types, characteristics, geometry :(٣٫٢) ٧٨ ……………………and depositional environments Subsurface facies types, characteristics and :(٣٫٣) ١١٣ ..……………………………approximate geometry ١٣٨ ...………Heavy mineral percentage and ZTR index :(٥٫١) Visually estimated and semi-quantitative of :(٦٫١) ١٤٤ …………………………………sandstone samples Clay mineral percentage of some Shendi Formation :(٧٫١) ١٦١ ……..……………………………………………samples Geochemical analysis of major oxides from the :(٨٫١) ١٦٨ ………….………………………………analysed samples

List of Plates

Plate Page ٥٨ .Facies types and characteristics of Shendi Formation :١ ٦٠ .Facies types and characteristics of Shendi Formation :٢ ٦٢ .Facies types and characteristics of Shendi Formation :٣ ٦٤ .Facies types and characteristics of Shendi Formation :٤ ١٤٠ .Some heavy mineral types of Shendi Formation :٥ ١٤٩ .Shendi Formation sandstone types and characteristics :٦ ١٥١ .Shendi Formation sandstone types and characteristics :٧

Chapter One Introduction :Location ١٫١ The study area is situated in Shendi province, River Nile state km north of Khartoum, bounded by the ٢٥٠ Northern Sudan. It is about River Nile from the west, Wadi El Mukabrab from the north and Wadi El square kilometers ٤٩٠٠ Hawad from the south. It covers an area of about ٣٤o to -٣٠ ٣٣o N and ٣٠ ١٧o to ٤٥ ١٦o and defined by coordinates of .(٢٫١&١٫١ .E (Fig ١٥ :Physiography and drainage pattern ١٫٢ A distinctive physiographic feature is represented by the appearance of the study area as almost a regular circular shape with dark tone on the photographs surrounded by the drainage patterns. In details the southern Bagrawyia area is characterized by the numerous occurrences of isolated sedimentary outcrops and a series to the east. From Bagrawyia to the north a gap of flat lying area is crossed by local wadies, up to around Umm Ali village where high elevated isolated Jebels appear once against. A few kilometers to the north , these Jebels form a flat lying and northerly extended sedimentary plateau. The rock units generally dip smoothly and gently to the north at Bagrawiya and Umm Ali areas but at the northern area they may have a flat lying tops. The drainage pattern in the area includes River Nile flowing north wards and seasonal major wadies such as El Mukabrab and El Hawad to the north and south of the areas respectively. These flow westerly to the Nile, while Wadi Abu Harik and Kahati at the east of the area running parallel to the Nile confluencing with Wadi El Mukabrab at the extreme NE of the area.

Minor dendretic seasonal water courses such as Wadi El Dan, Diayga, Terabyia and El Eish dissect the plateau and drain towards the Nile. :Climate ١٫٣ Arid to semi arid climate prevails in the area. The rainfall is rare and vegetation is represented by thorny trees mainly lining the wadies. The winter period extended from November to March with minimum ١٠oC. The summer period with maximum temperature of temperature of ٤٥oC follow the winter up to mid July where rainfall season begins with mm/y. September and October are generally ١٠٠-٥٠ an average of about hot during the day, warm at night with sometimes dust storm till November where the winter return back again.

:Previous work ١٫٤ Shendi Formation received considerable attention and studied by identified the (١٩٦٦) some geologist with different interests. Kheiralla (١٩٧١) quartzose sandstone around Shendi, subsequently Whiteman mapped the area as Shendi Formation for its type locality. studied Shendi Formation as a part of the so-called (١٩٨٣) Omer Nubian Sandstone Formation. He described its occurrence as gently dipping strata of sandy clayey nature and ferrigenous crust deposited in a graben structure by braided channel of medium to low energy. He certained the absence of the conglomerate and enrichment of lutite as distinctive remark. Moreover, he mentioned that the paleocurrent direction is mainly to the north and north east, but in the northern part of the area the paleoflow is reverced to the south and south east. Furthermore, he pointed out that two basins were created by pre- Cretaceous tectonic pattern; the eastern basins contains coarser sediments

derived from Butana plain and the western one with finer material brought in from south west. supported the idea of two basinal pattern from the (١٩٩٣) Ibrahim gravity anomaly at the area east and west of the Nile, attributing the distinctive abnormal positive Bougeoer anomaly to the widespread occurrences of ferricrete bands. ,studied the occurrences of some evaporites (halite (١٩٧٢) Madani gypsum and caliche) at Shendi and Omdurman Formation and suggested a hot dry climate prevailed during deposition of part of the Nubian sandstone in Sudan. studied the accumulation of lateritic (١٩٩٠) ,Germann et al weathering products (Kaolin, bauxitic ironstone and ironstones) in sedimentary basins of the northern Sudan. He attributed that to the limited tectonic activity together with warm humid climate that prevailed during the Mid-Cretaceous followed by period of tectonic instability which dispersed the weathering products to the depositional areas. identified fresh water vertebrate faunal assemblage (١٩٩٣) Werner collected from Shendi Formation indicating warm, wet biotopes with dense vegetation. He showed that there is a striking similarities of non- marine Upper Cretaceous African terrestrial vertebrate association which characterize the flood plain, swamps, small ponds and lakes. studied Shendi Formation and subdivided it (١٩٩٣،٩٥) Abdullatif into upper fluviatile and lower lacutrine deposits. The later consists of light grey bentonitic mudstones. studied the Sudanese intracratonic rift basins on the (١٩٩٧) Salama basis of geophysical survey, delineating NW-SE trending Cretaceous sediment troughs and mentioned that Wadi El Mukabrab in the northern part of the study area represents fault boundary.

studied the intracratonic basins evolution in the (١٩٩٨) Bussert central northern Sudan showing the contrast with typical cratonic sandstone sheets of north Africa which are dominated by uniform braided river sandstones. The sedimentary sequences of the northern Sudan show more pronounced lateral facies differentiation and contain higher amount m of ٣٠٠ of pedogenic, lacustrine and eolian sediments embracing up to the exposed Cretaceous sediments (Shendi Formation). During the latest Albian times the syn-rift phase was followd by thermal –sag phase and the basins expanded their arial extent beyond the graben structure, the outcropping sediments in the central northern Sudan deposited during this phase of basin evolution. Bussert (op,cit) also mentioned that Shendi- Atbara basin is a continental basin with internal drainage. Proximal areas were dominated by braided rivers with an increasingly meandering character that developed down stream and deposited their load in the basin centre within extensive floodplains, lodging playa lakes and swamps. investigated Shendi Formation as a part of the (١٩٩٧) Mohamed Nubian Sandstone Formation in Sudan. He studied the technical properties of bentonitic mudstone characteristics and industrial potentiality and suggested that the exploitation of these bentonitic mudstones as drilling mud seems to be technically and economically feasible. studied the sedimentology and engineering (٢٠٠٠) Osman properties of kaolintic mudstone at Umm Ali and Omdurman areas so as to construct and establish their origin and paleoenvironment to assess their technical properties and potentiality in refractory bricks and commenced their fair suitability. conducted a comprehensive work on the (٢٠٠٣) Mohamed occurrence, origin and industrial potentiality of kaolin deposits in

Central-Northern Sudan. He concluded that the kaolin is of high potential value.

:Objectives of the study ١٫٥ The study area is characterized by both bentonitic and a tanstein mudstones as well as ironstone deposits. The main aim of this work is to construct a conceptual model describing the depositional environments, possible source areas and paleogeography of Shendi Formation. This aim will be reached through the following: .Facies analysis .١ .Grain size analysis .٢ .Heavy mineral and Petrography .٣ .Clay mineralogy and Geochemistry .٤

:Methods of investigation ١٫٦ :Field methods ١٫٦٫١ During the field work adequate lateral and vertical profiles were carried out, described and classified into several lithological units on the basis of composition, grain size and sedimentary structures. The lateral profiles were photographed and mapped so as to delineate the bounding surfaces and architectural elements. :Subsurface data ١٫٦٫٢ Borehole lithological data were collected from Wadies and Groundwater Corporation and Geological Research Authority of Sudan profiles so as (١٤) GRAS). These boreholes were plotted and arranged in) to be classified into different facies units on the basis of lithology, grain size and colours.

:Laboratory methods ١٫٦٫٣ In order to study the obtained samples different laboratory techniques were carried out. These techniques include grain size, heavy minerals analysis, petrography, x-ray diffraction (XRD) for clay minerals and atomic absorption illustrate the (١٫٢) .spectroscopy (AAS) to determine the chemical composition. Fig plan of work.

Study area

Field work Laboratory work

Surface data Subsurface data Grain size analysis Heavy minerals analysis Petrographic analysis

Clay mineral analysis Geochemistry Lithofacies Analysis

Interpretation of the results

Model

.General plan of work :(١٫٢) .Fig

Chapter Two Regional Geology and Tectonic Setting

:Regional Geology ٢٫١ :Introduction ٢٫١٫١ The study area is entirely covered by Shendi Formation. Nevertheless, basement rocks have been exposed a few kilometers south of Wadi El Mukabrab juxtaposed to the west side of the a sphalt road to Atbara, where the area has shown scatter occupation of HudiChert on a peneplain .(١٩٨٨ mapped as Umm Ruwaba sediments (Roperson research have (٢٫١ In general the regional stratigraphic units in the area (Fig been arranged chronologically as follows: .Superficial deposits -٦ .Basaltic lavas -٥ .Umm Ruwaba Formation -٤ .HudiChert Formation -٣ .Nubian Sandstone Formation -٢ .Basement Complex -١

:Basement Complex ٢٫١٫٢ Shendi Formation is an outlier surrounded by the pre-Cambrian and Paleozoic crystalline rocks. South of Shendi, the basement is exposed at Sabaloka area; Butana plain extend to the east and Bayuda desert to the north and west. The Sabaloka basement complex is predominantly composed of grey gneisses with patchs of granulite and chronkite of pan-Africa age It was later intruded by poorly foliated .(١٩٨٨ ,Dawoud and Sadig) granitic rocks and post-orogenic younger granite, including ring complex .(١٩٧٧ ,١٩٧١ Almond)

To the east and south east, the Butana basement crop out comprising multi-metamorphosed quartzo-feldspathic to granitic high- grade gniesses and more extensive low-grade rocks mainly volcanogenic green schist which are locally intercalated by thin quartzite and marble bands. The post tectonic igneous suite is largely made up of granite and syenites exposed as isolated hills. Few mafic-ultramafic ridges represents an abducted mass belonging to the ophiolite zone of the eastern Sudan. .(١٩٨١ Son, et al) Basement rocks are exposed at Bayuda bounding Shendi Formation. In the western side the rocks are predominated by heterogeneous and poly-metamorphosed high-grade gneiss and migmatities interlayered with psammitic and pelitic metasediments ;١٩٧٥ ,introduced with syn, late and post tectonic granitoids. (Elrabba In the northern side, the above mentioned high-grade .(١٩٨٠ ,Dawoud Nile Craton has been structurally overlained by Nubian shield. The Nubian shield generally consist of Neoproterzoic volcano-sedimentary- ophiolitic assemblage metamorphosed as low-grade green schist facies and introduced by orogenic, unorogenic batholithic, plutonic dominantly .(١٩٨٣ ,of granodioritic, dioritic and granitic composition (Vail

:Nubian Sandstone Formation ٢٫١٫٣ to designate (١٨٣٧) The term “Nubian Sandstone” was introduced by Russegger

Paleozoic and Mesozoic sandstone of Nubian desert, south of Egypt. Since that

time a hot debate around the term has been risen. In general, according to Mckee

two main trends have been developed with many variation in details. The ,(١٩٦٣)

first trend is to restrict the name Nubian to the sandstone of Cretaceous age. The

second trend is to consider it a facies term and to apply it wherever sandstone

having certain general characteristic occur, regardless of age. These can include all

sandstone complex from Middle Cambrian to Cretaceous and even younger

(١٩٦٣) Furon & (١٩١٢) Ball ,(١٩٠٢) In this context, Blankenhorn .(١٩٤٣ ,Picard)

subdivided the Nubian sandstones into three units with the upper unit being the

true Nubian sandstone sensostricto of Upper Cretaceous age. The above

subdivision have been recently adopted and considered by Klitzsch and Squyres

as the three megacycles which the upper, so called, Nubian cycle of late (١٩٩٠)

Jurassic-Tertiary age. It is composed of interbedded fluvial and near shore facies in

NW Sudan. Shendi Formation belongs to this cycle and is entirely composed of

continental fluvial and lacustrine facies.

The Nubian sandstone, in it’s type locality, south west of Egypt (Bargat El Shab), is entirely composed of lenticular basal conglomerate followed by immature quartzitic sandstone topped by thin shale crust. It is characterized by cross-bedded structures with localized horizontal bedding and most common horizontal type of bounding surface overlying the basement and Upper Cretaceous marine strata. This surface is largely controlled by underlying surface of pre-Nubian northerly sloping shelf .(١٩٧٣;Issawi ;١٩٦٢ ,Mckee) used the term “arena delle Nubia” to designate siliceous (١٩٣٥) Desio sandstone in Libya of Devonian and Carboniferous age. Consequently he suggested that the term “Nubian sandstone” should be used to designate a facies and not a formation. Sanford .(١٩٠٩) The term “Nubian series” was first used by Beadnell Karkanis ,(١٩٦٣) Rodis, Hassan, Wahadon ,(١٩٤٨) Andrew, et al :(١٩٣٥) adopted the term and applied it to rocks in NW Sudan replacing (١٩٦٥) the term Nubian sandstone. This is to designate siliceous conglomertic sandstone with substantial clays of Upper Cretaceous age and recommended that the term “NST” should commonly be used to describe

a facies and therefore has no stratigraphic value. Subsequently the term “Nubian series” has been abandoned for the incorrect use of the term “series” which has a chronostratigraphic value and shouldn’t be used in a .(١٩٧١ ,lithostratigraphic sense (Whitman suggested that Nubian sandstone should be used as a (١٩٦٢) Said formation name for the rocks having similar lithologies to those of the Shukri ;(١٩٠٦) type area in Nubian. In this respect Said followed Hume .(١٩٤٥) and Shukri (١٩٤٦) and Said used the term (١٩٧١) and Whitman (١٩٦٦) In Sudan Kheiralla “Nubian sandstone” to designate the sediments of variegated colours lying unconformable on the basement and Paleozoic Formation comprising from conglomerates, sandstone, mudstones and overlain unconformably by HudiChert, Tertiary basaltic lavas or Quaternary sediments. recognized in the (١٩٦٦) On the lithological ground, Kheiralla Shendi-Khartoum area the following lithological types: :Conglomerates include -١ a- Pebble conglomerate. b- Intraformational conglomerate. :Sandstones include -٢ a- Merkhyiat sandstone. b- Quartzose sandstone. .Mudstones -٣ Kheiralla (op.cit) also differentiated quartzose sandstone for its non- pebbly, medium to fine grained, well sorted, poor degree of rounding and small clay silt fraction suggesting that might have been deposited slightly in different environment.

The above classical lithology quartzose sandstone type of Kheiralla for (١٩٧١) has been renamed as Shendi Formation by Whitman (١٩٦٦) it’s type locality in the Shendi district. described Shendi Formation as continental part of the (١٩٨٣) Omer Nubian sandstone sediments of clayey-sand nature where the absence of the conglomerate and the abundance of clays which were deposited at the centre of the basin are distinctive remark. Shendi (١٩٩٧) Mohammed ,(١٩٩٥) According to Abdullatif Formation is a continental sediments deposited in a fluviatile, fluvio- lacustrine and lacustrine environment. described Shendi Formation as being predominated by (١٩٩٣) Bussert fine-grained clastic of braided-meandering to meandering river, floodplain and minor lacustrine. suggested that (١٩٨٣) On the basis of the fossil wood from Umm Ali area Omer

;(١٩٩٠) Shendi Formation is of Cretaceous age. On the other had Wyeisk, et al

considered that Shendi Formation is a time equivalent to (١٩٩٠) Awad and Shrank

Omdurman and Wadi Milk Formation all being Albian-Cenomanian age (Table

.(٢٫١

:HudiChert Formation ٢٫١٫٤

HudiChert Formation is represented by loose fragments, boulders and cobbles are scattered, particularly north of the study area with increasing recorded (١٩٦٥) distribution to the north on flat vast areas. Karkanis insitu HudiChert capping hills west of the Nile in the Zeidab area and east of Sabaloka Jebels north of Khartoum. The type locality of the HudiChert is in a Wadi near Hudi station a few kilometers NE of the study area.

The boulders are mostly irregular ellipsoidal in shape, yellowish, grey whitish, brown, or pink in colour with cavities and pitted surface. They owe their bouldery appearance to weathering in the intensely hot climate at edge of the Savanna desert. The age of the HudiChert is not strictly .it’s of lower Tertiary (١٩٣٢) isochronus. Nevertheless, according to Cox However, there is strong probability that they are either of upper Eocene or lower Oligocene age and may have been deposited in shallow lakes .(١٩٧١ ,and Whitman ١٩٦٢ ,Berry)

:Umm Ruwaba Formation ٢٫١٫٥ The Umm Ruwaba sediments are exposed north of the study area juxtaposed to the south and north of Wadi Elmukabrab as pale dark grey (١٩٤٥) heavy clays with grits and gravels. Andrew and Karkanis proposed the term “Umm Ruwaba series” for thick superficial deposits that occur in central Sudan plain west of Kurmuk-ingessana-Moya ridge and surround the Nuba Mountains. the Umm Ruwaba (١٩٧١ in Whitman ١٩٤٨) ,According to Andrew sediments consist of unconsolidated sands, gravels, clayey sand which might have been deposited in a fluviatile and lacustrine regime, and are probably of Tertiary to Pleistocene in age.

:Basaltic Lavas ٢٫١٫٦ Tertiary basaltic lavas pierced the Nubian sandstone and occurs in so many places away from the study area. South of the Khartoum, Tertiary as well (١٩٤٨) Andrew ,(١٩٦٦) erruptions has recorded by Kheiralla also recorded them in ,(١٩٦٦) exposed rocks at J. Toria. Qureshi, et al some boreholes in Omdurman area. Basaltic lavas are also scattered in different places west of the valley such as at J. Umgulua, the lower part of Wadi Elmugaddum, J. Umm

Rihana and Glat Um Tudub, Hummar basin, other places at Bayuda desert, Gadaref and J. Mara. These basalts in Sudan may have been closely related to the updoming and extension associated with the .(٢٠٠١ ,formation of rift basins (Saad

:Superficial Deposits ٢٫١٫٧ The superficial deposits includes Wadies and Khor deposits which caurses the Jebels, recent fan deposits that emerged from the outcrops and consist of poorly sorted sediments redeposited from pre-existing sedimentary boulders, fragments and leached coarse and fine sediments. South to Bagrawyia area numerous mobile sanddunes (Qoz) consists of well sorted medium to-fine-grained sand, are covering the underlying Shendi Formation and extend to the east and north east to the river Atbara boundary. Substantial accumulation of Nile silt occurs as superficial deposits on both banks of the River Nile. They consist of very fine micaceous sands and silt that were deposited annually during the flood seasons.

:Tectonic Setting ٢٫٢ During the late Proteriozoic the Central Africa Shear Zone was initiated. It can be traced from Cameron trough of Central Africa, Chad to North Kordofan in Central Sudan and probably further into the Red Sea in NE Sudan representing one of the major shear zones of lithosphere Therefore, the Central .(١٩٨٧ weakness in Africa (Schandelmeier, et al Sudan lies in the eastern part of the Central Africa Rift System which extended from Benue Trough in Nigeria to the Atbara Rift in the eastern .(١٩٨٣ Sudan (Browne and Fairhead Along the Central Africa Shear Zone a series of NW-SE trending transtensional basins developed in response to intermittently reactivated

Jorgenson and ;١٩٨٧ pre-Cambrian discontinuities (Schandelmeier, et al .(١٩٨٩) Bosworth On the basis of geological and geophysical investigations, Bussert confirmed the existence of several deep (١٩٩٠) Wyeisk et al ;(١٩٩٣) et al ٢km) graben and half-graben structures. These structures, are located<) north of the central and northern Sudan rift segments within the region of Schandelmeier and ;١٩٩٣ the Central African Fault Zone (Bussert, et al .(٢٫٢) .Fig (١٩٩٠ Pudlo recommended that Shendi-Atbara sub-basin was (١٩٩٨) Bussert formed as isolated half-graben structure during upper most Jurassic to lower most Cretaceous time. At the beginning it was formed during north eastern extension to the west and Central Africa Rift System. This was followed by thermal-sag-phase and the basins expanded their areal extent beyond the limits of the graben structures. This outcropping sediments in the area represents the period of the basin evolution.

Chapter Three Surface and Subsurface Lithofacies analysis

:Surface Lithofacies Analysis ٣٫١ :Introduction ٣٫١٫١ This part of the study is based on the field work, where vertical and lateral profiles are arranged to delineate the sedimentary facies according to their lithology, structure and sedimentary textures. The bounding surface of each facies has been identified so as to built up their architectural elements and characterize the paleodepositional environment. Such elements will help to reconstruct the paleogeography as well as paleoenvironment and facies depositional model. Thirty three lateral and vertical profiles were examined from different localities, south of Bagrawyia, around Umm Ali village the centre of the study area and northern Fadnyia area with possible lateral correlation along the strike of the basin. The lateral profiles were photographed, mapped and described here. The localities of the vertical .(٣٫١) and lateral profiles are shown in Fig

:Description of the profiles ٣٫١٫٢ :(١) Profile a) consists from bottom to top, trough cross-bedded.٣٫٢ .This profile (Fig sandstone facies (St) overlain by horizontally bedded reddish to yellowish medium grained sandstone facies (Sh) with internal intensive rippled sandstone facies (Sr). ٤m in thickness suggest almost on upper flat bed from vertical accretion ,(Facies (Sh and weak traction. The facies (St) suggest architectural element (SB) of distal shallow braided stream sandy bedform. :(٢) Profile This profile represents coarsening-upward cycles from lower poorly exposed massive mudstone facies (Fm) at the bottom fine

laminated sandstone, siltstone and mudstone facies (Fl). The later is iron (٠٫٢٥m thick) rich ferricrete band, highly bioturbated and concretionary to trough cross-bedded medium sandstone facies (St) revealing erosional flatlying with slightly convex-up, fourth order bounding surface (architectural element CHs). The facies (St) is overlain by massive sandstone facies (Sm) which is reddish to brownish in colour. The lower cycle of facies (Fm) and (Fl) probably represents lake sequence wherever the upper cycle may represent shallow braided channel deposit. :(٣) Profile -c) consists of lower and upper coarsening.٣٫٢ .This profile (Fig upward cycles separated by thin ferruginous crust of poorly laminated ٠٫٢m thick. This crust (fine sandstones siltstone and mudstone facies (Fl is characterized by the present of some concretions and bioturbation structure. The lower cycle composed of interbedded massive mudstone ٠٫٥m thick, containing gypsum and ,facies (Fm) light grey in colour carbonate fractions. It is overlain by massive fine sandstone facies (Sm), ٠٫٥m ,(٠٫٧m thick. In turn overlain by massive grey mudstone facies (Fm thick. These lower unit assemblage seems to be laterally traceable. The upper cycle is composed of medium grained trough cross-bedded ١٫٥m in thickness with erosional evidence of sandstone facies (St). It is mudclast and flatlying fourth order bounding surface (architectural element (SB). This is overlain by massive medium grained sandstone facies (Sm). The lower cycle is probably of lacustrine sequence whereas the upper may represents shallow braided channel deposits. :(٦) Profile d) consists of sandstone facies. At the base it.٣٫٢ .This profile (Fig is yellowish in colour, medium-grained planar-cross bedded sandstone m thick. It is overlain by medium to fine yellowish rippled ٧٫٥ facies of ٥m thick. The later is overlained by (cross-laminated sandstone facies (Sr

planar cross-bedded medium grain sandstone facies (Sp). (Sp) facies is characterized by inlined third erosional bounding surface with architectural element (LA) may represents point bar deposits (Nami and .(١٩٧٨ ,Leader :(٧) Profile e) consists of sandstone facies. At the base it.٣٫٢ .This profile (Fig is medium-grained trough cross-bedded sandstone facies (St). This is overlain by horizontally bedded sandstone facies (Sh) which laterally changed to trough cross-bedded facies (St). The later has inclined third ٨m thick and order erosional bounding surface with mudclast. It is intensively rippled upward. Such bounding surface characterize lateral accretion element (LA)point bar sequence. :(٨) Profile m thick laterally traceable ٢ f) consists of.٣٫٢ .This profile (Fig fine laminated sandstone, siltstone and mudstone facies (Fl). It is overlain by massive to horizontally bedded medium grained brownish sandstone (١٫٥m thick facies (Fl ٢m thick. The outcrop capped by facies (Sh) of resembles iron rich ferricrete band. The horizontally bedded sandstone ٣rd order erosional surface (element facies (Sh) represent lateral accretion LA) and may characterize point bar in meandering channel style, where the lower part facies (Fl) with element (FFP) represent almost fluvio- lacustrine intercalated facies. :(٩) Profile ١m ,g) is composed of lower.٣٫٢ .The section of this profile (Fig thick, massive grey mudstone facies (Fm) overlain by well laminated reddish brown siltstone facies (Fsm). This is overlained by massive m thick and dominate the profile ٢٫٥ (mudstone lightish grey facies (Fm area. The profile is capped by poorly laminated bioturbated and concertionary fine sandstone, siltstone and mudstone facies (Fl) which is

iron rich ferricrete band. The profile represent coarsening –upward cycle, most probably characterizing deposition within deep lake facies (Fm) and lake shore facies (Fsm) and (Fl) having architectural elements of (FFD) and (FFP) respectively. :(١٠) Profile h) consists of fluvial facies sequence. At the .٣٫٢ .This profile (Fig base it is whitish kaolinitic medium-grained trough cross-bedded m thick, overlain by yellowish colour ١ sandstone facies (St). This is ٠٫٧m thick and in turn is (planar cross-bedded sandstone facies (Sp overlain by partially un exposed trough cross-bedded facies (St) representing concave-up ersional fifth order bounding surface. This is ٠٫٥ ٠٫٨m thick and followed by facies (Sp). The profile ended with about m thick fine laminated sandstone, silstone and mudstone facies (Fl) of probable overbank deposit. The assemblage represent architectural element (CHs) of shallow braided channel deposit. :(Wadi El Dan) ١٢ Profile This profile is composed of light grey yellowish and reddish up- of the section ٪٣٩٫٧ ward massive mudstone facies (Fm) forming about m. The mudstone is overlain by thin ٥٠٠ area and laterally traceable up to iron crust of brownish colour. The massive mudstone facies has an architectural element (FFD) of probably lacustrine origin whereas the iron crust may represent lake shore facies of coarsening upward units. :(١٣) Profile Two facies types are revealed here; trough cross-bedded sandstone facies (St) underlain by poorly laminated, highly bioturbated fine sandstone, sillstone and mudstone facies (Fl). The later is dark brown iron (m thick facies (St ١ rich crust un exposed to the foot. The upper unit is reddish to yellowish in colour medium grained representing erosinal third order bounding surface with channel basal lag deposit. Architectural

element (LA) is proposed characteristic of point bar of meandering style where facies (Fl) represented lake shore deposit. :(٢ J. Umm Bour) ١٥ Profile This profile represent coarsening and thickening-upward cycle from massive mudstone facies (Fm) and fine laminated facies (Fl) to ١٫٥ massive sandstone facies (Sm). The massive mudstone facies (Fm) is It is .(٤ photo ٣ m thick, grey in colour with cracks filling gypsum (Plate m thick developed fine laminated sandstone, siltstone and ١٢ overlain by mudstone facies (Fl). The later is friable and poorly cemented yellowish, reddish whitish in colour. The lower sequence is terminated by eroisonal lateral accretionary third order surface showing channel basal lag of intraclast and having (LA) architectural element. This upper unit characterizes point bar sequence whereas the lower units represents lake facies. :(lower part of J. Surruji) ١٦ Profile The lower part of J. Surruji represents a coarsening and thickening- upward cycle starting by grey massive mudstone facies (Fm) at the base. ١m thick massive poorly This facies is overlained by pale yellowish laminated fine sandstone facies (Sm) passing upwards into sharp inclined lateral accretion third order erosional bounding surface and planar cross- bedded sandstone facies (Sp). The later is dominated the profile area and composed of pale yellowish medium grained with (٪٥٣٫٣) architectural element (LA) point bar of meandering stream. The lower unit is probably of lacustrine origin. :(٢ J. Umm Bour) ١٧ Profile The profile represents coarsening upward cycle from massive (٪٨٩٫٣) yellowish grey mudstone facies (Fm), dominating the profile area ٠٫٢٣m) of laminated) and laterally traceable. It is overlain by thin bed siltstone facies (Fl). The later is iron rich ferricrete band, bioturbated and

concretionary. The massive mudstone facies (Fm) is probably of deep lake deposit, whereas the fine laminated siltstone may represent lake shore facies. :(Roadcut) ١٨ Profile This profile is composed of interbedded massive mudstone facies (Fm) and massive medium to coarse grained sandstone facies (Sm) of coarsening upward poly cycles. The facies (Fm) varied in colour from grey to pale grey at the base of the profile to yellowish at the top. The interbedded sequence is laterally traceable and slumped. The profile is topped by fine sandstone, siltstone and mudstone facies (Fl). The later is black in colour, iron rich ferricrete with oolitic structure and highly bioturbated. The coarse fraction dominated the profile area where the sand/mud ratio These tabular oriented sandstone bodies are bounded with fouth order .١ to ١٫٦ reach surface and architectural element (CR) representing crevasse deltaic distributary channels whereas the mudstones facies (Fm) may represent inter distributary bays, flood plain and small lakes. The facies (Fl) at the top of the sequence is probable represent over bank subenvironment. :٢٠ Profile This profile reflects also coarsening-upward cycles from massive mudstone facies (Fm) yellowish, pale reddish grey in colour dominating overlain by poorly laminated fine siltstone facies ,(٪٥١٫٥) the profile area (Fl). The later is iron rich ferricete band and shows slightly slump structures at the base. The iron bed is highly bioturbated and rooted. This sequence probably deposited within deep lake to lake shore sub environment. :٢١ Profile ١٫٦m thick This profile is entirely composed of fluvial facies from planar cross –bedded sandstone facies (Sp) overlain by trough cross- m thick with third order ١٫٢ bedded sandstone facies (St). The later is erosional bounding surface and architectural element (LA) characterizing point bar of meandering style. The sequence terminated by horizontally .m thick ٠٫٣ (bedded sandstone facies (Sh

:J. Mazmum ٢٥ Profile m thick ٢٫٣ The profile represents coarsening-upward cycle over m fine laminated sandstone ٠٫٣ from massive mudstone facies (Fm) to siltstone facies (Fl) overlain by massive sandstone facies (Sm). The later with erosional lateral accretion third (٥٧٫٦) is dominating profile area order surface and channel basal lag interclast of architectural element (LA). The (Sm) facies is yellowish reddish in colour, whereas (Fl) facies is whitish yellowish friable to pale grey in colour. The lower unit is probably of lake shore whereas the upper is fluvial point bar accretion. :٢٦ Profile This profile is entirely composed of well laminated siltstone and mudstone facies (Fsm). It is pale reddish, yellowish and whitish in colour. Near the top a thin bed of massive mudstone facies (Fm) observed. The ٨m and latterly traceable. This maximum thickness of the exposure is facies probably represent transitional fluvio –lacustrine lake shore facies where weak traction current developed the laminated structure. :(A,B) ٢٧ Profile This profile consist of poorly trough cross-bedded sandstone facies m thick channel, poorly sorted medium to coarse grained ٢٫١ St). It is) and reddish in colour. It is overlain by stacked channel trough cross- bedded sandstone facies (St) with erosional concave-up fifth order bounding surface. The later channel basal lag deposit here is angular to subangular fragments. The later is overlain by planar cross-bedded sandstone facies (Sp), poorly sorted with dispersed minor grits and covers The channel base is characterized by .(٪٣٩٫٦) of the profile area architectural element (CHs) of shallow braided channel. :٢٨ Profile This profile represented by trough cross-bedded sandstone facies m sedimentary cover. It is ٦ St) which is thickly bedded but burried with)

poorly sorted with angular to subangular fragments and friable reddish in colour. This is overlain by planar cross-bedded sandstone facies (SP) and erosional channel basal lag having extraclast and concave up fifth order rank. The architectural element (CHs/SB) is characteristically shallow braided stream deposit. The facies (Sp) is overlain by horizontally bedded ٢m thick, and the profile is terminated by sandstone facies (Sh) which ٠٫٨m thick fine sandstone and siltstone facies (Fl). The later iron rich and bioturbated over bank deposit. :٢٩ Profile This profile is dominated by trough cross-bedded sandstone facies (St) -from the profile area, reddish brown in colour, medium to coarse ٪٧١٫١ comprising grained with basal lag pebbles. This is overlain by planar cross-bedded sandstone .(m thick, characterized by architectural element (CHs/SB ١٫٣ facies (Sp) which is :٣١ and ٣٠ Profile The profile is characterized by trough cross-bedded sandstone of the profile area and reddish brown colour. It is ٪٨٢٫٦ facies (St) forms medium to coarse –grained containing channel basal lag of angular, ٠٫٨m rippled sandstone subangular pebbles. The profile terminated by facies (Sr). The sequence is characterized by architectural element (SB) of distal braided stream deposit.

:Facies type description ٣٫١٫٣ The above described profiles revealed eight facies assemblages which are: .St: trough cross-bedded sandstone facies -١ .Sp: planar cross-bedded sandstone facies -٢ .Sh: horizontally-bedded sandstone facies -٣ .Sm: massive sandstone facies -٤ .Sr: ripple cross-laminated sandstone facies -٥

.Fl: Laminated fine sandstone, siltstone and mudstone facies -٦ .Fsm: laminated siltstone and mudstone facies -٧ .Fm: massive mudstone facies -٨ :(Trough cross-bedded sandstone facies (St (١) This facies is composed of coarse to fine-grained sands whitish, yellowish and reddish in colour. It is poorly to moderately sorted and exhibit erosional evidence with from ٣٤٫٣ to ٥٫٩ ,٢٩٫٨ channel basal clasts. This facies occurs with percentage of recording an average thickness of ,(٣٫٦ .south Bagrawyia to north Fadnyia area (Fig ٣٫٩m to the north, respectively. These are laterally traceable northward and ١٫٣ ,٣٫٧ and usually stacked on top of each others. The fine grained units usually contain ripple cross-lamination within trough limbs most common at southern area. Generally D dunes and-٣ this facies suggest channel aggradation produced by migration of deposited within channel fill environment, whereas the ripples may indicate shallow .(٢ photo ١ channel environments (Plate :(Planar cross-bedded sandstone facies (Sp (٢) This facies composed of coarse to fine-grained sands, poorly to moderately sorted and whitish, yellowish to reddish brown in colour. It is usually friable. This facies sometimes contain extra basal lag (Fadnyia) or intraclast mudchips (Bagrawyia and Umm Ali) representing an erosional contact. This facies occurs either as single coset with an average m to the north or rarely stacked as forests of a ١ and ١٫٨ ,٤٫٧ thickness of -٢ few meter thick. The facies (Sp) suggest deposition from migration of ١ D. dnues deposited within bars complex of an upper flow regime (Plate .(٢ Photo .(Horizontally bedded sandstone facies (Sh (٣) This facies is composed of fine to medium-grained sands with ٤٫٤ to ٠٫٤ ,١٠٫٥ whitish yellowish and reddish colours. It forms about percent from south Bagrawyia to the northern Fadnyia area, with ٤m at Bagrawyia and laterally traceable for maximum thickness of several meters. Facies (Sh) can be considered as traditional facies between upper to lower flow regime and probably deposited within bars .(٦ photo top or crevasse-channels specially the laterally traceable (Plate١ :(Massive sandstone facies (Sm (٤)

This facies is composed of fine to medium-grained sands. It is whitish, yellowish and reddish in colour and poorly to moderately sorted. at around Umm Ali ٢٥٫٦ Facies (Sm) forms the maximum percentage of village. This facies may deposited from imbrication of sands in stagnant .(٣ photo ٤ water condition such as sedimentary gravity flow (Plate :(Ripple cross-laminated sandstone facies (Sr (٥) This facies consist of silty, fine to medium sandstone and occurs either as separate rippled cross-laminated or superimposed on trough cross-bedded units. The facies (Sr) widely occurs at Bagrawyia area with percentage of The intensive ripples .(٪٢٫٤) and less at northern Fadnyia area ١٨٫٧ occurs at the upper part of the profiles within point bars top and over bank environments. Thus it is very sensitive indicating change in flow condition and direction. Facies (Sr) suggest continuos sedimentation Similar description is .(١ photo ١ within the lower flow regime. (Plate .(١٩٨٤) given by Walker and Cant :(Laminated fine sandstone, siltstone and mudstone facies (Fl (٦) This facies is whitish, yellowish, reddish, brown to black in colour. It is friable in sand rich samples and hard in muddy units associated with (١٢m) ١ ironstone. The maximum thickness recorded from J. Umm Bour represents the sand rich unit, whereas those associated with ferricrete are and are concretionary and highly bioturbated. The (١٨ ٣٫٣m (Profile .(٪١٫٨) facies (Fl) preserved in minor amount at northern Fadnyia area ٣٠٠m (road cut section). Such The (Fl) facies laterally traceable to over facies may suggest deposition from suspension and weak traction as intercalated fluvio-lacustrine shallow lake shore units. This is indicated by the intensive burrowing, desiccation cracks and pedogenic alteration .(٧ photo ١ Plate) :(Fine siltsone and mudstone facies (Fsm (٧)

This facies is characteristically yellowish grey sand ometimes ٦m reddish, brown to black in colour. The maximum thickness is recorded at J. Muttmar. The yellowish colour can be attributed to the siliceous cement where as the grey one appear due to the richness of the clayey material. On the other hand the richness of iron reflects the dark colour and well indurated. This facies is probably deposited within lake .(٥ photo ١ shore facies (Plate :(Massive mudstone facies (Fm (٨) This facies is mainly forms the lower part of coarsening–upward ٥٠٠m through Wadi El Dan. The cycles and is laterally traceable up to ٤٫٢m. The facies (Fm) are maximum thickness recorded at J. Mazmum is grey, greenish, bluish, yellowish to pale reddish in colour. The grey beds contain gypsum specks, cracks filling gypsum and usually carbonate nodule. The facies (Fm) suggest deposition from suspension at the centre of the basin in stagnant conditions. Part of this facies is deposited with fluvial sequence at overbank and floodplain environments. The facies south ٪٧٫٩٥ ,at around Umm Ali village ٪٢٦٫٢ Fm) form an average of) .(٨ photo Bagrawyia area but are not recorded at the northern area (Plate١

:Bounding surfaces ٣٫١٫٤ The sandstone bodies are divisible internally into packets of genetically related strata by a hierarchically ordered set of bedding plains ١٩٨٢a). Eight types of bounding surfaces called bounding surfaces (Allen has been adopted and applied in this study so (١٩٩٦ ,١٩٨٨a,b ,of (Miall as to identify the architectural elements and to reconstruct the fluvial channel style. The first and second order surfaces occurs through all the profiles with no erosion evidence. The second order separate two different facies

types and thus indicate change in flow condition or direction. The third order rank dominated the profiles especially at the centre of the study area around Umm Ali village. Here it revealed as cross-cutting and gently inclined surface with erosional mud drapes. This surface is not observed at the northern part of the study area. Fourth order rank characterized either flatlying or convex-up erosional channel base and observed at the southern part of Bagrawia and north of Fadnyia area. Other occurrences of this surface includes minor channels such as chute channels, individual crevasse-splay lithosome within the flood plain and interdistibutary bays. Fifth orders characterized major sand sheets such as broad channels and channel-fill complex and generally its flat to concave-up with basal lag gravels as observed at the northern parts of the study area. The sixth order surface define a group of paleochannels that could Accordingly this .(١٩٩٦ ,be mapped as distinct stratigraphic units (Maill surface is suggested to separate the lower lacustine from the upper fluviatile members. identified the quartzose sandstone as a discrete (١٩٦٦) Kheiralla renamed (١٩٧١) lithological unit around Shendi. Subsequently Whiteman the unit as Shendi Formation. Based on lithofacies, grain size and heavy minerals. Many boundaries constitute seventh orders lithologic contrast a cross surface of different types that are likely to be significant (Maill, . Therefore, it’s possible to encompass the entire formation .(١٩٩٦ All the types of bounding surface has been illustrated within the lateral profiles.

:Architectural elements ٣٫١٫٥ Many facies criteria such as channel width/depth ratio, geometry of epsilon cross-bedding, vertical profiles and proportion of overbank facies

are not very sensitive indicators of fluvial style. Their interpretation is commonly ambiguous because similar deposits may result from different combination of geomorphic variables, that is why the proposed architectural element approach is thought to be more helpful. The architectural element is a group of sedimentary lithofacies which comprise subenvironment within the major environment such as channel and bars. It has been defined as a depositional system equivalent in size to or small than channel-fill and larger than an individual facies unit characterized by distinctive facies assemblage, internal geometry and Accordingly the study of architectural .(١٩٩٦ ,external form (Miall element requires study of bounding surfaces including their extent, nature, shape and facies relation. Therefore, the methodology of Miall is adopted, applied and illustrated in (١٩٩٦ ١٩٩٥a and ,١٩٩٤;١٩٨٨a,b) .so as to reconstruct the fluvial style (٣٫١) table

:Lithofacies associations ٣٫١٫٦ It is possible to subdivide the previously described eight facies types into three lithofacies associations includes one or more facies types represented by certain sedimentological characteristics and therefore each association belongs to different depositional environment such as: :Fluvial lithofacies association A (١) This association includes the following facies types: .(Trough cross-bedded sandstone facies (St (١) .(Planar cross-bedded sandstone facies (Sp (٢) .(Ripple cross-laminated sandstone facies (Sr (٣) .(Horizontally-bedded sandstone facies (Sh (٤) .(Massive sandstone facies (Sm (٥) :Transitional lithofacies association B (٢)

This association includes. laterally and vertically changed facies types. The facies types that are gradually changed from proximal fluvial to distal lacustrine are two types mainly deposited at lake shore environment due to the intercalated fluvio-locustrine processes. These facies are represented by laminated fine sandstone, siltstone and mudstone facies (Fl) and siltstone, mudstone facies (Fsm). Such facies types represent intensive bio and phytoturbation, dissication cracks and pedogenic alteration and it may typified the paleosoil horizon between .(٣٫٧) lower and upper members Fig :Lacustrine lithofacies association C (٣) The deep lake environment is characterized by deposition of massive mudstone facies (Fm) mainly from suspended load.

:Discussion and Interpretation of Depositional Environments ٣٫١٫٧ :Fluvial environment ٣٫١٫٧٫١ Continental fluviatile origin for the upper Shendi Formation has been adopted from the field work and constructed lateral and vertical profiles. This is in support of Adequate .(١٩٩٧) and Mohammed (١٩٩٨) Bussert ,(١٩٩٣) Abdullatif ,(١٩٨٣) Omer silicified and ferrugenized fossil wood and their prints collected in the field indicate Channel basal lag of .(١،٥ photo ٣ continental and fresh water environment (Plate intra or extraclast and cross-bedding poorly sorted lenticualr sandstone facies with rapid lateral facies change in addition of bio and phytoturbation indicates fluviatile who record (١٩٦٦) origin of sediments. This finding is in agreement with Kheiralla ferriecrete deposits with oolitic structures, ribbs and furrows that obviously indicates water lain sediments rather than eolian deposit. Moreover, Sedimentary structures, lithology and architectural elements recorded eight facies types support the suggestion of fluviatile origin. The trough cross-bedded sandstone facies (St) represent almost ,٥٥ base channel-fill deposits. Dominate the northern Fadnyia area with percentage of and ٢٩٫٨ whereas it is less frequent at the southern Bagrawyia area with percentage of .around Umm Ali village ٥٫٩ minor percentage of The planar cross-bedded sandstone facies (Sp) was probably deposited within longitudinal and point bars that also dominated at the at Bagrawia and ٪١٥٫١ ,٪٢٧٫٥ northern part of the study area recording around Umm Ali village. These two facies types clearly indicate ٪٥٫٣

relatively deep channel types at Fadnyia and shallow ones at Bagrawyia area. This is supported by the intensity and higher percentage of rippled sandstone facies (Sr) around the later area since the ripples coincide with the shallower depth. Minor channelized area around Umm Ali village were probably result from deep pre-existing lake environment at stagnant of massive (٢٨٫٦) conditions. This is supported by the higher percentage sandstone facies (Sm) which is, most probably deposited in quiet waters and imprecation of sands within stagnant environment. The overbank and floodplain sediments are represented by the facies Fl, Fsm and Fm. These facies were deposited from suspension and and occurs on a ٪١٫٨ weak traction. It is poorly persevered at Fadnyia wide range at Bagrawyia as well as around Umm Ali village reflects the (٤٩:١) The higher sand/mud ratio at Fadnyia area abundance of sand and intensive weathering within uncohesive bank and at ١١:١ high flow regime. On the other hand, the sand/mud ratio is Bagrawyia indicating relatively cohesive bank. Around Umm Ali village, is recorded reflecting restricted and cohesive bank (٠٫٧:١) high mud ratio within most probably meandering belt as well as dominated lacustrine .(٤٫٥ .environment. This is supported by the grain size analysis (Fig The above fluvial facies sequence reveals complete to incomplete -٢ fining-upward cycles with measured channel thickness range between ٦m at the northern-٥ ٥m at Bagrawyia and-٤ ,٣m around Umm Ali Fadnyia area confirming the subsequent channel degradation towards around Umm Ali village. The sedimentary strata at the northern parts of the study area were deposited under braided style. This depositional style were changed to most probably distal braided to meandering around Bagrawyia and to almost meandering nature around Umm Ali village.

:Lake environment ٣٫١٫٧٫٢ Lacustrine lithofacies is well developed at the lower part of Shendi mentioned that (١٩٩٣) Bussert and Werner .(١٩٩٣ ,Formation (Abdullatif continental fluviatile and closed basin with internal drainage sediments characterized Shendi Formation. The studied profiles confirmed the above mentioned conclusions on the basis of it’s dominating coarsening-upward cycles and mudstone ٩٠٠m. In contrast, the geometry which can be lateraly traceable to over fluvial facies lack the distinctive tabular bodies of lake or lake shore origin and the lenticular sandstone beds of rapid lateral facies change .(٣٫٢ become much more dominant (Table The absence of marine and lagoonal fauna and flora and the presence of fresh water faunas such as fishes, frogs, turtles, crocodiles, in the (١٩٩٣) snakes, amphibians and dinosaurs recorded by Werner study area support the assumption of fresh water lake environment. The clay mineralogy and geochemistry reveals partially smectite and Mohamed (١٩٩٣) rich mudstones. This is in support of Abdullatif The predominance of smectite within the lacustrine facies reflect .(١٩٩٧) subenvironment conditions where neogenesis is active within an alkaline confined environments within poorly drained area as a result of mentioned that recent ,(١٩٨٠) Singer .(١٩٤٩ ,aggradation (Millot researches in moderate alkaline lake indicates that preservation of smectite in lake sediments is facilitated by high alkalinity and moderate salinity. This is the most likely prevailing conditions creating smectite rich clays that characterized the lower Shendi Formation within alkaline and confined lake environment. carbonate (٤ Photo ٣ The evaporite facies gypsum specks (Plate ,and thin gypsum bed recorded by Medani (٨ Photo ٣ nodules (Plate may probably suggest slightly saline near shore that might have (١٩٧٢)

been associated with an alkaline lake water chemistry. This may indicate periods of sedimentological quiescence and paleosoil formation that characterize the transitional facies. The occurrences of kaolin rich clays might have followed periods of smectite sedimentation suggesting rather well drained areas within humid conditions prevailed at the source areas. This may testified periods of hydrollogicaly open lake with fluvial input possibly controlling the alkalinity and salinity during sedimentation of the upper member. The dominating grey colour mudstones within the lake sequence pointed towards reducing condition near the lake bottom. The absence of pyrite minerals, in spite of the iron rich environment, may indicates the overall oxidizing nature as well as the alkalinity effect. However, pyrite .(١٩٨٢ ,has been formed under acidic water chemistry (Maynard It has been suggested that Shendi subbasin was apparently shallow and subaerially exposed due to the intensive bio and phytoturbation, pedogenic facies comprising paleosoil horizon and mudcracks. This is supported by the thickness of the exposed mudstone beds which do not .٩٤٠m from the examined profiles ٤٫٥m and lateraly restricted to exceeds The above suggested depositional environments are represented by coarsening and thickening-upward sequences. These environments range from lacustrine to fluviatile meandering and even braided system along the periphery possibly representing zonal distribution of sediments. That is very much like the idealized lake model proposed by Reinck and Singh Such zonation is supported by the heavy mineral distribution .(١٩٨٠) from staurolite –kyanite dominate zone to inner zircon-rutile-tourmaline dominated zone with increasing maturation against the prephery (section were most (٣٫١ .The polymodal paleocurrent directions (Fig .(٥٫٥ probably responsible of such zonation.

represents the fluvial styles and typical proposed lake (٣٫١٢) .Fig model for the study area. :Paleoclimate ٣٫١٫٨ Arid to semi arid conditions are believed to have prevailed in Mid- Cretaceous (Albian-Cenomanin) in equatorial Gondwana including the This regional climatic pattern .(١٩٨٤ Batten ;١٩٨٠ Sudan (Boltenhagen seems to be locally interrupted. The concentration of lateritic weathering products such as kaolin and ironstone in the study area indicates warm, The .(١٩٩٣ ,Abdullatif ;١٩٩٠ ,humid tropical climate (Germann, et al on the basis of (١٩٩٣) same climatic pattern was confirmed by Werner recorded fresh water vertebrate faunal assemblage. suggested hot dry conditions due to the occurrences (١٩٧٢) Medani of gypsum and carbonate caliché at Shendi Formation. proposed tropical climate of the sahelian type (١٩٨٣) Omer depending on small size of Gymnosperm fossils and seasonal dryness on the basis of smectite and illite clays. Smectite rich mudstone and evaporate facies in the study area indicate onset arid to semiarid followed by dominance of warm humid climate during sedimentation of kaolinitic mudstone and ironstone .(٧٫٥ formation (section :Paleotopography ٣٫١٫٩ The paleotopography of the area can be deduced from the centripetal distribution of paleocurrent directions which are definitely controlled by the paleoslope in response to the tectonic regime created the basinal structure during Cretaceous time span. In this way, the southern area was slightly tilited and became peneplained during meandering evolution. The northern area had probably a steeper slope characterized by the braided channel style. Moreover, the paleogeographic evolution can be summarized from the smectite rich mudstone formed in a

pronounced relief at deep trough zones to Kaolinitic rich mudstone with gentle slope as well as ironstone formation within negligable relief and This .(١٩٩١ ,tectonic quiescence with strong leaching condition (Tucker is supported by the gradual channel degradation due to the both depositional and source area base level change and peneplaination. The paleogeography and climate were probably the main allocyclic events that acted to control the autocyclic processes and depositional environments.

:Sedimentary slump folds ٣٫١٫١٠ Sedimentary slump structure are well developed and exposed along ٤٫٣km NE of Umm All village and ,both sides of the road cut section ٤٠٠m. The slumped strata have been photographed extending for about and lateral profile were constructed from the eastern side of the road cut with vertical or subvertical fold axis. The ,(٣٫٩) .as shown partly in Fig show interbedded (٣٫٨ .Fig) ١٩ & ١٨ obtained lateral and vertical profiles small coarsening-upward sequences from massive mudstone facies (Fm) and massive sandstone facies (Sm). These facies were most probably deposited at lake shore environment within crevasses, flood plain and small lakes or delta plain facies including deltaic distributary channel facies (Sm) and bays massive mudstone facies (Fm) which may typify the pronounced relief. This is supported by the absence of complete lateral at the both sides of the road cut (٣٫٨ .Fig) ١٩& ١٨ correlation of profiles Similar slump structures have .(٢،٣ Photo ٢ and convolute bedding (Plate been recorded as important features of distrubutary channels (Stanely, .(١٩٧١ Laury ;١٩٦٦ Krinitzsky and Compton Field observations showed the occurrence of the slump structures such as the thinning of the mudstone beds within the slump zone and thickening away to the both sides, the occurrences of undisturbed

.(٢ photo ٤ sandstone beds beneath the slumped mudstones (Plate and Alen (١٩٧٠) Helwig ;(١٩٥٣) Kuenen ,(١٩٧١) According to Maas features that distinguish sedimentary slumps from purely tectonic (١٩٨٢) sedimentary structures include the following: .Deformed beds occur as a zone between undisturbed beds (١) .The upper part of the deformed bed is partly cemented (٢) Preferred orientation of the fold axes, if present are not related to (٣) any tectonic strike. Within a single slump the structural style may be irregular and a (٤) wide range of deformational structures my occur.

:Ferricrete bands ٣٫١٫١١ Ferricrete bands are a thick ferrungnous weathering crust which give evidence of a phase of an intensive lateritic weathering condition Such Ferricretes .(١٩٨٦ ,under tropical climate with dry season (Nahon are restricted in continental regions and attributed to one of the following origins: It is a residual formation by lateritic weathering process with (١) ferricrete being the top horizon of deep weathering profile of .١٩٨٦٠ Nahon ;١٩٧٩ ,saprolite and mottled zone. (Leprun reject the lateritic source, instead they (١٩٨٧) Bourman, et al (٢) could demonstrate that so-called saparalite which often consist of merely iron depleted parent rocks and flat Ferricrete are formed by absolute accumulation due to the lateral transport of iron rich solution, a suggestion which had already been put forward by .(١٩٦٧) Bremer for the sedimentary (١٩٩٣) The first origin is adopted by Abdullatif strata under consideration and supported by the occurrence of sand rich fluvial system, kaolin, ironstone and paleomicroflora which testify to

humid climate condition. Such climatic conditions favour lateritic weathering and activated hydrolitic process which led to the deep chemical leaching and accumulate in proximal fluvial and distal lacustrine environment. attributed the origin of ironstone of Shendi (١٩٩٠) Germann, et al Formation to the climatic conditions which prevailed during Mid- Cretaceous to Early Tertiary. They also showed that tectonic quiescence followed by periods of tectonic activity, redistribute and accumulate the weathering piles and concentrate them in morphological sink. During periods of high subsidence rate, the lateritic weathering supply would have been mixed with course clastic in flux. ,mentioned that, in humid tropical environments (١٩٨٦) Nahon distinctive soil can be formed that are characterized by almost total decomposition of weathering minerals and perhaps by desilicate and concentration of free iron horizon. also mentioned that soil weathering attacks the rocks (١٩٣٣) Omer in the source areas releasing iron and generating kaolinite. This climatic conditions enhanced by tectonic quiescence created the strong leaching conditions of the source areas and concentrated the ironstone within depositional environment. suggested that the oolitic and (١٩٦٤) Schellman ;(١٩٧٤) Harder pisolitic structures may have formed due to the early subsequent diagenesis or subaerial pedogensis process that were responsible from the conversion of clay minerals to amorphous Fe-Al -Si hydroxygel or colloidal coagulate. This will lead to the concentration of immobile Al- rich residue or pre-existing grains as a nucleus and mechanical lateral .(١٩٨٣ ,accretion of suspended particles (Maynard ٣٫٣m In the study area the maximum thickness of ferricrete bands reaches composing from poorly laminated fine sandstone, siltstone and mudstone (١٨ profile) facies (Fl). Highly bio and phyto- turbated with reddish brown to black colours,

Such .(٤ Photo ٤ and concretionary (Plate (٥ Photo ٤ dissication cracks (Plate characteristics most probably lead to suggest that the iron might have been deposited within oxic and shallow environment. However, facies and architectural element analysis show that the iron which is deeply associated as coagulate, with fine sediments of facies (Fl) and (Fsm) most likely to be deposited within over bank, flood .(٩،٢٠ plain and shallow lake shore environment (profile It has been suggested that the subsequent channel style degradation as meandering one and fluvio-lacustrine environment, may provide shallow sluggish circulation and suitable physicochemical condition (Fig. Due to the alkalinity/salinity of the lake shore water chemistry and .(٣٫١٠ fresh fluvial water input probably may facilitate the accumulation of ironstones that capped the outcrops and characterized the interface boundaries. The sedimentation of ironstone is widely attributed to allocyclic and autocyclic controls.

:Channel architectures and facies depositional model ٣٫١٫١٢ The construction of facies model help to delineate the local variations in fluvial style and the subtleties of their dependency on slight change in depositional controls including tectonism, climate, base level change, source area geology , vegetation .. etc. The studied profiles illustrate the occurrence of different fluvial styles. However, at the southern Bagrawyia area the profiles are dominated with architectural elements CHs, SB and LA representing the characteristics of both distal sheet flood braided and meandering styles. The sand bar architectural element SB is more likely indicates the distal braided bar system. The crevasse–splay, overbank and floodplains of this The higher .(٣٫٣ .style is similar to that of meandering river (Fig percentage of rippled sandstone facies (Sr) around Bagrawyia indicates agitation and shallow water condition more appropriate to the meandering style. Thus, the channel architecture is likely to be mixed braided and

this type of sediments can be (١٩٩٦) meandering. According to Miall .(٣٫١١ .considered as low sinuosity braided with alternate bars (Fig Further to the north, around Umm Ali village, the dominating lateral accretion architectural element (LA) are characterized by the well defined gently inclined third order bounding surface to the downstream direction and typically interpreted as point bar of meandering style. Nami mentioned that the occurrence of lateral accretion (١٩٧٨) and Leader element (LA) is a strong evidence of meandering channel specially if the sandstone bodies are of limited lateral extent with steep margins (Table The well developed floodplain, crevasse-splay deposits and shallow .(٣٫١ ٣m) supported this suggestion. Further to the north, the-٢) channel depths paleocurrent converted to the SE and revealed the presence of extensive sheets of in-channel lower flow regime dune consisting elements CHs and SB and dominated by lithofacies (St) and (Sp) with minor part of fine- grained floodplain deposits. The cyclic succession such as those common in proximal braided and meandering styles, are absent. Therefore, different styles of braided types such as shallow pernial braided platte deep pernial braided and even sheet flood ,(١٩٨٥،٧٨) type model of Miall distal braided types are also possible. shows the channel style architectures and the (٣٫١٢) .Fig approperiate facies depositional model of Shendi Formation. The generalized model composed of coarsening-upward cycles. The lower lacustrine dominated environment, supported by subsurface lithofacies analysis represent the bottom of these cycles. These cycles were changed upward to upper fluvial member possibly representing zonal distribution of depositional systems from inner lacustrine to fluviatile meandering and even braided system away to the basin prephery. Moreover, the conceptual model was characterized by heavy mineral zonation, clay geochemistry and mineralogical variations. This is testified the different

climatic and tectonic settings that prevailed during sedimentation of lower and upper cycles and acted as major allocyclic factors controlling the autocyclic processes and facies depositional model. :Subsurface Lithofacies Analysis ٣٫٢ :Introduction ٣٫٢٫١ Borehole lithological data have been collected, arranged and profiles, drawn parallel and perpendicular to the strike of the ١٤ plotted in basin so as to reflect their vertical and lateral facies distribution. Seven boreholes drilled by “Wadies and Groundwater ٢٧ profiles including ,٤ ٣٨٥m B. H Corporation” for drinking water with maximum depth of These boreholes never reached the basement with exception .(٦ Profile) with a (٥ Profile) ١٩ ٢٢٩m and B.H with a depth of (٤ Profile) ١٦ of B.H .(٣٫١٣ .٢٣٩m (Fig depth of obtained from the General Geological (١٤-٨) profiles ٧ The other (١٢m-٧) boreholes (١٥) Authority of the Sudan (Gras), include shallow ٣٥٠m) between the wells around-١٢٠) drilled at small scale spacing Umm Ali village. The aim is to evaluate and estimate the quality and .(٣٫١٤ .reserve of kaolinitic mudstone of Shendi Formation (Fig In each profile the thickness and percentage of each facies have been calculated and tabulated (Appendix B). Histograms are constructed from the calculated facies percentage to observe the dominant facies The study of the subsurface .(٣٫١٧ ,٣٫١٦ .types and their distribution (Fig lithofacies analysis has been carried out so as to define the lithofacies types and their association in order to reconstruct the subsurface paleodepositional environment which dominated the lower member of Shendi Formation.

:Profiles description ٣٫٢٫٢ :(١) Profile

This profile includes three bore holes N-S trending, ranged in depth ١٤٠m and located to the extreme south of the study area. The to ٨٧ from from the (٪٢٤٫١) profile has been dominated with coarse sandstone facies profile area. Kanker nodules associated with fine sandstone facies and The claystone is revealed with less amount .٦٫٣ attain percentage of In general the profile is characterized by rapid vertical and .(٪٢٢٫٦) lateral facies changes and difficulty of correlation. :(٢) Profile This profile is lies south of Umm Ali village and includes four is (٤) boreholes. The maximum depth recorded in this profile at borehole ٢٨٠m without penetrating the basement. The maximum thickness This profile is distinguished .(١٠٥m (B.H٥ recorded for claystone bed is by the absence of coarse sandstone facies and scarsity of medium whereas it is dominated by fine grained sediments (٪٤٫٥) sandstone facies Pedogenic facies is also recorded .(٪٤٥٫٣) as well as claystone facies .(٪١) with minor amount :٣ Profile Nine boreholes arranged in N-S trend represent nine facies (٢٧ assemblages including evaporite facies of thin gypsum bed (B.H This is plotted with exagerated scale due to) (١٩٧٢) ,recorded by Madani ١٩m). The maximum thickness recorded for the its shallow depth forming a (١١٠m (B.H١٢ claystone bed through the profile area is The carbonate caliche and kanker nodules .(٪٣٢٫٧) dominant facies type are clearly associated with fine sandstone and siltsone facies. The profile .٪٨٫١ is also consist of coarse sandstone facies forming :٤ Profile boreholes trending E-W possibly ٤ This profile include m ٢٢٩ perpendicular to the basin strike. The maximum depth recorded is which reached the basement. The maximum thickness recorded (١٦ B.H)

forming the dominant facies(m (B.H٢٦ ١٢٣٫٥ for the claystone bed is Slightly considerable amount of .(٪٤٤٫٢) through the profile area is also present. It represented by kanker and (٪٣٫٦) edogenic facies carbonate caliche associated with fine sandstone and sandy clayey facies. :٥ Profile N-S trending profile constructed from nine boreholes with a ٢٤٠m penetrating the basement at borehole maximum depth of The maximum thickness recorded for clayey sandstone facies is.(B.H١٩) The claystone facies dominated the profile area with .(٩٠m (B.H١٧ .(٪٦) and minor occurrence of coarse sandstone facies ٤٦٫٣ percentage of :٦ Profile boreholes possibly perpendicular to the ٦ This profile include The maximum .(m (B.H٤ ٣٨٠ strike of the basin. The maximum depth is and the claystone facies (١٦٧m (B.H٤ thickness of mudstone bed is with minor coarse and medium ٪٣٤ dominated the profile area recording .(٪١٫٧) sandstone facies and traces of pedogenic facies :٧ Profile N-E trending profile, include nine boreholes with nine facies types. m without penetrating the basement ٣٨٠ The maximum depth is (٪٤٤٫٤) The profile area is dominated with claystone facies .(B.H٤،١٠) The profile is dominated by .(١٠٥m (B.H٥ with maximum thickness of are mainly (٪٠٫٨) coarsening-upward cycles. Minor pedogenic facies associated with fine sandstone and siltstone facies. From the above described profiles nine facies types could be recognized on the basis of their lithology, grain size, colour and sorting including evaporite facies. :Coarse sandstone facies -١ Poorly sorted coarse sandstone facies with very minor pebbles content are recorded from the boreholes. This facies vary in thickness

٥٧٫٣m.The colour vary from whitish, yellowish to brown and to ٠٫٥ from reddish, whereas the grain morphology is angular to subangular and from the study area ٣٫٧ rounded. This facies occurs with percentage of and its probably deposited within fluvial regime. :Medium sandstone facies -٢ Poorly to moderately sorted sandstone facies occurs with subangular to rarely rounded grains. Its colour vary from whitish, yellowish, reddish, and brown due to the different cementing materials. ٣٧٫٣m whereas its to ١٫٥ The thickness of this facies range from This facies may have been .٢٫١ percentage through the entire area is deposited within fluvial regime. :Fine sandstone facies -٣ Moderately sorted fine sandstone facies mainly associated with pedogenic facies. This facies vary in colour from whitish, yellowish, brown reddish to black and rarely pale grey friable. The thickness vary from the study area. This facies might ٪١٢٫٦ ٨٣m and form to ١٫٨ from have been deposited within bar tops, crevasse-splay, levees or deltaic distributary channels (lake shore). :Sandy claystone facies -٤ This facies is mainly recorded as friable moderately cemented whitish, reddish brown, black and pale grey to greenish in colour. It to ١٫٥ from the study area and its thickness vary from ٪٧٫٤ covers ١٣٤٫٨m, also part of pedogenic facies associated with it. This facies may have been deposited within proximal overbank environment (lake shore). :Clayey sandstone facies -٥ Pale brown to black due to the iron content, also occurs as whitish from the ٪١٠٫٧ and pale grey when it is rich in kaolin. This facies covers m with minor pedogenic ١٢١٫٦ to ٣ study area ranging in thickness from

facies. This facies might have been deposited with proximal overbank environment (lake shore).

:Siltstone facies -٦ Reddish brown to black in colour associated with ironstone as has ٠٫٧ been recorded from the boreholes. This facies range in thickness from from the area. It has probably been ٪١١٫٩ ١٥m and covers about to deposited within proximal to distal overbank and floodplain environments (lake shore). :Mudstone facies -٧ Reddish brown to black and grey colour characterize this facies. It of the entire ٪٦٫٣ m and covers about ٢٥-١٧ ranges in thickness from area. The red brown mudstones was probably deposited within oxidized proximal areas whereas the grey one might have been deposited at deep lake environment especially the most thicker and grey colour position. :Claystone facies -٨ Sticky, whitish, voliet, grey, greenish and dark claystones are a widely spread through all the studied profiles representing the basal beds of the total facies ٪٤٦٫٥ of coarsening-upward cycles. This facies covers The grey and dark .(١٤٦m (B.H٥-٢ types and vary in thickness from colours may be due to the reducing environment and organic matter content. This facies was mainly deposited from suspended load within deep lake environment. :Evaporite facies -٩ Pedogenic facies such as carbonate nodules, caliche, kankers and gypsum occur and are mainly associated with fine sandstone and in less amount with clayey sand, sandy clay and siltstone facies. The gypsum This .(٠٫٥m thick (B.H٢٧ have been recorded as in bedded form reaching

facies indicates arid and semi arid environment with possible salinity near the lake shore. The pedogenic process is possibly associated with minor or no sedimentation and they may indicate the lower limit of evaporation switching. boreholes obtained from Gras representing the ١٥ The shallow profiles and drawn ٧ areas around Umm Ali village, are arranged in separately on small scale map taking in to account the abundance of claystone facies and the absence of coarse to medium sandstone facies. :١١ and ٨،٩،١٠ Profile boreholes arranged in an east –west ١٣ These four profiles include direction. Obvious coarsening–upward cycles dominated the profiles and includes four facies types with dominating claystone facies recording a .(٤،٧،٦ ٩m (B.H thickness of :١٤ and ١٣ ,١٢ Profile These profiles arranged with N-S trend. The maximum thickness ٩m and tend to be laterally oriented. The recorded for claystone is coarsening-upward cycles dominated the profiles. In general the profiles revealed five facies types as can be seen from the following summary: :Fine sandstone facies (١) of the ٪١٣٫٨ Moderately sorted fine sandstone facies is forms profiles area. It is whitish, reddish and brown in colour. The thickness This facies may have been deposited .(٦٫٥m (B.H٨ to ٠٫٣ ranges from within levees, crevasse-splay and distributary channels. :Sandy claystone facies (٢) This facies vary in colour from whitish, reddish brown to black when its iron rich. They are also whitish grey in colour when it’s

(١٫٤) probably rich in clay. This facies recorded with minor percentage of and probably may deposited within lake shore sub environment. :Clayey sandstone facies (٣) This facies vary in colour from whitish to pale grey and reddish brown to ٧٫٣m and occurs with an average to ٦٫٢ black. The thickness vary from of the total facies types. This facies may have been deposited ٪٩٫٤ of within lake shore environment. :Siltstone facies (٤) This facies vary in colour from pale grey to yellowish, reddish ٠٫١٥ brown to black due to the iron content. It is vary in thickness from This facies may deposited at .٢٤٫٥ ٥٫١m with an average percentage of to lake shore environment. :Claystone facies (٥) Sticky claystone with pale reddish, violet, pale grey and dark colour recorded from the boreholes. This facies partially covers the entire and vary in thickness ٥٠٫٩ profiles area with an average percentage of ٩٫١m. It might have been deposited within deep lake to ١٫٧ from environment from suspended load.

:Interpretation of Depositional Environment ٣٫٢٫٣ It is possible to group the above described nine facies types into three major lithofacies associations. .Lithofacies association A (١) This association includes coarse and medium to fine sandstone facies, and this types mainly represent the fluvial lithfacies association. :Lithofacies association B (٢) This association includes fine sandstone, siltsone, sandy claystone, clayey sandstone and mudstone facies representing the fluvio-lacustine lithofacies association.

:Lithofacies association C (٣) This association includes claystone facies mainly deposited from suspension load representing deep lacustrine deposits. From the constructed histogram it is clear that coarse and medium ٪٥٫٨ sandstone facies of association A occurs with minor percentage of while claystone facies of association C and intercalated facies of .respectively ٪٤٧ ,٤٦٫٥ association B predominate and represent Accordingly, this fact may illustrate that there is minor fluvial evidence within lacustrine dominated environment. While there is no record of fluvial facies within subsurface sediments around Umm Ali village (Fig. the area has been entirely covered with deep lacustrine lithofacies ,(٣٫١٧ and transitional fluvio-lacustrine intercalated (٪٥٠٫٩) association C Therefore, it can be suggested that the .(٪٤٩٫١) lithofacies association B central area was occupied with coarsening–upward sequence of typical .(٣٫١٨ .lake succession (Fig The occurrences of fine sandstone sheet geometry within the may (٣٫١٨ .lacustrine dominated environment at the central area (Fig probably suggest sedimentation within levee, crevasse-splay or deltaic distributary channels mostly associated with the lacustrine facies representing coarsening upward cycles and indicating periodic infilling or progradation of the lake environment. (٪٥٫٨) The minor amount of coarse to medium sandstone facies recorded from the subsurface of the area suggesting fluvial evidence. It has been recorded from the basin central, remotely situated, wells such as These profiles .(٪١٠٫٧) ٥ and profile (٪١٢) ٣ profile ,(٪٣٠٫٦) ١ profile may have been situated towards the basin prephery where marginally fluvial facies are represented. The close association of pedogenic facies with fine sandstone, siltstone and mudstone facies as well as the association B may indicate

shallow lake shore subenvironment with no or minor sedimentation rate without excluding direct evaporation since the climate was arid. This is ٠٫٥m) which may give the) supported by the record of bedded gypsum lake shore water slight saline affinity. On the other hand, the reddish brown to black colouration may suggest iron rich ferricrete bands and humid environment that prevailed at the source area with strong leaching conditions. The most grey colours suggest that they might have been deposited within reducing deep lake environment. may (٪٤٩٫١-٤٧) The higher percentage of lithofacies association B lead to suggest a vast and broad shallow lake shore subenvironment which can possibly be matched morphologically with low gradient (ramp- type) lake margin. This fact indicate the paleostructural style which developed as thermal-sag or stretched basin with gentle slope margin rather than strike slip or rifted basin. This is supported by the rapid vertical facies change rather than lateral one which is observed from Additional support to .(٣٫٢ subsurface and outcropping profiles (Table this evidence is the lack of surface alluvial fan or proximal braided stream deposit which characterize the lake margin in strike slip basins. The same .(١٩٨٩) conclusion has been reached by Bussert Generally the subsurface facies types, characteristics and geometry (Table. may probably lead to suggest that the lake was moderately large broad and (٣٫٣ shallow coinciding with the surface lithofacies analysis, as recommended by .(١٩٩٣) Abdullatif

Chapter Four Grain Size Analysis :Introduction ٤٫١ The grain size analysis gives an idea about the statistical measurements (central tendency, sorting, skewness and kurtosis) and helps in the classification of sedimentary rocks. This analysis together with other techniques also help to determine depositional environments. Contribution to this field of the study were provided by Krumbein ;(١٩٥٧) Folk and Ward ;(١٩٥٧،١٩٧٥) Petijohn ;(١٩٤٩) Inman ;(١٩٣٧) ;(١٩٦١،١٩٦٧) Friedman ;(١٩٥٧،١٩٦٤) Passega ;(١٩٥٨) Mason and Folk .(١٩٩١) Tucker ;(١٩٦٩) Passega and Byramjee

:Methods and procedures ٤٫٢ Thirty two representative samples were selected for grain size from (٢٣-٩) samples ١٥ ,from Bagrawyia (٨-١) analysis; eight samples .from Fadnyia area (٣٢-٢٤) around Umm Ali village and nine samples :Pre-analytical methods ٤٫٢٫١ The samples were carefully mortered in a porceline mortar and -١ then they were immersed in tap water. :Each sample was subjected to the following -٢ .hours ١٢ I. Shaking for about II. Disintegration by an ultrasonic bath for an hour. .٨٠oC III. Dried in an oven at .٤٠g were weighted from one quarters IV. Quartered and

:Analytical methods ٤٫٣ :Wet-sieving ٤٫٣٫١ Fourty grams of each sample were sieved by using set of sieves and ٠٫٠٦٣ ,٠٫١٢٥ ,٠٫٢٥ ,٠٫٥ ,١ ,٢ ,٤ arranged in descending order ٠،٠٣٢mm. These wet-sieving techniques has been a comodated by .٥ minutes and amplitude of ١٥ agitation and intermittent shaking for The sediments fractions retained in each sieve were collected carefully

and the sieve were throughly cleaned using a brush and the ultasonic .٨٠oC bath. These fractions were dried in the oven at

:Fine fractions analysis ٤٫٤

have been separated using (٢٣-١) samples ٢٣ .mm were treated using two techniques ٠٫٠٣٢ Fractions less than hydrometer test, whereas the other nine samples have been separated using atterberge test. :Hydrometer method ٤٫٤٫١ ٠٫٠٣٢mm were collected into a glass The fractions less than (١) sedimentation cylinder and distil water was added up to the level of .١٠٠٠mm The clyinder has been shaken for a period of few minutes agitation (٢) of the slurry and set the tube immediately in a convenient location. The hydrometer and equivalent temperature readings have been (٣) obtained by interval of times. The diameter and percentage of each fine particles have been (٤) calculated using the following formulae: Diameter = K √L/T Where: K = factor depending on specific gravity (Gs). T = time L = effective depth of hydrometer from calibration of hydrometer (HR٠٫٣٦-١٨٫٢٥) = L Where: HR = hydrometer reading N (١-TW (GS/GS/١٠٠ = Percentage passing Where: TW = total weight G.s = specific gravity N = correction reading = HR –HW. Where: HW = reading of hydrometer in salt.

:Atterberge method ٤٫٤٫٢ mm were collected into atterberge ٠٫٣٢ The fractions less than (١) .٣٠cm٣ cylinder and distil water was carefully added till The cylinder was carefully shaken for a few minutes. When the (٢) shacking was stopped, the commenced timing and the temperature were recorded. ٣٠cm٣ micron deposited and ٢ < hours all the fraction ٢٤ After (٣) ٢micron fraction was drained into column of water with dispersed a weight beaker. and the ٣٠cm٣ The cylinder was filled again with distill water up to (٤) procedure above was repeated for fraction grain size between .hours ٢ ٦micron and drained after-٢ minutes the cylinder liquid content poured off. This ١٥ After (٥) .micron ٢٠-٦ fraction represents grain size between The remaining fractions in the cylinder represents grain size (٦) .٣٢-٢٠ between All the samples were dried, weighted and their results added to the (٧) wet sieving results.

:Results and Interpretation of Grains Size Analysis ٤٫٥

After calculating and recording the weight percent and cummulative weight percent of each fraction retained by wet From these curves the .(٣&٤٫١،٢a,b) .sieving and hydrometer, atterberge, the cummulative frequency curves were drawn Fig cummulative weight percent of each fraction was plotted against it is grain size (in phi units) on probability paper. The percentiles were recorded. Consequently statistical parameters were ٩٥ and ٨٤ ,٥،١٦،٢٥،٥٠،٧٥ corresponding phi units for the .(formulae (Appendix C (١٩٥٧) calculated according to Folk and Ward

:Statistical parameters of the results ٤٫٦ :(Median (Md ٤٫٦٫١ ٪٥٠ This parameter measures the central tendency. It corresponds to mark of the cummulative frequency curve. The median values of the .phi ٣٫٣ to ١٫١ sedimentary sequence of Shendi Formation range from

:(Mean (Mz ٤٫٦٫٢ The mean offers a good picture of the spread of the cummulative frequency curve of a sample and it calculated by the following formula:

Mz = Ø + ٥٠ Ø + ١٦ Ø

٢٫٩ to ٠٫٨ The mean values of the analyzed sediments range from phi representing grain size from coarse to fine fraction. :(Sorting (σ ٤٫٦٫٣ Dispersion around central tendency determines sorting. Tails of distribution are taken to be environmentally sensitive and estimates of sorting have been designed to reflect them (Folk & Ward Op.cit). The standard deviation (σ) is given by the following formula:

Ø – ٩٥ Ø + ١٦ Ø – =σ Ø٨٤ ٥ ٤

Following the verbal description of the sorting values of Folk (Op.cit), one sample

falling with moderately sorted, whereas the other ٠٫٨ has the average value of

.٢٫٠ to ١٫١ samples are poorly sorted with “σ” value ranging from

:(Skewness (Sk ٤٫٦٫٤ The asymmetry of the distribution is measured by the Skewness which is determined by relative importance of the tails of distribution. Skewness is measured by the following formula:

٥٠ Ø ٢ – ٥ Ø + ٩٥ Ø + ٥٠ Ø ٢-١٦ Ø + ٨٤ Sk Ø [٥ Ø -٩٥ Ø]٢ =

According to the description of Skewness values of Folk (Op. cit), most of the determined sediments are generally negatively to very negatively- Skewed. Whereas the other samples are positivie to very positive-Skewed. Fadnya samples are generally course –Skewed whereas Bagrawyia and Umm Ali samples are fine-Skewed. :(Kurtosis (Kg ٤٫٦٫٥ Kurtosis is a measure of the peakedness of the distribution. If distribution is a flater than a normal distribution it is called platykurtic but if more peaked its called leptokurtic. The graphic formulae as follows:

٥ Ø – ٩٥ Kg = Ø

-٧٥ Ø] ٢٫٤٤ for the studied sample. This ١٣٫٨ to ٠٫٩ Kurtosis values range from indicates mesokurtic to extremely leptokurtic distribution. The values of the grain size parameter are shown in (Appendix C). :Scattered plot diagrams ٤٫٧ The scatter plot diagrams represent two different statistical The objective .(١٩٦٧ parameters plotted against each other (e.g. Fridman of these diagrams is to distinguish the various sedimentary environments. ,(٤٫٤a) .Scattered plot diagrams used here are sorting against mean Fig ٤٫٤b). The first scattered plots sorting) .sorting versus skewness Fig versus mean is used to differentiate river sediments from dune ones, whereas the second one distinguishes river sediments from beaches. For the above mentioned diagrams, all the samples are within the field of river sediments. This finding support the conclusion that has been .(٣٫١٫٧ obtained from the lithofacies analysis (section

:Histograms and smooth frequency curves ٤٫٨

The histogram and smooth frequency curves show the frequency of grains in each size class and usefully give an immediate impression of the grain size distribution, particularly whether the distribution is unimodal or bimodal. Histograms have been constructed for the representation of the selected samples in the study area by plotting weight percentage on the y- axis against one phi intervals on the x-axis. Smooth frequency curves are

derived from histograms by joining the mid points of histograms bars (Appendix D).

:Classification of the treated samples ٤٫٩ Grains size distribution of sedimentary rocks are often plotted on .(١٩٦١) constructed after Muller (٤٫٥ .triangular diagram (Fig This diagram indicate the main textual classes in the area. Thus, the plotted samples represents sand dominated at Fadnyia and muddy sand to silty sand at Umm Ali and Bagrawyia areas respectively. Therefore, this result briefly reflects the channel styles and processes at the same areas.

١ Plate

.Rippled cross –bedded sandstone facies (Sr), from Bagrawyia area (١) Trough cross-bedded sandstone facies (St) and planar cross-bedded (٢) sandstone facies (Sp) with obvious second order bounding surface. Fine siltstone and mudstone facies (Fsm) convoluted from road cut (٣) section. Lower lacustrine massive mudstone facies (Fm) and upper fluvial (٤) member trough cross-bedded sandstone facies (St) separated by shallow transitional facies (Fl) which is laminated siltsone iron rich ferricrete band from Bagrawyia area. Fine laminated siltstone and mudstone facies (Fsm) J. Muttman (٥) represents transitional facies. Horizontally-bedded sandstone facies (Sh) probably deposited as (٦) upper flat bed and bars top. Fine laminated sand, silt and mudstone facies (Fl) represents (٧) .(١ transitional facies (J. Umm Bour

Massive mudstone facies (Fm) represents deep lacustrine facies (J. Umm Bour (٨) .(٢

٢ Plate

Fine laminated sandstone-siltsone and mudstone facies (Fl) iron (١) rich ferricrete, from Bagrawyia area. Convoluted planar cross-bedded sandstone facies (Sp) from around (٢) Umm Ali village. Convoluted ripple and massive sandstone beds, probably deposited (٣) within paleoslope surface (nearby Umm Ali village). Massive mudstone facies (Fm) of deep lacustrine unit overlain by (٤) sandstone bed and represents load cast structure. .Massive sandstone facies (Sm) from around Umm Ali village (٥) Channel-fill trough cross bedded sandstone facies (St) overlain by (٦) rippled cross-laminated sandstone facies (Sr). ,(Typical channel-fill and bars sequence comprising the facies (St (٧) (Sp) and (Sh).

Massive mudstone facies (Fm) characterized by the appearance of slump (٨) structure and most probably represent lacustrine unit.

٣ Plate

near ٢ Trunk of ferrugenized fossil wood on top of J. Umm Bour (١) by Umm Ali village. Massive sandstone facies (Sm) overlain by massive mudstone (٢) facies (Fm) and some intercalated streaks of mudstones (fleather structure). Photograph from slumped zone (near by Umm Ali village).

Lower deep lacustrine massive mudstone facies (Fm) overlain by (٣) massive sandstone facies (Sm) locally represents slump structure or load cast. (road cut section). Massive mudstone facies (Fm) of deep lacustrine unit composed (٤) of fibrous gypsum filling the cracks representing subaerial .(١ exposure and pedogenic facies. (J. Umm Bour Branched fossil wood ferrugenized and well developed oolitic (٥) .٢ structures. Top of J. Umm Bour Fine laminated sandstone, siltstone and mudstone facies (Fl) iron (٦) rich ferricrete band extensively bio and phytoturbated (road cut section). Fine sandtone and siltstone facies bioturbated, characterized by (٧) the appearance of mud cracks and long the transitional facies. Massive mudstone facies (Fm) of deep lacustrine pedogenically (٨) modified unit representing carbonate nodules. ٤ Plate

Extensively bio and phytoturbated siltstone and mudstone facies (١) indicates shallow lake shore environment. Massive mudstone facies (Fm) sandwiched between massive (٢) sandstone beds. The mudstone bed shows slump structure with distinctive undisturbed lower massive sandstone bed. Fine massive sandstone facies (Sm) representing well developed (٣) vertical burrows probably indicating high rate of sedimentation. Fine siltstone and mudstone facies (Fsm) represents iron rich and (٤) well developed oolitic structures. Fine siltstone and mudstone facies (Fsm) showing cracks and (٥) shallow environment of sedimentation.

Stratigraphic sequence representing lower lacustrine massive (٦) facies (Fm) overlain by fine laminated sandstone and mudstone facies (Fl) as transitional facies which in turn overlain by upper fluvial massive sandstone facies (Sm). Ashtray structures within sandstone facies. It may indicates scouring and (٧) rapid channel switching on lacustirne mudstone facies (Fm).

٦ Plate

Photograph representing large polycrystalline quartz grain of (١) sutured contact with rare microlite inclusions probably of .(basement origin (X٢٠ Monocrystalline quartz with zircon grain inclusions, and (٢) .(overgrowth texture (X١٥ .(Monocrystalline quartz showing vacuole bubbles inclusions (X١٥ (٣) .Subangular to subrounded quartz grains rimmed with ironstones (٤) .(The grain represents rare or no inclusions (X١٠ Dominant monocrystalline quartz grains of angular, subangular to (٥) .(subrounded shapes and straight to andulose extinction types (X١٠ Rounded to subrounded quartz grains showing overgrowth texture (٦) .(and probable reworking evidence (X١٥ Polycrstalline grains probably representing fragmentation and (٧) .(intensive parting (X١٣ Iron welded subangular to subrounded quartz grains with rare or (٨) .(no inclusions (X١٥

٧ Plate

.(Rounded and elongated quartz grain types (X١٥ (١)

Angular to subangular water bearing quartz grain types and (٢) .(mudchips (X١٥ Dominant monocrystalline quartz grain with abundant mircolite (٣) .(inclusions and overgrowth texture (X١٥ Monocrystalline dominated, elongated and subrounded to rounded (٤) .(quartz grains with rare inclusions (X١٥ Dominanat monocrystalline quartz grains having tourmaline (٥) inclusion. Also large feldspar crystal showing simple tiwing .(X١٥) .(Ironstone rimmed quartz grains and large mudchips (X١٥ (٦) Subrounded to rounded quartz grain that seems to present vacuole (٧) .(bubbles inclusions (X١٥ Dominating coarse and medium quartz grains probably (٨) .(representing bimodal distribution pattern (X١٥

Chapter Five Heavy Minerals Analysis

:Introduction ٥٫١ The heavy minerals usually occurs as accessory grains in They are chiefly silicates and oxides, many .٪١ concentration of less than of which are very resistant to chemical weathering and mechanical ٢٫٩g/cm٣ abrasion. The specific gravity of heavy minerals is greater than In this view and .(٢٫٦gm٣) which is higher than of quartz and feldspars according to that their low concentration in sediments, heavy minerals are separated from loose sediments by suing heavy liquids such as bromoform, tetrabromoethene and thoulets solution which have an intermediate specific gravity. The study of heavy minerals represents an excellent tool in the interpretation of sediments provenance and its petrographic characters In connection with clay mineral study, heavy minerals .(١٩٨٥ ,Marton) are useful in paleogeogrophic reconstruction and determination of the setting of classic rock sequence within an orogenic cycles (Van Andel .(١٩٩٧ ,in Mohamed ١٩٨٧ Stattegger ;١٩٥٨

:Procedure ٥٫٢ The thirty two representative samples that have been selected for the sieve analysis, were also subjected to the heavy mineral investigation.

the following steps were ,(٤٫٣٫١ After the completion of sieving (section followed:

٠٫١٢٥mm was taken from – ٠٫٥ Sand fraction ranging form -١ each sample for heavy mineral analysis. This fraction range

represent the one in which most of the heavy mineral are typically concentrated. Normal magnet was used to separate the magnetic minerals -٢ portion from each selected fraction range. -Electromagnetic separator was used to separate the non -٣ magnetic mineral portion. The process of the separation was repeated several times to ensure that all the non-magnetic minerals were completely removed. was used to g/cm٣ ٢٫٩ Heavy liquid bromoform of density -٤ separate light fraction from the heavy fraction with aid of separation funnel, filter papers and acetone for washing. :Slide preparation ٥٫٢٫١ The heavy mineral slide have been prepared according to the following procedure: The number of each sample was written on a glass -١ slide. .Ribbon traverses were drawn on each glass slide -٢ .C°١٥٠ The glass slide was placed on a hot plate at -٣ .Few drops of Canada Balsam were placed on the slide -٤ After the Canada Balsam became more liquid and the -٥ air bubbles escaped out, suitable amount of heavy mineral fractions were distributed on each slide. Additional few drops of Canada Balsam were added -٦ on the slide inorder to cover the grains. A glass cover was carefully pressed on the slide until -٧ excess Canada Balsam was forced out around the prepheries of the slide.

Each slide was cooled and then cleaned by xylene to -٨ remove the excess Canada Balsam on the slide and the glass cover.

:Heavy mineral identification ٥٫٣ The identification of the heavy minerals is based on their textural and optical properties such as form, relief, cleavage, inclusions, colour, pleochroism, birefringence and extinction angle using the petrographic microscope. After the identification of the heavy minerals grains in each slide, they were counted and their percentage was calculated. Hubert grains should be ٣٠٠-٢٠٠ suggested that about (١٩٧٩) and Marton (١٩٧١) grains doesn’t clearly ١٠٠ counted, since the examination of less than reflect the confidence interval.

:Results ٥٫٤

The identified heavy minerals from Shendi Formation includes zircon, tourmaline, rutile, staurolite and kyanite. Their percentages are calculated and histograms are constructed to and maturation degree on the basis of (٥٫١ .reflect their distribution (Fig .(٥٫٢ .ZTR index (Fig :Zircon ٥٫٤٫١ Zircon is the most ultra stable heavy mineral observed through the studied slides and easily identified because of its distinctive optical properties. It is characterized by prismatic shape high relief bounded by opaque halos. The subrounded to rounded shape indicates long distance of transport and recycling effect. The elongated crystals show parallel extinction. In general, the grains are small with sharp outlines, colourless inside and first order grey interference colour with weak birefringence.

Zircon occurs in appreciable amount together with rutile and tourmaline probably because of the strong chemical stability and and almost equal(٪٢٫٧) maturation.It is recorded at Bagrawyia area (٪٦٫٥) to north Fadnyia area (٪٦٫٤) amounts around Umm Ali village :Tourmaline ٥٫٤٫٢ Tourmaline is the most abundant ultra stable mineral through the studied samples. It is easily recognized by its moderate relief, parallel extinction, lack of cleavage and faint pleochroism. Tourmaline is characterized by varieties of colours, from colourless, common brown, green, bluish to dark. The darkness is due to some iron content. It is also characterized by second order interference colour with moderate birefringence. The grain shapes range form slender prism with termination at one or both ends, tabular, six sided basal sections, egg- shaped ellipsoid to spherical ones. The heavy mineral tourmaline occur with almost similar percentages at the prophesies of south Bagrwyia and northern Fadnyia ٪٣٥٫٢ areas. It is even found in excess around Umm Ali village reaching of the total amount of the heavy mineral.

:Rutile ٥٫٤٫٣ The ultra stable heavy mineral rutile generally occur as small

,in Bireir ١٩٩٢ µ (Mange and Maurer ٢٠٠ grains which rarely exceed

They are characterized by their deep blood colour bounded by .(١٩٩٣ thick black halo due to its high refraction indices, and have high interference colour. The grain shapes are angular, subrounded to rounded. Knee shaped rutile were also observed. The crystals shows parallel extinction and high relief with distinctive pleochroism in plane polarize light. In general heavy mineral rutiles occur with equal percentage at whereas to the south of ٪١٧٫٢ central Umm Ali and north Fadnyia area .(٪٢٢٫٦) Bagrawyia area it is slightly higher

:Staurolite ٥٫٤٫٤

Staurolite is a metastable heavy mineral observed through the studied samples and occur with irregular, subangular and semi platy shape. Staurolite grains show ragged or etched out line and some inclusions inside. The colour range from pale yellow to strong golden yellow with faint pleochroism in plane polarized light and first order interference colour with weak birefringence. The regular fragments show parallel extinction and moderate relief. Staurolite shows similar distribution in the northern and southern but in much less around the central ٪١٤٫٦ parts of the area forming about Umm Ali area. :Kyanite ٥٫٤٫٥ Kyanite is the most abundant metastable heavy mineral in the study area. Kyanite grains are comparatively large, commonly angular, tabular and elongated with a combination of two sets of cleavage and parting.

Oblique extinction distinguish the kyanite minerals. It is colourless or rarely bluish distributed uneven and thus have faint pleochroism whereas the colourless are non-pleochroic. Kyanite minerals are characterized by high relief, weak interference colour and strong birefringence.

Generally the heavy mineral kyanites is distributed evenly at the and decrease to (٪٣٩٫٤ ) northern and southern parts of the study area .at the central area around Umm Ali village (٪٣١٫٢)

:Discussion and Interpretation ٥ .٥ The heavy minerals assemblage are greatly influenced by the geology of the source area, physiography, climate and depositional .(١٩٩٥ ,environment (Sheikh Omar five heavy minerals have been ,(٥٫٤ As already shown (section identified from the selected areas on the basis of their optical properties. These are kyanite, staurolite, rutile , tourmaline and zircon. These results .(١٩٨٣ ) and Omer (١٩٦٦) are similar to those obtained by kheiralla suggested that the ZTR index is an aid to the (١٩٦٢) Hubert quantitative definition of mineralogical maturity in a heavy mineral suite. These indices are useful as a scale of the degree of modification or maturity of the entire heavy mineral assemblage of sandstones. Accordingly, any increase in ZTR index is the result of the progressive dissolution of unstable minerals. Nevertheless, on the basis of ZTR index and the lateral distribution of the heavy minerals, an increasing maturation is noticed towards the central parts of the study area compared The .(٥٫٢ .to the prepheries; possibly representing zonal distribution (Fig zonal distribution concept is in agreement with analysis of paleocurrent which reveal polymodal or centripetal mode of (٣٫٤ .directions (Fig

distribution. In this respect it has been possible to delineate two heavy mineral zones on the basis of ZTR index as follows: :staurolite – kyanite dominated zone -١ This zone characterize the northern Fadnyia and southern Bagrawyia areas showing almost equal and low percentage of ZTR index respectively.Thus, the higher percentage of metstable ٪٤٦ and ٪٤٥ of heavy minerals compared to the stable ones can be attributed to the relative proximity of the source areas. Moreover, the kyanite distribution exceeds staurolite within the same zone. This may lead to suggest that the source area was dominated by the high grade metamorphosed rocks since the staurolite is relatively more stable than kyanite minerals. :Zircon-Rutile –Tourmaline dominated zone -٢ ZTR minerals are dominated the sediments of the central areas This .(٥٫٢ .Fig) ٪٦٠ around Umm Ali village where ZTR index reached progressive lateral increase in ZTR index can be a attributed to the long distance of transport combined with possible evolution of the channel style and depositional environment of lacustrine dominated region around Umm Ali village. However, reworking and advance weathering processes within the basin might have been facilitated by the agitation of water current which lead to the progressive dissolution of metastable heavy minerals and survival of more stable ones. The relatively higher percentage of zircon at northern Fadnyia may indicate that the (٥٫١ .compared to southern Bagrawyia areas (Fig northern sediments are relatively more mature rather than the southern ones. Such mature sediments may be transported for along distance or otherwise a result of the source area geology of metamorphic and recycled origin. The later was probably supported with low percentage of rutile within the northern areas.

Tourmaline is the most abundant ultra stable heavy mineral throughout the entire area with its different shapes (tabular, sub-rounded to rounded) suggesting metamorphic origin and reworking product of This interpretation is in .(١٩٨٥ ,sediments (Welsbroad & Nachmias .(١٩٩٣) agreement with Bireir showed that staurolite and kyanite can’t be (١٩٧٨) Greensmith derived form an area which is a source of tourmaline and rutile. This confirms the assumption based on the paleocurrent analysis that multiple source areas are supplying the sediments. The existence of both angular indicates (٥ and rounded forms of some stable heavy minerals (Plate .(١٩٨٤ ,mainly contribution form different sources (Lewis On the basis of the above mentioned points, it is clear that Shendi Formation may have been derived form medium to high grade metamorphosed rocks with contribution of older sediments and igneous rocks. This conclusion is supported by sandstone petrography (section .(٦٫٥ Moreover, the presence of ultrastable heavy minerals and the absence of unstable ones indicate to the maturation of sediments. Such mature sediments deposited under warm humid tropical climate and gentle slope relief on which the reworking are important for the concentration of ultrastable heavy minerals (Petijohn, The reconstructed paleogeographic and climatic settings supported the above .(١٩٧٥ interpretation during sedimentation of upper member.

Chapter Six Sandstone Petrography

:Introduction ٦٫١ The mature quartz arenite sands, a product of some allocyclic and autocyclic events, need to be studied carefully to erect the depositional environment and composition of the source rocks. This is because the petrography of a sandstone depends, in large measure, on the composition of the source rocks. ١٧th selected samples The petrographic studies are carried out on prepared at the Central Petroleum Laboratories (CPL). These samples are too soft so that a special impregnation technique is used as described below.

:Thin section preparation ٦٫٢ For the preparation of blue dye impregnation using magnetic -١ hours the following ١٢ C for°٥٠ stirrer on a hot plate at percentages were mixed:

.g waxline blue ١٥ litter caldofix resin with ١ -a ٧٫٥g waxline blue ml caldofix resin with ٥٠٠ -b ٣٫٥g waxline blue ml caldofix resin with ٢٥٠ -c The samples were put on a small plastic holders and the vacuum -٢ was set to evacuate both the chamber and rock samples from any gases in the pour spaces. The samples were collected and put in an oven to dry and -٣ .٤h Cfor°٨٠ ٨h or C for°٤٠ solidify under temperature of C to prepare to make it°٤٠ A hot gluing device was settled to -٤ ready for pasting the samples on the glass slides.

The precession cutting saw was adjusted with a moving a screw to produce a -٥

٧٠٠micron ١mm to thickness between

A stera grinding machine was used to grind the samples, using -٦ course and fine grinding spindles with aid of water flow. The ٢٠٠mm. The coarse grinding spindle is used to reach size of -٤٠) fine grinding spindle was used till the suitable thickness .micron) was reached ٣٠ in size was used with ٠٫٣١١ For polishing the slides aluminum powder -٧ polishing paper. The prepared samples were collected and washed with alcohol and -٨ labeled.

:Thin section description ٦٫٣ :(٦ -١) Fadnyia samples ٦٫٣٫١ The slides are dominantly composed of fine to coarse moderately sorted grains. The monocrystalline grains are the essential components Rock fragments .٪٠٫١ Feldspar crystal forms .٪٩٤٫٩ to ٨٩٫٧ with range of .١٫٣ to ٠٫٢ represent as soft pelitic mudchips with percentage range form The grain shapes are commonly subangular, rarely angular, subrounded to rarely rounded ones. Microlite inclusions commonly observed but vacuoles are rare. The composite and andulose extinction types are common with rare straight ones. Heavy minerals such as zircon, tourmaline and rutile are observed as inclusions and detrital grains. Over- growth textures are represented in considerable amount. The samples resemble mineralogical maturation and its quartz arenites.

:(١٢ -٧ ) Umm Ali samples ٦٫٣٫٢

Moderately sorted, fine to coarse grained samples represent Umm Ali area. They are almost mono and polycrystalline grains. The Rock fragment of .٪٨٩٫٢ to ٨٣٫٦ monocrystalline ones range from .Feldspars are not observed .٪١٫٣ to ٪٠٫٨٢ sedimentary origin form about The grains are angular, subangular to rarely rounded ones. Microlite inclusions commonly observed but vacuoles are rare. The andulose and composite extinction types represent the dominance with rare straight extinctions. Iron oxide rims some grains and cemented others. Overgrowth textures are rarely observed. The sample are mineralogicaly mature and it is quartz arenites.

:(١٧-١٣ ) Bagrawyia samples ٦٫٣٫٣

Bagrawyia samples represented by modoretly sorted, angular to subrounded and rare rounded grains. The polycrystalline grains of sutured Rock .٪١٦-١٢ contact occurs in considerable amount range from The .٪٠٫١ while, feldspars records ٪١٫٥ – ٠٫٩ fragments range form grains are fine to coarse and contains abundant microlite inclusions and rare vacuoles. Individual elongated crystal is observed. The andulose and composite extinctions is commonly observed. Iron oxides rims some grains and welded the others. Heavy mineral resembles minor inclusions and detrital grains. The samples are quartz arenite.

:Results ٦٫٤

The sandstone of Shendi Formation are fine to coarse grained, moderately sorted quartz arenite commonly subangular subrounded and rarely rounded, also individual elongated grains were observed. Plate shows the different characteristics of these quartz arenite (٧&٦) sandstones. The Bagrawyia and Umm Ali samples are characterized by of polycrystalline quartz. This (٪١٦ to ١٠) considerable amount with remarkable ٪٩-٤ percentage decrease at Fadnyia to the range of

overgrowth texture. Nevertheless all the analyzed samples are to some extend, homogeneous in their grain size, shapes, inclusions, extinction (٦٫١) types and maturation. Based on microscopic observations Table shows the visually estimated percentage of each sample constituents and that of ٪٠٫٩ where the average percentage of rock fragments is shows the (١٩٨٠) adopted from Folk (٦٫١) .Fig .٪٠٫٠٣ feldspars is estimated provenance of quartz.

:Discussion and Interpretation ٦٫٥

The above described thin sections revealed that the upper member of Shendi Formation has been entirely composed of mineralogically mature quartz arenite sandstone, characterized by the lack of feldspars and rock fragments content. The low percentage of rock fragments may represent a part of minor locally derived mudchips. There fore, detailed description on quartz grain habits, sorting, extinctions, inclusions, nature of grain contacts and textural maturity was carried out so as to erect the depositional environment as well as source area geology.

The mineralogical maturity of the upper member is also supported by the heavy minerals analysis due to the abundance of ultrastable suggesting the (٥٫٢ .minerals particularly around Umm Ali village (Fig remoteness of the source area and long distant of transport.

.Visually estimation and semi-quantitative of samples :(٦٫١) Table

Sample Location Facies Poly- Mono- Rock Feldspars

No. types Quartz Quartz Fragment

٠٫١ ١ ٩٤٫٩ ٤ Fadnyia St ١

٠ ٠٫٦ ٩١٫٤ ٨ Fadnyia St ٢

٠ ٠ ٩٣ ٧ Fadnyia Sp ٣

٠ ١٫٣ ٨٩٫٧ ٩ Fadnyia Sp ٤

٠ ٠٫٥ ٩٣ ٦٫٥ Fadnyia St ٥

٠ ٠٫٢ ٩٢٫٨ ٧ Fadnyia St ٦

٠ ٠٫٨ ٨٩٫٢ ١٠ Umm Ali St ٧

٠ ١٫٥ ٨٨٫٥ ١١ Umm Ali Sp ٨

٠ ١٫٢ ٨٦٫٨ ١٢ Umm Ali Sm ٩

٠ ١٫٤ ٨٣٫٦ ١٥ Umm Ali St ١٠

٠ ١ ٨٥٫٥ ١٣٫٥ Umm Ali Sp ١١

٠٫٢ ١٫٣ ٨٧٫٢ ١١٫٥ Umm Ali Sm ١٢

٠٫١ ٠٫٩ ٨٧٫١ ١٢ Bagrawyia St ١٣

٠ ٠٫٨ ٨٤٫٢ ١٥ Bagrawyia St ١٤

٠ ١٫٥ ٨٥٫٥ ١٣ Bagrawyia Sp ١٥

٠٫١ ١٫٠ ٨٥ ١٤ Bagrawyia Sm ١٦

٠ ٠ ٨٤ ١٦ Bagrawyia Sp ١٧

The studied thin sections are predominated with monocrystalline quartz the monocrystalline (١٩٨٠) Thus, according to Folk .(٦٫١ grains (Table quartz grains are derived from igneous plutonic, volcanic or vein quartz. The plutonic quartz is characterized by slightly andulose extinction and xenomorphic irregular subrounded shapes with no inclusions. Such .(٨ photo ٧ characteristics were rarely found (Plate

Minor constituent of quartz grains are easily identified with their angular shapes, straight extinction and clean water bearing surface .(٢ photo ٦ without inclusions and seem to be of volcanic source (Plate

The hydrothermal vein quartz is represented by rare percentage. They are characterized by andulose and composite extinction and .(٣ photo ٧ Plate & photo٧ .٦ common vacuole bubbles inclusions( Plate

A few grains of monocrytaline quartz are observed represented by subrounded to well rounded shapes. These are most probably derived from pre-existing older sedimentary rocks. This is supported by over- growth textures and reworking evidence observed particularly in the .(٦&٢ photo ٧ samples collected from Fadnyia areas ( Plate

The polycrystalline quartz grains of sutured contacts are observed in minor amount at Fadnyia samples and in considerable amount at Bagrawyia and Umm Ali areas. These are most probably derived from basement rocks where the fine to medium grain may have been derived from low grade metamorphic rocks and the coarse grains may have been .(١ photo ٧ originated from high grade ones ( Plate

The most common type through the studied thin sections, is the monocrystalline quartz grains that, are characterized by microlite

These .(٣ photo ٦ inclusions and andulose or composite extinction (Plate are most probably derived from basement rocks. The quartz grains which usually derived from basement terrains are polycrystalline. Since the identified quartz grains are monocrystalline it seems that reworking process disintegrate the polycrystalline grains to a monocrystalline ones. This is probably supported by the roundness and higher percentage of ,Amongst the later type .(٥ Plate & ٥٫٢ .ultrastable ZTR minerals (Fig the observed individual elongated or platy shapes seem to be of schistose That is because the quartz grains usually grow in .(١ photo ٦ origin (Plate between the micaflakes and oftenly exhibit platy shapes. Moreover, the monocrystalline quartz occurs with subangular to subrounded morphoglogies obviously indicates a source of medium to high grade metamorphic rocks. This is supported by the abundance of kyanite and .(٤ .٥ staurolite heavy minerals (section

Depending on the above evidences, the source area for upper member of Shendi Formation was most likely medium to high grade metamorphic rocks with contribution from older sedimentary and igneous rocks.

The moderate sorting and the wide varieties of the quartz grain morphologies, presence of mudchips and iron welded grains may suggest that the sandstones of Shendi Formation (upper member) are texturally immature supporting the fluvial evidence of upper member rather than texturally super mature eolian sediments. The conclusion may support the assumption of the weak and shallow channel styles, and coinside with distal braided and meandering one as well as effect of local lacustrine environments, which permits the clay matrix to fill the interstitial grain pores.

The quartz arenite rich sediments were mostly produced due to the warm humid climate prevailed at the source area and facilitate removal and destruction of unstable grains. The climatic evidence is supported by the occurrences of kaolinitic mudstones and ironstone formation (section Moreover, the slow sedimentation rate that characterize weak and .(٣٫١٫٨ shallow distal braided and meandering channel style may suggest the peneplaination and tectonic stability of the paleotopography during sedimentation of upper member. The same factors are in favour of strongly leaching condition of the source areas, evacuation and enrichment of orthoquartzite, kaolin and ironstone in the study area.

Chapter Seven Clay Minerals Analysis

:Introduction ٧٫١ Clay minerals belong to the phyllosilicate family and are typically Clays .(٢٠٠١ ,in Elamein ١٩٨٩ ,micron in size (Weaver ٢ smaller than can be formed either directly or indirectly from the decomposition of primary alumino silicates. Thus clay minerals give great information concerning processes and evolution of sequences (weathering, hydrothermal alteration, diagenesis and low grade metamorphism) of which they form a part. They may also reflect the sedimentation history better than any other component, because the characteristics of the clay minerals are to some extend, subject to changes depending on the .(١٩٩٧ ,in Mohamed ١٩٦٢ ,environment in which they are formed (Grim Hence, the study of the clay minerals provides information about paleogeography source areas, depositional environments and post depositional changes.

:Procedure ٧٫٢

A total of nine samples were selected so as to identify the clay mineral types on the basis of X-ray diffraction techniques (XRD). In µ from the selected ٢ order to extract the portion of the sediment less than samples, the following preparation procedures were followed.

:Sample preparation ٧٫٢٫١

The selected samples were carefully disaggregated -١ using a porcelain mortar and pestle.

Further disaggregation was done after scattering the -٢ .hours ٢٤ sample in distilled water and left for Distilled water was added continuously until hydraulic -٣ .sec was obtained/m٣ ٥٠ conductivity of about Distilled water with dispersed material was -٤ ٢٠٠٠ minutes under speed of ٢ centrifuged for µ ٢ < round/min to deposit fractions with diameter while clays remained in suspension. ٢µm were separated by The clay fractions less than -٥ sucking the suspended clay through filter candles. The separated clay fractions were collected in dishes -٦ .hours ٢٤ ٥٠oC for and then put into an oven set at For analytical purposes, a glass sample-holder was -٧ covered evenly by a wet sample, which is in turn covered by a thin glass sheet to obtain the basal reflection. hours so ٢٤ The samples were kept in a desicators for -٨ as to get arid of water. : XRD analysis ٧٫٢٫٢

Each of the selected samples was treated three times using the XRD. The first treatment was done for the sample after the dryness and the resultant pattern is called the air-dried XRD pattern. After this treatment, each sample was kept in a desicator containing ethylene-glycol hours and then analysed using the XRD. At last the ٤٨ solvent for a bout hours and analyzed for the third ٦ C for°٥٥٠ sample was heated under time which produces the C heated diffraction pattern.

X-ray ٥٠٠ XRD analysis were performed using a ssiemens D in the case of the °٣٣ to °٣ diffractometer. The scanning was done from

in the case °٢٠ air dried and glycolated samples. The scanning reach only of the heated samples. Through the whole analysis, the voltage and ٣٠m respectively, while the scanning speed ٤٠kv and current were kept a semiquantitative (١٩٧٦) minut. Following Thorez/°١ was kept at estimation of clay mineral constituents was carried out, accordingly the percentages of the different clay mineral types were calculated (Table .(٧٫١

:Identification of clay minerals ٧٫٣

Quantitative and qualitative identification of clay minerals have been obtained from nine samples (Appendix E). Koalinite is characterized by ٣٫٥A° peak. This peak is unchanged even after the sample is and ٧٫١ the treated with ethyleneglycol. The high rate of heating destroy the mineral. ١٧Ao ١٤A° peak. This peak increases to Smectite is characterized by the .١٠Ao after heating after the ethyleneglycol treatment and decrease to ١٠A° peak before and after Illite is characterized by an unaffected treating and heating process.

:Results ٧٫٤

The diffraction pattern analysis revealed different types of clay minerals which are tabulated with their percentage of distribution (Table :and includes (٧٫١

Kaolinite is the major clay mineral species throughout -١ to ٢٠٫٦ the studied area with a percentage varing from Such kaolinite is the most ubiquitous.٪١٠٠ aluminosilicate mineral in most soils and permeable bed rock in warm, moist regions formed as a residual

weathering product. Moreover, hydrothermal alteration of other aluminosilicates especially feldspars form this mineral. In addition, it can also form as a diagenatic mineral filling the pore spaces .(١٩٨٣ Singer, et al) Smectite is the second identified clay mineral in the -٢ .٪٤٠٫١ to ١٧٫١ samples with percentage ranging from The smectite is the real name for group of minerals both dioctahedral and trioctahedral that can expand and contract while maintaining two dimensional .(١٩٨٦ ,crystallographic integrity (Morre and Hower Illite also identified with a percentage ranging from -٣ Illite is formed in sedimentary rocks as .٪١٠٫٨ to ٧٫٥ pedogenic and weathering product and is also formed diagnostically as by product of weathering in hydrothermal and in metamorphic environment due to .(١٩٨٣ the burial condition (Singer, et al

Discussion and Interpretation ٧٫٥

in ١٩٨٨،١٩٩١ ) Tucker ;(١٩٥٨،١٩٨٩) According to Weaver in general the origin of the clay minerals can be (٢٠٠١ ,Elamein summarized in three points:

Inheritance or detrital form due to the weathering processes -١ of the parent rocks.

Neoformatin includes the clay minerals that was formed -٢ insitu within the depositional environment.

Tranformatin during diagenesis as a result of some stable -٣ physically conditions.

The distinction of clay origin should be based on the clay composition, structure, morphology, distribution and textural properties. Hence, the detrital origin has been adopted for the dominating kaolin and illite due to the peakedness or sharp diffraction patterns of the Part of the kaolin, may have been . (١٩٩٣,monocrystalline type (Bireir formed by neoformation processes.This kaolin occurs in the smectite dominated samples. On the other hand, the polycrystalline smectite may have been formed by neoformation within the basin (Bireir,Op.cit). This who suggested partial (١٩٨٣) result is in agreement with Omer authogenic origin for kaolinite and smectite occurs.

and Singer (١٩٧٩) Chamely ,(١٩٧٨) According to Greensmith the origin of kaolinite in clays can be attributed to the strong (١٩٧٥،١٩٨٠) leaching condition within tropical warm humid climate prevailing in the source area which maintained, from the high rainfall exceeding evaporation, removal of Ca, Mg, Na, K and Fe ions from the parent rocks and leaving high proportions of Al and Si as residual ions to form kaolinite. The actual leaching that has taken place is greatly effected by non climatic variables such as primarily topography, parent rock The .(١٩٨٠ ,characteristics and to limited extent the organisms (Singer gentle slope topography together with the minimal runoff are infavour of .(١٩٩٥,the strong leaching processes (Sheikh Omer

Suitable climatic condition to generate kaolinite has been recorded during Albian-Cenomanian time on the basis of occurrence of laterite, and the (١٩٩٠ ,bauxitic ironstone and ironstones (Germann et al vertebrate faunal assemblage form certain formation ( Werner et al, proposed tropical climate of the Sahelian type (١٩٨٣) Omer .(١٩٩٣

also recorded similar climatic (١٩٩٣ ) interrupted by dry season. Bireir pattern during sedimentation of upper Omdurman Formation (Merkhiyat member).

mentioned that kaolinite is more likely to be (١٩٥٨)Weaver dominant in fluvial environments. These are most likely the prevailing conditions responsible for occurrences of kaolinite characterizing the upper member of Shendi Formation. However, it might be suggested that the switching of external fluvial drainage into the lake basin began with high siliciclastic influx as a result of the prevailing climatic conditions at the source area. This could mostly be the case since the kaolin was supposed to be detrital in origin.

١٧٫١ In the studied samples, smectite also occur but in the range of and (١٩٩٧ ) Such clay mineral was interpreted by Mohamed. ٪٤٠٫١ to as smectite rich bentonitic mudstone characterizing the (١٩٩٣) Abdullatif lower member of Shendi Formation.

pointed that smectite has been formed under (١٩٧٨) Greensmith the following conditions:

.Evaporation exceeds precipitation -١ .leaching processes should be negligible -٢ :Alkaline condition should prevail to maintain low Al -٣ Si ratio. It is believed that kaolin requires an acid environment for its formation whereas alkaline environment favours formation of smectite Subsequently the alkaline environment .(١٩٩٧,in Mohamed ١٩٤٩ ,Millot) (١٩٩٣) ,to form the later has been suggested by Bohor and Triplehorn Arid to semiarid climate with high relief or .(١٩٧٩) and Chamely depression such as topography and poor or internal drainage preventing the removal of silica and alkaline earth ions, may have been appropriate

condition creating smectite clays. Moreover, in the study area Mohamed suggested that smectite has been derived from hydromorphic soil (١٩٩٧) by neogenesis and aggradations on the basis of the six possible origins of which all point towards detrital origin. In (١٩٨٩) smectite of Chamely mentioned that an important source of smectite is (١٩٧١) addition, Berner the alteration of volcanic glass with relatively high silica content characterized by very slow movement of water within swampy low lands in arid to semi arid regions.

and depending on the (١٩٩٣) For Bohow and Triplehorn environment in which the ash falls, it can change it to different clay minerals or be preserved for subsequent alteration. Thus, if the environment is acidic the ash usually altered to kaolinite and this forms a rock called a taustein, but if the environment is moderately alkaline the ash is usually altered to smectie and this forms a rock called bentonite. Incase of highly alkaline environment as in playa lakes, the ash usually altered to zeolite and K feldspars. It is therefore, obvious that the environment may control the creation of the different type of clays depending on its chemistry regardless of the parent rock characters.

Arid to semiarid climate has been suggested by many workers ;١٩٨٣ ,onset Albian-Cenomanian time from Omdurman Formation (Omer ,١٩٨٠ ,Bolten ;١٩٧٩ ,Nicolas and Bildgen ;١٩٩٠ ,Awad and Schrank Slightly alkaline .(١٩٧٢ ,and Shendi Formation by (Medani (١٩٨٤ ,Batten water chemistry had mostly prevailed associated with certain climatic conditions at the center of the pre-existing lake environment. Such conditions was most probably lead to neoformation of smectite clays within restricted internal drainage. Minor detrital portions are probably associated with suitable environment and weakly leaching condition due to the aridity of the climate.

The studied samples also revealed occurrences of some illite clays which are considered to be of detrital origin formed as a result of surface processes. This is simply because illitization of smectite usually begins .(١٩٨٣ ,٣km in burial conditions, (Singer et al-٢ and is completed form Mohamed ,(١٩٨٣) Similar interpretation has been reached by Omer .(٢٠٠٠) and Osman (١٩٩٧)

pointed out that illite may commonly be derived (١٩٨٩) Chamely in sedimentary basins from erosion and detrital supply of sloped and slightly weathered outcrops of metamorphic and plutonic source rocks. recommended that acid potassium rich condition (١٩٧٨) Green smith favours illite formation, rainfall and leaching should be only moderate to intermittent. Thus, it can be suggested that illites are climatically and topographically may characterize the hypothetical sedimentolgoical boundaries between smectite rich mudstone and kaolin rich mudstones of lower an upper members respectively. Hence, the predominance of these three clay minerals in one sample may indicates there is no abrupt contact or may suggest homogenous source area and or good mixing of fine (١٩٧٥) suspended materials within the basin. Moreover, Singer mentioned that kaolinite, which is formed in humid high leaching conditions, is more stable in its characteristics when the climate turns arid, whereas the opposite can degrade the smectite into kaolinite. This is possibly because degradation processes are faster than gradational ones. So it is most probable that the interference occurrences of these clays with lack of abrupt contact are due to the degradational or pedogenic processes. This is supported by major occurrences of pedogenic facies such as gypsum specks, carbonate caliche and iron nodules. Therefore, this may partially support the authogenic origin of kaolin and illites .(١٩٨٣) suggested by Omer

The numerous occurrences of kaolin rich clays indicates mafelsic to felsic sources, whereas illite and smectites are products of weathering of the basic and ultrabasic and their metamorphic equivalents (Chamely, Since the source area has not changed, the variation in clay .(١٩٨٩ mineral types could mostly be attributed to the variation in the paleoclimatic and tectonic settings that prevailed during sedimentation of lower and upper members of Shendi Formation.

Clay mineral percentages of some samples representing :(٧٫١) Table Shendi Formation.

Sample Profile No Facies Kaolin Smectite% Illite Non No type % % clays

- - - ١٠٠ Fm ,١٤ Upper part of profile ١ J.katla.

- - - ١٠٠ Fm ٣ Upper part of profile ٢

- ٧٫٥ ٤٠٫١ ٥٢٫٤ Fm ,٢٥ Lower part of profile ٣ J.mazmun

- - - ١٠٠ Fm ,١٥ Upper part of Profile ٤ ١.J.Umm Bour

- - - ٢٠٫٦ Fm ٣ Lower part of profile ٥

- ١٠٫٨١ ١٧٫١ ٧٢٫١ Fm ,Lower part of profile١٧ ٦

٢.J. Umm Bour

١٠٫١ - - ٨٩٫٩ Fl ,Upper part of Profile١٤ ٧

J. katla

- - - ١٠٠ Fm ١٨ Road cut of profile ٨

- ٩٫٩ ٣٩٫٣ ٥٠٫٥ Fm ,Lower part of profile١٤ ٩

J. Katla

Chapter Eight Geochemical Analysis :Introduction ٨٫١ Using geochemical analysis for the interpretation of depositional environments is a subject of a hot contraversory since the work of Gold- The idea is based on the fact .(١٩٥٧) and Degens, et al (١٩٣٢ ) schmidt that the accumulation of clay minerals as weathering products, may largely depend on the chemical conditions which characterize the depositional environments.

:Geochemical analysis ٨٫٢ Three geochemical methods were used.These are:

Spectrophotometric method for determination of SiO٢ -١

.and Al٢O٣ Atomic absorption spectroscopy for detection of -٢

.MgO and MnO ,Na٢O, K٢O, CaO, Fe٢O٣ .Determination of loss on ignition -٣

and Spectrophotometric method for determination of SiO٢ ٨٫٢٫١

:Al٢O٣

the samples were prepared as and Al٢O٣ a- For the determination of SiO٢ follows: mg of each sample was accurately weighed in a ٠٫٠٥ -١ gm sodium hydroxide and ١٫٥ nickel crucible with fused in electric oven. The fused sample was left to coal and distilled water -٢ th was added to fill up to ¾ the crucible volume. This .hours ٢٤ step took

The cooled sample was transferred into a flask -٣ ml ١٠ ml distilled water and ٢٥٠ containing conc.hydrochloric acid. liter by ١ The solution in the flask was completed to -٤ adding distilled water. The resultant solution represent the stock.

:determination, the reagents were prepared as follows b- For SiO٢ :Ammonium molybdate reagent solution -١

٢٥ml of ٠٫٥gm of ammonium molybdate were dissolved in

in a beaker. The mixture was transferred to a (١:٤)H٢SO٤ ١٠٠ml volumetric flask and diluted with distilled water to the mark and shaken . :٪١٠ Tartaric acid reagent solution -٢ ١٠gm of tartaric acid were dissolved in distilled water in a .١٠٠ml volumetric flask and diluted to the mark Reducing Solution :

This is a mixture of two solutions, A and B. Solution A was ml beaker ١٠٠ gm of sodium sulphite in a ٠٫٧ prepared by dissolving

sulphuric acid -٤-naphol-١-gm of amino ٠٫١ ml of H٢O ١٠ containing was added and allowed to dissolve. ٩gm of sodium metabisulphate were Solution B was obtained when ml beaker. Solution A and ٢٥٠ ml of distilled water in a ٧٥ dissolved in .١٠٠ml in volumetric flask mark B were mixed together and diluted to The three reagents were added as follows: ml of ammonium molybdate solution was added and ٢ -١ minutes ١٠ allowed to stand for

ml of tartaric acid solution ٥ minutes ١٠ After exactly -٢ were added and shaken well to mixes the content. ٢ml of the reducing solution were added and the -٣ ١٠٠ml by adding distilled volume was complete to water. ٣٠ The mixture was shaken and allowed to stand for -٤ minutes. The addition of the reducing solution changed the colour of the molybdosilicate complex from yellow to blue. The intensity of the blue solution was measured in the -٥ spectrophotometer by comparing with the standards and the blank solutions.

c- For Al٢O٣determination,the following reagents were prepared as follows: :Sodium acetate, buffer solution -١ ٢٠٠ml distilled ١٠٠gm of sodium acetate were dissolved in ٣٠ml of acetic acid was added and the .water in a beaker ml volumetric flask and the ٥٠٠ solution was transferred to volume was completed to the mark with distilled water. :Calcium chloride -٢

١٠٠ml of H٢O .ml beaker ٢٥٠ were weighed in a gm of CaCl٢ ٧ ١٥ml of conc. HCl were added and the mixture was heated to boiling and ٥٠٠ml for few minutes, then the solution was cooled and transferred to ١٠٠ml ١٥ml of each sample was transferred to .volumetric flask volumetric flask and the above reagents were added as follows:-

solution ٢ml of CaCl٢ -١ (٪١٠) ml of hydroxyammonium chloride solution ١ -٢ .(٪٠٫٧٥ ) ml of potassium ferricyanide solution ١ -٣ .minutes ٥ after (٪٤) ml of thioglycollic acid ٢ -٤

١٠ ١٠ml of buffer solution were added and kept for -٥ minutes ,were added (٪٠٫٠٥) ١٠ml of alizarin red Finally -٦ dilution with distilled water was made to the mark and the flask was shaken well. The intensity of the colour was measured in the -٧ (١٩٥٠ ,spectrophotometer ( Parker and Goddard

:d- Determination of Silica SiO٢

was based on the method devised by The determination of SiO٢ The stock prepared above was .(١٩٥٢،١٩٥٦) Shapiro and Brannock pipetted and ammonium molybdate was added to form the yellow molybdosilicate complex. Subsequents reductions of such complex to the molybdenium blue was affected by the addition of a solution containing sulphuric acid), sodium bisulphate and sodium -٤ naphol -٢ amino -١) sulphite. The intensity of the molybdenum blue solution was measured in the spectrophotometer together with the intensity of a blank and the standard silica solution. Atomic absorption spectroscopy(A.A.S) For determination of ٨٫٢٫٢ other major oxides: The atomic absorption spectroscopy is a method for determining the concentration of an element in a sample. Where each element is atomized and characterized by a light absorption wavelength. The following principles, instrumentation and and Zussman (١٩٨٩) procedure were summarized after Allen .(١٩٧٧) The instruments consist of an atomizing device, a light source and a detector. The sample is prepared as above in format solution and spirited into an ebulizer and atomized in a acetylene nitrous oxide flame,

where the metal ions are covered into the atomic vapour state. Radiation in the visible and ultra violet regions of the spectrum, characteristic of a particular element, is passed through the flame and decrease in intensity is measured using a detector after passing through amonochromatic. The degree of the absorption is a quantitative measure of concentration of the ground state atoms in the vapour. For determination of other major oxides, the following steps were followed for the preparation of each sample: gm of finely powdered sample was accurately ٠٫٥ -١ weighed in a teflon beaker. ml of ٥ ,(HF ٪٤٠) ml of hydrofluoric acid ١٠ -٢ perchloric acid

.٢ml of nitric acid were added respectively and (HClO٤ ٪٦٠) The mixtures were placed in a hot sand bath for the -٣ purpose of dryness. The same amount of acids were mixed again and -٤ added to the samples that did not dissolve. ٥N) was added to the) ml of hydrochloric acid Hcl ١٠ -٥ dissolved and cooled samples. The beaker was set on a sand bath a few minutes to dissolve the residue. ٥٠٠ml volumetric The solution was translated in to -٦ flask adding distilled water. The sample is ready now to run on atomic absorption spectroscopy.

:Determination of loss on ignition ٨٫٢٫٣

The loss on ignition (LOI) is defined as the decrease in weight of a rock sample on heating at high temperature as a result of liberation of the

and SO٢ volatile components like chemically bonded H٢O, OH, F,CO٢ The loss on ignition for each sample was determined as follows:

gm of sample powder were accurately weighted in a ١ -١

(platinum crucible ( ω١ The sample in the crucible was heated in a muffle -٢ .C for an hour°١٠٠٠ furnace at The heated sample was cooled in a desicator for about -٣ two hours. The sample was weighed after cooling to room -٤

(temperature (ω٢ The loss on ignition (LOI), was calculated according -٥ to the following equation:

ω1 −ω2 LOI = x100 ω1

.weight of sample before heating = where ω١

.weight of sample after heating = ω٢

:Results and Discussion ٨٫٣ The concentration of major oxides and loss on ignition in the .(٨٫١ analysed samples are shown in (Table

The geochemistry of the major elements is largely controlled by For example the silica .(١٩٩١ the clay minerals ( Bonjow and Dabard content reflects trend in geochemistry of clay deposit and probably the depositional environments.

– ٤٢٫٤) content in the treated samples range between The SiO٢ The low silica content samples typified the smectite rich clays .(٪٥٤٫٠٩ whereas the high silica content is related to kaolin rich clays. This can reflect the different in depositional environments with different paleotopographic and climatic settings. However, the smectite which authogenically formed within alkaline environment permits other cations

to be fixed within the crystal lattice influencing the concentration of

On the other hand, the kaolinite, which formed detritaly during.SiO٢

intense leaching conditions, all other cations may be dissolved and SiO٢ rich clays enriched in site of deposition.

These values are .٪٢٤٫٩ to ٢٠٫٦ ranges from The content of Al٢O٣ essentially controlled by the clay mineral types. Therefore, the relatively

can be correlated with kaolin rich clays while the higher content of Al٢O٣

,contents are related to the smectite rich clays. Unlike kaolinite low Al٢O٣ smectite is characterized by low Al/Si ratio due to the substitution of other cations such as Fe, Mg and Ca in the octahedral position (Sheik

Moreover, the higher content of Al٢O٣which depend on the .(١٩٩٥,Omer clay mineral types is in turn affected by the depositional environment. However, kaolinite is best formed under weathering regime where water is abundant and circulation is rapid, such as fluvial regime so that solute concentration remain low. Smectite, on the other hand, is more likely to be deposited in an area where drainage is slower and water is less Therefore, the solute concentration are .(١٩٧٨ ,abundant (Greensmith relatively high (lacustrine regime). with an average of ٪٧٫١ to ٦٫٨ The iron content range between in ٪٤٫٢ in smectite dominated samples compared with an average of ٪٦٫٩ kaolin rich samples. This iron may have probably been precipitated either .٢in oxidizing and reducing conditions respectively+٣or Fe+as Fe Therefore, the iron in smectite clays may be fixed within the crystal lattice under alkaline conditions, while in kaolinite may be formed due to the strong leaching and denudation of the source areas and persist as impurities during transport within depositional environment.Simillar .(٢٠٠٠) interpretation is given by Osman The high .٪٠٫٢ to ٠٫٠٧ The concentration of CaO is in the range of values are associated with the smectite clays whereas the low values

found in the kaolinite rich clays. This is clearly reflected in the chemical ٢cations can be a desorbed+formula of those minerals. However, Ca within the lattice of smectite from the alkaline and slightly saline environment. in smectite ٪٠٫٧٥ The concentration of Mgo shows an average of This .٪٠٫٢٩ rich samples compared to less concentration in kaolinite ٢in smectite typically indicate the+moderate concentration of Mg content ٢+moderate alkalinity of the lake water because the higher Mg On .(١٩٩٣,characterize the highly alkaline environments(Jones & Weir the other hand, kaolinite samples shows low concentration of MgO due to the strong leaching condition or probably to the parent rock characteristics.

and this can ٪٨٫٧ to ١٫٧ The concentration of Na٢O range between also be correlated to the clay mineralogical varieties. However, the smectite clays shows higher values than kaolinite. The higher

concentration of Na٢O in smectite clays, may be due to the alkaline

environment affinity, while the slightly high Na٢O in kaolins with a range can be attributed to the predominance of feldspars at the (٪٢٫٣-١٫٧) of source areas and intermediate characteristic of the parent rocks. This can (٪٠٫٥) and (٪٠٫٢٩) ٢+and Ca ٢+be correlated with the low values of Mg respectively in kaolin rich clays. The relatively high iron content with an in kaolin samples, may be due to the strong leaching (٪٤٫٣) average conditions the source area and fixation of iron during transport. It may also have appeared as impurities at the site of deposition(Osman,Op.cit). The loss on ignition values also can be correlated with the clay mineralogical composition. The samples with higher content of smectite clays are characterizing by relatively higher values of loss on ignition. Generally it is more likely to conclude that the major elements such as Si, Al, Ca, Mg, Fe and Na reflects the mineralogical control in clay

micron in size. Moreover, Mg, Fe, Ca, Na and Al ٢ fractions less than chemical elements may reflect the moderate alkalinity/salinity of lake water and fresh fluvial water of lower and upper members of Shendi Formation respectively.

Chapter Nine Summary and Conclusion

The study area which is occupied by (Albian- Cenomanian) sedimentary strata of Shendi Formation (Nubian Sandstone Group) is located north of Shendi from Wadi ElHawad up to Wadi Elmukabrab in .٧٠km across the north. It is approximately

The study is carried out so as to reconstruct the origin, facies types, paleogeography and paleodepositonal environments, through investigation of surface and subsurface lithofacies analysis, grain size, heavy mineral, petrography and clay mineralogy and geochemistry.

The outcropping lithofacies analysis reveals eight facies types on the basis of their lithology, grain size and sedimentary structures. The facies types are trough cross bedded sandstone facies (St), planar cross- bedded sandstone facies (Sp), horizontally-bedded sandstone facies (Sh), massive sandstone facies (Sm), ripple cross-bedded sandstone facies (Sr), laminated fine sandstone, siltstone and mudstone facies (Fl), laminated siltstone, mudstone facies (Fsm) and massive mudstone facies (Fm). The subsurface lithofacies analysis reveals nine facies assemblages on the basis of their grain size, lithology and colours which includes coarse sandstone facies (Cs), medium sandstone facies (Ms), fine sandstone facies (Fs),siltstone facies (Z), clayey sandstone facies (cS), sand clay-stone faces (sC), mudstone facies (M) claystone facies (C) and evaoprite pedogenic facies. Five architectural elements were identified within the formation includes, sand channel element (CHs) and sandy bed form (SB) which dominate Fadnyia and Bagrawyia areas respectively. Lateral accretion

element (LA), crevasse-channel element (CR) and fine deposit elements (FF) occur partially at Bagrawyia and dominate around Umm Ali village. The depositional environments are subdivided into lower lacustine and upper fluviatile members including three lithofacies associations, fluvial lithofacies association A, fluvio-lacustrine transitional association B and deep lacustrine association C. ٦th ,٧th order encompass the entire formation The bounding surface ٣rd ٤th and -٥th ,order rank separate between lower and upper members orders characterize channel surfaces and macroforms. Second and first orders separates meso and microforms respectively and dominated the profiles. The deep lacustrine lithofacies association C is characterized by massive mudstone facies (Fm) deposited from suspended load.The transitional lithofacies association B is dominated by fine laminated sandstone, siltstone and mudstone facies (Fl) and laminated siltstone and mudstone facies (Fsm). Moreover, the above lacustrine lithofacies dominate the subsurface strata comprising almost the subsurface facies types with part of minor fluvial sediments recorded from the basin prephery boreholes. The upper fluvial lithofacies association A is characterized by erosional channel surface, trough cross- bedded sandstone facies (St), planar cross-bedded sandstone facies (Sp), ripple cross-bedded sandstone facies (Sr), massive sandstone facies (Sm) and horizontally bedded sandstone facies (Sh) with some overbank and flood- plain fine sediments. The upper fluvial member of Shendi Formation represent the result of the work on different channels styles and subsequent degradation processes such as low-sinuosity braided channel at Bagrawyia area with some difference in details to meandering around Umm Ali village. Northwards it’s braided in style of platte type or shallow pernial braid.

The ironstone of Shendi Formation seems to be dissolved and strongly leached from the parent rock within warm humid climate activated by tectonic stability. It is then transported to the site of deposition where it has differentiated within oxic and shallow environments such as overbank, floodplain and lake shore facies. Such environments might have probably provided suitable physico-chemical conditions for its precipitation. Therefore, allocyclic and autocyclic control can be suggested for the iron sedimentation. The lake environment on which the sediments were centripetally accumulated as estuarine environment seems to be moderaltly large, broad and shallow due to intensive bio and phytoturbation, pedogenic facies comprising paleosoil horizon and mudcracks. This is supported by subsurface facies types, their characteristics, and geometry. The chemistry reveals onset alkaline and slightly saline near shore during hydrologically closed basin. This water type is subsequently become fresh water during hydrologically open lake that was controlled by fluvial input. The facies depositional model of Shendi Formation represents coarsening and thickening up-ward cycles. The zonal distribution of sediments was controlled by allocyclic and autocyclic sedimentation mechanisms. In addition, base level change is controlled by gradual decrease of tectonic activity leading eventually to dominance of an open or external drainage system during sedimentation of upper member. Such conceptual model is a constructed from subsurfaces lithofacies analysis, heavy mineral zonation and clay mineralogical and geochemical variation within lower and upper members of Shendi Formation. The lithofacies and grain size analysis show that the sandy sediments of the upper member are of fluviatile origin. Moreover, the grain size analysis reveals consistent gradation from silty sand to muddy

sand between Bagrawyia to Umm Ali areas. In contrast, the northern Fadnyia areas are sandy dominated. In this way, the grain size analysis support the lithofacies analysis typifying the fact that these areas were characterized by the existence of different channel styles. The heavy mineral zonal distribution are coincide with the paleoenvironment subdivisions. The kyanite-staurolite metastable minerals are outer zone coincide with the braided stream system. The inner ZTR dominated minerals zone coincide with the meandering and lake environments . The petrographic study reveals that the upper member of Shendi Formation is composed of mineralogically mature quartz arenite sediments characteristically lacking the feldspar and rock fragments. The wide varieties of the quartz grains morphology and the presence of mudchips indicate textural immaturity and fluvial origin of the upper member. Moreover, the quartz arenite rich sediments point towards warm humid climate and tectonic stability with long distance of transport. The same factors favoured lateritic weathering and activated hydrolytic processes which lead to preferential chemical leaching, evacuation and enrichment of orthoquartzite, kaolin and ironstone in study area. Thus, differentiation was achieved during transport and deposition owing to the grain size and facial differences within proximal fluvial and distal lacustrine environments where coarse fractions were separated from the fine ones. The clay mineral analysis reveals that the smectite was created by neoformation within alkaline environment during period of hydrological closed lake. On the other hand, kaolin and illites are detrital minerals supplied with fluvial input with minor authogenic grains occurs as pedogenic facies.

The centripetal paleocurrent direction indicate that circular high lands were surrounding. This leads to suggest that the sediments under consideration might have been derived from the Butana, Sabaloka and Bayuda desert. These source areas were dominated by medium to high grade regionally metamorphosed mafelsic to felsic affinity with contribution from older sedimentary and igneous rocks. This is confirmed by heavy minerals, sandstone petrography and clay mineralogy and geochemical analysis. The occurrences of smectite and evaporite facies characterize onset arid to semiarid climate during the deposition of the lower member (early Albian ? ) whereas the dominated kaolins, ironstones, quartzarenite and mature heavy minerals indicate warm humid ones during sedimentation of upper member (Albian - Cenomanian). The geochemical data show the overall inconsistency and the variation in accordance with the clay mineral composition. The relatively

is directly related to the kaolinite and SiO٢ higher percentages of Al٢O٣

and content. Therefore, the samples having high percentages of Al٢O٣

MgO, CaO, Na٢O characterize kaolin ,and low percentages of Fe٢O٣ SiO٢ rich clays of the upper member, whereas the samples having lower

MgO, CaO ,and high percentages in FeO٣ SiO٢ , percentages of Al٢O٣

.and Na٢O characterize the smectite rich clays of lower member

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