Sedimentological and Geochemical Characteristics’ of the Sediments West of Al Bauga Area, River Nile State, Sudan

Abstract

The study area lies about l O0 km n orth ofAtbara town in the River Nile State, in the central north of the Sudan, and located between latitudes 18° 00/ and l8" 18/ N and longitudes 33° 38/ and 33° 57/ E with an extending area about 300 kmz. The aim of the study is to determine the sedimentological, geochemical and hydrological characters to clarify the depositional palaeoenvironment. Four surface vertical profiles were examined in the field and several techniques were applied including grain size analysis, thin sections, x ~ ray diffraction, infrared spectroscopy, x — ray fluorescence, atomic absorption spectroscopy, spectrophotometer and water analysis. The lithofacies analysis of the Al Bauga sediments reveals seven facies types on the basis of their lithology, grain size and sedimentary stmctures. The depositional environments are including three different sedimentary facies sequences, fluvio-lacustrine, aeolian and fluvial sediments. Texturally most of the sandstones of Al Bauga sediments are medium grained, poorly to very poorly sorted, and fine skewed very leptokurtic with unimodal and bimodal size distribution. The grain size parameters coupled with the observed sedimentary structures indicated that the sandstones of Al Bauga sediments have been deposited in both fluvial and aeolian environments. The sandstones of Al Bauga sediments are characterized by a high concentration of SiO2 and are dominantly quartz arenite including a percent of suture polycrystalline quaitz and epidote which reflects an increased mineralogical maturity and indicated metamorphic source rocks. Clays are dominantly detrital kaolinite which suggested tropical humid paleoclimate. The lower part of Al Bauga sediments contains black phytoclast (black wood) high concentration of Sr and relatively high content of organic matter which suggested that these sediments were deposited under oxidizing condition in fluvial e lacustrine environment nearby the source area. The sediments sequence of the Al Bauga sediments is divided into five geochemical units using bulk geochemistry, especially those of iron, manganese, calcium and potassium. The unit boundaries are correlated with the significant lithologic boundaries. From the chemical data for groundwaters it is evident that the chemistry of groundwater from the region of Al Bauga sediments is controlled by the chemical and mineralogical composition of both the basalts and the basement rocks and the waters are medium to high saline. The groundwaters in the Al Bauga sediments area show the dominance of Ca+Mg,Na+K cations hydrochemical facies and HCO3,Cl+CO3 anion hydrochemical facies.