Red Sea Rifting Controls on Groundwater Reservoir Distribution: Constraints from Geophysical, Isotopic, and Remote Sensing Data

Highly productive wells in the Eastern Desert of Egypt are tapping fossil water in down droppes Upper Jurassic-Lower Cretaceous sandstone aquifers (Nubian sandstone) now underlying the Red Sea coastal plain and in sedimentary successions packaged within the Precambrian basement complex and proximal to the Red Sea coastal plain.  Reservoir development is related to the Red Sea rifting: (1) rifting was accommodated by vertical extensional displacement on preexisting NW, NNW, or N-S trending faults forming a complex array of titled half grabens and asymmetric horsts; (2) the down faulted blocks escaped erosion accompanying the uplift associated with Red Sea rifting and affecting the basement and overlying sequences.  Many down dropped blocks were identified and verified using satellite data (Landsat TM and SRTM), geologic maps, field and geophysical investigation (VES and magnetic).  Isotopic compositions (e.g., W. Ambagi dD: -35.4‰, dO¹⁸: -4.6‰; Dara dD: -48.1‰, d¹⁸O: -6.1‰; El Gona dD: -53.3‰, d¹⁸O: -0.35‰) of the ground water samples from these reservoirs are depleted (compared to modern precipitation) suggesting the aquifers were largely recharged in previous Pleistocene wet climatic periods.  Groundwater potential for investigated reservoirs is high: (1) estimated areal extent using remote sensing and GIS techniques is considerable (e.g., Ambagi: 84 km²; W. Queh: 14 km²; W. Gasus: 18 km²; Esl: 13 km²; Malha: 219 km²), (2) saturated thicknesses (from VES) are large (e.g., Ambagi: 100-130m; W. Queh: 30-40m; W. Gasus: 20-60m; Esl: 20-60m; Malha: 20-80m; Esh El Malahal: 25-250m), and (3) depth to water table (from VES) is shallow (e.g., Ambagi 10-25m; W. Queh: 90-130m; W. Gasus: 25-100m; Esl: 5-60m; Malha: 15-40m; Esh El Malahal: 15-80m).