RED SEA RIFTING CONTROLS ON AQUIFER DISTRIBUTION: CONSTRAINTS FROM GEOCHEMICAL, GEOPHYSICAL, AND REMOTE SENSING DATA

Highly productive wells in the Eastern Desert of Egypt are tapping fossil water in subsided blocks of Jurassic to Cretaceous sandstone (Taref Formation of the Nubian Sandstone Group) and Oligocene to Miocene sandstone (Nakheil Formation), now occurring beneath the Red Sea coastal plain and within the proximal basement complex. Aquifer development is related to Red Sea rifting: (1) rifting was accommodated by vertical extensional displacement on preexisting NW-SE to N-S trending faults forming a complex array of half-grabens and asymmetric horsts; (2) subsided blocks escaped erosion accompanying the Red Sea-related uplift. Subsided blocks were identified and verified using satellite data, geologic maps, field and geophysical investigations. Interpretations of Very Low Frequency (VLF) measurements across extensional faults bounding the subsided blocks suggest the faults might have acted as conduits for ascending groundwater from the subsided aquifers. Stable isotopic compositions of groundwater samples from these aquifers are depleted (δD: -19.3 to -53.9‰; δ¹⁸O: -2.7 to -7.1‰) compared to modern precipitation and samples from fractured basement and overlying alluvial aquifers (δD: -1.9 to -14.2‰; δ¹⁸O: -1.1 to -2.7‰), suggesting the subsided aquifers were largely recharged in Pleistocene times. Groundwater volumes in subsided blocks are large; within the basement complex of the Central Eastern Desert alone (excluding coastal plain aquifers) they are estimated at 3 x 10⁹ m³ and 10 x 10⁹ m³ for the Nakheil and Taref Formations, respectively. Results highlight the potential for identifying similar rift-related aquifer systems along the Red Sea – Gulf of Suez system, and in rift systems elsewhere.