Abstract:
Mica and heavy minerals in sediments of Egypt's Nile Delta are examined to test if measured proportions of these two mineral groups and their distributions can be used to define former coastline positions and their shifts in time and space during the Holocene. The premise of the study is based on the sufficiently different attributes of these two components, especially their shape, density, and size, which could induce their segregation and dissimilar dispersal patterns during sediment transport. To test this hypothesis, mineralogical data from more than 1400 samples from 87 sediment cores recovered across the northern third of the delta margin were analyzed. The marked contrast in both temporal and spatial distributions of high proportions of mica and of heavy minerals indicates distinct separation occurred primarily north of the central delta, in an area from similar to 45 km south of the present shoreline to similar to 10-15 km offshore during the time span considered. Additionally, detailed examination of core sediment types demonstrates a relationship between the proportions of these two mineral groups and proportions of clay, silt, and sand fractions in which they occur near the present coast. Mica is preferentially deposited with silt and clay landward of the modern shore, while heavy mineral concentrations are generally associated with coarser silt and sand near and seaward of the shoreline. Shifts of the Nile Delta margin have been triggered by natural processes leading to insufficient sediment replenishment: relative sea-level rise involving delta plain subsidence and shoreline erosion, and intensified human activity, especially during the past two centuries, such as construction of dams, barrages, and entrapment by the delta's expanded canal system. The approach used here helps define two of the three major earlier Holocene coastal shifts, and it could be used to measure ongoing and future landward shoreline advances onto the delta plain.
