Alluvial Stiff Muds (Late Pleistocene) Underlying the Lower Nile Delta Plain, Egypt: Petrology, Stratigraphy and Origin
Keywords:Alluvial plain, alluvial sands, coastal sediments, delta deposits, deltaic formation, Nile shelf, stratigraphic sequence
Stiff muds of late Pleistocene age, interbedded with alluvial sands, are buried beneath Holocene deposits of the Nile delta in Egypt. The origin of the muds are interpreted on the basis of the petrology, stratigraphy and distribution of radiocarbon-dated sections recovered in cores across the northern delta. Four distinct stiff mud sequences, or subfacies, are defined on the basis of lithology, sand-sized composition and clay mineralogy. These sequences include (I) calcareous nodule-, (II) gypsum nodule-, (III) shell- and (IV) plant debris-bearing stiff muds. They accumulated: I, in seasonally flooded inland sebkhas proximal to Nile channels, which were then covered with wind-blow sand; II, in somewhat more central sectors of such playas, more distal to flood channels; III, in more permanent salt lakes; and IV, in freshwater to low salinity marshes adjoining seasonally flooded playas. Sequence I, the dominant lithofacies, is concentrated in 14 geographically distinct Iithosomes. Sequence II occurs at or near the center of these lithosomes, while III and IV are more localized. Spatial and temporal distribution of muds suggest a predominance of incised channels in the north-central Nile alluvial plain, and more laterally migrating channels to the northeast and northwest.
Most stiff mud layers, deposited earlier than 34,000 and at ~28,000-22,000 and ~16,000- > 10,000 years BP, are correlated with several late Pleistocene aggradational phases of the Nile in upper Egypt. The more time restricted gypsum nodule-bearing sequence II accumulated primarily during a period of increased aridity. Lower alluvial plain muds are also correlated with deposits of equivalent age offshore, on the Nile Cone. Paleogeographic reconstructions indicate that River Nile sediments bypassed the northwestern delta plain and were deposited on the Cone. Nile stiff mud facies, primarily sequences I and III, are compared with those of equivalent age in Yangtze and Mississippi delta cores. Gypsum-rich subfacies (sequence II) are regionally more restricted to warm, arid regions such as the Nile. We postulate that stiff muds underlie Holocene sections in all major delta plains, having accumulated at times of eustatic Iowstands. Their petrology and distribution patterns are best explained in terms of depositional setting, fluvial sediment load and flood patterns, and especially, climate. The nature and configuration of stiff muds has practical applications for civil engineering and construction purposes in deltaic settings, and also for hydrocarbon exploration.