Abstract

Early to mid–Late Cretaceous Nubia Formation in and around the Natash Basin in southeastern Egypt incorporates a lower, regressive, nonmarine sandy facies 1, as much as 100 m thick; a middle, transgressive, paralic to marine facies 2, several tens of metres thick; and an upper, prograding, nonmarine to paralic sandy facies 3, about 120 m thick. These deposits extend northwestward across the Kharga arch into the large Dakhla Basin in central and southwestern Egypt, where facies 1 is several hundred metres thick and has an Aptian marine tongue. The Nubia Formation also has correlative lithofacies along the Gulf of Suez lowland in northeastern Egypt. There, facies 1 is of varying thickness and is an important petroleum reservoir.

Early Cretaceous to early Cenomanian facies 1 was deposited by high-energy, low-sinuosity braided streams during a low stand of sea level that persisted with only minor interruption for about 50 m.y. Sandy fining-upward fluvial sequences record intermittent aggradation and repeated development of paleosols. Deposits in the Dakhla Basin were dispersed northward from the deeply weathered Nubian Shield. Those in southeastern Egypt and in the Gulf of Suez lowland were dispersed southwestward and westward from uplands on the Eastern Desert and Arabian Platforms.

Southward-thinning late Cenomanian–Turonian marine facies 2 marks the climax of the first major Late Cretaceous Tethyan transgression. Waning of northward-dispersed detritus fostered development of three extensive beds of oolitic ironstone in the vicinity of Aswan, as well as a few thin lenses of ironstone in central, northern, and northeastern Egypt.

Distinctive tabular-planar cross-bedded facies 3 reflects renewed uplift of the Nubian Shield in late Turonian–Coniacian time and essentially continuous influx of quartzose sand for only a few million years. Facies 3 was spread across a prograding alluvial plain in migrating sand waves and channel bars in closely spaced non-sinuous distributary systems.

The older regressive facies 1 was succeeded rather abruptly by marine facies 2 as a result of waning detrital influx followed by low-energy transgression that developed only minor strand-line sand bodies. Prograding nonmarine facies 3 is separated from overlying transgressive Late Cretaceous marine deposits by a gradational ripple-bedded facies probably produced by gradual waning of detrital supply during continuing high stand of sea level.

The Nubia Formation and its correlatives in eastern Egypt reflect uplands and irregular topography along the Red Sea–Suez tract that had been a broad zone of deformation in late Precambrian time. The pattern of distribution of Early Cretaceous facies 1 in the Gulf of Suez lowland suggests that there may have been very little spreading along the Suez graben during Cenozoic opening of the Red Sea.

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