Stacked, Lower Miocene tide-dominated estuary deposits in a transgressive succession, Western Desert, Egypt

The net transgressive Lower Miocene Moghra Formation of Egypt is a sandy estuarine complex consisting of a series of stratigraphic units that reflect repeated transgressive to regressive shoreline movements across the Burdigalian (Lower Miocene) coastal landscape. The transgressive part of each unit is preserved atop a deep erosional scour surface, and consists of tidal-fluvial sandstones with tree logs and vertebrate bones that transition up to cross-stratified, tidal estuarine channel deposits and then to open-marine, shelf mudstones and limestones. In contrast, the regressive part is thinly developed and consists of thin-bedded, fossiliferous shelf mudstones that pass upward to thin, tide-influenced delta-front deposits. Each of the nine transgressive-regressive units of the Moghra Formation is capped by a river-scour surface that severely truncates the underlying regressive half-unit. Regional tectonic subsidence and an overall decreasing influx of clastic sediment accounts for the accumulation of the Moghra Formation and its overall transgressive character. The high frequency relative base-level changes reflected by the transgressive-regressive units (averaging <350 kyr) that punctuate the overall transgressive stratigraphic trend are thought to have been driven by (1) sea-level changes caused by recently-documented variations in East Antarctic ice-sheet volume during the Lower Miocene, and/or by (2) variation in the large-scale influx of sediment to the region (during continuous tectonic subsidence). The relative importance of the sea-level (eustatic fall) vs. supply drive (deep fluvial scour) mechanisms for producing the repeated and widespread Burdigalian incision surfaces in the Moghra succession cannot easily be determined. (c) 2012 Elsevier B.V. All rights reserved.