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.