Model for Development of Moghra Estuarine Complex

Abstract

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. Two main alternative models are suggested for Moghra Formation: (A) Erosion of a large (20–30 km wide, 20–60 m deep) master valley during a major Early Miocene sea-level fall, then infilling by repeated (4th-order time scale) transgressive–regressive cycles driven by (1) eustatic SL falls or by (2) autocyclicity, each during steady transgression into the valley and steady sediment supply; or by (B) subsiding (steady sea-level rise) tectonic depression (no large valley) infilled by T-R cycles. The T-R cycles were driven by (1) eustatic sea-level falls, by (2) short tectonic uplifts (to create updip erosion and downdip deltas as erosion products), or by (3) climate-driven sediment supply pulses (this would also contain some autogenic lobe shifting that would not be depression-wide). We prefer the climate-driven sediment supply pulses because we see no evidence of a large eroded valley, because eustatic falls would have been modest in early Miocene, and because there is no evidence of repeated regional tectonic uplifts to create the relative sea level falls. Climate changes are preferred because they are known to happen at high frequency and they are a major cause of sediment flux changes to basins.