Abstract

Facies and tectono-stratigraphic models for the tidally influenced Miocene Nukhul Formation are presented, based on outcrop data from Hammam Faraun fault block, Suez Rift, Egypt. Deposits of the Nukhul Formation are attributed to two linked depositional settings, offshore to shoreface and estuary settings, and were deposited during initial stages of rifting in hanging-wall depocenters of early-formed propagating fault segments. The offshore to shoreface deposits consist of variably bioturbated mudstones that pass gradationally upward to bioturbated bioclastic sandstones. The more landward estuary deposits can be separated into a tripartite division of estuary mouth, estuary funnel with bayhead delta, and upper estuary channel deposits. Estuarine processes generated a complex intercalation of lithologies, with both gradational and sharp facies transitions. In the estuary deposits, tidal ravinement surfaces are typically characterized by mudstones of the estuary-funnel association below, passing abruptly up to erosionally based estuary mouth sandstones. Maximum flooding surfaces are expressed by an abrupt erosional contact separating estuary-mouth sandstones below and estuary-funnel mudstones above.

Stratigraphic development was strongly influenced by the evolving early-rift structure. Depocenters were narrow (2-5 km wide) and elongate (< 10 km long) parallel to the strike of normal-fault segments. The shoreface shoal prevented wave energy in the estuary and increased the relative influence of tidal currents. The elongate, fault-controlled geometry of the depocenters confined the bayhead delta and further enhanced tidal influence. Stratal geometry reflects deformation associated with low-relief growth folds and surface-breaking faults that, together, formed part of an evolving fault array. This basin configuration and associated Nukhul stratigraphy is markedly different to tectono-stratigraphic models for crustal-scale tilted fault blocks that are applicable from late stages of rifting.

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