Lower Miocene Sequence Development in Northwestern Egypt and the U.S. Mid-Atlantic Continental Margin
A mismatch exists in the lower Miocene of northwestern Egypt between the ups and downs of global sea level, as determined from deep-sea oxygen isotope data, and the stratigraphic expression of those changes. Even if allowance is made for uncertainties in dating and an incomplete stratigraphic record, the sea-level changes responsible for the development of unconformity-bounded sequences at a timescale of 105-106 yr are not conspicuously sinusoidal in the manner normally assumed. Instead, the development of sequences appears to reflect an integration or low-frequency filtering of higher-frequency sea-level fluctuations. High-frequency cyclicity is likely expressed by complexities in facies stacking within sequences. Sequences of the Moghra Formation are incomplete and asymmetrical. They are typically no more than 60 m thick and bounded by erosional surfaces with as much as 15-20 m of relief. As might be anticipated at a site more than 50 km landward of the hinge zone of a Triassic-aged continental margin, the sequences are also transgressive-dominant. Rising sea level resulted in the development of new accommodation, deepening and backstepping of facies. Falling sea-level led to shoaling, bypassing and erosion. We infer that the preservation of in excess of 230 m of lower Miocene deposits, in spite of an overall sea-level rise of no more than about 10 m, reflects thermal subsidence engendered by Jurassic-Cretaceous rifting that modified the edge of Gondwana following the development of the Neo-Tethys ocean. A comparable sequence motif is observed in the lower Miocene of the U.S. mid-Atlantic margin. There, however, transgressive deposits at a similar paleogeographic location are still thinner and more lenticular. Sequences in the U.S. example are arranged laterally across a shelf that had been starved of sediment for tens of millions of years. Each sequence thickens markedly across the highstand rollover of the underlying sequence, and progradation with clinoforms is best developed where each sequence is thick. Comparable features in Egypt would likely be thinner and localized near the present-day Mediterranean shoreline.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California