Comparison of Modern Mississippi Fan with Selected Ancient Fans

G. Shanmugam, R. J. Moiola, J. G. McPherson, S. O'Connell

A comparison of the modern passive-margin Mississippi fan (DSDP Leg 96) with selected ancient active-margin fans reveals major differences in turbidite facies associations and seismic characteristics of the lower fan area. The lower Mississippi fan is composed of channel (facies B and F) and nonchannel sequences (facies C? and D), whereas lower fan areas of ancient active-margin fans are characterized by non-channelized, thickening-upward depositional lobes (facies C and D) with sheetlike geometry. An absence of depositional lobes in the lower Mississippi fan is also suggested by a lack of mounded seismic reflections. Continuous and parallel seismic reflections of the lower Mississippi fan may represent sheet sands, but not those of true depositional lobes. In mature pass ve-margin fans, long, sinuous channels develop as a consequence of low gradients and the transport of sediment with a relatively low sand/mud ratio, and these channels develop lenticular sand bodies. In contrast, channels in active-margin fans are short and commonly braided as a result of high gradients and the transport of sediment with a relatively high sand/mud ratio. Braided channels characteristically develop sheetlike sand bodies.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.