Mississippi Fan Morphology, Sedimentational History, Petroleum Potential

ABSTRACT The Mississippi Fan, in the northeast Gulf of Mexico, is similar in morphology and general growth patterns to other giant fans. The fan is an arcuate pile of clastic sediments derived primarily from the ancestral Mississippi River. The radius of the fan from its apex, at a water depth of 1200 m, to its margin (3400±m) is about 350 km. The upper fan contains a partly-leveed channel cut into older fan sediments and filled with late Pleistocene (Wisconsin) fine clastic sediments. The middle part of the fan, typical of many, is a massive complex of channels forming a crown or suprafan that rises up to 500 m above the surrounding fan surface. The lower fan has smooth, gentle slopes and depositional-type distributary channels. The upper sedimentary section on seismic profiles can be subdivided into three seismic-stratigraphic units. We term the youngest Unit A which on the mid-fan is over 3 km thick. It consists of a proximal facies of disrupted seismic zones and a distal facies of generally parallel reflections, separated by transparent zones. The former is interpreted to be channel, slump, and debris flow deposits; the latter, turbidite flows interbedded with hemipelagic sediments. Correlation to Deep Sea Drilling Project (DSDP) sites shows that Unit A represents a thick Pleistocene sequence. Indeed, the fan formed during the time represented by Unit A. Thus it is a Pleistocene feature. Two older intervals spanning the interval from middle Miocene to Pliocene were mapped to show the depositional patterns in this region prior to fan development. Computer generated isopach maps of each unit permit the following interpretation. The oldest, Unit C shows that in middle Miocene time the eastern Gulf of Mexico was not the site of major sediment input. From upper Miocene through Pliocene the section gradually thickens northward suggesting that a prominent depocenter existed to the north. The Mississippi fan was deposited on this sedimentary sequence. When the reservoir-type rocks, source beds, strato-structural traps, and the age-time-depth of burial relations of the fan are placed in perspective, the petroleum potential of this province remains an open question. INTRODUCTION The Mississippi Fan, located in the northeast Gulf of Mexico, is a broad, arcuate pile of sediments marked by a gentle outward bowing of the bathymetric contours (Fig. 1). The fan lies between the West Florida carbonate platform on the east and the Texas-Louisiana-Florida Continental Slope on the north and west. The lower portion of the fan bifurcates around the north-jutting salient of the Yucatan carbonate platform. The lower portions merge with the Florida Plain to the southeast and the Sigsbee Plain to the west. The pattern of deposition on the fan has changed with the fluctuation of sea level. During high stands the ancestral Mississippi River constructed a delta at one or another of its numerous mouths, and relatively little sediment has reached the fan. However, in the very late Pleistocene (18,000 V.B.P.), sea level was at least 85 m lower than at present (CLIMAP Project Members, 1976).