Lowstand Exposure and Drowning of a Mississippian Ramp and Its Seismic Signature

Robertson C. Handford

For more than a decade, stratigraphers have debated if seismic sequence boundaries and onlapping siliciclastic strata, which overlie carbonate platforms, form during sea-level lowstands or as a result of sea-level rises and drowning. Central to this debate is the question of whether or not the observed stratal discordance and onlap patterns are unique to the lowstand hypothesis, and if not, is the alternative drowning hypothesis valid? What has not been considered, however, is that both hypotheses may be correct.

The stratigraphy of a Mississippian carbonate ramp, which is exposed along the southern margin of the North American craton demonstrates a dual history of lowstand exposure and transgressive drowning. A major sequence boundary (SB345) developed at the end of Osagean time due to subaerial exposure at the top of a basinward prograding Lower Mississippian carbonate-ramp. The resulting hiatus lasted as much as 10 m.y. and left a chert gravel residuum across the shelf. In addition, deltaic and shoreface systems prograded from the northeast and occupied a lowstand accommodation zone along the basinward margin of the ramp near the present-day Mississippi embayment. These lowstand deposits form a relatively thick onlapping wedge of siliciclastic strata above the Lower Mississippian ramp and S 345. A late-Meramecian to Chesterian sea-level rise followed, forcing the siliciclastic depositional systems to retrograde shelfward. Concomitantly, carbonate sedimentation resumed across the flooded shelf and formed ooid and skeletal grain shoals. Carbonate sedimentation, however, was shortlived because large quantities of mud, derived from the Ouachita orogen to the south, encroached the shelf during the Chesterian sea-level rise, forming an onlapping succession of black shale. The rapid increase in accommodation space and the influx of suspended sediment and nutrients combined to submerge, suffocate, and poison the carbonate factory. As a result, two onlap surfaces developed: (1) a lowstand unconformity, and (2) a drowning surface. However, they lie so close to each other on the shelf that seismic discrimination is almost impossible. Synthethic seismic sections at 35-50 Hz display reflections onlapping only one surface, the lowstand unconformity, when in fact there are two surfaces of very different origin. This onlap only shows up in seismic data of 75-100 Hz. The paradox is that although the ramp was terminated by drowning, seismic evidence is missing at lower frequencies, and that the visible seismic onlap was due to lowstand exposure.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995