ABSTRACT: Effects of Sea Level Rise on River Delta Coasts

S. Penland, J. R. Suter, R. Boyd, S. J. Williams

Relative sea level in the northern Gulf of Mexico during the late Wisconsinian lowstand fell to depths of -130 m below present sea level, exposing the continental shelf and producing a set of shelf-margin deltas. Sediments were largely restricted to infilling the Mississippi canyon 18,000-10,000 years ago. Between 10,000 years ago and present, the Mississippi River was no longer totally confined to the canyon and proceeded to develop a series of shelf-phase delta plains on the outer to mid-continental shelf during the Holocene transgression.

The sediment volume supplied by the Mississippi River to its delta plain appears to have been unable to keep pace with the relative sea level rise that drove the Holocene transgression. Individual stillstand-coupled delta plains backstepped landward across the present continental shelf. The establishment of individual delta plains, built of smaller complexes by the delta-switching process, indicates the existence of several periods during which the rate of sea level rise slowed or achieved a stillstand during the Holocene transgression. As rates of relative sea level rise again increased, these earlier Holocene shelf-phase delta plains were transgressed and submerged, producing large sand shoals marking the former shoreline position. These Holocene shelf-phase delta plains progressive y onlap the Pleistocene Prairie unconformity.

Each shelf-phase delta plain lies on a ravinement surface and consists of a regressive and transgressive component. The regressive component is built predominantly of distributary sands encased in prodelta muds capped by freshwater marsh deposits. The second component of this shelf-phase delta sequence is transgressive and consists of lagoonal deposits overlain by a barrier shoreline or shelf sand body. Short periods of rapid sea level rise during the Holocene transgression, in combination with subsidence, led to the submergence of these delta plains, which can be recognized on the Louisiana continental shelf.

It appears that major stillstands took place between 10000-3000 years ago, therefore, less time was available for sea level to rise 20 m. As a result, the rate of rise must be higher than the average rate of 0.7 cm/yr, as indicated for the Holocene transgression. During this period, a 20-m rise in sea level occurred in which two stillstand and two transgressive events took place. Assuming the early (9000-10000 years ago) and late (4000-6000 years ago) Holocene stillstands each lasted 3000 years, only 2000 years are available to accomplish a 20-m rise in sea level at a rate of 1 cm/yr. If these transgressive events are only 500 years long, as some radiometric data suggest, the sea level rise rates would increase to 2 cm/yr. Combining these eustatic rates with the subsidence rates sugge ts that the threshold value for regional coastal erosion, land loss, and submergence to occur in the Mississippi River delta plain is at rates greater than 2 cm/yr.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990