Signatures of Climate and Sea-Level Change in Gulf of Mexico River Systems over the Last Glacial-Interglacial Cycle: A Source-to-Sink View

Mike Blum
Louisiana State University ([email protected])

Over the past 20 years, a variety of studies have examined the stratigraphic record of fluvial systems of the western Gulf of Mexico basin. These studies have traced deposits from upstream bedrock valleys to the lower reaches of the subsiding coastal plains, and correlated coastal plain deposits with incised valleys on the now-submerged shelf that have been imaged from seismic data. This paper discusses the evolution of fluvial systems over the last 100 kyr glacial-interglacial cycle, focusing on the signatures of climate and sea-level change in the stratigraphic record, and the flux of sediments to the shelf margin and beyond.
At the end of the last interglacial period, major rivers had constructed a succession of large alluvial-deltaic plains. With the onset of northern hemisphere glaciation, and corresponding glacio-eustatic sea-level fall, fluvial systems incised through highstand alluvial plains and extended across the newly emergent shelf. During the glacial period, when sea-level remained in mid-shelf or farther basinward positions, multiple episodes of channelbelt construction took place within bedrock valleys upstream, and extending through incised valleys on the coastal plain and shelf. These “falling stage” and “lowstand” channelbelts represent the conveyor belts that fed deltas constructed in mid-shelf or farther basinward positions.
With the glacial-interglacial transition, and corresponding sea-level rise, the lower reaches of river systems on the present-day coastal plain began to fill. Major episodes of channelbelt construction, incision, and stability correlate from upstream to downstream reaches, suggesting climate forcing of changes in discharge regimes and sediment supply as the primary control. However, in lower reaches of the coastal plain, and on the shelf, sea-level changes controlled stratigraphic architecture.
In today’s highstand world, fluvial sediment is sequestered within estuaries or on the inner shelf, and sediment delivery to the shelf margin is restricted to the glacial period when river mouths transit the shelf. Empirical modeling suggests that sediment yields (supply per unit area) were perhaps 20-30% less during the cooler glacial period, relative to the present interglacial. However, the merging of river systems as they transit the shelf during falling stage and lowstand increases the drainage area that contributes to individual river mouth point sources, such that there are fewer river mouth point sources to feed deepwater systems, but supply to individual river mouths and shelf-margin deltas actually increases.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008