THE HOLOCENE SEA-LEVEL CONTROVERSY ALONG THE GULF COAST: THE CASE FOR MIDDLE HOLOCENE SEA-LEVEL HIGHSTAND

Longstanding controversies in sea-level change for the Gulf of Mexico coast have reemerged, with contrasting models reinforced by recent work. On one side is the continual submergence model, with sea level not reaching present elevations until present-day. This model was defined in early work on the Mississippi delta, and has been reinforced recently by Tornqvist et al., who further argues the record from the delta is representative of the Gulf Coast as a whole. However, such an interpretation underappreciates the stratigraphic record of the depocenter, which includes some 900-1000 m of deltaic and shallow marine strata deposited since 2.2 ma, and which must have been accommodated by load-induced subsidence.

On the other side is the middle Holocene sea-level highstand model, where sea level essentially reached present elevations by ca. 6 ka, and may have oscillated within 1-2 m of present since that time. Prior to a few years ago, robust geochronological data had been presented in support of one or more highstands, but landforms and deposits that are difficult to explain without such a model have been repeatedly identified throughout the Gulf Coast, with the notable exception of the Mississippi delta. Now, two separate areas have recently produced data in support of sea level reaching present positions by ca. 6 ka. One site, along the central Texas coast, lies some 20 km inland within a tributary valley to Copano Bay. This site contains middle Holocene spits, with subtidal forams and crayfish burrows, that now lie up to 2 m above present sea level. A 2nd site, along the Alabama coast, consists of cross-cutting open marine beach-ridge complexes, and has been studied by 4 investigators. Middle to late Holocene ages are not disputed, although investigators interpret the significance of these features to sea-level history in different ways. However, it is not possible to form extensive subaerial beach-ridge plains such as these, from ca. 6-3 ka, without sea level being very close to present by that time.

Ongoing research along the non-subsiding Florida, Louisiana, and Texas Coasts, away from the subsiding Mississippi depocenter, will continue to test these contrasting models, as they have fundamentally different interpretations for Gulf Coast evolution, as well as current and future coastal restoration efforts on Mississippi delta.