Numerical Chronology of the Late Quaternary Gulf Coastal Plain; Barrier Evolution and an Updated Holocene Sea-Level Curve

Ervin G. Otvos
Department of Coastal Sciences and Gulf Coast Research Laboratory, University of Southern Mississippi, Ocean Springs, Mississippi 39566-7000; [email protected]

Luminescence dating for the first time provides a powerful new tool in establishing a broad chronological framework of late Quaternary coastal plain and river valley terrace deposits on the northern Gulf of Mexico. Dates 216-188 ka BP from the intermediate Montgomery coastal terrace coincide with Marine Isotope Substage 7e Interglacial highstand. A major entrenchment-erosion phase is inferred for the Pearl and Amite Valleys in the next, glacial stage, Marine Isotope Substage 6. The ranges of optically stimulated luminescence/ thermoluminescence dates from the Gulfport-Ingleside barrier and older Prairie coastal plain sectors, 135-116 ka, correspond to the Sangamon Marine Isotope Substage 5e (interglacial) interval. Prairie alluvium dated between ~100-28 ka BP was deposited during Eowisconsin Marine Isotope Substage 5d-5a and Wisconsin Marine Isotope Substage 4 and 3 from different fluvial sources at various time intervals. Alluviation resulted in a seamlessly merged collage of numerous flood plain surfaces that now form the Prairie coastal plain.

Only relatively small areas of the Prairie alluvium, deposited during elevated Sangamon Interglacial sea-levels are exposed in the surface. While erosional degradation and entrenchment was expected to have coincided with the extended periods of low Eowisconsin and Wisconsin sea-levels, terrace aggradation dominated, instead. The large alluvial sediment yield more than compensated for the base-level decline well inland from the contemporary shorelines. Enhanced deposition rates and the role of antecedent topography were similarly implicated in the formation of back-stepping late Quaternary nearshore valley-fill sequences. Increased sediment delivery resulted from intensive surface erosion during drier climate phases of sparser plant cover; eolian sand deposition has recurred repeatedly between ~62.0 and 5.0 ka. One-to-three terraces of aggradational and erosional origin formed in the entrenched valleys between ~90-20 ka BP, even during the late glacial maximum of record low sea-levels. A thick upper terrace aggraded in the Pearl Valley between ~60-30 ka and a lower, partially strath terrace formed near the present floodplain level between ~35-20 ka. The upper Amite Valley terrace lithosome, previously defined as late Holocene accumulated from ~50 to ~ −30 ka BP.

Emergence of a group of earlier Holocene mainland and island barriers by vertical aggradation was initiated between ~5.2-3.5 ka, at an elevation a few meters below modern sea-level (Galveston, Mustang Islands, the Mississippi-Louisiana barrier chain, ~5.2-4.0 14C ka; St. Vincent Island, ~ 4.5-3.5 ka; and the Morgan-Perdido barrier complex, ~5.0-3.5 ka). A much younger group, including the St. Joseph Bay area and Sanibel-La Costa barriers postdate ~3.0 ka 14C BP. Louisiana's chenier plain and Caminada strandplain, the Bolivar barrier and Mississippi's Belle Fontaine barrier spit also began about this time. Coastal ridge and scarp elevations, various sedimentary and geomorphic features even in alluvial units often are mistaken for proof of higher than present sea-levels. As recent northwest Florida data shows, the updated Gulf curve does not support mid -to- late Holocene highstands or prolonged stillstands either. Suggestions of multimillennial intervals of extreme hurricane activity, based on sand layers in estuarine muds are unsubstantiated.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana