Numerical Chronology of Pleistocene Coastal Plain and Valley Development; Extensive Aggradation During Glacial Low Sea-Levels

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Coastal Sciences, Gulf Coast Research Laboratory


Based on more than 60 newly acquired luminescence ages, field information, and sediment granulometry data, a comprehensive chronostratigraphic framework is presented for Pleistocene alluvial coastal plain and valley terraces and coastal barrier trends on the northern Gulf of Mexico between Texas and NW Florida. Luminescence ages 216-188 ka from the second youngest coastal terrace coincide with the highstand during the second youngest Pleistocene Marine Isotope Stage (MIS 7a). TL ages from the oldest Prairie alluvium and OSL-ages from the Gulfport barriers; 124-116 ka, are consistent with the Sangamon interglacial substage, MIS 5e. Forest assemblages of the last two Pleistocene highstands are very similar to the present hard pine-dominated arboreal flora, established under warm-temperate, humid conditions. Ages of the youngest, Prairie-Beaumont coastal terrace overlap with the Eowisconsin (MIS 5d-5a) and Wisconsin (MIS 4 and 3) stages, associated with a sea-level range of -80 to -30m. Coastal plain aggradation, not limited to interglacial highstands occurred during the much longer preglacial and glacial low sea-level stages. The Prairie-Beaumont coastal plain is a collage of seamlessly merged surfaces, aggraded between ca. 135 and 30 ka in several alluviation stages. Periodically dry conditions, associated with enhanced slope erosion and consequent increase in sheetwash, colluvial, and fluvial sediment flux may have induced substantial aggradation at significant distances inland from the coeval shorelines during depressed preglacial and glacial sea-levels. This balance between erosion and sediment delivery may explain the diminished control of lower Eowisconsin and Wisconsin sea-levels (base-levels) on coastal entrenchment and aggradation. The combined effects of alluvial aggradation and subsequent uplift, modified by surface erosion produced the present coastal plain topography. An early post-Sangamon phase of deep entrenchment involved the Amite, Sabine, Neches, and the Pearl Valleys in Eowisconsin and early Wisconsin times. Except for the universal impact of the deep Last Glacial Maximum (LGM) entrenchment, the alluviation-incision cycles that produced two to four sets of terraces of aggradational and strath origin did not occur in all valleys synchronously and with identical effects. In certain valleys terrace aggradation coincided with the LGM record lowstand and the following late Wisconsin transgressive deglacial bemicycle. In contrast with intensive valley filling, inferred for the MIS 6 glacial interval, Holocene backfilling is far from complete in most late glacial entrenched valleys. (c) 2004 Elsevier Ltd and INQUA. All rights reserved.

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Quaternary International



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