NORTHERN GULF PLEISTOCENE VALLEY EVOLUTION AND LUMINESCENCE CHRONOSTRATIGRAPHY; ROLE OF SEA-LEVEL, SEDIMENT SUPPLY, AND TECTONIC UPLIFT

Numerous OSL and TL-luminescence ages were obtained from Pearl, Amite, and Sabine River valley “Deweyville” terraces and the adjacent Late Pleistocene Prairie-Beaumont coastal plain. Aggradation of the southwest- and parts of the southeast Louisiana alluvial coastal plain took place during very low Early-to-Late Wisconsin sea-level stages. Sediment supply was abundant in associated dry climate phases. The present data may alter some earlier assumptions. Thus, the Pearl and Amite valleys do not display several downward-stepping aggradational terrace sets. Most Pearl and Amite terrace lithosomes are much older that the earlier suggested 26-15 ka B.P. “older Deweyville terrace,” respectively, Holocene ages of these valley terraces. Indicating the erosional (strath) origin of the Lower Terrace, the Upper and Lower Amite Terraces above the present bottomland surface were found to be of identical age. Vertical and lateral fluvial accretion, meander bight erosion, terrace and bottom floodplain beveling and gradual tectonic uplift were key factors. MIS 5-4 valley excavation phase coincided with a ca. 85-m eustatic sea-level decline. Most Pearl and Amite Valley Upper Terrace lithosomes aggraded between 60-30 ka during MIS 3. Upper Terrace surfaces may merge smoothly with adjacent open coastal plain surfaces, also dated 50-30 ka. Deep valley excavation and bottomland (lower terrace surface) beveling followed during the 25-45 m MIS 2 sea-level drop. Present valley bottoms (high, intermittent floodplains) are underlain/veneered by 19-6 ka alluvium, accumulated during the post-LGM sea-level rise.