ABSTRACT

Reconstruction of the late Pleistocene geologic history of southeastern Texas illustrates how climate and eustasy influenced facies architecture of incised valley fills at sites that are currently near sea level but were in far upstream positions when sea level was much lower. Walls and floors of fluvial terraces entrenched at three levels form a composite unconformable bounding surface that delineates each incised valley. The youngest terrace controls gradients, channel patterns, and positions of the modern rivers and surficial drainage. Late Pleistocene fluvial sediments are laterally accreted sand and gravel point-bar deposits 10-15 m thick that lack muddy overbank facies. In contrast, late Holocene-modern fluvial sediments are organic-rich suspended-load deposits that formed through asymmetrical channel fill and floodplain aggradation. Sandy channel facies are noticeably absent in these floodbasin muds that progressively onlap and bury older terraces downstream.

Geometries, ages, and facies of allostratigraphic units filling the incised valleys are different upstream and downstream of the position where modern floodplains onlap the oldest late Pleistocene fluvial terraces. These stratigraphic differences reflect the predominant fluvial-marine and climatic-eustatic regimes.