Abstract: Characteristics of Incised Valley Fills, Coastal Plain Rivers of Southeastern Texas

Robert A. Morton, Michael D. Blum, William A. White

Reconstruction of the late Pleistocene geologic history of the Sabine, Neches, Trinity, and San Jacinto Rivers in Texas illustrates how climate and eustasy influence 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.

Unconformable bounding surfaces at three levels of incision form the composite walls and floors of the incised valleys; the lowest terrace surface controls gradients of the modern rivers. Late Pleistocene (Deweyville) fluvial sediments are sand and gravel point-bar deposits 12-15 m thick. Muddy overbank facies are noticeably absent in these laterally accreted river terraces that are progressively inset and preserved along the valley walls and floors. In contrast, fluvial sediments of Holocene-modern coastal plain rivers are muddy, organic-rich suspended-load deposits that formed through vertical aggradation. Sandy channel facies are noticeably absent in these overbank deposits that progressively onlap and bury older Deweyville terraces downstream.

Geometries, ages, and facies of allostratigraphic units filling the incised valley are different upstream and downstream of the bayline, which is the position where modern floodplain sediments onlap the oldest late Pleistocene fluvial terraces. These stratigraphic differences reflect the predominant fluvial-marine and climatic-eustatic regimes. Maximum incision and valley-fill thickness is restricted to a zone basinward of the previous highstand shoreline. For these late Pleistocene valleys, maximum valley-fill thickness (30-40 m) is about one-third of the total lowering of base level during the Wisconsin maximum glaciation.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas