NON-SYMMETRIC RESPONSE TO BASE LEVEL RISE AND FALL: A CASE STUDY OF TWO ADJACENT FLUVIAL/DELTAIC SYSTEMS ALONG THE EAST TEXAS COAST

The equilibrium profile of an active floodplain is a direct measure of fluvial response to autocyclic and allocyclic controls. This study emphasizes the differences in the fluvial response of two adjacent East Texas fluvial/deltaic systems over the course of the last 4th order sequence stratigraphic cycle (100 kyr glacial-eustatic cycle). Numerous datasets, including 10 cores, 200 drillers’ water well logs, 25 cone penetrometer tests, 30 radiocarbon dates, seismic and ground penetrating radar data were integrated using GIS software (ESRIS’ Arcmap) to observe the fluvial response of the Brazos and Trinity fluvial/deltaic systems to base level changes.

Along the East Texas coast, the Brazos and Trinity incised valleys developed in response to base level fall. Starting at the coast, or barrier island, and extending updip away from base level control (~100 km inland) the volume of space excavated to create the incised valleys underlying the present day lower reaches of both of these fluvial systems converges on 20 km3. This signifies a strong similarity in fluvial response for both systems during the falling limb of highstand and the lowstand systems tracts. Yet the transgressive and early highstand aggradational fill of the Brazos and Trinity incised valleys represent the two end members of fluvial response to base level rise. The Brazos incised valley is overfilled (~21 km3), while the Trinity incised valley is underfilled (< half full).

Taken together, the non-symmetric equilibrium profile response for these adjacent fluvial/deltaic systems to base level rise vs. base level fall implies different autocyclic and allocyclic mechanisms are controlling fluvial response over one glacial-eustatic cycle. Eustacy appears to be the dominate control on the equilibrium profile as sea level falls and accommodation is destroyed (i.e. incision occurs), while sediment supply (in this case a combination of climate, drainage basin size, and surface geology) appear to be the dominate control on the equilibrium profile as sea level rises and accommodation is created.