Gravity-Driven Fan Facies in an Actively Accreting Point Bar: An Example from the Brazos River, Fort Bend County, Texas

The Yellowstone Landing point bar is an actively accreting, approximately 400 meter-long, arcuate sandbody with a maximum width of 30 meters in a meander bend of the Brazos River in Fort Bend County, Texas, U.S.A. In addition to abandonment geometries typical of sandy meandering streams, observations during low flow conditions reveal gravity-driven mass wasting processes at the terrace between the ridge and swale topography and the convex bank areas of the low flow point bar. Here, sediment is cannibalized and transported downslope in alluvial fans that interfinger with the lateral accretion surfaces of the active bar.

Gravity-driven deposition, or “gully erosion,” was recognized in the Brazos River by Bernard and Major (1956), but is generally absent in contemporary point bar depositional models. The fans are significant because they may increase facies heterogeneities, ultimately resulting in reservoir complexities. As a gravity-driven process, fan progradation must occur after rainfall events or during flood-waning or low flow conditions. To test the temporal and stratigraphic significance of fan facies, streamflow data from the United States Geological Survey (USGS) were monitored and compared to bar exposure. Fieldwork established gage-height cutoff values that estimate periods of fluvial nonproductive time, fluvial reworking time, and bankfull conditions. Mapping during varying stage conditions captured fan geometry and development, allowing for comparisons with analog data.

Analysis reveals the bar is subaerially exposed during the majority of the studied time (30 months, 66%). When fluvial sedimentation ceases, the bar is only modified by gravitational and/or aeolian processes. The most significant fan progradation occurs during falling-stage conditions, when water fills “ephemeral” trunk streams that drain gullies connecting the high-sloping ridge and swale topography from the lower, sandy reaches of the bar. Alluvial fan morphology is typified by concave up, lobate, distally fining, meter-scale sedimentary wedges that laterally coalesce and overlie the rising-stage deposits of the previous flood cycle. Results suggest that, locally, fan deposition is important in reworking point bar sediments, that fan facies are preserved, and that fans increase grain size heterogeneity.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California