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Fluvial systems are major sediment conveyers from source terranes to basinal sinks. Fluvial channel geometry results from interaction between fluid flow and sediment particles, and scales with water discharge and sediment flux. Point bar deposits formed in bends of meandering rivers provide a good proxy to estimate the depths of paleochannels (Ethridge and Schumm, 1978; Blum et al., 2013). Modern observations show that bankfull channel depth (or point bar thickness) well correlates with bankfull discharge and drainage area (Blum et al., 2013). However, this scaling relationship has not been rigorously tested in ancient rock records, especially for large passive margin basins that have diverse climate, tectonic, topographic, lithological and geomorphic regimes.
In this study, we try to test whether this scaling relationship applies to the large drainage systems in the Gulf of Mexico in early Miocene time. Point bar thickness data were collected from subsurface well logs that are within major paleo-fluvial axes. Point bar data from each major fluvial axis display a wide range of thickness and a combination of different river types, including point bar thickness from trunk stream, local small streams, and valley fills. The Paleo-Mississippi and Red rivers in eastern Texas-Louisiana have the thickest point bar deposits (23 m and 20 m), while Paleo Houston-Brazos and Guadalupe rivers in central Texas deposited the thinnest point bars in early Miocene (12 m and 14 m). Paleo-Rio Grande River has intermediate values, 16m. The results show a clear scaling relationship between the point bar thickness and paleo-drainage area, length of channel, and sediment supply rate. This study indicates that such point bar scaling relationship can be used to ancient sedimentary rock to constrain paleodrainage system size. In addition, the sediments deposited in point bars can be applied to predict the fan run out length and size where the seismic image quality does not allow one to detect such features beneath a salt canopy (e.g. lower Miocene subsalt reservoir in GOM deep-water basin).