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Point bar deposits of the Brazos River near Richmond, Texas, appear to be typical of an alluvial meandering stream of this size. They are the principal meander belt deposits and consist of a sequence of silt and fine sand grading downward to coarse sand and gravel. A typical section may be subdivided into four generalized zones, each characterized by a particular class of sedimentary structure: (1) small ripple (or small scale) cross-bedding, (2) horizontal lamination, (3) giant ripple (or medium scale) cross-bedding, and (4) poor bedding. The section is an offlap sequence deposited within the channel or the depositional (convex) bank area as the stream meandered laterally toward the erosional or caving (concave) bank. The average thickness of the total section is 55 feet which is equal to the average maximum depths of the river during flood stages.
Natural levee sediments deposited along the flood stage bank are 5 feet or less in thickness and are difficult to distinguish from the uppermost point bar sediments.
As the stream meanders within its belt, it produces a suite of deposits, consisting of sediments characteristic of point bars, natural levees, and fills of abandoned channels and oxbow lakes. The widths of the belts and related deposits, which are approximately 1.5 miles and 18-20 times the width of the stream, appear to be controlled primarily by the radius of curvature (600-2,400 feet) of the meanders.
Abandoned channel fills consist principally of laminated and bedded clay and silt. They are tortuous or arcuate in ground plan, a few hundred feet wide, and their cross sections are roughly U-shape. The deposits range from a few feet to approximately 40 feet thick and usually occupy positions within the upper two-thirds of the compounded point bar sections.
Flood basins within the alluvial plain of the Brazos are adjacent to the meander belts and are topographically lower, approximately 5 feet, than the uppermost point bar and natural levee sediments. The flood basin deposits are laminated and poorly stratified sandy clay and silt containing numerous soil zones and calcareous and ferruginous nodules.
During the Late Recent standing sea-level stage, approximately the past 3,500-5,000 years, the Brazos has developed and abandoned several meander belts within its alluvial plain, which is approximately 7 miles wide near Richmond. Most belt trends are approximately at right angles to the regional depositional strike. Crossbedding directions and grain orientation conform with various river current directions, and most are aligned closely with the trends of the depositional banks of the individual point bar deposits.
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