ORGANIC REMAINS OF POINT BAR DEPOSITS

Wood, finely divided plant fibers, land snails, reworked Upper Cretaceous Foraminifera, and fresh-water clams comprise most of the organic remains which occur in the Brazos River point bar deposits near Richmond. Logs and stumps of trees are common in the poorly bedded and giant ripple deposits (especially those of the channel crossovers,Figure 17), since these are usually laid down at or below the ground-water table. Most wood, stranded and buried on the higher parts of the point bar, is destroyed Trees buried in a standing position are common in swale fillings and other rapidly deposited point bar sediments. Numerous horizontal and cross laminae are rich in finely divided plant remains. Fresh-water clams (Figure 61) and reworked Tertiary and Cretaceous mollusks are extremely rare in the Brazos River deposits. Floating pieces of wood, plant fibers, numerous land snails, and reworked Foraminifera accumulate in the slack shallow waters near the inner bank. Many of these are stranded on the banks (Figure 55) buried in the clay drape, and subsequently covered with later deposits. Well-preserved Upper Cretaceous Foraminifera are very abundant, while poorly preserved Tertiary forms are very rare in many samples of horizontal laminae of very fine sand. The concentrations of Foraminifera in such laminae appear to be products of sorting on the bar. The excellent state of preservation of the Cretaceous forms, and the abundance and association with floating debris, suggest that they were essentially floated or transported as suspended particles to the sites of deposition from Cretaceous outcrops 100 miles upstream. Since these forms are derived from the Taylor and Navarro marls, either their chambers were never filled with much sediment, or the nature of the enclosing sediment and weathering conditions at the out crop favored removal of the chamber fillings. Air trapped in chambers would increase the buoyancy of Foraminifera in river currents. These forms are placered, in many cases, in small stream beds at the outcrops. It appears possible that large numbers of Foraminifera derived from these placers could be transported and distributed for long distances along the river banks by a single flood originating at the outcrop. A 30-foot-thick section of very fine-grained, marginal deltaic (Teche-Mississippi) Recent sediments cored off the central Louisiana coast contains well-preserved Cretaceous Foraminifera in abundance. Marginal deltaic sediments above and below this section are rich in Recent shallow-water Foraminifera. Since the nearest occurrences of Cretaceous outcrops are 300 miles inland from this locality these forms must have been transported from the outcrops to the site of deposition during a single major flood. By analogy, it is quite possible that a marine environment of deposition and an older age could be assigned to ancient point bar sands with calcareous cement and abundant reworked Foraminifera if other environ mental criteria are not employed.