“Sedimentology and Sedimentary Petrology
Spring Field Trip”

R. Douglas Elmore, Director, Robert and Doris Klabzuba Professor, School of Geology and Geophysics, The University of Oklahoma

Figure 1. Climbing ripples on a point bar leve, Quachita River Southern Oklahoma.

Figure 2. Growth fault in deltaic deposit.

Each spring the Sedimentology and Sedimentary Petrology class takes a fieldtrip to north Texas, Galveston Island, and the Brazos River delta. Our first stop is the Ouachita River just north of the Arbuckle Mountains where we examine point bar deposits of a meandering river (Figure 1). In north Texas we make several stops examining Pennsylvanian deltaic deposits, including an outcrop in the Dobbs Valley Sandstone (Strawn) along a railroad cut that displays a coarsening-upward deltaic sequence with growth faults (Figure 2).

After a day of examining the deltaic sequences, we drive to Galveston to observe beach and barrier island processes. We focus on the fact that the sand is always moving. The major processes are the longshore drift, which moves sand parallel to the beach, and the formation of washover fans, which moves the sand from the beach to the back part of the island during major storms. We also discuss and debate the various ways that cities and the Core of Engineers try to stop the movement of sand (e.g., geotubes, sea walls, etc.). Figure 3 shows the end of the Galveston seawall and the fact that the island is moving landward where there is no seawall. Figure 4 shows geotubes which only provide short term protection. We also dig trenches to investigate beach stratification (Figure 5). The last stop is the Brazos River Delta, a wave dominated delta.

Figure 3. The end of the Galveston sea wall. Note that the island has moved landward where there is no seawall. Also note the waves breaking on the offshore bars.

Figure 4. Houses protected with geotubes (black bags filled with dredge sand). These tubes were emplaced several years ago and were covered by vegetation just last year. Erosion has uncovered them. These houses will eventually be destroyed by storms. The geotubes are at best a short term fix.

If it is windy, we discuss the Bernoulli equation and the movement of sand grains by saltation (hopping and skipping). The Bernoulli Equation has an inverse relationship between velocity and pressure, and it is the reason that airplanes can fly. In terms of saltation, flow lines converge over the top of a grain which decreases the pressure. This produces a lift (less pressure on the top than on the bottom) and the grain pops up

Note the shell layer and dark oily sand in trench on the beach.

and is moved by the wind. We have the students get down on the sand to get their eyeballs right on the sand-air interface to observe the saltation. There is a thin zone of sand that is in constant movement.

Something unusual happened for the first time on this trip. When we asked the students to get down on the sand, half of them pointed their feet upwind and the other half pointed their feet downwind (Figure 6). This class was pretty good, but I have to wonder about the students who pointed their feet downwind!

Figure 6. Students examining the Bernoulli principle and saltation.