ABSTRACT

Although the source of the clastic sediments of the Brazos River Sandstone Member of the Garner Formation has not been positively established, it is generally believed that the sediments were derived from the Ouachita-Marathon geosyncline which during Strawn time was uplifted and eroded. The sediments were probably brought to the present outcrop area from the southeast, for the conglomerate zone of the member grades into marine sandstone to the northeast, northwest, and southwest. Because the conglomeratic portion of the outcrop is characteristic of a continental-marine environment and grades laterally into marine sandstone, it is assumed that the pebbles of the conglomerate were deposited from rivers whose energy decreased rapidly upon reaching the coastal area of the Strawn sea.

On the assumption that the dip azimuths of the cross-lamination point the direction in which the currents flowed, the azimuths of cross-beds within the Brazos River Member were measured and plotted as rose diagrams on a map of the outcrop. The current directions indicated by the readings infer the direction of source. From a composite rose diagram of the azimuths over the entire study area, it is suggested that the source area existed to the southeast, for few azimuths point in that direction.

The zones of preferred cross-bedding azimuths are closely related to features on the isopach and conglomerate-sandstone ratio maps. One zone, the azimuths of which point northeast, corresponds to the region of coarsest conglomerate and the thinner portions of the outcrop. The second zone, northwest of the conglomerate, coincides with a region of greatest thickness and nearly pure marine sandstone. The azimuths of the zone point both northeast and southwest suggesting that the currents flowed one direction and then the other in response to tides or seasonal variations.