ABSTRACT

The Mississippian Stanley Shale crops out along the Cossatot River in the Ouachita Mountains of western Arkansas. Here, exposures of deep water sandstones and shales, on recently established public lands, present a rare, three-dimensional look at sandstones of the usually obscured Stanley. The Cossatot River State Park Natural Area includes eleven miles of rugged and spectacular wooded, sandstone ridges, and cascading water. Cossatot Falls, a series of class IV and V rapids, is developed on massive- to medium-bedded quartz sandstones on the north flank of an asymmetric, thrust-faulted anticline.

In western Arkansas, the Stanley Shale is a 10,500-foot (3200 m) succession of deep water sandstone and shale. At Cossatot Falls, approximately 510 feet (155 m) of submarine fan channel sandstones, siltstones, and shales are exposed during low river stages. This section is composed primarily of sets of thinning-upward sandstone beds. With rare exceptions, the sandstones are turbidites, grading from massive, homogeneous, basal beds, upwards through festoon-crossbedded, thick beds, into rippled medium and thin beds. Sandstone sets are capped by thin shale and siltstone intervals. Regional, north-northwestward paleocurrent indicators are substantiated by abundant, generally east-west ripple crests, which are asymmetric to the north-northwest. Flute casts at the top of the sandstone sequence indicate an additional, east-west flow component.

Based on regional, lithologic characteristics, the sandstones at Cossatot Falls appear to be within the Moyers Formation. The Moyers is the upper sandstone unit of the Stanley and is an oil and gas reservoir in the eastern Oklahoma Ouachitas.