Abstract

Detailed geologic mapping of an exceptionally well-exposed clastic plug in northeastern Union County, New Mexico, provides a model for understanding the structure of the approximately 200 other less well-exposed clastic plugs that occur in Upper Triassic Dockum Group red beds in the Dry Cimarron Valley of northeastern New Mexico and adjacent parts of Baca County, Colorado, and Cimarron County, Oklahoma.

The clastic plug is about 300 feet in diameter and is roughly circular in outline. It is dominantly composed of chaotically-oriented fragments of brecciated Jurassic Exeter Sandstone and upper Sloan Canyon Formation (a part of the Dockum Group) that are dropped down into relatively undeformed lower Sloan Canyon Formation country rock. The middle member of the Exeter Sandstone is represented in the breccia, but the lower and upper members of the Exeter are not. A nearly continuous ring dike of quartz sandstone separates the breccia body from the surrounding country rock. Sandstone of similar nature occurs as mappable dikes and pods in the breccia body and generally fills between the individual breccia fragments. The same sandstone composes a 100-foot-diameter pipe that stands prominently above the surface towards one side of the structure. The pipe, dike and pod sandstones are fine-grained, relatively clean and lack stratification. However, the pipe displays faint vertical fluting. This particular clastic plug, as well as all the others in the region, is interpreted to have formed during or after or both during and after deposition of the middle member of the Exeter Sandstone and before deposition of the upper member of the Exeter. The clastic plug is interpreted to have formed by way of upward flow of water along a spring conduit.