CLASTIC DIKES WITHIN THE SWAN CREEK SANDSTONE, SOUTHWEST MISSOURI

The Swan Creek sandstone is an informal member of the Early Ordovician Cotter Dolomite in southwest Missouri. The Cotter generally is a peritidal carbonate, but laminae and interbeds of quartz grains become more prevalent near the top. The Swan Creek is the thickest of these sandstones and is present locally within the top ~20 m of the Cotter. The Swan Creek is a quartz arenite which displays herringbone and low-angle cross bedding, indicating a high-energy, near-shore depositional environment.

Clastic dikes emanating from the Swan Creek intrude the Cotter’s carbonate beds at various locations in southwest Missouri, but are ubiquitous in the type area near Sparta, MO. Here dikes of various thickness cut across cross bedding within the main sandstone body and with their own sets of laminae subparallel to the dike walls. In plan view the thinner dikes form an anastomosing network resembling both shrinkage cracks and boxwork weathering, but vertically they span >2m, and the material within the dikes is heavily cemented quartz arenite that clearly originated within the Swan Creek.

The age of the dikes is poorly constrained. They formed sometime during or after the Early Ordovician and before complete cementation of the main sandstone body. The larger dikes reach 15 cm in width and have a preferred northwest – southeast orientation, the same approximate orientation as the major faults in southwest Missouri. Thus, these dikes may be related to both local and regional tectonic events.