Structure of Brockman Hills, southwestern New Mexico

The Brockman Hills of southwestern New Mexico consist primarily of strongly folded quartzite and shale of the Lower Cretaceous Mojado Formation, and are a key part of the Laramide Cordilleran foldbelt. Folds trend west-northwest and are overturned to the north-northeast. Near the western end of the Brockman Hills the Mojado Formation and limestones of the Permian-Pennsylvanian Horquilla Formation have been thrust to the north-northeast over andesite of the lower Tertiary Hidalgo Formation. The Horquilla Formation is exposed as two small klippen surrounded by Hidalgo andesites and Quaternary alluvium. Minimum stratigraphic separation produced by the thrusting is approximately 25,000 ft (7,620 m). Where exposed, the thrust fault beneath the Horquilla dips 7 degrees south-southwestward suggesting large horizontal displacement. The Mojado-Hidalgo fault contact is not well exposed and could be steeper than the Horquilla-Hidalgo fault.