Volcanic stratigraphy, geochemistry, and structure of the Steeple Rock district, Grant County, New Mexico, and Greenlee County, Arizona
— Virginia T. McLemore, William C. McIntosh, and Robert Appelt

Rocks exposed in the Steeple Rock district consist of a sequence of Oligocene–Miocene (34-18? Ma) andesite, basaltic andesite, and dacitic lavas interbedded with sandstone, volcanic breccia, and rhyolitic ignimbrite. This sequence is locally intruded by intermediate–silicic plugs, dikes, and domes (33 and 28-18 Ma), some of which are associated with epithermal vein formation, brecciation, and faulting. The ignimbrites in the district are outflow sheets that were erupted from calderas in the Mogollon-Datil and Boot Heel volcanic fields. Extensional deformation of the volcanic rocks in the district produced a series of half-grabens and horsts with district-wide, northeastward dips of bedding planes and foliation. The stratigraphic nomenclature of two units in the area (Summit Mountain and Dark Thunder Canyon formations) is formalized in this report. The Summit Mountain Formation is approximately 240 m thick at the type locality and includes intrusive andesite and volcaniclastic sedimentary rocks, but the base of the unit is faulted. The Dark Thunder Canyon Formation consists of multiple gray to brown to purple to red porphyritic amygdaloidal andesitic to basaltic andesite lava flows with interbedded <28-Ma ignimbrites and volcaniclastic sandstones and is nearly 800 m thick at the type locality. Geochemical data suggest that pre 28-Ma (Summit Mountain and Bloodgood Canyon Tuft) and 28-20-Ma (Dark Thunder Canyon Formation and <28-Ma ignimbrites) volcanic rocks represent predominantly lithospherederived magmas, with increasing amounts of asthenosphere derived magmas from 28-20 Ma.