The Hopi Buttes area in the southern Navajo Country, Arizona, lies in a Pliocene basin of sedimentation and volcanism. The thin Pliocene sediments, known as the Bidahochi formation, are interbedded calcareous sand, clay, marl, tuff and other pyroclastic material. These sediments extend southeastward as far as the Zuni area, New Mexico. They overlie the Hopi Buttes erosion surface of low relief, a portion of which is now exhumed on the southern edge of Black Mesa. This surface may be widespread in northern Arizona. The Bidahochi formation and the erosion surface have evidently been deformed in Pliocene or post-Pliocene time.
The Hopi Buttes area, which lies at the lowest point of the structural basin of deformation, contains Pliocene volcanic rocks of alkalic composition greater in volume than the sediments, and over 200 closely spaced volcanic necks or diatremes ranging from 500 to 4000 feet in diameter, and arranged along a complex pattern of fractures.
Flows are few in number, for the volcanism produced pyroclastic débris and lava domes. Deep erosion reveals that explosions produced funnel-shaped pipes which were rapidly filled with explosion débris, with material poured in by streams or with lava and viscous agglomerate erupted from below. Small rims protected many diatremes for a time from filling by river-borne sediments so that fine ash, gypsum, and calcium carbonate collected in their crater lakes to be later buried by coarser material as stream aggradation continued. The structure of the diatreme fillings indicates that subsidence occurred after eruption.