Geologic map of the Hackberry Mountain Volcanic Center, Yavapai County, Arizona: Supplement 1 from "Geology of the Hackberry Mountain Volcanic Center, Yavapai County, Arizona" (Thesis)

The Hackberry volcano in central Arizona, is a large dacitic volcano of late Miocene age. Most of the Hackberry Mountain area is underlain by Miocene volcanic rocks, primarily basalt and dacite. The oldest are a heterogeneous group of basalt flows, the Hickey Formation, erupted from local cinder cones. They were superseded by a homogeneous group of basalt flows, the Thirteenmile Rock Basalt, erupted from vents east of the area. Pyroclastics from the Hackberry volcano intertongue with the upper part of the Hickey Formation and all of the Thirteenmile Rock Basalt. There were seven episodes of these pyroclastics, collectively termed the Towel Creek Tuff, that were formed early in the history of the Hackberry volcano. This unit contains numerous dacitic ignimbrites and air-fall tuffs. Coarse, interstratified breccias occur in the upper parts of these deposits; they are capped by reworked tuffs. The sequence of air-fall tuff, ignimbrite, and breccia suggests a Pelean style of eruption. After cessation of the pyroclastic activity, the volcano was intruded by a dacite stock, the Sally May Dacite. The stock probably extruded above the ground surface; because of its location, size (diameter of 8 km), petrography, and chemistry, the stock is believed to be the material of the magma chamber for the Towel Creek Tuff. The stock reached the surface without producing an attendant pyroclastic deposit. The fact that the stock rose to the surface without an accompanying paroxysmal eruption indicates that it was volatile-poor though still hot. Application of Stokes Law shows that it was unable to rise by buoyant forces alone; its upward migration apparently was facilitated by regional extension. The stock was hydrothermally altered at low temperature after its emplacement. Two rhyolite plugs intruded the stock. One was apparently the conduit for a rhyolite flow nearly eight kilometers in length. This plug was later disrupted, possibly by an eruption driven by fluids in the altered stock. The volcanic activity culminated with the extrusion of a thick dacite flow, the Hackberry Mountain Dacite. The Hackberry volcano was active for 2.2 to 3.9 m.y. The presence of basic xenoliths in the dacitic rocks suggests that its long life was the consequence of the injection of basaltic magma into the dacitic magma chamber. The dacite in all deposits of this volcano are petrographically similar. The Hackberry Mountain area forms the southeastern margin of the Verde Valley, a sedimentary basin in which the Verde Formation accumulated. Fluviatile sandstones of the Verde Formation intertongue with the upper Hickey and lower Thirteenmile Rock Basalt in the northwestern part of this area, and the Verde grades upward into lacustrine sediments younger than all the volcanic rocks, but still of Miocene age. The Verde Valley formed primarily by subsidence along northwest-trending normal faults, many of which occur in the area studied. The faults in the Hackberry area are commonly interconnected and have a net offset of 400 meters, downdropped southwest. The margin of the basin occurs here because the amount of subsidence decreases, and, more importantly, the Hackberry volcano formed a topographic boundary