Geologic map of the Little Horn Mountains 15' Quadrangle, Southwestern Arizona Authors Grubensky, M.J.; Demsey, K.A. Citation Grubensky, M.J. and Demsey, K.A., 1991, Geologic map of the Little Horn Mountains 15' Quadrangle, Southwestern Arizona. Arizona Geological Survey Map-29, map scale 1:50,000, 9 p. Rights Arizona Geological Survey. All rights reserved. Download date 07/10/2021 05:24:38 Link to Item http://hdl.handle.net/10150/629176 GEOLOGIC MAP OF THE LITTLE HORN MOUNfAINS 15' QUADRANGLE, SOUTHWESTERN ARIZONA by Michael J. Grubensky and Karen A. Demsey Arizona Geological Survey Map 29 1991 Geologic Map of the Little Horn Mountains 15' Quadrangle, Southwestern Arizona INTRODUCTION GEOWGIC SETTING Geologic studies by the Arizona Geological Survey The Tertiary volcanic rocks of the Little Horn in southwestern and west-central Arizona provide a Mountains quadrangle erupted from several adjacent framework for understanding the geologic evolution and vent complexes in and around the map area. This local known mineral deposits of the region. These studies assemblage is one of several volcanic-rock-dominated have partly focused on developing a regional Tertiary successions that record the construction of multiple stratigraphy and on understanding the volcanic and overlapping volcanic fields in southwestern and west- structural evolution of the rocks in the Phoenix 10 by 20 central Arizona between 32 and 13 Ma (Fig. 1; Crowe quadrangle. Before 1973, the geology of the Little Horn and others, 1979; Shafiqullah and others, 1980; Capps Mountains 15' quadrangle was largely unknown, except and others, 1985, 1986; Bagby and others, 1987; for information presented by Cousins (1973, 1984) and Grubensky and others, 1987; Stimac and others, 1987; Reynolds (1988). Grubensky and Reynolds, 1988; Grubensky, 1989). The Little Horn Mountains quadrangle is underlain Middle to late Tertiary crustal extension severely dis- by a diverse rock assemblage that formed during a sected these volcanic fields and their substrate, produc- complex geologic history spanning the Proterozoic or ing basin-bounded mountain ranges composed of fault Mesozoic to the present. The geologic column, from blocks of northeast- or southwest-dipping strata. Al- oldest to youngest, consists of Proterozoic and(or) though the pre-Tertiary crystal Iine substrate of the map Mesozoic schist and granitoid and Jurassic(?) metasedi- area is similar to that of adjacent mountains, the Tertiary mentary rocks; Tertiary volcanic and sedimen .rocks; volcanic sequence is different and regional volcanic units and latest Tertiary and Quaternary surficial d sits. are absent in the map area. The lack of regional deposits Pre-Tertiary crystalline rocks are sparsely exposed and, has hampered efforts to correlate Tertiary stratigraphy therefore, will not be discussed in detail in this report. and structure across the region and to constrain the The characteristics of correlative lithology in adjacent timing of crustal extension. Several consistencies, ranges, however, are described and interpreted in pub- however, have been recognized in southwestern and lished topical studies (Reynolds, 1980; Haxel and others, west-central Arizona: (1) Middle Tertiary volcanic 1985; Richard and others, 1988; Sherrod and Koch, deposits dominate the stratigraphy and are in a broad 1988; Tosdal and others, 1989). The surficial deposits range of structural dispositions, with younger deposits within the quadrangle are of either fluvial or eolian origin being consistently less tilted than older deposits; (2) and are primarily confined to low-lying basins. The limited evidence indicates that the older volcanic deposits regional characteristics and setting of surficial deposits were erupted before the onset of extensional faulting and in southwestern and south-central Arizona have been tilting; and (3) many of the youngest volcanic deposits summarized by Demsey (1988a) and Menges and and interbedded sedimentary rocks are flat lying and Pearthree (1989). were erupted during and after fault-block rotation. Tertiary deposits and structures are widespread in the Angular unconformities form several horizons in the map area. Middle Tertiary volcanic and sedimentary Tertiary stratigraphic succession of the Little Horn rocks are as much as 1 lcm thick and underlie most Mountains. Well-exposed sections within the quad- bedrock. The northwest trend of the basins and ranges rangle provide a valuable opportunity for constraining in the quadrangle is a product of middle to late Tertiary thetiming and rate of extensional deformation (Shafiqullah normal faulting. This report focuses on the local and others, 1980; Sherrod and others, 1987; Spencer and geologic relations in the Little Horn Mountains quad- Reynolds, 1989). They also provide an opportunity for rangle that increase the regional understanding of the describing the style of volcanism and the volcanic-sedi- middle Tertiary stratigraphic and structural history of mentary rock associations within an extended continental southwestern and west-central Arizona. terrane. Abrupt lateral and vertical changes in the Little 1 Horn Mountains stratigraphic succession are exposed at nic rocks of Royal Arch (see cross section B-B') and discrete locations within the quadrangle. The field rela- sandstone and conglomerate (see geologic map). Vent- tions, described below, indicate that volcanic construc- complex intrusions and cinder cones of this young basalt tion and structural uplift strongly influence the geometry are located north of Red Raven Wash (Fig. 3). of deposits within volcanic-rock-dominated basins. In the south-central and central areas of the map, eruptions from central vents constructed high-standing edifices, around which subsequent eruptive materials SlRATIGRAPHY from other vents were emplaced. The basalt of Oakland The stratigraphic succession in the Little Horn Mine is the dominant stratigraphic member only in areas Mountains consists of a ~ l-km-thick assemblage of near its vent. The Oakland mine lavas thin dramatically Tertiary volcanic rocks and minor amounts of sedimen- northward where they overlie thick sections of the basalt tary rocks that unconformably overlie Proterozoic or of Little Horn Mountains west and north of Hovatter Mesozoic metamorphic and igneous rocks (Fig. 2). This Road (see cross section A-A '). Oakland mine lavas largely volcanic succession is broadly a two-part se- conformably overlie or buttress directly against a pre- quence comprising a lower part of silicic lavas and cipitous stack of rhyolite of Nottbusch Valley to the east interbedded tuffs and an upper part of basalt lavas and and southeast (see cross section C-C'). interbedded rhyolite lavas. The sedimentary rocks Some rapid lateral stratigraphic changes in the Little include thin, discontinuous bedsets of limestone or Horn Mountains are apparently a result of differential locally thick sequences of pebbly sandstone and sedimen- uplift. In areas north of Cementosa Tanks, exposures of tary breccia. Only in the south-central part of the map the basalt of Oakland mine are confined on the northeast area do sedimentary rocks form a significant portion of and southwest by structurally and topographically high the stratigraphic section. volcanic rocks of Royal Arch and basalt of Little Horn The Tertiary succession varies in thickness and Mountains (see cross section A-A'; Fig. 3). These composition from northwest to southeast across the Little confining ridges compose the uplifted edges of tilted Horn Mountains quadrangle. The northeastern and blocks formed during the earliest phases of extensional northwestern areas are largely underlain by mineral- faulting (discussed below). The thick exposures of ogically monotonous basalt and basaltic andesite lavas, Oakland mine flows overlie the projection of a normal herein referred to as the basalt of Little Horn Mountains, fault that separates two tilted blocks. The basalt of Little that are gently tilted to flat lying. The basalt reaches its Horn Mountains may have also been partly confined maximum thickness in exposures between Coyote Peak between ridges of volcanic rocks of Royal Arch (see and Cementosa Tanks and pinches out southward against cross section B-B'). steeply tilted rhyolite lavas and interbedded tuffs, herein referred to as the volcanic rocks of Royal Arch, near STRUCTIJRE AND BASIN DEVELOPl\1ENT Cementosa Tanks, Royal Arch, and the southern end of Renegras Plain. The volcanic rocks of Royal Arch are Rocks in the Little Horn Mountains quadrangle are the oldest Tertiary volcanic rocks in the Little Horn exposed in several tens of narrow fault blocks, which Mountains and overlie, along a nonconformity, base- consist of northwest-striking strata of Tertiary volcanic ment rocks or sedimentary breccias derived from them. and sedimentary rocks. The strata are tilted along east- The volcanic rocks of Royal Arch are the dominant unit dipping, high- and low-angle normal faults that have in the southwestern part of the map area. Locally thick accommodated some of the crustal extension that oc- sequences of lavas and minor amounts of associated curred in southwestern Arizona during the middle pyroclastic rocks in exposures of the volcanic rocks of Tertiary (Spencer and Reynolds, 1989). Royal Arch suggest that they are near-vent deposits. Tertiary faults in the Little Horn Mountains range in In the central, north-central, and east-central areas, a orientation from nearly vertical to very low-angle and groupO) of flow-dome complexes of the rhyolite