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 of horizontal. Low-angle normal faults are exposed among Nottbusch Valley (see map legend) dominates the section steeply tilted, repeated sections of volcanic rocks of and iscommonly deposited on crystalline rocks, volcanic Royal Arch, the oldest volcanic unit in the area, that crop rocks of Royal Arch, or sandstone and conglomerate. out near Cementosa Tanks. Shallowly dipping faults that Sources of the rhyolite south of Hovatter Road and at juxtapose upper and lower portions of the volcanic rocks Coyote Peak include paired dikes and lava flows. of Royal Arch have no distinct morphologic expression The south-central area is dominated by a shield- and, therefore, may be located only where marker beds shaped vent area for the basalt of Oakland mine (see map may be followed and are displaced perpendicular to legend), which directly overlies the steeply tilted volca- strike. Low-angle fault zones are morphologically

2 prominent where the upper plate consists of Royal Arch Little Horn Mountains is largely absent, contains two rocks and the lower plate consists of recess-forming locally thick volcanic units: the rhyolite of Nottbusch metamorphic rocks because a prominent break in slope Valley and the basalt of Oakland Mine. Both units dip develops along the fault. Fault zones with metamorphic approximately SOto10"southwestward and unconforma- rocks in the lower plate commonly consist of an upper bly overlie steeply tilted volcanic rocks of Royal Arch. plate with a 3~ to l00-cm-thick, brecciated, calcite- These two thick volcanic piles conceal the northwest- cemented zone above a thin, hematite-stained, striated striking structural grain in the underlying tilted section surface and probably have the greatest displacement. and were apparently deposited during the last phases of Deformation in the lower plate consists of gouge or faulting (see cross sections A-A' and B-B'). microbreccia that commonly does not pervade more than In contrast to most other extended areas of south west- 1 m beneath the fault surface. ern Arizona, basins associated with extensional faulting High-angle faults are present throughout the map in much of the Little Horn Mountains quadrangle are area and are well exposed near Cementosa Tanks, where dominated by volcanic rocks rather than sedimentary they project along or crosscut older, low-angle normal rocks (Davis and others, 1980; Menges and Pearthree, faults. Recognition of the high-angle faults is based 1989; Spencer and Reynolds, 1989). In the northern half either on exposures of calcite-cemented or hematite- of the map area, widespread basaltlavas were emplaced stained breccia or on displacement of angular uncon- during and after faulting and tilting of the Royal Arch formities or marker horizons. Many of the younger rocks. Lavas of the basalt of Little Horn Mountains volcanic units in the map area are massive or were not probably accumulated quickly enough to cover the differentiated during mapping. Recognition of crosscut- landscape and fill almost completely any subsiding ting faults is, therefore, difficult. asymmetric grabens. Sedimentary rocks are scarce and Most faults are characterized by normal displacement consist of quiet-water deposits of limestone interbedded inferred to be middle Tertiary; a few do have reverse with the basalt of Little Horn Mountains. displacement. Repetition of gently dipping stratigraphic The tilted section in the southern half of the map area, sections dominated by the massive rhyolite of Nottbusch in contrast, includes Royal Arch volcanic rocks uncon- Valley, which is northwest of Nottbusch Valley, indi- formably('?) overlain by a southward-thickening package cates that large, high-angle normal faults are buried of sandstone and conglomerate, which potentially repre- beneath the alluvium. Some high-angle faults in the sent syntectonic sedimentary rocks. The apparent Little Horn Mountains quadrangle could be younger than southward change in composition of syntectonic deposits middle Tertiary because they apparently displace high- may be due to a topographic and structural high com- standing, pedogenic('?) carbonate horizons (unit Tpc) posed of Royal Arch rocks in the central part of the developed on the basalt of Oakland Mine. They could, map area that confined the flows of the basalt of Little therefore, be Pliocene or younger. Horn Mountains to the northern area. The southern half Stratigraphic and structural relations within individ- of the map area apparently lacked vents, and fault-block ual fault blocks indicate that part of the volcanic- rotation was accompanied by sedimentation. dominated Tertiary assemblage in the Little Horn Moun- tains quadrangle was accumulating before and during GEOCHRONOLOGY AND REGIONAL extensional faulting. Throughout the quadrangle, the CORRELA nON OF VOLCANIC UNITS volcanic rocks of Royal Arch were moderately to steeply tilted to the southwest during the initial, most intense Volcanic rocks in the Little Horn Mountains are phases of Tertiary extension. By the time the basalt of inferred to be early to middle Miocene, based on similar- the Little Horn Mountains was erupted, however, the ities in lithology and structural setting to geochronologi- intensity of faulting had waned significantly so that the cally dated volcanic rocks in adjacent ranges (Fig. 1). resulting structure changed from blocks that dip in one The dated middle Tertiary volcanic rocks in southwest- direction (southwest) to blocks that dip in multiple ern Arizona unconformably overlie Proterozoic and(or) directions, including southwest and northeast. Some Mesozoic metamorphic rocks, which also form the blocks experienced no additional tilting. Three large tilt lowest exposed structural units. The dated rocks are not domains are loosely distinguished in the map area, based metamorphosed and are deposited on or intercalated with upon the dips of flows of the basalt of Little Horn a veneer of conglomerate and sedimentary breccia Mountains: a nearly flat-lying domain in the northwest- derived from metasedimentary and igneous rocks below ern part, an east-dipping domain in the northeastern part, the basal unconformity. A volcanic lithology is com- and a southwest-dipping domain in the west-central part. monly the youngest type of bedrock exposed in south- The central portion of the map area, where the basalt of western Arizona. Middle Tertiary rocks are typically

3 tilted and faulted against crystalline rocks along low- to Kofa Mountains have yielded K-Ar ages of20.1 ± 0.6 high-angle normal faults, with younger members of the and 18.3 + 0.4 Ma (Shafiqullah and others, 1980; section exhibiting shallower dips than older members. Sherrod and others, 1987). The 20.1 Ma date may Available age constraints on volcanic and sedimen- correlate with the basalt of Little Horn Mountains, which tary rocks in the Little Horn Mountains quadrangle are is flat-lying in the northwestern part of the Little Horn poor. Fresh-water crustaceans from limestones in the Mountains quadrangle; the 18.3 Ma date may correlate Clanton Hills, south of the Little Horn Mountains, were with the basalt of Oaldand mine. broadly identified as Tertiary fauna (Ross, 1922). Two Middle Miocene, rhyolite-Iava-dominated, ash-flow K-Ar ages on volcanic rocks in the Gila Bend Mountains tuff-poor successions typical of the Vulture, Hiero- (Eberly and Stanley, 1978), which lie outside the Little glyphic, and Big Horn Mountains may correlate with the Horn Mountains quadrangle, have been tentatively rhyolite of Nottbusch Valley. If the basalts of Little Horn correlated with rock units in the map area, based on Mountains and Oaldand mine are indeed 20.1 and 18.3 lithologic similarities (Scarborough and Wilt, 1979). A Ma, respectively, then the rhyolite of Nottbusch Valley basaltic andesite lava that yielded a whole-rock date of is between 20.1 and 18.3 Ma, which is the general age 28.7 ± 4.2 Ma lies on strike with a ridge of the basalt of flows in the rhyolite-Iava-dominated successions to ofOaldand mine, although the sample may correlate with the northeast (Rehrig and others, 1980; Capps and the basalt of Little Horn Mountains. Low radiogenic others, 1985, 1986). argon in the basaltic andesite sample indicates that the age may be spurious. A rhyolitic tuff that is dated at 23.6 + ECONOMIC GEOWGY 1.6 Ma correlates lithologically with tuff interbedded with the volcanic rocks of Royal Arch that underlies the Mineral-resource studies in areas adjacent to the basalts of Little Horn Mountains and Oakland mine. We Little Horn Mountains quadrangle indicate a high proba- regard the age of the tuffs sample to be reasonable, based bility for undiscovered base- and precious-metal deposits on ages of ash-flow tuffs in the adjacent Kofa and Castle in nearby areas (Bagby and others, 1987; Adrian and Dome Mountains (Bagby and others, 1987). others, 1986). Mapping during the present study Geologic mapping in the Kofa and Castle Dome indicates that the Little Horn Mountains are in a geologic Mountains (Bagby and others, 1987) and Big Horn and setting that is similar to that of the Kofa and Castle Dome Vulture Mountains (Capps and others, 1985, 1986; Mountains and may, therefore, have significant potential Grubensky, 1989) indicates that the Little Horn Moun- for undiscovered base- and precious-metal deposits. tains are flanked by early Miocene, ash-flow-tuff-domi- Alteration and mineralization recognized during this nated volcanic fields on the west and middle Miocene, study are confined to a small northwest-trending strip of lava-flow-dominated fields on the east (Fig. 1). Litho- volcanic rocks of Royal Arch near the western edge of logie and stratigraphic correlations of vol canic rocks are the map area. The several-square-kilometer area in- critical until additional geochronologic work can be cludes map coordinates TM44-98-, -97-; TM45-98-, conducted. The Little Horn Mountains contain rock -97-; TM46-97-, -96-; and TM45-97- (see Fig. 4 for units that are lithologicall y comparable to those in nearby grid-zone explanation). In these areas, the volcanic rocks ranges to the west and east, although individual lava of Royal Arch are pervasively hematite-stained and flows are unlikely to be present in adjacent ranges. locally contain submillimeter-sized pyrite cubes Ash-flow tuffs within the volcanic rocks of Royal pseudomorphed by hematite and veins of white, gray, Arch in the Little Horn Mountains quadrangle miner- and greenish microcrystalline quartz. The altered and alogically resemble tuffs in the southern Kofa, Castle mineralized strip lies along the projection of an up-to-the- Dome, and Tank Mountains. K-Ar dates on several west, northwest-trending high-angle fault. Brecciation welded ash-flow tuffs in the Kofa and Castle Dome in the area is common, although some of the fragmenta- Mountains suggest they were erupted rapidly approxi- tion occurred before alteration and may be volcanogenic mately 22 Ma (Bagby and others, 1987). Some were in origin. This strip is adjacent to the highly mineralized erupted from calderas in the Kofa and Castle Dome Sheep Tanks mining district (Cousins, 1973, 1984). The Mountains (Grubensky and others, in press). Welded overlying basalt of Oaldand mine, however, is not tuff in the Little Horn Mountains quadrangle may have altered or mineralized. been erupted from these large, nearby calderas. Locally thick accumulations of basalt lava with minor CONCLUSIONS interbedded flows of rhyolite lava and tuff are common in the New Water and northern Kofa Mountains (Bagby The Little Horn Mountains quadrangle is dominated and others, 1987). Flat-lying basalt lavas in the northern by a thick assemblage of Tertiary volcanic rocks and

4 minor amounts of sedimentary rocks that were deposited REFERENCES before, during, and after middle Tertiary extensional Adrian, B.M., Smith, D.B., Vaughn, R.B., and Delaney, T.A., faulting. The oldest Tertiary rocks, the volcanic rocks 1986, Analytical results and sample locality maps of stream- of Royal Arch, are interpreted to predate tilting because sediment, heavy-mineral-concentrate, and rock samples from of consistent steep tilts and apparent absence of the Kofa National Wildlife Refuge, Yuma and La Paz Coun- ties, Arizona: U.S. Geological Survey Open-File Report 86- unconformities within the unit. In the northern part of 199,122 p., scale 1:100,000, 2 sheets. the map area, the Royal Arch rocks are overlain in Bagby, W.C., Haxel, G.B., Smith, D.B., Koch, RD., Grubensky, angular unconformity by the basalts of Little Horn MJ., Sherrod, D.R., and Pickthorn, L.B.G., 1987, Mineral resource assessment of the Kofa National Wildlife Refuge, Mountains, which show a range in dips from moderate Arizona: U.S. Geological Survey Open-File Report 87~, to shallow to flat lying and were apparently erupted 45 p., scale 1: 100,000, 6 sheets. during the late phases of extensional faulting. A thick Bull, W.B., in press, Geomorphic responses to climatic change: New York, Oxford University Press. sequence of sandstone and conglomerate that Capps, R.C., Reynolds, S.J., Kortemeier, C.P., and Scott, E.A., unconformably(?) overlies the Royal Arch rocks in the 1986, Geologic map of the northeastern Hieroglyphic Moun- southern part of the map area may also be syntectonic tains, central Arizona: Arizona Geological Survey Open-File deposits. The rhyolite ofNottbusch Valley and basalt of Report 86-10, 16 p., scale 1:24,000. Capps, RC., Reynolds, S.J., Kortemeier, C.P., Stimac, J.A., Oakland mine are flat lying to subhorizontal and are Scott, E.A., and Allen, G.B., 1985, Preliminary geologic interpreted as late- to postextensional deposits. maps of the eastern Big Horn and Belmont Mountains, west- Published reports suggest that abrupt changes in unit central Arizona: Arizona Geological Survey Open-File Report 85-14,26 p., scale 1:24,000, 2 sheets. thicknesses and wedging are fundamental characteristics Cousins, N .B.,1973, Relationship ofblack calcite to gold and silver of volcanic-rock -dominated successions in southwestern mineralization in the Sheep Tanks mining district, Yuma and west-central Arizona (Capps and others, 1985, County, Arizona: Tempe, Arizona State University, unpub- 1986; Grubensky and others, 1987, in press; Stimac and lished M.S. thesis, 43 p., scale 1:21,120. 1984, Gold, silver, and manganese mineralization in the others, 1987). Geologic relations in the Little Horn -----sheep Tanks mine area, Yuma County, Arizona, in Wilkins, Mountains quadrangle suggest that these variations are Joe, Jr., ed., Gold and silver deposits of the Basin and Range due to the presence of intrabasinal or basin-adjacent Province, western U.S.A.: Arizona Geological Society Di- gest, v . 15, p. 167-174. volcanic centers, the products of which are irregularly Crowe, B.M., Crowell, J.e., and Krummenacher, Daniel, 1979, distributed. This could be a result of several variables, Regional stratigraphy, K-Ar ages, and tectonic implications of including topography, flow composition and viscosity, Cenozoic volcanic rocks, southeastern California: American Journal of Science, v. 279, p. 186-216. eruption style and rate, and vent position. Davis, G.A., Anderson, J.L., Frost, E.G., and Shackelford, T.J., In southwestern Arizona, topographic relief associ- 1980, Mylonitization and detachment faulting in the Whipple- ated with extensional faulting may have controlled the Buckskin-Rawhide Mountains, southeastern California and distribution of some volcanic rock units. Lava flows of western Arizona, in Crittenden, M.D., Jr., Coney, P.J., and Davis, G.H., eds., Cordilleran metamorphic core complexes: the basalt of Oakland mine in the Little Horn Mountains Geological Society of America Memoir 13, p. 79-129. quadrangle were locally confined in narrow, structurally Demsey, K.A., 1988a, Geologic map of Quaternary and upper induced basins. The distribution of rock units in the Little Tertiary alluvium in the Phoenix North 30' x 60' quadrangle, Arizona: Arizona Geological Survey Open-File Report 88-17 , Horn Mountains, as well as contact relationships and scale 1:100,000. changes in thickness, has provided some insight into the __ 1988b, Quaternary geologic map of the Salome 30 x 60- surface morphology of a middle Tertiary extensional minute quadrangle, west-central Arizona: Arizona Geological Survey Open-File Report 88-4, scale 1:100,000. terrane. Eberly, L.D., and Stanley, T.B., 1978, Cenozoic stratigraphy and geologic history of southwestern Arizona: Geological Society of America Bulletin, v. 89, p. 921-940. ACKNOWLEDGMENTS Grobensky, MJ, 1989, Geologic map of the Vuhure Mountains, west-central Arizona: Arizona Geological Survey Map 27, The geologic map of the Little Horn Mountains 15' scale 1:24,000, 3 sheets. quadrangle was prepared as part of the Cooperative Grubensky, M.J., Haxel, G.B., and Koch, RD., in press, Geologic map of the Castle Dome Mountains, southwestern Arizona:' Geologic Mapping Program (CCXJEOMAP), which is U.S. Geological Survey Miscellaneous Investigations Map 1- jointly funded by the U.S. Geological Survey and 2138, scale 1:62,500. Arizona Geological Survey. Aerial photographs, which Grubensky, M.J., and Reynolds, S.1., 1988, Geologic map of the southeastern Vulture Mountains, west-central Arizona: Ari- augmented mapping in the field, were provided by the zona Geological Survey Open-File Report 88-9, 16 p., scale U.S. Bureau of Land Management in Phoenix. 1:24,000.

5 GEOLOGIC COLUMN FOR THE LITTLE HORN MTS. 15' QUADRANGLE, SOUTHWESTERN ARIZONA

pedogenic(?) carbonate rhyolite of Nottbusch Valley basalt of Oakland Mine

sandstone and conglomerate limestone EXPLANATION

basalt of little Horn Mts. limestone or carbonate sandstone and @ conglomerate ~ I Felsic lava

.rW------Felsic tuff ------I~=-~I ~ ~ ~ ~ ~ ...-t-J.- _ Maficlava , volcanic rocks of Royal Arch ~ .--".... ~ Breccia and conglomerate

~ Schist

"// \\::: _ It It Granite sedimentary breccia and 1 conglomerate Metased. and metavolc. Sedimentary and volcanic tE!?~~ granite and schist rocks of Slumgullion (\/\ unconformity

.-?"".. fault, inferred

Figure 2. Schematic geologic column of pre-Quaternary rocks in the Little Horn Mountains quadrangle. Graphical profile approximates the relative resistance to weathering of adjacent rock units. Total thicknesses of column and individual units are variable within the map area and locally are unknown because of complex structural disruption by high- and low-angle normal faults. The total column thickness is inferred to be approximately 1 kilometer or less.

8 2 miles N

S' A 4 kilometers

X 2:J.52

) . 100 C' ~ 1OO~.

• '200\. .?O80 . ~ ;g71 B x. ~100 •. / ., { • n@ ~

x 2~ I SoldiersTank • •\~3OO~'"'(\--J400l ------J (3. 01~ • Oakland• Mine )' N@X re----..... 400/c" 500

255<0(

EXPLANATION • Point where unit thickness measured on geologic map ~ Point where base of unit not exposed

2352 X Elevation, in feet, of geographic peaks shown on tapa. base

-----100" Isopach, in feet, of basalt of Oakland mine • -.-- Fault, exposed or projected on geologic map E9 Vent rocks of basalt of Oakland mine B ~ B' Line of cross section D'

Figure 3. Isopach map of the basalt of Oakland mine in the central part of the Little Horn Mountains. Faults portrayed in figure are younger than the basalt and do not delimit its original extent.

9 OliO lONE OUIONATlON: TO GIV[ A STANDARD.[fU[NC[ ON 12S THIS SH[ET TO N[ARUT 100 METUS 100.000 M. SQUAR[ 100NTlflCATlOII SAM'L[ 'OINT: OAKLANDMIN[

I...... 1tII... 1I•• lnrl., 100.000 ••• " IQUiri in ."klll Ute point Hn: TM I. lout. first VUTICAl,,1d lill' •• un of poi"t '" ,tad lA.C( fl,u," I.bttilll ttt. ~~ TM Ii'" Mitt.,. ill "" top or bottom ""'Ii'" 0.- ,ft ttle Ii", itstff: 49 [lliMtt. ten"-, 'I"" Jrid Nite t. pol"': 9 J.locat. fir" HORllONTAl.,id lin, .now 1tOiftf:,ftd 'IN URGE Ii..,," I.btfi•••,he II", ••••••. III tM 11ft Of rip' ""'lin. or IGNOIIlIllo SMALLU n•••• 01•• , ••••• 11". it"": trid ".", •••; tIMM I,. 'Of "Mill, [11I.,,,.1_ ""'" trI' H•• le ,..11I: IN I.n c:oortl'",t ••. U•• QNl Y " •• '"s LAItG£R"1"'" 01 tM arid ftumber; SAMn[ REfU[NC[: TM_3S - ••ph:368JOOO If reporti", ""'"' ••• ,••• ..,. 4irtdMNt. - ••rtfi. Crid loftl OtIi.".ttoft. II: 12ST_3S

Figure 4. Instructions for providing a standard reference on a topographic base prepared by the U.S. Anny Map Service. Grid references in text to the large mineralized areas include dashes in place of approximate tenths.

10

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