Stratigraphie Relations and U-Pb Geochronology of the Upper Cretaceous Upper Mccoy Mountains Formation, Southwestern Arizona

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Stratigraphie relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

RICHARD M. TOSDAL U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 PAUL STONE U.S. Geological Survey, National Center, Reston, Virginia 22092

ABSTRACT gent Mule Mountains thrust system in the lat- Stone and Pelka, 1989). The formation est Late Cretaceous (—70 Ma) marked the end records erosion of a Jurassic volcanic arc and A previously unrecognized angular uncon- of Mesozoic contractile deformation in the the exposure of a deeper crystalline-base- formity divides the Jurassic and Cretaceous area. ment terrane in the region to the north of the McCoy Mountains Formation into a lower and present outcrop belt (Pelka, 1973; Harding an upper unit in the Dome Rock Mountains INTRODUCTION and Coney, 1985). The formation was com- and Livingston Hills of western Arizona. The plexly deformed and regionally metamor- lower unit of the McCoy Mountains Formation The Jurassic and Cretaceous McCoy phosed at greenschist- and, locally, lower- consists of generally fine-grained quartzose Mountains Formation is a critical tectono- amphibolite-facies conditions during at least and volcaniclastic strata that were deposited stratigraphic unit in the Blythe-Quartzsite two episodes in the Late Cretaceous (Reyn- after cessation of Middle Jurassic explosive area of southeastern California and south- olds and others, 1986; Tosdal, 1986, 1990; volcanism. The basal contact of the lower unit western Arizona. The siliciclastic sedimen- Laubach and others, 1989). is disconformable in most places, but locally it tary and metasedimentaiy rocks that com- Although a broad stratigraphic and struc- has been interpreted to be gradational with the pose this formation are the only record of tural framework has been developed for the underlying silicic volcanic rocks. The upper some 80 to 90 m.y. in this part of the North McCoy Mountains Formation, its paleotec- unit is a fining-upward sequence of quartzo- American Cordillera between the cessation tonic setting is controversial. Different work- feldspathic and arkosic conglomerate and sand- of volcanism in the Middle Jurassic and de- ers have proposed that the formation was de- stone that records uplift of a northern source formation, regional metamorphism, and plu- posited in a transtensional basin formed along terrane. A tuff in the lower part of the upper tonism in the Late Cretaceous (Harding and the Late Jurassic Mojave-Sonora megashear unit has a U-Pb crystallization age of 79 ± 2 Coney, 1985; Reynolds and others, 1986; (Harding and Coney, 1985), in a back-arc rift Ma (Late Cretaceous). Rocks of the lower unit Tosdal, 1990). The formation, furthermore, basin that was an extension of the Upper Ju- are deformed by pre-80 Ma thrust faults of the lies in the center of an easterly oriented zone rassic and Cretaceous Bisbee basin in south- generally southward-vergent Maria fold and of deformation and metamorphism that is eastern Arizona and northern Mexico (Dick- thrust belt, which bounds the outcrop belt of anomalous with respect to generally north- inson and others, 1989), and in a basin formed the McCoy Mountains Formation on the westerly structural trends characteristic of in the foreland of an eastward-advancing Ju- north. The upper unit is exposed only south of the cordillera in southern California and Ar- rassic and Cretaceous fold and thrust belt the fold and thrust belt. izona. Thrust faults which border the forma- (Drewes, 1991). In this report, we reinterpret We interpret the intraformation unconform- tion on both the north and south record tec- the paleotectonic setting of the McCoy ity in the McCoy Mountains Formation to have tonic transport inward toward the center of Mountains Formation on the basis of new developed where rocks of the lower unit were its outcrop belt, and perhaps toward the cen- stratigraphic and U-Pb geochronologic infor- deformed adjacent to the southern margin of ter of its depositional basin (Spencer and mation derived from studies primarily in the the Maria fold and thrust belt. The upper unit Reynolds, 1990; Tosdal, 1990). Upper plates Livingston Hills and Dome Rock Mountains, of the formation is interpreted as a foreland- of the thrust fault systems are crystalline ter- Arizona (Fig. 1). This new information indi- basin deposit that was shed southward from ranes, the northern of which is continuous cates that an unconformity divides the for- the actively rising and deforming fold and with the area now occupied by the Colorado mation into two major units in these ranges, thrust belt. The apparent absence of an equiv- Plateau. reflecting a previously undocumented, tec- alent unconformity in the McCoy Mountains The McCoy Mountains Formation crops tonically induced break in deposition, and Formation in adjacent California is presuma- out in several ranges from the Coxcomb that the upper unit of the formation is largely, bly a consequence of the observed westward Mountains, California, on the west to the if not entirely, of Late Cretaceous age, sig- divergence of the outcrop belt from the fold Black Rock Hills, Arizona, on the east nificantly younger than inferred by previous and thrust belt. Continued southward short- (Fig. 1). It consists of as much as 8 km of workers. These findings support a new pale- ening deformed the entire formation under sedimentary and metasedimentary rocks otectonic interpretation that links deposition greenschist- and, locally, amphibolite-facies whose protoliths were deposited in fluvial, al- of the upper part of the McCoy Mountains conditions soon after the upper unit was de- luvial-fan, and lacustrine environments Formation, and the development of the in- posited. Tectonic burial beneath the north-ver- (Pelka, 1973; Harding and Coney, 1985; traformational unconformity, to deformation

Geological Society of America Bulletin, v. 106, p. 476-491, 11 figs., 2 tables, April 1994.

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EXPLANATION e« >< *v * Alluvium (Quaternary) a Dome Rock sequence (Jurassic)

Volcanic and sedimentary rocks (Tertiary) Monzonitic and granitic rocks (Triassic and Triassic?) £3.

VO vo Granitic rocks (Cretaceous) Sedimentary rocks (Early Mesozoic)

McCoy Mountains Formation (Cretaceous and Jurassic) Sedimentary rocks (Paleozoic)

Apache Wash facies (Cretaceous and Jurassic) Wy/Zß Gneiss, schist, and granitoids (Proterozoic)

* « = - " u '' Granitic rocks of the Kitt Peak-Trigo Peaks super-unit (Late and Middle Jurassic) LV Thrust fault-dashed where projected Contact Fault Figure 1. Generalized geologic map of the Blythe-Quartzsite area, southeastern California and southwestern Arizona. Areas of enlarged geologic maps enclosed in boxes. BM, Big Maria Mountains; BR, Black Rock Hills; CD, Castle Dome Mountains; CM, Chuckwalla Mountains; CX, Coxcomb Mountains; DR, Dome Rock Mountains; GW, Granite Wash Mountains; HA, Harquahala Mountains; HM, Harcuvar Mountains; KM, Kofa Mountains; LC, Little Chuckwalla Mountains, LH, Livingston Hills; LM, Little Harquahala Mountains; MA, Little Maria Mountains; MC, McCoy Mountains; MM, Mule Mountains; NW, New Water Mountains; PL, Plomosa Mountains; PM, Palen Mountains; PV, Palo Verde -J -J Mountains; TM, Trigo Mountains; TP, Trigo Peaks. Adapted from Tosdal (1990) and references listed therein.

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and uplift on folds and thrust faults along the sents a widespread clastic sequence not re- erally, both units have homoclinal southward northern margin of the formation's present stricted to any basin in the Blythe-Quartzsite dips. outcrop belt. area. In contrast, strata equivalent to the Our discussion of the stratigraphy of the This report uses the time scale of Harland upper part of the formation are apparently McCoy Mountains Formation applies specif- and others (1990). restricted to the Blythe-Quartzsite area, ically to the Dome Rock Mountains (Fig. 3) with the possible exception of conglomer- and Livingston Hills (Fig. 4), where the lower REGIONAL FRAMEWORK atic strata in the Chocolate Mountains that and upper units of the formation are well de- were correlated with the upper part of the fined and sharply differentiated. Harding and The northern and eastern outcrop limit of McCoy Mountains Formation by Richard Coney (1985) and Stone and Pelka (1989) the McCoy Mountains Formation in Califor- (1992a). have described the McCoy Mountains For- nia and Arizona is the Jurassic and Creta- The McCoy Mountains Formation has mation in the Palen and McCoy Mountains ceous Maria fold and thrust belt (Fig. 1), a been divided into two facies, a thick basin (Fig. 1). In the ranges in our study area in north-dipping zone of polyphase ductile de- facies and a thinner Apache Wash facies Arizona, the sedimentary rocks are metamor- formation and regional metamorphism at (Fig. 2) (Harding, 1982; Harding and Coney, phosed and cut by several generations of conditions ranging up to the amphibolite fa- 1985; Richard and others, 1987). The basin cleavage (Tosdal, 1986). Our description re- des (Reynolds and others, 1989; Spencer and facies crops out along the entire length of the fers to their protoliths unless otherwise Reynolds, 1990). Most structures in the belt outcrop belt from the Coxcomb Mountains necessary. are of Cretaceous age (Reynolds and others, on the west to the Black Rock Hills and Little 1986; Spencer and Reynolds, 1990), with few Harquahala Mountains on the east (Fig. 1). In STRATIGRAPHY Jurassic structures being preserved (Hamil- contrast, the Apache Wash facies crops out ton and others, 1987; Tosdal and others, only in the New Water, southern Plomosa, Lower Unit 1989). The belt separates Proterozoic crystal- and Little Harquahala Mountains (Fig. 1). line basement in its upper plate from polyde- Thrust faults usually separate the two facies The lower unit (Fig. 2) is a distinctive as- formed Mesozoic sedimentary and volcanic where they are present in the same range sociation of interbedded maroon mudstone, rocks in its lower plate. Highly deformed Pa- (Harding and Coney, 1985; Richard and oth- siltstone, white- to cream-colored quartzose leozoic cratonal sedimentary rocks and their ers, 1987; Sherrod and Koch, 1987), except sandstone, quartzite-clast conglomerate, and Proterozoic basement are present within the for one locality in the southern Plomosa brown arenaceous limestone (basal sand- fold and thrust belt. The Paleozoic cratonic Mountains where the Apache Wash facies stone member 1) that grades upward into stratigraphy consists of the formations that unconformably overlies the basal strata of the green-gray volcaniclastic rocks (basal sand- are presently found on the Colorado Plateau, basin facies (Richard, 1992b). The Apache stone member 2) consisting of lithofelds- some 150 km to the northeast. Whether or Wash facies is generally considered to be cor- pathic sandstone, siltstone, and mudstone. not the McCoy Mountains Formation origi- relative with some part of the basin facies, Homogeneous mudstone and siltstone inter- nally extended northward from its present but the available petrologic and stratigraphic bedded with minor sandstone and conglom- outcrop belt is not known, as Mesozoic rocks data allow conflicting correlations (Harding, erate are common in the uppermost part of in the area have been eroded (Spencer and 1982; Harding and Coney, 1985; Richard and the unit (mudstone member). Lithic grains in Reynolds, 1990). On the south, the McCoy others, 1987). Because the stratigraphic rela- basal sandstone member 2 are both volcanic Mountains Formation lies mostly beneath the tions between the two facies are uncertain, and sedimentary, and all feldspar is plagio- south-dipping Mule Mountains thrust system we consider the Apache Wash facies in this clase (Harding and Coney, 1985). The dis- of latest Cretaceous age (Tosdal, 1990). report to be a distinct stratigraphic unit rather tinctive maroon color of mudstone in the In California, the McCoy Mountains For- than a simple facies of the McCoy Mountains basal sandstone member 1 is replaced by pur- mation lies south of the Maria fold and thrust Formation. Our use of the term "McCoy ple and gray with increasing metamorphic belt, whereas in Arizona, the formation con- Mountains Formation" in this report refers grade, owing to the growth of phengitic(?) verges eastward into the fold and thrust belt specifically to the basin facies. muscovite. Diorite sills intrude rocks in basal in the southern Plomosa, Granite Wash, and Harding and Coney (1985) divided the Mc- sandstone member 1 in the Dome Rock Harquahala Mountains (Fig. 1) (Harding and Coy Mountains Formation into six members. Mountains (Fig. 3). Inferred depositional en- Coney, 1985; Reynolds and others, 1986, The lower three members consist of quartz- vironments are fluvial and locally lacustrine, 1991; Laubach and others, 1987; Richard and ose and volcaniclastic rocks formed from and paleocurrent data indicate a northern others, 1987). To the south of the Blythe- sediment deposited by southward-flowing source terrane (Robison, 1980; Harding and Quartzsite area, the Winterhaven Formation fluvial systems (Robison, 1980; Harding and Coney, 1985). in the Trigo and Chocolate Mountains (some Coney, 1985). These members have been re- Throughout the outcrop belt of the McCoy 50 km to the south), the sedimentary and vol- ferred to informally as the "lower unit" Mountains Formation, the basal contact of canic rocks of Slumgullion in the Castle (Fig. 2) (Tosdal and others, 1989). The upper the lower unit with the underlying Middle Ju- Dome Mountains (some 30 km south of the three members, which consist of conglomer- rassic Dome Rock sequence is generally dis- Kofa Mountains), and unnamed strata in the ate, feldspathic sandstone, and siltstone de- conformable (Pelka, 1973; Robison, 1980; central Kofa Mountains (Fig. 1) are consid- posited predominantly in an alluvial-fan en- Harding and Coney, 1985; Richard and oth- ered correlative with the lower part of the vironment south of a northern source terrane ers, 1987; Tosdal, 1988). In several ranges in McCoy Mountains Formation (Haxel and (Harding and Coney, 1985), together form an Arizona and California, however, the contact others, 1985; Tosdal and others, 1989; Rich- upward-fining depositional sequence that has has locally been interpreted as gradational ard, 1992a). These correlations imply that at been referred to informally as the "upper (Harding and Coney, 1985; Tosdal, 1988; Ad- least the lower part of the formation repre- unit" (Fig. 2) (Tosdal and others, 1989). Gen- ams and others, 1991). One of the inferred

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Dome Rock McCoy, McCoy Harquahala New Water Mts. and Palen, and and Little and Plomosa Livingston Coxcomb, Palen Harquahala Mts. Hills and Dome Mts. Mts. Bock Mts., This study Livingston Stone and Richard and Harding and Coney Pelka (1989) others (1987) Hills (1985), Reynolds and others, (1986) Harding and Coney (1985) Conglomerate member Member L (structurally repeated) Member K Not exposed Member J Member I Not Siltstone exposed Member H member c .o © 13 Member G F £ <0 Upper Sandstone @ .s S unit member Member F b .c Red Hills w © Conglomerate conglomerate i CD member o- o Mudstone Siltstone Member E .8 s member § <= member Ranegras S fi Basal sandstone Member D Lower member Sandstone member 2 O to unit Member C member o Basal sandstone Member B ái Conglomerate member 1 Member A member Dome Rock sequence of Tosdal and others (1989) Dome Rock sequence of Tosdal and others (1989)

© U-Pb sample location © Fossil angiosperm Figure 2. Generalized stratigraphie nomenclature of the McCoy Mountains Formation of Harding and Coney (1985), showing various subdi- visions of the basin facies and the Apache Wash facies of Reynolds and others (1986).

gradational contacts crops out in the eastern (Fig. 3). As there is no evidence in the upper Hills (Fig. 4), about 150 m of different parts of Dome Rock Mountains where the base of the part of the Dome Rock sequence for syn- the lower unit crop out beneath the upper unit formation overlies concordantly tuffaceous depositional faulting, we infer that the (Tosdal, 1988); the base of the formation is and sedimentary rocks that represent the present fault reactivated an older high-angle not exposed in the range. In the southern Plo- stratigraphically highest part of the subjacent fault that was active after explosive volcan- mosa Mountains, some 5 to 10 km to the Dome Rock sequence (unit Jds in Fig. 3) ism and volcaniclastic sedimentation and north of the Livingston Hills (Fig. 1), the (Harding and Coney, 1985). Less than 1 km prior to deposition across its trace of the low- lower unit that crops out beneath the base of to the south, however, across a Late Ceno- est quartzose sandstone in the basal member the upper unit is as much as 1,400 m thick zoic fault with left separation lying along of the McCoy Mountains Formation. These (Robison, 1979). Cross-beds and graded bed- Copper Bottom Pass, the base of the lower conclusions imply that the previously in- ding indicate that parts of the lower unit in unit of the McCoy Mountains Formation dis- ferred gradational contact between the lower this range are overturned. conformably overlies tuffs that form the low- unit of the McCoy Mountains Formation and est part of the Dome Rock sequence (unit the subjacent Dome Rock sequence north- Upper Unit Jdm in Fig. 3). Highly schistose rocks along east of Copper Bottom Pass must be a dis- the contact southwest of Copper Bottom conformable contact. The upper unit is a fining-upward sequence Pass are interpreted to be a metamorphosed In the study area, the thickness of the consisting of combinations of brown-, tan-, weathering horizon. In contrast to the Dome lower unit varies. In the Dome Rock Moun- and gray-weathering conglomerate, sand- Rock sequence, member thicknesses in the tains, it is about 2.4 km thick on the east and stone, siltstone, and mudstone (Fig. 2). Rare lower unit of the McCoy Mountains Forma- thins progressively to the west until it is less silicic pyroclastic rocks and mafic to interme- tion are maintained across the present fault than 10 m thick (Fig. 3). In the Livingston diate lavas are interbedded with the clastic

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Figure 3. Generalized geologic map of the southern Dome Rock Mountains, southwestern Arizona. See Figure 1 for location.

rocks (Miller, 1966; Harding, 1982; Laubach feldspathic, typically arkosic, and was de- The stratigraphy of the conglomerate and others, 1987). Diorite dikes intrude the rived from volcanic, plutonic, metamorphic, member, which forms the lowermost part of unit in the Dome Rock Mountains (Fig. 3). and cratonic sedimentary terranes (Harding the upper unit (Fig. 2), is important to inter- The upper unit is inferred to record chiefly and Coney, 1985). Quartzose and volcani- preting the basal contact. In the Livingston alluvial-fan, fluvial, deltaic, and lacustrine en- clastic sandstone, which is abundant in the Hills, the conglomerate member in the north- vironments (Harding and Coney, 1985). Dep- lower unit, is minor in the upper unit. Rocks western part of the range consists of about osition records an abrupt increase in the relief of the upper unit are generally of coarser 1,200 m of conglomerate, with rare interbeds and crustal depth of the source terrane to the grain size than those of the lower unit. These of sandstone (Fig. 4). The exposed thickness north (Harding and Coney, 1985). compositional and textural changes in the of the member decreases along strike to the The clastic rocks of the upper unit are com- clastic rocks between the two units are abrupt east. In the Dome Rock Mountains, the con- positionally distinct from those of the lower in both the Livingston Hills and the Dome glomerate member reaches a thickness of unit. Sandstone of the upper unit is largely Rock Mountains. about 1,600 m in the eastern part of the range,

480 Geological Society of America Bulletin, April 1994

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FIGURE 3 and it thins to about 200 m in the southwestern part of the range (Figs. 3 and 5). In the EXPLANATION thicker sections in this range, the member Alluvium (Quaternary) consists of about 800 m of siltstone, sandstone, and lesser conglomerate that is over- Tc Conglomerate (Tertiary) lain by about 800 m of sandstone and conglomerate with rare siltstone (Harding, 1982). Tv Volcanic and sedimentary rocks (Tertiary) Across the range from northeast to south- Kd Diorite (Cretaceous) west, siltstone and sandstone in the basal part of the member disappear, and conglomerate KJd Diorite and andesite (Cretaceous or Jurassic) dominates the member (Fig. 5). McCoy Mountains Formation of Harding and Clast populations low in the conglomerate Coney (1985) (Cretaceous and Jurassic)— member are heterogeneous and contain var- iable percentages of Paleozoic marble and Upper unit—Divided into: quartzite; Jurassic quartz-phenocrystic tuffs; rare Mesozoic feldspathic sandstone, quartz KJmst Siltstone member arenite, and quartzite-clast and carbonate- chip conglomerate; and rare granitic rocks of KJms Sandstone member probable Proterozoic and Jurassic age. The percentage of granite clasts increases upsec- ;Kjmò:-. Conglomerate member—Locally contains mappable horizons of: tion in the member. Clast size in conglomerate and detritus in sandstone beds coarsens »RJmcg Granite clast-rich conglomerate and upsection in the conglomerate member in the arkosic sandstone western Livingston Hills. In the eastern Dome Rock Mountains, conglomerate and KJmcp Phyllite and metamudstone sandstone dominate the upper part of the member (Figs. 3 and 5). In the upper part of KJmcc Calcareous sandstone and quartz the member and — 1 km above the base of the porphyry-rich conglomerate thicker sections, such as those in the western Livingston Hills and eastern Dome Rock Lower unit—Divided into: Mountains, beds consist almost entirely of KJmm Mudstone member granite and K-feldspar detritus, and arkosic sandstone and gritstone are common (Hard- Basal sandstone member 2 KJm2 ing, 1980, 1982). These granite-detritus-rich and arkosic beds form a distinct horizon IffTvjff Basal sandstone member 1 (Fig. 3), which in the northeastern Livingston Jkt Kitt Peak-Trigo Peaks super-unit of Tosdal and Hills overlies rocks of the lower unit (Fig. 6A; others (1989) (Late and Middle Jurassic)— see below). Granitic rocks Sources for several of the clast types in the conglomerate member are known. Clasts of Dome Rock sequence of Tosdal and others (1989) quartz-phenociystic tuffs are lithologically (Middle Jurassic)— similar to rocks of the Dome Rock sequence. Clasts of Mesozoic quartz arenite, quartzite- Jds Siltstone, sandstone, and conglomerate clast conglomerate, and lithofeldspathic sand- Tuff and tuffaceous sedimentary rocks stone are lithologically similar to the rocks in basal sandstone members 1 and 2 of the lower Massive quartz porphry tuff unit. Diorite, porphyritic granodiorite, and granite cobbles and boulders in conglomerate Contact beds in the Livingston Hills are lithologically Intramember contact identical to plutonic rocks of the Middle Ju- rassic Kitt Peak-Trigo Peaks super-unit, Fault—Dotted where concealed which is found throughout the Blythe- Quartzsite area (Fig. 1) (Tosdal and others, Low-angle normal fault—Dotted where concealed 1989) and to Proterozoic granites. The dom- Thrust fault—Dotted where concealed inant granitoid clast type in the Dome Rock ,20 Mountains is nondistinctive. In the Living- Strike and dip of bedding ston Hills, granite clasts in the upper part of —• Asymmetric syncline the member are as much as 1 m in diameter, whereas those in the Dome Rock Mountains • U-Pb sample location are rarely larger than 20 cm in diameter.

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QTs Sedimentary rocks (Quaternary and Tertiary?) Tv Volcanic rocks (Tertiary) McCoy Mountains Formation of Harding and Coney (1985) (Cretaceous and Jurassic)— Upper unit— KJmst Siltstone member Figure 4-Genera,ized maP of the Livins ston Hills, southwestern Arizona. See Figure 1 for KJms Sandstone member location. KJmo: Conglomerate member Lower unit— KJm2 Basal sandstone member 2 KJmi Basal sandstone member 1 Contact Intramember contact Fault—Dashed where inferred, dotted where buried

-A Reverse fault 52 Strike and dip of bedding—Ball indicates facing is known Strike and dip of overturned bedding

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SW NE overlain by granite-clast conglomerate that contains abundant angular quartz-arenite Fault Tuff Complex Locality fault zone clasts within a few meters of its base. The contact between the two units here has a mi- Sandstone member nor amount of relief, only a few meters. Quartz-arenite intervals in basal sandstone member 1 along the contact form small paleotopographic highs, whereas the mudstone intervals form paleotopographic lows. Clast- and matrix-supported breccia, composed of angular quartz-arenite cobbles in a matrix of reworked maroon mudstone and finer Quartz Basal grained quartz arenite, are interbedded with porphyry sandstone Basal granite-clast conglomerate adjacent to the pa- member 1 sandstone | leotopographic highs. member 2 The angularity between the two units and the presence of clasts of underlying rocks in member the basal conglomerate beds confirms that - — — — — —Lower Unit — 1—1 the contact is an angular unconformity, as Sandstone, siltstone, and minor conglomerate originally interpreted by Miller (1966). As- suming that the thicknesses of the various B^I Conglomerate and sandstone — km members of the formation measured by Har- ding (1980, 1982) in the Dome Rock Moun- Siltstone with minor sandstone tains and Livingston Hills are appropriate measures, as much as 3 km of stratigraphic Contact 1 rr section may be missing across the contact. 2 The missing section includes most of the km lower unit and basal strata of the upper unit. In the northwestern Livingston Hills Figure 5. Simplified distribution of clastic rocks along strike within the conglomerate member (Figs. 4 and 7), conglomerate at the base of of the McCoy Mountains Formation of Harding and Coney (1985) in the Dome Rock Mountains. the upper unit of the McCoy Mountains For- Intermember contacts in the lower unit are shown for reference. Stratigraphic position of the Late mation is in contact with an isolated and up- Cretaceous tuff is shown. Vertical scale is exaggerated. Thicknesses in the eastern part of the right section of lithofeldspathic sandstone, formation are after Harding and Coney (1985), whereas those in the western part are estimated pebble conglomerate, and minor siliceous from the geologic map. The anastomozing fault system lying along Ehrenburg Wash has been siltstone and limestone that we assign to basal simplified. sandstone member 2 of the lower unit. The conglomerate member consists of generally massive conglomerate and minor sandstone INTRAFORMAHONAL bedding in the otherwise massive conglom- that face southward. The rocks are over- p p UNCONFORMITY erate (clast-rich horizons, crude clast imbri- turned and dip steeply (60 -90 ) northward at cation, lenticular grain size variations, and the contact; less than 0.5 km to the south, Livingston Hills rare interbeds of quartz arenite-clast breccia) however, the beds are upright and dip 40°-60° indicates that the rocks dip 60°-90° south- southward (Fig. 7). Clasts of basal sandstone In the northeastern Livingston Hills, the ward and locally are overturned to the north. member 2 are present in the conglomerates. contact between the lower and upper units of The conglomerate beds lie subparallel to the Field relations suggest that the contact be- the McCoy Mountains Formation crops out contact. tween the upper and lower units at this local- along the north flank of a strike ridge (Figs. 4 The contact between the lower and upper ity is either an angular unconformity or a pre- and 6). Here, quartz arenite and maroon silt- units is depositional except toward the east metamorphic fault, but the limited exposure stone of the lower unit (basal sandstone mem- end of the ridge, where some fault gouge is and structural complications make a defini- ber 1) on the north abut against granite-clast found. Where maroon mudstone is present tive interpretation difficult. conglomerate on the south along this steeply beneath the contact, the contact is marked by Rocks of the McCoy Mountains Forma- dipping contact (Figs. 6A and 6B). The gran- a 1-m-thick (or thinner) horizon of maroon tion in the northwestern Livingston Hills are ite-clast conglomerate is interpreted as equiv- mudstone-chip breccia with a purple sericitic separated from those in the northeastern Liv- alent to rocks that form the upper part of the matrix. In these places, maroon mudstone ingston Hills by a northwest-striking, largely conglomerate member of the upper unit that chips are present in the overlying granite- concealed fault of unknown displacement crops out along strike to the west. Sparse clast conglomerate. Where quartz arenite is (Fig. 4). The original lateral relations between channels and graded beds indicate that the present beneath the contact, a bed, no more rocks at the two localities are thus uncertain. conglomerate faces southward. The rocks of than a few meters thick, of quartz arenite- The structural contrast between the south- basal sandstone member 1 dip 40°-65° south- clast breccia with a purple sericitic matrix is dipping section of basal sandstone member 1 ward and are truncated at the contact. Rare present along the contact. This breccia is beneath the conglomerate member in the

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EXPLANATION

Qal Alluvium (Quaternary) McCoy Mountains Formation of Harding and Coney (1985) (Cretaceous and Jurassic) Upper unit— pi Conglomerate member—Locally includes: (KJmcb| Quartz arenite-clast breccia Lower unit— Ifflmil Basal sandstone member 1—Locally includes: Quartz arenite Contact Fault—Dotted where concealed I 70 Strike and dip of bedding gfh Strike and dip of overturned bedding

TJJT Direction of stratigraphic top I- H Line of cross section

Figure 6A. Enlarged geologic map of the northeastern Livingston Hills, southwestern Arizona. See Figure 4 for location.

northeastern Livingston Hills and the north- that basal sandstone member 1 was faulted these rocks could have been deposited syn- dipping section of basal sandstone member 2 against basal sandstone member 2 before chronously with a reverse growth fault or beneath the conglomerate member in the deposition of the conglomerate member, above a blind thrust. The present fault, which northwest suggests the presence of a signifi- which then overlapped the fault contact be- cuts the conglomerate member, would be cant discontinuity between these two sec- tween those units. The abrupt westward the result of continued movement or tions. The structural orientation of the largely thickening of the conglomerate member sug- reactivation. south-dipping conglomerate member is simi- gests that displacement on such a fault would lar in both areas. The 1,000-m-thick section of have been up to the northeast. The buried Dome Rock Mountains polymictic conglomerate that underlies the fault seems best interpreted as a thrust fault distinctive horizon of granite-clast conglom- because the overturned bedding in the lower Bedding in the lower and upper units is erate in the northwestern Livingston Hills, unit in the southern Plomosa Mountains to generally concordant in the Dome Rock however, is absent in the northeastern Liv- the immediate north implies that shortening Mountains. Along strike within the range, ingston Hills. There, granite-clast conglom- affected those rocks, and because the fanning however, members of the lower unit are grad- erate directly overlies basal sandstone mem- dips in the conglomerate member in the ually overstepped from northeast to south- ber 1. Our interpretation of these relations is northwestern Livingston Hills suggest that west by the conglomerate member at the base

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mation and with rocks of Slumgullion in the Castle Dome Mountains. These units are interbedded with Middle Jurassic(?) silicic vol- Conglomerate A s— member Figure 6B. Schematic canic rocks or are intruded by Middle Juras- N cross section across uncon- sic^) granodiorite (Haxel and others, 1985; formable contact between Tosdal and others, 1989). The other two Basal Sandstone rocks of basal sandstone members of the lower unit of the McCoy Contact member 1 member 1 and granite- Mountains Formation are undated, but are clast conglomerates of older than thrust faults that are intruded by Quartz conglomerate member of Late Cretaceous granites in the Granite Wash arenite the McCoy Mountains Mountains (Reynolds and others, 1991). Formation. Angular trun- These granites have been dated at about 80 cation of the older rocks at Ma by U-Pb and 40Ar/39Ar methods (Rich- the contact and the sub- ard, 1988; DeWitt and Reynolds, 1990). We parallelism of conglomer- therefore assign the lower unit a Jurassic and ate beds with the contact Cretaceous(?) age. are emphasized. Cross section is drawn across un- Upper Unit conformity at west end of Granite-clast Basal strike ridge shown in A. Poorly preserved angiosperm wood fossils conglomerate Quartz breccia in the upper part of the upper unit of the Mc- arenite- Coy Mountains Formation in the Palen and clast breccia McCoy Mountains indicate that these rocks are no older than late Early Cretaceous (Fig. 2) (Pelka, 1973; Stone and others, 1987; of the upper unit (Figs. 3 and 5). In the south- these faults, varying thicknesses of the lower Stone and Pelka, 1989). The occurrence of west part of the range (Fig. 8), only 10 m of unit are preserved beneath the base of the clasts of Middle Jurassic granitoids in the up- muscovite-quartz schist and epidote-quartz conglomerate member. One premetamorphic per unit in the Livingston Hills is consistent gneiss and schist separate metatuffs of the fault cuts part of the upper unit as shown by with this age assignment. The upper unit is no Dome Rock sequence from the base of the the thickening of strata at the base of the up- younger than 70 ± 4 Ma, on the basis of the upper unit. These metasedimentary rocks are per unit as it crosses a fault that now forms minimum age of the post-depositional Mule likely equivalent to basal sandstone member the axial surface of a north-facing asymmetric Mountains thrust system (Tosdal, 1990). 1 (base of the lower unit), although they could anticline (Fig. 8). Overlying conglomerate Uranium-lead geochronologic data on zir- alternatively be equivalent to metasedimen- beds are not folded and do not thicken across con from a metamorphosed tuff in the upper tary rocks at the top of the Dome Rock se- the fold hinge. Deposition of the conglomer- part of the conglomerate member of the Mc- quence that underlie basal sandstone mem- ate member at the base of the upper unit thus Coy Mountains Formation in the Dome Rock ber 1 farther northeast (unit Jds in Fig. 4). If overlapped and continued after an episode of Mountains refine the age assignment (Fig. 3). the latter stratigraphic correlation is correct, faulting that deformed the lower unit of the The tuff is interbedded conformably in a sec- then the lower unit would be absent here, and McCoy Mountains Formation and the basal tion of sandstone, siltstone, and subordinate the base of the upper unit would directly strata of the conglomerate member. Kinemat- pebble conglomerate that is flanked by cob- overlie the Dome Rock sequence. Regardless ics of these faults are not known. ble conglomerate (Fig. 5). Evidence that the of the proper stratigraphic correlation of the rock is a tuff and not an intrusive sill includes rocks intervening between the subjacent AGE OF THE McCOY MOUNTAINS fragmental blastophenocrysts of embayed metatuffs and the upper unit, most, if not all, FORMATION quartz, sanidine, and plagioclase (Fig. 9A); of the lower unit is absent locally in this part evidence for reworking and rounding of blas- of the range. Furthermore, clasts of tuff and Lower Unit tophenocrysts at the top and lenticular tips of quartz arenite derived from the Dome Rock the tuff bed (Fig. 9B); and normal grading of sequence and basal sandstone member 1, re- The basal part of the lower unit of the blastophenocrysts (Tosdal, 1988). The rock spectively, in addition to typical granitic McCoy Mountains Formation is generally as- has a rhyolitic composition according to im- clasts and K-feldspar detritus, are present in signed a Jurassic age based on the interpre- mobile element plots (Fig. 10; Table 1). conglomerates at the base of the upper unit. tation of a locally gradational contact be- Zircons separated from the tuff are of two These relations define an unconformity at the tween basal sandstone member 1 and silicic types. Orange-tinted, euhedral zircons, which base of the upper unit across which between volcanic rocks of the subjacent Dome Rock compose 99% of the population, are inter- 1 and 2.4 km of rocks may be missing in the sequence. The volcanic rocks are known or preted to be igneous. Rounded, lavender- southwest Dome Rock Mountains. inferred to be of Middle Jurassic age (Harding tinted zircons constitute the rest and are inter- Also in the southwest part of the Dome and Coney, 1985; Busby-Spera and others, preted to be xenocrysts. The latter zircons Rock Mountains (Fig. 8), premetamorphic 1990; Adams and others, 1991; R. M. Tosdal, appear identical to xenocrystic zircons found faults, here southward dipping, cut basal unpubl. data 1988-1992). Indirect support in pyroclastic rocks in the region, such as the sandstone members 1 and 2, but only locally stems from the correlation of the basal sand- tuffs of the Dome Rock sequence that underlie cut the basal part of the upper unit. Across stone member 1 with the Winterhaven For- the McCoy Mountains Formation in the

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Interbedded sandstone and phyllite Sandstone blocks Interbedded sandstone _ A' and • conglomerate

0 100 200 Conglomerate ' \ Basal sandstone meters member ? member 2

EXPLANATION

Qal I Alluvium (Quaternary) McCoy Mountains Formation of Harding and Coney (1985) (Cretaceous and Jurassic) Upper unit— KJmc; Conglomerate member Lower unit— LKJm Basal sandstone member 2 Jnyi Basal sandstone member 1 Contact Fault—Dotted where concealed , Strike and dip of bedding 50 U° Strike and dip of overturned bedding Strike and dip of foliation ^N,, Direction of stratigraphie top I Line of cross section

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EXPLANATION

Qal Alluvium (Quaternary) Dome Rock sequence (Middle Jurassic)

Te Conglomerate and sandstone (Tertiary) Jdt Tuff and sedimentary rocks 7-TT- Massive quartz porphyry tuff KJd Diorite (Cretaceous or Jurassic) Jdm* Contact McCoy Mountains Formation of Harding and Coney (1985) (Cretaceous and Jurassic) Intramember contact

Upper unit— Fault—Dotted where concealed

KJms Sandstone member Premetamorphic fault Conglomerate member— II u Low-angle normal fault .T- - Granite-rich upper interval Strike and dip of lithologic layering KJmcg, Micaceous lower interval Strike and dip of schistosity, showing bearing and RjrocVn 28 plunge of intersection lineation Lower unit— h H A A Line of section KJm2 Basal sandstone member 2 Kjmiif Basal sandstone member 1

Figure 8. Geologic map and schematic cross sections of southwestern outcrops of the McCoy Mountains Formation of Harding and Coney (1985) in the Dome Rock Mountains, southwestern Arizona. See Figure 3 for location.

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Figure 9. Photomicrographs of (A) metamorphosed rhyolitic tuff and (B) slightly reworked metamorphosed tuff at easternmost tip of dated horizon. A, albitized sanidine; P, plagioclase; Q, quartz; and S, sanidine. Scale bar equals 1 mm.

Dome Rock Mountains. Most xenocrystic zir- conventional methods. Exclusion of the one others, 1988). The presence of xenocrystic cons were removed by handpicking before conventional data point that suffers from zircons in the tuff is consistent with this analysis. poor analytical precision in the measured observation. Five discordant zircon fractions from the 2H6Pb/204Pb ratio changes the lower intercept metamorphosed tuff define a chord on a U-Pb age by about 0.2 m.y. From these data, the TECTONIC AND PALEOGEOGRAPHIC concordia diagram (MSWD = 1.6; Ludwig, tuff is interpreted to be of mid-Late Creta- INTERPRETATIONS 1983), with a lower-intercept age of 79 ± 2 Ma ceous age. No direct significance is attached and an upper-intercept age of 1,630 ± 230 Ma to the upper-intercept age of 1,630 ± 230 Ma An angular unconformity has been shown (Table 2; Fig. 11). One zircon fraction defin- because of the limited spread in the analytical to divide the McCoy Mountains Formation ing the chord was air abraded, using the tech- data and their distribution at the lower end of nique of Krogh (1982), and another was the discordia array. The early Proterozoic leached, following a procedure modified from age, however, does agree generally with Krogh and Davis (1975) by Busby-Spera and known basement-rock ages in southern Cal- TABLE 1. MAJOR-, MINOR-, AND TRACE-ELEMENT GEOCHEMISTRY OF METAMORPHOSED RHYOLITE others (1990). Both analytical techniques are ifornia and west-central Arizona (Silver, TUFF IN THE CONGLOMERATE MEMBER OF THE McCOY MOUNTAINS FORMATION, DOME ROCK used to detect subsequent Pb loss from zir- 1971; Wooden and Miller, 1990; Chamberlain MOUNTAINS, SOUTHWESTERN ARIZONA con, an event that can be excluded in this and Bowring, 1990) and with ages of inherited case because the isotopic data for these zir- or xenocrystic zircons in Mesozoic rocks in Sample TUK25 TM 893 con fractions plot on the discordia array de- the Mojave Desert region and Blythe-Quartz- site area (Calzia and others, 1986; Foster and Si02 76.0 72.4 fined by the zircons that were analyzed by Ti02 0.16 0.19 A1203 12.9 14.1 Fe203 0.81 1.12 FeO 0.11 0.11 MgO 0.26 0.30 80 r CaO 1.11 2.21 Na,0 3.99 1.66 K Ô 3.02 6.43 • Rhyolite 2 P205 <0.05 <0.05 MnO <0.04 0.04 + H20 0.40 0.57 Comendite and H,0" <0.01 0.04 Rhyodacite pantellerite CÒ, 0.06 0.07 and dacite Total 98.82 99.27 Figure 10. Trachyte Ba 560 1,250 6 0 _ 7 — Ti02 diagram for U-Pb zircon- Ce 88 100 La 48 50 Andesite / -*^rTrachyTrachy-- s' dated metatuff. Fields for volcan- Nb 25 32 andesite I- / ^ andesite ic rocks are after Winchester and Rb 105 230 Sr 240 240 Phonolite Floyd (1977). Y 20 30 Zr 120 150 Suba ka ine basalt Basinite, trachybasimte, Note: major-element and volatile analyses in weight percent nephehnite are by X-ray spectroscopy. Minor- and trace-element analyses in Alkali basalt parts per million (ppm) are by energy dispersive X-ray spectroscopy. Analytical techniques are described in Baedecker (1987): analysts are A. Bartel, T. Fries, and J. Taggart, U.S. Geo- 0.01 0.1 1.00 Zr/TiO, logical Survey.

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TABLE 2. U-Pb GEOCHRONOLOGIC DATA FOR METAMORPHOSED RHYOLITE TUFF (TUK 25) IN THE SOUTHERN DOME ROCK MOUNTAINS, SOUTHWESTERN ARIZONA

Mineral Fraction* Weight 206pbt 238u Observed ratios5 Atomic ratios** Ages (error) Ma++ (mg) (ppm) (ppm) OTpb 21W, 207 207p,,. 206p , 207 2«a 208pb Pb< b 207BJ, Pb* 2«pb »Pb 206pb 23ÈQ 235Ü 206Pb* 235Ü 206pb,

Zircon MC 17.2 8.3 673 249 0.1133 0.4027 0.014154 0.10620 0.054420 90.6 ± 0.5 102.5 ± 2.0 388 ± 22 N(>102) 6.4 7.0 586 192 0.1298 0.4381 0.013711 0.10093 0.053389 87.8 ± 0.5 97.6 ± 2.2 345 ± 26 MFR 11.0 10.0 853 1054 0.0672 0.2960 0.013590 0.09977 0.053245 87.0 ± 0.5 96.6 ± 1.8 339 ± 21 MFA 5.8 9.3 801 267 0.1067 0.4265 0.013337 0.09513 0.051732 85.4 ± 0.5 92.3 ± 1.3 274 ± 16 MF 8.3 9.3 820 250 0.1096 0.4314 0.013125 0.09182 0.050736 84.1 ± 0.5 89.2 ± 5.7 229 ± 80

*N = nonmagnetic and M = magnetic at 1.1 amps and 0° side slope on a Franz Isodynamic separator; 102 denotes grain size in microns; C = coarse-grained split; F = fine-grained split; A = abraded for 30 minutes following procedure outlined by Krogh (1982); R = residue from hydrothermal leaching experiment of zircon where zircons were leached in warm concentrate HF for two weeks and the residue was processed according to standard dissolution techniques. +Radiogenic Pb. Sample dissolution and ion exchange chemistry modified from Krogh and Mattinson (1987). Zircon Pb blank averaged 0.2 ng 206Pb. 5Observed ratios collected on FarTady cups were corrected for 0.125% per unit mass fractionation, based on replicate analyses of NBS 981. Uncertainties in the measured 208Pb/206Pb and ^Pb/^Pb ratios are less than 0.1%, the uncertainty in the measured 206Pb/2O4Pb ratios are between 0.5% and 10%. Isotopic data measured on Finigan-Mat MAT 261 mass spectrometer at the U.S. Geological Survey in Menlo Park. "Atomic ratios calculated using the following constants: 238U/S5U = 137.88; H5U = 0.98485 x 10"9 yr-1; 238U = 0.155125 x 10"® yr"1. Observed ratios corrected for common Pb ratios 208:207:206:204 Pb of 39.2:15.67:18.8:1 determined in feldspar. Uncertainties in the 206Pb*/238U ages are generally better than 0.5%, and in 2n7Pb*/23sU ages are better than 1.0%, although for two fractions, poor measuring statistics on the 206Pb/204Pb ratio make the uncertainty no better than 3.5%. All errors are reported to 2

cated in the Granite Wash Mountains. The ever, that the angularity and stratigraphic ef- upper unit is not deformed by thrust faults of fect of the unconformity diminish from east the belt, although faults of uncertain kinemat- to west into California within the present out- ics cut the base of the upper unit in the Dome crop belt of the formation. It is possible that Rock Mountain, and syndepositional thrust- a lithologic break within the basal part of the ing may have occurred in the southern Plo- conglomerate member in the McCoy Moun- mosa Mountains to the immediate north of tains (Reynolds and others, 1989) represents the Livingston Hills (Fig. 1). High sedimen- a hiatus equivalent to the intraformational un- tation rates for the upper unit are implied by conformity in the ranges in Arizona. Addi- its syntectonic deposition in alluvial-fan and tional work is needed to investigate this fluvial environments and the locally coarse possibility. texture of the conglomerate member (Miall, We propose that the unconformity within Figure 11. Part of U-Pb concordia diagram, 1978; Harding, 1982). The formation, in- the McCoy Mountains Formation in Arizona showing U-Pb isotopic data for metamor- cluding both lower and upper units, was formed in response to south-, southwest-, phosed tuff (TUK 25) in conglomerate member deformed and metamorphosed before and southeast-directed shortening during of the McCoy Mountains Formation of Hard- about 70 Ma, when it was tectonically bur- early Late Cretaceous deformation in the ing and Coney (1985), Dome Rock Mountains, ied beneath the northeastward-verging Maria fold and thrust belt. Rocks as young as southwestern Arizona. Ellipses represent error Mule Mountains thrust system (Tosdal, the lower unit of the formation in Arizona in the analytical data at 2

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outcrop belt in California from the edge of shortening as the Maria fold and thrust belt ACKNOWLEDGMENTS those parts of Maria fold and thrust belt that encroached into its foreland deposit. The were deformed in Late Cretaceous time entire formation was then tectonically bur- Part of this work was included in the Ph.D. (Fig. 1). ied beneath the northeast-vergent Mule thesis of R. M. Tosdal at the University of We interpret the upper unit of the McCoy Mountains thrust system in the latest Cre- California, Santa Barbara. Support for this Mountains Formation to be a foreland-basin taceous (Tosdal, 1990). work was provided by various programs of deposit shed from an uplifting and, possibly, The upper unit of the McCoy Mountains the U.S. Geological Survey, including the evolving Maria fold and thrust belt. The fact Formation occupies a similar stratigraphic Pacific-Arizona Crustal Experiment (PACE) that sources for clasts in the conglomerate position in western Arizona and southeastern project. Additional support was provided by member in the lowermost part of the upper California as the Fort Crittenden Formation the National Science Foundation Grant unit are present in the Maria fold and thrust does in south-central and southeast Arizona EAR81-15730 awarded to J. C. Crowell. This belt and its upper plate supports this model. (Dickinson and others, 1989). There, the Fort work has benefitted from discussions with In addition, the 1-m-diameter granite boul- Crittenden Formation was deposited uncon- many colleagues. Kelly Oliver prepared the ders and the 1,200-m thickness of conglom- formably over folded Upper Jurassic and zircon and feldspar separates for U-Pb erate that forms the lowermost member of Lower Cretaceous Bisbee Group in the geochronologic analysis. Reviews and com- the upper unit in the Livingston Hills indicate Upper Cretaceous (about 75 to 85 Ma). ments by L. E. Harding, S. M. Richard, S. J. that the source region was nearby, an inter- Deposition filled basins that formed during Reynolds, and J. H. Stewart helped to im- pretation supported by the proximity of folds early stages of shortening deformation. prove the manuscript. and thrust faults to the immediate north in the Continued shortening involved rocks of southern Plomosa Mountains. The upward the Fort Crittenden Formation. The strik- REFERENCES CITED coarsening in the conglomerate member in ing parallels between these rocks and the Adams, B.N.F., Busby-Spera, C. J., Stone, Paul, and Mattinson, the Livingston Hills and Dome Rock Moun- J. M., 1991, New stratigraphic & chronologic controls in the upper unit of the McCoy Mountains For- Mojave-Sonora Desert: Evidence from the Palen Mts. (south- tains furthermore implies progressive uplift mation raises the possibility that a similar eastern Mojave): Geological Society of America Abstracts with Programs, v. 23, no. 5, p. A250. and exhumation of a northern source terrane, tectonic setting characterized the Late Baedecker, P. A., ed., 1987, Methods for geochemical analysis: an observation that is also consistent with a U.S. Geological Survey Bulletin 1770, 150 p. Cretaceous across southern Arizona and Busby-Spera, C. J., Mattinson, J. M., Riggs, N. R., and Schermer, rising highland created by a southward-ad- southeastern California. E. R., 1990, The Triassic-Jurassic magmatic arc in the Mo- jave-Sonoran Deserts and the Sierra-Klamath region; simi- vancing fold and thrust belt. The eastward larities and differences in paleogeographic evolution, in Har- wood, D. S., and Miller, M. M., eds., Paleozoic and early thickening of the conglomerate member in CONCLUSIONS Mesozoic paleogeographic relations; Sierra Nevada, Klam- the Dome Rock Mountains and its westward ath Mountains, and related terranes: Geological Society of America Special Paper 255, p. 325-337. thickening in the Livingston Hills also sug- Calzia, J. P., DeWitt, Ed., and Nakata, J. K., 1986, U-Th-Pb age The McCoy Mountains Formation of and initial strontium isotopic ratios of the Coxcomb grano- gest that a depositional trough or basin of un- southwestern Arizona and southeastern Cal- diorite, and a K-Ar date of olivine basalt from the Cox- known orientation developed during deposi- comb Mountains, southeastern California: Isochron/West, ifornia consists of two stratigraphic units de- no. 47, p. 3-8. tion of the conglomerate member in the area posited in different tectonic settings. In Ari- Chamberlain, K. R., and Bowring, S. A., 1990, Proterozoic that now lies between the Dome Rock Moun- geochronologic and isotopic boundary in NW Arizona: Jour- zona, the contact between these two units is nal of Geology, v. 98, p. 399-416. tains and Livingston Hills. Finally, the dom- Davis, G. M., 1985, Geology of the southern Plomosa Mountains sharp and marked by an angular unconform- [M.S. thesis]: Tempe, Arizona, Arizona State University, inance of sandstone and siltstone in the upper ity. In California, no ofrvious unconformity is 159 p. two members of the upper unit suggests that DeWitt, Ed., and Reynolds, S. J., 1990, Late Cretaceous plutonism present. The lower unit was apparently de- and cooling in the Maria fold and thrust belt, west-central tectonism waned in the Maria fold and thrust Arizona: Geological Society of America Abstracts with Pro- posited as part of a widespread, southward- grams, v. 22, no. 3, p. 18. belt during their deposition in the Late Cre- Dickinson, W. R., Fiorillo, A. R., Hall, D. L., Monreal, Rogelio, flowing fluvial system established during the Potochnik, A. R., and Swift, P. N„ 1989, Cretaceous strata of taceous (post-79 Ma). waning stages of Jurassic volcanic activity. southern Arizona, in Reynolds, S. J., and Wilt, J. C., eds., Geologic evolution of Arizona: Arizona Geological Society Penetrative cleavages, folds, and growth Fluvial deposition may have continued into Digest, v. 17, p. 463-483. Drewes, Harald, 1991, Description and development of the Cordil- of greenschist- to amphibolite-facies met- the Cretaceous. In contrast, the upper unit is leran orogenic belt in the southwestern United States and a Late Cretaceous syntectonic deposit that northern Mexico: U.S. Geological Survey Professional Pa- amorphic mineral assemblages in the en- per 1512, 92 p. tire McCoy Mountains Formation along was shed from an uplifted terrane, beginning Foster, D. A., Harrison, T. M., and Miller, C. F., 1989, Age, in- heritance, and uplift history of the Old Woman-Piute batho- the length of its outcrop belt indicate re- before, and continuing after, 79 ± 2 Ma. The lith, California, and implications for K-feldspar age spectra: newed Late Cretaceous tectonism in the available geochronology implies that the up- Journal of Geology, v. 97, p. 232-243. Hamilton, Warren, Tosdal, R. M„ Stone, Paul, and Haxel, G. B., Blythe-Quartzsite area. Several genera- per unit was deposited after the lower unit 1987, Mesozoic tectonics of southeastern California, in Davis, G. H., and VandenDolder, E. M., eds., Geologic diversity of tions of crosscutting cleavage form a struc- began to be deformed and intruded by gra- Arizona and its margins: Excursions to choice area: Arizona nitic rocks in Arizona. The unconformity in Bureau of Geology and Mineral Technology Geological Sur- tural arch that cuts the homoclinal south- vey Branch Special Paper 5, p. 337-350. ward-dipping rocks (Harding and Coney, the McCoy Mountains Formation in Arizona Harding, L. E., 1980, Petrology and tectonic setting of the Living- ston Hills Formation, Yuma County, Arizona: Arizona Ge- 1985; Tosdal, 1986; Stone and others, is proposed to result from deformation in the ological Society Digest, v. 12, p. 135-145. Harding, L. E., 1982, Tectonic setting of the McCoy Mountains 1987). The cleavages are the dominant Maria fold and thrust belt where it intersects Formation, southeastern California and southwestern Ari- the present outcrop belt of the formation. In zona [Ph.D. thesis]: Tucson, Arizona, University of Arizona, structural fabrics that affect the entire for- 197 p. mation in its outcrop belt. Thrust faults are this model, deposition of the upper unit of the Harding, L. E., and Coney, P. J., 1985, The geology of the McCoy not associated with this episode of defor- McCoy Mountains Formation throughout the Mountains Formation, southeastern California and southwestern Arizona: Geological Society of America Bulletin, mation. In the structural culmination in the Blythe-Quartzsite area resulted from uplift in v. 96, p. 755-769. Harland, W. B., Armstrong, R. L., Craig, L. E., Smith, A. G„ and Dome Rock Mountains, north-dipping fab- the Maria fold and thrust belt. Continued or Smith, D. G., 1990, A geologic time scale, 1989: Cambridge, renewed metamorphism and deformation af- England, Cambridge University Press. rics cut south-dipping fabrics (Tosdal, Haxel, G. B., Tosdal, R. M., and Dillon, J. T., 1985, Tectonic setting 1986, 1990). The younger, north-dipping fected rocks of the upper unit soon after their and lithology of the Winterhaven Formation, a new Mesozoic stratigraphic unit in southeasternmost California and south- fabrics are related to continued southward protoliths were deposited. western Arizona: U.S. Geological Survey Bulletin 1599,19 p.

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Knapp, J. H., and Heizler, M. T., 1990, Thermal history of crys- Laubach, S. E., 1988, Geologic setting of Mesozoic and Ce- izona and adjacent southeastern California: Journal of Geo- talline nappes of the Maria fold and thrust belt, west-central nozoic metamorphism in Arizona, in Ernst, W. G., ed., Met- physical Research, v. 95, no. Bl, p. 539-555. Arizona: Journal of Geophysical Research, v. 95, no. B12, amorphism and crustal evolution of the western United States Spencer, J. E., and Reynolds, S. J., 1991, Tectonics of mid-Tertiaiy p. 20049-20073. (W. G. Rubey Volume 7): Englewood Cliffs, New Jersey, extension along a transect through west-central Arizona: Tec- Krogh, T. E., 1973, A low contamination method for the hydro- Prentice-Hall, p. 466-501. tonics, v. 10, p. 1204-1221. thermal decomposition of zircon and extraction of U and Pb Reynolds, S. J., Spencer, J. E., Laubach, S. E., Cunningham, Dick- Stone, Paul, and Pelka, G. J., 1989, Geologic map of the Palen- for isotopic age determinations: Geochimica et Cosmochim- son, and Richard, S. M., 1989, Geologic map, geologic evo- McCoy Wilderness Study Area, Riverside County, Califor- ica Acta, v. 37, p. 485-494. lution, and mineral deposits of the Granite Wash Mountains, nia: U.S. Geological Survey Miscellaneous Field Studies Krogh, T. E., 1982, Improved accuracy of U-Pb zircon ages by the west-central Arizona: Arizona Geological Survey Open-File Map MF-2092, scale 1:62,500. creation of more concordant systems using an air abrasion Report 89-4, scale 1:24,000, 46 p. Stone, Paul, Page, V. M., Hamilton, Warren, and Howard, K. A., technique: Geochimica et Cosmochimica Acta, v. 46, Reynolds, S. J., Spencer, J. E., Laubach, S. E., Cunningham, Dick- 1987, Cretaceous age of the McCoy Mountains Formation, p. 637-649. son, and Richard, S. M., 1991, Geologic map and sections of southeastern California and southwestern Arizona: Geology, Krogh, T. E., and Davis, G. 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MANUSCRIPT RECEIVED BY THE SOCIETY JULY 28,1992 Geological Society Digest, v. 16, p. 35-51. Spencer, J. E., and Reynolds, S. J., 1990, Relationship between REVISED MANUSCRIPT RECEIVED APRIL 29,1993 Reynolds, S. J., Richard, S. M., Haxel, G. B., Tosdal, R. M., and Mesozoic and Cenozoic tectonic features in west-central Ar- MANUSCRIPT ACCEPTED MAY 5, J993

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