The geology of the McCoy Mountains Formation, southeastern California and southwestern Arizona LUCY E. HARDING* Conoco Inc., 3500 General DeGaulle Drive, New Orleans, Louisiana 70114 PETER J. CONEY Laboratory of Geotectonics, Department of Geosciences, University of Arizona, Tucson, Arizona 85721 ABSTRACT (south side) onto McCoy Basin rocks. Struc- mountain range, 240 oriented cores taken for tural analysis and paleomagnetic data strong- paleomagnetic analysis, and structural data col- The McCoy Mountains Formation is a 7.3- ly suggest that the thrusting over and folding lected along measured section lines and basin km-thick metasedimentary sequence exposed of the McCoy Basin rocks took place in Late margins. The detailed results of the paleomag- in at least 6 mountain ranges in southeastern Jurassic time as a single event. The fault netic study are presented elsewhere (Harding California and southwestern Arizona. The sil- along the southwestern margin of the McCoy and others, 1983). iciclastic McCoy Mountains Formation is Basin is thus apparently a Jurassic terrane deposited on, and interbedded at its base boundary and may be a northern extension of Geographic Extent of the with, a Jurassic volcanic terrane and is in- the Jurassic Mojave-Sonora megashear. If McCoy Mountains Formation truded by undeformed Upper Cretaceous(?) true, proposed early Tertiary accretion of the plutons. It has been assigned a Cretaceous- Tujunga terrane must have occurred farther The McCoy Mountains Formation is here Paleocene age on the basis of fossil angio- to the southwest. redefined to include (1) the McCoy Mountains sperm wood found within the upper third of Formation of Miller (1944), Greene (1968), the sequence. Presently known exposures of INTRODUCTION and Pelka (1973), as exposed in the McCoy, McCoy Mountains Formation define a west- Coxcomb, and Palen Mountains, California; northwest-trending basin 140 km long by 25 Named forty years ago (Miller, 1944), the (2) equivalent strata in the Dome Rock km wide which was filled from the north by McCoy Mountains Formation remained until Mountains, Arizona (informally referred to as predominantly alluvial processes. The McCoy recently an enigmatic Mesozoic sedimentary ter- the Livingston Hills Formation by Crowl, 1979, Mountains Formation is divided into six rane exposed in at least six mountain ranges in and Marshak, 1979); (3) the continental red bed members, each containing distinctive stratig- southeastern California and southwestern Ari- deposits of Miller (1966, 1970), exposed in the raphy and sedimentary petrology. Sandstone zona. These Mesozoic, clastic, sedimentary rocks area near Crystal Hill in the New Water Moun- consists of quartz, feldspar, and lithics, and are monotonous in lithology, exceedingly thick, tains, Arizona (also called Mesozoic "Red Beds" each member has a different composition. and are very different from the classic Mesozoic by Robison, 1979, 1980); and (4) some of the The source evolved from volcanic-sedimen- sequence exposed on the Colorado Plateau, only exposures of Miller's (1966, 1970) Livingston tary to plutonic-sedimentary composition 250 km to the northeast. They lie at the very Hills Formation (including all exposures in the with time. Much thinner, laterally equivalent southwestern edge of cratonic North America Livingston Hills, Arizona). As redefined, the clastic sequences are now known on the along a major discontinuity or terrane boundary McCoy Mountains Formation is thus exposed in craton adjacent to the basin's northern separating autochthonous North America from the Coxcomb, Palen, and McCoy Mountains, margin. suspect terranes (Coney and others, 1980) to the California, and in the Dome Rock, southwestern The McCoy Mountains Formation is ho- southwest. The role of the McCoy Mountains Plomosa, and New Water Mountains and in moclinal and south-dipping, and its upper- Formation in the tectonic evolution of south- Livingston Hills, Arizona (Fig. 2). Possibly re- most strata are deformed into a north-verg- western North America has not been under- lated rocks are exposed in the Granite Wash and ing, overturned syncline with sympathetic stood. Little Harquahala Mountains and in the Black south-dipping cleavage. Northern exposures The results of a multidisciplinary attack on Rock Hills, Arizona, -100 km northeast of the of McCoy Mountains Formation and under- this major tectonic problem, using methods from study area. Strata which may be thinner, lateral lying Jurassic volcanic terrane contain a stratigraphy, sedimentary petrology, regional equivalents of the McCoy Mountains Formation north-dipping cleavage and south-verging, tectonic analysis, structural analysis, and paleo- (Harding, 1978,1980,1982) are best exposed in overturned folds. Sympathetic to the cleav- magnetism are presented here. Effort was con- the Apache Wash region of the southern Plo- ages and folding are north- and south- centrated in the McCoy, Palen, and Coxcomb mosa Mountains. These rocks were named the bounding faults which limit McCoy Moun- Mountains in California, and in the Dome Rock Livingston Hills Formation by Miller (1966, tains Formation exposures and place the Mountains, southern Plomosa Mountains, and 1970). North American craton (north side) and the Livingston Hills in Arizona (Fig. 1). Data pre- The Mesozoic McCoy Mountains Formation Mojave-Sonora Complex or Tujunga terrane sented here are the result of >23 km of strati- consists of distinctive clastic sequences at least graphic section measured by Jacob Staff, >500 7.3 km thick. The type section is exposed in the samples collected at maximum intervals of McCoy Mountains (Miller, 1944; Pelka, 1973). *Present address: Department of Geology, Mid- 50-75 m for petrographic analysis, paleocurrent The presently known exposures of McCoy dlebury College, Middlebury, Vermont 05753 direction indicators measured throughout each Mountains Formation occupy a broad west- Geological Society of America Bulletin, v. 96, p. 755-769, 11 figs., 2 tables, June 1985. 755 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/6/755/3445017/i0016-7606-96-6-755.pdf by guest on 25 September 2021 1 Tv Tertiary volcanic rocks I Mm J Mesozoic McCoy Mountains Fm. Mil Ki Late Cretaceous (?) plutonic rocks J Ma j Mesozoic rocks of Apache Wash IMI | Mi 1 Mesozoic plutonic rocks f~ Mu j Mesozoici?) metasedimentary rocks | Mgn [ Augen gneiss, age unknown Jv I Jurassic volcanic rocks Figure 1. Tectonic map of the McCoy Basin, California and Arizona. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/6/755/3445017/i0016-7606-96-6-755.pdf by guest on 25 September 2021 California // Arizona Basin bounding faults Jp Jurassic Palen Fm. main, secondary Other faults : Pz : I Paleozoic rocks, undivided Orientation of foliation and as- sociated lineation-, bedding, up- PßPy Precambrian basement right and overturned Folds, upright and overturned Precambrian, Paleozoic and Mesozoic -U--U- Depositional contact, stipple on rocks of North America, undivided younger rocks §MSf| Mojave - Sonora composite terrane 5 10 Figure 1. (Continued). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/6/755/3445017/i0016-7606-96-6-755.pdf by guest on 25 September 2021 758 HARDING AND CONEY Figure 2. Summary of pre- The McCoy Mountains Formation is divided vious nomencla ture applied into six members, each with different lithology to the McCoy Mountains and sandstone petrology (Fig. 4). The lower Formation. References: (1) McCoy three members, basal sandstone members 1 and Mtns. Green (1968); (2) Pelka 2 and the mudstone member, are equivalent to (1973); (3) Miller (1944); (4) L 7000 Pelka's (1973) units 1-4. The upper three Marshak (1979), Crowl members, the conglomerate, sandstone, a id silt- (1979); (5) Miller (1966, stone members, are equivalent to Pelka's (1973) ' \Dome Rock ¡~ - 6000 1970). \ Mtns. Livingston ÏÏ units 5-14 and to Miller's (1970) conglomerate, ÜÜÄ graywacke, and siltstone members of the Living- Palen / - 5000 ston Hills Formation. The similarity in expo- Mtns. / sures of the coarse clastic McCoy Mountains LIVINGSTON Formation over a strike distance of 140 km is 4000 McCOY HILLS remarkable. Figure 5 shows the correlator! of the six members at the different exposures. MOUNTAINS FORMATION -3000 Thickness and contacts of the measured sections from the McCoy Mountains Formation are Coxcomby FORMATION Mtns.y Missing ¡3 summarized in Table 1. 2000 New H In order to confirm correlation between expo- Water < Mtns. E sures and to provide information abcut the -1000 source of the McCoy Mountains Formation, CoijSinental vRed point counts (400 points minimum per thin sec- Bed CALIF. ARIZ. )eposits tion) were made on the best-preserved sand- West East stones distributed laterally and vertically within the formation. The point-count data are sum- marized in Table 2. Grain parameters ars those northwest-trending band 140 km long by 25 north-south-trending mountain ranges. Because of Dickinson (1970). Figure 6 is a stratigraphic km wide (Harding and others, 1982). Geology of this pattern of exposure, east-west facies presentation of the point-count data frorr Hard- and deformation to the south and west of the changes are well exposed; north-south changes ing (1978, 1982). McCoy Mountains Formation exposures are are not. Jurassic volcanic rocks rest below, and The McCoy Mountains Formation hits been described by Powell (1981a, 1981b), Tosdal are interbedded with, the base of the McCoy subjected to low-grade