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.
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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).
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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 metamorphism, resulting (1982), and Haxel and Dillon (1978). Mountains Formation (Figs. 3 and 4). The stra- in the presence of sericite, calcite, albite, chlorite, tigraphie top of the McCoy Mountains Forma- epidote, sphene, biotite, and specular hematite as STRATIGRAPHY AND tion is nowhere exposed. In various places, the secondary minerals. Commonly, these second- SEDIMENTARY PETROLOGY present top of the section is truncated by a re- verse fault, ends in the core of an overturned
The McCoy Mountains Formation is south- synclinal fold, is covered by alluvium, or is in- 7 km dipping and homoclinally exposed in a series of truded by a pluton. SILTSTONE MEMBER
km JURASSIC McCOY MOUNTAINS
VOLCANIC ROCKS FORMATION km SANDSTONE MEMBER
km
CONGLOMERATE MEMBER km
km MUDSTONE MEMBER
BASAL SANDSTONE: km MEMBER 2
SANDSTONI: MEMBER 1 JURASSIC VOLCANIC ROCKS
Figure 3. Basal contact of the McCoy Mountains Formation on the Jurassic volcanic ter- Figure 4. Generalized stratigraphie column rane, northern McCoy Mountains. Contact dips -40° to the south (right). View is to the east of the McCoy Mountains Formation, type from the Palen Mountains. Maximum relief is -700 metres. section, McCoy Mountains, California.
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ary minerals account for 20% or more of the sequence of Jurassic volcanic rocks (Fig. 3) (dis- 1973; Miller, 1966). Relief along the basal con- volume of the rock (interstitial material in Table cussed below in the section on Regional Geol- tact is <3 m. Nowhere is angular unconformity 2). Thin sections with the least amount of inter- ogy and Structure). Horizons 1-3 m thick of exposed between the McCoy Mountains Forma- stitial material were chosen for point counting, rhyodacite porphyry with magnetite-bearing tion and the Jurassic volcanic terrane. The but to achieve a distribution of data from the cross-strata and quartzite clasts are commonly rhyodacite porphyry intrudes other Jurassic vol- entire sedimentary terrane, some thin sections interbedded with the basal 10-20 m of McCoy canic rocks but has not been found intruding the with >20% interstitial material had to be Mountains Formation in the Dome Rock McCoy Mountains Formation. The rhyodacite counted. The interstitial material most likely Mountains. Near the basal contact with the porphyry is believed to be an intrusive-extrusive formed from feldspar and from quartz-poor overlying McCoy Mountains Formation, the volcanic rock deposited concurrently with the lithic grains (Harding, 1982). rhyodacite porphyry is locally internally struc- basal sediments of the McCoy Mountains tureless but more commonly shows evidence of Formation. Basal Contact a volcaniclastic origin or reworking in the form of incorporated quartzite clasts, cross-strata, and Basal Sandstone Members 1 and 2 The McCoy Mountains Formation locally stratification in the McCoy and Palen Moun- rests both disconformably and nonconformably tains. Rocks of volcaniclastic origin are common The relative thicknesses of basal sandstone on rhyodacite porphyry, part of a heterogenous throughout the Jurassic volcanic terrane (Pelka, members 1 and 2 vary from range to range, but together the two members comprise the lowest 1,600 m of the McCoy Mountains Formation.
TABLE 1. THICKNESS AND CONTACTS OF MEASURED SECTIONS FROM THE McCOY MOUNTAINS FORMATION Basal sandstone member 1 thins to the west and is absent in the Coxcomb Mountains, the west- Location Thickness* Base* Top* Reference ernmost known exposure of the McCoy Moun- McCoy Mountains Formation, less distinctive in the eastern exposures of the McCoy Mountains 7,340 depositional reverse fault Harding (1982) McCoy Mountains Formation, and rocks sim- McCoy Mountains Formation, ilar to it were not described in the New Water Palen Mountains 4,539 depositional covered Harding (1982) Mountains by Robison (1980). Section is McCoy Mountains Formation, 2,000 Coxcomb Mountains (estimated) depositional intrusion Harding (1982) faulted out above basal sandstone member 1 in the New Water Mountains, and so it is uncertain McCoy Mountains Formation, Dome Rock Mountains 5,510 depositional syncline core Harding (1982) whether basal sandstone member 2 was depos- McCoy Mountains Formation, ited there. Livingston Hills 2,880 fault syncline core Harding (1982) Basal sandstone member 1 is between 0 and McCoy Mountains Formation, 1,000 1,000 m thick and consists of a sequence of New Water Mountains (estimated) covered covered Robison (1980) interbedded conglomerates, sandstones, silt- •At location where section was measured. stones, and mudstones. Near its base, the se- quence contains interstratified rhyodacite por- phyry horizons. The quartzofeldspathic min- eralogy of the rhyodacite porphyry is only slightly mixed with the interstratified, basal, quartz-rich sandstones. The conglomerate, sandstone, siltstone, and ? FOLD mudstone of basal sandstone member 1 are ar- ranged in fining-upward cycles 5-20 m thick, 5000m - SILTSTONE MEMBER ALLUVIUM similar to those described by Robison (1980) from Miller's (1970) continental red bed depos- « SANDSTONE MEMBER ?- its. The base of each cycle is marked by an K o o° .o: erosional surface. The top unit of most cycles is CONGLOMERATE £<7 a o t calcareous. Cut-and-fill structures and laminated 3000m - J&ßi MEMBER 3 •hZp; layers of pebbly gravels with much interstratified fts .-?? •OgkÈ FAULT silt and sand are common. Clasts are found con- MUDSTONE MEMBER centrated in wedge-shaped beds (channel lag MISSING deposits?) or floating in sand matrix. Festoon BASAL SANDSTONE FAULT ^ MEMBER 2 cross-bedding is common in the sandstone (thus BASAL SANDSTONE the large amount of paleocurrent data obtained öftrer MEMBER 1 from basal sandstone member 1) and may re- V i. M " -"1 T > present point bar deposits. Fresh exposures of > JURASSIC VOLCANICS •> V "S- > Vi. , i- ± 7 V 1/ A V sandstone and conglomerate matrix are light tan COXCOMB PALEN McCOY DOME ROCK LIVINGSTON MTNS. MTNS. MTNS. MTNS. HILLS to pink-tan where iron oxide and calcareous NEW WATER cement are present. Where the cement is mostly MTNS. silica, the rocks are light to medium gray. The Figure 5. Stratigraphie correlation of the six members of the McCoy Mountains Formation. sandstone and conglomerate are interbedded 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 760 HARDING AND CONEY with purple-gray ;ind gray-green siltstone and A change in sandstone and cement composi- Palen Mountains include quartzite, sandstone, mudstone. The siltstone and mudstone exhibit a tion marks the contact between basal sandstone marble, schist, silicic volcanic, and granitic rocks penetrative north-dipping (45°) cleavage that members 1 and 2. The contact is gradational, for of pebble and cobble size. The mixed-clas': lith- obscures their primary internal structures. Cores quartz-rich sandstones occur as much as 200 rn ologies support a geologically heterogenous for the paleomagnetic study of Harding and oth- above this contact. Siltstone and mudstone 1:;- source area. ers (1983) were taxen from the purple-gray silt- thologies remain unchanged. Massive gray sand- stone and mudstone lithology. stone of basal sandstone member 2 contains Mudstone Member Sandstone in basal sandstone member 1 is feldspar (34%), quartz (34%), and lithic grains quartz-rich (87%), contains a small lithic com- (32%) (Table 2; Fig. 6). Lithic grains are both The mudstone member (900 m thick) con- ponent (7%), and a small amount of unzoned volcanic and sedimentary, and all feldspar is sists of thick intervals of gray siltstone and mud- plagioclase feldspar (6%) (Table 2; Fig. 6). The plagioclase, commonly zoned, and euhedral. stone interbedded at regular intervals with thin sandstone contains; no K-feldspar and almost no The section is resistant to weathering and forms horizons of sandstone and conglomerate. The volcanic lithic fragments or zoned plagioclase. a high ridge crest with steep slopes. The compo- siltstone and mudstone are commonly phyllitic The chief source is interpreted as having been sitional change between basal sandstone mem- and locally consist of pure sericite. Internal crystalline basement, which provided the mono- bers 1 and 2 does not affect the fining-upward structure in siltstone and mudstone consists of a crystalline quartz. Cratonic sediments provided cycles of conglomerate, sandstone, siltstone, and penetrative north-dipping, closely spaced cleav- the sedimentary li:hic grains and quartzite. The mudstone, although the proportion of conglom- age. Sandstone and conglomerate horizons have plagioclase feldspar came either from the base- erate decreases upsection. The presence of these an erosional base and occur every 20-100 m in ment or from the Jurassic volcanic terrane. cycles is one of the few indications that the rocks the section in groups of 2 or 3 beds separated by Conglomerate from basal sandstone member 1 are still fluvial deposits. The upper 50 m of basal siltstone or mudstone. Thickness of the sand- contains quartzite, sandstone, and marble clasts, sandstone member 2 consists of coarsening- stone and conglomerate beds ranges between 50 suggesting a component of Paleozoic cratonic upward cycles of distinctive green-gray sand- cm and 10 m. Sandstone beds commonly have a sedimentary rocks within the source region. stone, green conglomerate, purple and gray- layer of pebbly conglomerate 10-50 cm thick at Point-count data collected by Robison (1980) green siltstone, and gray, highly fractured the base. The sandstone also contains pebble from the continental red bed deposits of Miller mudstone. Some of the gray-green siltstone has stringers and small-scale, fluvial cross-beds de- (1970) have a composition similar to data col- an internal structure similar to flow banding of fined by magnetite-rich sand. The sandstone lected here from basal sandstone member 1. rhyolite. The coarsening-upward sequences may beds are lenticular and pinch out laterally over a This similarity further substantiates stratigraphic represent lacustrine fan deltas. distance of Vi km or less. Conglomerate clasts correlation between the continental red bed Sandstone compositions suggest that the include quartzite, silicic volcanic, and pliyllitic deposits and basal sandstone member 1 of the source was mainly volcanic. Subordinate expo- mudstone rocks and are commonly flattened McCoy Mountains Formation. Paleocurrent sures of sedimentary rocks and basement pro- and rotated into the north-dipping fcliation data from basal sandstone member 1 show cur- vided the quartzite and quartz sericite compo- plane. Green chlorite blebs occur in the foliation rents flowing to the south (Fig. 7). The sequence nent as well as some of the quartz. Conglom- plane as well. Primary sedimentary structures is probably of fluvial origin (Robison, 1980). erate clast lithologies from this member in the are very rarely preserved in this part of the sec- TABLE 2. POINT-COUNT DATA FROM THE McCOY MOUNTAINS FORMATION, AVERAGED BY MEMBER Mountain range, member P,B1 P.B2 P.MS P,C M,B1 M,B2 M,MS M,C M.SA M.SLS D.B1 D,B2 D,MS D,C D,SA L,C L,SA L.SLS Percentage of total rock interetital material Percentage of total framework mica Percentage of QFL Q 92 19 43 63 81 41 51 53 63 36 87 43 39 56 39 34 35 33 F 4 51 20 29 8 24 14 46 34 40 6 28 27 29 56 55 54 47 L 4 30 37 8 11 35 35 1 3 24 7 29 34 15 5 11 11 20 Percentage of L, 4 38 41 9 13 41 44 3 6 28 9 30 39 23 5 15 13 22 1. Volcanic-felsitic 0 24 31 10 0 11 24 0 2 32 1 68 34 10 0 25 8 48 2. Volcanic-microlithic 0 30 7 2 0 16 11 0 0 13 0 21 8 7 39 23 68 26 3. Volcanic-lathwork 0 0 0 0 0 0 0 0 0 0 0 0 0 9 56 0 1 3 4. Volcanic-hypabyssal 0 14 14 9 0 0 0 0 2 10 0 0 1 6 0 0 0 0 5. Clastic-quartz sericite 100 10 39 56 86 59 43 38 61 30 79 11 46 59 0 23 9 9 6. Quartzite 0 21 9 16 14 13 21 55 22 15 20 0 11 9 5 15 7 9 7. Tectonite-metasedime:itary 0 0 0 6 0 0 0 5 13 1 0 0 0 0 0 0 0 0 8. Indeterminate 0 0 0 1 0 1 1 2 0 0 0 0 0 0 0 14 7 5 Percentage of (Lt-8) Qp (6) 0 21 9 16 14 13 21 56 22 15 20 0 11 9 5 17 8 9 Ls (5 + 7) 100 10 39 63 86 59 43 44 74 31 79 11 46 59 0 27 10 9 Lv (1 + 2 + 3 + 4) 0 68 52 21 0 27 35 0 4 55 1 89 43 32 95 56 82 82 Plagioclase/total feldspar 1 1 1 0.87 1 1 1 0.42 0.84 0.88 1 1 1 0.78 0.95 0.78 0.7.1 Number of samples 2 5 8 10 6 4 4 8 8 8 7 1 6 9 3 4 6 1 Note: P= Palen Mountains, M = McCoy Mountains, D = Dome Rock Mountains, L = Livingston Hills, B1 = Basal Sandstone Member 1, B2 = Basal Sandstone Member 2, MS = Mudstone Member, C = Conglomerate Member, SA = Sandstone Member, SLS - Siltstone Member. Grain parameters are from Dickinson (1970). Unaveraged data are found in Harding (1978, 1982). 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 GEOLOGY OF McCOY MOUNTAINS FORMATION, ARIZONA 761 tion. Bedding has been transposed on a local intervals and showed southeast-flowing currents. trices of the two conglomerates are petrograph- scale within the mudstone member. The mud- The fluvial cross-strata and the fluvial nature of ically indistinguishable. stone member forms distinctive low, rounded members above and below the mudstone The brown conglomerate consists of inter- hills and a low ridge crest. member indicate deposition in a fluvial envi- bedded sandstone and conglomerate with only The mudstone member sandstone contains ronment. The fine-grained nature of the mud- minor amounts of siltstone. The brown color is quartz (33%), plagioclase (20%), and lithics stone member may suggest a lacustrine envi- derived from the presence of limonite after py- (35%) (Table 2; Fig. 6). Lithics are both volcanic ronment as well. rite disseminated in the matrix and concentrated and sedimentary in origin. These data suggest a along bedding planes and fractures. Clasts in the complex source area containing a volcanic ter- Conglomerate Member brown conglomerate consist of quartzite, sand- rane diluted with cratonic sedimentary rocks stone, and, rarely, siliceous volcanic rocks. and some underlying basement. The volcanic The conglomerate member (1,600-2,000 m The blue-gray or gray conglomerate (1,200- terrane supplied the plagioclase and volcanic thick) begins abruptly, with light brown inter- 1,600 m thick) also consists predominantly of rock fragments; the cratonic cover and basement bedded conglomerate and sandstone deposited sandstone and conglomerate with minor source provided most of the quartz and all of the on gray phyllitic siltstone and sandstone of the amounts of siltstone and is correlative with quartzite and sedimentary lithic grains. The rare mudstone member. The brown conglomerate Miller's (1970) conglomerate member in the paleocurrent indicators found in the mudstone and sandstone (100-450 m thick) grade up- Livingston Hills. Interbedded with the blue-gray member (Fig. 7) came from small-scale (10 cm wards into blue-gray or gray interbedded con- sandstone and conglomerate are horizons, 10 cm thick), fluvial-type cross-strata within the sandy glomerate and sandstone. The sandstone ma- to 1 m thick, of sandstone with a brown or purple matrix, occurring every 100 m or so in the section. The brown and purple sandstones McCOY MTNS. are commonly mixed with the blue-gray sand- stone to form intermediate colors. The sand in 50 100% these intervals consists mainly of zoned, eu- LIVINGSTON HILLS (HARDING, 1978) O 50 100 % 4000 111 V)< m 2 o 3000 te u. (0 oc UJ H 2000 tu 1000 Figure 6. Stratigraphie chart of Q-F-L and Qm-F-Lt point-count data, McCoy Mountains Formation. Grain parameters are from Dickinson (1970). B1 = Basal Sandstone Member 1, B2 = Basal Sandstone Member 2, MS = Mudstone Member, CGL = Conglomerate Member, SA = Sandstone Member, SLS = Siltstone Member. 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 762 HARDING AND CONEY 2; Fig. 6) is petrologically distinctive due to the present. Bedding is marked by grain-size appearance of K-feldspar in the section for the changes. This part of the section is heavily first time and to a low percentage of lithic grains. veined with limonite, quartz, and calcite It is K-feldspar is present only as either microcline or not resistant to weathering, typically forming perthite, suggestive of a deep crustal (basement) steep rubble-covered slopes, and it is expos>;d for source. Detrital mica is common throughout this only a short distance along ridge crests. A pre- member. Major petrologic and lithologic change viously undiscovered fossil-wood log was iound occurs between the conglomerate and mudstone within the sandstone member in the McCoy members, suggesting a change in tectonic setting Mountains (Harding, 1982). Pelka (1973) col- midway through deposition of the McCoy lected fossil wood from the conglomerate and Mountains Formation. Rugged relief in the siltstone members in the McCoy Mountains. source area probably produced the coarse clas- Sandstone from the sandstone member has a tics of the conglomerate member and exposed composition of quartz, 46%; feldspar, 48%; deep crustal rocks containing microcline. Source lithics, 6% (Table 2; Fig. 6). K-feldspar is present relief then decreased during deposition of the in all samples. These data suggest continued remaining McCoy Mountains Formation mem- erosion of basement in the source area, together bers, although microcline continued to be sup- with a widening of the source, to include vol- plied to the basin. In contrast, major relief and canic rocks and sedimentary cover. Sandstones deep crustal rocks are not required by the with zoned, euhedral plagioclase are present in lithology and petrology of the members below the sandstone member of the Dome Rock the conglomerate member. Mountains and may have been air-fall tuffs, in- Clast types in the conglomerate member vary dicating active volcanism nearby. The sandstone from range to range, reflecting a heterogenous member is poorly exposed, highly fractured, and 21 DATA source area. Percentages of volcanic and granitic cleaved; it yielded no paleocurrent data. The en- POINTS vironment of deposition for the sandstone 49 DATA Siltstone clasts change independently of one another, sug- POINTS Member gesting that the two lithologies are not closely member is thought to be alluvial fans reworked Basal Sandstone Member 1 associated in the source area. The abundance of by fluvial processes. Evidence for this inteipreta- granitic clasts closely parallels that of the sedi- tion includes the presence of fossil wood, the Conglomerate interbedded sandstones, siltstones and pebble Member mentary and metamorphic clasts, suggesting that these three rock types were spatially associated conglomerates, and the inferred depositional en- 0 in the source area. vironments of the underlying conglomerate member and overlying siltstone member. DATA POINTS Paleocurrents obtained from the conglomer- ate member come from the sandstone interbeds Figure 7. Paleocurrent data, McCoy and show south-flowing currents (Fig. 7). The Siltstone Member Mountains Formation, showing directions of coarse-grained nature of the conglomerate current flow. Data are segregated both by member, the lenticular bedding, and the similar- The siltstone member (500-1,400 m thick) is mountain range (A.) and by member (B.). ity of clast lithology and sandstone matrix sug- lithologically diverse. It is named for the rippled gest that it represents a sediment flood in the siliceous siltstone and fine-grained sandstone form of a series of alluvial fans reworked by found near its base. The lower 500 m of the hedral plagioclase crystals mixed with varying braided streams. siltstone member is correlative with Miller's amounts of quart:, plutonic feldspar, and lithic (1970) siltstone member in the Livingston Hills. grains typical of the conglomerate member Sandstone Member The upper 900 m of the siltstone member is sandstones. They therefore may be tuffaceous. exposed only in the McCoy Mountains (Fig. 5). The intervals are more laterally persistent The conglomerate member grades lithologi- Gray sandstones and siltstones of the siltstone (0.5-1.5 km) than non-volcanic, feldspar-con- cally upward into the sandstone member member are interbedded, interlaminated, and taining beds, and this, along with their contrast- (1,000-1,500 m thick), but sandstone composi- highly fractured. The siltstones are siliceous and ing color and mineralogy, suggests a volcanic tions remain similar throughout both members. display well-preserved asymmetric, transverse, origin. The sandstone member consists of siltstone, climbing ripples. Above the fine-grainec. sand- The conglomerate member exhibits lenticular sandstone, calcareous mudstone, and conglom- stone and siliceous siltstone in the McCoy bedding which can be followed, at most, 0.5 km erate. Sandstone, conglomerate, and siltstone are Mountains is a 20-m-thick, cross-bedded sand- along strike. Sandstone beds are poorly sorted medium gray when fresh and are commonly stone which contains silicified fossil-wood logs with respect to grain size, are 2-3 m thick, and stained with hematite. The calcareous mud- described by Pelka (1973). The cross-strata are pinch out laterally. Locally, magnetite-rich sand stones are light gray on fresh surfaces, weather- defined by layers of magnetite sand which has layers define small-scale fluvial cross-beds. Con- ing to light orange-brown. The calcareous been altered to limonite. The logs are 0.5-1 m in glomerate cuts channels into underlying sand- mudstone occurs both as discrete horizons and diameter and 0.5-3 m in length. The section stone and fines upward into sandstone as well. as elongate pods oriented within the foliation above the fossil-wood-bearing sandstone con- The proportion of sandstone relative to con- plane. Conglomerate clasts are highly variable in sists of sandstone with subordinate conglomer- glomerate increases upsection. composition. They consist mainly of siliceous ate and siltstone. Conglomerate contains silicic Sandstone from the conglomerate member volcanic rocks, but clasts of quartzite, sandstone, volcanic, granite, quartzite, carbonate, and (quartz, 51%; feldspar, 40%; lithics, 9%) (Table marble, and phyllitic mudstone rocks are also biotite-hornfels clasts all flattened into the south- 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 GEOLOGY OF McCOY MOUNTAINS FORMATION, ARIZONA 763 dipping foliation plane. Sericite is present in one leocurrents flowed from north to south (Fig. 7). Kaibab(?) Limestone, with the porphyritic interval in such large amounts that the rock ap- For the upper three members, rugged relief and volcanic clearly intrusive into the Paleozoic pears white from a distance. Secondary biotite is exposure of deep crustal rocks are suggested by rocks (Miller, 1970). The bases of the McCoy abundant in all the sandstones. Conglomerate the coarseness of the lithologies and presence of Mountains Formation and the thinner Mesozoic channels cut into sandstone beds; otherwise, sed- microcline. Paleocurrents again show southerly sequence thus may correlate, because both are imentary structures are very hard to find. The flow. All lithologies represented by McCoy interbedded with porphyritic volcanic rock. The section is penetratively foliated by a south- Mountains Formation sandstones (quartz-rich petrology of basal sandstones interstratified with dipping mineral cleavage. cratonic sedimentary rocks, Jurassic volcanic the volcanics differs in the two sequences (Hard- Sandstone from the siltstone member contains rocks, deep-seated crustal rocks, and shallow- ing, 1982). approximately equal amounts of quartz (35%), level plutons) are exposed along the northern The petrology and lithologic succession of the feldspar (43%), and lithic grains (22%) (Table 2; margin of the formation's exposures. This ob- thinner Mesozoic sequence (conglomerate to Fig. 6). Both K-feldspar and zoned plagioclase servation, in addition to south-directed paleo- sandstone to siltstone to sandstone) is similar to are present. The siltstone member appears to currents, strongly suggests a northern source for the upper three members of the McCoy Moun- have had a geologically complex source, con- the McCoy Mountains Formation: off the North tains Formation (conglomerate, sandstone, and taining volcanic rocks, basement, and cratonic American craton. siltstone members) (Harding, 1982; Miller, cover. 1966). The individual lithologies are easily dis- Paleocurrent data from the rippled siliceous STRATA PROXIMAL TO THE tinguishable in the field, however (Harding, siltstones and fossil-wood-bearing sandstone in- McCOY MOUNTAINS FORMATION 1978). Within the numerous thrust sheets dicate south-southwest-flowing currents (Fig. mapped by Miller (1970) along the western es- 7). A deltaic or lacustrine environment is sug- Exposed in the Apache Wash and Ramsey carpment of the southern Plomosa Mountains, gested for the lower siltstone member on the Mine areas of the southern Plomosa Mountains the two Mesozoic sequences are beautifully in- basis of fresh-water brachiopods collected from (see Miller, 1966, for map and locations) and in terleaved but never occur together within the the siltstone member in the Livingston Hills the Little Harquahala Mountains (Richard, same thrust sheet. Differences in sandstone pe- (Harding, 1978, 1980), interlaminated silt and 1982) is a second Mesozoic sedimentary se- trology between the two sequences include the sand, graded bedding, and climbing ripples. quence which likely correlates with the McCoy presence of chert in the thinner Mesozoic se- Fossil-wood logs deposited in trough cross-strata Mountains Formation (Harding, 1978, 1982). quence (not yet found in the McCoy Mountains present the clearest evidence of fluvial origin This Mesozoic sedimentary sequence is much Formation) and the presence of K-feldspar as (point bar deposits?) of the upper part in the thinner than the McCoy Mountains Formation orthoclase in the thinner Mesozoic sequence siltstone member. and was named the Livingston Hills Formation (McCoy Mountains Formation sandstone con- by Miller (1966, 1970). This nomenclature is tains K-feldspar only as perthite or microcline). Top of the Section confusing for two reasons. (1) The Livingston The thinner Mesozoic sequence thus may corre- Hills Formation, named for exposures in the late to the upper three members of the McCoy Exposures of the stratigraphic top of the Livingston Hills by F. K. Miller (1966), is iden- Mountains Formation. Nondeposition or depo- McCoy Mountains Formation have not been tical to the upper half of the McCoy Mountains sition and subsequent erosion of rocks equiva- found. The uppermost strata of the generally Formation of W. J. Miller (1944) and should be lent to the lower three members of the McCoy homoclinal, south-dipping McCoy Mountains named as such. (2) The thinner Mesozoic se- Mountains Formation in the thinner Mesozoic Formation are deformed into a north-verging, quence, correlated by F. K. Miller to the Living- sequence is possible but does not explain the dif- overturned syncline. The core of the syncline ston Hills Formation, may correlate to either the ferent petrologies of rocks interbedded with the exposes the top of the measured sections. The lower or upper part of the McCoy Mountains porphyritic volcanic rocks. Age control on the inverted limb of the syncline is cut by a south- Formation. It was informally named the Apache volcanic rocks might help this correlation prob- dipping reverse fault which places overturned Wash Formation by Harding (1982) because it lem. Rocks transitional to the two sequences McCoy Mountains Formation and Jurassic vol- is easily distinguished from the McCoy Moun- have not yet been found. Due to this and to the canic rocks on top of the section. In the McCoy tains Formation as a map unit; because there lack of age control, it is possible that the two and southwestern Dome Rock Mountains, the exist differences in petrology, current directions, sequences either are not the same age or else top of the section is truncated by a reverse fault geographic distribution, and thickness between were deposited in unrelated tectonic settings and with no overturned syncline exposed. the two sequences; and because of the problem thrust into their current proximity. of correlation between the sequences. Summary Both the thin Mesozoic sequence and the REGIONAL GEOLOGY McCoy Mountains Formation are interbedded AND STRUCTURE The 6-member McCoy Mountains Formation at their bases with a Jurassic porphyritic vol- has an internally consistent stratigraphy along canic rock (quartz porphyry of Miller, 1970; 140 km of east-west exposure. The source for rhyodacite porphyry of Pelka, 1973). The por- The McCoy Mountains Formation is exposed the lower three members contained quartz-rich phyry appears to have a similar composition in as a giant, south-dipping homocline, its upper- sediments, coeval volcanics, and possibly crys- its exposures near the base of the sedimentary most strata deformed by an overturned, north- talline rocks due to the high quartz content. sequences, but no detailed petrology or radio- verging syncline (Figs. 1 and 8). The northern Orthoclase is missing from the lower three metric age-dating has been done to confirm this part of the homocline in each range is deformed members' sandstones. Sedimentary structures correlation. Zircons have not been found in the by a north-dipping, penetrative mineral cleav- and the predominance of fine-grained sand- porphyritic volcanic rock, although they have age. This cleavage is present in all units which stones and siltstones suggest mainly fluvial with been searched for. The thin Mesozoic sequence are structurally below the McCoy Mountains local lacustrine environments of deposition. Pa- is exposed in depositional contact on Paleozoic Formation, from its depositional contact on 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 764 HARDING AND CONEY DOME ROCK MOUNTAINS commun.). The Mojave-Sonora terrarie in- cludes the Tujunga and Baldy terranes of Blake and others (1982) and the San Gi.briel- Joshua Tree terranes of Powell (198la, 1981b). Rocks to the north of the McCoy Basin are characteristic of the Paleozoic North American craton. The McCoy Basin thus lies at or on the edge of the North Amer- ican craton. The following discussion is based on structural data collected within and along the margins of the McCoy Basin and includes observations by other workers which could N McCOY MOUNTAINS not, until recently, be put into a regional framework. -5 km McCoy Basin Geology - 0 km The McCoy Mountains Formation is depos- ited everywhere on a thick, intrusive-extrusive - 5 km volcanic terrane. This contact is locally noncon- formable or disconformable, but nowhere is an- gular unconformity observed. Two age deter- minations have been reported from the under- lying volcanic terrane. A U-Pb, Late Triassic or Early Jurassic apparent age was assigned by L. T. Silver (cited in Miller, 1970) in the central part of the southern Dome Rock Mountains, and K-Ar determinations on plagioclase by D. Krummenacher yielded apparent ages of 66, 140, and 176 m.y. (cited in Pelka, 1973). The volcanic terrane is believed to be Early Jurassic in age and ranges from basaltic to rhyolitic in composition (Marshak, 1979, 1980). The Palen Formation of Pelka (1973) and LeVeque (1981, 1982) is exposed structurally below some of the Jurassic volcanic terrane and is intruded by it as well. Exposed in Palen Pass in the Palen Mountains, the Palen Formation consists of three members: lithofeldspe.thic ar- O 5 10 enite, polymictic conglomerate, and quartzose KMS arenite. The total apparent thickness is -1,200 m; however, the base is not exposed, the top is intruded by Jurassic porphyry, and the section is Figure 8. Schematic structure sections drawn through the Dome Rock, McCoy, and Palen intensely folded (Pelka, 1973; LeVeque, 1982). Mountains, McCoy Basin. See Figure 1 for legend. The quartz monzonite of Middle: Camp Mountain intrudes the Jurassic volcanic terrane Jurassic volcanic rocks northward to the graphic in that the basin was a deep locus of in the northern Dome Rock Mountains (Wil- north-bounding thrust fault of Figure 1. The deposition. son, 1960; Crowl, 1979). The pluton is de- southern exposures of McCoy Mountains In addition to McCoy Mountains Forma- formed by a north-dipping foliation sympathetic Formation are deformed by a south-dipping tion, the structural McCoy Basin contains to the north-bounding thrust fault (Figs. 1 and 8) mineral cleavage, sympathetic to the over- rocks of the Jurassic volcanic terrane, the and to cleavage in other rock units of the north- turned syncline, and present all the way to the Palen Formation, meta-igneous and metased- ern McCoy Basin. Assuming that all of the south-bounding fault (Mule Mountains Thrust) imentary rocks of the northern Dome Rock cleavage resulted from the same deformational of Figure 1. All of the rocks between these Mountains, and the quartz monzonite of event, the age of this deformation should be a two bounding faults are deformed by north- Middle Camp Mountain. The unfoliated Cox- minimum age for emplacement of the quartz or south-dipping cleavage and are here referred comb pluton intrudes the foliated McCoy Basin monzonite pluton. to as rocks of the McCoy Basin. The term rocks. The foliated McCoy Mountains Formation is "McCoy Basin" is structural in that it refers The rocks to the south of the McCoy Basin intruded by the undeformed Coxcomb Pluton in to the "pod" o f McCoy Mountains Formation are here grouped into the so-called Mojave- the southern Coxcomb Mountains (Greene, deformed between bounding thrusts and strati- Sonora terrane (L. T. Silver, 1982, personal 1968). The Coxcomb Pluton has yielded appar- 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 GEOLOGY OF McCOY MOUNTAINS FORMATION, ARIZONA 765 ent ages of 71-65 m.y. (K-Ar, biotite, Arm- age deforms the rocks. The boundary between strong and Suppe, 1973; Calzia, 1982) and a the cleavage domains is characterized by an ~ 1- preliminary apparent age of 145 m.y. (Rb-Sr km-wide transition zone, wherein the north- whole rock, Calzia, 1982). dipping cleavage fans into the south-dipping Nearly flat-lying middle Tertiary volcanic cleavage by a gradual change in structural atti- rocks rest with angular unconformity on the tude. This cleavage transition zone contains McCoy Mountains Formation in the southeast- rocks with locally transposed bedding. The ern Livingston Hills (Miller, 1966) and are ex- boundaries of the transition zone are not sharp posed locally in the northern Palen Mountains and do not follow a stratigraphic horizon. The (Pelka, 1973). With the exception of these two transition zone occurs within the mudstone small areas, Tertiary volcanic rocks are absent member in the Dome Rock Mountains and from the McCoy Basin of Figure 1. Tertiary within the sandstone member in the McCoy low-angle normal faults such as described by Mountains. Throughout the change in cleavage Martin and others (1982) are absent from the orientation, bedding remains south- or south- McCoy Basin but may be present in association east-dipping (Fig. 1). with the north-bounding thrust in the central Accompanying the cleavage is a lineation Plomosa Mountains of Miller (1970) and in the which plunges moderately to the northwest in Palen Pass area of Pelka (1973) and LeVeque the northern cleavage domain and moderately to (1982). the southeast in the southern cleavage domain Basement to the McCoy Basin rocks is no- II. (Fig. 10). The lineation is defined by mineral where exposed. A North American cratonic streaking in sandstone and siltstone, by long axes basement is suggested by roof pendants of a Pa- of stretched pebbles in conglomerate, and by leozoic protolith in the quartz monzonite of fold hinges of folds to which the cleavage is axial Middle Camp Mountain. Quartzite and fossilif- planar. The orientation change occurs within the erous marble clasts in conglomerates of the cleavage transition zone: the lineation rolls from McCoy Mountains Formation, together with northwest to southeast as cleavage changes from south-directed paleocurrents, suggest a proximal north- to south-dipping. cratonic source along the northern McCoy Basin Folds to which the mineral cleavage is axial margin. The proposed correlation (Harding, planar occur in both northern and southern 1982) between the thick McCoy Mountains cleavage domains. These folds are best exposed Formation of the McCoy Basin and much near the north- and south-bounding faults thinner Mesozoic rocks exposed in Apache Figure 9.1. Poles to cleavage orientations, (Fig. 1). Folds verge to the north in the southern Wash in the southern Plomosa Mountains (de- McCoy Mountains Formation. A. Palen cleavage domain; in the northern cleavage do- posited on Paleozoic Kaibab(?) Formation of Mountains, B. McCoy Mountains, C. Dome main, the folds verge to the south. Examples of the North American craton) suggests an addi- Rock Mountains. II. Poles to bedding orien- the north-verging folds are well displayed in the tional link between the McCoy Mountains tations, McCoy Mountains Formation. southern Livingston Hills, southern Dome Rock Formation and North America. These relations A. Palen Mountains, B. McCoy Mountains, Mountains, southern Palen Mountains, and indicate that the McCoy Mountains Formation C. Dome Rock Mountains. southern Coxcomb Mountains. At each of these was very likely deposited on, or adjacent to, the localities, the McCoy Mountains Formation (as North American craton. well as underlying Jurassic volcanic rocks in the Cleavage orientations in the McCoy Basin Coxcomb Mountains) is folded into a north- can be divided into north-dipping and south- verging, overturned syncline. South-verging dipping cleavage domains (Fig. 9). The folds within the northern cleavage domain are northern domain begins at the north-bounding well exposed in the northern Palen Mountains fault in each range and goes south to midway (LeVeque, 1982) and in the northern Dome within the homoclinal McCoy Mountains Rock Mountains. Fabric data collected Formation. The southern domain extends from throughout the McCoy Basin and bounding the south-bounding fault in each range north to upper plates imply that the cleavage, tilting, and the midsection of the McCoy Mountains folding of the McCoy Basin rocks are sympa- Formation. thetic to the north- and south-bounding faults. The cleavage in both domains is predomi- In addition to the folds, cleavage, and line- nantly a mineral cleavage with a clear recrystal- ation described above, the McCoy Basin is de- lization fabric. Minerals responsible for the formed by later, nonpenetrative brittle folding foliation include sericite, chlorite, and, locally, and faulting that preserves the vergence asso- biotite. In quartz-rich sandstone, the cleavage is ciated with the older deformation. In the McCoy a fracture cleavage. In the Dome Rock Moun- Mountains, the McCoy Mountains Formation is tains where most carefully observed, the north- also deformed by large-scale open folds with and south-dipping cleavages were never seen to Figure 10. Trend and plunge of mineral lin- north-northwest-plunging axes (Pelka, 1973). cut one another. Thin-section examination sup- eations, McCoy Mountains Formation, Dome These folds fold the south-dipping cleavage. ports the field observation that only one cleav- Rock Mountains. High-angle faults trending northwest in the 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 766 HARDING AND CONEY Dome Rock and Mew Water Mountains, north- rocks were informally named the Apache Wash marked by several south-dipping layers of my- northwest in the Palen and McCoy Mountains Formation (Harding, 1982). They have a min- lonite and has been named the "Mule Mountains and Livingston Hills, and east-west in the Liv- imum thickness of 2 km, in contrast to the min- Thrust" (Fig. ll)(Crowell, 1981; Tosdal, 1982; ingston Hills deform the McCoy Basin (Fig. 1). imum 7.3-km thickness of the McCoy Moun- Vedder and others, 1982). Because of the kick of These faults are brittle in nature. Left or right tains Formation within the major basin, and age control on the meta-plutonic rocks, their re- separation with a maximum offset of 2 km is have different internal stratigraphy and petrol- lationship with the underlying McCoy Basin seen on the faults. The differing senses of separa- ogy as well, as discussed above. In the southern rocks cannot properly be called a thrust fa alt. tion of the faults suggest that the faults are nor- Plomosa Mountains, the thinner Mesozoic se- The meta-plutonic rocks may correlate with mal faults, although they have most commonly quence is deposited disconformably on the Pa- the crystalline rocks of the San Gabriel and been interpreted as strike-slip faults (Miller and leozoic Kaibab(?) Formation and is interbedded Joshua Tree terranes of Powell (1981a, 1581b). McKee, 1971; Crowl, 1979). at its base with an intrusive-extrusive quartz These terranes, together with the Orccopia Despite the cleavage, folding, and faulting af- porphyry (Miller, 1970), probably equivalent to Schist terrane and Vincent thrust of Haxel and fecting the McCo}' Basin, small-scale deforma- the rhyodacite porphyry in the upper part of the Dillon (1978), are contained in the Mojave- tion of rocks is no" severe. Throughout most of Jurassic volcanic terrane. The quartz porphyry Sonora Complex, a name suggested by L. T. the McCoy Mountains Formation, primary sed- intrudes the upper Paleozoic Kaibab(?) Forma- Silver (1982, personal commun.). These same imentary structures are still visible. Sandstone tion. These complex, but clearly exposed, rela- rocks are included in the Tujunga and Baldy commonly contains <20% interstitial material, tionships occur in a thrust plate which is terranes of Blake and others (1982). To date, allowing accurate determinations of detrital overlain by allochthonous crystalline rock and neither North American craton rocks nor the modes. underlain by the McCoy Mountains Formation McCoy Mountains Formation has been found and Jurassic volcanic rocks of the McCoy Basin. south of the south-bounding fault. The McCoy Geology of the North Margin The Livingston Hills Formation of F. K. Miller Basin separates two regions of different geologic of the McCoy Basin (1966, 1970) thus includes rocks of the pre- histories and rock types and therefore was de- viously named McCoy Mountains Formation of posited on, or adjacent to, a terrane boundary In direct contrast to the enigmatic nature of W. J. Miller (1944) and probably time- (Coney and others, 1980). Powell (1981a) sug- rocks within the McCoy Basin, mountain ranges equivalent Mesozoic clastics deposited directly gested that the extension of the south-bounding to the north contain familiar Precambrian, Pa- on the North American craton. These two se- fault to the west may have been obliterated by leozoic, and lower Mesozoic rocks of the North quences are now found thrust over one another. intrusion of Mesozoic plutons. American craton. Structural geometry suggests Flat-lying volcanic rocks isotopically dated at The south-bounding fault has a characteristic that the boundary is a north- to northeast- 19-20 m.y. (K-Ar, biotite and hornblende) geology, as described below (Figs. 1 and 81. The dipping, south-verging thrust fault, placing Pre- (Miller and McKee, 1971; Eberly and Stanley, uppermost strata of the south- and southeast- cambrian, Paleozoic, and lower Mesozoic rocks 1978) unconformably overlie the upper thrust dipping McCoy Mountains Formation are; typi- onto rocks of the McCoy Basin (Harding and plate within the southern Plomosa Mountains. cally exposed in a north-verging, overturned, others, 1982). The fault zone is complex (Miller, Low-angle normal faulting of middle Tertiary synclinal fold. This fold is overlain by a south- 1970; Harding, 1982; LeVeque, 1982), generally age (Martin and others, 1982; Hamilton, 1982, dipping fault which places overturned basal dips 20-50° northward, and may locally be for example) is probably present in the complex sandstone member 1 and/or conglomerate overprinted by later low-angle faulting in the upper plate structures of the northern Palen, member rocks onto the top of the McCoy southern Plomosa and northern Palen Moun- Maria, northern Dome Rock, and northern and Mountains section. It is noteworthy that these tains. This boundary between cratonic North southern Plomosa Mountains as well. two members have the most distinctive petro- America and McC oy Basin rocks cannot be fol- logic and lithologic characteristics of all mem- lowed far outside the study area because of poor Geology of the South Margin bers of the McCoy Mountains Formation. exposures. Along irend of the fault to the west of the McCoy Basin Volcanic rocks resembling the Jurassic volcanic are crystalline rocks of the Joshua Tree and San rocks (beneath the McCoy Mountains Forma- Gabriel terranes of Powell (1981a, 1981b), and The McCoy Basin rocks are separated from tion) are then found structurally above, and in- to the east are Ternary volcanic rocks (Wilson, meta-plutonic rocks to the south by a south- terleaved with, the overturned part of the 1960). The amount of horizontal movement dipping fault. Deformation related to the south- McCoy Mountains Formation with both deposi- along the fault zone is unknown but could be bounding fault system occurs in a belt at the tional and faulted contact. Rocks of the Mojave- very substantial (Harding, 1982; Reynolds and southern end of the Coxcomb, Palen, McCoy, Sonora Complex terrane lie structurally above others, 1980). Mesozoic(?) plutons are involved and Dome Rock Mountains and Livingston the Jurassic volcanic rocks, separated by several in faulting of the northern McCoy Basin bound- Hills, as well as in the northern end of the Mule zones of mylonitized rock. ary in all exposure!;. Mountains (Wilson, 1960; Pelka, 1973; Cro- At some time during the deposition of the well, 1981; Tosdal, 1982). A single cross section AGE CONSTRAINTS McCoy Mountains Formation, sediments oth- through the belt is nowhere exposed, but based erwise confined to the region underlain by thick on the map distance between northernmost and The foliated McCoy Mountains Formation is Jurassic volcanic rocks were deposited on Pa- southernmost structures related to the fault sys- interbedded at its base with Jurassic volcanic leozoic and underlying Precambrian rocks of the tem in the Mule and McCoy Mountains, respec- rocks and is intruded by the undeformed Juras- North American craton to the north of the tively, the belt has a structural thickness of sic(?) or Upper Cretaceous (Armstrong and McCoy Basin. These sections of Mesozoic clastic — 12 km. In the Mule Mountains, this fault is Suppe, 1973; Pelka, 1973; Calzia, 1982) Cox- 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 GEOLOGY OF McCOY MOUNTAINS FORMATION, ARIZONA 767 the McCoy Mountains Formation is thus brack- JURASSIC McCOY MOUNTAINS eted by the loosely dated Lower Jurassic vol- VOLCANIC ROCKS FORMATION canic sequence below it and the deformation of late Middle to middle Late Jurassic, accompan- ied by precipitation of specular hematite. In addition, consistency of directions of mag- netization from both north and south cleavage domains and the sympathetic relationship of these cleavages with the thrust faults which bound the McCoy Mountains Formation sug- gest that all of the deformation of the McCoy Mountains Formation occurred during a single event. In addition to the major deformation of Jurassic age, evidence of later, minor deforma- tion is common. The later deformation(s) af- fected the McCoy Basin and upper bounding plates but have resulted in only minor distur- bances of the earlier fabric. No evidence of Figure 11. The Mule Mountains Thrust, northern Mule Mountains, places meta-plutonic major faulting associated with later deformation rocks onto overturned Jurassic volcanic rocks and McCoy Mountains Formation. View is to of the McCoy Basin or bounding upper plates the southwest. The thrust dips -23° to the southwest. has been found. REGIONAL IMPLICATIONS comb Pluton in the southern Coxcomb Moun- Mountains Formation in the McCoy, Palen, and tains. Fossil wood, possibly angiosperm, has Dome Rock Mountains. Data and results are The conclusions of this work are that the been collected from several localities within the presented by Harding and others (1980, 1983) 7.3-km-thick McCoy Mountains Formation was upper third of the McCoy Mountains Formation and are briefly summarized here. deposited in a subsiding basin above sea level but is generally poorly preserved. This fossil The paleomagnetic data, constrained by geo- during Early to Middle Jurassic time. The basin wood has been used to assign a Late Cretaceous logic evidence, indicate that the McCoy Moun- formed upon and is partly coeval with a Lower or Paleocene age (Pelka, 1973; Miller, 1970; tains Formation was deposited, deformed, and Jurassic volcanic terrane. This basin was then Robison, 1980; Hayes, 1970) to the McCoy then magnetized. Microscopic examination of quickly deformed and overthrust from both the Mountains Formation even though Jurassic vol- samples from sites where a characteristic natural northeast and southwest sides. canic rocks are interbedded at its base. New col- remanent magnetization (NRM) was isolated The original size and shape of the McCoy lections of the wood, recently examined, have shows that specular hematite is the only abun- Basin is still obscure. The only demonstrable been reidentified as angiosperm (S. R. Ash, dant opaque mineral. Textural evidence further equivalents of the McCoy Mountains Formation 1981, written commun.). There is considerable indicates that the specular hematite was formed are much thinner sequences known nearby to disagreement, however, regarding the age range by postdepositional chemical precipitation, most the northeast which clearly lie depositionally on of angiosperm wood. The uncertainties include likely in response to a mild metamorphism. The lower Mesozoic and Paleozoic rocks of cratonic (1) disagreement as to the age of the oldest angi- resulting paleomagnetic pole (not corrected for North America (our present interpretation is osperm pollen, (2) discrepancies between the structural tilt) (\p = 57.7°N, 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 768 HARDING AND CONEY Canyon of northern Arizona and the ranges of words, a terrane boundary, what does it repre- and northwestern South America. The pre posed southeastern Arizona directly up to the edge of sent? We will address some of the available Mojave-Sonora megashear would cut obliquely the McCoy Basin. These rocks are never seen options. across this widespread magmatic terrane at a again southwest of the McCoy Basin. These last- The boundary might be the northwestern ex- very low angle. What is more, the fault system is known exposures of Paleozoic rocks are typical, tension of the Mojave-Sonora megashear of presumed to have been active during the mag- in facies and thickness, of interior cratonic North Silver and Anderson (1974), a proposed north- matic activity. For a considerable distance in America and not of the Cordilleran miogeocline. west-trending, left-slip transform fault system northwestern Mexico and southwestern United In other words, if we are at the extreme south- which may have linked spreading centers in the States, the proposed transtensional fault system western edge of cratonic North America, the Gulf of Mexico with plate margins along the would have been in an intravolcanic arc setting. Cordilleran miogeocline is missing. Evidence western edge of North America. The fault is This could then explain the occurrence of sim- that these same cratonic rocks underlie the presumed to have been active in Early to Middle ilar magmatic material on both sides of the pro- McCoy Basin coines from paleomagnetic data Jurassic time. Movement on the fault system is posed Mojave-Sonora megashear. Some of the suggesting that the McCoy Basin was deformed suggested to be at least 800 km. This offset thus material could, furthermore, have been erupted, adjacent to cratonic North America and from explains the occurrence, near the cities of Ca- deposited, or emplaced during final ptoises of xenoliths of Paleozoic(?) marble occurring in the borca and Hermosilla in Sonora, Mexico, of late movement on the fault system. presumed Jurassic quartz monzonite of Middle Precambrian and Paleozoic sediments with It has recently been proposed that the suspect Camp Mountain. It must be emphasized, never- thicknesses and lithologies similar to those found terranes (Coney and others, 1980) found today theless, that Paleozoic and older rocks are not in the Cordilleran miogeocline far to the southwest of the McCoy Basin are part of a known to crop out anywhere within the McCoy northwest of the McCoy Basin (Anderson and composite accreted terrane termed the "Tujunga Basin as defined here. Silver, 1979). We believe that if the concept of a terrane" (Blake and others, 1982; Vedder and We have not worked in the region southwest Mojave-Sonora megashear is accepted, the only others, 1982). It has been suggested, on the basis of the McCoy Basin. Much of the work that place to run it across southwestern Arizona and of paleomagnetic and geologic relations, that others have done there is still in thesis form adjacent southeastern California, where it pro- final accretion of this terrane (which included (Powell, 1981b; Haxel, 1977; Dillon, 1975, jects, is along the southwestern edge of the the Salinian block) may have been as recent as etc.), and much work is still in progress. The McCoy Basin. early Tertiary time. The northwestern-bounding region is reported to be characterized by Pre- If all of the above is accepted, the McCoy suture of the proposed Tertiary accretion in cambrian crystalline rocks, ages and lithologies Basin could be interpreted as being a left trans- southwestern Arizona and adjacent California is of which are nol. entirely correlative with ex- tensional rift basin along the transform fault. the southwestern boundary of the McCoy Basin. posed Precambrian rocks just northeast of the The size, shape, and subsidence history of the The paleomagnetic and structural data presented McCoy Basin. P.utonic rocks of Jurassic and basin are not inconsistent with such a model, at in this paper, if correct, imply, of course, that the Cretaceous age intrude the older rocks. There least as proposed by Aydin and Nur (1982). If, discontinuity is of Jurassic age. Therefore, the are scattered occu rrences of the Orocopia Schist indeed, the McCoy Basin did form as a rhombo- Tertiary accretionary boundary implied by (Ehlig, 1981; Haxel and Dillon, 1978), assumed chasm during movement on the megashear, it Vedder and others (1982) is either incorrect, to be of oceanic affinity, and equally scattered must have been quickly subjected to compres- cryptic, or occurs some place else. sedimentary roclcs (as yet undated), usually sive regional stress which presumably reacti- Finally, we should address the fact that a termed the Winlerhaven Formation, and vol- vated the bounding transtensional faults into number of workers familiar with the geology of canic rocks of presumed Jurassic age. The Oro- opposite verging thrust faults after movement on southwestern Arizona and adjacent California copia Schist is usually described as being the megashear ceased. In any event, the lack of have written that the McCoy Mountains Forma- structurally below all other rocks, separated any proven distal equivalents of the McCoy tion is of Late Jurassic to Cretaceous or even from them by the Vincent-Chocolate Moun- Mountains Formation southwest of the basin, as Paleocene in age (Crowell, 1981; Robison, tains Thrust. In our view, none of the sedimen- well as the lack of evidence (in our view) of a 1979, 1980; Hamilton, 1982). It is often men- tary rocks can to; directly correlated, unequiv- source for the McCoy Mountains Formation tioned informally that it might broadly correlate ocally, with anything either within or northeast southwest of the basin, would be consistent with with the Bisbee Group of southeastern Arizona of the McCoy Basin without presently unjusti- large-scale, left-lateral translation along the (Hayes, 1970). It is correct that, to date, the fied interpretation. The same is certainly the case southwestern margin of the basin. Any possible published geologic age brackets of the McCoy with the Orocopia Schist. The Jurassic volcanic source rocks or distal equivalents of rocks in the Mountains Formation are the Lower Jurassic and plutonic rocks occur both northeast, within, McCoy Basin would be as much as 800 km to volcanic rocks interbedded at the base and the and southwest of the McCoy Basin. There has the southeast, now buried beneath the Tertiary crosscutting pluton in the Coxcomb Mountains been much Tertiary low-angle listric normal lavas of the Sierra Madre Occidental in northern of probable Late Cretaceous age. Two argu- faulting southwest of the McCoy Basin (for ex- Mexico. Similarly, rocks presently adjacent to, ments, one geologic and the other paleomag- ample, Garner and others, 1982). Listric normal and southwest of, the McCoy Basin might have netic, have been presented here; they encourage faults also are known northeast of the McCoy been as much as 800 km to the northwest. As us to reject that correlation and the younger age. Basin (for example, Frost, 1981; Reynolds, for the similarity of Jurassic volcanic and plu- The transitional nature of the passage from 1982), but for some still unclear reason, this type tonic rocks northeast, southwest, and within the Lower Jurassic volcanic rocks into the McCoy of faulting has not been recognized within the McCoy Basin, it must be pointed out that rocks Mountains Formation suggests that they are part McCoy Basin itself. of this general age and aspect are known from of the same sequence. We find it somewhat ex- If the southwestern boundary of the McCoy southern California, southern Arizona, across cessive that >100 m.y. was needed to deposit Basin is such a profound discontinuity, in other Mexico, and even into parts of Central America 8 km of commonly coarse, conglomeratic fluvial 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 GEOLOGY OF McCOY MOUNTAINS FORMATION, ARIZONA 769 Crowell, J. C., 1981, An outline of the tectonic history of southeastern Califor- v. 12, p. 123-133. sediments. Many have maintained that dating nia, in Ernst, W. G., ed., The geotectonic development of California Martin, D. L., Krummenacher, K., and Frost, E. G., 1982, K-Ar geochrono- the metamorphism of a rock by a paleomagnetic (Rubey Volume 1): Englewood Cliffs, Prentice-Hall Inc., p. 583-600. logic record of Mesozoic and Tertiary tectonics of the Big Maria- Little Crowl, W. J., 1979, Geology of the central Dome Rock Mountains, Yuma Maria-Riverside Mountains terrane, in Frost, E. G., and Martin, D. L., pole, thus further constraining the age of the County, Arizona [M.S. thesis]: Tucson, Arizona, University of Arizona, eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River 76 p. region, California, Arizona, and Nevada (Anderson-Hamilton Volume): McCoy Mountains Formation protolith to Dickinson, W. R., 1970, Interpreting detrital modes of graywacke and arkose: San Diego, Cordilleran Publishers, p. 518-549. within the middle of the Jurassic, is an un- Journal of Sedimentary Petrology, v. 40, p. 695-707. Miller, F. K., 1966, Structure and petrology of the southern half of the Plomosa Dillon, J. T., 1975, Geology of the Chocolate and Cargo Muchacho Moun- Mountains, Yuma County, Arizona [Ph.D. thesis]: Stanford, California, orthodox approach. We do not agree, but until tains, southeastemmost California [Ph.D. thesis]: Santa Barbara, Cali- Stanford University, 107 p. fornia, University of California, 405 p. 1970, Geologic map of the Quartsite quadrangle, Yuma County, new data are developed and published, we will Doyle, J. A., 1977, Patterns of evolution in early angiosperms, in Hallam, A., Arizona: U.S. Geological Survey Quadrangle Map GQ-841, scale stand with the data presented here. ed., Patterns of evolution as illustrated by the fossil record: Amsterdam, 1:62,500. Elsevier, p. 501-546. Miller, F. K., and McKee, E. H., 1971, Thrust and strike-slip faulting in the We have believed from the outset that the 1978, Origin of angiosperms, in Johnston, R. F., Frank, P. W., and Plomosa Mountains, southwestern Arizona: Geological Society of Michener, C. D., eds., Annual Review of Ecology and Systematics, v. 9, America Bulletin, v. 82, p. 717-722. principal role of our work is to provide new data p. 365-392. Miller, W. J., 1944, Geology of the Palm Springs-Blythe strip, Riverside Eberiy, L. D., and Stanley, T. B., Jr., 1978, Cenozoic stratigraphy and geologic County, California: California Journal of Mines and Geology, Quar- on a very thick and long, enigmatic sequence of history of southwestern Arizona: Geological Society of America Bul- terly Chapter of the State Mineralogist's Report XL, v. 40, no. I, rocks exposed over -10,000 km2 in southwest- letin, v. 89, p. 921-940. p. 11-72. Ehlig, P., 1981, Origin and tectonic history of the San Gabriel Mountains, Pelka, G. J„ 1973, Geology of the McCoy and Palen Mountains, California, ern Arizona and adjacent California, which are central Transverse Ranges, in Ernst, W. G., ed„ The geotectonic devel- southeastern California [Ph.D. thesis]: Santa Barbara, California, Uni- opment of California (Rubey Volume I): Englewood Cliffs, Prentice- versity of California, 160 p. deployed in a very conspicuous, important re- Hall Inc., p. 254-283. Powell, R. E., 1981a, Geology of the crystalline basement complex, eastern gional tectonic setting. The results and some of Frost, E. G., 1981, Structural style of detachment faulting in the Whipple Transverse Ranges, southern California: Constraints on regional Mountains, California, and Buckskin Mountains, Arizona, in Stone, C., tectonic interpretation, in Howard, K. A., Carr, M. D., and Miller, the implications of our work were as astonishing and Jenney, J. P., eds., Arizona Geological Society Digest, v. 13, D. M., eds., Tectonic framework of the Mojave and Sonoran Deserts, p. 25-29. California and Arizona: U.S. Geological Survey Open-File Report 81- to us as they have been to some others. We have Gamer, W. E., Frost, E. G., Tanges, S. E., and Germinario, M. P., 1982, 503, p. 87-89. no vested interest in any particular interpretation Mid-detachment faulting and mineralization in the Trigo Mountains, 1981 b, Geology of the crystalline basement complex, eastern Transverse Yuma County, Arizona, in Frost, E. G., and Martin, D. L., eds., Ranges, southern California: Constraints on regional tectonic interpreta- or model, but until new evidence is presented, Mesozoic-Cenozoic tectonic evolution of the Colorado River region, tions [Ph.D. thesis]: Pasadena, California, California Institute of Tech- California, Arizona, and Nevada (Anderson-Hamilton Volume): San nology, 441 p. we feel comfortable with the data presented Diego, Cordilleran Publishers, p. 158-171. Raven, P. H., and Axelrod, D. I., 1974, Angiosperm biogeography and past here. We must admit that we believe that it is Greene, R. P., 1968, Metamorphosed McCoy Mountains Formation, Coxcomb continental movements: Missouri Botanical Garden Annals, v. 61, Mountains, California [M.S. thesis]: Santa Barbara, California, Univer- p.539-673. most consistent, as now understood, with the sity of California, 50 p. Reynolds, S. J., 1982, Multiple deformation in the Harcuvar and Harquahala Hamilton, W. A., 1982, Structural evolution of the Big Maria Mountains, Mountains, west-central Arizona, in Frost, E. G., and Martin, D. L., concept of a Mojave-Sonora megashear. Only northeastern Riverside County, southeastern California, in Frost, E. G., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River re- further work will finally resolve remaining prob- and Martin, D. L., eds., Mesozoic-Cenozoic tectonic evolution of the gion, California, Arizona, and Nevada (Anderson-Hamilton Volume): Colorado River region, California, Arizona, and Nevada (Anderson- San Diego, Cordilleran Publishers, p. 137-142. lems in the tectonic evolution of North Ameri- Hamilton Volume): San Diego, Cordilleran Publishers, p. 1 -27. Reynolds, S. J., Keith, S. B„ and Coney, P. J., 1980, Stacked overthrusts of Harding, L. E., 1978, Petrology and tectonic setting of the Livingston Hills Precambrian crystalline basement and inverted Paleozoic sections em- ca's southwestern margin. Formation, Yuma County, Arizona [M.S. thesis]: Tucson, Arizona, placed over Mesozoic strata, west-central Arizona, in Jenney, J. P., and University of Arizona, 57 p. Stone, C., eds.. Studies in western Arizona: Arizona Geological Society 1980, Petrology and tectonic setting of the Livingston Hills Formation, Digest, v. 12, p. 45-51. ACKNOWLEDGMENTS Yuma County, Arizona, in Jenney, J. P., and Stone, C., eds., Studies in Richard, S. M., 1982, Preliminary report on the structure and stratigraphy of western Arizona: Arizona Geological Society Digest, v. 12, p. 135-145. the southern Harquahala Mountains, Yuma County, Arizona, in Frost, 1982, Tectonic significance of the McCoy Mountains Formation, E. G., and Martin, D. L., eds., Mesozoic-Cenozoic tectonic evolution of southeastern California and southwestern Arizona [Ph.D. thesis]: the Colorado River region, California, Arizona, and Nevada (Ander- Thanks to D. Laetz and P. McAlaster, who Tucson, Arizona, University of Arizona, 197 p. son-Hamilton Volume): San Diego, Cordilleran Publishers, p. 235-242. provided assistance in the field. Discussions with Harding, L. E., Butler, R. F„ and Coney, P. J., 1980, Paleomagnetic evidence Robison, B. A., 1979, Stratigraphy and petrology of some Mesozoic rocks in for a Jurassic age assignment on a 6 km thick sedimentary terrane in western Arizona [M.S. thesis]: Tucson, Arizona, University of Arizona, G. H. Davis, W. R. Dickinson, G. Haxel, D. G. southwestern Arizona and southeastern California: Geological Society 138 p. of America Abstracts with Programs, v. 12, p. 109. 1980, Description and analysis of Mesozoic "Red Beds," western Ari- Howell, R. A. LeVeque, H. W. Pierce, R. E. Harding, L. E„ Coney, P. J., and Butler, R. F., 1982, Stratigraphy, sedimentary zona and southeastern California, in Jenney, J. P., and Stone, C., eds.. Powell, S. J. Reynolds, S. M. Richard, L. T. petrology, structure and paleomagnetism of the McCoy Mountains-Liv- Studies in western Arizona: Arizona Geological Society Digest, v. 12, ingston Hills Formation, southeastern California and southwestern p. 147-154. Silver, and R. M. Tosdal were very helpful. Spe- Arizona: Geological Society of America Abstracts with Programs, v. 14, Silver, L. T., and Anderson, T. H., 1974, Possible left-lateral early to middle p. 170. Mesozoic disruption of the southwestern North American craton mar- cial recognition is due to R. F. Butler, who was Harding, L. E., Butler, R. F„ and Coney, P. J., 1983, Paleomagnetic evidence gin: Geological Society of America Abstracts with Programs, v. 6, involved in all stages of this research. Funding for Jurassic deformation of the McCoy Mountains Formation, south- p. 955. eastern California and southwestern Arizona: Earth and Planetary Steiner, M. B., 1978, Magnetic polarity during the Middle Jurassic as recorded came from National Science Foundation Grant Science Letters, v. 62, p. 104-144, in the Summerville and Curtis Formations: Earth and Planetary Sciencc Haxel, G., 1977, The Orocopia Schist and the Chocolate Mountain Thrust, Letters, v. 38, p. 331-345. EAR80-18500; from an Asarco Fellowship Picacho-Peter Kane Mountain area, southeastemmost California Tosdal, R. M., 1982, The Mule Mountains Thrust in the Mule Mountains, awarded to L. Harding; and from J. W. Cagle of [Ph.D. thesis]: Santa Barbara, California, University of California, California, and its probable extension in the southern Dome Rock 277 p. Mountains, Arizona: A preliminary report, in Frost, E. G., and Martin, Conoco Inc. Haxel, G., and Dillon, J., 1978, The Pelona-Orocopia Schist and Vincent- D. L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado Chocolate Mountain Thrust system, southern California, in Howell, River region, California, Arizona, and Nevada (Anderson-Hamilton D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the volume): San Diego, Cordilleran Publishers, p. 55-60. western United States: Pacific Coast Paleogeography Symposium 2, Vedder, J. G., Howell, D. 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