<<

Making the Southern Margin of Laurentia themed issue

Stratigraphy and age of Upper strata in north-central , : Southwestern Laurentian record of crustal extension and tectonic transition

David J. Mauel1,*, Timothy F. Lawton1, Carlos González-León2, Alexander Iriondo3, and Jeffrey M. Amato1 1Department of Geological Sciences, State University, Las Cruces, New Mexico 88003, USA 2Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, , Sonora, 83000, México 3Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla Querétaro, 76230, México

ABSTRACT derived from arc rocks region, refi ne temporal limits of Upper Jurassic and exhumed Caborcan basement, Paleo- strata and the unconformities bounding them, Stratigraphy, sedimentology, and geo- zoic–Lower Jurassic sedimentary cover, and and shed light on the stratigraphic relationships chronology of the Upper Jurassic Lower intermediate volcanic of the Glance Conglomerate, a synorogenic unit Formation in north-central Sonora, Mexico, rocks. Revised stratigraphy of the Cucurpe- inferred to establish the locations and timing of provide new insights into rift- Tuape region indicates that several con- extensional basins in the region. This study was ing along the southwestern margin of Lau- glomeratic units, formerly interpreted as carried out within a main study area and within rentia. The Cucurpe Formation is the fi ll of Late Jurassic pull-apart basin deposits, are a broader general study area (Fig. 1) where the Altar-Cucurpe Basin. This basin devel- not of Late Jurassic age. comparative reconnaissance geology was com- oped upon attenuated crust of the – pleted. The main study area is located ~13 km Middle Jurassic continental arc and was INTRODUCTION northwest of the village of Cucurpe along the part of the -Cucurpe seaway; a nar- eastern fl ank of Sierra de Cucurpe, where an row marine embayment oriented parallel to extensive Lower Jurassic to Lower Cretaceous The Late Jurassic marked changing interplate and located west of the trough. section is exposed. Although structurally dis- dynamics in the Cordillera of the southwestern The Cucurpe Formation unconformably turbed, this section is intact and contains a com- U.S. and northwestern Mexico. Incipient open- overlies Middle Jurassic arc assemblages plete section of the Cucurpe Formation. ing of the Gulf of Mexico during the breakup and represents upward-coarsening marine of Pangaea was coincident with Late Jurassic prodeltaic deposits. New U-Pb zircon geo- METHODS continental rifting and marine incursion in chronology and a ammonite north-central Sonora. Opinions vary as to the Research in the main study area included (Idoceras cf. I. densicostatum) constrain its dominant tectonic mechanisms that initiated geologic mapping at a scale of 1:25,000, strati- age to between ca. 158 and 149 Ma. Detrital this rifting. Some workers infer that exten- graphic measurement, U-Pb zircon geochronol- zircon ages from the unconformably over- sion resulted from rollback of the subduct- ogy, and petrography of the Cucurpe Formation lying Lower Cretaceous Bisbee indi- ing oceanic slab (e.g., Lawton and McMillan, and basal strata of the overlying Bisbee Group. cate a maximum depositional age of 139 ± 1999; Dickinson and Lawton, 2001b), whereas In the general study area, local mapping and 2 Ma (2σ error), demonstrating a hiatus of others infer transtensional extension associated comparative stratigraphy were accompanied by at least 10 m.y. between Jurassic and Cre- with the Mojave-Sonora megashear (MSM; compilation of previous mapping (Plate 2) to taceous strata. Detrital zircon ages and e.g., Anderson and Nourse, 2005; Busby et al., unify the stratigraphic nomenclature and iden- petrographic data indicate the provenance 2005). sedimentary successions tify problem areas. of Cucurpe Formation and lowermost Bis- exposed in the Cucurpe-Tuape region of north- Sandstone thin sections for point counts bee strata. The lower part of the Cucurpe central Sonora (Fig. 1, Plates 1–2) provide were selected on the basis of quality, minimal was derived dominantly from Middle Juras- details con cerning volcanic activity, basin sub- alteration, and grain size. Each thin section was sic volcano-sedimentary successions. The sidence, and basement uplift during this criti- stained with sodium cobaltinitrate. The Gazzi- upper part of the Cucurpe Formation was cal period of transition. Dickinson method was utilized for point count largely derived from syneruptive volcanic We present new data on the stratigraphy, sedi- analysis (Ingersoll et al., 1984; Zuffa, 1980). A material equivalent to the Ko Vaya vol- mentology, and age of Upper Jurassic strata of single operator performed all point counts with canic suite of southern and north- the Cucurpe Formation and review stratigraphi- a node spacing of 0.66 mm for medium-grained ern Sonora. Lowermost Bisbee strata were cally adjacent units of Middle Jurassic and Early sandstones and a node spacing of 0.99 mm for Cretaceous age. Upper Jurassic strata are signif- coarse-grained sandstones. Point count param- *Present address: Alaska Division of Geological icant because they record onset of major exten- eters, raw count data, and normalized modal & Geophysical Surveys, 3354 College Road, Fair- banks, Alaska 99709, USA; email: david.mauel@ sion and marine incursion into northern Sonora. percentages of framework grains are presented alaska.gov. Our data improve the lithostratigraphy of the in Supplemental Table 11.

Geosphere; April 2011; v. 7; no. 2; p. 390–414; doi: 10.1130/GES00600.1; 13 fi gures; 2 tables; 2 plates; 2 supplemental tables.

390 For permission to copy, contact [email protected] © 2011 Geological Society of America

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113°W 8 10 111°W 0 50 100 km

2 Tucson 32°N MSM 10 32°N 19 Mexico

Explanation Glance Conglomerate Nogales

Ko Vaya suite upper Altar High Formation Cucurpe (Nourse, 2001) ? Formation SB A CA Papago domain Magdalenaa general 2 study area Mojave-Sonora MSM main SM and MSM megashear study approximate Gulf area Cucurpe area of Plate 2 of 30°N N EV LC LT 30°N 113°W 111°WTuape

Figure 1. Outcrops of strata known or inferred to be of Late Jurassic age in southern Arizona and northern Sonora (adapted from Tosdal et al., 1989). Rectangles indicate general and main study areas. Localities: A—Altar; CA—Cerros El Amol; EV—El Venado; LC—Rancho La Colgada; LT—Rancho La Tesota; SB—Sierra El Batamote; SM—Rancho San Martin.

U-Pb geochronology was performed for 1σ level, and errors on igneous crystallization means of these groups were calculated using detrital zircons using the laser ablation multi- ages are reported at the 2σ level. Analytical Isoplot 3.0 (Ludwig, 2003). We employ the collector induced-coupled-plasma mass spec- results, age data, and global positioning system time scale of Walker and Geissman (2009). trometer (LA-MC-ICPMS) at the University (GPS) coordinates for each sample are included of Arizona. Geochronology on tuffs was com- in Supplemental Table 22. GPS coordinates use GEOLOGIC SETTING AND pleted at the Stanford/U.S. Geological Survey the 1927 North American Datum for Mexico. TECTONIC MODELS (USGS) facility using the sensitive high- Only detrital zircons in coherent age groups of resolution ion microprobe (SHRIMP). Sample three or more were considered representative Mesozoic tectonic events proposed for the preparation used standard mineral crushing and for obtaining maximum depositional ages due southwestern margin of Laurentia include large- separation techniques. Analytical errors asso- to problems that can arise from discordance scale sinistral truncation and translation (MSM), ciated with the LA-MC-ICPMS facility at the (e.g., Dickinson and Gehrels, 2009b). The continental arc magmatism, arc extension and University of Arizona are detailed elsewhere youngest coherent age groups from detrital zir- rifting, exotic arc or fringing arc accretion, post- (Gehrels et al., 2008). Errors on ages of indi- con analysis were determined using the Detrital rift thermal subsidence, and Late Cretaceous vidual detrital zircon grains are reported at the Age Pick program of Gehrels (2006). Weighted metamorphism and magmatism. The evidence for and existence of several of these events 1Supplemental Table 1. PDF file of Sandstone 2Supplemental Table 2. Excel fi le of SHRIMP data remain controversial (Molina-Garza and Iriondo, point count parameters. If you are viewing the PDF and LA-ICPMS data. If you are viewing the PDF of this paper or reading it offl ine, please visit http:// of this paper or reading it offl ine, please visit http:// 2005). The following summarizes some of the dx.doi.org/10.1130/GES00600.S1 or the full-text article dx.doi.org/10.1130/GES00600.S2 or the full-text article hypotheses concerning the Mesozoic tectonic on www.gsapubs.org to view Supplemental Table 1. on www.gsapubs.org to view Supplemental Table 2. evolution of southwestern Laurentia.

Geosphere, April 2011 391

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EXPLANATION Arizona, Ko Vaya super-unit Ko Vaya Peaks super-unit –Trigo Cargo Muchacho super-unit (allochthonous) Sand Well Formation and “Gu Achi Formation and “Gu Sand Well sequence,” Upper Jurassic or Cretaceous? Orocopia Schist McCoy Mountains Formation Fresnal Canyon and Dome Rock sequences (Early and Middle Jurassic) includes San Martin Formation Jurassic Cratonic Triassic(?)–Lower assemblages of southeastern California and southwestern Arizona Jurassic Granitoids undifferentiated Mazatzal (1.70–1.60 Ga) transition Yavapai-Mazatzal (1.80–1.70 Ga) Yavapai transition Mojave-Yavapai Mojave (2.0–1.8 Ga) Caborca block (2.0–1.8 Ga) Glance Conglomerate (Late Jurassic and Cretaceous); locally includes entire Bisbee Group Ko Vaya Suite (ca. 158–146 Ma); includes Ko Vaya Altar Formation, “Artesa sequence,” Upper and correlatives Cucurpe Formation (-) Barranca and El Antimonio groups and Barranca and El correlative Basomari and Lily formations Jurassic) (Triassic-Lower et al. (2004); the full-sized Jurassic Granitoids and Supracrustal Rocks Paleoproterozoic Basement Provinces 34 N N 30°N 32°N 170 170 185 area of Plate 2 171 D CB

100 km T RH 167 10 LM H W

50 Mojave-Sonora megashear Absolute age in Ma (dated by various methods) U.S. Interstate Highway Mexican Highway U.S. or Mexican city location Antonio fault Imuris lineament/San Thrust fault US-Mexico border 152 E 174 CH 170 Tuape C SC 150 Imuris Nogales La Lamina

0 RLT PT SDM Tucson SR 111 W 111 19 166 LA SDC 189 170 111°W P EL TM CRA SM ? 169 RS LG 165 159 CED Magdalena S 2 BM FC SSR 159 SN ST CA 170 RL 149 146 AM 10 Altar GA Phoenix 149 KV CM ~150 SB V 165 CR ~150 SH SS 175 SG PDC Caborca 170 Sonoita SDA 160 ~165 153 G BT 113°W 113°W 176 ~166 8 10 Quitovac PQ 225 178 LH of Gulf ed from Stewart et al. (1986); Reynolds et al. (1986); Chepega (1987); Stephens (1988); Tosdal et al. Tosdal Stewart et al. (1986); Reynolds Chepega (1987); Stephens (1988); ed from GW California KM NR BSM NW PM CD 154 165 2 DR TR PI 173 164 MM 160 MC CMM 115°W 165 CHM NR = Neversweat Ridge Mountains NW = New Water = Pajarito Mountains P = Palen Mountains PA PDC = Pozo de Serna PI = Picacho PM = = PT PQ = Puerto Blanco and Quitobaquito Mnts RH = Red Hills = Rancho La Garapata RL Tesota = Rancho La RLT RS = Roskruge and Bell Mountains S = Sasabe SB= Sierra el Batamote SC = Sawmill Canyon Alamo = Sierra del SDA SDC= Sierra del Caracahui SDM= Sierra de la Madera SG= Sierra La Gloria SH = Sheridan Mountains SM = Sierrita Mountains SN = Sil Nakya Hills SR = SS = Sierra San Manuel SSR = Sierra de Santa Rosa = Santa Rosa Mountains ST Hills Tombstone = T Mountains Tucson TM = Mountains Trigo TR = Hills V = Vekol W = PA 115°W le of Plate 1, please visit http://dx.doi.org/10.1130/GES00600.S3 or the full-text article on www.gsapubs.org to view Plate 1. the full-text article on www.gsapubs.org or le of Plate 1, please visit http://dx.doi.org/10.1130/GES00600.S3 CX 165 Karlstrom et al. (2004); Anderson et al. (2005); Haxel (2008); and González-León (2009). For et al. (2004); Karlstrom fi PDF Plate 1. Paleoproterozoic basement provinces and Jurassic granitoid and supracrustal rocks in southeastern California, southern and Jurassic granitoid supracrustal rocks basement provinces Plate 1. Paleoproterozoic may be exposures smaller rocks; younger or older locally include minor Mauel, 2008). Mapped areas and northernmost Sonora (from slightly exaggerated in size. Modifi (1989); Rodríguez-Castañeda (1988, 1991, 1994, 1996, 1999); Gastil et al. (1991); Nourse (1994); (1995); Iriondo ET 165 10 LOCATION ABBREVIATIONS LOCATION AM = Artesa Mountains AM = BM = Baboquivari Mountains BSM = Buckskin Mountains = Bates Mountains BT C = Cucurpe Amol CA= Cerros el CB = Cabullona Basin CED = Cerro el Dieciseis CH = CHM = Chocolate Mountains CM = Comobabi Mountains CMM = Cargo Muchacho Mountains CD = Mountains CR = Cerro Rajón = Cobre Ridge area CRA CX = Coxcomb Mountains D = DR = E = Empire Mountains = Estación Llano EL Ranges Transverse = Eastern ET FC = Fresnal Canyon Sequence G = Achi Mountains = Gu GA Mountains GW = Granite Wash H = KM = Hills KV = Ko Vaya Avispas = Sierra las LA LG = LH = Little LM = Lone Mountain area M = MC = McCoy Mountains MM = Big and Little Maria Mountains 34°N 32°N

392 Geosphere, April 2011

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Stratigraphy and age of Upper Jurassic strata in north-central Sonora,4 Mexico

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Con Re References Anonymous, 2000, Benjamin Hill H12-A89, Sonora: Servicio Geológico Anonymous, 2000, Cucurpe H12-B71, Sonora: Servicio Geológico Mexicano, Carta Geológico-Minera, scale 1:50,000, 1 sheet. sheet. Anonymous, 2000, H12-B61, Sonora: Servicio Mexicano, Carta Geológico-Minera, scale 1:50,000, 1 sheet. 1 Ana H12-A69, Sonora: Servicio Geológico Geológico Mexicano, Carta Geológico-Minera, scale 1:50,000, Anonymous, 2000, Santa north- Tuape, 1987, Reconnaissance geology of Chepega, J.R., Jr., Mexicano, Carta Geológico-Minera, scale 1:50,000, 1 sheet. A., Morales-Morales, H., and Orantes-Contreras, González-Gallegos, central Sonora, Mexico [M.S. thesis]: University of Pittsburgh,139 p. H12-B73, Sonora: Servicio Geológico Mexicano, 2003, Arizpe, Sonora centro- V., González-León, C., 1978, Geología del área de Carta Geológico-Minera, scale 1:50,000, 1 sheet. 1998, Mass-gravity deposits and T.H., Anderson, McKee, M.B., and septentrional [B.S. thesis]: Hermosillo, Universidad de Sonora,71p. structures in the Lower Cretaceous of Sonora, Mexico: Nourse, J.A., 1995, Jurassic-Cretaceous paleogeography of the p. 1516–1529. 110, America Bulletin, v. Geological Society of Magdalena region, northern Sonora, and its influence on the metamorphic cor complexes, in Jacques- Tertiary positioning of C., González-León, C.M., and Roldán-Quintana, J., eds., Ayala, 1992, Geológia de la A., and Minjarez, V.A., Studies on the Mesozoic of Sonora and adjacent areas: Palafox, J.J., Mendoza, America Special Paper 301, p. 59–78. Geological Society of Tuape region de la Sierra Caracahui, Sonora, Mexico: Boletin del Rodríguez-Castañeda, J.L., 1988, Estratigrafia de la region n.1, p. 19–34. Departmento de Geologia, Uni-Son, v.9, Autónoma de México, Sonora México: Universidad Nacional Rodríguez-Castañeda, J.L., 1990, Relaciones estructurales en la parte 7, p. 52–66. Institutode Geologia, Revista, v. centro-septentrional del estado de Sonora: Universidad Nacional 9, p. 51 Autónoma de México. Instituto Geologia, Revista, v. estado de Sonora, Tuguachi, Rodriguez , J.L., 1994, Geológia del area El brittle- Rodríguez-Castañeda, J.L.,1996, Late Jurassic and mid-Tertiary n. 1, 11–28. Mexico: Revista Mexicana de Ciencias Geológia, v.11, ductile deformation in the Opodepe Region, Sonora, Mexico: en la Terciaria Cretácica y Autónoma de México, Instituto Tectónica Universidad Rodríguez-Castañeda, J.L., 2002, Nacional Geologia, Revista, v 13, n. 1, p. 1–9. Alto de Cananea, Sonora norte-central margen suroeste del Autónoma de Universidad Nacional Doctoral]: México, D.F., L.A., and Castro Escárcega, J.J., 1999, Saracachi [Tesis Terán-Ortega, México, 217 p. L.A., Siqueiros-López, C.J., and Guzmán-Espinoza, J.B., H12-B72, Sonora: Servicio Geológico Mexicano, Carta Terán-Ortega, Geológico-Minera, scale 1:50,000, 1 sheet. H12-B62, Sonora: Servicio Geológico Aberhan, M., and Teresa 2003, Santa T.F., A.B., González-León, C.M., Lawton, Villaseñor, Mexicano, Carta Geológico-Minera, scale 1:50,000, 1 sheet. 2005, Upper Jurassic ammonites and bivalves from the Cucurpe Formation, Sonora (Mexico): Revista Mexicana de Ciencias 22, n. 1, p. 65–87. Geológicas, v. Mine in production Abandoned mine Prospect Map symbols

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394 Geosphere, April 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/2/390/3714949/390.pdf by guest on 25 September 2021 Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico

Mojave-Sonora Megashear and Jurassic magmatic arc consist dominantly of (Haenggi and Muehlbuerger, 2005; McKee Basement Ages calc-alkaline to alkaline granitoids and rhyo- et al., 2005). A Late Jurassic to litic to dacitic ash fl ow tuffs and fl ows locally bathymetric high known as the Aldama platform The MSM was originally proposed to explain interbedded with quartz arenite and sedi- (Haenggi, 2002) separated these Late Jurassic the apparent distribution of Proterozoic basement mentary strata with abundant volcanic clasts marine depocenters and was likely contiguous provinces in northern Sonora (Plate 1; Silver and (Plate 1; May and Haxel, 1980; Hardy, 1981; with the Cananea high along the Sonora-Arizona Anderson, 1974; Anderson and Silver, 2005). Haxel et al., 1985; Segerstrom, 1987; Tosdal boundary (McKee and Anderson, 1998). Two Precambrian basement age provinces were et al., 1989; Palafox et al., 1992; Nourse et al., The basal conglomerate member of the Bis- recognized in northwestern Sonora, a northeastern 1994; Nourse, 1995; Rodríguez-Torres et al., bee Group, the Glance Conglomerate (Ransome , group with ages of 1.7–1.6 Ga, and a southwestern 2003; González-León et al., 2005; Haxel et al., 1904), is widely exposed in ranges of southeast- group with ages of 1.8–1.7 Ga (i.e., the Caborca 2005; Leggett, 2009). Interstratifi ed eolian and ern Arizona and north-central Sonora (Plate 1), Block; Plate 1; Anderson and Silver, 1979). This shallow-marine quartz arenite indicate that the where its dominantly alluvial deposits vary basement age distribution led Silver and Ander- Middle Jurassic arc was low standing and occu- greatly in thickness, clast composition, texture, son (1974) to propose that 1.8–1.7 Ga rocks in pied a graben depression which acted as a trap contact relations, and age (Bilodeau et al., 1987; Sonora were displaced sinistrally along the MSM for quartz-rich sand transported southwest from Tosdal et al., 1989; Nourse, 1995). The strati- from rocks of similar age in the Mojave Desert the –Middle Jurassic erg complex graphic relation of the Glance Conglomerate region (Plate 1). Correlation of the overlying Neo- of the Plateau (Bilodeau and Keith, to the marine Upper Jurassic strata of northern proterozoic–Paleozoic strata from both regions 1986; Busby-Spera, 1988). The arc trend from Sonora remains in question; the Glance is com- supports this assertion (e.g., Stewart, 2005). These northern Sonora to a southeastern segment monly identifi ed on the basis of coarse texture, relationships led Anderson and Silver (1979) to extending to Guatemala, termed the Nazas arc, stratigraphic position, and/or unconformable propose that the Caborca block translated 700– has alternately been interpreted as continuous relation with older rocks (Tosdal et al., 1989). 800 km along the MSM during the Late Jurassic (Bartolini et al., 2003) or as offset by the MSM In southern Arizona, stratigraphic variation of to reach its present location. Evidence in support (Jones et al., 1995). clast composition records unroofi ng of adjacent of Late Jurassic translation includes (1) deforma- uplifted fault blocks (Bilodeau et al., 1987). tion of fossiliferous Oxfordian strata along the Upper Jurassic Igneous and Supracrustal The lower parts of the Glance Conglomerate fault in central Sonora, (2) 158 Ma hypabyssal Rocks of Southern Arizona and Sonora locally contain silicic tuffs with ages of 147 ± rocks displaced along the fault, (3) emplace- 6 Ma (K-Ar, biotite), 149 ± 11 Ma (Rb-Sr, ment of the undeformed 148 Ma Independence Upper Jurassic rocks of the U.S.-Mexico whole-rock), and 151 ± 2 Ma (Rb-Sr, whole dike swarm near the fault, and (4) crosscutting border region are less studied and correlations rock) (Marvin et al., 1978; Kluth et al., 1982). relationships among deformed and undeformed are more tenuous than those of the Upper Trias- These ages, if correct, indicate that the Glance plutons in the central Mojave Desert that indicate sic to Middle Jurassic magmatic arc rocks. The is generally younger than or correlative with deformation between 164 and 149 Ma (Anderson Upper Jurassic includes the Ko Vaya Suite, the the upper parts of the Upper Jurassic strata we and Silver, 2005). Other workers have proposed Cucurpe Formation, the Glance Conglomerate, describe here. Some workers interpret Glance similar truncation and translation of the Caborca and various correlative units (Fig. 1 and Plate 1). Conglomerate deposition as the result of rifting block in late Paleozoic to Triassic time (Stevens Late Jurassic age assignment until recently has and back-arc extension related to the opening of et al., 2005; Dickinson and Lawton, 2001b). been based solely on stratigraphic position due the Gulf of Mexico (Bilodeau, 1982) rather than This alternate Early to to a lack of geochronological data. transtension. (ca. 281–241 Ma) sinistral strike-slip fault has The Ko Vaya Suite consists of granitic rocks, been termed the California- transform characterized in part by textural and composi- Changes in Inter-plate Kinematics during (Dickinson, 2000; Dickinson and Lawton, 2001b). tional heterogeneity and alkaline tendencies, Late Jurassic Time Late Jurassic basin geometry and subsidence and associated sedimentary and volcanic rocks have been attributed to transtension along releas- (Haxel et al., 2008). The supracrustal rocks of Most workers agree that the Late Juras- ing bends of the MSM. Anderson and Nourse the Ko Vaya Suite have also been termed the sic was a time of transition from the tectonic (2005) described upward-fi ning Upper Jurassic “Artesa sequence” (Tosdal et al., 1989), which regime of the Late Triassic to Middle Jurassic. conglomerate gradationally overlain by fi ner- contains a number of lithologies with common Potentially interrelated tectonic events pro- grained Bisbee Group strata and interpreted domi- lateral facies changes and local unconformities. posed for the Late Jurassic include: (1) Initia- nant fault orientations as representing pull-apart The most widespread and indicative lithology tion of continental rifting along the southwest- geometries. A Jurassic history for these faults, interbedded within volcanic tuffs and fl ows is ern margin of Laurentia following an episode which typically exhibit Late Cretaceous displace- laminated or well-bedded immature volcanic of extensive caldera development and emplace- ment, is inferred on the basis of an unconformity litharenite (Tosdal et al., 1989). ment of silicic tuffs (“quartz porphyries”) by between Upper Jurassic(?)–Lower Cretaceous The Cucurpe Formation and related marine 168 Ma (Riggs et al., 1993); (2) trenchward strata. Rifting and subsidence in the pull-apart shales and turbidites in north-central Sonora con- migration of the Jurassic arc from ca. 157 to model occurred between 162 and 148 Ma. tain Oxfordian–Tithonian ammonites (Rangin , 148 Ma; and (3) rapid increase in absolute 1977; Imlay, 1980; Almazán-Vázquez and northward motion of western North American Late Triassic to Middle Palafox-Reyes, 2000; Villaseñor et al., 2005). from ca. 160 to 125 Ma. Jurassic Magmatic Arc These and correlative strata to the south (Lyons, A mechanism proposed for initiation of Late 2008) represent a northwest-trending arm of the Jurassic rifting is westward rollback of the By the Late Triassic, arc magmatism had Mar Mexicano termed the Arivechi-Cucurpe Faral lon slab or a pre-Farallon slab by steep- developed along the southwestern margin of seaway, which lay roughly parallel to, but west ening of the slab-subduction angle (Lawton Laurentia. Rocks of the Late Triassic to Middle of, the Chihuahua trough in central Chihuahua and McMillan , 1999; Dickinson and Lawton,

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2001b), which resulted in passive intra- and STRATIGRAPHY AND CORRELATION exposed; rather, unit contacts are commonly back-arc extension within the upper plate, OF JURASSIC–LOWER CRETACEOUS faults, and stratigraphic sections are them- asthenospheric upwelling, and resultant partial STRATA, NORTH-CENTRAL SONORA selves faulted (Plate 2). Therefore the strati- decompression melting of the elevated astheno- graphic column is a composite section based spheric mantle. Attendant lithospheric melting Lower Jurassic–Lower Cretaceous strati- on relations observed at various locations in produced mafi c and effusive silicic volcanic graphic units of the general study area include, the general study area. Conglomerate identi- rocks interbedded with Upper Jurassic conglom- in ascending order, the Basomari, Rancho San fi ed as Glance directly underlies the Morita erates during slab retreat; and continued upwell- Martin, Cucurpe, Rancho La Colgada, Morita, Formation in the northern part of the general ing, resulting from return mantle fl ow following Mural, and Cintura and La Juana formations study area at a locality where strata of known slab retreat, caused decompression partial melt- (Plate 2 and Fig. 2). Nowhere in the general Late Jurassic age do not crop out; we discuss ing of the asthenosphere and produced basalts study area is this succession continuously the Glance with the Cucurpe Formation below. with ocean-island chemical affi nities. Our revised stratigraphic column is correlated The apparent polar wander path of the North with strata of adjacent regions on the basis of American plate reveals an abrupt change in the Cocospera Formation published and new biostratigraphic and geo- Dusky or medium red, poorly sorted, rate and direction of plate movement during polymictic conglomerate, sandstone, chronologic data (Fig. 3). Late Jurassic time (Beck and Housen, 2003). siltstone, and porphyritic to aphanitic We refer to the sedimentary basin in which andesite. Contains Lower Cretaceous Earlier in the Jurassic (ca. 200–160 Ma), North limestone blocks, slabs, and olistoliths. Upper Jurassic marine and continental syn-rift America slowly rotated clockwise and moved Unconformity strata accumulated as the Altar-Cucurpe Basin, La Juana Formation north with respect to the paleomeridian. This (>270 m; Albian) Gray shale, which lay south of the Cananea high. The was followed by rapid northwestward absolute conglomerate and sandstone. geographic extent of the Alter-Cucurpe Basin motion of western North America from Late is more narrow and elongate than that of the Cintura Formation Jurassic to Early Cretaceous time (ca. 160– (>860 m; Albian) Fluvial red siltstone, “Bisbee fl ank basin” of Dickinson and Lawton 125 Ma; Beck and Housen, 2003). The begin- conglomerate and sandstone, (2001a) and extends farther to the southeast commonly containing pedogenic calcite ning of rapid Late Jurassic North American nodules. beneath Cenozoic cover of the Sierra Madre plate movement coincided with the onset of Occidental. Defi nition of the Altar-Cucurpe marine deposition in the Altar-Cucurpe Basin. Mural Limestone Basin distinguishes Jurassic syn-rift from (160–800 m; Aptian-Albian) Shallow Lower Cretaceous post-rift deposits, which marine limestone and shale. Commonly are separated by an angular unconformity. It Early Cretaceous Extension and includes upper rudistid boundstone unit. also takes into account new data presented in Subsidence within the Bisbee Basin Morita Formation (300–1100 m; Aptian) Fluvial red this paper and Mauel (2008) indicating that a siltstone, conglomerate, sandstone, and thick Upper Jurassic marine section, commonly uncommon limestone beds with oysters. The post-Glance Bisbee Group of south- Common calcite nodules. Uncommon tightly folded and partly correlative with the eastern Arizona was originally divided, in tuff beds in upper part. Glance Conglomerate, underlies Lower Creta- ascending order, into the Morita Formation, the ceous strata along a trend that passes generally m Rancho La Colgada Formation Mural Limestone, and the Cintura Formation e (0–160 m; Aptian) Marine sandstone. southeast from just west of the community of t (Ransome , 1904). Subsequent studies have Unconformity Altar, past Cucurpe, and on to the southeast to documented the presence of correlative strata e Cucurpe Formation embrace outcrops of black shale described by r (>1000 m; Oxfordian-Tithonian) in southwesternmost New Mexico (Mack s Volcaniclastic marine turbidity-current Rodríguez-Castañeda (1994), which we include et al., 1986; Lawton and Harrigan, 1998; and debris-flow deposits and shale, in the Cucurpe Formation. The Bisbee fl ank commonly slumped and folded. Lucas and Lawton, 2000; Lawton, 2004), Contains uncommon Oxfordian, basin was defi ned to include only thin Glance southeastern Arizona (Bilodeau 1978, 1982; Kimmeridgian and Tithonian Conglomerate sections (Dickinson and Lawton, ammonites. Composed of overall Dickinson et al., 1986, 1989; Lawton and upward-coarsening succession. 2001a) and so lacks the structural distinction of Olmstead, 1995), and north-central Sonora a deeply subsided Late Jurassic depocenter to (Jacques-Ayala, 1989, 1995; González-León, Unconformity (?) which we ascribe the basin here. 1994; González-León and Lucas, 1995; Law- Rancho San Martin Formation ton et al., 2004; Peryam, 2006). The Bisbee (ca. 600 m; Middle Jurassic) Lower and Middle Jurassic Strata Interbedded mafic flows, laminated Basin of northern Sonora has been defi ned as lacustrine limestone,volcanic feldspathic the “Bisbee fl ank basin” on the basis of its thin- litharenite, volcanic lithic clast Basomari Formation conglomerate and quartz arenite. ner (<100 m) intervals of Glance Conglomer- The Basomari Formation is exposed in the Unconformity (?) ate than in Arizona, which was occupied by the main study area north of Rancho Basomari Basomari Formation “Bisbee core basin” (Dickinson and Lawton, (ca. 800 m; ) Thickly (Figs. 4 and 5), where it consists of ~750 m 2001a). These thinner, more fl uvial, Glance bedded granule to boulder of thickly bedded granule to boulder muddy conglomerate containing basement intervals and the Ko Vaya Suite lie within the clasts and with muddy matrix support matrix-supported conglomerate with inter- Papago domain (Fig. 1), a region characterized intercalated with maroon to dark gray calated maroon to dark-gray volcanic litharenite volcanic litharenite, siltstone and by its general lack of exposed autochthonous uncommon tuff beds. and siltstone beds (González-León et al., 2009). Proterozoic–Paleozoic rocks (Anderson et al., Conglomerate beds contain clasts of intermedi- 2005). Post-Glance Early Cretaceous deposi- 0 1000 2000ate volcanic rocks, 3000 gneiss, granite, 4000diorite, and 5000 tion in the Bisbee Basin records post-exten- Figure 2. Generalized stratigraphic column uncommon quartzite. The Basomari Formation sional thermotectonic subsidence (Dickinson of Mesozoic units exposed in study area of is interpreted as a fl uvial and alluvial conti- et al., 1989; McKee et al., 2005). north-central Sonora. nental deposit based on lithology and absence

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Benigno (7)

Benevides Loma Plata Group

Cuchillo

La Casita Navarrete Chihuahau Las Vigas Chihuahua Trough 2 5 J J 0 J 0 1 3 J J K (6) Navajo age Morrison LA-ICPMS on tuff Carmel Cedar Entrada Wingate Kayenta

Morrison Temple Cap Group Canyon Glen Summerville/Curtis ? (5) (part) (part) U-Bar Finish Mojado quartz Hell-To- (MM-1) Broken Jug SW New Mexico monzonite Hell-To-Finish Piper Gulch Tuff of Pajarito Tuff Mustang tuff ? NE and Cave Mount

Onion

SE AZ Quartz Sonora Glance? Saddle Crystal Porphyry Wrightson Mural Morita Cintura of N Sonora (4) and NW S Arizona and Suite "Artesa Group SW AZ Sonora Ko Vaya Topawa Topawa sequence" ? (3) ? ? lower upper Rancho Cucurpe Mural Morita Cintura La Juana San Martin Cucurpe Cucurpe, Sonora Basomari Rancho La Colgada Maximum depositional age from U-Pb detrital zircon analysis (LA-ICPMS) ? ? (2) de Glance? Morita Rosa Arroyo Santa Sierra Sasabe Cintura Martin (?) Altar, Sonora Altar, Nourse, 1995) Rancho San Cucurpe (?) or (part of Altar Fm of ? ? (1) ? McCoy Sequence member?) Mountains mudstone McCoy member 1) Dome Rock McCoy Basin Mountains member 2 and (basal sandstone (quartz porphyry) (basal sandstone and Lawton (2001b); (12) Dickinson et al. (1989), McMillan (1999); (13) González-León (2009). (1998); (6) Kowallis et al. (2001); (7) Haenggi (2002); (8) Peryam (2006); (9) Anderson et al. (2005); (10) Barth (2008) (1998); (6) Kowallis et al. (2001); (7) Haenggi (2002); (8) Peryam (2006); (9) Figure 3. Correlation of stratigraphic units in north-central Sonora with those of other regions in the southwestern United Sta regions of stratigraphic units in north-central Sonora with those other 3. Correlation Figure of inferred tectonic events and basin-forming mechanisms, Early Jurassic–Early Cretaceous time. Sources of data: (1) Barth et a time. Sources tectonic events and basin-forming mechanisms, Early Jurassic–Early Cretaceous of inferred y Barragán et al. (1998), González-León (2009); (3) Peryam (2006 (1995), García et al. (2005); (2) Jacques-Ayala Spencer Riggs et al. (1993), Lawton and Olmstead (1995), Haxel (2005); (5) Lawto et al. (2007), Leggett (2009); (4) adapted from Albian Aptian SHRIMP U-Pb on tuff U-Pb on tuff SHRIMP or flow Tithonian Berriasian

Barremian Oxfordian Valanginian Hauterivian

Age

Kimmeridgian

Pleinsbachian

Late Middle Early Early

Cretaceous Jurassic Age (Ma) 200 140 150 160 110 170 180 190 120 130 100

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of (González-León et al., 2009). Paleo- as the type section. This section is structurally Nal Neogene surficial deposits zoic(?) micritic limestone olistoliths(?) tens to complex, incomplete, and consists of ~161 m of hundreds of meters in length along strike crop ammonite-bearing upper Oxfordian tuffaceous Miocene to Holocene Nsv sedimentary and volcanic rocks out in southwestern exposures. The Basomari sandstone, shale, and siltstone. Subsequently, (includes Baucarit Formation) Formation was formerly considered Late Juras- Rodríguez-Castañeda (1991) assigned Upper sic (Stephens, 1988); however, a dacitic tuff Jurassic ammonite-bearing strata of the area to Pv Paleogene volcanic rocks ~130 m from its top yielded an age of 189.2 ± the La Colgada Formation. Following the rule 1.1 Ma (U-Pb zircon SHRIMP; Leggett et al., of priority in the North American Stratigraphic Pi Paleogene intrusive rocks 2007; Leggett, 2009), indicating that this unit is Code (NACSN, 1983), we refer to Upper Juras- at least partly correlative with the Lower Juras- sic strata of the Cucurpe-Tuape region as the Kbc Bisbee-clast conglomerate sic Sierra de Santa Rosa Formation (Fig. 3). The Cucurpe Formation. (Cocospera Fm?) Basomari is in fault contact with the Rancho The Cucurpe Formation consists of more Kbu Undifferentiated Bisbee Group San Martin Formation in the main study area than 1000 m of thinly bedded shale, mudstone, (Leggett, 2009). tuffaceous siltstone and sandstone, and subor- Kc Cintura Formation dinate granule-pebble conglomerate beds. Its Rancho San Martin Formation lower part contains intercalated thin beds of Kmu Mural Limestone The Rancho San Martin Formation is best black, gray, and reddish gray shale, tuffaceous exposed in the main study area southeast of siltstone and mudstone, less common volcanic Morita Formation Rancho San Martin (Fig. 5) and along Arroyo litharenite, and rare andesitic fl ows. Locally, Kmo El Cajón west of Rancho La Tesota (Fig. 6). the Cucurpe Formation coarsens up-section Rancho La Colgada In the main study area it consists of ~700 m into meter to decimeter scale intervals of amal- Klc Formation of volcaniclastic conglomerate, eolian quartz gamated medium-to coarse-grained volcanic arenite, lacustrine limestone, and interbedded litharenite and volcaniclastic granule to pebble Jc Cucurpe Formation basaltic andesitic and dacitic fl ows (Leggett, conglomerate beds which commonly exhibit 2009). The lowest 250 m consists of well-sorted inverse to normal grading, cross stratifi cation, Jsm Rancho San Martin Formation fi ne-grained quartz arenite intercalated with horizontal laminae, scour surfaces, black to moderately sorted, fi ne- to medium-grained, gray subrounded “fl oating” shale intraclasts, Jeb El Bajio Porphyry (Leggett, 2009) volcanic and feldspathic litharenite, overlain and rare fl ute and load casts at bed bases. Con- by an ~50-m-thick interval of dacitic ash fl ow glomerates are typically matrix supported, with Jb Basomari Formation tuff with an age of 168.4 ± 1.2 Ma (U-Pb zir- dominant rhyolite clasts. Fine- grained intervals con SHRIMP; Leggett et al., 2007; Leggett, Paleozoic(?) Allochthonous(?) locally contain uncommon belemnites, bivalves, Pzls limestone block (Stephens, 1988) 2009). Overlying the tuff is ~250 m of pebble to silicifi ed wood, rare bone fragments, cobble volcaniclastic conglomerate with inter- and locally abundant ammonites. The Cucurpe pCm Precambrian Micrographic granite bedded andesitic fl ow breccia above which an Formation is unconformably overlain by the ~40 m interval of laminated algal boundstone Precambrian Layered saussurite Rancho La Colgada Formation in the southern pClsg gabbro (Stephens, 1988) grades upward into ostracod-bivalve wacke- part of the general study area and by the Morita stone (Leggett, 2009). The uppermost 110 m Formation in the central part (Plate 2). La Lamina metaleucogranite pClmg (Stephens, 1988) consists of pebble to cobble volcaniclastic con- Oxfordian–early Tithonian ammonites indi- glomerate interbedded with discontinuous beds cate the general age range of the Cucurpe For- Precambrian metagranite and pCmg related rocks (Stephens, 1988) of fi ne-grained quartz arenite (Leggett, 2009). mation (Rangin, 1977; Villaseñor et al., 2005). The Rancho San Martin Formation represents One ammonite and U-Pb ages of three reworked eolian, alluvial fan, and lacustrine deposits in an silicic ash fall tuffs provide new age constraints Syncline, approx. located extensional basin within or adjacent to the active for the Cucurpe Formation and corroborate its Measured section Middle Jurassic Nazas arc (Leggett, 2009). In Late Jurassic age. Two of these siliceous tuffs Reverse fault the main study area, the Cucurpe Formation (3–03-LL-21 and 5–21-CM02) were collected Reverse fault, approx. located unconformably overlies the Rancho San Martin ~150 m and ~20 m, respectively, from the top of Lithologic contact Formation. the formation in the main study area, where it is Lithologic contact, approx. located Upper Jurassic Strata unconformably overlain by the Morita Forma- Normal Fault tion (Fig. 5). Sample 3–03-LL-21 produced 12 Normal fault, approx. located Cucurpe Formation concordant grains with a weighted mean age of Normal fault, approx located and ? The Upper Jurassic Cucurpe Formation is 152 ± 3 Ma and sample 5–21-CM02 yielded 21 queried widespread in the general study area (Plate 2), concordant grains with a weighted mean age of Strike and dip of bedding 65 where it is typically isoclinally folded and 150 ± 1 Ma (U-Pb zircon SHRIMP; Fig. 7). A faulted. The strata were fi rst described by third tuff (sample 5–22-CU-01), collected north- Strike of vertical bedding Rangin (1977) near Rancho La Colgada (Fig. 1), west of Rancho La Colgada (Fig. 8), near the Strike and dip of overturned bedding 65 who assigned them a late Oxfordian age on type locality of the Cucurpe Formation, yielded Geochronology sample DM1-6-8 the basis of ammonites in the section. Araujo- 15 concordant grains with a weighted mean age Mendieta and Estavillo-González (1987) later of 152 ± 2 Ma (U-Pb zircon SHRIMP; Fig. 7). All Figure 4. Explanation of map symbols and assigned strata near El Venado (Fig. 1) to the three tuff samples have Late Jurassic ages that fall stratigraphic units for maps of Figures 5, 6, Cucurpe Formation and designated the locality on the Kimmeridgian–Tithonian boundary. and 8.

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An ammonite discovered in a micritic lime- stone clast of a belemnite-bearing, micrite- ′ 110°50 W and volcanic-clast conglomerate bed in the 40 main study area was identifi ed as Idoceras cf. Pv 36 Kbc pCmg 60 I. densicostatum, of early Kimmeridgian age 39Kbu Pi (Villaseñor et al., 2005). This clast-supported Pv Nsv 55 pClsg pClsg bed lies ~760 m above the base of the Cucurpe 50 Formation. Approximately 10–20 m upsection, pCmg uncommon, lithologically similar, meter-scale Nsv micritic limestone blocks contain soft-sediment pClmg pCm folds (Mauel, 2008). Soft-sediment deforma- 40 tion indicates that the ammonite-bearing micrite 52 Jc 38 clast is intrabasinal and penecontemporaneous pClmg 45 with the coarse-grained deposits that contain it. 60 58 Geochronology of tuffs that bracket these beds Pv 52 57 55 supports this assertion. 38 56 70 75 Glance Conglomerate 16 12 ? 52 25 30°25 The primary exposures of the Glance Con-

N Kmu ′ 32 Kmo glomerate in the general study area are northeast ′ 21 N of Magdalena near Cerro Azul (Plate 2), where 20 30°25 45 its extent remains uncertain. The onlapping 61 21 21 ? and lithologically similar Upper Cretaceous 12 22 DM9-4-3 30 Cocospera Formation is also widely exposed 21 35 24 in the area and disagreement exists as to the 23 32 32 20 23 69 5-21-CM-02 Nsv identity of these two units (e.g., Nourse, 1995; 46 79 DM1-6-54 35 Terán-Ortega and Castro-Escárcega, 1999). 3-03-LL21 80 B 30 44 The Glance Conglomerate near Cerro Azul 56 20 38 40 44 contains more than 1000 m of interbedded vol- 16 88 54 70 ? 39 canic pebble to cobble conglomerate, volcanic Jc 56 48 60 A litharenite, mudstone, quartz feldspar tuff and ammonite 60 80 80 55 agglomerate, and feldspar porphyry (McKee 35 49 40 Nsv ? 60 31 and Anderson, 1998) with a general upward- 80 65 40 6 64 70 50 60 fi ning trend (Nourse, 1995). The clast popula- 68 80 56 86 8 65 22 tion of the Glance is dominated by quartzite, 52 40 56 43 Jsm? rhyolite, and quartz arenite (Nourse, 1995). ? 18 33 18 The Glance Conglomerate near Cerro Azul is 65 42 67 5243 15 overlain by the Morita Formation on a contact 32 50 variously interpreted as depositional (Nourse, 36 61 50 32 67 38 1995; Peryam, 2006) or structural (McKee and 82 Jsm 15 Pi DM1-6-8 Anderson, 1998). In the main study area, the DM1-6-10 Cucurpe Formation contains a basal conglomer- 80 69 Jeb 30°22 ate, ~2 m thick, composed of rounded pebbles, N 30 ″ cobbles, and boulders of volcanic litharenite ′ 30 30 ′ Pv and quartz arenite derived from the underlying ″

N Rancho San Martin Formation. The conglomer- 40 60 60 30°22 Jb 22 ate concordantly overlies andesite fl ow breccias 30 24 31 of the Rancho San Martin Formation. Our new N 30 Pzls 19 19 14 40 age constraints indicate that this thin conglom- erate is the probable Glance correlative there. ? 60 Previous workers have correlated a nearby base- 12 51 Jc? 80 2 kilometers Jc ment clast-bearing conglomerate to the Glance 54 36 41 19 45 58 (Nourse, 1995; Anderson and Nourse, 2005); 83 54 however, recent geochronology has shown that ′ ′ ″ 110°50 W 110°47 30 W unit to be Early Jurassic (i.e., the Basomari For- Figure 5. Geologic map of eastern fl ank of Sierra de Cucurpe near Rancho San Martin, mation; Leggett, 2009). Similar depositional ~13 km north-northwest of the village of Cucurpe (main study area). Adapted from Stephens contacts, although not well exposed, and lack- (1988). Explanation of symbols in Figure 4. ing a conspicuous basal conglomerate, are pres- ent in the southern part of the general study area. Near Rancho La Tesota, along Arroyo El Cajón,

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110°52′30″W 110°50′W 2 kilometers 31 N 24 35 Pv

43 30°08 N 30 ″ 45 ′ ′

26 Nsv 45

15 ″ 83 N 30°08 Jc Nsv 37 49 20 4-PIM-15 43 35 19 50 45 85 71 29 4-PIM-12 66 Jsm Klc Jsm? 70 76 55 83 79 70 Jc Nal 68 80 84 85 79 84 79 87 73 83 Jc Kmo 69 69 82 Pv 84 88 Klc 75 74 81 Kml Kmo 74

67 83 73 83 70 Kml 19 79 11 Kc Nsv Nsv 79

Kc 30°06 N ″ ′ 15 15 ′ Kc 61 ″ N 23 30°06 110′52′30″W 110°50′W

Figure 6. Geologic map of area near Rancho La Tesota in southern part of general study area. Adapted from Chepega (1987) and Palafox et al. (1992). Explanation of symbols in Figure 4. See Mauel (2008) for geochronologic sample details.

and at a locality ~14 km southeast of Cucurpe, (González-León et al., 2001). It is the lowest succession. Volcanic litharenite beds commonly Upper Jurassic shale concordantly overlies stratigraphic unit of the Bisbee Group in the have trough cross-bedding, horizontal laminae, interbedded andesitic fl ows and volcanic-clast general study area and is present only in south- and maroon siltstone intraclasts. Sandstone and conglomerates of the Rancho San Martin For- ern exposures, where it exhibits both faulted and lenticular channel conglomerate beds of fl uvial- mation, but the contact intervals are not exposed unconformable contacts with the Cucurpe For- deltaic origin are abundant in the lowermost (Figs. 1 and 5 and Plate 2). mation (Plate 2). Sandstone beds of the Rancho Morita Formation. Clast populations include La Colgada Formation commonly contain hori- rhyolite, andesite, quartzite, and uncommon Suprajacent Lower Cretaceous Strata zontal laminae and planar cross-beds (facies limestone. The Morita Formation fi nes upward association E; Tables 1 and 2) and were depos- and is conformably overlain by the Mural Strata younger than Upper Jurassic strata ited in foreshore and shoreface environments Limestone. A tuff in the uppermost Morita For- described here include the Rancho La Col- (Peryam, 2006). The Rancho La Colgada For- mation along Arroyo San Joaquin southwest gada, Morita, Mural, Cintura, and La Juana mation is gradationally overlain by the Morita of Rancho La Tesota (Fig. 6) has been dated formations. The Rancho La Colgada Formation Formation but warrants formational identifi ca- at 115.5 ± 0.7 Ma (U-Pb LA-MC-ICPMS; unconformably overlies the Cucurpe Formation tion due to its different bedding styles, color, Peryam, 2006). in the southern part of the general fi eld area, near and inferred depositional setting. The Rancho Rancho La Colgada (Plate 2 and Fig. 8), whereas La Colgada Formation has been assigned to the Mural Limestone the Morita Formation overlies the Cucurpe upper Barremian–lower Aptian on the basis of The Mural Limestone consists of ~800– Formation in the central part of the study area detrital zircon young grain ages (Peryam, 2006). 900 m of shallow-marine limestone and shale. near Rancho San Martin (Fig. 5) and the Glance Carbonate lithologies include oyster-bearing Conglomerate in the northern part of the general Morita Formation wackestone-packstone and rudistid boundstone study area (Plate 2). The Morita Formation is composed of 300– which represent discontinuous moundlike bio- 1100 m of cyclic fl uvial sequences containing herms. Siliciclastic intervals are dominated by Rancho La Colgada Formation pebble conglomerate, volcanic litharenite, and gray to black shale containing ammonoids and The Rancho La Colgada Formation consists siltstone. Maroon to greenish-gray siltstone trigonids. Six formal members of the Mural of less than 150 m of fi ne- to coarse-grained, with common ~1–2 cm calcareous pedogenic have been identifi ed in the Cucurpe-Tuape locally conglomeratic sandstone, siltstone, nodules and uncommon asymmetrical ripples region. In ascending order these members are and sandy bivalve-bearing limestone beds of facies association F (Table 2) dominates the (1) Cerro La Ceja, (2) Tuape Shale, (3) Los

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data-point error ellipses are 2-sigma data-point error boxes are 2-sigma Mean 206Pb/238U age = 3-03-LL-21 0.14 3-03-LL-21 Rancho San Martín 152 ± 3 Ma (2 sigma) MSWD = 0.35, n = 12 Mean = 152 ± 3 Ma MSWD = 0.35, (12 of 15) 0.12

Pb 0.10 206

Pb/ 0.08 207

0.06

180 170 160 150 140 130 120 0.04 34 38 42 46 50 54 170 160 150 140 130 120

data-point error ellipses are 2-sigma data-point error boxes are 2-sigma

206 238 0.068 5-21-CM-02 Mean Pb/ U age = 5-21-CM-02 Rancho San Martín 150 ± 1 Ma 0.064 (2 sigma) MSWD = 1.2, n = 21

0.060 Pb 0.056 206

Pb/ 0.052 207 190 180 0.048 170 160 150 140

0.044 Mean = 150 ± 1 Ma MSWD = 1.20, (21 of 24)

0.040 33 35 37 39 41 43 45 47 49 180 170 160 150 140 130

data-point error ellipses are 2-sigma 0.08 data-point error boxes are 2-sigma 5-22-CU-01 Mean 206Pb/238U age = 5-22-CU-01 Arroyo El Potrero 152 ± 2 Ma (2 sigma) MSWD = 1.19, n = 15 0.07 Pb

200 180 160 140

206 0.06 Pb/ 207 Mean = 152 ± 2 Ma 0.05 MSWD = 1.19, (15 of 19) 170 160 150 140

0.04 36 38 40 42 44 46 48 195 185 175 165 155 145 135 125 238U/206Pb 206Pb/238U Age (Ma)

Figure 7. U-Pb zircon geochronology of three tuff beds in the Cucurpe Formation. Sample 3–03-LL-21 is from the upper part of the formation and sample 5–21-CM-02 is from the uppermost part of the formation, both near Rancho San Martin. Sample 5–22-CU-01 is from Arroyo el Potrero, northwest of Rancho La Colgada.

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Coyotes, (4) Cerro La Puerta, (5) La Espina, and cally coherent, other workers have interpreted facies of the Mural as deep-marine deposits (6) Mesa Quemada (Lawton et al., 2004). Fos- Mural exposures near Cerro Azul and adjacent that form a matrix encasing biostromal blocks sils indicate a latest Aptian to mid-late Albian areas (Plate 2) as mass-gravity deposits shed shed from the Cananea high and small-scale age for the Mural Limestone, which is conform- from the Cananea high (Fig. 1) (e.g., McKee folds within the Lower Cretaceous section as ably overlain by the Cintura Formation. and Anderson, 1998; McKee et al., 2005; likewise indicative of mass-wasting processes. Although the Mural Limestone is inter- Rodríguez-Castañeda, 1997, 2002). The alloch- McKee and Anderson (1998) also concluded preted here as autochthonous and stratigraphi- thonous interpretation regards black shale that interbedded shallow-water oyster-bearing facies must be allochthonous on the basis of stratigraphic proximity to deep-marine facies. 110°42′30″W 110°40′W Alternatively, Lawton et al. (2004) considered 30 folds in the Lower Cretaceous section to be of 21 Pv 47 28 tectonic origin. Moreover, more recent strati- 58 52 45 Nal 34 graphic and sedimentologic studies indicate that the black shale intervals of the Mural Limestone 10 13 30 81 occupy consistent stratigraphic positions with 47 41 Pv respect to biostromal carbonate facies through- out north-central Sonora and that the shales Pv 5-22-CU-01 53 46 are marine shelf deposits, rather than deep- Jc 60 22 marine facies (Lawton et al., 2004; González- 31 70 León et al., 2008). The revised interpretations 75 56 65 30 of fold origin, stratigraphic consistency, and 70 50 depositional setting obviate the need for deep- 47 32 33 41 water emplacement of shallow-water facies by 55 39 18 3-03LC-18 mass wasting. 42 45 86 64 42 Cintura Formation

Nsv 49 The Cintura Formation ranges from greater 30 Nal 7 14 than 400 m thick in the northern part of the general study area (Bennett, 1993) to more than Pv 25 Nsv 30°12 2000 m near Tuape (Plate 2), where it was pre- N ″ 15 Pv Jc viously termed simply unit E (Chepega, 1987). ′ 30 30 ′ The Cintura Formation is dominated by maroon ″ Jc Jc N to gray siltstone, mudstone, and shale with com- 30°12 66 15 RLC mon calcareous nodules of pedogenic origin, Pv asymmetrical ripples, and horizontal laminae as 22 Klc well as subordinate discontinuous interbedded 47 17 Klc cross-bedded sandstone and pebbly conglomer- Pv 53 Klc ate beds interpreted as marine tidal fl at and fan- delta deposits, respectively. Calcareous nodules Kmo 53 eroded from the thick fi ne-grained intervals 46 dominate conglomeratic lags at the bases of 42 Kmo sandstone beds. Fossil wood and logs as long as 25 m have been reported within intercalated 26 sandstone-conglomerate intervals southeast of 42 Cerro Azul (Bennett, 1993). Nsv Kmu Nal Nsv 37 N 24 10 51 La Juana Formation 10 70 78 The La Juana Formation provisionally refers to a >270 m unit which conformably overlies and interfi ngers with the upper part of the Cin- 35 tura Formation just west of Rancho Santa Mar- 2 kilometers 51 46 garita (Plate 2). The La Juana Formation consists

Kc 30°10

N of thin- to medium-bedded sandy gray to pink- ′ 30 64 ish gray locally silicifi ed limestone, siltstone, Nsv ′ N 21 30°10 gray fi ssile shale, gray to brownish gray fi ne- to medium-grained sandstone, and conglomer- Kc 23 8 43 Nal 110°42′30″W 110°40′W ate lenses (>20 m in length). Clasts within the conglomerate are dominantly limestone and Figure 8. Geologic map of Rancho La Colgada and vicinity. Adapted from Chepega (1987) subordinate quartz arenite. Similar strata overly- and Villaseñor et al. (2005). RLC—Rancho La Colgada. Explanation of symbols in Figure 4. ing the Cintura Formation in the eastern part of

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TABLE 1. LITHOFACIES DESIGNATIONS AND INFERRED DEPOSITIONAL PROCESSES OF CUCURPE, RANCHO LA COLGADA, AND LOWER PART OF MORITA FORMATIONS Symbol Facies Sedimentary structures Depositional process Gct Clast-supported stratified gravel Trough cross-beds and pebble lags Minor fl uvial channel fi ll Gcm Clast-supprted gravel No evident grading, local weak planar clast fabric Plastic debris fl ow Gms Muddy matrix-supported gravel or Massive with no internal stratifi cation; dark gray Muddy slump/debris fl ow pebbly sandstone mudstone matrix; local intraclasts of Fsm and Fl Gmm Matrix-supported gravel or pebbly Weak grading, locally with weak planar clast fabric; Suspension settling(?) sandstone beds commonly amalgamated; local broad shallow scours and lenticular beds; mudstone intraclasts at bed bases Gmg Contorted or deformed beds of Inverse to nomal grading; mudstone intraclasts and Plastic debris fl ow sandstone/shale pebbles; sandy matrix; beds commonly amalgamated; local broad shallow scours at bed bases and lenticular beds Csb Sandstone, fi ne- to very coarse grained Contorted bedding or other soft-sediment deformation Slump Sm Sandstone, fi ne- to very coarse grained Massive or with faint lamination; local mudstone Sandy debris fl ow intraclasts Scg Grain-supported siltstone and fi ne- to Beds with normal grading Suspension settling very coarse grained sandstone locally pebbly Sr Sandstone, fine- to very coarse grained Ripple cross-lamination Lower fl ow regime traction current Shm Sandstone, fi ne- to very coarse grained, Horizontal lamination Upper fl ow regime traction current locally pebbly Shm Sandstone, quartzose, fi ne- to medium- Hummocky cross-stratification Storm wave depositio n grained Fsm Sandstone, siltstone, mudstone Massive beds with pedogenic calcareous nodules Fluvial overbank deposition and soil development Fr Sandstone, siltstone, mudstone Ripple cross-lamination Lower fl ow regime traction current Fl Sandstone, siltstone, mudstone Fine lamination Suspension settling Fsf Shale, mudstone Fissile shale and mudstone Suspension settling Note: Lithofacies symbols and descriptions adapted from Miall (1996).

TABLE 2. FACIES ASSOCIATION DESIGNATIONS AND INFERRED DEPOSITIONAL SETTINGS OF CUCURPE, RANCHO LA COLGADA, AND LOWER PART OF MORITA FORMATIONS Facies associations Symbol Bedding thicknesses Depositional process Dominantly Fsf with common interbeds of A Very thin to thin (1–20 cm) Pelagic/hemipelagic suspension settling Fl, Fr, and uncommon Scg Interbeds of Fl, Fr, Fsf (~80%) and less B Very thin to thin (mostly 1–20 cm; Low-density turbidity currents; deposition common Csb, Sm, Sh interbeds; sorting less commonly in mm-scale beds); by traction bottom current reworking variable; uncommon scoured bases; Bouma Tc-e common; Tb-e less bed contacts commonly parallel and common; Ta rare. less commonly undulating to form pinch and swell bed geometry; rare sole marks Dominantly Gmg, Gmm, Sh, Sm; beds C Medium to very thick (mostly Sandy debris fl ows or high-density commonly amalgamated; common 20–200 cm); Bouma Ta present turbidty currents (nonturbulent mudstone intraclasts (1–70 cm) with in normally graded beds lacking plastic fl ows) and turbidity currrents planar fabric; less common Fsf, Fl, Fr fl oating mudstone intraclasts. (suspension setting); amalgamation intervals or interbeds; common lenticular beds may represent progressive aggradation with scoured bases; rare load/fl ute casts during quasi-steady (hyperpycnal) fl ow Csb and/or Gms D Variable Debris fl ows, slumps, submarine slides, block fall Dominant St, Sh, and Shm interbedded with E Sandstone beds 10–30 cm thick, Storm wave deposition and lower to siltstone. Common lags of rounded 1–7 cm commonly in amalgamated bedsets upper fl ow regime traction currents, quartzite, rhyolite and chert pebbles; local pebbly 5 m thick. Siltstone beds a few cm probably as rip and longshore currents foresets. Oyster and other mollusc fragments to 2 m thick. Upward-coarsening in shoreface and nearshore settings common in lags. Shm bed tops burrowed. successions 10–15 m thick common. Gct and Fsm F Medium to very thick Channel fi ll and overbank deposits with paleosols of fl uvial coastal plain

the general study area (Plate 2), although lacking (Fig. 9) indicates that Late Jurassic marine pro- detrital zircon age group (n = 13; Fig. 10). conglomerate, contain the upper Albian oyster deltaic deposition was succeeded after a hiatus This age suggests a hiatus of at least 10 Ma Ceratostreon walkeri (R.W. Scott, 2001, written by Early Cretaceous fl uvial to marginal marine between the Rancho San Martin and Cucurpe commun.). deposition (Villaseñor et al., 2005; Peryam, formations. The maximum depositional age of 2006; Mauel, 2008). the upper part of the formation is 149 ± 1 Ma SEDIMENTOLOGY AND Both upper and lower contacts of the Cucurpe (sample DM1–6-54; Figs. 9 and 10), consistent STRATIGRAPHY OF THE Formation are unconformities. The basal con- with tuff U-Pb ages in the formation indicating CUCURPE FORMATION tact is overlain by a thin conglomerate layer. A ages of 150–152 Ma (Fig. 7). The concordant sample (DM1–6-8) collected 15 m above the upper contact with the Morita Formation north Stratigraphy of the Cucurpe Formation base of the formation (Figs. 5 and 9) yielded of Rancho San Martin is marked by an abrupt and lower part of the Morita Formation in the a maximum depositional age of 158 ± 1 Ma shift from marine prodeltaic deposits to fl uvial main study area and near Rancho La Colgada (2σ error) on the basis of its youngest coherent channel and overbank deposits. A detrital zircon

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Arroyo La Cumarosa and vicinity. Dominant Rancho La Colgada. Dominant Composite section A and B Figure 5 facies Figure 8 facies association association 139 ± 2 Ma (2-sigma) 30 km 150 ± 1 Ma (2-sigma) E 152 ± 3 Ma (2-sigma) 800 am (?) C Morita 1400 Formation F C Unconformity (?) D B 700 C Rancho B 1300 La Colgada am C Formation C B am 600 E B am C 1200 B(?) (?) D upper 500 B 149 ± 1 Ma (2-sigma) informal 1100 am C member B of Cucurpe Formation 400 lower am C Tithonian fauna am 1000 BC B 300 A am C 900 Kimmeridgian fauna ? undocumented near am C Rancho La Colgada 200 Figure 9. Measured strati- 800 am graphic sections and lithofacies C upper early Oxfordian associations of the Cucurpe Kimmeridgian C Base of upper member 100 fauna Formation in Arroyo La Cuma- ammonite A of Cucurpe Formation rosa near Rancho San Martin 700 C A and near Rancho La Colgada. C A lower 0 sh sltst cngl C informal vffmcss B member 600 am ? C of Cucurpe Section at Rancho La Colgada begins A Formation directly south of Arroyo La Angostura; C thick succession of shale with isoclinal folds underlies measured section 500

Symbols fossilized wood chaotic or convolute beds 400 belemnite ripple cross-stratification ammonite horizontal laminae floating shale or am bed amalgamation mudstone intraclast flame structure oblate concretions sole mark 300 plant fragments evidence of faulting A mollusc fragments lava flow oyster fragments sill cngl conglomerate sltst siltstone 200 ss sandstone sh shale

152 ± 3 Ma (2-sigma) U-Pb age (SHRIMP) on tuff 139 ± 2 Ma (2-sigma) Maximum depositional age from DZ U-Pb age (LA-ICPMS) 100

Basal clast-supported pebble-boulder conglomerate bed of the Cucurpe Formation contains volcanic lithic and quartz 158 ± 1 Ma (2-sigma) arenite clasts derived from Rancho San Martin Formation 0 Unconformity sh sltst cngl

vffmcss Rancho San Martin Formation

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175 near Rancho La Colgada were divided into litho- a pseudo-matrix around more competent grain lower Morita Fm (DM9-4-3) 165 facies adapted from the fl uvial lithofacies of Miall types such as quartz and feldspar. This altera- (1996) modifi ed to better suit prodeltaic deposits tion and grain deformation commonly make 155 and lithofacies associations modifi ed from Ricci- volcanic grain type and grain boundaries dif- 145 Lucchi (1975) and Shanmugam (1997) based on fi cult to distinguish. Volcanic lithic fragments 135 bedding characteristics, sedimentary structures, are assigned to fi ve groups: (1) felsitic (Lvf); lithology, and stacking patterns (Table 1; Mauel, (2) lathwork (Lvl); (3) vitric (Lvv); (4) micro- Age (Ma) 125 weighted mean = 139 ± 2 Ma 2008). Lithofacies were grouped into associa- litic (Lvm); and (5) hypabyssal and plutonic 115 tions inferred to represent (1) pelagic and distal (Lvp). Felsitic grains are the most common of box heights are 2-sigma 105 MSWD = 1.8, n = 9 prodeltaic deposits, (2) interchannel prodeltaic the volcanic lithic grain types. Felsitic grains overbank/levee deposits, (3) prodelta channel generally exhibit a tuffaceous, uncommonly 95 deposits, (4) mass wasting/slump/slide deposits, eutaxitic, character altered to a microcrystal- 210 (5) shallow-marine deposits, (6) fl uvial channel line feldspar groundmass that accepts sodium upper Cucurpe Fm (DM1-6-54) and overbank deposits, and (7) alluvial deposits cobaltinitrate stain. Lathwork grains are the 190 (Table 2). second most common lithic volcanic grain type. Plagioclase laths within these are both 170 SANDSTONE PETROLOGY parallel and randomly oriented and typically 150 set within a cryptocrystalline groundmass. Point counts were performed on samples of Microlitic grains contain subhedral to euhedral,

Age (Ma) 130 the Cucurpe, Rancho La Colgada, and Morita commonly blocky feldspar microlites in a weighted mean = formations within the main study area and felted, trachytic, or hyalopilitic texture. Vitric 110 149 ± 1 Ma near Ranchos La Colgada, San Martin, and La volcanic grains consist of glass or altered glass box heights are 2-sigma MSWD = 0.75, n = 59 Tesota to determine sandstone compositions and and locally contain feldspar microlites. Hyp- 90 identify stratigraphic petrographic variation, or abyssal and plutonic igneous rock fragments petrofacies. include microgranular subhedral to anhedral weighted mean = 190 158 ± 1 Ma intergrowths of feldspar crystals plus or minus Monocrystalline Grain Types quartz and granitic grains with myrmekitic or 180 box heights are 2-sigma MSWD = 1.5, n = 13 graphic textures. 170 Monocrystalline framework grains are domi- Sedimentary lithic grains (Ls) and metamor- nated by quartz and feldspar. Monocrystalline phic lithic grains (Lm) are less abundant than 160 quartz grains (Qm) include grains with undu- volcanic lithic grains. Sedimentary lithic grains Age (Ma) lose and straight extinction. Quartz grains with include siltstone, very fi ne-grained sandstone, 150 undulose extinction commonly contain vacuole and shale clasts. Chert (Qp) was counted in a 140 trains and Boehm lamellae indicative of plu- separate category and was distinguished from lower Cucurpe Fm (DM1-6-8) tonic quartz (Folk, 1974). Monocrystalline fi nely crystalline volcanic lithic grains on the 130 quartz grains with straight extinction and rare basis of a clear appearance in plane polarized embayments indicate a volcanic quartz compo- light. Metamorphic lithic grains (Lm) constitute Figure 10. Weighted means of youngest nent of varying abundance. Common fractured the least common polycrystalline grain type and coherent detrital zircon age populations in Qm grains likely indicate deep burial. Syntaxial include grains with a tectonite fabric and meta- the basal Cucurpe Formation (DM1–6-8), quartz overgrowths are common in the Rancho quartzite grains of polycrystalline quartz exhib- upper Cucurpe Formation (DM1–6-54), and San Martin, La Colgada, and Morita formations iting strongly sutured boundaries. basal Morita Formation (DM9–4-3) near but are uncommon in the Cucurpe Formation. Rancho San Martin. Number of grains per Monocrystalline feldspar grains are dominated Diagenesis fi gure indicated by n. by unzoned plagioclase exhibiting albite and Carlsbad twinning. These grains commonly Sandstones examined in this study exhibit show partial to complete replacement by cal- characteristics indicative of metasomatism and sample (DM9–4-3) collected from a basal con- cite and fi nely crystalline white mica. Feldspars a discernable cement paragenesis. Cement stra- glomeratic sandstone in the Morita Formation are typically blocky with local authigenic over- tigraphy indicates a typical paragenetic sequence (Figs. 5 and 9) yielded a young coherent age growths. The presence of potassium feldspar of (1) zeolite interstitial cement and replacement group (n = 9; Fig. 10) with a weighted mean age grains is evident from both staining and visible of volcanic lithic grains, (2) calcite cement and of 139 ± 2 Ma (2σ error; Neocomian), indicat- tartan plaid twinning indicative of microcline. partial replacement of feldspars, and (3) iron ing a hiatus of ca. 10 Ma at the contact. Near oxide cement. The extent of each of these dia- Rancho La Colgada, the Cucurpe and overlying Polycrystalline Grain Types genetic processes varies between samples but the Rancho La Colgada formations are separated by sequence is consistent. Evidence of albitization a low-angle angular unconformity (Villaseñor Polycrystalline framework components of feldspar grains includes chessboard twinning et al., 2005). There, the Cucurpe Formation also include volcanic, sedimentary, and metamor- (e.g., Walker, 1984) and patchy cobaltinitrate displays greater deformation than the overlying phic lithic grains. Of these, volcanic grains are staining of relict potassium feldspar. Sodium ions Bisbee Group strata. by far the most abundant. Volcanic lithic grains for the albitization process were likely derived Strata of the Cucurpe Formation and lower- (Lv) are typically partially altered to zeolite(?), from the breakdown of unstable volcanic lithic most Bisbee Group in the main study area and white mica, and calcite and commonly exist as grains. Albitization of grains likely infl uenced

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relative plagioclase and potassium feldspar arenites and volcanic litharenites. Sandstones to material generated by volcanism (subaqueous abundances and resulting position on QmPK from lower Bisbee Group strata are composi- or subaerial) coeval with sedimentation (Critelli sandstone provenance ternary plots; therefore an tionally more mature than the Cucurpe Forma- and Ingersoll, 1995). The quartzofeldspatho- interpretive emphasis on plagioclase-potassium tion and include subarkoses, lithic arkoses, feld- lithic petrofacies, represented by sandstones feldspar ratios was avoided. spathic litharenites, and volcanic litharenites. of the lower Bisbee Group, fall mainly within recycled orogen fi elds of the QmFLt and QtFLt Sandstone Composition Petrofacies provenance ternary plots (Fig. 11). Three general petrofacies are defi ned here The paleovolcanic petrofacies includes the Sandstones range from compositional sub- from point count data. The paleovolcanic fi ve lowest sandstone samples from the Cucurpe arkose through lithic arkose and feldspathic and neovolcanic petrofacies, restricted to the Formation and two samples from the Morita For- litharenite to sublitharenite. Sandstones of the Cucurpe Formation, fall dominantly within mation. These feldspathic volcanic litharenites Rancho San Martin Formation, specifically magmatic arc fi elds of the QmFLt and QtFLt and volcanic litharenites have total quartz (Qt) because of their eolian origin, are atypical of this provenance ternary plots (Fig. 11), but differ contents of 9%–49%. Quartz framework grains typically more volcanic lithic-rich unit. Cucurpe in quartz content and general character of vol- of this petrofacies are commonly well rounded Formation sandstones are feldspathic lith- canic lithic grains. The term neovolcanic refers and lack quartz overgrowths. This petrofacies

Qt Qm

Recycled orogen quartzofeldspatholithic provenances petrofacies

Continental block provenances quartzofeldspatholithic Recycled orogen petrofacies provenances

basal Cucurpe basal Cucurpe (DM1-6-8) Continental block (DM1-6-8) provenances paleovolcanic petrofacies neovolcanic Magmatic arc Magmatic arc petrofacies provenances neovolcanic provenances petrofacies paleovolcanic petrofacies basal Morita basal Morita (DM9-4-3) (DM9-4-3) F L F Lt Morita Fm Qp Qm Rancho La Colgada Fm basal Cucurpe (DM1-6-8) upper Cucurpe Fm lower Cucurpe Fm Rancho San Martin Fm quartzofeldspatholithic (eolian units) petrofacies petrofacies fields

Recycled orogen provenances paleovolcanic M petrofacies ix Increasing maturity ed or stability from continental block provenances

neovolcanic petrofacies Increasing ratio of plutonic/volcanic components in magmatic arc provenances Lvm Ls P K basal Morita Magmatic arc (DM9-4-3) provenances

Figure 11. Compositional ternary plots of selected Mesozoic sandstones. Provenance fi elds after Dickinson and Suczek (1979).

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has a greater variety of volcanic lithic frame- peak at 158 ± 1 Ma (2σ error) of 13 zircons Jurassic arc assemblages. Both samples have work grains than the neovolcanic petrofacies (Fig. 10) is interpreted as the maximum deposi- zircon age populations of ca. 2750–2650 Ma, with some examples containing a greater pro- tional age of the sample. ca. 1850–1550 Ma, ca. 1400–1000 Ma, ca. 650– portion of lathwork to felsite framework grains. 550 Ma, ca. 450–400 Ma, and ca. 190–165 Ma The stratigraphically lowest sample of the Upper Cucurpe Formation (Fig. 12), indicating that the majority of Paleo- Cucurpe Formation falls in the recycled orogen zoic and Proterozoic zircons in the lower part fi elds of the QmFLt and QtFLt provenance ter- Sample DM1–6-54 was collected from a of the Cucurpe Formation were derived from nary plots (Fig. 11) due to its greater abundance succession of amalgamated tuffaceous pebbly eolianites interstratifi ed in the Middle Juras- of well-rounded quartz grains lacking quartz sandstone beds of the neovolcanic petrofacies in sic arc assemblages. Late Oxfordian recycling overgrowths. the upper part of the Cucurpe Formation com- of these sands probably occurred by erosion of The neovolcanic petrofacies is restricted to posed primarily of angular to subangular felsite arc rocks exposed along the fl anks of the Altar- the upper Cucurpe Formation. This petrofacies and lesser plagioclase grains as well as sub- Cucurpe Basin. The age range of these eroded arc includes feldspathic volcanic litharenites and rounded dark gray shale intraclasts. Uncommon sequences is ca. 190–165 Ma, indicated by the volcanic litharenites which have extremely low bubble-wall shards are visible in thin section. The prominent zircon population in the basal Cucurpe quartz abundances ranging from less than 1% sample contains a unimodal coherent age group Formation (Fig. 12). Paleozoic and Proterozoic to 3%. Felsite volcanic framework grains that of 59 zircon grains with a weighted average age zircon populations within Jurassic erg deposits typically exhibit greater size and angularity than of 149 ± 1 Ma (early Tithonian), statistically of the Colorado Plateau have been interpreted the subordinate monocrystalline feldspar and indistinguishable from tuff ages higher in the sec- as derived from Appalachian and other distant quartz grains overwhelmingly dominate these tion (Figs. 7, 9, and 10); accordingly, we interpret Laurentian sources and transported westward sandstones. Relict bubble wall shards are visible this as the depositional age of the sample. by trans conti nental river systems (Dickinson within the matrix of less altered samples. and Gehrels, 2009a). Subsequently, these grains The quartzofeldspatholithic petrofacies is Basal Morita Formation were transported southwest into the Middle generally restricted to the lower Bisbee Group. Jurassic arc by prevailing winds (Bilodeau These subarkoses, volcanic lithic arkoses, and A sandstone (DM9–4-3) from the base of and Keith, 1986). Local Meso protero zoic and feldspathic litharenites exhibit greater com- the Morita Formation, collected within 10 m Paleoproterozoic basement (e.g., Iriondo et al., positional maturity and more diverse volcanic of a well-exposed Morita-Cucurpe contact in 2004; Anderson and Silver, 2005) potentially and sedimentary lithic grains than other petro- the main study area near Rancho San Martin exposed along rift shoulders may have served as facies defi ned here. Although albitization of (Fig. 3), is a coarse-grained lithic arkose, rich an additional source; for example a ca. 1.4 Ga framework grains is common in the Cucurpe in volcanic lithic fragments. The sampled bed zircon population varies somewhat in abundance Formation, the quartzofeldspatholithic petro- is intercalated within pebble conglomerate con- between samples (Fig. 12). facies appears to contain a greater abundance taining pebble lags of fl uvial origin. The zircon Detrital zircon data from the upper member of potassium feldspar grains, some of which ages in DM9–4-3 range from latest Archean to of the Cucurpe Formation, consisting of a single exhibit tartan twinning. Uncommon graphic Early Cretaceous (Neocomian) with age groups age group of 59 grains near 149 Ma, corrobo- granite grains are also present in sandstones of at ca. 1780–1701 Ma, ca. 1184–1167 Ma, rate the interpretation of syneruptive detritus in this petrofacies. ca. 262–231 Ma, ca. 222–207 Ma, and ca. 194– the neovolcanic petrofacies (Figs. 10 and 12). 137 Ma (Fig. 12). Neovolcanic detritus was likely introduced by DETRITAL ZIRCON ANALYSES contemporary volcanism related to emplacement DISCUSSION of the Ko Vaya volcanic suite (e.g., Haxel et al., New data constrain the depositional age and 2008). Massive infl ux of Kimmeridgian neo- reveal the provenance of Jurassic and earli- Petrography, U-Pb zircon geochronology, and volcanic detritus into the Altar-Cucurpe Basin est Cretaceous strata in the general study area. revised Jurassic–Early Cretaceous stratigraphy corresponded temporally with an increased These data include U-Pb SHRIMP ages on tuffs of the Cucurpe-Tuape region provide insights abundance of ash-fall tuffs in correlative upper and LA-MC-ICPMS U-Pb detrital zircon ages. into the Mesozoic tectonic regime of southwest- Kimmeridgian strata of the Morrison Formation ern Laurentia. Petrographic and U-Pb detrital (Turner and Peterson, 2004). This coincidence Basal Cucurpe Formation zircon data from the Cucurpe Formation and potentially resulted from shifting paleowind lower Bisbee Group strata provide a record of patterns as North America drifted northward The stratigraphically lowest geochronologic nearby arc activity, rift volcanism, and basement from the trade winds (late Middle Jurassic– sample (DM1–6-8) from the Cucurpe Forma- uplift. Our stratigraphic revision indicates that Oxfordian? ) into the westerly winds (Peterson, tion in the main study area is a fi ne-grained sub- many sedimentary units in the region previously 1988). Northward drift of North America thus angular tuffaceous subquartzose volcanic lithic assigned to the Late Jurassic are of other ages. appears to have been coeval with a late Kimmer- sandstone of the quartzofeldspatholithic petro- idgian increase in rift-related volcanic activity. facies collected ~15 m above the base of the Sediment Provenance The quartzofeldspatholithic petrofacies of the formation. Proterozoic zircon age populations lowest Bisbee group strata was derived from a are represented at ca. 2673, ca. 1808–1544, Integration of petrographic and detrital zircon mixture of recycled sedimentary, extinct mag- 1332–1024, and 811–574 Ma with a prominent data provides a robust means of revealing sedi- matic arc, and basement sources indicated by peak at ca. 600 Ma. Phanerozoic age groups mentary provenance. Comparison of detrital zir- lower Morita Formation zircon populations are at ca. 441–412 Ma (–earliest Devo- con data from the lowermost Cucurpe Formation (Fig. 12). The lower Morita contrasts with nian), 344–256 Ma (Mississippian–Permian), and a quartz arenite from the Rancho San Martin Jurassic samples in lacking Neoproterozoic and ca. 187–157 Ma (Early to Late Jurassic) Formation (Leggett, 2009) indicates derivation “pan-African” ages and containing a broader (Fig. 12). The sample’s youngest coherent age of the paleovolcanic petrofacies from Middle array of Triassic–Early Cretaceous arc-derived

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A B 10 Guerrero “Grenville” Cordilleran-Nazas arc lower Morita Fm (DM9-4-3) 12 8 “Yavapai- n = 38 Mazatzal” 10 East Mexican arc 8 1.4 Guerrero Accretion 6 Ga 6 lower Morita Fm (DM9-4-3) 4 4

Number “Mojave” Number n = 62 2 “Pan- 2 African” 0 100 150 200 250 300 350 400 450 500 550 0 500 1000 1500 2000 2500 3000 3500 4000 30 Cordilleran-Nazas arc 6 25 upper Cucurpe Fm (DM1-6-54) 4 n = 2 20 upper Cucurpe Fm (DM1-6-54) 2 n = 62 Number 15 Number 0 500 1000 1500 2000 2500 3000 3500 4000 10

5 10 East Mexican arc “Pan- African” lower Cucurpe Fm (DM-1-6-8) 8 0 “Grenville” n = 59 100 150 200 250 300 350 400 450 500 550 6 12 “Yavapai- Cordilleran-Nazas arc Mazatzal”

Number 4 10

8 Laurentian lower Cucurpe Fm (DM-1-6-8) 2 “Mojave” craton 6 n = 34

4 0

Number “Gondwanan” arc East 500 1000 1500 2000 2500 3000 3500 4000 2 Mexican arc 0 10 100 150 200 250 300 350 400 450 500 550 “Grenville” Rancho San Martin Fm (Leggett et al., 2009) 8 12 “Yavapai- n = 72 Rancho San Martin Fm Mazatzal” “Mojave” 10 quartz arenite 6 1.4 Ga (Leggett et al., 2009) plutons Laurentian 8 craton n = 27 Number 4 6 “Pan- Cordilleran-Nazas arc African” 4 “Gondwanan” arc Number East 2 2 Mexican arc 0 0 100 150 200 250 300 350 400 450 500 550 500 1000 1500 2000 2500 3000 3500 4000 Age (Ma) Age (Ma)

Figure 12. Detrital zircon probability distribution spectra and histograms of a Rancho San Martin Formation quartz arenite (Leggett, 2009), two samples of the Cucurpe Formation, and a sandstone of the lower Morita Formation. (A) Spectra for Phanerozoic grain ages (<550 Ma). (B) Spectra for grain ages >500 Ma. Number of grains per fi gure indicated by n.

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grains. This is interpreted as a result of dilu- relatives. As discussed, some workers propose against or stratigraphically underlies the Morita tion by new, increasingly infl uential sediment Glance deposits represent distinct subbasins Formation there. Although Jurassic correlations sources in the Early Cretaceous indicated by zir- within the hypothetical MSM strike-slip system. remain problematic between the Cucurpe and con populations at ca. 262–190 Ma and ca. 144– However, the marine deposits of the Cucurpe Altar regions and the Glance Conglomerate has 132 Ma (Fig. 12). Proterozoic zircon popula- Formation and its correlatives must also be been mapped on the northeastern fl ank of the tions at ca. 1780–1701 Ma, ca. 1450–1400 Ma, considered in any paleogeographic and tectonic Sierra El Chanate (Jacques-Ayala, 1992), we and ca. 1296–1015 Ma were likely recycled in reconstructions. consider these fi ner-grained strata to represent part from local sedimentary rocks and basement Upper Jurassic deposits of Sonora defi ne a the former northwestern extension of the Altar- rocks. The lack of “pan-African” and Paleo- marine trough known as the Arivechi-Cucurpe Cucurpe Basin. The northeastern limit of the zoic grains indicates that these zircons were not seaway (Haenggi and Muehlberger, 2005), Altar-Cucurpe Basin in the general study area is derived from distant Appalachian sources (e.g., which includes the Altar-Cucurpe Basin. This located roughly along the present-day San Anto- Dickinson and Gehrels, 2009a); furthermore, narrow embayment formed as a result of Late nio fault/Imuris lineament (Plate 2), where allu- uncommon micrographic granite grains and Jurassic rifting along the former trend of the vial deposits of the Glance Conglomerate appear microcline suggest a local basement source for Middle Jurassic continental arc. The basal allu- to grade laterally and abruptly into marine turbi- some of these grains. Micrographic 1.1 Ga gran- vial conglomerate of the Cucurpe Formation dite and debris-fl ow deposits of the Cucurpe For- ites have been documented in the main study directly overlain by distal slope to prodeltaic mation. The role of the San Antonio fault/Imuris area and in the Caborca block (Anderson and deposits that grade in turn to channel and inter- lineament in Mesozoic tectonics is also evident Silver, 2005; Amato et al., 2009). channel slope overbank/levee deposits record by its northern projection which also crudely Paleozoic–Early Jurassic and Early Creta- rapid early rift subsidence and marine transgres- defi nes the northeastern edge of the Papago ceous zircon populations in the basal Morita sion followed by local progradation of marine domain, the boundary between the Bisbee core Formation indicate a change in provenance prodelta deposits. The upper part of the Cucurpe and fl ank basins, which is defi ned by Glance relative to that of Jurassic depositional systems. Formation (Kimmeridgian–Tithonian) was Conglomerate thickness trends and depositional Early Mesozoic (ca. 230–190 Ma) zircons are greatly infl uenced by coeval volcanism. During character, and the apparent northeastern limit rare to absent in Jurassic samples. These Trias- its deposition, ash fall tuffs likely blanketed the of the Ko Vaya Suite (Haxel et al., 2008). The sic–Lower Jurassic grains were derived from land surface and water-lain tuffs were deposited southwestern limit of the Altar-Cucurpe Basin magmatic rocks of the early Cordilleran-Nazas and reworked within the basin. Erosion, rainfall, lies somewhere near the El Pimiento and Santa arc and/or recycled forearc deposits (e.g., and prolonged fl ooding events within the basin Margarita fault zones (Plate 2), the southern extent Gonzalez-Leon et al., 2005, 2009) exposed catchment likely choked river systems feeding of Cucurpe Formation outcrops. Thus the basin on the Caborca block. Early Permian–Triassic the Altar-Cucurpe basin with eruptive material appears to overlap the boundary of the Mazatzal (ca. 285–230 Ma) zircon grains were derived which resulted in deposition of sediment-laden and Caborcan basement provinces (Plate 1), from East Mexican arc rocks (Sedlock et al., neovolcanic hyperpycnal fl ows. which may lie as much as 50 km north of the 1993; Torres et al., 1999), Upper Permian to Distribution of inferred Upper Jurassic proposed trace of the Mojave Sonora megashear Lower Triassic plutons in southern California marine strata correlative to the Cucurpe For- (Amato et al., 2009). and (Walker, 1988; Miller and Glazner, mation approximates the former extent of the Near the top of a section interpreted as correl- 1995; Barth et al., 1997; Stewart et al., 1997), Altar-Cucurpe Basin. West of the general study ative with basal member 1 of Harding and Coney local Permian plutons of the Caborca block and south of the trace of the MSM near Cerro (1985) of the McCoy Mountains Formation in (Iriondo and Arvizu, 2009; Arvizu et al., 2009), el Dieciséis, strata lithologically similar to the the of southwestern Ari- and/or recycled Triassic–Lower Jurassic forearc Cucurpe Formation have been assigned a Late zona (Plate 1), an andesite fl ow has yielded an age deposits exposed on the Caborca block (Gehrels Jurassic age (Plate 1; González-Gallegos, 2006). of 154.4 ± 2.1 Ma (U-Pb zircon; Spencer et al., and Stewart, 1998; González-León et al., 2005). Northwest of this location near Altar, the locally 2005). It remains unclear how these continen- No nearby Early Cretaceous (ca. 144–132 Ma) metamorphosed, dominantly fi ne-grained sedi- tal deposits relate to the Altar-Cucurpe marine ages have yet been reported for the Cordilleran- ments and uncommon micrite of the Upper Altar depocenter, but detailed sedimentology of inter- Nazas continental arc; however, the offshore Formation (Los Corrales and Bateyera mem- calated sedimentary strata of the Ko Vaya Suite, Guerrero arc was active during this period (Busby bers) are exposed along a northwestern trend also known as the “Artesa sequence,” has the et al., 1998). Air fall tuffs could have been depos- from Cerros El Amol to Sierra El Batamote potential to shed light on any inter connectivity ited on the continent and subsequently transported (Plate 1; García y Barragán et al., 1998; Nourse, between these deposystems. Common “lami- into the basin. However, andesite clasts from a 2001). Detrital zircon geochronology near Sierra nated and well-bedded immature volcanic grey- fl uvial conglomerate bed in the lower Morita For- El Batomote has indicated that many conglom- wacke” in this unit (Tosdal et al., 1989) may mation have produced an average age of ca. 140 ± eratic intervals previously assigned to the Lower represent further extension of the marine basin 3 Ma (Peryam, 2006). These clasts were derived Altar Formation (Nourse, 2001) are more likely or transitional marginal marine facies. from either an undocumented Early Cretaceous Late Cretaceous rather that Late Jurassic in age volcanic suite or a segment of the Guerrero ter- (Jacques-Ayala et al., 2009). However, along Infl uence of Revised Correlations rane accreted to North America by Aptian time. Arroyo El Charro near Sierra El Batamote, a on Distribution of Upper Jurassic siliceous tuff (?) collected from a sedimentary Conglomerate and Slide Blocks, and Their Basin Type, Extent, and Signifi cance succession previously mapped as upper Altar Relation to the Mojave-Sonora Megashear Formation has produced an age of 164 ± 1 Ma Previous studies of Late Jurassic basins (U-Pb SHRIMP; Mauel, 2008). This succes- Mesozoic conglomerates of several ages are and paleogeography in the U.S.-Mexico bor- sion lithologically resembles well-bedded and present in north-central Sonora. They tend to be der region focused on continental alluvial fan fi ne-grained marine intervals of the Cucurpe compositionally similar due to the dominance deposits of the Glance conglomerate and its cor- Formation. It either is structurally juxtaposed of arc source rocks. Our new geochronology

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and revised regional stratigraphy (Figs. 2 and as discussed, the Basomari Formation is Early Cretaceous in age. Reinterpretation of the stratig- 3) indicate that several conglomerates, slide Jurassic in age (Leggett, 2009) and therefore raphy in this area indicates that these units rep- blocks, and olistoliths previously interpreted as was not deposited in a Late Jurassic pull-apart resent the Cucurpe, Rancho San Martin, Morita, deposits of Late Jurassic age (Chepega, 1987; basin. The Morita Formation, which contains a and Mural formations, respectively (Mauel, Stephens, 1988) are in fact of different ages. lower fl uvial conglomeratic interval, is no older 2008). Chepega interpreted unit B as an olistolith These deposits have been used in support of than 139 ± 2 Ma (2σ error) based on geochonol- that slid into accumulating Cucurpe sediments Late Jurassic pull-apart basin models (e.g., ogy presented here. These observations indicate and was subsequently buried by continuing Anderson and Nourse, 2005). that the ~2-m-thick alluvial conglomerate bed Cucurpe sedimentation. Although faulted, these In the main study area, near Rancho San directly underlying the marine shale deposits strata share common bedding orientations, are Martin, the Basomari and Morita formations of the Cucurpe Formation is the only remaining generally in correct stratigraphic order, and do were formerly assigned to the La Cumarosa Epi- candidate for the Glance Conglomerate at this not require a slide block interpretation to explain clastic sequence of inferred Late Jurassic–Early location on the basis of age, apparent alluvial their genesis. The only Upper Jurassic strata Cretaceous age (Stephens, 1988). The Basomari origin, and stratigraphic position. at this location are turbiditic marine prodeltaic Formation contains common Proterozoic base- In the Tuape quadrangle, near Rancho La deposits of the Cucurpe Formation. ment clasts and a possible Paleozoic olistolith Tesota, Chepega (1987) tentatively interpreted These examples illustrate the importance measuring over 1000 m in length. However, each of his units A, B, C, and D as Jurassic–Early of having proper age constraints on Mesozoic

40°N 100°W 120°W N 110°W 90°W Morrison intracratonic basin 0 500 km

d 40°N in w o le a p

d in ow pale North American Plate 30°N

Mogollon Highlands Rift Shoulder

? McCoy basin d C n h ih ? wi u * o a le h * L a Cananea High u ** p a Trough a te ? J Aldama u ? Platform ra ? La Mula Island 30°N ss AC Guerrero Arc Terrane *ic a S *rc? Sabina paleolatitude ? ? Mar Coahuila s Mexicano Island Basin 30°N Caborca block Nazas volcani co ? mplex Farallon c Plate Mezcalera Plate 120°W 110°W 20°N ? 100°W

Non-marine Magmatic arc Basement cored Ko Vaya rift sedimentary basin rift shoulder related volcanic Marine sedimentary High standing Continental center basin basement block lithosphere

Figure 13. Interpreted paleogeography of southwestern North America during the Late Jurassic. ACS—Arivechi-Cucurpe seaway. Yellow diamonds represent Upper Jurassic marine shale exposures after Lyons (2008). Asterisks indicate location of Upper Jurassic plutons after Stewart et al. (1986), Sedlock (2003), and Barth et al. (2008). Paleolatitude and paleowind directions after Peterson (1988). Adapted from Dickinson and Lawton (2001b).

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conglomerates in the region prior to interpreting is divided into informal lower and upper mem- Lawton and Jeffrey Amato. Student research grants, dominant basin-forming mechanisms. Restric- bers. The lower member is ~755 m thick and the awarded in 2003 to the fi rst author by the Geologi- tion of Upper Jurassic conglomerate to the more upper member is ~715 m thick ~13 km north- cal Society of America Research Grants Program and the American Association of Petroleum Geologists areally restricted exposures of Glance Con- west of Cucurpe in north-central Sonora. The Cucurpe Formation unconformably Foundation, also provided funding for this research. glomerate casts doubt on the widespread distri- George Gehrels and others at the University of Ari- bution of pull-apart basins, and thus on a causal overlies the Middle Jurassic (ca. 170 –165 Ma) zona LaserChron Center, suported by NSF Grant link to the MSM. Rancho San Martin Formation. New U-Pb zir- EAR-0443387, assisted in detrital zircon geochronol- ogy and Joe Wooden at Stanford University facilitated We infer alternatively that Late Jurassic crustal con geochronology on tuffs and sandstones indi- cate that the Cucurpe Formation was deposited SHRIMP analyses. Discussions with Tom Peryam extension and basin development recorded by and William Leggett provided valuable details for the Ko Vaya volcanic suite and Cucurpe Forma- between ca. 158 and 149 Ma (early Oxfordian– interpretation. This manuscript benefi ted from the tion were not a result of transtensional tectonics early Tithonian). These data corroborate pre- insightful and constructive comments of Bill Dickin- associated with a transcurrent fault system, viously reported biostratigraphic ages from son and two anonymous reviewers. Rancho La Colgada, and extend the base of the but rather resulted from changing dynamics REFERENCES CITED of the subduction zone along the southwestern Cucurpe Formation to early Oxfordian age. margin of Laurentia (Fig. 13). Although the New detrital zircon and sandstone petrography Almazán-Vázquez, E., and Palafox-Reyes, J.J., 2000, exotic nature of the offshore Guerrero terrane indicate that the Cucurpe Formation contains Secuencia samítica del Jurásico Tardío con amoni- two petrofacies, termed paleovolcanic and neo- tas del género Subdichotomoceras (Kimmeridgiano) remains debated, its accretion in the region of expuesta al oriente de Arivechi, Sonora, México: NW Mexico in the interval ca. 147–130 Ma is volcanic, determined by the relative abundance Cuarta reunión sobre la geología del noroeste de indicated by the unconformity between strongly of syneruptive volcanic detritus in the formation. México y areas adyacentes: Universidad Nacional Sediment sources for the paleovolcanic petro- Autónoma de México, Instituto de Geología, Estación folded Upper Jurassic strata and less deformed Regional del Noroeste, Libro de Resúmenes, Publica- strata of the La Colgada and Morita formations facies, which dominates the lower member of the ciones Ocasionales, p. 1–2. Cucurpe Formation, were Middle Jurassic vol- Amato, J.M., Lawton, T.F., Mauel, D.J., Leggett, W.J., that unconformably overlie the Jurassic. 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Southward younging of arc initiation along a truncated continental margin: Tectonics, v. 16, p. 290–304, doi: coarsening succession of prodeltaic marine ACKNOWLEDGMENTS 10.1029/96TC03596. slope debris-fl ow and turbidity-current deposits Barth, A.P., Wooden, J.L., Jacobson, C.E., and Probst, K., The primary source of funding for fi eld and analyti- 2004, U-Pb geochronology and geochemistry of the that contain abundant neovolcanic detritus and cal expenses was from U.S. National Science Foun- McCoy Mountains Formation, southeastern Califor- interbedded tuffs in its upper part. The formation dation Grant EAR-02-29565 awarded to Timothy nia: A Cretaceous retroarc foreland basin: Geological

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