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Downloaded from gsabulletin.gsapubs.org on April 5, 2011 Geological Society of America Bulletin Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico Fred W. McDowell, Jaime Roldán-Quintana and Ricardo Amaya-Martínez Geological Society of America Bulletin 1997;109;1349-1360 doi: 10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes Geological Society of America Downloaded from gsabulletin.gsapubs.org on April 5, 2011 Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico Fred W. McDowell* Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Jaime Rold‡n-Quintana Instituto de Geolog’a, Universidad Nacional Aut—noma de MŽxico, Apartado Postal 1039, Hermosillo, Sonora 83000, MŽxico Ricardo Amaya-Mart’nez Departamento de Geolog’a, Universidad de Sonora, Hermosillo, Sonora 83000, MŽxico ABSTRACT Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These deposits accumulated within basins located throughout the eastern two-thirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora. The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The domes apparently formed when viscous lava welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with, and sheds clasts laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections. K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation by the major river of Sonora has exposed a full section of the volcanic and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections. The mafic lava flows, dominantly basaltic andesite, are similar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico *E-mail: [email protected] and the mid-Tertiary Datil-Mogollon volcanic field in southwestern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state. A convergent-margin magmatic arc was active in the present-day Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 Ma with little coeval volcanism. INTRODUCTION Much of western North America has been affected by extension since middle Tertiary time. Regional descriptions of the history and geometry of this extension have focused primarily on the Great Basin, where two phases have been recognized (Zoback et al., 1981; Eaton, 1982). The first phase began as early as 35 Ma (Gans et al., 1989), and was characterized by relatively high strain rates that produced extreme rotations along relatively closely spaced listric fault systems. In much of the region, the extension direction was westsouthwestÐeast-northeast. During late Miocene time (after ca. 10 Ma), extension in the Great Basin was characterized by lesser degrees of block rotation along more widely spaced brittle normal faults. Locally, strain rates were much lower, and the direction of extension changed to northwest-southeast. The latter phase produced the pattern of elongate ranges and basins that characterizes the Basin and Range physiographic province. Virtually the entire state of Sonora lies within the Basin and Range province, which continues to the south at least into central Mexico (Henry and Aranda-G—mez, 1992; Stewart and Rold‡n-Quintana, 1994). The modern topography of Sonora consists of linear ranges oriented north-northwest and separated by broad alluvial valleys. In the western portion of the state, the ranges have been largely submerged and the bounding faults mostly concealed by a thick veneer of young alluvium. As a consequence, the distribution of young basins does not correspond well to the original Neogene topography. In contrast, in central and eastern Sonora a deeper level of erosion has enhanced the topographic contrast between north- to northnorthwestÐoriented ranges and valleys, and has exposed some of the rangebounding faults. GSA Bulletin, October 1997; v. 109; no. 10; p. 1349Ñ1360; 6 figures; 2 tables. 1349 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. 1350 Gans (1997) has focused on the Santa RosaÐSan Nicol‡s area (Fig. 2) in the eastern portion of our study area, where we have made only reconnaissance examinations. Gans concluded that extension was in progress there by 27 Ma and was largely completed by 20 Ma. BAUCARIT AND LISTA BLANCA FORMATIONS Although our emphasis is on the genetic significance of the stratigraphic sections, the rock units correspond in part to two formations that have been described for many years within the literature of Sonoran geology. Dumble (1900) was the first to describe the occurrence of clastic sediments filling linear basins in Sonora. King (1939) applied the name of B‡ucarit Formation to these rocks to designate a sequence of Òslightly indurated, wellbedded sandstones, conglomerates, and some clays. In the lower part of the formation, where fully developed, there are one or more basalt flows, interbedded with basalt agglomerate.Ó Clasts include virtually all older rocks that are locally exposed in the adjacent highlands. The type locality 110° 111° S 112° 109° ARIZONA 31° SONORA T M CHIHUAHUA Stewart and Rold‡n-Quintana (1994) compiled the distribution of stratal tilts and major normal faults for the state of Sonora. In addition to delineating the 10Ð15 km width of ranges and basins, the compilation showed that rotation of strata was generally consistent within three specific domains. These domains are separated by synformal or antiformal boundaries and accommodation zones. The area discussed in this paper includes their eastern domain, in which dips are generally east to east-northeast from 60¡ to less than 5¡, their central domain, in which dips are mostly west to southwest from 55¡ to less than 5¡, and the antiformal boundary between the domains. For the most part the tilted strata are middle Tertiary to Neogene age. Rocks as young as 10 Ma cap tilted blocks, but the most extreme stratal tilts occur in rocks older than 10Ð11 Ma (Stewart and Rold‡n-Quintana, 1994). Another extensional event that affected the western margin of Sonora during Neogene time is the development of the Gulf of California extensional province (Stock and Hodges, 1989). The most prominent aspect of this province is the oblique extension that occurred during the main phase of opening of the Gulf, from 5.5 Ma to the present. An earlier interval of more orthogonal opening began as early as 15 Ma in some areas of the Gulf, as indicated by differential rotation of dated volcanic horizons, and by incursion of marine sediments into the developing proto-Gulf of California (Smith, 1991). Gulf extension may have begun as an extensional domain within the Basin and Range before evolving into a distinct tectonic province (Henry, 1989). In northern Sonora the oldest deformation related to Tertiary extension has been reported from metamorphic core complexes (Nourse et al., 1994). These are found in the areas of Magdalena, Tubutama, Sas‡be, Mazat‡n, Puerto del Sol, and the northern portion of the Aconchi batholith (Fig. 1). Timing of mid-crustal extension within the Sonoran core complexes is from 25 to 18 Ma (Nourse et al., 1994). Detachment faults produced large extensional basins that contain Tertiary sediments with volcanic interbeds. In the Magdalena basin, a sequence of latite flows at the base of the sedimentary section has K-Ar ages of 27 and 22 Ma (Miranda-Gasca and DeJong, 1992). In the Tubutama basin, a 22 Ma latite is at the base of a section of Miocene lake deposits that are covered in angular unconformity by a sequence of red conglomerates. The conglomerates are overlain by a 7 Ma basalt flow (G—mez-Caballero et al., 1981). The objective of this paper is to present new information about Neogene basins formed by extensional faulting in Sonora. This extension produced grabens and half-grabens that accumulated sedimentary and volcanic fill. Because these rocks constitute the primary record of extension in Sonora, we describe their field relations, emphasizing the age and nature of the volcanic rocks. The study covers the entire known east-west extent of the clastic sedimentary deposits at lat 28¡ to 29¡N (Fig. 2). This area is well to the east of the Gulf of California vicinity, and is south of known exposures of metamorphic core complexes. The study is reconnaissance in nature and attempts to examine the rock record in the context of its relationship to Neogene extension. The primary focus is on the well-exposed clastic deposits within the basin of the Rio Yaqui, SonoraÕs major permanent river (Figs. 2 and 3). Also examined in some detail are exposures in the area of Suaqui Grande, 30 km to the west of the Rio Yaqui. Results of reconnaissance examinations of scattered exposures both to the east in the YŽcora area and in the Arroyo Los Pilares, and to the west near the town of La Colorada in the northern Sierra Bacatete, and at Cerro Lista Blanca (Fig. 2) are also included. In addition to our field studies we have included the work of CochemŽ and Demant (1991) in the Tep—ca area and in reconnaissance farther to the east; the study of Bockoven (1980) near YŽcora; and that of AmayaMart’nez et al. (1992) at Cerro Lista Blanca (Fig. 2). Bartolini et al. (1994) examined volcanic and sedimentary rocks related to extension in Sonora covering an area much larger than that of this report. A concurrent study by 45 A 30° PS MZ H IT 29° Fig. 2 16 GU LF OF Y 28° G CA LI FO RN IA Baucarit * A 0 50 km O AL N SI Figure 1. Map of Sonora, showing the distribution of Neogene continental clastic basins (shaded). Locations of Highways 45 and 16 are shown. Abbreviations: HÑHermosillo; GÑGuaymas; YÑYŽcora; ITÑIsla Tibur—n. Italicized bold letters mark locations of metamorphic core complexes (SÑSasabe; TÑTubutama; MÑMagdalena; AÑ Aconchi; PSÑPuerto del Sol; MZÑMazat‡n). An asterisk is placed at the type locality of the B‡ucarit Formation. The rectangle encloses the area of Figure 2. Geological Society of America Bulletin, October 1997 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 TERTIARY EXTENSION IN SONORA, MEXICO Hermosillo 110° 111° 109° 29° 16 R. Yaqui La DII Colorada RIO YAQUI BASIN 45 30 WESTERN LOCALITIES DIII * 28,29 AIII Co. Lista 24 Blanca DIV BI DI BII DVI DV 11 * * SG BIII BIV * SUAQUI AI GRANDE AREA AV Sierra Bacatete Sierra Libre 5 AIV Onavas CI Tepoca San 16 Nicolas 14 AII CII CIII Yecora * 15 EASTERN LOCALITIES Sierra Santa Ursula 28° 0 50 km Guaymas Figure 2. Location map for part of south-central Sonora showing geographic features mentioned in the text. Shaded areas indicate major Neogene clastic sedimentary deposits; asterisks show known outcrops of distinctive amphibole-bearing intermediate-composition igneous rock (see text). Letters paired with Roman numerals (e.g., A IV) indicate locations of stratigraphic columns displayed in Figure 4, AÐD. Numbered dots are locations for dated samples not indicated in Figure 4. SGÑSuaqui Grande. A dot-dashed line shows the border between the states of Sonora and Chihuahua. The solid lines are the locations of Highways 16 and 45, and the dotted line locates the Rio Yaqui and its reservoir system. designated by King (1939) for the B‡ucarit Formation is the Arroyo Cedros, near the town of B‡ucarit (Fig. 1) about 100 km to the south of the area covered in this paper. A subdivision of the B‡ucarit Formation into an upper clastic and lower volcanic member was originally applied by King (1939). Lava flows and volcanic breccias of basaltic to andesitic composition are prominent within the lower member of the B‡ucarit Formation, particularly within the eastern part of its known distribution. Typically, the volcanic rocks are conformable, either immediately beneath or intercalated within the oldest sedimentary layers. The lava flows are characterized by brecciated bases, and by altered olivine phenocrysts. Some also contain augite and hornblende as phenocryst phases. Although King (1939) interpreted the contacts between B‡ucarit Formation and older rocks either as unconformities or as thrusts, we have observed only normal fault contacts or unconformities. The sedimentary rocks have a patchy distribution and probably did not form continuous cover throughout their area of deposition. The lack of paleocurrent studies and minimal information about internal stratigraphy preclude reliable reconstruction of original depocenters for the B‡ucarit Formation. These depocenters probably were closed continental basins, within which the coarsest strata were locally derived and accumulated in alluvial fans or talus deposits. The centers of deposition correspond only partly with present-day basins. Clastic sedimentary deposits similar to those of the B‡ucarit Formation have now been recognized throughout much of the province of parallel ranges and valleys in Sonora (Fig. 1). Where present as tilted strata in uplifted blocks, the significance of the deposits is obvious. Elsewhere, it may be difficult to discern them from younger Pliocene to modern unconsolidated alluvial deposits. Their only distinguishable characteristic is com- monly a greater degree of induration than is observed within the younger deposits. The identification is especially difficult in western Sonora, where little exhumation has occurred. As far as we are aware, no sedimentary deposits that can be related to those of the B‡ucarit Formation have been recognized to the west of the area shown in Figure 2. Age information contained within the B‡ucarit Formation is significant with respect to the history of extension in Sonora. Extremely limited vertebrate remains suggest a Neogene or Quaternary age for the upper portion of the B‡ucarit Formation (King, 1939). Palynologic study of fine-grained sediments within the upper part of the B‡ucarit Formation to the northeast of Hermosillo has yielded estimates ranging from Pliocene to Pleistocene age (Martinez-Hern‡ndez, 1984). Obviously, the time interval represented by deposition of the B‡ucarit Formation is poorly defined. Lista Blanca division was first designated by Dumble (1900) for a conglomerate and volcanic sequence at Cerro Lista Blanca (Fig. 2), to which he assigned a tentative Triassic age. Later, King (1939) attributed a Cretaceous age to the section. Morales et al. (1990) established a Tertiary age by showing that volcanic rocks at Cerro Lista Blanca lie conformably above conglomerates that they reassigned to the B‡ucarit Formation. They also reported a K-Ar age of 10.4 ± 0.2 Ma for a latite flow within the Lista Blanca section. Also at Cerro Lista Blanca, Amaya-Mart’nez et al. (1992) described a 48 m thick section of rhyolite lava flows and tuffs, ignimbrite and tuffaceous sandstone, all conformably above conglomerates of the B‡ucarit Formation. The sequence dips uniformly 25¡ to the southwest. We concur with the consensus of these later workers that the conglomerates originally included by Dumble (1900) and King (1939) within the Lista Blanca Formation are instead probably correlative with the B‡ucarit Formation. Geological Society of America Bulletin, October 1997 1351 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. 109°30' 28°43' 109°37' pre-N pre-N Nsl III 60 65° 6 Nsu Qal pre-N Qal GEOLOGY AND GEOCHRONOLOGY OF SPECIFIC LOCALITIES Nsl EXPLANATION Qal Introduction To 24 Nsu younger deposits T-Q Nsl pre-N Ns–Neogene clastic rocks 18 Nsu–upper clastic rocks Nsl–lower clastic rocks Qal pre-N–older rocks pre-N 29 IV 35 7 Sample location with age Nsl Nsu 10 Highway 16 Qal Rio Yaqui 0 30 20 Nsl Normal fault with direction of dip 3 km T-Q Nsl N On 70° Nsu 25 I T-Q Nsl 10 Nsl II 15 Nsu 7 33.5 Ma Ns LD Ns V pre-N pre-N Qal 28°24' 109°29' Figure 3. Generalized geology of Rio Yaqui basin. For clarity, a stipple has been added to the outcrop area of the upper clastic rocks (Nsu). Roman numerals mark the locations of columns in Figure 4A. TOÑTonich’; ONÑOnavas; LDÑLa Dura. The lava flows within the Lista Blanca Formation are likely to be close to their source, although no vents have yet been recognized. One major source is probably located within the area of Cerro Lista BlancaÐnorthern Sierra del Bacatete (Fig. 2), where the section is thickest and lava units are the most prevalent. No caldera source for the ignimbrite components of the Lista Blanca Formation has yet been recognized. It is possible that the ignimbrites could have originated in western Sonora, where thicker Neogene volcanic exposures have been described from the GuaymasÐSierra Santa Ursula areas (Mora-Alvarez, 1992, 1993; Sheridan et al., 1979). A thick section of felsic volcanic rocks is also present in the Sierra Libre (Fig. 2). 1352 It is obvious that the formal definitions and the lateral extents of both the B‡ucarit and Lista Blanca Formations remain only vaguely determined. Based in considerable part on reviewersÕ responses, we have abandoned attempts to assign the sedimentary and volcanic rocks that we describe to these formations. In order not to detract from our emphasis on the genetic relation of these deposits to extensional faulting, we have avoided using the terms B‡ucarit and Lista Blanca Formations throughout the remainder of this paper. However, these names have been applied in previous studies within much of the area that we describe. When the nature and extent of the these formations is better understood, their formal redefinition should be undertaken. In this section we describe the stratigraphic and timing relationships that we have examined within the study area. Field observations from specific localities are portrayed on the generalized stratigraphic columns of Figure 4 (AÐD). These are grouped by subareas that are located in Figure 2. The discussion below emphasizes only general relationships. In order to establish the timing of clastic sedimentation within Neogene extension-related basins, and by inference the time of extension, we have obtained K-Ar ages for 30 volcanic rock samples. Many of these are tied stratigraphically into the generalized columns of Figure 4. Locations for the others are given on Figures 2 and 3. All of the data are in Table 1, where the samples are organized according to the major geographic subdivisions, and by stratigraphic position within those subdivisions. Samples are numbered consecutively to coordinate them with the figures and text di scussion. Potassium was determined by flame photometry, using lithium as an internal standard and sodium as a buffer. Radiogenic argon was analyzed by isotope dilution using on-line extraction and purification with a 3-in gas-source mass spectrometer operated under computer control. Further details were given in McDowell and Mauger (1994). In the previous section two specific associations for the volcanic rocks were given: mafic lava flows that occur either at the base or within the basal parts of the clastic sedimentary sections, and rhyolitic ignimbrites and lava flows that cap the coarser and thicker parts of the sections. In addition, we have recognized a third distinctive and widespread igneous rock type associated with the lower parts of the clastic sections. This rock type is a porphyritic lava flow of intermediate composition that contains striking phenocrysts of euhedral dark brown amphibole, with opaque rims of variable thickness. Andesine and, in some cases, clinopyroxene also are present as phenocrysts. The matrix, either glassy or microlitic, is dominated by plagioclase. The typical field occurrence is massive domes for which external contacts are rarely exposed. Less commonly the rock occurs as tabular lava flows. Locally, the lava flows and domes underlie clastic sedimentary layers. Individual flows and domes are laterally discontinuous, indicative of a relatively high initial viscosity. Our interpretation is that this amphibolebearing magma welled up at locations along developing faults at graben margins. We regard this rock type as an important marker for the progress of extensional faulting in this area of the Sonoran Basin and Range, and have indicated its known outcrops in Figure 2. Rio Yaqui Basin The Rio Yaqui basin is located in eastern Sonora about 180 km southeast of the city of Hermosillo (Fig. 2). The present course of the Rio Yaqui follows a Neogene paleobasin that is from 6 to 7.5 km wide and more than 35 km long (Fig. 3). The valley is one of the few localities in which both the up- Geological Society of America Bulletin, October 1997 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 A III Arroyo el Salto I Arroyo el Obispo 9 10 well consolidated conglomerates and sandstones in 3-6 m-thick layers; section includes 5-10 m-thick rhyolitic tuffs and a 280 m-thick sequence of basaltic andesite flows and breccias S.M.O. volcanic rocks 25.3±1.6 Ma (lava dome) III Arroyo Molina rhyolitic ignimbrites, lava flows, and breccias clastic sediments, conformably above mafic flows section dips 50°W 3 27.5±1.1 Ma (mafic flow) unconformity unconformity S.M.O. volcanic rocks K volcanic and intrusive rocks unconformity 27.1±0.9 Ma (ignim.) S.M.O. volcanic rocks II Arroyo las Palomas 12.3±0.3 Ma conformable contact conglomerates section dips 25°SE mafic lava flows amphibole-bearing intermediate composition lava dome K-T intrusive and volcanic rocks 20.2±0.4 Ma (mafic flow) unconf. 6 12 section dips 50-62°E 12.0±0.8 Ma rhyolitic lava flows and ignimbrites unconformity polymictic conglomerates with unsorted, subrounded clasts; no mafic flows section dips 40°NE 12.5±0.3 Ma (ignim.) well consolidated, pebble-boulder conglomerate in 0.5-1 m-thick strata with local sandy intervals; four intervals of mafic flows and breccias containing plag. and oliv. phenocrysts section dips 20-30°E 2 II Rancho el Pozo rhyolitic ignimbrites and lava flows cobble-boulder conglomerate, unsorted, subrounded clasts section dips 6°E unconformity poorly consolidated, thin bedded boulder conglomerate, some sandy horizons 1 B I Cerro Vigia 4 km north of Suaqui Grande chaotic mixture of mafic flows and breccias with pockets of clastic sediments section dips 30-35°W normal fault contact (fault dips W) 8 rhyolitic volcanic rocks unconformities sandstones with subordinate conglomerates with basaltic andesite lavas section dips 65°E IV Arroyo Cajon Bonito K-T intrusive rocks IV Arroyo Taraicito amphibole-bearing intermediate lava flow and breccia K volcanic rocks conglomerates and lava flows of basaltic andesite section dips 40°E 200 m 63.6±1.5 Ma K-T intrusive and volcanic rocks V La Dura 0 coarse clastic sediments 4 200 m 20.5±1.1 Ma (lava dome) intermediate composition dome intrudes lower section, has brecciated carapace that has shed blocks into clastic section; upper sediments lap onto dome I Cerro Lista Blanca 0 Cocheme and Demant, 1991 Gans (1997) III Rancho El Churi II Yecora Bockoven, 1980 14,17,16.7,17.4 locally derived andesitic lava flows, flatlying 14.4±0.3 Ma 23.3±1.1 Ma section dips about 20°W 200 m 0 Paleozoic rocks IV Pozo de Leyva basaltic lava flow 20 8.9±0.4 Ma conformable contact air fall tuffs, ignimbrites K-T intrusive rocks mafic lava flows conformable contact conglomerates ? VI E. Sierra el Bacatete V N. Sierra el Bacatete 25 Bockoven, 1980 23.2±0.8 Ma 12.7±0.3 Ma 12.3±0.6 Ma conformable contact coarse clastic sediments dip is about 20°W unconformity 18.4±0.4 Ma 19 III Arroyo los Pilares 18 rhyolitic ignimbrite 26 S.M.O. volcanic rocks basaltic andesite lava flows, interbedded with rhyolite lava flows and ignimbrites thin tuff clastic sediments section dips 15°W lower mafic lava flows clastic sediments section dips 5°W (?) conformable contact mafic lava flows with interbedded rhyolitic ignimbrites (SMO volc. rocks) rhy. ignimbrite 27 conformable contact coarse clastic sediments dip is about 20°W unconformity coarse clastic sediments, generally flat lying,with no interbedded volcanic rocks unconformity 17 13 10.4 Ma 21 11.5±0.3 Ma 22 12.8±0.6 Ma 23 12.2±0.3 Ma II NW La Colorada rhyolite ignimbrites and lava flows coarse clastic sediments dip is about 30°W C I Tepoca D rhyolitic ignimbrite spherulitic lava flow intermediate lava flow conformable contact conglomerates 11.5±0.9 Ma rhyolitic ignimbrites lithic tuffs section dips about 20°W conformable contact conglomerates unconformity K-T intrusive rocks ? 200 m 0 Figure 4. Grouping of stratigraphic columns by areas. (A) Rio Yaqui basin. (B) Suaqui Grande area. (C) Eastern localities. (D) Western localities. Stratigraphic positions of K-Ar dated samples are shown with ages and circled numbers keyed to Table 1. Ages in brackets are from Bartolini et al. (1994) and Gans (1997). SMOÑSierra Madre Occidental. Geological Society of America Bulletin, October 1997 1353 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. per and lower parts of the clastic section are well exposed (Rold‡n-Quintana and McDowell, 1992). Within the Rio Yaqui basin five sections were studied in detail (Fig. 4A), but intense faulting allowed reliable thickness determinations for only two of them. Eight K-Ar ages provide chronologic control on clastic sedimentation within the Rio Yaqui basin (Table 1, samples 1Ð8): none of these ages are from north of the Highway 16 bridge (Fig. 3). The clastic sediments were deposited unconformably on early Paleozoic sedimentary rocks, on granitoid or related volcanic rocks of the Late CretaceousÐearly Tertiary batholith (sample 8; 64 Ma; Fig. 4A, column IV), or on mid-Tertiary volcanic rocks that presumably are outliers of the Sierra Madre Occidental volcanic field. Ages of 27.1 Ma (sample 6; Fig. 4A, column I) and 33.5 Ma (7; Fig. 3) are consistent with ages for the felsic components of that field from the YŽcora area and from localities farther to the east (Bockoven, 1980; McDowell et al., 1990; McDowell, 1993; McDowell and Mauger, 1994). They provide an older time limit for deposition of the clastic sediments within the basin. The lower part of the clastic section crops out in a continuous belt for about 20 km along the western margin of the Rio Yaqui basin (Fig. 3), where it dips east 10¡ to 30 ¡ toward the center of the basin. In the southern part of the basin, discontinuous outcrops are also present along the eastern margin. The lower section is composed of clastic and volcanic rocks in about equal proportions. The clastic rocks include conglomerates and sandstones, and subordinate siltstones. The volcanic rocks are lava flows and breccias, dominantly of basaltic andesite and andesite, but some basalt may be present. The lava flows have well-developed breccias at their bases and otherwise are vesicular with phenocrysts of plagioclase and olivine, mostly altered to iddingsite. Some lava flows also contain augite as phenocrysts. In many localities the mafic lava flows compose the lowest part of the section. The overlying section is typically conformable and consists of interbedded lava flows and clastic layers. We can recognize no distinction between the basal and intercalated lava flows, and we assign both to the clastic-volcanic section. In the Arroyo el Obispo, west of the Rio Yaqui (Figs. 3 and 4A, column I), a 500 m section dips 20¡ to 30¡ to the east and comprises mafic lava flows and breccias alternating with conglomerate and sandstone. The upper contact is placed at the base of a prominent rhyolitic ignimbrite dated as 12.5 Ma (sample 1, Fig. 4A, column I). This is the youngest igneous rock present in this part of the Rio Yaqui basin. A K-Ar age of 20.2 Ma (sample 2, Fig. 4A, column I) was obtained for a sample from the lowest of four intervals of mafic lava flows. This interval has some sedimentary strata below it. Thin exposures of the lower part of the section are present on the east side of the Rio Yaqui basin. At Arroyo las Palomas (Fig. 4A, column II), mafic lava flows (27.5 Ma; sample 3) lie directly on a pluton of the Sonoran batholith; there is no basal sedimentary interval. Layers of clastic sediment are present above the lava flows. At Arroyo Cajon Bonito (Fig. 4A, column IV), undated mafic lava flows are chaotically mixed with pockets of clastic sedimentary rocks. In the northern part of the Rio Yaqui basin, in Arroyo el Salto (Fig. 3), the lower part of the clastic section is about 650 m thick and rests unconformably on a (Tertiary?) amphibole-bearing lava flow of intermediate TABLE 1. K-AR AGES Index Sample Rio Yaqui Basin Capping rhyolites 1 SO 31(R) Ignimbrite Interbedded mafic flows 2 SO 33 3 SO 48 Basaltic andesite Amphibole-bearing lava-dome 4 SO 82 5 SO 4 Pre-Neogene rocks 6 SO 32(R) Ignimbrite 7 SO 101 tuff 8 SO 57 Andesite flow Suaqui Grande area Capping rhyolites 9 SO 76 Ignimbrite 10 SO 75 Ignimbrite Interbedded mafic flow 11 SO 28 Andesite Amphibole-bearing lava-dome 12 SO 106 SO 106 1354 Mineral K (%) 40*Ar (%) 40*Ar × 10–6 (scc/gm) 3146.40N 639.45E Alkali feldspar 4.465 4.573 73 77 2.215 2.181 12.5 ± 0.3 3146.20N 639.45E 3144.30N 643.70E Whole rock 1.396 1.395 1.344 1.330 73 70 68 71 1.121 1.083 1.400 1.479 20.2 ± 0.4 3138.05N 637.65E Amphibole 0.7160 Amphibole 0.5481 0.5566 0.6013 0.5751 0.5145 0.5441 0.5537 20.5 ± 1.1 3157.25N 623.05E 55 36 57 53 60 3146.75N 636.60E 3141.01N 641.92E 3156.35N 645.05E Alkali feldspar 0.5477 0.5436 0.6308 0.6373 1.406 1.393 25 22 50 52 89 90 0.5731 0.5853 0.8417 0.8215 3.468 3.574 27.1 ± 0.9 3135.45N 594.20E 3135.25N 593.95E Alkali feldspar 4.176 4.170 3.091 63 73 71 67 2.034 1.863 1.493 1.481 12.0 ± 0.8 3143.95N 615.00E Whole rock 1.739 1.703 78 68 1.545 1.616 23.5 ± 0.8 3145.53N 605.61E Amphibole 0.6110 0.6269 Plagioclase 0.2900 0.2841 0.7699§ 0.6401 0.5850 0.5318§ 0.3443 0.3012 25.3 ± 1.6 3145.53N 605.61E 26 27 54 19 27 28 UTM coordinates (km) Whole rock Plagioclase Whole rock Alkali feldspar Geological Society of America Bulletin, October 1997 Age† ± 1s (Ma) 27.5 ± 1.1 25.4 ± 0.5 33.5 ± 0.8 63.6 ± 1.5 12.3 ± 0.3 28.7 ± 2.7 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 TERTIARY EXTENSION IN SONORA, MEXICO TABLE 1. (Continued) Index Sample UTM coordinates (km) Eastern localities Capping andesite flow 13 SO 14 SO 14 Basal mafic flows 14 15 16 17 SO 40 Basaltic andesite SO 41 Basaltic andesite SO 19 3 21 3 Basaltic Andesite 3148.39N 669.42E 3148.39N 669.42E 3135.10N 670.05E 3123.00N 665.25E 3147.85N 697.05E 3138.3N 709.8E Pre-Neogene rocks 18 3 21 2 Rhyolite flow 3 21 2 Rhyolite flow 19 3 20 4 Ignimbrite 3138.8N 710.5E 3138.8N 710.5E 3139.8N 724.0E Western localities Capping volcanic rocks 20 SO 60 Basalt flow 21 LB-2 Rhyolite? flow 22 SO 110 Rhyolite flow 23 SO 108 Rhyolite ignimbrite 24 SO 67 Rhyolite Ignimbrite 3144.50N 556.55E 3148N 562E 3148.18N 562.15E 3148.95N 562.95E 3161.35N 573.65E 25 26 27 28 SO 61 Rhyolite? flow SO 69 Rhyolite flow SO 70 Rhyolite ignimbrite SO 9 Rhyolite ignimbrite 3139.95N 559.35E 3177.40N 545.25E 3186.80N 539.25E 3173.45N 574.15E 29 SO 111 3173.08N Rhyolite ignimbrite 574.49E Amphibole-bearing lava flow 30 SO 68 3181.70N 541.05E K (%) 40*Ar (%) 40*Ar × 10–6 (scc/gm) Age† ± 1s (Ma) 2.517 2.569 0.2015 0.1983 67 66 13 20 1.449 1.406 0.1357 0.1326 14.4 ± 0.3 1.614 1.609 0.1607 0.1561 1.501 1.482 1.669 1.680 69 67 16 17 77 64 40 52 1.058 1.090 0.1731 0.1706 1.320 1.345 1.174 1.226 6.882 6.776 0.6349 0.6360 0.6282 0.6953 0.6697 0.6634 66 69 55 54 52 56 6.345 6.490 0.5646 0.5479 0.5962 0.6129 Whole rock 0.3292 Plagioclase 0.6615 0.6575 0.4375 0.4371 5.546 5.497 4.094 3.680 4.116 7.198 7.176 4.558 4.532 3.345 3.263 4.502 4.498 25 30 41 32 21 24 80 82 11 14 0.1096 0.1175 0.2927 0.3001 0.2167 0.2186 2.638 2.605 1.798 2.000 3.057 3.395 2.227 2.260 1.629 1.526 3.856 4.221 3.846 2.054 2.136 0.6140 0.6103 Mineral Whole rock Plagioclase Whole rock Plagioclase Whole rock Whole rock Biotite Plagioclase Plagioclase Plagioclase Alkali feldspar Alkali feldspar Alkali feldspar Alkali feldspar 4.269 4.235 80 85 65 71 13 13 86 84 81 60 64 Plagioclase 0.6520 0.6503 26 51 Alkali feldspar Alkali feldspar Alkali feldspar 17.2 ± 1.6 17.1 ± 0.4 27.7 ± 1.4 22.8 ± 0.5 18.4 ± 0.4 24.0 ± 0.4 22.4 ± 0.5 23.3 ± 1.1 8.9 ± 0.4 11.5 ± 0.3 12.8 ± 0.6 12.2 ± 0.3 12.3 ± 1.2 11.5 ± 0.9 12.7 ± 0.3 12.3 ± 0.6 22.6 ± 1.2 12.6 ± 0.4 24.0 ± 0.6 *Radiogenic argon (scc/gm= cm3 at STP per gram of sample). †Decay constants: λ = 4.963 × 10-10yr-1; λ -10 -1 40 -4 β ε+ε′ = 0.581 × 10 yr ; K/K = 1.167 × 10 . §Value not used in age calculation. composition. Four separate intervals of basaltic andesite lava flows and breccias, along with felsic tuff horizons, are interbedded with conglomerates, sandstones, and siltstones (Fig. 4A, column III). Felsic tuffs are more prominent there than in the Arroyo el Obispo section. The Arroyo el Salto section is tilted eastward at dips of 50¡ to 62¡ (Fig. 3), probably resulting from local rotation. To the southwest of La Dura (Figs. 3 and 4A, column V), a 20.5 Ma amphibole-bearing intermediate-composition dome (sample 4) displays a significant interrelationship with the clastic section. The outcrop can be traced continuously from a massive lava dome into a brecciated carapace with a steep frontal flank, and finally into a volcanic breccia from which a tongue of reworked clasts extends as a lens into the sedimentary section. The contemporaneity of the volcanic and sedimentary units is unmistakable at this locality, and we suggest that such a relationship is generally accurate. Another amphibole-bearing intermediate-composition lava dome from a locality about 10 km to the northwest of Arroyo el Obispo has been dated as 25.4 Ma (sample 5; Fig. 2). The upper part of the clastic section occupies the largest area of the Rio Yaqui basin, extending about 16 km in a north-south direction (Fig. 3). In the central part of the basin, the rocks form prominent mesas with general tilts of 6¡ to 10¡ to the east. In general the tilt of the beds decreases gradually from the lower to the upper portions of the Neogene sections. Recent Geological Society of America Bulletin, October 1997 1355 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. fluvial erosion by the Rio Yaqui has produced very steep cliffs and pillars tens of meters high. Rock types include sandstone and poorly to wellcemented conglomerates, but no igneous rocks. There are excellent exposures near the town of T—nichi, in the northern part of the basin, and where the bridge of Highway 16 crosses the Rio Yaqui (Fig. 3). Near the highway bridge, Barrera-Moreno and Dom’nguez-Perla (1987) measured a thickness of about 150 m for a section consisting of sandstones in beds 0.5 m thick, and pebble to cobble conglomerates, either as massive layers or as beds from 0.5 to 10 m thick. The conglomerates are unsorted and polymictic, and include angular to subangular clasts of granitoid, basalt, andesite, rhyolitic tuff, chert, and limestone in an arkosic matrix. The base of this section is not exposed, and the thickness could be several hundred meters. At Arroyo el Obispo, a 200-m-thick section of poorly consolidated thinbedded pebble to cobble conglomerate with some sandy horizons (Fig. 4A, column I) is generally conformable on a 12.5 Ma ignimbrite horizon. In the northern Rio Yaqui basin at Arroyo el Salto (Fig. 4A, column III), 40 m of conglomerates tilted 6¡ to the east rest in angular unconformity on the older clastic rocks, which dip 60¡E. Suaqui Grande Area Near Suaqui Grande extension-related deposits were examined at four localities (Figs. 2 and 4B). The deposits unconformably overlie Upper Cretaceous volcanic and volcaniclastic rocks, granitoids, or middle Tertiary volcanic rocks. Thickness ranges from about 75 m at Arroyo Molina (Fig. 4B, column III) to greater than 250 m at Arroyo Taraicito (column IV), and as much as 300 m at Cerro Vigia (column I). The basal units are either sedimentary rocks or basaltic andesite lava flows. The lava flows are absent from the section at Cerro Vigia. The clastic rocks generally dip toward the northeast from 40¡ at Cerro Vigia (column I) to 65¡ at Rancho el Pozo (column II). All of the sections examined are capped by a considerable thickness (as much as 200 m) of rhyolitic ignimbrites and lava flows. These units generally dip more gently to the east and are in angular unconformity with the units beneath them. At Rancho el Pozo (column II), however, the relationship is conformable. Other significant outcrops of the clastic sedimentary rocks are present 12 km south of Suaqui Grande, along the main arroyo. These were not examined in detail. Four K-Ar ages were obtained for volcanic rocks associated with these deposits (Table 1, samples 9Ð12; Fig. 4B). A mafic flow 7 km northeast of Suaqui Grande is dated as 23.5 Ma (sample 11, Fig. 2). Although no full section was examined at this locality, the sample is from near the base of a clastic section. A sample of amphibole-bearing intermediate-composition lava from a dome at Cerro Vigia (Fig. 4B, column I), 4 km to the north of Suaqui Grande is 25.3 Ma (sample 12; using the more precise amphibole age). To the west of Suaqui Grande, at Rancho el Pozo (Fig. 4B; column II), two units of the overlying rhyolitic section were dated: the lower unit is 12.3 Ma (sample 10) and the upper is 12.0 Ma (sample 9). Localities to the East of the Rio Yaqui Basin On his reconnaissance map, King (1939) showed major exposures of Tertiary continental clastic deposits in areas to the east of the Rio Yaqui basin, extending into the main part of the Sierra Madre Occidental volcanic field (Fig. 2 and 4C). He considered these deposits to belong to the B‡ucarit Formation. Some of these areas were described by Bockoven (1980), by CochemŽ and Demant (1991), and Gans (1997), and were examined by us only briefly. In the Sierra Madre, these clastic deposits have a tuffaceous matrix, and angular clasts of various volcanic rocks. We suspect that in some cases these may be caldera fill rather than related to extension. K-Ar 1356 and Ar-Ar ages for volcanic rocks in close stratigraphic proximity to these clastic layers have been published by Bockoven (1980), CochemŽ and Demant (1991), Bartolini et al. (1994), and Gans (1997). Due to the lack of capping felsic rocks from areas to the east of the Rio Yaqui basin, opportunities to obtain younger time limits to deposition of these sedimentary rocks are limited (Fig. 4C). At Tep—ca, 25 km to the east of the Rio Yaqui, CochemŽ and Demant (1991) described a 250-m-thick section of locally derived flat-lying andesitic lava flows that overlies approximately 100 m of clastic sedimentary rocks that dip about 15¡ to the west (Fig. 4C, column I). Additional mafic volcanic rocks underlie the clastic sedimentary rocks in this area. The sedimentary section contains a minor amount of interbedded mafic lava flows and reworked tuffs, and is capped by a thin white tuff layer. For the overlying volcanic rocks, Bartolini et al. (1994) cited a whole-rock K-Ar age of 14.0 Ma and a hornblende age from an andesite porphyry of 17.2 Ma, for samples taken Òto the northwest of Tep—ca,Ó and Gans (1997) gave Ar-Ar ages of 16.7 and 17.4 Ma for samples near Tep—ca (Fig. 4C; column I, ages in brackets). Our whole-rock age for one of the flows of this sequence (sample 13) is 14.4 Ma, whereas the plagioclase phenocrysts give 17.2 Ma. We regard the whole-rock age as more reliable owing to its higher potassium content (Table 1). From approximately 15 km to the northeast of Tep—ca, Gans (1997) obtained three whole-rock Ar-Ar ages (22.6, 26.4, and 26.4 Ma) for lava flows that underlie clastic sedimentary deposits. In this area, the entire section dips about 55¡ to the northeast. Farther to the east there are no lava flows above the conglomerate layers. In the YŽcora valley (Fig. 4C, column II), gently dipping to flat-lying conglomerates are the youngest rocks exposed, except for minor young fluvial deposits (CochemŽ and Demant, 1991; Bockoven, 1980; McDowell, 1993). The Neogene section lies in angular unconformity on a westward-dipping section of basaltic andesite lava flows interlayered with rhyolitic flows and tuffs. To the west of the YŽcora valley, mafic lava flows that underlie clastic sedimentary rocks have whole-rock ages of 27.0 Ma (Bartolini et al., 1994) and 22.8 Ma (sample 16). Along the eastern valley margin, the underlying volcanic rocks have ages of 18.4 Ma and 23.2 Ma (mean of biotite and plagioclase ages) (Fig. 4C, column II; samples 17 and 18). Note that preliminary age results for samples 17, 18, and 19 were given in Bockoven (1980). The easternmost locality of clastic sedimentary deposits is at Arroyo los Pilares (Figs. 2 and 4C, column III), about 15 km to the east of YŽcora (Fig. 2). The section is gently dipping, has a minimum thickness of a few tens of meters, and includes no mafic volcanic rocks. The Neogene section is generally conformable with underlying basaltic andesite lava flows, and is interbedded with rhyolite lava flows and tuffs. One K-Ar age for a rhyolite tuff is 23.3 Ma (sample 19). During reconnaissance into unmapped areas to the south of Highway 16, southwest of the town of San Nicol‡s (Fig. 2), we noted extensive exposures of coarse clastic rocks interlayered with mafic lava flows. The total thickness of these sections is unknown. Just to the south of San Nicol‡s, these sedimentary rocks were deposited on plutons of the Sonoran batholith and on a felsic tuff with a K-Ar age of 54 Ma. About 13 km due south of Tep—ca, a whole-rock age of 17.1 Ma was obtained for a basaltic andesite lava flow (sample 14) (Fig. 2). Gans (1997) determined a whole-rock Ar-Ar age of 18.0 Ma from the same locality. He placed this flow within the clastic sedimentary section. Bartolini et al. (1994) gave groundmass K-Ar ages of 19.5 and 16.3 Ma for mafic lava flows from nearby localities. Gans (1997) determined ages of 22.3 and 26.1 Ma for mafic lava flows a few kilometers to the east, at or near the base of the sedimentary section. About 14 km farther to the south, plagioclase separated from a mafic lava of unknown stratigraphic position gave a plagioclase K-Ar age of 27.7 Ma (sample 15). Thirty-five km to the south of Tep—ca, near the town of Nuri, an estimated Geological Society of America Bulletin, October 1997 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 TERTIARY EXTENSION IN SONORA, MEXICO minimum thickness of 2000 m of coarse conglomerates are interbedded with abundant mafic lava flows. Western Localities To the west of Suaqui Grande (Fig. 2), basin and range structure is less well exposed, in part due to cover by younger clastic deposits. Exposures of Neogene conglomerates are generally thinner and lack interbedded volcanic rocks. The sections dip gently (up to 20¡) to the west. No mafic lava flows have been noted intercalated within clastic sedimentary rocks from any areas to the west of the Suaqui Grande area. However, at many localities the base of the section is not exposed. Where the lower contact is visible, the conglomerates lie directly and unconformably on Late CretaceousÐearly Tertiary granitoids or on Mesozoic and Paleozoic sedimentary units. The distinction of the Neogene rocks from younger clastic deposits is based primarily on the observation that they are capped conformably by felsic volcanic rocks. Six sections have been examined in this portion of the study area (Figs. 2 and 4D). At Cerro Lista Blanca (Figs. 2 and 4D, column I), Bartolini et al. (1991) reported a thickness of more than 100 m for clastic sedimentary rocks, but elsewhere the exposed thickness is significantly less. The clastic sections are everywhere overlain conformably by an interval of rhyolitic ignimbrites and lava flows that is about 50 m thick at Cerro Lista Blanca (Amaya-Mart’nez et al., 1992). There is only one K-Ar age from this western area that provides an older time limit for deposition of clastic sediments. A porphyritic lava flow-breccia sampled 4 km to the south of La Colorada (Fig. 2; sample 30) is 24.0 Ma. This brecciated flow underlies clastic sedimentary rocks and is petrographically similar to the amphibole-bearing intermediate composition rock that forms prominent domes and lava flows in the Rio Yaqui basin and Suaqui Grande areas. It is unknown whether additional sedimentary layers are present beneath the flow at this locality. The remaining K-Ar ages from the western localities are for rocks of the overlying rhyolitic volcanic section, and thus provide younger limits to the time of clastic sedimentation. These ages are very uniform throughout the western area. At Cerro Lista Blanca (Fig. 4D; column I), we obtained ages for the lowest (12.2 Ma; sample 23) and highest (12.8 Ma; sample 22) units in the rhyolitic section. Although the K-Ar ages do not agree with stratigraphic order, their analytical uncertainties overlap (Table 1). A third age of 11.5 ± 0.3 Ma (sample 21) is for a sample lithologically similar to the youngest unit, but of unknown stratigraphic position. Bartolini et al. (1994) published a younger K-Ar age of 10.4 ± 0.4 Ma from the Cerro Lista Blanca section. At Pozo de Leyva (Fig. 4D, column IV), 10 km to the west of Cerro Lista Blanca, a capping basaltic lava flow is 8.9 Ma (sample 20). This flow conformably overlies the westernmost documented exposure of Neogene clastic deposits. From the thick section of rhyolites overlying clastic sediments in the northern Sierra el Bacatete (Fig. 4D, column V), sample 25 is 11.5 Ma. Northwest of the town of La Colorada (Figs. 2 and 4D; column II) a rhyolitic ignimbrite that overlies the sedimentary rocks is 12.3 Ma (sample 27). A published K-Ar age from the same general location is 11.6 Ma (Bartolini et al., 1994). At Rancho el Churi (Fig. 4D, column III), about 5 km south of La Colorada, a 20-m-thick rhyolitic ignimbrite is 12.7 Ma (sample 26) and is conformably above an 80-m-thick section of clastic sedimentary rocks. Capping felsic rocks have been sampled for dating at scattered localities between Suaqui Grande and Cerro Lista Blanca (Figs. 2 and 4D). Some of these are small outcrops that overlie clastic sedimentary rocks. At one location, an ignimbrite gave an age of 12.3 Ma (Fig. 2; sample 24). At prominent ignimbrite exposures along Highway 16, one sample gave an age of 22.6 Ma (Fig. 2; sample 28), a value that is probably too old. A resampling of the same unit from a nearby location gave a more reasonable age of 12.6 Ma (sample 29). We have no explanation for the older age, and believe it to be spurious. Bartolini et al. (1994) reported a K-Ar feldspar age of 12.8 Ma from a nearby locality. GEOCHEMISTRY Six new major-element analyses have been obtained for samples of mafic lava flows from within the study area. K-Ar ages for the analyzed flows range from 8.9 to 27.7 Ma. No analyses are available for the amphibolebearing lava flows and domes of intermediate composition that are associated with the lower parts of the clastic sections. Table 2 contains the original oxide values prior to any normalizations for volatiles and adjustments for iron oxidation. The analyses were recalculated volatile free and adjusted for Fe+3/Fe+2 = 0.15 for plotting in Figures 5 and 6. On a plot of total alkalies against silica (Fig. 5), these compositions (solid squares) are basaltic andesite to andesite, with one exception (most mafic solid circle), which is basalt. Using the classification of Le Maitre (1989), the compositions straddle the boundaries between subalkaline (basaltic andesite and andesite) and alkaline (trachybasaltic andesite and trachyandesite) fields. In addition to our single analyses from the Tep—ca area, CochemŽ and Demant (1991) published seven major-element analyses for lava flows from the Tep—ca section. All of these flows clearly overlie a significant thickness of clastic sedimentary rocks. These analyses are plotted as open squares in Figures 5 and 6. All of the analyzed Tep—ca flows are andesitic in composition. Also plotted in Figures 5 and 6 are major-element analyses for 12 samples of mafic lava flows that occur above rhyolitic volcanic units of the Sierra Madre Occidental volcanic field from near YŽcora (Bockoven, 1980Ñfilled diamonds; CochemŽ and Demant, 1991Ñopen diamonds). These flows underlie clastic sedimentary rocks. Both on the plot of total alkalies-silica and on a plot of MgO against SiO2 (Fig. 6), it is apparent that there is complete overlap of compositions for mafic-intermediate lava flows that cap felsic sections in the Sierra Madre field, and those that underlie or are interbedded with the oldest clastic sediments in our study area. In contrast, there is a distinct difference between these compositions and those of Neogene mafic lava flows from volcanic sequences in western Sonora, adjacent to the Gulf of California. Those analyses have been summarized by Mora-Alvarez (1992) and are plotted as solid circles in Figures 5 and 6. The following section provides further discussion, and extends these comparisons to include regional suites of analyzed mafic-intermediate igneous rocks that are plotted as fields in Figure 5. TABLE 2. WHOLE-ROCK CHEMICAL ANALYSES SO 60 SO 28 SO 48 Location: Pozo Leyva Suaqui Grande Rio Yaqui 8.9 Ma 23.5 Ma 27.5 Ma Age: SO 14 Tepoca 14.4 Ma SO 40 SO 41 Nuri Nuri 17.1 Ma 27.7 Ma 47.79 57.41 50.68 60.17 53.30 55.08 SiO2 2.10 0.74 1.58 0.76 1.29 1.09 TiO2 15.75 17.08 16.88 16.76 16.96 17.11 Al2O3 6.50 5.69 8.91 2.47 4.88 6.81 Fe2O3 FeO 4.96 0.40 0.83 2.35 2.95 0.50 MnO 0.19 0.09 0.14 0.08 0.13 0.12 MgO 6.85 2.99 4.16 2.18 4.60 3.38 CaO 10.04 6.44 8.24 5.08 7.30 6.51 3.30 3.80 3.50 3.57 3.67 3.58 Na2O 0.37 2.04 1.56 2.97 1.96 2.26 K2O 0.32 0.26 0.99 0.27 0.53 0.42 P2O5 0.50 0.75 0.92 0.32 0.59 0.90 H201.28 0.91 0.92 1.33 0.76 0.98 H2O+ 0.15 0.02 0.04 0.07 0.03 0.02 CO2 TOTAL 100.10 98.62 99.35 98.38 98.95 98.76 Notes: All analyses in percent. SO 60 = Olivine basalt; SO 28 = Fine-grained andesite; SO 48 = Olivine basaltic andesite; SO 14 = Pyroxene andesite; SO 40 = Olivine-pyroxene basaltic andesite; SO 41 = Olivine-pyroxene basaltic andesite. Geological Society of America Bulletin, October 1997 1357 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. DISCUSSION 10 TA 8 %K2O+Na2O TBA TB 6 4 BA B A 2 0 45 49 53 61 57 65 %SiO2 Figure 5. Plot of total alkalies against silica, after International Union of Geological Sciences (IUGS) (Le Maitre, 1989) for mafic and intermediate composition Neogene volcanic rocks from south-central Sonora. Labeled fields are: BÑbasalt; BAÑbasaltic andesite; AÑandesite; TBÑtrachybasalt; TBAÑtrachybasaltic andesite; TAÑtrachyandesite. Solid symbols are used for analyses from this report (Table 2), Bockoven (1980), and Mora-Alvarez (1992). Open symbols are data from CochemŽ and Demant (1991). Squares represent volcanic rocks associated with clastic sedimentary sections, diamonds are used for capping mafic lavas in the western Sierra Madre Occidental volcanic field, and dots are samples from the Circum-Gulf volcanic province (see text). Various lines enclose data from surrounding regions discussed in the text as follows: solid lineÑmid-Tertiary (20-30 Ma) mafic and intermediate rocks from the Mogollon-Datil field of southwest New Mexico (Davis and Hawkesworth, 1993); dash-dot lineÑSCORBA (Cameron et al., 1989); dotted lineÑNeogene (< 20 Ma) mafic rocks from the Mogollon-Datil field of southwest New Mexico (Davis and Hawkesworth, 1995); and dashed lineÑalkalic basalts related to extensional faulting in the Nazas area, Durango (Aguirre-D’az and McDowell, 1993), and Trans-Pecos Texas (Henry and Price, 1986). 8 %MgO 6 4 2 0 45 50 55 60 65 %SiO2 Figure 6. Plot of MgO against SiO2. Sources of data and use of symbols are the same as for Figure 5. 1358 The association of volcanic strata with coarse clastic deposits that accumulated in developing extensional basins provides a means to date the progress of extension in south-central Sonora. In the Rio Yaqui basin, where the thickest and most complete sections of these deposits are found, they are punctuated by the appearance of prominent rhyolite ignimbrites. These rhyolites are dated as 12.5 Ma, and they cover the coarsest and thickest lower two-thirds of the clastic strata. Although most of the basin fill is thus older than 12.5 Ma, the presence of additional clastic material above the ignimbrites and the 25¡ E tilt of the upper section demonstrate that basin-andrange style brittle extension continued in the late Miocene. In the northern Rio Yaqui basin, where the rhyolite strata are absent, there is a distinct unconformity within the clastic section at about the same stratigraphic level. It is not clear whether these observations reflect two discrete episodes or continuous extension. In areas to the west of the Rio Yaqui basin, the rhyolites become thicker and individual units more numerous. Our K-Ar ages for these volcanic rocks have a narrow range, from 12.8 to 11.5 Ma for 11 samples taken over an area of 100 × 50 km. Three published K-Ar ages (Bartolini et al., 1994) from within the same area are generally consistent, but have a longer range, from 12.8 to 10.4 Ma. For the thickest section of these rocks at Cerro Lista Blanca, available K-Ar ages range from 12.8 to 10.4 Ma. These results indicate that deposition of the greatest thickness of clastic sediments associated with extension was completed in the region before 12.8 Ma, but that some deposition and tilting continued to occur, at least in some areas. Neither the K-Ar ages nor the continuity of outcrop allow specific correlation of these rhyolitic units across the study area. Highprecision Ar-Ar dating, perhaps along with systematic paleomagnetic measurements, will be needed to fully exploit the potential of these rocks as time and stratigraphic markers. In western Sonora, to the west of the area shown in Figure 2, volcanic rocks equivalent in stratigraphic position and age become thicker and more diverse. In contrast, clastic sedimentary deposits are not evident to the west, although in many places the base of the volcanic section is not exposed. In western Sonora, McDowell et al. (1993) assigned volcanic rocks of this general age and character to the circum-Gulf volcanic province. Within the Sierra Santa Ursula and surrounding ranges (Fig. 1), an age range of 23 to 11.5 Ma has been established for the felsic volcanism of this province (Mora-Alvarez, 1993). The area of Tep—ca, about 40 km to the west of YŽcora, provides the easternmost locality where volcanic rocks overlie extension-related clastic sedimentary strata (CochemŽ and Demant, 1991). These rocks, andesite rather than rhyolite in composition, have K-Ar and Ar-Ar ages between 17.5 and 14.5 Ma (CochemŽ and Demant, 1991; Bartolini et al., 1994; Gans, 1997; Table 1). Deposition of clastic sedimentary rocks was completed in the Tep—ca area before about 17 Ma. Dating of volcanic rocks associated with the lower parts of the clastic sections indicates when extension began in south-central Sonora. One complication arises from the ambiguity in stratigraphic position and in composition (see the following) between basaltic andesite lava flows that directly underlie all clastic strata and those that cap the rhyolitic sections of the Sierra Madre Occidental volcanic field. Where such lava flows are directly and conformably overlain by sedimentary strata, and where the overlying section contains interbedded lavas of similar aspect, we have assigned them to the clastic section. Our K-Ar dating of these rocks indicates that formation of structural basins and accumulation of coarse clastic sediments began locally as early as 27 Ma (Fig. 4). The distribution of these ages, however, suggests that each basin had a distinct history of development. We find no Geological Society of America Bulletin, October 1997 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 TERTIARY EXTENSION IN SONORA, MEXICO systematic geographic pattern to that development. The most comprehensive age controls are from the southern portion of the Rio Yaqui basin. There, distal units from the Sierra Madre Occidental volcanic field that underlie clastic sediments have ages of 33 and 27 Ma. At Arroyo las Palomas (Fig. 4A; column II) a basal mafic lava flow that we have assigned to the sedimentary section is dated as 27 Ma. Thus, the beginning of basin evolution at this locality is possibly 27 Ma. Elsewhere in the basin, mafic flows in a similar stratigraphic position are dated as 20 Ma. Further dating is warranted for the northern section at Arroyo el Salto (Fig. 4A; column III), where numerous volcanic layers bounding and within the clastic section have been documented. To the west near Suaqui Grande a mafic lava flow, probably at the base of the clastic section, is dated as 23.5 Ma. From areas to the south of Tep—ca, available K-Ar and Ar-Ar ages for mafic flows are between 16 and 28 Ma (Table 1; Bartolini et al., 1994; Gans, 1997). Apparently, flows 19 Ma and younger are intercalated within clastic sedimentary sections, whereas those 22 Ma and older are in a basal or nearbasal position. East of Tep—ca, extensional basins were superposed on the western margin of the Sierra Madre Occidental volcanic field, and only older limits to the beginning of extension can be dated. The youngest volcanism in this part of the Sierra is generally about 24 Ma, somewhat younger than that within other parts of the field (McDowell et al., 1990). East of YŽcora (Fig. 4C; column II), this volcanism is characterized by an intercalation of felsic ignimbrites and lava flows with massive lava flows of basaltic andesite that are as young as 18 Ma (Bockoven, 1980; CochemŽ and Demant, 1991; McDowell, 1993). At Arroyo los Pilares (Fig. 4C; column III), a thin section of clastic sedimentary rocks overlies all volcanic rocks. Although no dated locality is nearby, a general older limit for deposition of these sediments is 24 Ma. Another opportunity to establish timing for the beginning of extension is from dating of restricted but massive amphibole-bearing domes and lava flows of intermediate composition that are closely related to faulting at the margins of the basins. These occur as discontinuous masses along or near the margins where the greatest thicknesses of clastic sedimentary rocks are found. Thus, they appear to mark the active side of half grabens. K-Ar ages from three localities are 25.3, 25.4, and 20.5 Ma. Near La Colorada, in the western portion of the area shown in Figure 2, a similar, intermediatecomposition flow breccia is 24.0 Ma. Limited age results from northern Sonora are consistent with the timing we have determined for deposition of clastic sediments. Bartolini et al. (1994) showed that volcanic layers that predate sedimentary deposits are as young as 24 Ma. Most of their dated volcanic units intercalated with sedimentary layers are 21 to 23 Ma, but one age as young as 13 Ma was quoted. Their ages for overlying volcanic rocks are much younger at 5.5 and 7 Ma. Bartolini et al. (1994) also summarized K-Ar ages for volcanic rocks from coastal Sonora, including many published by Gastil and Krummenacher (1977). Most of those rocks appear to belong to the Miocene convergentmargin arc that existed in the Gulf region from 24 to about 11.5 Ma (Stock and Hodges, 1989; Sawlan, 1991; Mart’n-Barajas et al., 1994). No continental clastic deposits are known to be stratigraphically associated with these rocks. Neogene sedimentary rocks on Isla Tibur—n (Fig. 1; Neuhaus et al., 1988) are partly marine and related to the initial opening of the Gulf of California rift system. Neither ages nor major-element compositions provide a basis for distinguishing basaltic andesite lava flows that occur within or near the base of the clastic sequences from those that occur as capping lava flows in the Sierra Madre Occidental volcanic field. On a diagram of total alkalies versus SiO2 (Fig. 5), these compositions completely overlap. Also shown in Figure 5 are fields that encompass analyses for 30Ð20 Ma mafic to intermediate lavas of the Mogollon-Datil volcanic field in southwestern New Mexico (Davis and Hawkesworth, 1993), and for capping basaltic andesite lava flows (Southern Cordilleran basaltic andesites [SCORBA]) from a regional study across the Sierra Madre Occidental volcanic field (Cameron et al., 1989). It is evident that the compositions of all of these groups are similar with respect to alkali and silica contents. The only significant difference might be that the SCORBA suite occupies a more restricted compositional range. Wark (1991) showed that, at the T—mochic Volcanic Center in the central portion of the Sierra Madre Occidental, capping basaltic andesite lavas equivalent to SCORBA have chemical and isotopic compositions that support a genetic relationship to the underlying felsic volcanic section, and that they therefore probably represent the final stage of volcanism in the Sierra Madre Occidental volcanic field. Another group of analyzed rocks plotted in Figures 5 and 6 are of mafic lava flows associated with thick volcanic sequences adjacent to the Gulf of California (the Circum-Gulf volcanic province of McDowell et al., 1993). These data were summarized and discussed by Mora-Alvarez (1993). The stratigraphic setting of the Circum-Gulf lavas is similar to that for mafic lavas in the Sierra Madre Occidental volcanic field, that is, they occur directly above felsic lavas and tuffs. However, they are significantly younger, having K-Ar ages from 11 to 8.5 Ma (Mora-Alvarez, 1993). As is clear from Figures 5 and 6, they have lower concentrations of alkalies and are higher in MgO at equivalent SiO2 content. One analysis reported in Table 2 (sample SO 60) is from the westernmost locality described in this study (Pozo de Leyva, Fig. 4D, column IV). It is a true basalt, unlike the basaltic andesites of the remaining analyzed samples. We place the Pozo de Leyva lava flow within the Circum-Gulf group, because of its similarity in age, although it has lower silica content than others of that group (Fig. 5). Other analyses plotted as fields in Figure 5 include post-20 Ma mafic rocks from the Datil-Mogollon province (Davis and Hawkesworth, 1995), and 20Ð24 Ma mafic lava flows from Trans-Pecos Texas (Henry and Price, 1986), and from Nazas, Durango (Aguirre-D’az and McDowell, 1993). The latter two groups are clearly associated with brittle extension in their respective areas. They are alkali basalts and tend to have higher alkali contents than the Neogene basaltic andesites from Sonora (Fig. 5). CONCLUSIONS (1) The early stage of Tertiary extension in south-central Sonora is marked by deposition of clastic sediments in discontinuous linear grabens and half-grabens. These basins are particularly well exposed throughout the eastern two-thirds of the state. In their easternmost occurrence these tectonic basins were superimposed on the western margin of the Sierra Madre Occidental volcanic field. (2) Potassium-argon dates of mafic flows conformably beneath the sedimentary column are as old as 27 Ma, and may reflect the earliest time of Tertiary extension in south-central Sonora. Dated mafic flows within the basal portion of the clastic sedimentary section, as well as amphibolebearing intermediate-composition domes and lava flows that occur along the basin margins, indicate that extension was underway in most basins by 24 Ma and across the entire area by 20 Ma. This timing is generally coincident with the development of metamorphic core complexes in northern Sonora and parts of Arizona, Nevada, and Utah (Nourse et al., 1994, and references cited therein). It also coincides with the development of a 24 to 11.5 Ma magmatic arc in western Sonora and Baja California, in the area of the present-day Gulf of California. Early extension in central Sonora was geometrically in a back-arc position. (3) The massive lower portions of the clastic sedimentary sections typically are from 100 to 500 m thick. Throughout much of the region these sections are capped, generally conformably, by felsic volcanic rocks having KAr ages of 12.8 to 11.5 Ma. This narrow age range provides a convenient time and stratigraphic marker within the period of extension. It does not nec- Geological Society of America Bulletin, October 1997 1359 Downloaded from gsabulletin.gsapubs.org on April 5, 2011 MCDOWELL ET AL. essarily indicate that there were two discrete periods of extension. Continued sedimentation and rotation of beds younger than 11.5 Ma show that extension continued in late Miocene time. During this time extension was coeval with initial evolution of the Gulf of California extensional province. (4) The composition of mafic-intermediate lava flows at and near the base of the clastic sedimentary sections is similar to that of the capping basaltic andesite lava flows from the Sierra Madre Occidental volcanic field. This resemblance includes the regional suite known as the Southern Cordilleran basaltic andesites (SCORBA), and mafic to intermediate lava flows dated as 20 to 30 Ma from the Datil-Mogollon volcanic field of southwestern New Mexico. All of these compositions are distinct from those of 11 to 8.5 Ma capping mafic lavas in the Gulf of California extensional province, and mafic lava flows younger than 20 Ma from the Datil-Mogollon field. They are also compositionally distinct from 20 to 24 Ma alkali basalts both from Trans-Pecos Texas and from the Nazas area of northern Durango. The Texas and Nazas rocks are true alkali basalts (Fig. 5) that are clearly associated with extensional fault systems. ACKNOWLEDGMENTS This study was supported by National Science Foundation grants EAR9204635 and EAR-8720380 to McDowell, and by CONACYT Project 1204T9203 to Rold‡n-Quintana and Amaya-Mart’nez. Sample LB-2 was collected by Edward Erlich, and samples 3 20 4, 3 21 2, and 3 21 3 were collected by Neil Bockoven. Gabriela Mora-Alvarez, Gerardo Aguirre-D’az, Andrew Bowen, and Tony Faucett helped with mineral separations and with the K-Ar analyses. An earlier version of this manuscript was critiqued thoroughly by Christopher Henry and John Stewart. Bulletin editor Myron Best and reviewers John Dilles and Phillip Gans provided careful and thoughtful reviews that guided the final revisions of this manuscript, as did Chris Henry in an additional review. Gans also provided a preprint of his manuscript in press. REFERENCES CITED Aguirre-D’az, G. J., and McDowell, F. 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V., 1989, Pre-Pliocene extension around the Gulf of California and the transfer of Baja California to the Pacific Plate: Tectonics, v. 8, p. 99Ð115. Wark, D. A., 1991, Oligocene ash-flow volcanism, northern Sierra Madre Occidental: Role of mafic and intermediate-composition magmas in rhyolite genesis: Journal of Geophysical Research, v. 96, p. 13389Ð13411. Zoback, M. L., Anderson, R. E., and Thompson, G. A., 1981, Cainozoic evolution of the state of stress and style of tectonism of the Basin and Range province of the western United States: Royal Society of London Philosophical Transactions, ser. A, v. 300, p. 407Ð434. MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 1, 1996 REVISED MANUSCRIPT RECEIVED MARCH 5, 1997 MANUSCRIPT ACCEPTED APRIL 2, 1997 Printed in U.S.A. 1360 Geological Society of America Bulletin, October 1997