Revista Mexicana de CiVenoclciaasn iGsmeo alóngdi cmaas,r ivn.e 1 i8n,c nuúrmsio. n2s, 2in0 0th1e, pS.a 1n1 L1o-1r2en8z o Archipelago, Gulf of California 111 Late Miocene volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California, Mexico Felipe de Jesús Escalona-Alcázar*, Luis Alberto Delgado-Argote, Margarita López-Martínez, and Gabriel Rendón-Márquez Departamento de Geología, División Ciencias de la Tierra, CICESE, Km. 107 Carr. Tijuana-Ensenada, C.P. 22860, Ensenada, Baja California. *Present address: Instituto de Ecología y Medio Ambiente de Zacatecas, GODEZAC, and Unidad Académica de Minas, Metalurgia y Geología, Universidad Autónoma de Zacatecas, e-mail: [email protected] ABSTRACT During Late Miocene time extensional deformation and restricted volcanic activity related to the vanishing Miocene volcanic arc occurred in the margins of the northern part of the present Gulf of California. As a result, local basins with periodical influence of marine water were formed in some areas like Bahía de los Ángeles, Bahía Las Ánimas, Ángel de la Guarda Island and the San Lorenzo Archipelago. The stratigraphic sequence of the San Lorenzo Archipelago is formed by a series of evaporitic and clastic deposits that overlie a crystalline basement and indicate alternate transgressive and regressive periods. Minor effusive andesitic volcanism, normal faulting and sedimentation occurred simultaneously; the end of local sedimentation coincides with an increase of basaltic to andesitic volcanic activity 5 ± 1 Ma ago. The stratigraphic sequences of San Lorenzo Island and eastern Sierra Las Ánimas in the peninsula are correlative. In the Sierra Las Ánimas, the volcanic and coarse-grained sedimentary rocks predominate, and represent the western limit of a basin. In the Sierra Las Ánimas, the age of andesitic lava underlying the sedimentary rocks is 7.8 ± 0.2 Ma, bracketing the age of the marine sedimentary sequence in the Late Miocene (approximately 7.8 to 5 Ma). The present position of the northern part of the archipelago with respect to the sierra indi- cates that the islands moved towards the southeast as a rigid block, along the San Lorenzo and Partida fault zones. Keywords: Late Miocene, Pliocene, volcanism, marine incursions, San Lorenzo Archipelago, Gulf of California. RESUMEN Durante el Mioceno Tardío, las márgenes de la parte septentrional del Golfo de California actual experimentaron una deformación extensional y actividad volcánica restringida relacionada con el arco volcánico miocénico en extinción. Como consecuencia, en áreas como Bahía de los Ángeles, Bahía Las Ánimas, Isla Ángel de la Guarda y el Archipiélago San Lorenzo, se formaron cuencas locales con influencia de incursiones periódicas de agua marina. La secuencia estratigráfica del Archipiélago San Lorenzo está formada por series de depósi- tos clásticos y evaporíticos que sobreyacen a un basamento cristalino e indican periodos alternados de transgresiones y regresiones. Sedimentación, volcanismo efusivo andesítico restringido y falla- miento normal son contemporáneos en el archipiélago; la finalización de la sedimentación coincide con un incremento en la actividad volcánica andesítica y basáltica fechada en 5 ± 1 Ma. Las secuen- cias estratigráficas de la Isla San Lorenzo y el oriente de la Sierra Las Ánimas en la península son correlacionables. En la Sierra Las Ánimas predominan las rocas rocas volcánicas y sedimentarias de grano grueso del extremo occidental de la cuenca formada por la sierra y el archipiélago. La 112 Escalona-Alcázar et al. edad de una lava andesítica que infrayece a las rocas sedimentarias es 7.8 ± 0.2 Ma, restringiendo la edad de la secuencia sedimentaria marina al Mioceno tardío (aproximadamente entre 7.8 y 5 Ma). La posición actual de la parte norte del archipiélago con respecto a la sierra indica que las islas se movieron como un bloque rígido hacia el sureste a lo largo de las zonas de falla San Lorenzo y Partida. Palabras clave: Mioceno tardío, Plioceno, volcanismo, incursiones marinas, Archipiélago San Lo- renzo, Golfo de California. INTRODUCTION REGIONAL GEOLOGIC FRAMEWORK The San Lorenzo Archipelago is one of a group of An overview of the geology of the State of Baja large islands detached from either the peninsular or the California and the islands of the northern Gulf of Cali- continental mainland, as a consequence of the extension fornia is presented in Gastil et al. (1975). These authors and right lateral slip faulting during evolution of the Gulf show that Tertiary volcanic and sedimentary rocks, of California. During the entire process, basins with ranging in age from Miocene to Recent, occupy exten- ocean-like floor developed in the central and southern sive areas of the eastern margin of the peninsula, and rest parts of the Gulf, while smaller and restricted basins on Paleozoic metamorphic rocks and Cretaceous formed along the eastern margin of the Baja California granitoids (Gastil et al., 1975). peninsula. The 27 km-long archipelago formed by the The regional metamorphic basement is composed San Lorenzo and Las Ánimas islands is located 20 km of a metasedimentary sequence and metalavas of Devo- northeast of Bahía San Rafael, in the central part of the nian age. Paleozoic environments of deposition in the Gulf of California (Figure 1). Previous works postulated northeastern margin of the peninsula range from conti- that the main stage of the volcanic activity in the islands nental slope to basin (Campbell and Crocker, 1993). The was Miocene (Gastil et al., 1975; Desonie, 1992). How- sequence that crops out in the Sierra La Asamblea corre- ever, Gastil et al. (1979) suggested that a sequence lates with similar sequences from northern Baja Califor- formed by basaltic lava flows with interstratified marine nia and southern Sonora (Canal de Ballenas Group, in sediments in the San Lorenzo Island could be Pliocene. Campbell and Crocker, 1993). The metamorphic rocks The volcanosedimentary sequence of the archipel- are intruded by granitic Cretaceous plutons, that crop out ago is similar to that exposed in the eastern side of the extensively in the northern part of the peninsula to lati- Sierra Las Ánimas. There, the basement is made up of tude 28°N (Gastil et al., 1975). Intrusive rocks are absent Paleozoic metamorphic and Cretaceous granitic rocks. between Bahía San Rafael and Bahía Las Ánimas. The western side, and part of the top of the Sierra Las However, at latitude 28.3°N, plutons crop out along a Ánimas, is covered by felsic pyroclastic rocks, and ande- 30 km wide, E-W oriented continuous strip (Figure 1). sitic to basaltic lavas ranging in age from middle to late After a reconstruction of the paleoposition of the archi- Miocene (Delgado-Argote and García-Abdeslem, 1999). pelago, 60 km northwest of its present position, based on On the eastern side of the sierra, the volcanic rocks seem the closure of the major basins of the Gulf of California to be younger and are closely related to local sedimen- (Delgado-Argote, 2000), the granitic rocks of southern tary basins (Delgado-Argote, 2000). San Lorenzo Island are located East of the Sierra Las The only available geologic map of the San Ánimas, being part of the same plutonic complex. The Lorenzo Archipelago is at a scale of 1:250,000 (Gastil volcanic and sedimentary sequence of the island can also et al., 1975). In this map, Paleozoic schist and quartzite, be correlated with part of the Late Miocene (?) strata Cretaceous tonalite, and Miocene felsic volcanic rocks described in the northern and central parts of Bahía Las form the main lithologic units. Ánimas (Vázquez-Jaimes, 2000). In this paper we present new isotopic ages data of According to Sawlan (1991), the age of the calc- volcanic rocks from the San Lorenzo Island, a strati- alkalic arc-related volcanism in the eastern margin of the graphic correlation with the volcanosedimentary sequen- peninsula ranges from 24 to 12.5 Ma. Arc-like volcanic ce of eastern Sierra Las Ánimas, and a description of the activity has occurred continuously around the Gulf since stratigraphic units of the San Lorenzo and Las Ánimas Late Miocene time, and it has been documented in the islands, with emphasis on the Cenozoic cover. Also, we Puertecitos area (Martín-Barajas et al., 1995), Ángel de present the results of a provenance analysis and our la Guarda Island (Delgado-Argote, 2000), San Esteban interpretation of the sedimentary depositional environ- Island (Desonie, 1992), La Reforma-El Aguajito-Tres ment. Vírgenes volcanic complex (Demant, 1981; Sawlan, Volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California 113 USA G ulf of Pacific México Ángel de la Guarda I. Cali Ocean f o r ni Tiburón I. a BFZ TFZ 29ºN NSB BA TB SLA PFZ BAN SSB SAS SPMFZ Figure 5 SLFZ San Esteban I. Figure 2 San Lorenzo Archipelago 28º30'N BSR San Francisquito SPMB 0 12.5 25 km 114ºW 113ºW Plio-Quaternary sedimentary rocks Late Tertiary acidic volcanics (includes alluvial, lacustrine and shallow (includes lava and pyroclastic flows). marine deposits). Late Tertiary sedimentary rocks C retaceous granitoids. (includes continental and marine deposits). Paleozoic and Mesozoic metamorphic rocks (includes Triassic-Jurassic sandstones and shales, Late Tertiary basaltic rocks. and Paleozoic metasediments). Normal fault Fossil transform fault Spreading center Transform fault Structural lineament Fossil spreading center Figure 1. Lithologic units and regional structural features of central Gulf of California and Baja California peninsula. Abbreviations: BA: Bahía de los Ángeles, BAN: Bahía Las Animas, BFZ: Ballenas Fault Zone, BSR: Bahía San Rafael, NSB: North Salsipuedes Basin, PFZ: Partida Fault Zone, SAS: Sierra Agua de Soda, SLA: Sierra Las Animas, SSB: South Salsipuedes Basin, SLFZ: San Lorenzo Fault Zone, SPMB: San Pedro Mártir Basin, SPMFZ: San Pedro Mártir Fault Zone, TB: Tiburón Basin, TFZ: Tiburón Fault Zone (modified from Delgado-Argote, 2000). 114 Escalona-Alcázar et al. 1991, Capra et al., 1998; Garduño-Monroy et al., 1993), Neogene sediments in the largest islands of the northern La Purísima area (Sawlan, 1991) and at the Mencenares Gulf of California, as well as in the Loreto, Santa volcanic field and Coronado Island (Bigioggero et al., Rosalía, San Felipe, Laguna Salada and Imperial Valley 1995). In some of these places, this post-subduction, areas are Late Miocene to Pliocene in age (Smith, 1991). localized volcanism has a structural control associated with the evolution of the Gulf of California (Sawlan, 1991; Bigioggero et al., 1995). Methods Most of the exposed lithologic units between Bahía de los Ángeles and Bahía San Rafael are Neogene Since topographic maps of San Lorenzo and Las volcanic rocks (Delgado-Argote, 2000; Figure 1). Geo- Ánimas islands are not available, geologic mapping was logically, the best-known region near the study area is conducted with aerial photographs (INEGI, scale Bahía de los Ángeles, where the oldest recognized rocks 1:20,000). Structural mapping and sampling of represen- resting on the granitic basement are 18.8 ±(cid:31) 1.0 Ma old tative lithologic units was made during field verification andesitic flows. The andesitic flows are pillowed and of the photogeologic map. For the petrographic analyses, include undifferentiated sandstones in the interpillow we adopted the modal point counting method of Gazzi- cavities. The andesitic flows are covered by 14.0 D i ckinson (more than 500 points per section; Ingersoll ± (cid:31)0.1 Ma old ignimbrites and 12.1 ±(cid:31) 0.1 Ma old basaltic et al., 1984); for volcanic rocks, crystals less than flows (Delgado-Argote et al., 2000 and Delgado-Argote 0.1 mm long were considered matrix. Texture names and García-Abdeslem, 1999). The islands of the Gulf of used for igneous and metamorphic rocks follow California have been poorly studied. Regional geologic MacKenzie et al. (1991) and Phillpotts (1989) respec- mapping of Angel de la Guarda Island was initially made tively, and the classification and nomenclature used for by Gastil et al. (1975), and later, Escalona-Alcázar and sedimentary rocks is that of Adams et al. (1991) and Delgado-Argote (1998) conducted a detailed study in the Pettijohn et al. (1987). region of El Paladar, located in the southwestern coast of Volcanic rock samples were dated in the Laborato- Ángel de la Guarda Island, where the stratigraphic rio de Geocronología of CICESE by the K-Ar method. sequence is made up of air-fall tuffs deposited in a low The potassium analyses were run by duplicate on a Cole- energy marine environment, andesitic lavas and dacitic Parmer flame photometer Model 2655-00 and in a pyroclastic flows. The close spatial relationships be- Thermo Jarrell AA-Scan 1 atomic absorption spectrome- tween the andesitic lavas and dacitic domes dated at 3.0 ter. Sample preparation techniques are described in – 0.1 Ma (Delgado-Argote, 2000), indicate that the vol- Mora-Álvarez and Moreno-Rivera (1994) and Moreno- canic rocks located in the southern part of the island Rivera and López-Martínez (1996). Argon isotopic extruded contemporaneously. Late Pliocene volcanic analyses were made by triplicate with a MS10 mass activity is widespread in the central part of the Gulf of spectrometer, and sample preparation techniques are California and it has also been documented in the San after Moreno-Rivera and López-Martínez (1996). Esteban (Desonie, 1992), Tiburón (Gastil and Krummen- acher, 1977; Gastil et al., 1979) and Coronado (Bigioggero et al., 1995) islands (Figure 1), and in Cerro STRATIGRAPHY OF THE SAN LORENZO AND Mencenares (Bigioggero et al., 1995) and Puertecitos LAS ÁNIMAS ISLANDS (Martín-Barajas et al., 1995) in the peninsular mainland (the last three areas are not shown in figures). Paleozoic (?) greenschist facies metamorphic rocks Marine fossil invertebrates with Caribbean affini- and a tonalitic pluton crop out extensively in the central ties are found in the present gulf region since middle and southern part of San Lorenzo Island forming the Miocene times (Smith, 1991). Marine incursions proba- crystalline basement. Metamorphic rocks of the central bly occurred during the early Miocene, since fossilifer- part of San Lorenzo Island include metapelites, metadio- ous marine sediments occur in the matrix of 17.7 r i t e and metalavas (Figure 2). Metadiorite from the base ±(cid:31) 0.6 Ma old brecciated andesitic to dacitic flows em- of the metamorphic sequence shows poorly defined placed into sandstones in the northeastern part of Ángel foliation, and it is composed mainly by plagioclase de la Guarda Island (Delgado-Argote, 2000). These (An -An ), biotite and quartz (sample 3, Table 1). 28 40 rocks, together with the pillow lavas of the Sierra Las Metapelites occupy the upper part of the sequence in Ánimas, which probably are also associated with marine central and western San Lorenzo Island, where foliation environments, are the oldest evidences of marine incur- is well defined by the distribution of biotite and silli- sions in the Gulf area. Mollusk distribution around the manite (sample 4, Table 1); locally, euhedral andalusite Gulf of California indicates that during Middle Miocene and poorly defined crenulation cleavage is also observed. time the proto-gulf extended as far north as the Salton In the western part of San Lorenzo, the metasedimentary Trough (Smith, 1991). Bivalve-bearing Middle Miocene rocks are cut by E-W trending felsic dikes less than marine sediments have been documented in Tiburón 20 cm thick; the uppermost part of the metamorphic Island (Gastil et al., 1979; Smith, 1991) and Bahía de los sequence in the eastern part of San Lorenzo Island is in- Ángeles (Delgado-Argote et al., 2000). Most of the truded by a 2 to 4 m thick andesitic sill (sample 2, Table Volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California 115 112°55' 112°50' 112°45' 114º W 113º W 112º W LAS ANIMAS ISLAND N Las Ballenas ChannelÁndgeeG llIuasalradnad TI Sonora29º N BA 28°40´ 28°40' PUERSGTARTEOCRHIOO GULF OF CALIFORNIA GuerrBearjoa CNaleifgorrnoia PeBniAnsNulaSaBlsiSpuRedes ChannelSan LoSreaGnnuz lEof soAtfe Crbcaahlniifpo Ire.nliaago28º N AE SAN LORENZO ISLAND AA Figure 2a Alluvial and beach deposits S A L Volcanic and sedimentary rocks SI P U E Marine sediments: Gypsum beds DE sandstones and conglomerates S C H A Granitic rocks N N E Metamorphic rocks L Normal fault Oblique fault 28°33´ 28°35' Structural lineament 0 1 2 4km Figure 2. Simplified geologic map of San Lorenzo and Las Ánimas islands. AA= Arroyo Las Águilas and AE= Arroyo Los Esqueletos. Table 1. Petrographic synthesis of representative intrusive and metamorphic rocks from San Lorenzo Island. Classification and L C Coord Qz Plg K Bio Chl Calc Sil And PlgMz PlgF HbMz HbF Indif. Texture 112°52.2'/ Tonalite. Granular 1 V-98-4 29.4 56.7a 1.5 11.6c 0 x 0 0 0 0 0 0 0 112°52.2' anhedral. 112°49.3'/ Andesitic sill. Inequi- 2 X-98-32 1.6 0 0 0 0 0 0 21.9a 37.4a 6.2c 19.3c 12.8 112°49.3' granular anhedral. 112°50.0'/ Metapelite. Shistose 4 X-98-18 42.7 0 0 29.4 0 0 18.6 9.2c 0 0 0 0 0 112°50.0' recrystallized 28°38.0'/ Metadiorite. 3 X-98-17 4.9 42.5 0 29.1c 23.2 x 0 0 0 0 0 0 0 112°49.5' Recrystallized ABBREVIATIONS: L= laboratory sample number, C= field sample number. CONTENT: And= andalusite, Bio= biotite, Calc= calcite, Chl= chlo- rite, HbF= hornblende phenocrysts, HbMz= hornblende matrix, Indif.= undifferentiated (possibly quartz and/or feldspar), K= potassium feldspar, Plg= plagioclase, PlgF= plagioclase phenocryst, PlgMz= plagioclase matrix, Qz= quartz, Sil= sillimanite. MISCELLANEOUS: a= hydrothermal al- teration, c= chloritization, x= abundance <1%. 116 Escalona-Alcázar et al. 112º55'W Las Animas Island N G ulf of C Quaternary alif or ni a Alluvial and beach deposits Pliocene S Estrecho Upper lithic tuff alsi Purgatorio p San Lorenzo Island u Pumice and lithic tuff e Brecciated lithic tuff des 38° 39° 25° C h Crystalline tuff a n n Basalt el 28°40'N Lapilli-rich crystalline tuff Andesitic lavas 39° Normal fault Late Miocene 5±1 Ma K-Ar, whole rock Marine sedimentary sequence Oblique fault AE composed by gypsum, Structural lineament sandstone and conglomerate 30° Dike Gypsum beds. 26° AA Fossiliferous locallity Paleozoic 12° Dip and strike of strata 16º 18º 32° Metasedimentary rocks 0 500 1000 m 3.8±0.3 Ma K-Ar, whole rock 15° 43° Figure 2a. Detailed geologic map of northern San Lorenzo and Las Ánimas islands. See Figure 2 for location and abbreviations. 1). A strong silicification and brecciation extends coast. From the central part of San Lorenzo Island, the approximately 3 m around the sill. Since the metamor- evaporitic beds thin rapidly towards the northwest and phic sequence of San Lorenzo Island is lithologically gradually to the northeast. In the western coast the first similar to that described in the Sierra La Asamblea five meters of the basal section contains two interbedded (Campbell and Crocker, 1993) and the Sierra La Liber- silicified sandstone strata and an andesitic dome cuts the tad (Delgado-Argote and García-Abdeslem, 1999), we gypsum deposit. The dome is markedly argillitized and assume that they are part of the same lithodemic unit. oxidized (sample 5, Figure 3, Table 2). The southern part of the San Lorenzo Island is On the eastern coast, the central part of the formed by a biotite-hornblende bearing tonalite pluton evaporitic sequence contains interbedded sandstones and that intrudes the metamorphic rocks. Contact relation- a 60 cm-thick tuff horizon. The evaporite deposits of the ships are sharp, schist screens are scarce, and moderate top of the western coast sequence are interbedded with foliation is locally observed. A petrographic analysis of a yellow and gray arcosic arenite; differences in color and representative tonalite is given in Table 1. Andesitic granulometry indicate lateral facies change (samples 6 dikes 40 to 50 cm thick intrude the pluton in the western and 7, Table 3, Figure 3). The 25 to 60 m thick yellow side of San Lorenzo. sandstone (Unit M, Figure 3) is comprised of 10 cm to The basement of the northern part of the San 1 m thick, well-consolidated beds. This is a well-sorted Lorenzo Archipelago is unconformably overlain (locally deposit varying from mature to submature sand and by normal faults) by a tertiary volcano-sedimentary cemented by calcite. To the southeast, outcrops of the sequence (Figures 2 and 2a). The bottom of the sequence yellow sandstone changes laterally to a gray sandstone is formed by a gypsum unit, which is 5 to 30 m thick on (Unit L, Figure 3) showing thickness variations from 30 the western coast, and less than 7 m thick on the eastern to 60 m, with beds 10 cm to 1 m thick. Both, the yellow Volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California 117 COMPOSITE STRATIGRAPHIC COLUMN OF ISLA SAN LORENZO 30-40 m 35 LEGEND 34 30-40 m 33 A) Upper lithic tuff 32 31 B) Pumice and lithic tuff C) Brecciated lithic tuff >240 m 20-30 m 223890 DEF))) CLBAaranypsdsaieltllastiil-tlriiicnc ehf l tocuwrfyfss/talline tuff 20-30 m 27 G) Dacitic domes 3A.8n–de0s.3it iMc fal o(wwhole rock)H) Basaltic andesite 26 I) Limestone 25 K-Ar Basaltic flow 5–1 Ma (whole rock) J) Tuff 10-20 m 2243 K - A r K) Polimictic conglomerate 10-15 m 22 L) Gray sandstone 21 2109 M) Yellow sandstone 18 N) Gypsum 17 Biotite-hornblende 16 O) bearing tonalite 15 K-Ar 30-40 m 7 - 1 0 m 1134 * P) Andesitic sill 12 11 Q) Greenschists 30-50 m 1 0 -15 m 10 9 8 20-30 m 7* 6 SIMBOLOGY 5-30 m 5 Disconformity >100 m 34 Facies change 2 Vertical scale >200 m 0 50 m 1 Figure 3. Composite stratigraphic column of San Lorenzo Island. Right side numbers indicate petrographic samples, K-Ar dated samples (italics), and asterisk indicates location of fossil samples. Units (G) of andesitic lavas and dacitic domes, (O) tonalite and (Q) greenschists (the last two units are out of scale). For clarity, domes are shown as part of unit (G). and gray sandstones gradually change upwards to a granite, schist, dacite, andesite, tuff and minor quartzite polimictic conglomerate. The transition zone is more and basalt in a coarse-sand matrix. In the southwestern than 1.5 m thick, and it is characterized by 30 cm thick part of San Lorenzo Island, near to the top of the unit, alternating strata of sandstone (sample 7, Figure 3, Table the conglomerate includes a tuff deposit (Unit J, Figure 3) and conglomerate. The gray clastic deposits in the 3) covered by basaltic andesite lava (Unit H, Figure 3) transition zone are cemented by calcite, and are poorly whose petrographic characteristics are shown in Table 2 sorted. One sample from this zone contains fragments of (samples 10 and 11, respectively). The conglomeratic pectinids and benthic foraminifers (Figures 4a and 4b, unit is covered by gypsum beds that, in turn, are overlain respectively). The conglomeratic unit is reddish brown to by yellow arcosic arenite (sample 12, Table 3). Near the brown, 30 to 50 m thick, formed by 1 m wide strata, and mouth of the Arroyo Las Águilas (Figure 2), this sand- including conglomeratic sandstones (Unit K, Figure 3). stone contains interbedded calcareous mudstone (Unit I, It is composed of poorly sorted subangular fragments of Figure 3) with fragments of bivalves (pecten) (Figure 118 Escalona-Alcázar et al. Table 2. Petrographic synthesis of selected volcanic rocks from San Lorenzo and Las Ánimas Islands. LAVA FLOWS L C Coordinates Mz PlgMz PlgF HbMz HbF CpxMz CpxF OlMz OlF Op R.C. Textures 28°39.18’/ Porphyrotrachytic and 29 V-98-15 46adv 17 22gy 3 6agop 0 3g 0 0 3 A 112°51.5’ pilotaxitic 28°39.2’/ Porphyritic and 26 V-98-12 51adv 17 23gy 0 3go 2 4ag 0 0 x A 112°51.5’ pilotaxitic 28°38.7’/ Porphyrotrachytic, 25 V-98-1 33 21 32 2 6 0 1 0 0 5 A 112°51.7’ seriated and poikilitic Porphyrotrachytic, 28°39.5’/ 24 X-98-27 49o 11 28g 6 5g 0 0 0 0 x B seriated and 112°52.4’ pilotaxitic 28°39.6'/ Porphyritic and 23 X-98-26 50adv 25 19g 1 5go x 0 0 0 x B 112°52.3' hyalopilitic 28°39.1'/ 21 V-98-11 35o 33 20 4 5aop 0 x 0 0 2 A Porphyrotrachytic 112°51.7’ 28°39.0'/ Hypocrystalline, 20 V-98-10 63aev 9 14a x 3o 0 x 0 0 x A 112°51.8' seriated and axiolitic 28°39.6'/ Porphyritic and 19 X-98-25 56a 21 17g 3o 5op 0 0 0 0 0 A 112°52.2' pilotaxitic 28°39.3'/ Porphyritic and 18 V-98-5 51o 15 23agy 3 7ag 0 x 0 0 x A 112°52.2' pilotaxitic 28°38.5'/ Porphyro-trachytic 11 X-98-22 22 35 8ay 13 3 0 0 6o 12o 1 AB 112°50.5' and pilotaxitic DOMES L C Coordinates Mz PlgMz PlgF HbMz HbF CpxF Op R.C. Textures 28°40.5'/ 31 X-98-28 88s x 3 x xa 1co 0 D Porphyritic 112°53.6' 28°39.2'/ Hypohyaline and 28 V-98-14 54dov 22 16gy x 7go 0 2 D 112°51.5' trachytic 28°38.3'/ Porphyritic and 5 X-98-19 43adev 19 9 2o 10op 0 4 D 112°50.1' hyalopilitic TUFFS L C Coordinates Mz PlgMz PlgF HbMz HbF Qz BioMz Pmz Lt.V. Lt.M. Op U Textures 28°40.5'/ 35 X-98-29 62o x X 0 0 1 xa 0 36 0 0 Tls Fragmental 112°53.1' 28°41.5'/ Fragmental and 34 X-98-35 33cetv x X 0 0 0 0 39c 28 0 0 Tpl 112°54.2' vitroclastic 28°41.5'/ 33 X-98-34 58adtv 0 X 0 0 0 0 5 36 0 0 Tpl Fragmental. 112°54.2' 28°41.5'/ Vitroclastic and 32 X-98-33 67av 1 X 0 0 0 0 21 10 1.1 0 Tpl 112°54.2' fragmental. 28°39.2'1/ Porphyritic and 30 V-98-16 43 28 19gy 3 6ag 0 0 0 0 0 x Tlb 12°51.5' pilotaxitic 28°39.2'/ 27 V-98-13 48dov 19 13 3a 3ap 1 0 0 10 0 3 Tc Fragmental. 112°51.5' 28°39.4'/ 22 X-98-24 49adv 10a 14a 0 0 1 4a 0 21 0 x Tl Fragmental. 112°52.4' 28°39.0'/ Fragmental and 15 V-98-9 51av x 12 0 0 0 2c 17 7 7 0 Dt 112°51.8' vitroclastic 28°38.5'/ Hypocrystalline and 10 X-98-23 58adv 31 5 0 0 3 0 0 x 0 0 Dt 112°50.5' fragmental. Volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California 119 Table 3. Petrofacies analyses of selected sedimentary rocks from San Lorenzo Island. Classifi- L C Coord. Qm Qp K Plg Lp Lv Lm M.P. Mz Calc Sc Mi Bi. Red. Sph. Size cation Cgl with 28°40.5'/ 500-700 Mz of Lit- 17 X-98-31 112°51.9' 0 0 0 3.1 4.4 28.7 45.2 0 0 18.7 0 0 0 SR LS m m hic arenite 28°40.5'/ Conglome- 16 X-98-30 x 0 0 x x x x 0 x x 0 0 0 SR-SA LS > 2 mm 112°51.9' rate matrix 28°38.9'/ Limestone 14 V-98-8 0 0 0 0 0 0 0 0 0 100 0 0 0 0 0 0 112°51.8' (micrite) 28°38.9'/ 1.0-2.0 Lithic 13 V-98-7 112°51.8' 3.5 4.6 x 10.2 5.8 9.1 4.9 x 4.2 32.7 0 1.2 23.2 SA-SR HS m m arenite 28°38.9'/ 200-350 Arcosic 12 V-98-6 112°51.9' 2.6 7.2 0 32.4 3.1 8.1 2.7 0 3.8 1.7 25.7 12.7 0 SA-SR LS m m arenite 28°38.5'/ 80-150 Arcosic 9 X-98-21 112°50.5' 2.1 18.0 x 39.4 1.6 x 2.4 0 8.6 x 26.1 0 0 SA-SR HS m m arenite 28°38.5'/ 100-300 Arcosic 8 X-98-20 112°50.3' 6.3 7.6 x 25.5 19.0 x x x 7.9 0 23.6 5.1 0 SR-SA HS m m arenite 28°38.5'/ 150-900 Arcosic 7 V-98-3 112°51.1' 8.9 4.3 0 25.8 0 15.5 3.9 3.2 12.9 23.1 0 1.4 x SA LS m m arenite 28°38.7'/ 100-150 Arcosic 6 V-98-2 112°51.2' 20.2 6.1 x 25.1 0.5 6.3 6.3 0 10.5 13.9 0 9.8 0 SA LS m m arenite ABBREVIATIONS: L= laboratory sample number, C= field sample number. CONTENT: Bi.= bioclasts, Calc= calcite, K= potassium feldspar, Lm= metamorphic lithic, Lp= plutonic lithic, Lv= volcanic lithic, Mi= mica, M.P.= heavy minerals, Mz= matrix, Plg= plagioclase, Qm= monocrystalline quartz, Qp= polycrystalline quartz (includes chert), Sc= silica. MISCELLANEOUS: Sph.= sphericity, HS= high sphericity, LS= low sphericity, Red. = rounded, SA= subangular, SR= subrounded, x= abundance <1% (x in sample 16 just indicates the presence of components). 4c), cirripeds (Balanus sp.) (Figure 4d), bryozoans Marine sediments are overlain by a subaerial (Figure 4e), echinoderms (Eucidaris?) (Figure 4f), tube volcanic sequence composed at its base of porphyritic worms (Figure 4g) and gastropods (Epitonium sp.; andesitic lava flows with plagioclase (An to An ) and 16 37 Figure 4h). The calcareous mudstone layer is 5 m thick hornblende phenocrysts (Table 2, samples 18 to 21). and, near its upper contact it is interbedded with a 7 to Plagioclase commonly shows a spongy structure that 10 m thick air fall tuff (Unit J, Figure 3) showing normal suggests thermal unbalance (Cox et al., 1979). The ande- gradation of lithic fragments and reverse gradation of sitic lava flows are interbedded with pyroclastic deposits pumice fragments (sample 15, Table 2). The tuff is and occasionally with basaltic lavas. Unit F (Figure 3) covered in erosional unconformity by a coarse grained is represented by a widely exposed pyroclastic deposit yellow sandstone which, in turn, is unconformably over- extending along the Arroyo Los Esqueletos (Figure 2a). lain by a similar yellow sandstone. In the eastern coast of Lithic fragments of this unit are formed by dacite, ande- San Lorenzo Island, both sandstone units change site and tuff, ranging in size from 4 to 6 cm, although, laterally to polimictic conglomerates (unit K, Figure 3). occasionally they can reach 25 cm in diameter (sample Along this coast, conglomerates have more homogene- 22, Table 2). Underlying the tuff is a basaltic lava flow ous clast sizes. The sandy-matrix (sample 16) of one of (sample 23, Table 2; Unit E in Figure 3 and Figure 2a) these conglomerates (sample 17) is described in Table 3. that is brecciated and mixed up with the pyroclastic unit. ABBREVIATIONS: L= laboratory sample number, C= field sample number. MINERALOGY: BioMz= biotite matrix, CpxF= clinopyroxene phenocryst, CpxMz= clinopyroxene matrix, HbMz= hornblende matrix, HbF= hornblende phenocryst, Lt.M.= metamorphic lithic, Lt.V.= volcanic lithic, Mz= matrix, OlF= olivine phenocryst, OlMz= olivine matrix, Op= opaque minerals, PlgMz= plagioclase matrix, PlgF= plagioclase phenocrysts, Pmz= pumice, Qz= quartz. R.C.: ROCK CLASSIFICATION OR LITHOLOGIC UNIT (Figure 3): A= andesite, AB= basaltic andesite, B= basalt, D= dacite, Dt= tuff deposit, Tc= crystalline tuff, Tl= lithic tuff, Tls= upper lithic tuff, Tlb= brecciated lithic tuff, Tpl= pumice and lithics tuff. MISCELLANEOUS: a= hydrotermal alteration, , c= chloritization, d= desvitrification, e= spherulites, g= glomerocrysts, o= oxidation, p= pseu- domorph, s= silicification, t= shards, v= glass , y= spongy structure, x= abundance <1%. Dated samples are in italics. 120 Escalona-Alcázar et al. a) b) c) d) e) f) g) h) Figure 4. Fossil fauna of the marine sequence of San Lorenzo Island. From sample 7 are: a) fragment of pecten and b) benthonic foraminifer; from sample 13 are: c) fragment of pecten, d) cirriped (Balanus sp.), e) bryozoan, f) echinoderm (Eucidaris?), g) shell drilled by tubeworms and h) gastro- pod (Epitonium sp.). A K-Ar age of 5 ± 1 Ma (Table 4) was obtained on lithic fragments are less than 5 cm in diameter, while basalt sample 24 (Table 2) taken from the massive dacitic fragments can be as large as 50 cm across. The central part. dacitic fragments are petrographically similar to those In the southern part of the Arroyo Las Águilas from a nearby dome (sample 28, Table 2). Both, the tuff (Figure 2a) andesitic lava flows cover the basaltic unit. and the dacitic dome are overlain by andesitic lava flows Andesite shows a porphyritic texture with plagioclase (sample 29, Table 2) that, in turn, are covered by a brec- phenocrysts (An -An ) and hornblende (sample 25 and ciated monolithologic lithic tuff composed of dacitic 16 35 26, Table 2). A whole rock K-Ar analysis of sample 25 fragments (unit C, Figure 3 and sample 30, Table 2). yielded a 3.8 – 0.3 Ma age (Table 4). In the same arroyo In northern San Lorenzo and Las Ánimas islands a a pyroclastic flow overlies the lavas (sample 27, Table 2; pumiceous lithic tuff (unit B, Figure 3) is widely unit D, Figure 3). This flow includes lithic fragments of exposed (samples 32, 33 and 34, Table 2). This unit is andesite and dacite in a brecciated matrix. Andesitic locally covered by the lithic tuff of unit A (Figure 3) that