CENOZOIC VOLCANIC AND TECTONIC EVOLUTIONOF BAJA CALIFORNIA SUR. MEXICO

Brian P. HausbackDepartment of Geology and Geophysics

University of CaliforniaBerkeley. CA 94720

ABSTRACT

Tertiary to Recent volcanic and sedimentary rocks in Baja Califor-nia Sur record the geologic events caused by the interactions of lithos-pheric plates, notably the transition from subduction to strike-slipmotion between the North American and adjacent oceanic plates.

As the Farallon Plate was subducted beneath North America dur-ing the early Tertiary, Baja California was a stable marine continentalshelf receiving volcanic detritus from the active Sierra Madre Occiden-tal calc-alkaline volcanic arc in western Mexico. Volcanic activitymigrated westward and arrived along what is now eastern Baja Califor-nia about 24 m.y. ago accompanied by uplift of the Baja platform.

Calc-alkaline volcanism dominated the geologic events in BajaCalifornia Sur from 24 m.y. to about 12 m.y. The focus for this vol-canic activity lay along the eastern margin of the present peninsula ofBaja California. The volcanic and volcaniclastic materials that constitutethe Isidro and Comond Formations were shed to the west from thearc, first onto a shallow marine shelf, then onto a network of volcanicand detrital nonmarine fans built across Baja California. These arc-related volcanic rocks range from andesitic flows and lahars to rhyoliticash-flow tuffs. The abundance of rhyolitic volcanics generally decreasesto lhe north from the Baha de La Paz region.

Subduction ceased off Baja California by 12 m.y. ago, closely fol-lowed by the extinction of the Comond volcanic arc. Consumption ofthe Farallon Plate resulted in the juxtaposition of the Pacific Plate withBaja California along the Tosco-Abreojos fault. Lack of depositionalunits above the Comond volcanics indicates a separation of the penin-sula from the easterly volcanic and sedimentary sources probably due tograben formation along the locus of a proto-Gulf of California.

Coeval with the development of the proto-Gulf of California, aseries of basalts yielding dates from 12.4 to aI least 0.47 m.y. eruptedfrom widely distributed local sources in Baja California Sur. Theselargely alkalic basalts are associated with the extensional structuralregime and further document the transition from the earlier period ofcalc-alkaline volcanism that accompanied the mid-Tertiary subduction.

Structurally, the southern Baja California Peninsula was a gen-erally stable continental margin during the Miocene arc volcanism. Onemajor discontinuity, the La Paz Fault, was active prior to the Mioceneand evidence shows that normal, east-side down, and leflateral strike-slip motion has occurred along this structure from the late Miocene topossibly Recent times.

INTRODUCTION

The southern portion of the Baja California Peninsula provides anexcellent opportunity to study the genesis, growth and extinction of awell exposed Tertiary volcanic arc. To date, most models of the tec-tonic evolution of this continental margin are based on offshore mag-netic, bathymetric and seismic geophysical data, with few constraintsfrom the terrestrial geology. This study focuses on the volcanology,geochronology and geochemistry of the Cenozoic stratigraphy in thesouthern portion of Baja California Sur(fig. 1) in order to correlate thegeologic consequences of lithospheric plate interactions and to helpdefine the geologic events resulting from these plate motions.

The major focus of this report is the Comond Formation of BajaCalifornia Sur. Detailed studies of the Comond and the associatedstratigraphy were undertaken in the La Paz area, with reconnaissancenorth and south of this area through most of figure 1. In addition,detailed work in the area srrbunding La Puri'sima has been completed(Mclean and Hausback, 1984).

in Frizzell, Virgil A. Jr.. ed., 1984, Geology of the BqiaCalifonia PJninsul: Pacific Section S.E.P.M.. Vol. 39. o. 219-236 219

The physiography of the southern peninsula is dominated by theSierra de la Giganta, an elongate northwest-trending, asymmetrically-shaped mountain chain. Several peaks along the rugged Sierra crestreach elevations of over 1000 meters. The position of the divide aver-ages less than 10 kilometers from the eastern shoreline resulting in aprecipitous Gulf of California escarpment, and a western flank gentlydraining to the Pacic Ocean (fig. 2). The elevation of the Sierradeclines gently to the south until the mountains disappear into the low-lands of the La Paz area and then abruptly rise again to form the Sierrade la Laguna of the Cabo(Cape) province.

PREVIOUS WORK

Geologic reconnaissance of the southern peninsula was under-taken by several early explorers who concentrated their studies on themarine sedimentary stratigraphy of the region: Gabb (1882), Darton(1921), Heim (1922), and Beal (1948). More recently, McFall (1968)described the Comond volcanics of the Bahia Concepcin area. Gastiland others (1979) described the geochemistry and geochronology of thecircum-gulf volcanic rocks on a reconnaissance basis. This studyamplifies and reinterprets Gastil's record of volcanism for Baja Califor-nia Sur between Baha Concepcin and La Paz.

STRATIGRAPHY

The deserts of Baja California Sur contain a well exposed succes-sion of Cenozoic volcanic and sedimentary rocks that record the geolo-gic events of this active plate margin. The prominent Sierra de laGiganta is built of the largely undeformed volcanic and volcaniclasticrocks of the Miocene Comond Formation. Along the western andeastern margins of the Sierra there are local exposures (figs. 1 and 3) ofunderlying Tertiary marine sediments. Exposures of the basementrocks are resticted to the periphery of the southern peninsula andmake up most of the rugged Cabo province.

Pre-Tertiary Basement

The basement of Baja California Sur is similar to that found in thePeninsular Ranges batholith of Alta California, U.S A. and Baja Califor-nia Norte. This batholith is composed of a chain of Mesozoic graniticbodies and accompanying metasedimentary roof pendants structurallysutured to a western belt of oceanic crustal rocks.

Pre-Tertiary basement rocks are sparsely exposed in Baja Califor-nia Sur, although the southern tip of the peninsula, the Cabo province,is dominated by exposures of Pre-Tertiary, mostly Mesozoic, basementterrains (Gastil and others, 1978). Exposures of basement outside theCabo province in the southern part of the peninsula occur primarilyalong the coasts in isolated exposures. McFall (1968) reported 78 m.y.granodiorite at Baha Concepcin (locality 1, fig. l). Northwest ofLoreto exposures of tonalite have yielded a date of 144 m.y. (locality 2,fig. 1; Gastil and others, 1978; Chvez, 1978). The nature of rhe base-ment below Arroyo La Puri'sima, northwest of Loreto (locality 3, fig.i), is indicated by partially melted xenoliths of diorite and gneiss in ayoung basalt flow. Between Loreto and La Paz, at Punta San Telmo(locality a, flg. 1), there are exposures of meta-sediments, "schists andgneisses" (Diaz, 19'l'l). Exposures of Mesozoic ophiolitic and island-arc rocks are present on Isla Santa Margarita, Isla Magdalena, the Vis-caino Peninsula and Isla Cedros along the west coast of the Peninsula(Blake and others, this volume). The southernmost Cabo provincestructural block, sutured to the peninsula by the north-south trendingLa Paz fault, is a highland of Mesozoic crystalline rocks containing

220 Hausback, Brian P.

numerous metamorphic pendants. Potassium-argon dates on the grani-tic rocks of the Cabo block yield ages of 70 to 109 m.y., including abiotite granite that yields 93 m.y. ages by both the K-Ar and U-Pbmethods (Frizzell, V.A., Ort, K., and Mattinson, J.M.; written comm.,1983). A layered hornblende diorite along the La Paz fault zone yieldsa K-Ar date of 115 m.y.(locality 5, fig i). Gastil and others (1976,1978) report additional 70 to 98 m.y. K-Ar ages as well as two Paleo-zoic age determinations from the Cabo block.

The Cretaceous marine sedimentary rocks of the Eugenia andValle Formations overlie the crystalline basement. These rocks cropout only on the Viscaino Peninsula north of the study area and aresimilar to the Great Valley Sequence of Alta California, probablyformed frorn materials shed from the exposed Mesozoic batholithichighland.

Figure 2. Landsat image of the Bahi de La Paz region; crest of theSierra de la Giganta (SG), La Paz (LP), La Paz Fault (LPF).

'j'{" " Tepetate Formation

":The Cretaceous rocks of BajaCalifornia Sur are overlain by lower

Tertiary marine sedimentary rocks, of which the oldest encountered inthis study is the Tepetate Formation (Heim, 1922). The Tepetate isexposed only on the western side of the Peninsula in two areas: west ofthe Baha de La Paz, and considerably farther north in Arroyo Mesqui-tal, northwest of La Pursima. Cursory examination of the sandstonesshow them to be foram-bearing feldspathic wackes derived mainly froma granitic source with little or no volcanic detritus. These sandstonescommonly contain abundanf Discoqtclina, a disk-shaped foraminilera upto 1 cm or more in diameter. The microfgLuna of this formation werestudied by Knappe (1974) who concluded that this sequence of sand-stone and shale is the result of upper bathyal to mid-bathyal slopedeposition during the early Eocene.

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Salto Formation

Localized subaerial volcanic activity on the eastern side of BajaCalifornia may have occurred prior to and possibly coeval with marinedeposition of the San Gregorio Formation. the stratigraphic unit gen-erally found overlying the Tepetate Formation. McFall (1968) eportedan age of 28.1 + 0.9 m.y" for a tuff in what he refers to as the SaltoFormation which he considers to be the oldest portion of his "ComondGroup" at Baha Concepcin. Gastil and others (1979) report a date of27.5 + 0.8 m.y. on a basalt within the marine sedimentary sectionnorth of San Juan de la Costa. In addition, a subaerially depositedash-flow tuff near Punta San Telmo, probably lying stratigraphicallybelow the San Gregorio Formation, yields a date of 28.0 r- 0.7 m.y. Ineach case these volcanic units are associated with cross-bedded redsandstones. Allhough widely dispersed, these essentially coeval Oligo-cene volcanic rocks and cross-bedded sandstones of the Salto Forrnationconstitute a pre-Comond portion of the Tertiary stratigraphy localizedalong the eastern margin of Baja California Sur that is slightly olderthan or coeval with the San Gresorio Formation.

San Gresorio Formation

The San Gregorio Formation (Beal, 1948) unconformably overliesthe Tepetate Formation and is well exposed on the east and west sidesof the Sierra de la Giganta west of Bahia de La Paz, as well as in lim-ited exposures in several arroyo bottoms in the La Purisima region.These rocks have also been referred to as the Monterey (and Monter-rey) Formation due to their lithologic similarity to the younger MioceneMonterey Formation of Alta California (Darton, 1921; Heim, 1922;Mina, 1957; Escandn, 19781 Ojeda, t979). The exposed thickness ofthe formation is about 100 m in Arroyo San Hilario (Ojeda, 1979),127m at San Juan de la Costa (Escandn, 1978) and at least 72 m in the LaPursima area. In the La Puri'sima region the formation consists oi asequence of interbedded, commonly phosphatic, silicious shale, diatom-ite, pelletoidal phosphatic sandstone and rhyolite tuff. In the SanHilario area the lithoiogies are similar but less diagenically silicified thanat La Pursima. The Gulf coast exposures at San .luan de la Costa (fig.4) of the San Gregorio Formation contain tuffaceous sandstone, silt-stone (mildly diatomaceous), ostracod-bearing sandstone, and pel-letoidal phosphatic sandstone (commonly containing abundant sharkteeth and marine mammalian bohes). The presence of abundantsmooth-shelled ostracods probably indicates an estuarine brackish waterenvironment (Pokorni, 1978). This, together with abundant marinemammalian bones, rare solitary corals, pelecypod coquinas, and localcross- bedding, strongly suggests deposition of much of the San Gre-gorio Formation at San Juan de la Costa in a shallow marine, near-shore environment. The occurrence of coquinas and cross-beddingincreases upward in the San Juan de la Costa section, perhaps indicatinga gradual uplift. In the La Pursima region the benthic foram assem-blage indicates that the San Gregorro Formation was deposited at upperbathyal depths of 2000-1500 m (Mclean, Barron, and Hausback,1984), and is directly overlain by the shallow marine Isidro Formation,suggesting either an abrupt period of uplift or a gap in the depositionalrecord.

Pebble conglomerates in the upper 15 m of the San Gregorio For-mation at San Juan de la Costa contain metamorphosed volcanic peb-bles. These pebbles are strongly silicified, well-rounded clasts ofwelded rhyolite tuff, andesite, rvhite (vein ?) quartz, and minorquartzo-feldspathic sandstone. These rocks contain local groundmasssericite and epidote, locally replacing plagioclase. This meta-volcanicpebble component appears in the upper San Gregorio Formation,extends. through the Isidro Formation and terminates in the lowerComond Formation. The occurrence of the meta-volcanic pebbleconglomerates is widespread throughout the study area, notably foundat the base of the Isidro Formation in all known exposures. The peb-bles were probably shed westward onto the Baja California shelf fromuplifted exposures of the middle Cretaceous to Eocene volcanic com-plex developed along the western Mexican mainland.

The age of the San Gregorio Formation has been a topic of debatefor the better part of a century but is now known to be late Oligocene.Early workers in the region (Darton, 1921; Heim, 7922; BeaI, 1948;Mina, 1957) suggested ages ranging from Eocene to mid-Miocene, pri-marily based on lithologic similarity to the Monterey Formation of AltaCalifornia. Vanderhoof (1942) discovered fossil remains of the sea cow

22t

Cornwallius resulting in the first concrete evidence of an Oligocene age.Hausback (1982) determined a K-Ar radiometric age of 25.5 m.y. (lateOligocene) on biotite from a tuff bed in the San Gregorio Formation atArroyo San Hilario. Four additional tuffs in the San Gregorio Forma-tion in the La Puri'sima area have yielded K-Ar dates ranging from 27.2Io 23.4 m.y. (table i), consistent with their relative stratigraphic posi-tion. In addition to radiometric evidence, diatoms at La Pursima indi-cate a late Oligocene age for the formation (Mclean, Barron, andHausback, 1984), Well preserved coccoliths, Cyclicargolithus abisectus(Mller) and dictyococctes bisectus (Hay and others), from the sameArroyo San Hilario locality as the dated biotite-bearing ash also indicatea late Oligocene age for the San Gregorio Formation (David Bukry,written comm., 1982). Furthermore, Shelton Applegate (pers. comm.,1983) suggests that the shark fauna in the Arroyo San Hilario exposureof the San Gregorio Formation is of Oligocene age.

The dates on silicic ashes of the San Gregorio Formation showthem to be coeval with voluminous calc-alkaline volcanism of the adja-cent Sierra Madre Occidental of western Mexico. McDowell and Keizer(1977) and McDorvell and Henry (1983) document the occurrence of avoluminous north-south elongate volcanic arc in the Sierra MadreOccidental that may have given rise to distal water-laid ashes such asthose deposited in the San Gregorio Formation. This volcanic arc wasactive from 32 to 23 m.y. ago, during which time it gradually migratedtoward the western edge of the continent.

Isidro Formation

Deposition of the Isidro Formation (Heim, 1922) marks the ini-tiation of local volcanism in eastern Baja California during earliestMiocene time. Along the Gulf coast, at San Juan de la Costa, thisformation is composed of a 37 m section of green-colored water-laintuff, pebbly sandstone, oyster coquina and conglomerate intercalatedwith reworked pink tuff (fig. 6). This locally intense green color is dueto clinoptilolite (Edward Montgomery, pers. comm., 1981) coatings ongrains and is apparently restricted 10 the marine deposits.

On the west side of the Sierra de la Giganta, in the San Hilarioarea, the Isidro Formation is only about 15 to 25 m thick, mainly com-posed of pebble conglomerate and sandstone, contains minor pelecypoddebris, and lacks primary volcanic deposits. This thin unit has beenincluded with the lowermost Comond Formation in figure 3.

The northernmost exposures of the Isidro Formation near LaPursima are composed of bioturbated yellow to brown fine-grainedtuffaceous sandstone and siltstone that attain a stratigraphic thickness ofup to 80 m. Pelecypod, gastropod and barnacle fossils are locally abun-dant. Cross-bedding indicates strong current action similar tooccurrences of these rocks to the south. The fossil and sedimentologi-cal data suggest neritic water depths, probably of a lagoonal depositionalenvironment; (Judy Smith, this volume). Field relations indicate thatthe Isidro Formation interfingers with the nonmarine lowermostComond Formation.

The age of the Isidro Formation is well constrained by K-Ar dateson the enclosing stratigraphy. In Arroyo San Hilario the Isidro Forma-tion unconformably overlies the San Gregorio Formation which con-tains a 25.5 m.y. ash bed. In the San Juan de la Costa area the Isidrobeds are overlain by the lowermost Comond formation containingtuffs dated at 21-22 m.y. (fiC. 5). Further north in the La Puri'simaregion the Isidro Formation unconformably overlies San Gregorio bedsas young as 23.4 m.y. and at Pursima Vieja, 16 km northwest of LaPursima, the Isidro Formation grades vertically upward into nonmarineComond beds containing a 14.5 m.y. basalt flow. So, it appeas that inthe Bahih de La Paz region the Isidro Formation is lowermost Miocene,between 25.4 and 22 m.y. but in the La Purisima region the Isidroprobably has a longer age span of lower to middle Miocene, between23.4 and 14.5 m.y. Undoubtedly, the Isidro is, in part, coeval with theComond Formation and orobablv reDresents the shallow marineequivalent of the Comond.

Figure 3. Following page. Geologic map of the Bahi de La Pazregion. Note that the thin Isidro Formation on the west side of theSierra de la Giganta has not been mapped and is included with thelower member of the Comond Formation. On the west side of theBaha de La Paz only the base of the San Juan tuff has been mapped;its bulk is included in the Tcu member.

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.Comond Formation

The Comond Formation was named by Heim 0922) for the vol-caniclastic sandstones and conglomerates exposed in the deep canyonwa"llliihe"illag of Comond, west of Loreto. Heim stated that theComond Formation thickened to the east and included "basaltic brec-cia" in the crest of the Sierra de la Giganta at Cerro Giganta. Prior toHeim's work the Comond Formation was informally known as theMesa Sandstone (Gabb, 1882, and Darton, 1921). More recently, theComond designation has been liberally applied to the entire Miocenesequence of volcanic ancl volcanicla3tic rciCkS'of the Sirra de la Giganta(Beah1948; Mia, 1957). This study restricts the name Comond For-mation to the Miocene arc-derived silicic and intermediate compositionvolcanics and volcaniclastics that make up the bulk of the Sierra de laGiganta and eicludes the late Miocene and younger capping basali flowsrelated to post-arc rifting.

The widespread Comond Formation dominates the geology ofBaja California Sur. The formation overlies the Isidro Formation and inmost exposures the transitiort'is gradational from the shallow marinefsird-Itda to the subaerially deposited Comond strata. Comondrocks extend as far south as La Paz and are exposed continuously tojust north of the 28th parallel, north of which equivalents are founddiscontinuously through Baja California Norte and into eastern AltaCalifornia and Arizona (Glaznet and Supplee, 1982). To the west thedistal sandstones of the Comond Formation thin across the gentlePacific slope and generally disappear beneath Quaternary alluviumbefore reaching the western shore; to the east, the massif of theComond rocks, 1500 m or more in thickness at numerous localities, issharply truncated by the Gulf of California escarpment. The ComondFormation is described below in detail for the Baha de La Paz regionand in reconnaissance fashion to the north.

Lithologies

The Comond Formation is composed of interbedded volcaniclsandstones and conglomerates, rhyolitic ash-flow tufs, and andesitici lahars and lava flows (g. 5). The most abundant of these are volcanic

sandstone and conglomerate. These immature, fragmental, brown togray rocks form lensoidal to tabular beds often displaying planar lamina-

Hausback. Brian P.

Figure 6. Aerial view to the west of the San Juan de la Costa area showing nearly flat-bedded stratigraphy; San Gre-gorio Formation (SG), Isidro Formation (I), and lower part of the Comond Formation (C) including a prominentcliff-forming andesitic breccia or lahar (ab).

tions with thin interbeds of tuffaceous siltstone. The sandstones andconglomerates are composed of disaggregated volcanic rocks: poorly-sorted bubble-wall shard glass, crystal fragments, and andesitic to rhyol-itic lithic debris. Trough cross-bedding, massive bedding and reverse-graded bedding are common. Frequently, the sandstones andconglomerates are rich in pumice pebbles and the sandstones supportoutsized rounded cobbles and boulders of andesite. These features sug-gest braided stream and debris flow depositional mechanisms. TheComond Formation lacks fossil remains and was probably deposited asa set of detrital fans on the western flank of a north-south chain of vol-canoes.

The next most abundant lithologies in the La Paz region are therhyolite ash-flow tuffs which are interbedded throughout the ComondFormation. The tuffs comprise about 75 percent of the stratigtaphy onIsla Espiritu Santo, 20 percent at San Juan de la Costa, but are absentwest of San Hilario suggesting easterly source areas for the tuffs. Indi-vidual flow units range from 6 to at least 77 meters in thickness. Theash-flow deposits are commonly pink-colored, massive, pumicious,crystal-vitric, poorly welded tutrs with basal airfall tephras. On IslaEspritu Santo the ash-flow tuffs contain welded portions with blackbasal vitrophyres up to 15 m thick. Only one major ash'flow tuff in theLa Paz section, the newly named La Paz tuff, is strongly welded andnone are strongly welded in the San Juan de la Costa section. One ofthe largest of these tuffs, the San Juan tuff (newly named for its expo-sures qt San Juan de la Costa), has a reconstructed volume of at least31 km'pf uninflated rhyolitic m4gma exposed west of Baha de La Paz(57 km' of expanded 1.37 glcmt tuff; assuming magma density of 2.5g/cm'). Accounting for the rifted easterlJ portion of this tuff, the ori-ginal magma volume was at least 55 km', similar in magnitude to thevolume of the climactic eruption of Crater Lake caldera in Oregon(Bacon. 1983).

Based on phenocryst mineralogy, Sr, Rb and Zr ace elementgeochemistry (fig. 7) and magnetic stratigraphy constrained by K-Argeochronology, two general tuff correlations can be made between thethree geographically separated areas in the Baha de La Paz region (figs.3 and 5). The prominent San Juan tuff appears on Isla Esprritu Santoas one or both of the associated flow units of member Tcj. Also, thethick, welded La Paz tuff can be found just south of San Juan de laCosta as a distal. thin. unwelded ash-flow tuff.

TUFFCORRELATIONS

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Figure 7. Sr, Zr, Rb trace element correlation data for the San Juantuff in the San Juan de la Costa region west of Baha de La Paz and onIsla de Espiritu Santo; also the La Paz tufl in the La Paz arca and in thecoastal section south of San Juan de la Costa. Analyses by X-rayfluorescence.

Figure 8. Outcrop of an andesitic lahar in the lower Comond Forma-tion of the San Ju4n de la Costa area. The hammer is 0.5m long forscale. Note the matrix-supported angular blocks and the massive topoorly.stratified charactef of the deposit.

227

' The final major category of Comond rock types are the geneti-cally associated andesitic lahars and lava flows. The lahars form mas-sive tabular sheets up to about 40 m in thickness and locally grade intolava flows (autobreccias). The lahars consist of breccias of largelymonolithologic hypersthene andesite with angular clasts commonly upto I m or more in diameter. The clasts float in a groundmass of gray tobrown, poorly sorted, sand-sized andesitic detritus (fig. 8). The laharslocally fill deep paleochannels. Where these lahars overli tuff andtuffaceous sandstone the extreme invertd density relationship fre-quetly has resulted in the formation of large-scale, probably syn-depositional, loading structues -- blocks (commonly tens to hundredsof meters in dimension) of lahar sinking into what must have beenunconsolidated sandstone, and the sandstone flaring upward into theandesite bodies as large dikeJike structures. Local baked and siliciedzones along the base of the breccia units and in the immediately under-lying tuffaceous sandstones indicate that these andesitic breccias wereemplaced at high temperatures. The andesite breccias are thus probablythe result of hot volcanic debris flows associated with strato-volcaniceruptions. Laterally the breccias grade into conglomerates and sand-stone units.

Transport Directions

The sedimentary and volcanic transport directions of theComond Formation deposits are derived from several lines of evi-dence and indicate divergent directions between the La Paz and SanJuan de la Costa areas (frg. 9). Clast imbrications and cross-beddingindicate fluvial transport of the Comond sandstone and conglomerateto the west near San Juan de la Costa are, and to the south in the LaPaz area. Lateral fining of andesitic breccia and maturation intoconglomerate and sandstone occur along these same directions. Inaddition, isopachs of the prominent San Juan tuff show a general thin-ning to the west and indicate a source area to the east of the presentGulf shoreline, probably to the east of the Isla Espritu Santo. Similarfacies gradations of the andesitic clastic units between Loreto and VillaInsurgentes also indicate a southwesterly to westerly transport direction.In the La Purrsima area, Mclean and Hausback (1984) report fluvialComond transport directions to the south and southwest.

Figure 9. Compilation of Comond Formation volcanic and sedimen-tary transport direction data. Rose diagrams determined from pebbleimbrication and cross-bedding direction data in the lower ComondFormation;tic units; dashed lines are isopaches of the San luan tuf.

Ooen=wesf of ^abohode - a^Lo Poz, n= 2l

^dAt'J'.'oo,;,?l 3j:,", " s."

n=7

Ya\ \PACIFICOCEAN

N

I

ffi,#-\8

BAH'A de

V\V/V

228

Throughout much of the Sierra de la Giganta the westward-sloping mesas are capped by lahars of andesitic breccia, conglomerate,and sandstone with well-preserved surface morphologies displaying lowamplitude crescentic pressure ridges convex to the west, indicating tran-sport in that direction. These debris flows once flooded down channelsdeveloped on the detrital fans built along the western flank of theComond volcanic chain. After deposition the dense, blocky flowsbecame lithified and very resistant. Subsequent erosion of less com-petent surrounding sandstones has left these lahars as caps to many ofthe east-west elongate mesas.

In summary, the Comond volcanic and volcaniclastic units wereshed-O[fEfctes gring alng ihe eait side of what is now theB-aj Peninsula onto the southern Peninsula along trnsport diretionsvaryitg-frbm west to south. Th volcanics wele.!-rqt shed into a shal-low marin nvironmnt, depositing. the Isidro,Formation and- plogres-sivelv built the Comond subarial volcanic and volcaniclastic apronwesl;ard across Baia California.

Facies Models

The volcanic and volcaniclastic rocks of the Comond Formationdisplay regular and predictable lateral gradations in grain size, lithologicassemblages, and welding of ash-flows. These variations can be appliedto a nonmarine volcanic facies model developed by Vessel and Davies(1981). The facies, or assemblages, ofan active continental stratovol-cano are:

Volcanic Core Facies: lavas, airfall ash and colluvial deposits.Proximal Volcaniclastic Facies: largely monolithologic volcanic

breccias and airfall ashes.Medial Volcaniclastic Facies: interbedded breccias, conglomerates,

and coarse-grained sandstone.

Distal Volcaniclastic Facies: sandstone and conslomerate litholo-gies.

This model has beeri simplified for the Comond Formation in thisstudy by combining Vessel and Davies' proximal and medial facies intoone facies, here called "proximal"(fig. l0). The core of the volcanicarc extended along the eastern margin of Baja California prior to rifting.The volcanic axis is presently offshore through southernmost Baja Cali-fornia and indications of its proximity are found in several localitiesalong the east coast. Between Baha Concepcin and Loreto severalhornblende-bearing hypabyssal andesitic porphyries cut the ComondFormation. Because these stocks are coeval with Comond eruptives(t9. + 0.9 m.y., locality 6, fig. l; 20.0 + 2.0 m.y., locality 7, McFall,1968) and cut the lowermost part of the section, these intrusions andtheir associated dike systems probably represent subvolcanic Comondmagma chambers and feeders. This chain of porphyritic stocks alongeastern Baja extends to the south where porphyries of similar lithologyare found at Punta San Telmo (locality 4, fig. 1) and possibly at LosDolores (locality 8, fig. 1).

The Pichilingue area, immediately north of La Paz, containsanother portion of the volcanic core facies composed of andesite lavasgrading into voluminous monolithologic breccias. In addition, a majorwelded tuff, the La Paz tuff, occurs there. This is the southernmostextension of the Comond volcanic core facies exposed on the Penin-sula as the zone is apparently truncated by the La Paz Fault. Nooutcrops of the Comond Formation have been found during recon-naissance studies in the Cabo province. The northernmost tip of IslaCerralvo, east of La Paz, contains a remnant capping exposure of asequence of probable Comond equivalent ash-flow tufs, one of whichis densely welded, and sedimentary rocks (J.G. Smith and M.G.Sawlan, pers. comm., 1983). The southern continuation of the volcaniccore facies may have been ofset along the La Paz Fault in a left-lateralmotion (fig. l0), as suggested by offshore volcanic stratigraphy outlinedby Normark and Curray (1968) and by recent seismic events (Molnar,r973).

Figure 10. Generalized volcanic facies model of the Comond Forma-tion in Baja California Sur. Facies designations modified from Vesseland Davies (1981), see text.

On Isla Espiritu Santo the presence of andesite flows and severalstrongly welded ash-flow tuffs belonging to the core facies indicates amuch nearer-source environment than to the west at San Juan de laCosta, where the proximal facies contains unwelded tuffs interbeddedwith a section of sandstone, conglomerate, and andesitic breccias.Further west, the Comond stratigraphy is best characterized as distalfacies containing mainly sandstones with few ash-flow tuffs, breccias, orcoarse-grainbd conglomerates.

Hausback, Bran P.

229

Table 1. Potassium Argon Age Data

Sample LocationNumber Lat./Long

Rocktype

Mineral o/oKDated + std. dev

40Ar* 4oAr*x 10-l I o/o

mol./gm

Agem.y.

i 2 std. dev.

StratigraphicUnit/Comments

Late Miocene to Recent Basalts383-16-1 26'05'10"N Alkalic

112'01'36"W Basaltw.R 3.087 0.2503

+ 0.1174.4 0.47 + 0.09 Canyon-Filling Flow,

Rancho Escondido383-8- 1 26"16'36"N Alkalic

112'02'18"W Basaltw.R 2.r76 0.3270

+ 0.0924.6 0.87 + 0.13 Cinder Cone Flank

Flow, NE of La Pursima383-9-l 26'15'32"N Alkalic

112'05'58"W Basaltw.R. 2.585 3.747

+ 0.04256.2 8.3 + 0.3 Capping Flow between

La Pursima & Paso Hondo383-10-1B 26'29'08"N Alkalic

112'07'40"W Basaltw.R 1.803 3.492

+ 0.04156.9 11.1 + 0.8 Canyon-Filling Flow

A. San Gregorio383-10-1A 26"29'08"N Basalt

1 12'07'40"Ww.R. 0.948

r 0.0132.052 55.5 12,4 t 0.4 Oldest Capping Flow

A. San Greeorio401 25'26'1 1 "N Alkalic

111'31'18"W Basaltw.R. 3.584

J 0.0173.346 40.0 5.4 + 0.1 N. side of Highway

32 km NE of Insurgentes438 24"54'34"N Alkalic

111'05'00"W Basaltw.R 3.961 2.560

+ 0.01421.6 3.7 + 0.1 5.5 km W of Los Torres

443 25"15'59"N Alkalic111'30'51"W Basalt

w.R. 3.096 2.232i 0.002

64.6 4.2 + 0.1 Mesa Nombre de Dios,26 km E of Insurgentes

Comond Formation, West of Bahia de La Pazt62 24"24'40"N Andesite Plag.

I10"56'00"W flow0.5096 t.ll2

+ 0.0155.0 12.5 * 1.4 Tch, San Hilario lava

Bjl 24"22'24"N Rhy. ash- Plag.110'41'35"W flow tuff

0.9234 z.1'.trr 0.0010

40.9 17.2 + 0.6 Tcu. Devitrified tuff.San Juan de la Costa

28 24"21'37"N Rhy. ash- Glass110"41'31"W flow tuff

4.7 3r+ 0.067

13.48 20.3 16.4 r 0.6 Tcj, San Juan tuff

Bj2 24'21'37"N Rhy. ash- Sanid.ll0'41'51"W flow tuff

8.1 79+ 0.016

25.15 93.0 17.6 + 0.1 Tcj, San Juan tuff,San Juan de la Costa

Bj2 24'21'37"N Rhy. ash- Bio110"41'51"W flow tuff

6.27r+ 0.070

19.68 41.8 13.9 + 0.5 Tcj, San Juan 1uff,San Juan de la Costa

38 24'20'44"N Rhy. ash- Bio.110"40'42"W flow tuff

6.87 s

r 0.07025. 15 86.2 2t.0 i 0.4 Tcl,6kmSof

San Juan de la Costa178 24"26'49"N Rhy. ash- Glass

110"46'55"W flow tuff4.353

+ 0.02416.36 37.4 21.5 + 0.4 Tcl, A. Tarabillas

Prob. same as 38425 24'19'08"N Pumicious Bio.

110"39'05"W ruff6.555

+ 0.00524.90 7 4.3 21,.8 + 0.2 Tcl, Coastal exposure

N of A. Camarn57^ 24'18'22"N Andesitic W.R.

110"39'02"W lahar,clastt.7 47

+ 0.0396.923 4t.5 22.7 t 1.7 Tcb, N side of

A. Sauzoso578 24"18'22"N Andesitic Plae

110'39'02"W lahar.clast0.143

+ 0.023.099 52.1 23.9 x. 0.7 Tcb, N side of

A. Sauzoso

286 24"28'49"N Andesite W.R110"23'00"W flow

t.733f 0.006

4.983 37.3 16.5 0.3 Tcf. W side of Island

292A 24'31'25"N Rhy. ash- Bio110'20'33"W flow tuff i 0.035

78.1 21,2 + 0.02 Tca, NE shore ofIsland

5.649 20.86

Comond Formation. LaPaz Area24'07'33"N Rhyodacite Plag.

110'17'13"W lava0.7544 2.509

I 0.0224.9 l9.l !. 1.2 Tcr, SW end of

Cerro Atravesado

239 24'10'20"N Rhyodacite Plag.110'16'36"W lava

0.692t 2.318+ 0.0053

80.5 19.2 + 0.5 Tcr, 2 km NE ofCemetery

383 24'11'37"N Rhyodacite Glass110'17'33"W Obsidian

3.848 t3.22I 0.019

80.2 19.7 t 0.2 Tcr, basal breccia,Coromuel

45 24'09'24"N Rhy. ash- Plag110'16'27"W flow tuff

0.531 1.670+ 0.006

49.2 18.0 I 0.6 Tcc, Coromuel Tuff,Quarry, NE La Paz

45 24"09'24"N Rhy. ash- Bio.ll0"l6'27"W flow tuff

5.684 19.62+ 0.040

64.5 19.8 t 0.3 Tcc. Coromuel Tuff.Quarry, NE La Paz

45 24"09'24"N Rhv. ash- Glass 3.869 13.48 65.5 20.0 + 0.4 Tcc. Coromuel Tuff.110'16'27"W flow tufl + 0.032 Quarry, NE La Paz

110'17'21"W Pumice + 0.011 Cemetery, NE La Paz266 24"16'22"N Rhyolite Glass

110'19'39"W Tephra4.462 16.06

+ 0.01636.6 20.6 ! 0.2 Basal Tephra, La Paz tuff,

Ferry Terminal

429 24"16'40"N Andesite W.R.110'19'23"W lava

1.831 6.4'.t7+ 0.014

76.2 20,3 t 0.4 Tcb, E of PlayaPichilineue

Bj3 24'17'12"N Andesitic Plagll0"l8'20"W lahar. clast

0.1996 0.648+ 0.0110

10.7 18.6 t 2.4 Tcb, Directly above Bj4,Cerro El Indio

230 Hausback, Bian P.

Table I -- continued

SampleNumber

LocationLaf./Long.

4orx

x 1o-ll4oAr*

o/o

Rocktype

Mineral o/oKDated -+- std. dev

Agem.y.

+ 2 std. dev

StratigraphicUnit/Comments

mol./

Comond Formation, LaPaz Area, continued

ffiash- Bio. 6.159 2l .83110'15'41"W flow tuff + 0.071

37.1 20.3 + 0.5 Tcs. NW of Salinas (El Coyote),50 m below Tcb

BioBj4 24"17'12"N Rhy. ash-110"18'20"W flow tuff

6.748 25.89+ 0.060

68.1 22.0 + 0.4 Tcs, S base of Cerro El Indio.top of Tcs

238

394

Plag.dump, S ol'

24"08'32"N Rhy. ash- Bio. 6.969 30.48 50.6 25.0 + 0.6 Tcs. base ol small hill110"16'16"W flow ruff + 0.042 s of quarry

eomond Formation, Loreto -- La Pursima Region

112'10'11"W Basalt + 0.052 Pursima Vieja

24'09'05"N Rhy. ash-110"15'49"W flow tuff

0.398 i.609+ 0.0i2

20.i 23.2 ! 1.6 Tcs. Base of hill at

5 82-3- I 25"58'05"N Andesitic110"29'35"W porphyry

Hbl 0.319r 0.004

I .081 16.7 19.4 + 0.9 Old Loreto Grade;locality 6, fig. I

San Gregorio FormationBio26C 24'21'13"N Rhyolite

110"59'40"w tuff6.881

r- 0.02830.60 86.2 25.5 + 0.4 A. San Hilario; previously

pub. 25.4 i 0.2 m.y.28-3 -8 26"07'00"N Rhyolite

112'11'02"W tuffBio. 4.48

+ 0.01I 8.34 73.7 23.4 + 0.3 A. La Purisima,

La Ventana

28-3- I 6 26"07'00"N Rhyolite112"11'02"W tuff

Glass 3.7 6-F 0.03

t5.7r 85.9 23.9 + 0.4 A. La Pursima,La Venetana

482-26-7 26"11'50"N Rhyolite112'03'23"w tuff

Bio. 6.7 6+ 0.03

29.90 69.8 25.3 + 0.3 A. La Pursima,Pump House area

HM-3-13 26"09'57"N Rhyolite1i2"14'15"W tuff

Glass 3.64+ 0.04

17.28 92.'l 27.2 + 0.6 A. San Gresorio

383-5-3 25'18'56"N Rhy. ash-110'57'09"W flow tuff

Sanid. 6.366+ 0.069

31.12 79.7 28.0 + 0.7 Salto Fm, Punta San Telmo,upper of 2 tuffs

Basement Rocks02

110'i0'20"w Diorite f 0.001 La Paz Fault zoneaoKdecayconstants:Le+tre':0.581 x 10-10/yr, tB:4.962 x 10 lo/year;aoK/TotalK:1.16? x 10 4. Abbrevia-tions: W.R. : whole rock, Plag. : Plagioclase, Sanid. : sanidine, Bio. : biotite, Hbl. : hornblende, Rhy' : rhyol-ite, A. : arroyo, Fm.. : Formation, * : radiogenic.

In several areas along the Baja Peninsula lateral facies variationsin the Comond Formation can be traced directly in outcrop. Thesegradations are commonly from core facies of coarse andesitic breccias,with or without lava flows, to proximal facies of coarse-grainedconglomerates and sandstones, and frnally to distal facies sandstonesand finer-grained conglomerates. This gradation occurs along asouthwesterly transect through most areas of the Sierra de la Gigantaand is extremely well displayed in a southerly traverse from Pichilingueto La Paz.

Age

The Comond Formation was deposited during the early to mid-dle Miocene. Twenty-seven K-Ar radiometric dates on various volcanicminerals, glass and whole rock samples range from 25.0 to 14.5 m.y.and have detailed the chronology of this formation (table l).

Several stratigraphic units were independently dated by usingmore than one component (mineral or glass separate). Biotite-glasspairs generally produced concordant ages. Because of this and theirattendant high potassium contents, biotite and glass ages are consideredthe most reliable. Although sanidine ages are few because of the gen-eral dearth of this potassic phase in the rhyolites, they are also con-sidered reliable. Plagioclase was dated twice in conjunction with othercomponents; once from the Coromuel tuff, the other from the Provi-dencia rhyodacite, both in the La Paz area. In these experiments themean data on plagioclase is up to 9 percent younger than associatedglass and biotite dates. Dalrymple and Lanphere (1969, p. 169) showseveral examples of dated volcanic biotite-plagioclase pairs in which theplagioclase is generally younger (up to 100/o) than the biotite.

The oldest dates on the Comond Formation in the Baha de LaPaz region are from rocks found in isolated exposures of the loweststratigraphic unit in the La Paz area, the Satinas member (25.0 + 0.6m.y. biotite; 23.2 + 1.6 m.y. plagioclase). Another older date of 23.9+ 0.7 m.y. was determined from an andesite clast in the andesite brec-cias near San Juan de la Costa. These clasts are older than underlyingtuffs, suggesting that older clasts in the lahar may have been inheritedfrom previous volcanic events. It appears that voluminous mid-Terliaryvolcanism along Baja California initiated about 25 m.y. ago, first depo-siting in a shallow marine environment (the Isidro Formation) followedby the accumulation of the subaerial Comond Formation.

The bulk of Comond volcanics in the La Paz region is from 25to l7 m.y. old. However, younger andesitic lava flows of the Comondunconformably cap the stratigraphies on the Isla Espritu Santo and nearSan Juan de la Costa, and have whole rock ages of 16.5 m.y. and 12.5m.y., respectively. Both of these younger flows comprise the presentsurface and were apparently never overlain by succeeding rock units,hence marking the end of the period of voluminous volcanic and vol-caniclastic inundation of the southern Peninsula.

In the northern portion of Baja California Sur the Comond For-mation ranges from22.9 + 2.7 m-y. (Gastit, and others, i979) to about15 m.y. (Jim Smith, pers. comm., 1983). Thus, the activity of theComond volcanoes appears to be approximately synchronous along thelength of Baja California Sur, although possibly older in the southern-most area by l-2 m.y. Furthermore, the youngest date of the San Gre-gorio Formation in the northerly La Purisima aea (23.4 m.y.) isyounger than the oldest Comond at La Paz, suggesting the possibilityof a northward progression of volcanic events from La Paz to LaPurisima.

Bulk Composition

The Comond volcanics form a suite of lithologies ranging fromlow-silica andesite to high-silica rhyolite. The volcanic arc that existedalong the present eastern shoreline of Baja California produced a fargreater proportion of rhyolites in the Bahia de La Paz area than in theregion to the north; rhyolite tuffs form a major part of the La Paz stra-tigraphy and become fewer in number northward, entirely disappearingfrom the section at Punta San Marcial. The Comond Formation tothe north is almost entirely andesitic with local rhyolitic ash-flow tuffsoccurring east of Bahia Concepcin. In the vicinity of La Paz and SanJuan de la Costa the paucity of dacitic volcanic products results in abimodal andesite-rhyolite sequence (fig. ll). This may be attributed tothe fact that dacites tend to be associated with volcanic edifices in theform of domes and viscous lavas, uncommonly forming far-traveledfluid lavas or pyroclastic flows. With the Gulf of California east-side-down faulting of the majority of the volcanic vents in Baja CaliforniaSur the dacite component has been selectively removed.

The bulk composition of the Comond suite in the La Paz regionis typical of continental volcanic arcs. Major elements (table 2) areplotted against silica in figure 11. An alkali-lime index of 60 defines acalc-alkaline classification (Peacock, 1931). These analyses plotted onMiyashiro's (1974) FeO*/MgO vs. SiO" diagram also indicates a calc-alkaline classification. The Comond vlcanic chemistry plotted on anAFM diagram (fig. 12) displays a "differentiation" trend similar to thatof the Cascade calc-alkaline continental arc volcanics, showing no ironenrichment.

23r

AFM DIAGRAM eo*

COMONDU

VOLCAN I CS

Andesite,53-63% SiOzDccite. 63-70 % SiOz

Rhyolile,>7O% SiOe

NozOlK go

Figure 12. AFM diagram of the Comond Formation volcanic rocks inthe Baha de La Paz region. For comparison, the dashed line is a sum-mary of chemical data from lavas of the Cascade range of northwesternUnited States (Carmichael and others, 1974). FeO* is total ironreported as FeO.

MAJOR ELEMENT VARIATION DIAGRAMS

oa = Comond volconic rocks (25- 12.5 my)p = Pursimo Viejo lovo of the Comond

.^= Alkolic bosolts (2.4my ond younger)

r8

t6ro

t48

t26

4

(l)Exo'ed

=

2

olo

I

6

4

2

6o6

4 4

2 2

o 60 70 60 70wr. % SiOz

Y

o. MgO

a

otr. ^olo oRL)V

On@o -o oo o oo-o.Po

,t':Q-^ af ^^:^1iile " al.=x \IJc)

iile-" a ^-T-tY^?faA a X49-o.- --a a_d"-?Wo-Q aa4or\

o :E \-ar= (2Q+NozOl lI !n=r^ II9-ar=(2Q+Noeol lf ! o\...I iE: n-oooo = coo I i .Qn'BqDs.50

o^qhoo^3

o od ";op ,:o

o

O^^

- .to.Bb?g^Ooo ._.

"a^ ^*\) "oqo (Qgs&u

oo.Co c ob-p J$"s38S% "

Figure 11. Major elemental oxides versus SiO" for volcanic rocks in Baja California Sur. Determinations from X-rayfluorescence methods. Note, the general differnce in trends between the younger series of alkalic basalts and the olderComond Formation. FeO* is total iron reported as FeO.

232 Hqusback, Briqn P.

Young Alkalic Basalts

Unconformably overlying the Comond Formation is a series ofmid-Miocene to Holocene basalts that are clearly different from theolder Comond andesites and rhyolites. These basalts occur ascanyon-filling flows as well as units capping the gently westward-tiltedmesas of the Sierra de la Giganta and are most voluminous andwidespread to the north of Punta San Marcial. The basalts occursporadically in isolated exposures as far south as Los Torres (locality 9,fig. 1) but do not occur in the Baha de La Paz region. They apparentlyissued from scattered local cinder cone source vents.

These basalts have completely different compositional signaturesand trends compared to those of the Comond volcanics, as indicatedin figure 11. On the alkalies vs. silicaclassification ofKuno (1959) infigure 13, the basalts plot in the alkaline classification, off-trend fromthe Comond calc-alkaline or high-alumina volcanic suite.

Petrographically, the alkali basalts are locally vesicular, fine-grained, gray to brown, holocrystalline intergranular lavas. They typi-cally contain clinopyroxene and olivine microphenocrysts commonly setin a groundmass of sanidine, plagioclase, phlogopite, Fe-Ti oxides and(aenigmatite or priderite ?). These basalts are best classified as trachy-basalts and are similar to Pliocene and Pleistocene minette lavas in theColima Graben area near Guadalajara, Mexico (Allan and Carmichael,1983). Associated with this primarily alkalic basalt sequence are unitsthat are less alkaline, lacking the groundmass sanidine and phlogopite.

K-Ar ages of 72.4 m.y. to 0.47 m.y. (table 1) demonstrates thatthese lavas are younger than the entire Comond Formation of BajaCalifornia Sur. At Pursima Vieja a 14.5 m.y. alkalic basalt is interbed-ded with Comond volcanic sandstones. The similarity of thisComond-age lava to the bulk composition of the younger alkalicbasalts possibly suggests that it is a transitional eruptive unit betweenthe older Comond arc volcanics and the younger alkalic basalt series.

qA"PAaA aAA

r=Bosolt Series (lZ4my ond younger)

= Comond Formotion (25- 12.5 my)

50 60 70Wt. % SiOz

80

Figure 13. Alkalies versus silica for the volcanic rocks of Baja Califor-nia Sur. Alkalic-High Alumina classification from Kuno (1959).

STRUCTURAL GEOLOGY

Baja California Sur is a coherent crustal block that has suffered lit-tle deformation since the early Tertiary. Generally, the topography ofthe southern Peninsula gently slopes to the west about 1". The beddingof the Comond Formation generally follows this westward dip, whichprobably represents the original depositional attitude of the volcaniclas-tics shed westward onto a broad network of alluvial fans draining theeastern chain of volcanoes.

Several deformational events occurred during the Tertiary, eachresulting in minor warping of the stratigraphy. Contacts between theTertiary formations are generally concordant but locally show angulardiscordances where the underlying formation has been folded into gen-tle flexures. The latter point is well-illustrated by the San GregorioFormation which was gently deformed into north-trending open folds in

faUte Z.neprese"tComond Formation, San Juan de la Costa

Member Tch Tcj Upper Tcllava pumice Tcb Hbl-tuff

Sample # 162 I 10B t76A 444sio,)Tio;AtrolFeO*-MnOMgoCaONaroKroPFscf (pm)Sr (ppm)

Toral Wt. o/o

57.841.01

16.795.7 6

0.093.7 |6.403.612.330.26

585

97.86

73.7 40.34

13.031.040.040.101.142.8s5. 15

142088

97.54

55.180.96

18.686.990.133.267.863.301.490.20

492

98.10

66.940.66

14.903.340.071.81

2.59

2.80

9646282

97.89

Comond Formation, Isla Espritu SantoMember

Sample #

Tcf Tcelava basal288E 272A

Tcdtuff283

Tcjpumlce2748

to

sio,Tio'.)AtrolFeO*-MnOMgoCaONarOKroPFscI (pm)Sr (ppm)

Total Wt. o/o

s6.t20.93

I 6.686.400.124.027,t72.932.t00,29

618

96.82

69.6i0.48

t4.322.690.050.271.694))4.33

1 409223

97.80

72.910.r5

t2.46t.6'l0.051.041.452.544.08

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7 4.21 7 5.6s0. l0 0.0413.59 tt.921.07 0.920.07 0.030.56 0.070.95 0.522.82 2.654.66 5.86

60.94 60.640.75 0.16

r7 ,20 17.706.t2 6.190.09 0.092.00 t.795.72 5.963.54 3.461.90 1.910.17 0.22

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Sample #

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438

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Samples analysed by X-ray fluorescence, Univ. of Calif., Berkeley.

Figure 14. View to the northwest, at the mouth of Arroyo Sauzoso,south of San Juan de la Costa. Note, tilted lower Comond Formationsandstone and tuff overlain by undeformed upper Comond bedsincluding the San Juan tuff (SJT).

233

Figure 15. View downstream in Arroyo Camarn, south of San Juan dela Costa. Note, contorted lower Comond Formation sandstone andtuff.

the area of Arroyo La Pursima and was subsequently covered by therelatively flat-lying Isidro Formation. In the Baha de La Paz regionthere is only the slightest regional warping of the Comond, Isidro, SanGregorio, and Tepetate Formations.

There is clear evidence for localized syn-depositional deformationwithin" the Cronif formation. In Arroyo L Pursima an abruptanglai hisciildnc is present within the sandstone and conglomeratesequence. In the Baha de La Paz region the undeformed upperComond rests unconformably upon locally intensely sheared andfolded underlying tufs and sandstones (fies. 14 and l5). This deforma-tion is found in two areas: along the Gulf coast south of ArroyoCamarn and in Arroyo El Abra along the main peninsular highwaynea km 69 to the north of La Paz. Although deformational structuresappear to be locally chaotic, slickensides measured on subhorizontalfault planes south of San Juan de la Costa roughly indicate asouthwest-northeast shear direction. In addition, large detached lithicblocks along these subhorizontal fault planes show a general imbricationdirected west to southwest along the Gulf coast and to the northwest(also perpendicular to local fold axes) in the Arroyo EI Abra area. Thisdeformation probably indicates a syn-depositional, generally westwardgravitational sliding of immense slabs of lower Comond Formation.

La Paz Fault

The La Paz Fault is a prominent strand of a major north-trendingtrans-peninsular fault system developed in the Cabo province. Thisfault juxtaposes the southern granitic Cabo province highland to thevolcanic-dominated southern peninsula. The existence of the La PazFault was originally proposed by Beal (1948). Normark and Curray(1968) suggested that the fault was a wide structural zone with bothleftlateral strike-slip movement and Quaternary normal, east-side-downmovement based on bathymetric and offshore dredge-haul data in thesouthern cape area. Fault plane solutions for two 1969 earthquakes onthis southern offshore extension indicate a large component of left-lateral motion on the La Paz fault (Molnar, 1973). Beal (1948) andNiemitz and Bishoff (1981) inferred the northerly extension of the LaPaz fault along the major topographic escarpment of about 800 mheight to the north of La Paz.

Geologic studies in the La Paz area substantiate the existence ofthis major fault zone. South of La Paz the fault is a single 2 km widezone of sheared mylonitized granite and monzonite with local zones ofhornblende cataclasite containing disseminated pyrite. These rocks,which must have been deformed beneath at least several kilometers ofoverburden, demonstrate the deep-seated nature of the fault and ind!cate that subsequent uplift and erosion has brought these lithologies tothe surface.

North of LaPaz fhe fault zone is hidden beneath a north-trendingvalley filled with apparently unfaulted Pliocene (?)'and Quaternary allu-vium and alluvial terrace gravels. At the north end of the pichilinguepeninsula the fault is covered by an undeformed sequence of alluvialconglomerates and sandstones known informally as the Punta Coyotegravels. The youngest portion of this conglomerate sequence contains alocalized shallow marine assemblage with abundant pelecypod and gas-tropod fauna of late Pliocene to Recent age (Judy Smith, writtencomm., 1983). A coral from the marine portion of the punta Coyotegravels has yielded a uranium-series date of 146,000 + 9,000 years(Barney Szabo, *ritten comm., 1984). The structurally undisturbednature of these beds indicates that this portion of the La paz Fault hasprobably been inactive since late Pleistocene time; however, ComondFormation rocks near the fault zone are generally contorted, indicatingpost-middle Miocene fault movement. The relative inactivity of thisportion of the fault zone is of great importance to the 200,000 peopleof La Paz who live directly downstream from a dam currently underconstruction astride the La Paz Fault. It must be reiterated thatmoden seismic events have been recorded on the offshore extensionof this fault (Molnar, 1973).

A strand of the fault reappears on the eastern side of the IslaEspritu Santo where itjuxtaposes mylonitized granitic rocks on the eastwith the Comond Formation on the west. At this locality ample evi-dence exists for post-Comond normal fault movement along thisstructure. The island has an abrupt eastern escarpment and volcanicunits resting atop the granitic basement have been vertically down-dropped over 300 meters to the east.

The scarcity of Comond volcanic deposits overlying the graniticbasement to the east of La Paz, as well as the occurrence of sedimen-tary and volcanic transport directions parallel to the fault escarpmentstrongly suggest that the granitic area was already a highland duringComond deposition, and therefore had been uplifted prior to the-Miocene. During the Comond volcanic period the granitic block prob-ably formed a high relief valley wall, confining most of the volcanics tothe La Paz aea. The abrupt termination of volcanic core facies of theComond Formation against this granitic highland, without evidence ofsub-volcanic intrusions into the granitic rocks, combined with observa-tions south of La Paz cited above, suggest that the southern volcanicextension of the Comond Formation has been translated to the northby left-lateral strike-slip movement of the La Paz Fault (fig. l0).

In summary, prior.to the Miocene, the southern Cabo provincewas uplifted along the La Paz Fault forming an eastern granitic high-land. After the Comond Formation was deposited both an easterlydowndropping and possibly left-lateral strike slip motion occurred alongthe fault. The timing of this post mid-Miocene fault movement in theLa Paz arca is poorly constrained and may, in part, continue to Recenttimes. Current activity is documented by earthquakes along the south-ern offshore extension of the fault.

234 Hausback, Brian P.

Gulf Normal Faultins

An ubiquitous set of Gulf-downdropping normal faults, cut theBaja Peninsula. These structures generally trend Nl5-35W withnumerous arcuate extensions varying widely in orientation. The faultsdisplay local extensional brittle deformation, typically have sharp faultplanes with steep easterly dips, and commonly die out along strike.The period of faulting appears to be entirely post-Comond in age andactivity probably continues to the present since the faults, in general,are morphologically young, and at least one cuts soils west of La Paz.The most prominent of these struclures in the Baha de La Paz regionis the San Juan fault which has a vertical offset of about 0.5 km at SanJuan de la Costa. South of Los Dolores, (locality 10, fig. l). a majornormal fault with a vertical offset of at least 100 m beheads the upperarroyo drainages at the Sierra crest, creating a series of internally-draining playa lakes. On Isla Espi'ritu Santo the abundant normal fault-ing merges in orientation with the La Paz Fault, suggesting that thepost-Comond normal offset along the La Paz Fault both on IslaEspritu Santo and near La Paz is a reactivation of the old structure dueto Neogene tensional stresses.

Associated with the northwest-trending normal faults is a conju-gate series of faults and major joints trending N60'80E. These arenearly vertical structures with normal offsets up to 100 m or more.Best displayed in the La Paz and Isla Espritu Santo areas, they slice thetopography into a series of elongate tilted mesas.

TECTONICS

Interest in the tectonics of Baja Catifornia began with AlfredWegner (192D, who proposed that Baja California had been originallyattached to the Mexican mainland. Although the modern picture of thetectonic evolution of Western North America is based to a large extenton offshore geophysical data, an understanding of the onshore geologyof Baja California contributes an integral and important part to thisstory.

McKenzie and Morgan (1969) and Atwater (1970) outlined thegeneral tectonic evolution of western North America and concludedthat during the early Tertiary the Farallon plate was being subductedbeneath western Mexico. The collision of the Pacific-Farallon spreadingridge with the subducting North American-Farallon boundary (about 29m.y. ago), created a zone of right-lateral, strike-slip faulting along thecoastline that expanded both north and south, progressively convertingthe subducting plate boundary into a transform interplate shear zone.

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Mammerickx and Klitgord (1982) detailed these tectonic eventsfor the eastern Pacific-Baja California region and delineated severalperiods of lithospheric plate interactions that bear on the geologic his-tory of Baja California. From 25 to 12.5 m.y. the narrow southern por-tion of the Farallon plate, called the Guadalupe plate, began to breakup and began a pivoting subduction off Southern Baja California (fig.16a). This geometry required subduction in the Bahia de La Pazregion, with a northerly decreasing subduction rate toward the axis ofrotation which was probably in northen Baja California. At 12.5 to l1m.y. a major plate reorganization began (flg. 16b) and included the fol-lowing: 1) the East Pacific Rise had rotated parallel with the westernBaja coast; 2) subduction ceased synchronously along the length of BajaCalifornia Sur; 3) several fragments of the Guadalupe plate werestranded along the west coast; and 4) the right-lateral strike slip motionbetween Baja California and the Pacific Plate began along the Tosco-Abreojos Fault.

The period following this plate reorganization is one of greatimportance in understanding the process of detachment of Baja Califor-nia from mainland Mexico. Geometrically, the Pacic plate was slidingnorthwestward with respect to North America. The Tosco-AbreojosFault (fig. l7a) formed approximately 12.5 m.y. ago and became thetransform fault boundary between the Pacific oceanic plate and theNorth American continental plate which included Baja California at thattime. Transform faults such as these form parallel to the relativemotion between adjoining plates, which, for the Pacific-North Americanplate movement at that time had an azimuth of about 325'. After theformation of this right-lateral fault 12.5 m.y. ago the azimuth of relativemotion between the two plates progressively swung to the west (fi9.17b). This change resulted in an inability for the Tosco-Abreojos Faultto accommodate the entire interplate motion; the increasingly westwarddrift of the Pacific plate required either a reorientation of the boundaryfault or attendant extensional motion. There is no evidence of a west-ward azimuth-shift of the Tosco-Abreojos Fault nor of rift basins open-ing along this structure. The necessary extension probably took placealong the axis of the present Gulf of California -- initiating a proto-Gulf, or zone of graben formation. This would have been a likely locusfor extension to occur since throughout the previous 10-12 m.y. it hadbeen the core of the Comond volcanic arc, therefore a zone of heated,intruded, fractured and generally weak crust. By 3.5 m.y. beforepresent, most of the motion between the Pacific Plate and the Mexicanmainland transferred from the Tosco-Abreojos Fault to the proto-Gulf,then developing along eastern Baja California. The Baja Peninsula wassevered from North America and became sutured to the Pacific plate.Sea floor basalts began emerging in the spreading mouth of the Gulf ofCalifornia, and a new set of more westwardly-oriented transform faultsand spreading basins were generated along the evolving Gulf of Califor-nla.

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Figure 17. (a) Modern fault map of western Mexico showing the activetransform fault system within the Gulf of California and the now rela-tively dormant Tosco-Abreojos Fault; after Spencer and Normark(1979). (b) Pacific versus North American Plate motion through time(dashed line). Data points with error bars from Blake and others(1978). The Tosco-Abreojos Fault initiated about 12.5 m.y. ago parallelto interplate motion (square). Since then, this plate motion hasbecome more westerly directed requiring proto-Gulf extension (pat-tern) in addition to the Tosco-Abreojos Fault movement. By about 3.5m.y. ago the Tosco-Abreojos Fault had become relatively inactive andthe proto-Gulf rift system continued to evolve into the modern Gulf ofCalifornia.

GEOLOGIC HISTORY AND INTERPRETATION

During the late Oligocene, Baja California Sur was a structurallystable marine shelf attached to western Mexico, shallowing eastwardwith a shoreline near the present Gulf of California coast. Upon thisplatform the San Gregorio shales, diatomites, and phosphorites weredeposited, incorporating a large volume of silicic volcanic airfall asherupted from the then active Sierra Madre Occidental volcanic arc. Thefocus of volcanic activity of the Sierra Madre was migrating westwardand arrived in the vicinity of eastern Baja California at the onset of theMiocane epoch. Symptomatic with this westward volcanic migrationwas the crustal heating and uplift of western Mexico, including the Baja

235

platform. Uplift generated the westward shedding of older volcanicbasement detritus, recorded in Baja California as an influx of foreignmeta-volcanic pebbles. The calc-alkaline volcanic chain arrived alongwhat is now eastern Baja California about 24 m.y. ago, and quicklyinundated the remnant shallow marine platform with a large volcanic-volcaniclastic apron along the length of the Baja Peninsula. Withgreater subduction rates in the south this arc produced large volumes ofsilicic volcanics, whereas the proportion of silicic volcanics decreased tothe north where subduction was slower. By 16 m.y. ago volcanicactivity began to wane and by 12.5 m.y., synchronous with the halt ofsubduction, the final products of the volcanic arc were shed onto thesouthern peninsula near San Juan de la Costa. Arc volcanism lasteduntil about l5 m.y. in the Loreto region and may have been transitionalto the next phase of dominantly alkalic basalt volcanism. The demiseof arc volcanism was nearly synchronous along Baja California Sur andreflects the proposed synchronous end of subduction along the entirelength of Southern Baja California (Menard, 1978; Mammerickx andKlitgord, 1982). There is no indication of a progressively southwardterminalion of volcanism as would have been predicted for a southerlymigrating triple junction and sequential end of subduction from northto south. After subduction ceased, the boundary between the Pacificand North American plates became a major transform fault along thewest coast of Baja with extension along this margin geometricallyrequired. Lack of volcanogenic deposition onto the southernmost BajaPeninsula after 12.5 m.y. ago indicates a separation of the peninsulafrom easterly volcanic sources with probable graben formalion along theaxis of the then extinct chain of volcanoes. The formation of thisproto-Gulf is well documented in the southernmost Gulf of Californiawhere, on Maria Madre Island, a major subsidence occurred after thedeposition of mid-Tertiary volcanics and prior to the deposition of 8.5m.y. marine diatomites (McCloy, 1982; McCloy and Ingle, 1985).

Alkalic basalts erupted along Baja California Sur coeval with, andprobably as a result of the rifting of the proto-Gulf of California,extending from 12.4 m.y. to at least 0.47 m.y.

The southern Peninsula has generally been struclurally stablethroughout the Tertiary. The granitic Cabo province was probablyalready upthrown along the La Paz Fault prior to the early MioceneComond deposition. Post 12 m.y. movement along the La Paz Faultprobably includes the downdropping of the Cabo block and signicantleft-lateral strike-slip faulting. The Gulf of California srructuraldevelopment has been accomplished along postComond (post 12m.y.) normal faults, likely active today.

ACKNOWLEDGEMENTS

Many people have helped and influenced me throughout my stu-dies of Baja California. I would especially like to thank my advisor,Garniss Curtis, as well as Bob Drake and Joachim Hampel for their gui-dance, discussions and technical help throughout this project. My edu-cation at Berkeley has been vastly broadened through my interactionswith several of my fellow graduate students: Mary Gilzean, GregHarper, James Allan, Jim Luhr, Bill Chvez, and Alan Dieno, whohave unselfishly supported me through thick and thin. My har is of toKorda Cordes, my friend who skillfully engineered and typed thismanuscnpt.

Fond memories of fieldwork in Baja are highlighted by rhe timesthat I have spent with my field partners: Hugh Mclean, AndrewArnold, and Tom Green. These fine folks have shared with me many acelestial desert evening of vagabond thoughts and have taught memuch about Baja California. I would like to thank several other people:Clark Blake, Virgil Frizzell, David Howell, Jim Ingle, Ceil McCloy,Mike McWilliams, Mike Sawlan, Jim and Judy Smith, and Chris Utterof the U.S. Geological Survey and Stanford University who haveassisted me in many ways and have seeded me with their trueenthusiasm for life and science.

Logistical and financial support for fieldwork in Baja California hasbeen generously provided by Ingenieros Guillermo P. Salas, SalvadorCasarrubias, Hugo Cortez, and Antonio Castro of the Consejo deRecursos Minerales, as well as Ingenieros Francisco Escandn, CarlosChacn, and Alfonso Martinez of Roca Fosfrica Mexicana, andIngeniero Diego A. Crdoba of the Univrsidad Ncional Autnoma,Mexico. In addition, my work has been, in part, funded by the Geo-logical Society of America, Penrose grants and the University of Cali-fornia.

236 Hausback, Brian P.

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. York, 376 pp.*l Spencer, J.E., and Normark, W.R., 1979, Tosco-Abreojos fault zone: a

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