Neogene sedimentary evolution of Baja California in relation to regional tectonics

Journal of South American Earth Sciences 12 (1999) 589±605

Neogene sedimentary evolution of Baja California in relation to regional tectonics J. Helenes a,*, A.L. CarrenÄo b a

Departamento de GeologõÂa, Centro de Investigaciones Cientõ®cas y de EducacioÂn Superior de Ensenada, Carretera Ensenada-Tijuana Km. 107, 22830, Ensenada, Baja California, Mexico b Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Circuito Exterior, C.U. DelegacioÂn CoyoacaÂn, 04510 DF, Mexico

Abstract During the Neogene, the tectonic and sedimentary evolution of the Baja California Peninsula followed four stages: (1) during the early Miocene (22 Ma), the initiation of transform motion between Paci®c and North American plates, caused a rapid subsidence in the Continental Borderland Province and in some adjacent areas.This subsidence coincided in time with with a global rise in sea level. At this time, the eastern and southern parts of the peninsula did not show any evidence of subsidence. (2) During the middle Miocene (12 Ma), normal and strike slip faulting migrated eastward, causing subsidence in the northern part of the Gulf of California, where the oldest Tertiary marine sedimentary rocks were deposited. The areas in central Baja California Sur and the central part of the Gulf itself received abundant volcanic deposits related to continental extension. (3) During the late Miocene (8 Ma), the western margin of the Peninsula changed to a slightly compressive regime, while the northern part of the Gulf contained a marine basin with upper bathyal environments. The central area of the Gulf continued receiving abundant volcanic deposits, while the Los Cabos block received marine sedimentation, correlatable with sedimentary units reported from the continental margins in Nayarit, Jalisco and MichoacaÂn. (4) Beginning in the early Pliocene (5 Ma), the present con®guration of the Gulf of California developed through right-lateral strike slip and extension in the Gulf itself. Since Pliocene times, the Gulf presents widespread marine sedimentation with deep basins reaching lower bathyal depths. # 1999 Published by Elsevier Science Ltd. All rights reserved. Resumen Durante el NeoÂgeno, la evolucioÂn tectoÂnica y sedimentaria de la penõ nsula de Baja California presenta las etapas siguientes: (1) Durante el Mioceno temprano (22 Ma), el inicio de fallamiento de transcurrencia en la regioÂn debida a la interaccioÂn de las placas del Pacõ ®co y de Norte AmeÂrica, causo una subsidencia raÂpida en la Provincia de Borde Continental y algunas areas adyacentes. Esta subsidencia coicid en tiempo con una elevacioÂn global del nivel del mar El oriente y sur de la penõ nsula no parecen haber sido afectadas por esta subsidencia; (2) durante el Mioceno medio (12 Ma), el fallamiento normal y de transcurrencia emigro hacia el oriente, originando subsidencia en el aÂrea norte del Golfo de California, en donde se depositaron los sedimentos marinos terciarios maÂs antiguos del Golfo. En regiones hacia el centro y sur del presente Golfo de California se acumulaban depoÂsitos volcaÂnicos abundantes, relacionados con extensioÂn continental. (3) durante el Mioceno tardõ o (8 Ma), la margen occidental de la penõ nsula cambio a un reÂgimen de compresioÂn tectonica leve, mientras que el norte del Golfo contenõ a una cuenca marina con paleoambiente batial superior. El aÂrea central del Golfo continuo recibiendo abundantes depoÂsitos volcaÂnicos, mientras que el bloque de Los Cabos recibõ a sedimentacioÂn marina correlacionable con estratos marinos de la margen occidental de Nayarit, Jalisco y MichoacaÂn; (4) a partir del Plioceno (5 Ma), el Golfo de California se desarrollo por fallamiento dextral y expansioÂn. Desde el Plioceno, el Golfo presenta abundantes sedimentos marinos con cuencas que alcanzan profundidades de ambiente batial inferior. # 1999 Published by Elsevier Science Ltd. All rights reserved.

* Corresponding author. Tel.: +1-61-744-501 x26032. E-mail addresses: [email protected] (J. Helenes), anacar@ servidor.unam.mx (A.L. CarrenÄo). 0895-9811/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 9 9 ) 0 0 0 4 2 - 5

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1. Introduction 1.1. General setting The Baja California Peninsula and the adjacent Gulf of California are striking features in northwestern Mexico, whose general geologic history is well understood. The geology of the peninsula itself (Darton, 1921; SantillaÂn and Barrera, 1930; Beal, 1948; Mina, 1957; Lozano-Romen, 1975; Gastil et al., 1975) and the geophysical characteristics of the sea ¯oor surrounding it (van Andel, 1964; Atwater, 1970; Curray and Moore, 1984; Lonsdale, 1989, 1991) have been used to help unravel this history. However, many details of this history are now being reinterpreted, among them are the earliest history of the gulf itself (Karig and Jensky, 1972) and the di€erent stages of the separation of the peninsula from mainland Mexico. The main regional features of Baja California (Fig. 1) include: the Lower Tertiary volcanic rocks of the Sierra Madre Occidental; the Cretaceous granites of the Peninsular Batholith, Transverse Ranges and Sierra Nevada; the Mesozoic sedimentary rocks of the Sierra San AndreÂs and Coast Ranges; the mixed oceanic-continental basement of the Continental Borderland Province, and the Upper Tertiary marine sedimentary rocks found in several places in and around the peninsula. By Oligocene times, these Mesozoic and Lower Tertiary features were already in place in western North America. During the late Cenozoic, a combination of large-magnitude extension, rotation and strike slip faulting resulted in the present con®guration. The main objectives of this paper are to compile and summarize the known Neogene sedimentary record in the Baja California Peninsula region, and to relate this record to the proposed tectonic models. We expect to help clarify some of the aspects of the initial stages of the Gulf of California and some of the relationships among di€erent areas in the region. To accomplish these objectives, we present ®rst the most important aspects of the tectonic models proposed for the region. Then, we present the Neogene sedimentary record, and ®nally we point out and illustrate the relation among these two data sets. 2. Tectonic framework This region has been shaped by the interaction of two important tectonic elements, the North American and Paci®c plates (Fig. 2). Several tectonic reconstructions have been proposed, based on paleomagnetic information from the Paci®c Ocean ¯oor (Atwater, 1970, 1989; Atwater and Molnar, 1973; Mammerickx and Klitgord, 1982; Engebretson et al., 1985; Lonsdale,

Fig. 1. Main regional features and location of points of interest.

1991; Bohannon and Parsons, 1995). All of these models state that during Early Tertiary times, the Gulf of California did not exist, hence the Baja California peninsula was part of Mexico's mainland block. From the models presented by these authors, the following is a summary of the most important aspects for Baja California. From Late Cretaceous times and through most of the Early Tertiary, subduction of the Farallon plate was taking place in southwestern North America. This regime is preserved in the geologic record of Baja California, northwestern Mexico and southwestern California. It is represented by large scale granitic intrusions (i.e. Sierra Nevada and Peninsular Batholith), metamorphic belts (i.e. Continental Borderland), accretionary complexes (i.e. Franciscan Complex), forearc sedimentary rocks (i.e. Great Valley Sequence and the Cretaceous sedimentary sequence

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northern part of the actual Gulf of California were low enough to receive sea water. Given the palinspastic resonstructions proposed on the basis of geophysical data (Lonsdale, 1991; Bohannon and Parsons, 1995), passage of sea water was precluded from the north or west by the presence of the plutonic belt, and must have entered from the south, in a manner similar to today's Gulf of California. The precise location of this seaway is still uncertain, but considering the extent of the main outcrops of the Penisular Batholith, it was probably located south of the Sierra San Pedro Martir, but somewhere to the north of the Los Cabos block. By late Miocene time, subsidence became widespread in the Gulf itself. Initially, subsidence was more pronounced in the northern part of the actual Gulf of California (Dokka and Merriam, 1982; Stock and Hodges, 1989; Stock et al., 1996; Winker and Kidwell, 1996), but general extension is recorded in the central and southern parts of the gulf, during this interval, with the central part of the Gulf receiving marine sediments until Pliocene times (CarrenÄo, 1982, 1983; Smith, 1989; Dorsey et al., 1997).

Fig. 2. Most important tectonic elements in the region.

from Sierra San AndreÂs) and volcanism (i.e. Sierra Madre Occidental). In Oligocene times, the Paci®c-Farallon spreading ridge approached the continental margin somewhere in southern California. From the Oligocene and through most of the Miocene, compressive events and strike slip faulting occurred in the Paci®c margin of the continent, west of the Peninsular Batholith (Atwater, 1970; Spencer and Normark, 1989; Lonsdale, 1991). During the earlier part of this interval, continental extension took place to the east of the Batholith (Crowe, 1978). This extension continued and the strike slip faulting migrated to the east of the Batholith (Crowe, 1978; Dokka and Merriam, 1982; Barrash and Venkatakrishnan, 1982; Stock and Hodges, 1989), so that by middle Miocene times, some areas in the

Fig. 3. Areas with known sedimentary record from Oligocene to Pleistocene. The palinspastic reconstruction shown here in Fig. 3 and 7 to 11, is based on reconstructions presented by Lonsdale (1991), and Bohannon and Parsons (1995).

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Also in the late Miocene, but farther to the south, marine sedimentary rocks were being deposited in the Islas Marias (ChinÄas-Robisel, 1963; CarrenÄo, 1985; McCloy et al., 1988) and at Punta Mita (Jensky, 1975) in mainland Mexico. These sedimentary rocks were most probably deposited in an embayment located to the south of the proto-Gulf. Marine deposition on oceanic crust in the southern part of the Gulf itself began in late Pliocene times. According to the magnetic lineations, the southernmost part of the Gulf of California opened approximately 5 million years ago (Atwater, 1970, 1989; Ness et al., 1981; Bohannon and Parsons, 1995), separating the Los Cabos block from mainland Mexico. 3. Sedimentary record The Neogene tectonic evolution just mentioned, left a sedimentary record in the peninsula and nearby areas. To better understand the signi®cance of the Upper Tertiary sedimentary record from the Baja

California region, it can be grouped into six areas (Fig. 3), where the sedimentary record is homogeneous within them, but their geologic histories are di€erent from each other. These areas are: Southern California±Northwestern Baja California (SC± NWBC), Bahõ a Tortugas (BT), La Purõ sima-El Cien (LP±EC), Salton Sea±TiburoÂn Island (SS±TI), Santa Rosalõ a±Loreto (SR±L), Los Cabos±Punta Mita (LC± PM). The stratigraphic nomenclature in the region is very large, complicated, the names of the units are only of local importance, and many of the sedimentary stratigraphic units have not even been named formally (Table 1). We feel that these names have no bearing in a regional reconstruction like the one we intend to accomplish in this paper. To simplify and clarify the presentation of the data and our interpretation, we will not mention the names of the stratigraphic units. Instead, we will discuss their paleoenvironmental and paleogeographic signi®cance. The reader interested in the stratigraphic nomenclature is referred to the original authors for each area.

Table 1 Generalized correlation of the sedimentary stratigraphic units in Baja California and nearby areas

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3.1. Southern California±Northwestern Baja California This area (Figs. 3 and 4) includes from the southern side of the Transverse Ranges to approximately 100 km

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south of the international border and includes the northern part of the Continental Borderland Province. During Oligocene times, the stratigraphic record to the north of Southern California (Bandy and Arnal, 1969; Addicott, 1972; Webb, 1981; Lagoe, 1987a,b; McLean and Wiley, 1987) indicates the presence of deep marine basins in the Southern San Joaquin Valley, around Bakers®eld. The Oligocene stage in Southern California (Los Angeles and Ventura basins, and the Santa Ynez Mountains) and most of the Continental Borderland region is represented by non-marine Oligocene sedimentary rocks. After 30 Ma, extensional tectonics created deep marine basins which contain as much as 8 km of Neogene sedimentary rocks (Ingle, 1973, 1980; Blake et al., 1978). Along the western edge of northwestern Baja California, shallow marine basins developed in the early Miocene, (Minch et al., 1970; PeÂrez-GuzmaÂn, 1985; Ashby, 1989) and reached their maximum extent during middle Miocene times. In the late Miocene, all these basins started ®lling up so that by the beginning of the Pliocene, some of the easternmost basins in the area, such as the Los Angeles basin, were already full and the coastline prograded westward (Ingle, 1973, 1980). 3.2. BahõÂa Tortugas

Fig. 4. Lithostratigraphic columns and paleoenvironments from representative areas in Southern California, northern Baja Califronia and western Vizcaino Peninsula. Sources used in this compilation are mentioned in the text.

This area (Figs. 3, 4 and 5) is located in the northwestern portion of the SebastiaÂn Vizcaõ no Peninsula (Fig. 3), at the southern end of the Continental Borderland Province. Neogene sedimentary rocks are widely exposed in this area (Mina, 1957; Troughton, 1974; Robinson, 1975), in Cedros island (Kilmer, 1977, 1979), and in several o€shore localities (Normark et al., 1987). These rocks are assigned to an early to late Miocene unit (Helenes and Ingle, 1979; Applegate et al., 1979; Helenes-Escamilla, 1980; PeÂrez-GuzmaÂn, 1985; Moreno-Ruiz and CarrenÄo, 1994) and to a Miocene±Pliocene unit (Smith, 1984, 1986), and unconformably overlie Cretaceous rocks. Tertiary stratigraphy in the northwestern part of the SebastiaÂn Vizcaõ no Peninsula is characterized by lack of Paleogene rocks, angular unconformities below and above the Neogene formations, and widespread tilting and/or faulting of the latter units. Volcanic material and diatomaceous mudstones and laminated diatomites are characteristic in the upper parts of the early to late Miocene formation, while the Miocene± Pliocene formation consists of arenaceous, shallowwater sedimentary rocks in all the sections reported. In contrast, the sedimentary sequence underneath the SebastiaÂn Vizcaõ no basin, to the northeast, presents a di€erent stratigraphic column. This area has been drilled by PEMEX (Mina, 1957; Madrid-Solõ s, 1979;

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Fig. 5. Lithostratigraphic columns and paleoenvironments from the Vizcaino Peninsula and representative ares in western Baja California Sur. Sources used in this compilation are mentioned in the text. For legend see Fig. 4.

Helenes, 1984), and contains an almost continuous marine sedimentary sequence from Albian to middle Eocene. All the localities in the central and southern parts of the VizcaõÂ no basin, present a hiatus encompassing from middle Eocene to Pliocene. The Miocene is represented only in the northern and western parts (PEMEX, personal communication, 1980; Normark et al., 1987). The presence of the Paleogene strata indicates a more quiet tectonic history in this part of the area, allowing these strata to be deposited and/or preserved. The lack of Neogene sedimentary rocks indicates that the area was probably uplifted sometime after the middle Eocene and did not receive sediments until the Pleistocene. 3.3. La PurõÂsima±El Cien This area (Figs. 3 and 5) includes the western side of Baja California from San Ignacio in the north to El Cien in the south, and extends to the Gulf side in San

Juan de la Costa, at the western margin of the La Paz Bay. In general, throughout this area, a sedimentary hiatus encompasses from late Eocene to early Oligocene. In the northern part of the area, near San Ignacio, middle to late Miocene sedimentary rocks representing shallow marine environments (Mina, 1957; Smith, 1984; McLean and Barron, 1988) rest unconformably over middle Eocene sedimentary rocks (CarrenÄo and Cronin, 1993; Squires and Demetrion, 1992, 1994). Slightly to the south, near La PurõÂ sima, the middle Eocene rocks unconformably underlie upper Oligocene diatomaceous strata (Beal, 1948; Hausback, 1984a,b; Kim, 1987; Kim and Barron, 1987), which are in turn unconformably overlain by middle Miocene sedimentary rocks (Heim, 1922; Smith, 1984). In the southern part of the area, in the vicinity of El Cien, the stratigraphic sequence includes at the bottom a Cretaceous to middle Eocene marine sedimentary unit (Heim, 1922; Fulwider, 1976), unconformably overlain locally by Oligocene, phosphorite-bearing sedimentary rocks (Applegate, 1986; CarrenÄo, 1992b; Schwennicke, 1992). On the western side of the area, near El Cien, this Oligocene unit (Fischer et al., 1995) and represents outer neritic marine facies. While in San Juan de la Costa, to the east (approximately 50 km to the north of La Paz), this unit is thicker than on the western side, and represents shallower marine to paralic facies. On the western side of the area, the middle Eocene rocks are unconformably overlain by late Miocene to Pliocene, shallow-marine clastics (Heim, 1922; Mina, 1957; Smith, 1992). Some sedimentary rocks indicative of non-marine environments are found in the eastern part of the area (Applegate, 1986). Unfortunately, the uncertainty of their age and precise stratigraphic position with respect to the phosphorite-bearing Oligocene to early Miocene strata, preclude con®dent correlation with other strata (CarrenÄo et al., 1997). 3.4. Salton Sea±TiburoÂn Island This area (Fig. 3 and 6) includes from the Salton Sea in the north to TiburoÂn Island in the south, and it is particularly important to help understand the evolution of the Gulf, because it contains the most extensively studied series of outcrops and the oldest dated tertiary marine sedimentary rocks in the Gulf area, east of the batholith. Oligocene sedimentary rocks described from this area include mostly non-marine conglomerates and fanglomerates (Woodard, 1974; Eberly and Stanley, 1978; Winker and Kidwell, 1996). The oldest marine sedimentary rocks reported are those from TiburoÂn island (Smith et al., 1985; Smith, 1991b), which have middle Miocene radiometric (12±13 Ma) and paleonto-

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Fig. 6. Lithostratigraphic columns and paleoenvironments from representative areas inside the actual Gulf of California. Sources used in this compilation are mentioned in the text. For legend see Fig. 4.

logical ages. At that time, non-marine rocks were also being deposited in a large area from San Felipe, Baja California to Phoenix, Arizona (Eberly and Stanley, 1978). Afterwards, the area received at least two more marine transgressions. The ®rst one occurred in late Miocene to early Pliocene times, and deposited marine sediments as far north as Fish Creek and Lake Mead in the northern part of the SS±TI area (Ingle, 1974; Blair, 1978; Quinn and Cronin, 1984). By late Miocene times, the transgression was well established and contained a well developed marine basin, occasionally anoxic, with depths of more than 150 m in the San Felipe area (Boehm, 1982, 1984; PeÂrez-GuzmaÂn, 1985; Winker and Kidwell, 1996), and with neritic to intertidal facies in the Puertecitos area (Martin-Barajas et al., 1997). The second marine transgression reached the area in the late Pliocene (Schremp, 1981; Quinn and Cronin,

1984; VaÂzquez-HernaÂndez et al., 1996), and is represented in several basins with paleoenvironments ranging from upper bathyal to transitional in the western side of the Gulf of California (Stock et al., 1996; MartõÂ n-Barajas et al., 1997). Strata deposited during these transgressive events, usually contain abundant reworked fossils from Cretaceous strata (Merriam and Bandy, 1965; Fleming, 1993, 1994; VaÂzquez-HernaÂndez et al., 1996; Helenes-Escamilla, 1997). Additionally, some workers have reported the presence of Oligocene to Miocene microfossils in samples from wells and outcrops, otherwise assigned to a Pliocene age in the area (Cotton and Vonder Haar, 1981; Helenes-Escamilla, 1997; CarrenÄo, 1998, unpublished data). The presence of these microfossils could be related to reworking of middle Miocene material from a deeper facies of unseen strata correlative with the Tiburon Island

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deposits. These unseen sedimentary rocks could be found in the subsurface of Sonora, or underlying the late Miocene San Felipe diatomites, or o€shore underlying the deposits from the Colorado delta. There is also the possibility that they could have been completely eroded. 3.5. Santa RosalõÂa±Loreto This area (Figs. 3 and 6) is located in the central part of the Gulf and contains sedimentary rocks which indicate a very active tectonic history during the Neogene, and probably also during the Oligocene. The oldest Tertiary sedimentary rocks in this area are red-colored, cross-bedded sandstones, which represent ¯uvial environments. The upper part of these beds contain tu€aceous material dated as 28.1 2 0.9 Ma old (McFall, 1968). These deposits are unconformably overlying a Cretaceous granitic complex, and are probably related to the initial stages of a continental rift. After a long interval (approximately 50 million years) of erosion or non-deposition, during the last part of the Oligocene the area began to subside slowly along what is now the Gulf of California (McFall, 1968). A marine basin formed in the area in early Pliocene times and reached its maximum extent by late Pliocene times. Marine sedimentary rocks were deposited in Santa Rosalõ a (Wilson, 1948; Applegate and EspinosaArrubarena, 1981; CarrenÄo, 1982, 1983; Smith, 1989), in the ConcepcioÂn Peninsula (Wilson, 1948; Smith, 1991a,b; Johnson et al., 1997; Ledesma-VaÂzquez et al., 1977), and in the vicinity of Loreto (McLean, 1988, 1989; Umhoefer et al., 1994; Zanchi et al., in press; Piazza and Robba, 1994; Dorsey et al., 1997). Strata found near Santa Rosalõ a contain planktonic foraminifera indicative of open marine conditions with maximum depths of 200±500 m. At the ConcepcioÂn Peninsula, Neogene strata are represented by clastic alluvial to neritic deposits, with chert and limestone beds in the upper part. The limestone and chert beds contain molluscs and mangrove roots, and were deposited in shallow, marine environments in a relatively small, sheltered embayment (Johnson et al., 1997). Near Loreto, the Pliocene is represented by clastics deposited in deltaic to shallow marine environments. This clastic sediments were largely controlled by contemporaneous faulting (Dorsey et al., 1997). Middle to late Pliocene marine deposits are found in Punta Chivato and in the islands between Santa Rosalõ a and Loreto, although the precise correlation of some of these beds with the strata in the areas on land mentioned before is uncertain. The islands San Lorenzo and Carmen contain Pliocene strata, and probably also the Monserrat and Coronado islands (Durham, 1950).

3.6. Los Cabos±Punta Mita This area (Figs. 3 and 5) includes the southernmost part of the Peninsula of Baja California; the Deep Sea Drilling Project Sites 474, 475, and 476; the Tres Marõ as islands; and Punta Mita, to the northwest of Puerto Vallarta. In the southernmost part of the Peninsula of Baja California, the marine Neogene has been described from the Trinidad Graben (Pantoja-Alor and CarrilloBravo, 1966; McCloy, 1984; CarrenÄo, 1992a; CarrenÄo and Segura-Vernis, 1992; Martõ nez-GutieÂrrez and Sethi, 1997), or San Jose del Cabo Basin. In this area, a well dated late Miocene, marine diatomaceous unit is conformably overlying non-fossiliferous, red-beds representing continental environments and whose age is unknown. The age of these red-beds is debatable and has been interpreted as middle to late Miocene based on their stratigraphic position. However, the sedimentary characteristics and conformable contacts between the red-beds and the diatomaceous strata indicate that they belong to the same transgressive cycle. So, most probably the red-beds are also late Miocene in age. The Neogene transgression reached its maximum in late Miocene times. Afterwards, a regressive cycle started, with shallow marine sedimentation present in the area at least until the end of the early Pliocene. Farther to the northeast, inside the Gulf of California, the islands San JoseÂ, and Cerralvo also contain marine Pliocene sedimentary rocks (Hanna and Hertlein, 1927; Durham, 1950). The sedimentary rocks sampled from the DSDP sites 474, 475 and 476 contain evidence of marine environments in the area, from late Pliocene to Pleistocene times (Aubry et al., 1982). DSDP site 474 cored a marine sequence overlying oceanic crust; whereas sites 475 and 476 contained continental conglomerates (Curray et al., 1982). The Isla Marõ a Madre contains a Neogene sequence unconformably overlying Cretaceous rocks (ChinÄasRobisel, 1963). Miocene volcanic rocks are also part of the Neogene sequence in this island. According to CarrenÄo (1985) and McCloy et al. (1988), the Neogene sedimentary sequence includes a mid(?)-Miocene nonmarine to shallow marine sandstone, unconformably overlain by upper Miocene laminated and massive diatomaceous mudstones, and siltstones. These strata are unconformably overlain by lower Pliocene sandstones and siltstones, cropping out in the main road to Balleto, and which are in turn, unconformably overlain by lower Pliocene to Pleistocene (?) limestones and siltstones. All these units are unconformably capped by Pleistocene deposits. Other late Cenozoic marine sedimentary rocks have been reported from farther south in mainland Mexico.

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In Punta Mita, north of Puerto Vallarta, Miocene marine microfossils have been found (Jensky, 1975) in sedimentary rocks interlayered with 10 Ma basalts (Gastil et al., 1978). Other Late Cenozoic marine sedimentary rocks have been reported from farther south in mainland Mexico. Approximately 115 km to the south of Puerto Vallarta, the Campo Acosta basin contains marine sedimentary rocks with Late Cenozoic mollusks and Ophiomorpha-type burrows (Durham et al., 1981). 4. Tectonic and sedimentary evolution As mentioned before, throughout the Early Tertiary, the entire Baja California Peninsula was located next to mainland Mexico, while the oceanic Farallon plate was being subducted under the continental North American plate. From Paleocene to at least middle Eocene, marine sedimentation took place in a deep marine forearc basin, with bathyal depths, in most of the region now occupied by the state of Baja California Sur. At this time, there was no sedimentation on most of the northern state of Baja California, or received only continental deposits like those in the vicinity of Tijuana. However, marine Paleogene sedimentation took place in the central part (Rosario embayment). Paleogene marine sedimentary rocks are present in the western part of Southern California (Los Angeles Basin) and in the eastern and western parts of the entire Borderland Province (Blake et al., 1978; Helenes, 1984). Most of the Oligocene and younger tectonism in Baja California is the result of the crustal accretion of the northern East Paci®c Rise (Lonsdale, 1991). As fragments of the subducted Farallon plate became attached to the Paci®c plate, they created extension and strike slip in the western side of southern California and Baja California during late Oligocene and early Miocene times. Ultimately, beginning in the middle Miocene, these tectonic stresses jumped eastwards and caused oceanic spreading in the Gulf of California (Bohannon and Parsons, 1995). These tectonic events are thus related to the sedimentary and tectonic history of the peninsula, and we interpret this history as follows. 4.1. Late Oligocene (30 Ma, Fig. 7) During the Oligocene, to the northwest of Baja California, the Paci®c-Farallon spreading center was either being subducted or very close to the North American continental margin. The subduction of young, thick and buoyant oceanic crust under the western continental margin of the North American plate, probably pushed the continental margin up, and com-

Fig. 7. Reconstruction of tectonic elements and sedimentary record for beginning of late Oligocene (30 Ma).

bined with a eustatic drop in the sea level (Haq et al., 1988), caused a widespread marine regression in the region. From late Oligocene to earliest Miocene, the region from northwestern Baja California Sur, to Southern California received continental and shallow marine sedimentation. In the southern San Joaquin Valley, near Bakers®eld, California (east of the Vancouver Plate on Fig. 7), the paleoenvironments changed to marine towards the west. To the south, the BT area does not contain any Oligocene sedimentary rocks, while the LP±EC area, contained a marine basin deepening to the west. The resulting marine sedimentary rocks are now being mined for phosphates. In the meantime, the central part of the Gulf of California, in the SR±L area, received cross-bedded sandstones, which are probably sedimentary rocks related to an initial continental rift-

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to receive marine sediments, representing deeper facies towards the west.

Fig. 8. Reconstruction of tectonic elements and sedimentary record for early Miocene (22 Ma). See text for explanation on the right-lateral, strike slip fault located east of the BT area. For legend see Fig. 7.

ing phase, after a long sedimentary hiatus (Cretaceous±Oligocene). Farther east, during this interval, the northern part of the Gulf (SS-TI) and the southern tip of Baja California (LC±PM) do not contain evidence of sedimentation. 4.2. Early Miocene (22 Ma, Fig. 8) During this time, marine deposition took place o€shore California and in the northwestern part of Baja California Sur, where the BT area contains important data regarding the evolution of the region (HelenesEscamilla, 1980; Normark et al., 1987). Just before approximately 21 Ma, a rapid transgressive event in the area caused the deposition of marine sediments. This marine transgression must have been very rapid in the west to locally allow direct deposition of middle bathyal shales above tilted and partially eroded Cretaceous sandstones. The transgression probably originated by subsidence related to the inception of strike slip motion west of the peninsula, re¯ecting a change to subduction of older, deeper crust (Crouch, 1981; Normark et al., 1987; Lonsdale, 1991). Additionally, a contemporaneous eustatic rise, which probably represents the Third Order Cycle TB-1.5 (Haq et al., 1988), also increased the rate of subsidence. This transgressive event is not re¯ected in the Sebastian Vizcaino Bay, since it was not a€ected by tectonism. Farther south, the LP±EC area continued

4.2.1. The La Bamba Fault The right-lateral, strike-slip fault shown to the east of the BT area is proposed here as the contact between the eastern (Baja California) and the western (Sebastian Vizcaino Peninsula) area. This fault is a southern trace of the Cedros Island Fault, which is located to the east of Cedros Island (Krause, 1965, Fig. 5, pro®le G; Normark, 1977, Fig. 2, pro®les I and II; Normark et al., 1987, Fig. 4, section F). A continuation of the Cedros Island Fault can be traced from seismic sections, to the south in the Sebastian Vizcaino Bay (Normark, 1974, Fig. 5; Calderon-Riveroll, 1978, Fig. 19). This trace is aligned with the western end of Playa Malarrimo in the southwestern corner of the Sebastian Vizcaino Bay, and corresponds to the La Bamba fault. This fault has been mapped along the eastern side of Punta Eugenia, in the northern portion of the Vizcaino peninsula (Hickey, 1984). Unfortunately there is not enough information to precisely locate these fault zones farther to the south, under the sands of the Vizcaino Desert. However, we believe that this fault zone probably continues to the south, towards a point to the west of Punta Abreojos, and probably joins the ToscoAbreojos Fault Zone (Spencer and Normark, 1979, 1989) o€shore. 4.3. Middle Miocene (12 Ma, Fig. 9) At this time, an important change in the basic distribution of tectonic stress took place. The extensional regime migrated from the western to the eastern side of the peninsula, where normal faulting and extensive volcanism are related to continental rifting. At approximately 12 Ma, subduction along the western margin of the peninsula ceased. This change caused transform motion (Tosco-Abreojos Fault Zone) to become more signi®cant in the area (Spencer and Normark, 1979, 1989; Yeats and Haq, 1981; Normark et al., 1987). Additionally, sedimentation rates exceeded subsidence rates in the local basins, probably caused by thermal extension in the Proto-Gulf and accompanying uplift of the peninsula (Normark et al., 1987). During the middle Miocene, TiburoÂn island in the Northern Gulf area shows the oldest evidence of marine sedimentation. These shallow marine deposits can be considered the oldest documented proto-Gulf deposits. Micro- and megafossils from the marine deposits in the SS±TI indicate mixed anities to warmer, Paci®c-Caribbean waters, and to cooler, California Current waters. The fossil assemblages identi®ed in this area are di€erent than those from

J. Helenes, A.L. CarrenÄo / Journal of South American Earth Sciences 12 (1999) 589±605

o

o

115 W

105 W

Phoenix

Middle Miocene (ca. 12 Ma)

31o N

o

31 N

Pioneer F. Z.

Hermosillo

Los Angeles Murray F. Z.

o

o

25 N

25 N

PACIFIC PLATE

599

Bahía Tortugas Guadalupe Plate

Molokai F. Z.

. S hirlyeF. Z

Pto. Vallarta

16 o

o

19 N

19 N

0

K m

500

o

115 W

Cocos Plate

o

105 W

Fig. 9. Reconstruction of tectonic elements and sedimentary record for middle Miocene (12 Ma). Note location of the hypothetical seaway from the San Ignacio area northwards in the Gulf, and the continental sedimentation in the LC±PM area. For legend see Fig. 7.

Southern California, which have only cooler North Paci®c anities (Ingle, 1974; Smith, 1991a,b). The tropical and subtropical characteristics of the faunas described from strata in this area indicate that the sea water occupying these basins came from the south. During this interval, to the south of the peninsula, in the LP±EC and the SR±L areas, terrestrial sedimentation was accompanied by extensive volcanism (Smith, 1992; Dorsey et al., 1997). At the southern tip of the peninsula, the LC±PM area received ¯uvial and alluvial sediments, also related to normal faulting in a continental rifting setting (Martõ nez-GutieÂrrez and Sethi, 1997). 4.3.1. Hypothetical seaway We propose here the presence of a seaway in the central part of the peninsula. This idea does not have any other merit than our re¯ections on the mixed cool-tropical characteristics of the fossil assemblages mentioned above and the distribution of marine depos-

its in the north and non-marine deposits in the center and south of the Gulf of California. These conditions present a predicament in regards to the path followed by the sea water entering the northern area in middle and late Miocene times. Apparently, a northern seaway is out of the question on the basis of the tropical and subtropical characteristics of the fossil assemblages mentioned above. Additionally, palinspastic reconstructions based on geophysical data (Lonsdale, 1991; Bohannon and Parsons, 1995. Fig. 15-D), propose a barrier in the northern part of the Gulf, between the Paci®c Ocean and the Basin and Range to the east. This barrier was formed by a batholithic belt formed by the Sierra Nevada, eastern Transverse Ranges and Peninsular Batholith. This barrier was in place and has provided clastics to the western margin of the Peninsula since Late Cretaceous times. To the west of the peninsula, in latest middle to earliest late Miocene times (approximately 10 Ma), the

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BT area contained a deep marine basin where diatomaceous mudstones and laminated diatomites were being deposited. The basin was located in an upwelling area, with high organic productivity, similar to the Recent basins o€ southern California and in the Gulf of California. Diatom assemblages contained in the diatomites indicate a marked in¯uence of cool waters with only the occasional presence of some warmer species. Quantitative comparison of late Miocene diatom assemblages from a number of DSDP sites and outcrop sections ranging from northern California to the southern tip of Baja California, were used to locate the approximate southern limit of the cool water oceanic current somewhere between the Vizcaino Penisula and the southern tip of Baja California during the late Miocene (Esparza-Alvarez, 1997; Esparza et al., 1998). Many of the species indicative of the cool California Current were also present during the middle Miocene, suggesting that the paleoceanographic conditions were similar then. A southern seaway, similar in some respects to today's Gulf, seems more appropriate to explain the known paleontological data. However, the non-marine characteristics of the sedimentary units in the central and southern parts of the eastern margin of the peninsula makes it dicult to locate this seaway. In addition to the non-marine nature of the sedimentary rocks present in the SR±L area, the northern and central areas of Baja California Sur were covered by large amounts of volcanic and volcaniclastic material (Hausback, 1984a,b; Sawlan and Smith, 1984). This material most probably precluded passage of sea water from the vicinity of Santa Rosalõ a to the south. There is no clear evidence of a west-to-east seaway across the peninsula during middle Miocene times. However, the shortest way to join middle to late Miocene marine sedimentary rocks, from west to east across the Baja California Peninsula, is through San Ignacio. This locality, just north of la Purõ sima, received marine sedimentation during middle Miocene times (Smith, 1984). Sea water entering through this area could have the appropriate mixture of tropical and temperate faunas and ¯oras to account for the mixed assemblages observed in the northern Gulf sites. Furthermore, tholeiitic basalt ¯ows dated as 10 Ma, cover a large area to the south and west of San Ignacio (Sawlan and Smith, 1984). Near San Ignacio, these basalt ¯ows were deposited on top of marine sedimentary rocks with middle Miocene faunas. Some workers have interpreted the origin of these ¯ows as somewhere east of San Ignacio (Sawlan and Smith, 1984). However, newer data and interpretations (Delgado-Argote, L., personnal communication, 1998) indicate the presence of several vents located along the area covered by the ¯ows. The presence of these vents

Fig. 10. Reconstruction of tectonic elements and sedimentary record for late Miocene (8 Ma). For legend see Fig. 7.

and the tholeiitic nature of the basalts suggest a zone of weakness in the crust that coincides approximately with the proposed seaway. 4.4. Late Miocene (8 Ma, Fig. 10) In late Miocene times, continental faulting and volcanism continued in the Gulf area, and spreading started in the southern tip of Baja California, where the northern part of the East Paci®c Rise was located near the Los Cabos block (Lonsdale, 1991). By late Miocene times, the marine sedimentation spread out in the Gulf area. The northern SS±TI area contained the San Felipe basin, with minimum depths of 150 m. At this time, the SR±L and LP±EC areas received marine sedimentation in shallower environments. To the west, the BT area presents an unconformity between early±late Miocene and latest Miocene sedimentary rocks. During the interval from approximately 8.5±7 Ma, the marine sedimentary rocks were uplifted, partially eroded, and tilted. Then, in the latest Miocene (approximately 7 Ma), neritic sediments were unconformably deposited on top of the tilted, diatomaceous strata. In the south, the LC±PM area contain sedimentary sequences representing a transgressive±regressive cycle which started in late Miocene and ®nished in Pliocene times. This area presents a similar, but younger evolution, to that from the northern gulf area. Apparently, this area remained attached to Mexico until the latest Miocene (ca 5 Ma), and separated when the mouth of

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601

474, 475 and 476. The LC±PM area, the southern islands (San Jose and Cerralvo) inside the Gulf, and the Tres Marõ as islands present almost continuous marine sedimentation until the upper part of the Pliocene, when these areas were uplifted above sea level. Since late Pliocene times, the present con®guration of the region developed through motion along strike slip faults and spreading centers in the Gulf itself.

Acknowledgements

Fig. 11. Reconstruction of tectonic elements and sedimentary record for beginning of Pliocene (5 Ma). For legend see Fig. 7.

the Gulf opened. After that it attached itself to the rest of Baja. Contemporaneous sedimentary rocks are reported from the Paci®c margin in mainland Mexico. If the southern part of the Gulf opened at 5 Ma, then the sea water for the 12±6 Ma old marine sedimentary rocks present in the norhern part of the Gulf must have entered through a di€erent pathway. 4.5. Miocene±Pliocene (5 Ma, Fig. 11) At the end of the Miocene, the strike slip stresses were located inside the Gulf and new intercontinental part of the spreading center was being created to the south of Los Cabos (Lonsdale, 1991). These conditions are linked to the change in the relative motion of the Paci®c and North American plates at approximately 5 Ma (Mammerickx and Klitgord, 1982; Normark et al., 1987; Lonsdale, 1991). Tectonically smaller events are recorded on the western side of the peninsula, as indicated by uplift and tilting of the Pliocene marine sedimentary rocks in the Vizcaino peninsula. From the beginning of the Pliocene, the Northern Gulf, Central Gulf and San Jose del Cabo Trough areas contain marine sedimentary rocks which essentially represent the actual Gulf of California. The older magnetic lineations along the Tamayo Transform Fault correspond to Chron 2A, dated as 3 Ma (Lonsdale, 1991), thus indicating that creation of oceanic crust started in the Pliocene, when the actual mouth of the Gulf was opening. In here, bathyal marine sediments were being deposited in the DSDP sites

We thank CONACyT and UNAM (grant num. INI02995) for the ®nancial help during the initial stages of this research. We also recognize those who have helped us through the years of research in Baja; particularly Judy T. Smith and James C. Ingle Jr., of Stanford University for their encouragement and guidance. We would specially like to acknowledge and thank the critical reviews by A. Martin±Barajas CICESE and Cathy Busby, University of California, also to G. Tolson, UNAM, for his editorial critical review. We must also warmly thank ourselves for the e€ort in preparing this paper in such a short time.

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