Lower Cretaceous Alisitos Formation at Punta San Isidro: Coastal sedimentation and volcanism

Formación Alisitos del Cretácico temprano en Punta San Isidro: Sedimentación costera y vulcanismo

Jonathan L. Payne¹, Markes E. Johnson² and Jorge Ledesma-Vázquez³*

¹ Department of Earth and Planetary Sciences. Harvard University 20 Oxford Street, Cambridge, MA 02138, USA.

³ Facultad de Ciencias Marinas Universidad Autónoma de Baja California Apartado postal 453 Ensenada, CP 22800, Baja California, México. *E-mail: jledesma@uabc.mx

Recibido en abril de 2003;
aceptado en marzo de 2004.

Abstract

The Lower Cretaceous (Aptian-Albian) Alisitos Formation is well exposed at Punta San Isidro and adjacent sea cliffs on the Pacific shores of Baja California near Eréndira, Mexico. Continuous coastal outcrops define a local stratigraphic succession less than 100 m in total thickness, including repetitious tuff, sandstone and conglomerate units interbedded with discontinuous limestone beds rarely more than 2.5 m thick. The limestone beds are biostromal units that include scattered oysters and/or corals, as well as units dominated by the rudistid bivalve, Caprinuloidea perfecta. Two distinctive conglomerate units are composed of andesite cobbles colonized by encrusting oysters in a quasi rocky-shore setting. One sandstone unit includes abundant fossil wood with tree limbs up to 55 cm long and 5 cm in diameter. Proximal volcanic activity is indicated by a series of dikes that cut through pyroclastic beds and lead to a 10-m thick andesitic flow that caps the succession at Punta San Isidro. Compared with thicker intervals of the Alisitos Formation elsewhere in Baja California that are dominated by andesitic flows and offshore limestone, the Punta San Isidro sequence offers a window on a back-reef environment adjacent to a paleoshore that received pyroclastic lahars from a terrestrial origin or mass flows from shallow submarine explosions. Recovery of marine life and the renewal of a carbonate substrate followed successive episodes of volcanism and massive erosion along an active coastline. This scenario is very different from depositional processes of the Upper Cretaceous (Campanian-Maastrichtian) Rosario Formation that cannibalized and subsequently formed a regional unconformity against tilted Alisitos strata with substantial topographic relief in the Eréndira region.

Key words: Rocky shorelines, rudist limestone, Alisitos Formation, arc volcanism.

La Formación Alisitos del Cretácico temprano (Aptiano-Albiano) se halla expuesta en Punta San Isidro y en los cantiles adyacentes a las costas del Pacífico de Baja California cerca de Eréndira, Mexico. Continuos afloramientos costeros definen la sucesión estratigráfica local con menos de 100 m de grosor total, incluyendo unidades repetitivas de tobas, areniscas y conglomerados intercalados con capas de calizas discontinuas raramente con más de 2.5 m de grosor. Las capas de calizas son unidades biostromales que incluyen ostiones y/o corales esparcidos, así como unidades dominadas por el bivalvo rudista Caprinuloidea perfecta. Dos singulares unidades de conglomerados están compuestas de guijas de andesita colonizadas por ostiones encrustantes en un ambiente costero semi-rocoso. Una unidad de arenisca incluye abundante madera petrificada con ramas hasta de 55 cm de largo y 5 cm de diámetro. Actividad volcánica en las cercanías está indicada por una serie de diques que cortan a través de capas piroclásticas y conducen a un flujo andesítico con 10 m de grosor, el cual sobreyace la sucesión en Punta San Isidro. En comparación con intervalos más gruesos de la Formación Alisitos en otras localidades de Baja California, que también están dominadas por flujos andesíticos y calizas marinas, la secuencia de Punta San Isidro ofrece una ventana hacia un ambiente ante-arrecifal adyacente a una paleocosta que recibió lahares piroclásticos de origen terrestre o flujos masivos provenientes de explosiones submarinas someras. La acumulación de vida marina y la renovación de un sustrato de carbonato fue la continuación a sucesivos episodios de volcanismo y erosión masiva a lo largo de una línea de costa activa. Este escenario es muy diferente a los procesos de depositación de la Formación Rosario en el Cretácico tardío (Campaniano-Maastrichtiano), la cual canibalizó y subsecuentemente formó una discordancia regional contra los estratos inclinados de la Alisitos, con sustanciales relieves topográficos en la región de Eréndira.

Palabras clave: líneas de costa rocosa, caliza de rudistas, Formación Alisitos, vulcanismo de arco.

Introduction

The Lower Cretaceous Alisitos Formation of northern Baja California, México, has attracted prior attention for its extensive volcaniclastic strata (Tardy et al., 1993; Morán-Zenteno, 1994) and its massive fossil-bearing limestone (Allison, 1955; 1974). Different units of the Alisitos Formation, including widespread andesitic flows, form the main part of the range that extends along the Pacific coast between Punta Banda near Ensenada in the north and Sierra Calamajui to the south (Wetmore et al., 2002). Thus, these rocks record the early stratigraphic history regarding the development of the western part of northern Baja California.

Rocks of the Alisitos Formation exposed near Punta San Isidro in the Eréndira region form the substratum of an active rocky shoreline that reoccupied the same position within a few hundred meters several times during the last 115 million years. The local succession of strata represent an unusual overlap of rocky-shore deposits that date from the Early Cretaceous, Late Cretaceous, Pleistocene, and Holocene, all in the same vicinity (Johnson and Hayes, 1993; Clark and Johnson, 1994; Zwiebel and Johnson, 1996; Johnson et al., 1996). Although outcrops ofseveral rocky-shore deposits are closely clustered, characteristics of the Alisitos Formation show that the coastal environment during the Early Cretaceous had little in common with later rocky shores. The difference was episodic influx of volcanic materials from the nearby Alisitos-Teloloapan arc (Tardy et al., 1993). Subsequent coastal evolution in the Eréndira region was quiescent in terms of volcanism compared with that of the early Cretaceous.

No previous studies of the Alisitos Formation address the influence of arc volcanism on the shallow-water, near-shore environment. Allison (1955, 1974) researched the Early Cretaceous fauna of the Alisitos Formation, but focused his studies at Punta China (40 km north of Punta San Isidro), where limestone units are uninterrupted for a thickness over 100 m. Other studies on Alisitos strata have placed much less emphasis on paleontology. Outcrops of the Alisitos Formation around Punta San Isidro offer a unique and previously overlooked perspective on the near-shore setting of the Alisitos-Teloloapan arc. A late Early Cretaceous rocky shore developed there in which marine organisms struggled to survive in a setting frequently engulfed by volcaniclastic debris and other clastics, the volume of which is significantly greater than found in younger strata (Johnson et al., 1996).

The goals of this contribution are to test the succession at Punta San Isidro against different predictive models. Carey and Sigurdsson (1984) provide one model that features volcanic arc sedimentation. Coogan et al. (1972) present another model based on the organization of Cretaceous rudistid reefs in eastern Mexico. Heikoop et al. (1986) consider the effects of proximal volcanism on organisms from a variety of substrates and water depths. Lockley (1990) reviews additional material on the unusual preservation of biological material in volcanic deposits. The Alisitos Formation at Punta San Isidro is of particular interest, because it presents an opportunity to apply a wide range of models to the interpretation of Cretaceous environments in a near-shore, volcanic setting.

Geological setting and study location

Allison (1974) assembled a general stratigraphic section for the entire Alisitos Formation that can be divided into four major lithologically distinct units (fig. 2). The 3,000-m thick lowest unit consists of thinly bedded tuffs interlayered with andesite sills and flows. The tuffs are overlain by 2,000 m of andesite flows, and andesitic pyroclastic and epiclastic rocks. Most of the uppermost 1,000+ m of the Alisitos Formation is dominated by biohermal limestones interbedded with andesitic pyroclastic and epiclastic rocks. Units bearing non-marine fossils were found locally at the top of the formation. Limestone is not limited to the uppermost part of the Alisitos Formation. Allison (1974) noted that in certain places south of Punta China, limestone can be found lower in the stratigraphic sequence.

The Alisitos arc segment and its oceanic basement was exotic to North America prior to its accretion in the late Early Creataceous at 108-115 Ma (Wetmore et al., 2002), but there remains some disagreement over the direction of subduction (Tardy et al., 1993; Sedlock et al., 1993; Morán-Zenteno, 1994). After it joined onto North America, the arc terrane was transported to the north, probably by Eocene time. Paleomagnetic studies indicate that the Baja California peninsula was subject to considerable northward transport relative to the North American craton after the Early Cretaceous. The magnitude of this northward displacement has been proposed as 15° of latitude, or slightly upwards of 1,200 km (Beck, 1991; Filmer and Kirschvink, 1989; Sedlock et al., 1993; Morris et al., 1986).

The Alisitos Formation is structurally complex, exhibiting tight folding, faulting, and intrusion by Upper Cretaceous igneous rocks (Silver et al., 1963; Gastil et al., 1975). Bedrock exposures often are deeply weathered. This contribution deals exclusively with strata exposed along the coast from 1 km southeast to 4 km northwest of Punta San Isidro, which is situated 4 km northwest of the village of Eréndira (fig. 1). In turn, Eréndira is located approximately 70 km southeast of Ensenada. The topography of the region is covered by the Puerto San Isidro quadrangle (index reference: H11B32) at a scale of 1:50,000. Complications due to structure and weathering usually posed by the Alisitos Formation are absent at Punta San Isidro, where outcrop exposures are continuously eroded along the shore and fault offsets are minor.

Geologic age constraints

Although almost entirely confined to limestone, the fauna of the Alisitos Formation offers the best tool for determining the relative age of the succession. Allison (1955) was the first to use biostratigraphic correlations in a systematic fashion to date the Alisitos Formation. He described the gastropods found through 160 m of strata at or near Punta China, the type area of the Alisitos Formation. He determined the age of the Alisitos as Aptian to Albian, based on the presence of the foraminifera Orbitolina texana (Roemer). Allison (1955) noted that O. texana is common both at Punta China and Punta San Isidro, thus providing strong evidence for the equivalent ages of Alisitos strata at Punta China and the area of this study.

Previous studies at Punta San Isidro

Kirk and MacIntyre (1951) were the first to recognize the fauna of Punta San Isidro as distinctly Aptian-Albian in age. Allison (1955) included a small subset of data from Punta San Isidro in his study of Cretaceous gastropods found in coeval limestones at Punta China. The best evidence for physical correlation of the Alisitos Formation over long distances is a zone of 90 to 150-m thick biohermal caprinid limestone that can be traced from Punta China inland and south to San José, approximately 80 km away. Fossils at Punta San Isidro clearly belong to the Alisitos Formation (Kirk and MacIntyre, 1951; Allison, 1955), but Silver et al. (1963) cautioned that correlation between the thick caprinid limestone and lesser outcrops at Punta San Isidro is unclear. Allison (1974) included specimens from Punta San Isidro in his study of the bivalves of the Alisitos Formation, but did not clarify the lateral relationship between the strata of Punta San Isidro and Punta China, where he collected a majority of his samples. Ledesma-Vázquez et al. (1989) prepared a stratigraphic section for the Alisitos Formation immediately south of Punta San Isidro, but did not localize the section within the broader formational column, as a whole.

Field methods

The main sequence from the Alisitos Formation considered in this study occurs as a continuous stratigraphic succession immediately south of Punta San Isidro. Three lesser but repetitive sections isolated from one another also occur north of the point within a distance of 4 km. The local stratigraphy was mapped on the Puerto San Isidro quadrangle using a meter tape and a Brunton compass. Samples from each stratigraphic unit were collected in the field and later prepared as thin sections for compositional analysis at Williams College.

Results

Description and correlation of key stratigraphic units

Fieldwork at Punta San Isidro focused on the compilation of an uninterrupted succession that spans strata approximately 90 m in thickness (fig. 3). The Alisitos Formation is especially well exposed in coastal cliffs between Punta San Isidro and a location 300 m south of the point. The southernmost 100 m of exposure are formed by hypabysal andesite. The northern edge of this body is overlain by the stratigraphically lowest local sedimentary unit within the Alisitos Formation. Strata above this level consist of an interlayered succession of clastics, volcaniclastics,and limestones. The highest unit is marked by a thick andesite flow with columnar joints that forms the resistant headland of Punta San Isidro and several adjacent sea stacks (fig. 4). A detailed description of some key stratigraphic units from the southern section is provided, as follows.

Unit 4

Unit 4 is a well-sorted sandstone that includes a conglomerate lens 1.2 m thick. The conglomerate is cemented by sparry calcite. The clasts are almost entirely andesitic and basaltic in composition. Unit 4 also includes traces of limestone and very minor shell fragments. These shell fragments, in addition to oyster-encrusted cobbles (see fig. 5a for comparison), are stratigraphically the lowest marine fossils to be located in the sequence. The earliest occurrence of limestone is represented by a mass of rudistid bivalves attached to an andesite boulder (fig. 5b).

This unit represents the earliest limestone ledge exposed at Punta San Isidro. It is 2.4 thick and occurs directly above a 7.3-m thick tuffaceous sandstone entirely devoid of fossils. The most distinctive features of the limestone unit include presence of abundant gastropods (Aptyxiella sp.) and a network of serpulid worm tubes. The tubes occur parallel to the bedding plane in bunches (fig. 5c). Other fauna include scattered oyster shells, abundant echinoid spines, and rare scleractinian corals.

Unit 10

Sandstone with conglomerate lenses containing andesite clasts constitute Unit 10. This unit, which is 6.75 m in thickness, exhibits low-angle cross bedding. Fossil tree limbs are present in this unit. Limbs up to 55 cm long and 5 cm in diameter are exposed on bedding planes (fig. 5d) in broken slabs at the base of the sea cliff. Intact in strata within the outcrop, however, a lag deposit of more than 13 limbs occurs 4.6 m above the base of the unit. The length of these limbs can not be determined because they are exposed along transversal sections.

Unit 11

With a thickness of 1.4 m, unit 11 is formed by conglomerate composed of well-rounded andesite clasts in a sandy matrix. These clasts are 5 to 15 cm in diameter, imbricated, and commonly exhibit encrustation by oysters (fig. 5a). The fragments of valves cemented to these andesite cobbles are, unfortunately, insufficient for specific or generic identification.

Unit 12

Unit 12 consists of a 2.5 m-thick limestone (fig. 6) composed almost entirely of the rudistid bivalve Caprinuloidea perfecta. The bivalves are not in life position. This unit is remarkable for its lack of clastic debris. It occurs as a laterally discontinuous bank directly above an andesite conglomerate, and is overlain by tuffaceous sandstone (Unit 13), but contains only a minor percentage of clastic debris.

Unit 19

This limestone unit, 1.7 m in thickness, includes scattered scleractinian corals. It is significant as a marker bed that allows the succession to be traced across a zone of shallow faults with fault planes that dip to the southwest (fig. 7). This is the only part of the succession south of Punta San Isidro that is faulted.

Units 21-25

These tuff units are important because they also occur along the coast north of Punta San Isidro, where they strike parallel to the coast for approximately 3 km. They are the principal units of correlation in the region. The total thickness of these units is more than 26 m. They are poorly sorted, exhibit bedding on a scale of centimeters to tens of centimeters, and contain several distinct andesitic and basaltic lithologies. The most striking feature of these units is the presence of platy limestone clasts. The maximum size of these plates reaches 10-20 cm in diameter in unit 22. This unit also contains intact bivalve fossils that have no surrounding limestone matrix. The absence of a limestone matrix demonstrates that the shells were incorporated into the deposit either as living organisms or as loose shells on the sea bottom.

Correlation with other local beds

The first isolated outcrop of the Alisitos Formation beyond Punta San Isidro is located about 0.75 km to the northwest near El Buey. The best evidence for relating this outcrop to the master section at Punta San Isidro is a 1.3-m-thick tuff unit bearing limestone clasts including possible corals corresponding to unit 22 (fig. 8). This bed is bracketed by a pair of units bearing only andesite clasts. The full sequence correlates physically with units 21-23 at Punta San Isidro.

A greater thickness of Alisitos strata amounting to 15 m is exposed about 500 m north of El Buey. There, the succession also includes a limestone-bearing tuffaceous conglomerate that matches well with a comparable interval at Punta San Isidro. Lower within this sequence, lahar deposits include large fossil oysters. Stratigraphically below the tuffaceous conglomerate occurs an alternation of limestone and clastic beds that physically corresponds to units 15-19 at Punta San Isidro.

The greatest thickness of Alisitos strata north of Punta San Isidro occurs 4 km to the northwest towards Las Minas. This exposure is dominated by a 17-m-thick tuff that is lithologically similar to the tuffaceous conglomerate of units 21-25 from the master section at Punta San Isidro. Correlation of isolated sequences with the upper part of the master stratigraphic section at Punta San Isidro is interpreted mainly on the basis of characteristic limestone units (fig. 8).

Discussion

Carey and Sigurdsson (1984) provide a model for volcanogenic sedimentation in back-arc basins. The model applies to volcaniclastic deposits in both the back-arc and fore-arc settings. It incorporates elements of sedimentation by explosive volcanism (e.g. lahars, surges, ashfalls), fluvial and aeolian reworking, and subaqueous mass-flows. The authors emphasize that a volcaniclastic apron forms along the side of the volcanic arc, similar to an extended submarine fan. They show that during arc volcanism a significant source of sedimentation is primary volcaniclastic influx, including the subaqueous deposition of ashfalls. The sides of the arc are covered in a thick apron of material that includes primary volcanogenic debris, epiclastic sediment from the erosion of the arc, as well as biogenic and aeolian material. The variety of sedimentary processes acting simultaneously produces deposits that do not exhibit the classic proximal to distal (or near-shore to off-shore) relationships existing in deltas and submarine fans. In this context, fossil evidence is especially important for establishing water depth and position relative to paleoshore in the Alisitos Formation. Carey and Sigurdsson (1984) argue that subaerial volcanic arcs result in deposits with many overlapping volcaniclastic facies interspersed with lesser accumulations of biogenic and epiclastic material. This is clearly the case at Punta San Isidro. The greatest accumulations are of volcaniclastic and epiclastic material, interbedded with thin and dirty limestones. The variety is also great, ranging from conglomerate (units 3 and 5) to tuff (units 7 and 8) and sandstone (units 10 and 30). The degree of reworking also varies greatly between beds, from undisturbed tuff in unit 7 to well-rounded sand and pebble clasts in unit 10.

Fossil evidence presents an overwhelming argument for the near-shore nature of the Alisitos deposits. The oyster-encrusted andesite cobbles within units 4 and 11 (fig. 5a) function as surrogate rocky shores. These are described by Libbey and Johnson (1997, p. 219) as a type of rocky shore in cameo, where organisms encrust individual cobbles or boulders surrounded by a matrix such as lime sand. The occurrence of encrusted cobbles at two stratigraphic levels in the Punta San Isidro area indicates close depositional proximity to the shoreline. The mass of rudistid bivalves cemented to an andesite block in a conglomerate lens in unit 4 (fig. 5b) also demonstrates the existence of a nearby incipient rudistid reef. The presence of a major shell bank is confirmed by unit 12 (fig. 6), which is composed almost entirely of the rudistid bivalve Caprinuloidea perfecta.

While encrusting oysters suggest the presence of a rocky-shore environment at Punta San Isidro during local deposition of the Alisitos Formation, the fossil tree limbs in unit 10 confirm that the paleoshore existed along the margin of a volcanic shore that supported terrestrial life. The lag deposit of limbs in unit 10 shows that they were transported by fluvial action and deposited in shallow water. These limbs occur within a few meters stratigraphically below the encrusting oysters in unit 11.

The limestone units of the Alisitos Formation at Punta San Isidro are instructive. They vary greatly in terms of fauna preserved and facies represented. None of the limestone units, however, exceed 3 m in thickness. The thin limestone of Punta San Isidro stands in sharp contrast to those at Punta China where, according to Allison (1974, p. 33), "uninterrupted thicknesses of carbonate rock ... may exceed 200 m [in total thickness]." From these massive biohermal limestones, Allison (1955, 1974) showed an impressive compilation of faunal assemblages. Allison (1974) listed 88 species of bivalvia alone, and confided that his list was not exhaustive. His vision of the shallow marine waters during deposition of the Alisitos Formation is not one of tectonic upheaval. He states that "the only recognized environmental change of major significance occurs at the top of the section, south of Punta San José, where freshwater fossils occur in abundance" (Allison, 1974, p. 45). Busby et al. (2003) describe gentler slopes on a volcano-bounded shallow-water marine basin that allowed pyroclastic flows to enter the sea with integrity, and supported extensive buildups of bioherms 200 km south of Eréndira at Valle San Fernando. In contrast to Punta China and Valle San Fernando, the stratigraphy of the Alisitos Formation at Punta San Isidro indicates that local environmental changes were common. These environmental shifts were the result of changes in volcanic activity and changes in the fluvial system delivering clastic material to the paleoshore.

Context of a back-reef model

Coogan et al. (1972) provide a facies model of a marine shelf during mid-Cretaceous time based upon the Tamabra and El Abra limestones, which are oil source-rocks in eastern Mexico. Their model shows the distribution of fauna on a shelf dominated by barrier rudistid reefs and smaller patch-reefs. The limestone units of Punta San Isidro present an opportunity to evaluate this model. The Tamabra and El Abra limestones formed, however, on a large, shallow shelf free of any volcanic activity. The effects of volcaniclastic sedimentation can be approximated using the study of Heikoop et al. (1996). They demonstrate that near-shore, hard bottom communities often experience nearly complete destruction following a volcani-clastic event, while offshore, soft-bottom communities are relatively unharmed.

The basic faunal elements present in the succession at Punta San Isidro correspond to the model put forth by Coogan et al. (1972). Unit 9 is the limestone that appears to represent the shallowest water, perhaps closest to shore. This accounts for the presence of serpulid tubes (fig. 5c) and abundant gastropods (Aptyxiella sp.), as well as transported oyster shells. Unit 8, directly beneath this limestone, is represented by a thick tuf-faceous sandstone that may be terrestrial or marine in origin. The unit above the serpulid limestone (unit 10) is a thick sandstone that recorded more sediment influx from terrestrial sources, rather than a significant change in water depth. This interpretation is supported by the presence of fossil wood within the unit (fig. 5d). The rudistid bank (unit 12) may reflect an increase in water depth. The fact that it directly overlies a conglomerate with oyster-encrusted cobbles, however, suggests it is more likely a near-shore deposit derived from a patch reef. The presence of such reefs is suggested by a mass of rud-ist bivalves on a block of andesite (fig. 5b) that occurs lower in unit 4 only 20 m laterally from oyster-encrusted cobbles in the same stratum. This block is either an incipient rudistid mound, or a block detached from a small patch reef during a storm. The multiple associations of encrusting oysters and rudistid deposits provides evidence for ecological succession, as well as close lateral proximity. The encrusting oysters are the first organism to take advantage of the hard substratum. They are succeeded by rudistid bivalves, which subsequently dominate the environment. High rates of volcaniclastic sediment influx recorded at Punta San Isidro interrupted the pattern of ecological succession by continually smothering the environment.

Limestone beds from units 14 and 16 also represent a back-reef environment that was subject to a considerable influx of terrestrial sediment of local volcanic origin. The presence of oyster shells and solitary corals is the strongest evidence for back-reef deposition (Coogan et al., 1972). Units 26 and 29 also represent similar environments, but subject to much lower rates of influx of clastic sediments. These younger limestone beds have relatively clean micrite matrices and abundant fossils, as opposed to the presence of clay particles, sanidine, plagioclase, andesite, and quartz found in units 14 and 16.

Context of fossils in volcaniclastic deposits

As noted by Lockley (1990), fossil assemblages are recorded in marine "volcanogenic mass-flow deposits" but these accumulations are rare in the geological literature. Lockley (1990) cites one example of this phenomenon from Miocene deposits in New Zealand, and identified another such deposit from the Ordovician of Wales. Preservation of fossils in such deposits is surprisingly good, including unbroken bryo-zoan fronds. Plessis et al. (1978) also described the excellent preservation of leaves in volcanic ash on the floor of a bay on Ua Pou in the Marquises Islands. They believe that the leaves were entrained in lahars that remobilized volcanic ash on the slopes of the volcano during heavy rains. The lahars then spread as large fans that were deposited along the bottom of the bay.

Despite the success of Lockley (1990) to establish the various ways in which volcanism affects the biostratigraphic record, his survey documents few results regarding preservation of fossils in volcanogenic marine mass-flow deposits. Fisher (1984), however, provides a model for the subaqueous deposition of lahars. This model gives the framework to understand the preservation of articulated bivalves within a lahar deposit north of Punta San Isidro (fig. 8). The most likely of possibilities discussed by Fisher (1984) involves the triggering of water-soaked debris and the occurrence of heavy rain during an eruption. This scenario is consistent with the evidence, because the units in question are associated with shallow-water limestone and clastic facies.

Comparison with Alisitos strata elsewhere

The rich fossil assemblages described by Allison (1955, 1974) at Punta China imply a more off-shore environment compared to San Isidro on the basis of faunal diversity and the thickness of the limestone units. No fossil wood or other terrestrial fossils, and no encrustation of cobbles by oysters are reported from Punta China.

Beggs (1983) described facies from the Alisitos Formation of west central Baja California (fig. 1) that are primarily distal. He interpreted only a single interval of Alisitos strata designated as El Progreso Formation as the result of shallow-water deposition. No evidence of shoreline deposits was noted by Beggs (1983). El Progreso Formation also contains considerably less pyroclastic and epiclastic material than the Punta San Isidro exposures, suggesting deposition farther from shore. Beggs (1983) observed that conglomerate units in the Alisitos Formation vary in clast size, structure, and degree of rounding, but all clasts are reworked from the volcanic rocks in the Alisitos Formation. His findings in this regard are consistent with the present study. The clasts from conglomerate beds of the Punta San Isidro area are subangular to subrounded, and composed almost entirely of andesite and volcaniclastics from the Alisitos Formation. They also conform to the description of Beggs (1983) in matrix composition and oxidation of clasts. Beggs (1983) denotes one such lithology as a lahar breccia-conglomerate. He interprets such units as the product of lahars originating on the steep sides of volcanoes that spread into sheet flows along either flat terrain or in shallow marine water.

The relative position of strata at Punta San Isidro within the greater Alisitos Formation is still uncertain, but probably corresponds to the upper part of the formation redescribed by Alllison (1974) in the Punta China area (fig. 2). This is the part of the succession in which limestone is most prevalent. The Alisitos fauna found at Punta San Isidro has not been identified to the species level. A more thorough catalogue of this fauna would provide a useful comparison to the data of Allison (1955, 1974). It would also provide stronger evidence for establishing differences and similarities in marine environments preserved at Punta China and Punta San Isidro.

Cretaceous sea-level change and regional tectonics

A major time gap exists between the Alisitos Formation of Aptian-Albian age and the Rosario Formation of Campanian-Maastrichtian age. No record of the intervening Cenomanian to Campanian stratification exists at Punta San Isidro. According to the Cretaceous record of eustasy compiled by Haq et al. (1987), sea level rose globally through Albian time. A sharp drop in global sea level is recognized many places around the world on the basis of unconformities between upper Albian strata and younger strata, as shown in North America, South America, Eurasia, and Africa (Fernández-Mendiola and García-Mondéjar,1997, table 2). These trends are consistent with the thick, uninterrupted accumulations of relatively shallow water facies in the Alisitos Formation of the Eréndira area. During post-Albian time, the Peninsular Ranges batholith was widely emplaced in Baja California (Silver et al., 1963; Gastil et al., 1975). In the Eréndira area, strata that belong to the Alisitos Formation are tilted as much as 65o out of the hori-zonal (fig. 7) due to later deformation. The unroofing and exhumation of the vast batholith occurred in coastal districts by Campanian time. Basal strata belonging to the Rosario Formation were deposited across upturned Alisitos strata and against the flanks of granodiorite rocks in the Eréndira area (Gastil et al., 1971; 1975).

Fieldwork for this project was initiated in January 1996 and contributed to Payne's honor's thesis at Williams College. Acknowledgment is made to the donors of the Petroleum Research Fund (American Chemical Society) for full support of this project through grants 22904-B2-C and 27325-B8 to Johnson at Williams College. The authors are grateful to two anonymous reviewers whose comments helped to improve the manuscript.

References

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