Rhyolite Domes and Subsequent Offlap of Pliocene Carbonates on Volcanic Islets at San Basilio (Baja California Sur, Mexico)

Abstract

San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km² bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin’s faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma.

1. Introduction

Examples of volcanic edifices with associated marine sedimentary deposits are rare from within the Gulf of California or along the adjacent peninsular Baja California shore. Forty named islands in the Gulf of California entail some component of igneous rocks, mostly associated with block-faults and the uplift of basement Cretaceous granodiorite or surface flows of Miocene andesite [1]. Among the many gulf islands, however, a few exhibit volcano superstructures. Isla San Luis is a small island (4.5 km²) in the upper gulf region that features a large, half-eroded volcanic crater and a pair of intact rhyolite domes [2]. High-energy hydromagmatic eruptions from the volcano’s earliest stage are regarded as Holocene in age. In the central gulf region, Isla Tortuga (11.4 km²) is a basaltic shield volcano that retains a central crater [3]. A third example is Isla Coronados (7.5 km²), which projects the classic shape of a stratovolcano covering 70% of the island with andesite flows. The youngest of these is about 160,000 years old [4]. In contrast to San Luis and Tortuga, Isla Coronados is notable for extensive fossil deposits that include late Pleistocene coral reefs on the volcano’s south flank [5]. Fossil deposits on San Luis and Tortuga are slim to non-existent.

Larger islands in the Gulf of California reveal paleoshores marked by well-defined unconformities between Pliocene limestone and older igneous rocks [6]. Variations in intertidal to shallow-subtidal limestone deposits are found in relation to such unconformities, but no scenario previously described for fossil beds within the Gulf of California is connected to a Pliocene volcano that was active at the time. Evidence in support of contemporaneous development would include physical proximity between fossil beds and a discernable volcanic structure, as well as possible interruptions by lava flows or ash falls.

This paper is an outgrowth of geological mapping and detailed stratigraphic logs regarding the Pliocene to Pleistocene history of landscape development in the San Basilio basin on the gulf shores of Baja California Sur, Mexico. Located north of the greater Cerro Mencenares volcanic complex [7], heretofore this scenic but remote area has received little attention. Our several goals are inter-related in design in order to: (1) show the extent of effusive volcanism associated with rhyolite domes, (2) sample microfossil foraminiferal content gauging the paleobathymetry of buried volcanics, (3) describe late-stage carbonate facies formed by Pliocene macro-faunas on the periphery of steep-sided volcanic islets and other rocky shorelines, as well as (4) survey the emplacement of faults related to the tectonic evolution of the adjacent Pliocene Gulf of California. Comparisons are made with carbonate deposits from volcanic islands that were contemporaneously active elsewhere in other parts of the world, particularly in open oceanic settings.

2. Geographical and Geological Setting

Located 30 km north of Loreto by sea along the Gulf of California (Figure 1A), the San Basilio embayment is surrounded by lands that occupy approximately 25 km² on the north side of the Cerro Mencenares volcanic complex with its commanding shield rising more than 600 m above sea level (Figure 1B). The region can be reached by boat from Loreto or by land on secondary ranch roads east from Mexican Highway 1. The main bay opens to the southeast into the Gulf of California. It has a coastline ringed by steep rocky bluffs interspersed with sand beaches that collectively trace through an arc of 6 km. Jagged spires of rhyolite are distinctive features both at the coast, most notably at Punta Mencenares, as well as rising through the water within the bay. These reach heights typically four to five meters above the water line. A smaller bay called Ensenada Almeja, opens northward onto the Gulf of California below the outlet of the San Juanico arroyo (Figure 1B). It is framed by massive rhyolite cliffs on opposite shores, but terminates at a wide sand beach. At both ends of the beach, canyons with north-south orientations cross inland toward the large embayment to the south. The central region is heavily dissected by tributaries that converge as arroyos feeding into the San Basilio estuary on Ensenada San Basilio.

Previously published sources on the geology of the San Basilio area are few and incomplete. The Cerro Mencenares volcanic complex (Figure 1B) is the primary focus in the most detailed study by Bigioggero et al. in 1988 [7]. Geologic maps that accompany that study [7] (Figure 2 and Figure 3 therein) extend to San Basilio, where volcanics are assigned to an older Pliocene unit shown as the San Juanico. The treatment of sedimentary rocks is inconsistent, because those mapped at a smaller scale are shown as limestone in the San Basilio basin [7] (Figure 2, therein), whereas the same areas portrayed in a more detailed map [7] (Figure 3, therein) are identified as sandstone. More than a dozen late-stage dome structures and related vents are attributed to a younger Pliocene unit shown as the San Basilio-San Juan. Rhyolitic domes in the peripheral San Basilio basin are generalized without showing eruptive vents. Likewise, a series of north-south oriented faults are mapped across Cerro Mencenares, but none extend into the San Basilio area. Data from detailed stratigraphic logs (including sedimentary rocks) are incorporated into the Cerro Mencenares study [7], but those sections are restricted to the south and more distal flank of the volcanic complex 15 km away from San Basilio.

The only other available source on San Basilio is a published abstract [8], in which emphasis is given to rhyolitic domes said to have formed under submarine conditions during the middle to late Pliocene. Therein, three stages of volcanism are outlined, including the formation of hyaloclastite breccia, followed by escaping ash, and concluding with bed-load modifications to the substrate by bottom currents and/or tidal action. Analysis using foraminifera to gauge water depth was cited, but without documentation. No mention is made of macrofossils, either for biostratigraphic or facies control.

3. Methods

The methods applied to the present study of San Basilio geology entailed the recording of stratigraphic descriptions and graphic logs to provide details on the intercalation of sedimentary and volcanic rocks at key localities. In addition, volcanic vent locations were mapped and the trend lines of the area’s major faults were traced with attention to fault offsets. Foremost, lithology was mapped during ground traverses with access from roads newly routed during the last few years. In the case of inaccessibility due to the most rugged topography, a drone (Dragon 4™) was flown to film lithotypes and record changes in elevation. All such data were plotted on a base map derived from the Mexican federal government map for Santa Rosa (G12A78). Contour intervals from the government map were scanned and traced to yield a project map with 20-m contours. A 50-gm sample from mudstone in a position above hyaloclastite breccia was collected for microfossils. Foraminifera preparation and interpretation follow the standard techniques [9]. Mega-fossil identifications follow the standard work on Pliocene and Pleistocene paleontology and stratigraphy in the Gulf of California [10].

4. Results of Geologic Mapping

4.1. Volcanic Events

Discerning the overall relationship of igneous rocks, volcanoclastic sediments, and sedimentary strata is complicated by the rugged topography around San Basilio (Figure 2A), but geological mapping confirms the basic outline of key events (Figure 2B). Banded rhyolite is the most pervasive lithology exposed across approximately half the mapped area. High sea cliffs south of Punta Mencenares and around the peninsulas on the north side of Ensenada San Basilio are formed by banded rhyolite. A large rhyolite dome (Figure 2A,B) commands a central position in the basin, rising more than 220 m above sea level.

The region’s most remarkable igneous lithology is demonstrated by hyaloclastite, prominently exposed in low cliffs on the south shore of Ensenada San Basilio around Rancho Santa Ana (Figure 3A). Juxtaposition of individual brecciated blocks of hyaloclastite up to 40 cm in cross-section show variable orientations in the pattern of banding (Figure 3B) typical of re-sedimentation following quench fragmentation. Jigsaw-fitted hyaloclastite is a descriptive term sometimes applied to this condition. Additional outcrops with hyaloclastite deposits also occur below rhyolite flows and intervening siltstones at Rancho Santa Ana (Figure 4A,B). Accumulations of volcanic ash up to 20 m in thickness also cap the hills above Rancho Santa Ana (Figure 3C), segregated well above rhyolite flows (Figure 4A).

Volcanic events recorded on the northwest margin of the study area are more difficult to relate stratigraphically to banded rhyolite. Thick ash beds are isolated in laterally extensive cliffs on the north bank of the San Juanico arroyo (Figure 2B). Located eastward on the south shore of the Boca San Juanico, basalt or andesitic basalt shows distinct pillow structures more than a meter in diameter.

4.2. Inter-Relationship of Sedimentary Events

Alternating mudstone and sandstone layers occur on the north side of Ensenada San Basilio exhibited by a thick cliff section above a tree on the shore festooned with boating regalia left by visitors (Figure 5). The detailed description for this section is provided in Appendix A, Section 1.

The succession is typical of the dominant sedimentary strata across the western and southern shores of Ensenada San Basilio (Figure 2B). In contrast, shore exposures close to the Santa Ana ranch cabin and cliffs above the nearby corral show that sandstone strata with a variable thickness up to 3 m are highly irregular in lateral continuity. Overlying rhyolite flows (Figure 4A,B) demonstrate irregular contacts and local bulges, which suggest that the rhyolite came into sudden contact with cold water or water-saturated sediment. Additionally at the shore, a second sandstone layer is preserved between the top of the rhyolite flow and bottom of ash beds that truncate the sandstone (Figure 4A). In bluffs above the corral, the surface of the rhyolite flow that buried the first sandstone is deeply eroded and there is no surviving evidence of the second sandstone.

Rhyolite flows are localized and laterally discontinuous compared to interbedded sandstone and siltstone strata that dominant exposures along the south shore. A clay-rich sample collected near the base of the thick sandstone–siltstone sequence (500 m east of the Santa Ana ranch cabin) immediately above hyaloclastite yielded a diverse fauna of 28 benthic foraminifera (

Table 1. Benthic foraminifera species diversity and dominance from a 50-gr. sample collected on the south shore of Ensenada San Basilio (26°20′54″ N and 111°25′ 14″ W). Abbreviations: L = littoral-inner shelf biofacies (0–50 m); O = outer shelf to shelf edge biofacies (50–150 m); U = upper bathyal biofacies (150–500 m).

Species	Number of Species	%	Bathymetry
Bolivina acuminata Natland	5	1.61	O
Bolivina cf. B. spissa Cushman	1	0.33	U
Bolivina interjuncta (Cushman)	65	21.45	U
Bolivina pacifica Cushman & Mcculloch	10	3.30	U
Bolivina sp.	1	0.32	O
Bulimina denudata Cushman & Parker	6	1.98	O
Bulimina marginata d’Orbigny	30	9.90	O
Buliminella cf B. basispinata R & K Stewart	2	0.66	O
Buliminella tenuata Cushman	6	1.98	U
Eubuliminella curta (Cushman)	20	6.60	O
Cancris carmenensis Natland	2	0.66	O
Cassidulina cf C.limbata Cushman & Hughes	2	0.66	U
Cassidulina corbyi Cushman & Hughes	1	0.33	U
Cibicides mckannai Calloway & Wissler	22	7.26	O
Cibicides spp.	7	2.31	O
Discorbis isabelleana (d’Orbigny)	3	0.99	L
Elphidium crispum (Linne)	1	0.33	L
Elphidium poeyanum (d’Orbigny)	1	0.33	L
Falsocibicides gallowayi Cushman & Valentine	2	0.66	L
Gyroidina rothwelli Natland	1	0.33	U
Hansenica soldanii (d’Orbigny)	1	0.33	U
Nonionella cf. N. atlantica Cushman	2	0.66	L
Patelina sp.	2	0.66	L
Planulina ornata (d’Orbigny)	8	2.64	O
Reusella aequa Cushman & MaCulloch	1	0.33	L
Virgulina cf. V. schreibersiana Czjzck	4	1.32	O
TOTAL	303	100%

) and a lesser fauna of five planktonic foraminifera (

Table 2. Planktonic foraminifera species diversity and dominance from the same 50-gr. sample (see Table 1).

Species	Number of Species	%
Globigerina eamesi Blow	83	31.56
Globigerinoides bulloideus (Bolli)	26	9.0
Globigerinoides conglobatus (Brady)	1	0.05
Globigerinoides ruber d’Orbigny	150	57.04
Globorotalia menardii (d’Orbigny)	3	0.15
TOTAL	263	99.7

). No age diagnostic planktonic species were detected and, therefore, no age estimation was possible. Relevance of the benthic foraminifera with respect bathymetry at the time of interference by hyaloclastite from below is interpreted in the discussion section. Calcareous nannofossils were noted, but not studied.

Eastward nearly as far as Punta Mencenares (Figure 2B), stretches of sandstone–siltstone bluffs rise between 40 and 50 m above sea level (Figure 6A). Bedding planes exposed near the base of the sequence exhibit extensive networks of horizontal burrows belonging to the trace fossil Thalassinoides sp. (Figure 6B). The sandstone–siltstone succession at this locality is capped by flat-lying limestone beds that thicken southward in excess of 20 m. The Pliocene echinoid Clypeaster bowersi recovered from the base of these limestone beds is an index fossil [10], the relative age significance of which is reviewed in the following discussion section. Limestone strata on the west side of the San Basilio basin (Figure 2B) also are underlain by silty sandstone bearing extensive horizontal burrows of the trace fossil Thalassinoides sp.

Exposures of rhyolite in the steep side walls of Ensenada Almeja (Figure 2B) rise 40 m above sea level. The extreme plasticity of banded rhyolite is illustrated by a deep fold with a downward-dipping axis ≥ 4 m preserved in a shallow sea cave on the east side of Ensenada Almeja (Figure 7A). Limestone caps the rhyolite massif along a north-south ridgeline that runs 0.5 km inland to the north from the Spanish Contessa’s house. The limestone features a basal conglomerate eroded from the underlying rhyolite. Rhyolite cobbles and boulders from 20 to 50 cm in diameter are encased in a limestone matrix (Figure 7B) that includes external molds of the large bivalve Periglypta multicostata preserved in growth position nestled among rhyolite clasts (Figure 7C). Typically, such a scenario suggests a former rocky shore and the life habit of this extant species is reviewed in the following discussion section. Below the same north-south ridgeline to the east, limestone outliers formed by abundant pecten shells sit on rhyolite as isolated deposits (Figure 2B).

4.3. Exhumed Volcanic Necks and Related Limestone

Several volcanic vents or exhumed volcanic necks are marked on the geological map for the San Basilio basin (Figure 2B). Three examples are represented by spires of vertically fractured rhyolite that rise above the waters of the San Basilio embayment as sea stacks. Two such clusters can be visited at Punta Mencenares on approach by boat from the southeast into the main bay.

A third example is well within the bay below the Spanish Contessa’s house (Figure 2B), where a bridge of bedded limestone projects outward 125 m from the coast separated from the closest rhyolite spires by 90 m of open water. Carbonate layers that plank the bridge appear flat-lying near the shore, but bend upward at a high apparent angle closer to the rhyolite sea stacks to form a synsedimentary ramp structure (Figure 8A). The exposure at this locality features an unconformity between the local rhyolite basement and overlying Pliocene sedimentary sequence. A detailed description of strata compiled for this site (Appendix A, Section 2) records a dip of 12° about 10 m below the top of the 35-m thick succession. A graphic representation of the rock column is provided in Figure 9.

Strata in the lower part of the succession are dominated by shell coquinas. Northward dipping basal layers (Figure 8B) consist of disarticulated pecten shells and rare oysters densely packed with evidence of shell imbrication. Unusual examples of pecten shells incorporated within mud balls up to 20 cm in diameter are noted (Appendix A, Section 2, Unit 3). Below the coquinas, a thick volcanic breccia with sub-rounded blocks up to 1.5 m in diameter changes over to a volcanic agglomerate with rounded pebbles to small boulders of rhyolite up to 30 cm in diameter (Figure 9, units 1 and 2). Blocks of welded ash as much as 40 cm in cross-section are found in some upper limestone layers (Appendix A, Section 2, Unit 5). A marker bed formed by volcanic ash interrupts the sedimentary succession at a horizon 6.8 m above the base (Section 2, Unit 6). Layers of sandstone with scattered, small pecten shells follow above the marker bed with interbedded sands and volcanic ash becoming more prevalent, thereafter.

Above the shore about 500 m west of the Spanish Contessa’s house (Figure 2B), another example of vertically fractured rhyolite is exposed in cliffs that offer an east-west cross-section, also including limestone beds formed by abundant fossil pectens on opposite flanks. The scale of this feature is commensurate with the more dramatic scenario of the rhyolite sea stacks and related sedimentary bridge below the Spanish Contessa’s house (Figure 8A).

Mostly intact, the largest rhyolite dome in the San Basilio basin exhibits a superstructure that rises more than 220 m above sea level roughly 1 km inland from the bay’s west shore (Figure 2A,B). Viewed from the northwest (Figure 10A), part of the structure’s core is exposed in a cliff face to reveal about 60 m of vertically-fractured rhyolite consistent with sea stacks in the bay. Explored on foot to the 100 m-elevation, the big dome features a limestone rim that follows in a ~4-m thick band around its northern and western slopes. Fossil oysters are a substantial component of the limestone. Closer examination of the vertiginous north wall was performed using a drone, which gave a clear image of conglomerate beds approximately 2-m thick stratigraphically wedged between the top of the rhyolite and the limestone, but also revealing a lateral facies relationship (Figure 10B). As much as 20 m of sandstone and interbedded limestone follow above the interface of conglomerate and limestone (Figure 10A), with horizontal stratification apparent across the north face of the dome structure. In this instance, the superposition of thick sandstone directly above limestone is unusual on a regional basis. If sandstone once blanketed the limestone plateau in the northern part of the San Basilio basin, it was subsequently removed by erosion. The upper part of the big dome features ~15 m of andesite flows. Twin vents (Figure 2B) are discernable at the top of the dome with the aid of drone surveillance. The more southerly vent is apparent as viewed from the northeast from across the bay, as it sits within a distinctly cratered depression.

4.4. Regional Faults

Arroyos that coalesce on the San Basilio estuary at the center of the San Basilio basin are dendritic in organization. Overall, they trace a drainage pattern that empties to the east. In contrast, a strong overprint of fault lines exists on the basin’s periphery that controls narrow valleys with mainly north-south orientations. Two distinct fault patterns cross one another. A north-south fault valley descends to Punta Mencenares in the southeast part of the basin (Figure 2B). On the opposite side of the basin, another north-south valley cuts across the north-east peninsulas to link the Almeja and San Basilio embayments. A parallel north-south valley also runs between adjacent limestone-capped plateaus starting inland from the coast about 1 km east from the mouth of the San Juanico arroyo (Figure 2B).

Two parallel faults that strike from N 28° E to S 28° W cross through the north-east peninsulas (Figure 2B) and may extend to the center of the San Basilio basin. A substantial offset of about 10 m on a normal fault lifted a rhyolite dome on the outer part of the north-eastern peninsula to a higher elevation (Figure 11). Another set of parallel faults follow a similar trend crossing through the south shore of Ensenada San Basilio. Rancho Santa Ana along the southern edge of the bay sits within a half-graben structure that accommodates a broad beach and low coastal plain with superior exposures of re-sedimented hyaloclastite. It is likely these faults continue northeast into the bay.

5. Discussion

5.1. Stages of Volcanism and Basin Development

The spatial distribution of hyaloclastite, banded rhyolite, and vertically fractured rhyolite spires suggests multiple stages of development in the San Basilio basin. Ponza and Gavi are islands off the west coast of Italy in the Tyrrhenian Sea, where iconic examples of subaqueous lavas are well described as distinguished by hyaloclastite [11]. There, felsic lavas with high-silica content resulted from effusive submarine eruptions. Hyaloclastite derives from quench fragmentation, where magma is chilled by water to produce fractured glass under thermal stress. Jigsaw-fitted hyaloclastite is exemplified on Ponza, where rotation of quenched clasts relative to one another is interpreted to occur during intrusion of new magma in an expanding submarine lava dome [11]. Laminar flow banding in rhyolite is influenced by migration of gasses released by crystallization during highly viscous extrusion. As observed in the east wall of Ensenada Almeja (Figure 7A), recumbent folds in banded rhyolite are interpreted as the result of lava extruded in a radial pattern under the influence of gravity as it flowed outward from a nearby vent. In contrast, vertically fractured rhyolite spires are exposed by differential erosion as volcanic necks at the center of rhyolite domes.

The earliest widespread phase of volcanic activity in San Basilio bay involved the creation of hyalosclastite as incipient domes grew beneath extensive marine mudstone and sandstone, examples of which are well exposed in discontinuous outcrops around the perimeter of the bay (Figure 2A). Exposures of re-sedimented hyaloclastite are especially well preserved on the south and west shores of Ensenada San Basilio, where overlying sandstone–siltstone strata are left intact. Less well represented on the north shore west of the Spanish Contessa’s house (Figure 2B), a thick wedge of interbedded sandstone and siltstone strata is correlated with the more extensive deposits on the west and south shores (Figure 5; Appendix A Section 1). Likely by means of explosive rupture from below and subsequent submarine erosion, the sandstone–siltstone facies disappeared from the approximately 4 km² area of present-day San Basilio bay under the influence of strong bottom currents.

The next phase of volcanic activity involved expansion of numerous rhyolite domes in the central and northern parts of the San Basilio basin, typically distinguished by flow banding. Several large domes coalesced, especially in the northern part of the basin. The largest, free-standing dome with two well preserved vents occupies the central part of the basin (Figure 2B). The top of the rhyolite core in the large central dome was exposed to subaerial erosion, as indicated by rhyolitic conglomerate that inter-fingers with limestone around its perimeter. The same facies relationship is found on the flank of another rhyolite dome north of the Spanish Contessa’s house. Smaller domes sat beneath volcanic islets within the yet flooded San Basilio embayment. Sea stacks formed by vertically fractured rhyolite mark the vent locations of those islets today. The final stage of volcanic activity entailed intermittent explosions that sent large amounts of ash into the air recorded by extensive tuff layers both on the north and south sides of the bay. In particular, the large rhyolite dome at the center of the San Basilio basin was the locus of renewed volcanic activity that brought surface flows spilling over the margins of the dome from a pair of vents.

5.2. Paleobathymetry

Following the protocol for Recent and subfossil benthic foraminifera [9], the sample collected from the base of sandstone–siltstone strata east of Rancho Santa Ana was found to be dominated (66.61%) by species characteristic of an outer and inner biofacies (Table 1), most likely deposited near an upper bathyal depth as great as 150 m under conditions of reduced oxygen. This interpretation is based on the presence of almost equal proportions of Bolivina bicostata and B. interjuncta. High abundances of the later species are reported from the upper bathyal oxygen minimum zone along the East Pacific Margin [12], as well as from the outer shelf biofacies (73–152 m) in the Gulf of California [9]. Scarce occurrences of other upper bathyal, low-oxygen species such as Buliminella tenuata, Bolivina pacifica, and B. sp. cf. spissa also support such a depth interpretation. In this case, the abundance of outer-shelf edge species (53.04%) and the presence of rare upper bathyal species places the sample at a maximum depth between 100 and 150 m, probably within an environment with slightly low content of dissolved oxygen. Essentially, this finding also delimits the bathymetric setting soon after the re-sedimented hyaloclastite deposits formed after highly explosive hydromagmatic conditions. Our result is in general agreement with the undocumented bathymetric estimate of 110 m for initial basin development previously reported in abstract form [8].

Extensive sandstone–siltstone strata with abundant Thalassinoides trace fossils are interpreted as having formed within an ecological window below fair-weather wave base but above the level of oxygen starvation that provided sufficient oxygen for the burrow makers [13]. At a maximum water depth of 120 m, burrow inhabitants still had enough oxygen but also were sheltered against severe storms that otherwise might destroy their habitat. Hence, the Thalassinoides habitat is considered to range no deeper than 120 m, an estimate that fits well in position above the sample of benthic foraminifera from the base of the sandstone–siltstone succession in more poorly oxygenated waters.

Capstone carbonate strata in scattered outliers and entire plateaus across the northern part of the study area (Figure 2B) feature abundant bivalves often dominated by fossil pectens. Seventeen pectinid species from Pliocene strata in Baja California have been described [10]. Poor preservation makes it difficult to pin down accurate identifications from any of the Basilio pectenids, although Aequipecten abietis was found to be the most abundant representative from Pliocene strata in the nearby El Mangle area to the south [14]. Living pectens are entirely subtidal in distribution and a common species such as Argopecten circularis with fossil representation in Baja California is known to occupy water depths from 1 to 135 m [15] (p. 87). On the basis of stratigraphic position, the pecten beds that follow after sandstone–siltstone layers with the Thalassinoides facies thrived in shallower waters subject to frequent disturbances above normal wave base.

The basal conglomerate composed of banded-rhyolite cobbles and boulders on the ridgeline north of the Spanish Contessa’s house is especially significant for inclusion of fossil bivalves [10] belonging to the venerid Periglypta multicostata. This species is the largest venerid bivalve living today in coastal habitats from western Mexico to Peru. It is restricted to a sandy substrate nestled among rocks at extreme low tide [15] (p. 161). Hence, there is strong support for interpretation of an inter-tidal rocky shore that commenced with deposition of Pliocene mixed conglomerate and limestone in the San Basilio area.

Unique to the San Basilio basin, the carbonate bridge below the Spanish Contessa’s house (Figure 8A) is of particular interest because nothing like it is reported elsewhere from the Pliocene of Baja California. Evidence from the 35 m succession of interbedded limestone, sandstone, and ash beds within the sequence (Figure 9; Appendix A, Section 2) suggests it accumulated coeval with but subjected to the disruptive growth of a small volcano, the neck of which is exhumed in nearby sea stacks. The upward turn in strata at the end of the broken bridge is interpreted as mainly syndepositonal in origin, but also having been disturbed by magma expansion from below that led to direct volcanic warping of the overlying strata. Hence, the surviving configuration represents the erosional remnants of a sedimentary ramp structure that formerly surrounded the volcano on all sides.

Evidence in support of a steeply inclined surface on the submarine flank of a volcano derives from bands of shell coquina formed by densely imbricated pectens, as well as mud balls that also incorporate pecten-shell debris. Gravity flows were likely to have been caused by shaking in the immediate area during the expulsion of volcanic ash. Although unrelated to a volcanic setting, similar phenomena including transported shell coquinas and mud balls was interpreted as a gravity slide off a pre-existing shelf in the Pliocene of Isla Monserrat [16]. Evidence to support volcanic co-development at San Basilio is credible on the basis of sizable tuff blocks incorporated within limestone layers, in addition to a 30 cm thick ash bed that appears 6.80 m above the base of the sedimentary package. The succeeding limestone and sandstone beds incorporate an increasing fraction of volcanic ash.

The structural integrity of the carbonate bridge and rhyolite sea stacks is preserved in a small half graben topographically below the prominent ridgeline of limestone north of the Spanish Contessa’s house (Figure 2B). As such, the carbonate bridge represents the seaward extension off a rocky shore, interfacing with a small volcano. Elsewhere around the bay, hyaloclastite is intimately related only with overlying beds of sandstone and siltstone (Figure 4A,B) deposited earlier in a deeper-water setting during a more intense phase of volcanic activity.

5.3. Biostratigraphic Correlation

Most of the limestone in the San Basilio basin is regarded as the final stage of basin development prior to dissection by faults, whether in succession above sandstone strata or in direct contact on an erosional surface with rhyolite. The sea urchin index fossil Clypeaster bowersi from basal limestone in the southeast part of the basin has a wide distribution in Pliocene strata throughout Baja California [10]. In the scheme of concurrent range zones from the thick Pliocene succession at Arroyo Blanco on Isla del Carmen, this species has been placed [17] at the top of the Lower Pliocene as previously defined [10]. In formal stratigraphic nomenclature, this level is equivalent to the upper part of the Zanclean Stage in the Lower Pliocene.

On the basis of other considerations including fault relationships, San Basilio limestone was likely to have spanned the lower part of the succeeding Piacenzian Stage (further elaborated in discussion, below).

5.4. Fault Patterns and the Pliocene Gulf of California

North-south faults in the San Basilio basin mirror the many north-south faults previously mapped by Bigioggero et al. [7] (Figure 3, therein) on the Cerro Mencenares volcanic complex adjacent to the south. Moreover, the 5 km long El Mangle lineament [14] (Figure 2, therein) connects with the north-south fault that exits into San Basilio bay near Punta Mencenares (Figure 2B). These north-south faults conform to the pattern of east-west crustal extension during an early tectonic stage in the opening of the proto-gulf that ended as late as 3.5 million years ago [18].

At the south end of the Mangle fault block, an unusual scarp with a fault trend oriented N 55° W to S 55° E, was named El Coloradito [14]. That orientation runs parallel to active transform faults in the present-day Gulf of California and is regarded as an early manifestation of transtensional tectonics in the Gulf of California. No fault trends of this kind are apparent in the San Basilio basin, but faults with an orientation N 28° E to S 28° W through the north-east peninsulas (Figure 2B) are offset from El Coloradito by sixty-three degrees. A full ninety degrees would bring those faults into alignment with spreading zones in the present-day Gulf of California. The short-fall, however, does not eliminate a connection with transtensional tectonics that stressed the basin in another dimension during the earliest stages of a new tectonic regime. In that case, the final events to affect the San Basilio basin occurred no earlier than about 3.5 million years ago. Carbonate ramps at El Mangle prior to offset by El Coloradito fault were dated to 3.3 million years ago on the basis of tuff beds near the base of the ramps [14]. By extension, this approximate date would place the some of the thick limestone beds in the adjacent San Basilio basin within the lower part of the Pliocene Piacenzian Stage.

5.5. Comparisons with Other Volcanoes in the Gulf of California

Rhyolite domes outside the San Basilio basin are rarely found within or adjacent to the Gulf of California. The prime example is Isla San Luis in the upper gulf region (Figure 1A) with a pair of intact rhyolite domes that measure from 0.5 to 1 km in diameter and reach a maximum elevation 175 m above sea level. Surface deposits on the domes include rhyolitic bombs that have an obsidian crust indicative of subaerial activity [2]. Isla Tortuga, located in the midriff area of the gulf (Figure 1A) is a shield volcano composed of Hawaiian-style tholeiitic basalt [3]. The island’s superstructure rises more than 200 m above sea level and exhibits a central crater with a diameter of 1250 m. Including that part of the superstructure below sea level, the complete Tortuga volcano matches the entire San Basilio basin in scale. Closer to Loreto (Figure 1A,B), Isla Coronados is a classic stratovolcano with a superstructure that rises more than 275 m above sea level and features well-preserved andesite flows. It is roughly two-thirds the size of Isla Tortuga above the waterline, more than enough in size to occupy San Basilio bay. No older than Holocene in age, San Luis and Totuga are very young, whereas the youngest lava flows on Coronados date to the later Pleistocene at 160,000 years in age [4].

Among the three island examples, only Coronados includes a substantial fossil record represented by a coral reef in a paleo-lagoon on its southern, leeward side [5]. Given the older, mid-Pliocene status of the rhyolitic domes and volcanoes in the San Basilio basin, it is remarkable so much of the carbonate bridge and related volcanic slope deposits has survived (Figure 7A). Compared to the Pleistocene coral reef on Coronados which dates between 121,000 and 127,000 years in age and reached mean sea level due to upward growth [5], the pecten coquinas and mud balls from gravity slumps on the slope of a small volcano accrued in deeper waters, but still likely close to fair-weather wave base.

5.6. Comparison with Oceanic Volcanoes Elsewhere

Oceanic islands are mostly volcanic in origin composed of basalt. Aside from fossil coral reefs that form the basis of atolls around former volcanic islands, few such volcanoes are well understood from the literature as retentive of marine sedimentary deposits with any paleoecological value. Ponza and Gavi in Italy’s Pontine archipelago are noteworthy for their rhyolitic lava domes [11], for example, but their island flanks preserve no marine sediments or megafossils. Much inspired by the available literature on Italian volcanoes, Charles Darwin was among the earliest naturalists to draw attention to non-reef limestone preserved as shallow, subtidal deposits around basalt islands such as Santiago in the Cape Verde archipelago [19]. Fossil rhodoliths secreted by coralline red algae contributed to substantial Pleistocene deposits against basalt rocky shores on Santiago and other, older islands in the Cape Verde, Madeira, and Azores islands. Much to Darwin’s disappointment, the basaltic Galapagos off the coast of Ecuador failed to reveal similar carbonate deposits during his visit there in 1835. However, island carbonates are present in the Galapagos, as for example Holocene beach sands preserved as over-wash deposits between basalt flows on Isla Sombrero Chine [20]. Perhaps the best example of a volcano-slope deposit similar to the Pliocene shell coquinas and mudballs from San Basilio was described from the middle Miocene of Ilhéu de Baixo in the Madeira archipelago [21]. In particular, a carbonate facies dominated by broken branches of the fossil coral Pocillopora madreporacea occur within a gravity slide on the steep submarine flanks of a volcano with prevalent Surtseyan deposits from frequent ash eruptions.

6. Conclusions

Observations from geologic mapping and interpretations with respect to inter-related patterns of volcanism and sedimentation support the following conclusions at the level of a reconnaissance overview of landscape evolution in the San Basilio basin.

1. The earliest widespread, post-Miocene phase of volcanic activity commenced with submarine domes filled with hyaloclastite that impinged on overlying siltstone and sandstone strata. Examples of re-sedimented hyaloclastite are documented for the first time in Mexico’s Baja California, matching classic textbook examples of hydro-magmatic volcanism in Ponza, Italy.

2. Based on bathymetric analysis of foraminifera with nearly equal proportions of Bolivina bicostata and B. interjuncta from mudstone at the base of the siltstone-sandstone succession above hyaloclastite, seawater at the time of magmatic injection was no more than 150 m deep under conditions of reduced oxygen. Better oxygen levels in silty sediments at a slightly shallower water depth supported organisms that made extensive Thalassinoides burrows.

3. A subsequent phase of volcanic activity occurred with introduction of rhyolite domes characterized by extensive flow banding sometime after destruction and erosion of sandstone and siltstone strata at the center of the paleobay at San Basilio. Substantial domes were emplaced in the central and northern parts of the basin.

4. Widespread Pliocene limestone overlies rhyolite domes exposed to wave erosion that led to intertidal conglomerates inhabited by the venerid bivalve Periglypta multicostata. This large bivalve is extant and dwells nestled among cobbles at extreme low tide. Another carbonate facies off former rocky shores is represented by pecten bivalves that lived in water depths above wave base on rhyolite substrate where sorting and imbrication of disarticulated shells was common.

5. A unique variation on the pecten-shell facies is preserved as a slope cover around a small volcano with an exhumed neck of vertically fractured rhyolite in San Basilio bay. Influenced by magma expansion from below as well as shaking during expulsion of volcanic ash, gravity slumps on a steep slope are indicated by densely imbricated shells and mud balls. Volcanic activity continued apace during carbonate deposition registered by sizable tuff blocks and other volcanic detritus incorporated within the limestone, as well as emplacement of a tuffaceous marker bed.

6. North-south faults define valleys in the present-day landscape that are attributed to east-west extension during an early phase of tectonism in the Gulf of California ending roughly 3.5 million years ago. Residual effects from the changeover to transtensional tectonics are suggested by cross-cutting faults with a trend N 28° E to S 28° W. The sea urchin index fossil Clypeaster bowersi from the base of San Basilio limestones indicates a position in the upper part of the Zanclean Stage (Lower Pliocene). Ongoing limestone deposition prior to transtensional faulting was likely to have concluded at a level equated with the lower Piacenzian Stage (Upper Pliocene) based on dated tuff in strata from the nearby El Mangle to the south.