The Vizcaino fore-arc basin accumulated approximately 4 km (∼13,000 ft) of upper Albian–middle Eocene siliciclastic marine sedimentary rocks derived from the Peninsular Ranges in Baja California. Data from eight exploratory wells document the micropaleontological content and lithological characteristics of these rocks. The strata studied represent mostly neritic–upper bathyal marine environments and overlie a basement composed of Cretaceous granitic rocks, or Aptian–Albian volcaniclastic sedimentary rocks correlative with the Alisitos Formation. We recognize four major depositional sequences within the basin that are related to the regional geology. The basal Albian–Turonian sequence 1 represents the initiation of fore-arc basin sedimentation, contains continental conglomerates that change to bathyal shales, and correlates with the lower part of the Valle Group of the Vizcaino Peninsula. Sequence 2 is Coniacian–Paleocene, includes basal conglomeratic sandstones grading into Maastrichtian bathyal shales, and usually overlies a Coniacian–Santonian unconformity. Sequence 2 is represented at the surface by the Rosario Group in northwestern Baja California and the upper part of the Valle Group in the Vizcaino Peninsula. Sequence 3 is Paleocene–middle Eocene, represents continuity of fore-arc sedimentation in neritic–upper bathyal conditions, is capped by a major unconformity, and correlates with the Sepultura and Bateque Formations to the north and south of the basin, respectively. The uppermost Miocene–Pliocene sequence 4 is composed of marine sandstone–siltstone unconformably overlying sequence 3 and is correlative with the Tortugas Formation that represents sedimentation after the end of subduction of the Farallon plate beneath the North America plate.
The western margin of North America was a continuous convergent zone during subduction of the Farallon plate from the Early Cretaceous–Paleogene. During this interval, the tectonic activity in Baja California changed from an extensional arc system to a contractional arc apron (Busby et al., 1998; Centeno-García et al., 2011). In the course of this change, the westernmost margin developed the Vizcaino fore-arc basin (VB) in central Baja California (Figure 1), where a sedimentary infill greater than 3 km thick (>9,842 ft) was deposited. This important stratigraphic record provides new information with which to evaluate tectonic models and the depositional history of southwestern North America (Gastil et al. 1975, 1978; Rangin, 1978, 1986; Barnes, 1984; Bottjer and Link, 1984; Johnson et al., 2003).
Several reports have addressed the stratigraphy and sedimentology of strata cropping out along the Pacific margin of Baja California and southern California (Kilmer, 1977, 1984; Patterson, 1978; Beggs, 1984; Boles and Landis, 1984; Kimbrough, 1984, 1985; Yeo, 1984; Cunningham and Abbott, 1986; Vázquez-García and Schwennicke, 1996; Kimbrough et al., 2001; González-Barba, 2002; Miranda-Martínez and Carreño, 2008). However, a thicker, larger, and mostly undisrupted part of this sedimentary basin lies beneath the continental shelf of central Baja California in the VB (Figure 1). Lithostratigraphic, chronostratigraphic, and paleoenvironmental data from this subsurface record provide time constraints for regional stratigraphic and tectonic events that marked the evolution of the southern part of the fore-arc basin during the Early and Late Cretaceous and Paleogene.
An exploration program of Pemex® (the Mexican oil company) from 1951 to 1981, drilled several exploration wells in western Baja California, however only limited stratigraphic data from these wells have been published (Lozano-Romen, 1975; Alvarado-de-la-Tejera, 1976; Cavazos-Prado, 1976; Diaz-Cuevas, 1976; González-García, 1976; Ramos-García, 1976; Helenes, 1984). According to these reports, the VB contains a sedimentary succession that spans the late Albian–middle Eocene (Lozano-Romen, 1975). Well C penetrated 3700 m (12,139 ft) of siliciclastic rocks and did not reach basement in the deepest part of the VB (Figure 1).
This report presents data and interpretations from lithology as well as micropaleontological and palynological analyses carried out in cuttings and core samples from eight wells in the VB. Based on these results, we construct a detailed chronostratigraphic framework and propose a paleobathymetric evolution of the basin that help constrain evolutionary models of this continental margin. In addition, we propose a sequence stratigraphic context regionally correlated with the formal stratigraphic units cropping out onshore from California and south into the Vizcaino Peninsula (Figure 1).
The VB lies on the western margin of the Baja California Peninsula (Figure 1) that is south of the continental borderland province (Emery, 1960; Krause, 1964, 1965) and northeast of the Vizcaino Peninsula (Gastil et al., 1975; Lozano-Romen, 1975). The eastern margin of the fore-arc basin lies parallel to the western shoreline of Baja California south of Punta Canoas (Figure 1). To the west, the continental margin extends to the Vizcaino Peninsula and Cedros Island. To the south, the VB extends into the coastal plain confined between the Sierra San Andrés and the Peninsular Ranges batholith (PRB) in Baja California. To the south, a subsurface structural high known as the Lagunitas structural high (Lozano-Romen, 1975) separates the VB from the Purísima Basin to the south (Figure 1).
Regional gravimetric anomalies (Lozano-Romen, 1975; Alvarado-de-la Tejera, 1976) show the approximate location and extent of the VB and its southern limit in the Lagunitas high (Lozano-Romen, 1975, figure 2 as “Plano 2”; Alvarado-de-la Tejera, 1976, figures 3, 4, 6, and 7). A southeast-trending gravimetric minimum offshore of Punta Canoas extends south to offshore of Guerrero Negro, roughly following the northwest–southeast axis of the basin (Figure 1), whereas an almost east–west gravimetric maximum shown south of Guerrero Negro indicates the approximate position and alignment of the Lagunitas structural high (figure 2 in Lozano-Romen, 1975).
Two stages of arc magmatism formed the PRB in Baja California (Silver and Chappell, 1988). The first stage was an arc apron composed of Lower Cretaceous intermediate to mafic volcaniclastic plutonic and sedimentary rocks known as the Alisitos arc (Santillán and Barrera, 1930; Allison, 1974; Busby, 2004). The second stage produced an Andean-type continental arc during the Late Cretaceous. Subsequent uplift and erosion have exposed the underlying plutons from southern California to Baja California (Gastil et al., 1975; Silver and Chappell, 1988; Ortega-Rivera et al., 1997). These rocks separate the regional geology into prebatholithic, batholithic, and postbatholithic rock belts. The prebatholithic rocks are represented by a small number of Paleozoic–Lower Cretaceous metasedimentary and volcaniclastic pendants (Gastil, 1983).
Regional Mesozoic–Cenozoic Stratigraphy
Subduction-related stratigraphic units exposed in northwestern Baja California include the Lower Cretaceous Alisitos Group (Beggs, 1984), the Upper Cretaceous Rosario Group (Kennedy and Moore, 1971), the Paleocene Sepultura Formation, and the Eocene Delicias and Buenos Aires Formations. Postsubduction sedimentation includes the Miocene Rosarito Beach and the Pliocene–Pleistocene Cantil Costero Formations (Figure 2). To the southwest, in the Vizcaino Peninsula and Cedros Island, the Mesozoic stratigraphic succession is different and related to an accreted island arc terrane (Sedlock, 2003).
The Lower Cretaceous Alisitos Group (Santillán and Barrera, 1930; Allison, 1974; Beggs, 1984) includes volcanic and volcaniclastic sedimentary strata of island-arc affinity. This unit has been interpreted as an extensional volcaniclastic plutonic island arc that fringed the continental margin of North America (Busby, 2004). The Alisitos Group is composed mainly of andesitic–rhyolitic volcaniclastic rocks (Beggs, 1984; Wetmore et al., 2003; Busby, 2004) with massive biohermal limestones formed when volcanic activity decreased by the end of the Early Cretaceous (Suárez-Vidal, 1987).
After collision of the Alisitos arc with the western margin of North America, the Upper Cretaceous Rosario Group (Kennedy and Moore, 1971; Patterson, 1978; Yeo, 1982; Miller and Abbott, 1989) was deposited in the El Rosario fore-arc basin (Figure 1). These strata represent deposition on a continental margin (Gastil et al., 1975; Busby, 2004) as indicated by the arkosic–lithic composition with fossil evidence that indicates continental–upper bathyal environments in the type area near El Rosario (Kilmer, 1963; Patterson, 1978; Boehlke and Abbott, 1986). Upper Cretaceous stratigraphic sections with similar lithologic and depositional settings are described north of El Rosario, along the western margin of Baja California (Acosta, 1966; Yeo, 1982; Maestas et al., 2003). Farther north in San Diego County, the marine Upper Cretaceous Point Loma and Cabrillo Formations are composed of siltstone and sandstone (Kennedy and Moore, 1971; Gastil and Higley, 1977; Nilsen and Abbott, 1979, 1981; Yeo, 1982) and are part of the Rosario Group.
The Paleocene Sepultura Formation includes marine rocks that crop out in small areas in westcentral Baja California (Figure 2; Santillán and Barrera, 1930; Gastil et al., 1975; Zinsmeister and Paredes-Mejía, 1988; Abbott et al., 1993; Helenes and Téllez-Duarte, 2002; Téllez-Duarte et al., 2006). Eocene marine sedimentary rocks are scarce in central Baja California although outcrops of the shallow marine sandstones assigned to the Buenos Aires or Delicias Formations have been described in the northwestern part of the peninsula (Flynn, 1970; Carreño and Smith, 2007). Farther north, in the San Diego Embayment of southern California, Eocene marine sedimentary rocks are assigned to the La Jolla Group (Kennedy and Peterson, 1975) and are partially correlative with the Delicias Formation. No Oligocene marine deposits have been reported in the northern Baja California Peninsula.
A clastic wedge of Neogene deposits punctuated with volcanic rocks includes the middle Miocene Rosarito Beach Formation that crops out in the northwestern part of the peninsula, from the United States–Mexico border south to Ensenada (Figure 1). This unit consists of basalt flows interbedded with pyroclastic and marine epiclastic deposits (Minch, 1967; Carreño and Smith, 2007; Salinas-Márquez et al., 2016). In addition, the highly fossiliferous Pliocene Cantil Costero Formation unconformably caps the Maastrichtian Rosario Group north of El Rosario (Carreño and Smith, 2007).
As mentioned above, the stratigraphy of Mesozoic rocks exposed on the Vizcaino Peninsula and Cedros Island is different from that in northern Baja California (Figure 2). In the Vizcaino region, Triassic–Jurassic ophiolite and related plutonic rocks compose the basement and are tectonically juxtaposed alongside Early Cretaceous–Cenozoic marine strata (Mina, 1957; Lozano-Romen, 1975; Helenes, 1984; Abbott et al., 1995; Busby, 2004). The oldest Mesozoic rocks exposed on Cedros Island and the Vizcaino Peninsula include a Triassic–Jurassic ophiolitic complex at the base of the section (Finch and Abbott, 1977; Rangin, 1978; Pessagno et al., 1979; Whalen and Pessagno, 1984; Moore, 1985; Whalen and Carter, 2002). A Jurassic tectonic mélange unconformably overlies the Jurassic ophiolitic rocks (Boles and Landis, 1984; Kilmer, 1984; Kimbrough, 1984, 1985). On the Vizcaino Peninsula, Triassic cherts and Jurassic volcaniclastic rocks overlie the ophiolitic rocks (Hickey, 1984; Kilmer, 1984; Moore, 1984, 1985; Kimbrough, 1985). Cretaceous, shallow- to deep-water marine clastic deposits of the Valle Group rest unconformably on basement rocks (Mina, 1957; Barnes, 1984; Berry and Miller, 1984; Patterson, 1984a; Smith and Busby, 1993; Kimbrough et al., 2001). In the Vizcaino Peninsula, the Valle Group forms two subbasins in the north–south direction, which indicates the unconformity has significant structural relief. Paleocene–lower Eocene marine terrigenous deposits crop out only in the southern part of the Vizcaino Peninsula (Abbott et al., 1995; Carreño and Smith, 2007), whereas Miocene–Pliocene strata of the Tortugas Formation are exposed mainly in the northwestern part of Vizcaino Peninsula and in Cedros Island (Mina, 1957; Robinson, 1975, 1979; Kilmer, 1977, 1979; Helenes-Escamilla, 1980, 1984). Our study documents the stratigraphic succession in the fore-arc basin and complements the patchy stratigraphic record previously studied onshore.
MATERIALS AND METHODS
Lithology in wells was interpreted based on gamma-ray, spontaneous potential, and resistivity logs, which were verified with mud-log reports from Pemex (Figure 3A, B). Samples for micropaleontological analysis mostly represent fine-grained lithofacies and were selected based on their distribution along the well. We analyzed 439 cuttings and 8 core samples from 9 exploratory wells, which were processed and analyzed for palynology and foraminifera. Age and paleobathymetric assignments (Appendix 1, supplementary material available as AAPG Datashare 104 at www.aapg.org/datashare) are based on age ranges of the planktonic and benthonic foraminifera as well as on dinoflagellates and spores recovered from these samples (Appendix 2, supplementary material available as AAPG Datashare 104 at www.aapg.org/datashare). Individual chronostratigraphic tables from each well present the paleontological ages assigned and the paleobathymetric ranges indicated by the micropaleontological assemblages observed. Taxonomy used follows Stewart and Pearson (2000) (PLANKRANGE) for foraminifera, Fensome et al. (2008) (DINOFLAJ2) for dinoflagellates, Perch-Nielsen (1985) for calcareous nannofossils, and the various references mentioned for age determination of pollen and spores.
Palynological Processing and Analyses
Samples for palynology were processed following standard treatment (Wood et al., 1996), which included treatment with HCl, HF, and sodium polytungstate (specific gravity = 2.0), without oxidation. The residue was then sieved through 125-μm and 15-μm mesh sieves, and strew mounts were prepared with the residue retained in the 15-μm mesh sieve. One slide per sample was analyzed under the microscope, and numerical abundances of marine and terrestrial palynomorphs were recorded.
The assigned ages are based mainly on the known stratigraphic range of the species of dinoflagellates and planktonic foraminifera reported here. Spores and pollen grains were also observed, and their known stratigraphic ranges are noted. The absolute ages of the palynomorph ranges are calibrated after Gradstein et al. (2012).
Stratigraphic ranges used for the dinoflagellates were compiled from Drugg and Stover (1975), Williams and Bujak (1985), Wrenn et al. (1986), Helby et al. (1987), Haq et al. (1988), Matsuoka and Bujak (1988), Powell (1992), and Williams et al. (1993, 2004). Most of the stratigraphic ranges used here for the terrestrial palynomorphs are reported in Germeraad et al. (1968), Pares-Regali et al. (1974a, b), Müller et al. (1987), and Lorente (1986). Additionally, stratigraphic ranges of all palynomorphs are complemented with information contained in the computer databases: TAXON (R. L. Ravn, 1993, personal communication) and the Palynodata datafile (White, 2008). The absolute ages of the palynomorph ranges are calibrated with Gradstein et al. (2012).
Palynological Marine Index
The initial weight of the samples was low (mean = 9.4 g), and thus recovery was generally low to moderate; for this reason, no statistical analyses were done on the microfossil assemblage. Nevertheless, we use a palynological marine index (PMI) as an auxiliary tool with the paleoenvironmental interpretations (Helenes et al., 1998). The PMI is calculated using the following formula: PMI = (DM/DC+1) × 100, where DM = diversity or richness of marine palynomorphs (dinoflagellates and acritarchs) and DC = diversity or richness of continental palynomorphs (pollen and spores). Null values of PMI indicate samples without marine palynomorphs and are interpreted as representing continental environments. Low values of the PMI indicate deposition in transitional environments, whereas higher values indicate marine deposition. Larger values generally correspond to neritic environments as indicated by benthonic foraminifera. The PMI is useful when comparing values from adjacent samples. Large differences indicate changes in depositional environments and help in the definition of sequence boundaries and maximum flooding surfaces.
Foraminiferal Processing and Analyses
Samples for foraminiferal analyses were prepared according to the normal oxidation procedure (Thomas and Murney, 1985) including washing of the samples with detergent and treatment with warm hydrogen peroxide (H2O2). The residue was sieved (0.063 mm) and the larger fraction dried and analyzed. The entire residue was scanned for specimens and numerical abundances of the taxa were recorded. Stratigraphic ranges used here for the planktonic foraminifera are those shown in Sliter (1968), Kennett and Srinivasan (1983), Bolli and Saunders (1985), Caron (1985), Toumarkine and Luterbacher (1985), Premoli-Silva and Sliter (1999), and Premoli-Silva et al. (2004). Additionally, age assignment was also assigned after considering information in the computer database PLANKRANGE (Stewart and Pearson, 2000).
Paleobathymetric interpretations are based primarily on information from benthonic foraminifers following Sliter and Baker (1972) in combination with lithologic and palynological data. We also consider that the planktonic/benthonic (P/B) ratio increases with increasing water depth (van der Zwaan et al., 1990), with lower P/B ratios commonly occurring beneath coastal water and in deep-water environments below the foraminiferal lysocline (Akimoto, 1994). Integration of all micropaleontology and lithology data and considerations allowed the subdivision of the paleoenvironments according to the following depth intervals: continental above sea level; transitional between high and low tides; inner neritic 0–50 m (0–164 ft); middle neritic 50–100 m (164–328 ft); outer neritic 100–200 m (328–656 ft); upper bathyal 200–500 m (656–1640 ft); and middle bathyal 500–2000 m (1640–6562 ft).
Consensus ages for the samples from each well were assigned on the basis of micropaleontological data presented here and, in some cases, by integrating the information provided by Pemex (Figures 3 and 4). The absolute ages of the species reported here have been taken from the references mentioned above with ages determined by the correlation of faunal chronozones with radiometric dates and magnetic polarity chronozones (Haq et al., 1988; Gradstein et al., 2012). Appendix 1 (supplementary material available as AAPG Datashare 104 at www.aapg.org/datashare) contains biostratigraphic information for each well presented in Tables S1–S8 (wells A–H) (supplementary material available as AAPG Datashare 104 at www.aapg.org/datashare) with the main biostratigraphic markers for each interval and their assigned age ranges in Ma. In addition, Appendix 2 (supplementary material available as AAPG Datashare 104 at www.aapg.org/datashare) contains the complete names and assigned stratigraphic ranges of the taxa used in this study, separated by biological group.
Terrigenous deposits in the fore-arc basin range from conglomerates to mudstones with few calcareous units. Fossil recovery ranged from good to poor, depending on the lithology of the sample. Volcaniclastic sedimentary rocks were barren of microfossils, and conglomerate and sandstone lithofacies contained continental and some marine palynomorphs, whereas siltstone and shale intervals contain well-preserved planktonic and benthonic foraminifera, dinoflagellates, pollen, and spores.
The stratigraphic columns drilled in the VB contain microfossils ranging in age from Albian (113–100.5 Ma) to Miocene–Pliocene (23–3.6 Ma). A plutonic basement was drilled and dated (unpublished data from Pemex) in wells B (135±9 Ma K/Ar, feldspar) and H (102±10 Ma K/Ar, feldspar). Six wells drilled Albian–Cenomanian conglomeratic intervals at the base, but wells D and E drilled basal Albian volcaniclastic deposits overlain by bioclastic limestones that are in turn overlain by basal conglomerates. On top of these basal strata, a succession of mainly sandstone, siltstone, and mudstone deposits with subordinate conglomeratic horizons constitutes the fore-arc basin fill. Most wells contain an unconformity between Turonian and Campanian strata, except wells C and F. A regional unconformity is also evident between middle Eocene and Miocene–Pliocene strata. The following summaries highlight important aspects and occurrences of the biostratigraphic ages that we recognized and that were subsequently used to construct a biochronologic framework for the VB.
Strata with this age were clearly recognized in wells A, C, and D, whereas lithological and stratigraphically correlative levels from the rest of the wells contain biostratigraphic data indicative of either Turonian–Albian or Cenomanian–Albian age ranges.
In well B, the interval from 2380 to 2535 m (7808–8317 ft) contains fossils with an age range from Albian–Berriasian (100.5–145 Ma). Because the age range is an uncertainty, and considering the correlation of this interval with wells A and C, we interpret the age of this interval as Turonian–Albian (89.8–113 Ma). This interpretation is supported by the early Albian–Berriasian (126–144 Ma) radiometric age of the underlying igneous rock.
In well D, core samples in the interval 1224–1255 m (4016–4118 ft) contain fossils with an age range from late Albian–late Pennsylvanian (101.7–299 Ma). Again, because the age range is an uncertainty and considering the correlation of this interval with well E, we interpret the age of this interval as being no older than Albian (<113 Ma).
Strata with age ranges that include the Cenomanian were found in all wells. Well C has the strongest evidence for an interval of Cenomanian age—at depths from 2350 to 2505 m (7710–8218 ft). This interval is assigned a Cenomanian–latest Albian (93.9–101.9 Ma) age range.
Strata with this age are clearly indicated in wells C and F. The rest of the wells contain strata with age ranges that include the Turonian.
Strata of this age are recognized in a short interval (1680–1700 m [5512–5577 ft]) in well A.
Strata with this age were not clearly defined, and only thin intervals of mudstone–siltstone strata with age ranges including the Coniacian were found in some wells. The Coniacian is almost certainly missing in wells A, C, F, G, and H, whereas in wells B, D, and E this age was not identified.
Strata with this age are clearly identified in wells A, B, C, F, and G. In wells E and H, the Campanian is probably included in intervals with assigned age ranges from Maastrichtian–Campanian or Campanian–late Albian.
Strata of this age were clearly recognized in all wells, except D, in which the Maastrichtian is not recognized because of a lack of samples in the interval from 455 to 985 m (1493–3232 ft), located between Paleocene and Turonian strata.
Strata of this epoch were clearly recognized in seven of the eight wells studied, but no evidence of a Cretaceous–Paleocene unconformity was found. However, wells B and F drilled only parts of the Paleocene. Well B does not contain lower Paleocene strata, and well F contains an upper Paleocene unconformity.
On top of the Paleocene interval, strata of early to middle Eocene age were clearly recognized in wells A, B, F, G, and H. Rocks of this age unconformably underlie strata of Miocene–Pliocene age in wells A and D. In well D, the 115- to 150-m (377–492-ft) interval has an age range of Oligocene–middle Eocene. However, this mudstone interval is interpreted here as being middle Eocene because it shares the characteristic alternating mudstone and siltstone of these rocks. A Miocene–Pliocene sandy upper interval unconformably overlies Eocene strata, expressing an unconformity previously recognized in the region (Martín et al., 2015).
Strata representing this epoch unconformably overlie Eocene sedimentary deposits in wells A and D, in which a hiatus of 24.3–24.8 Ma is recognizable. In well E, the uppermost interval (0–95 m [0–312 ft]) was deposited in continental to shallow marine environments sometime from Eocene–Miocene (Ypresian–Messinian; 50.3–5.33 Ma). So, this interval could be correlated with the uppermost sequence 4. In other wells, strata of this age are not recognized; this is probably because of either erosion or the strata not being sampled because of the casing of the upper 200 m of well.
In addition, the uppermost sample (80–85 m [262–279 ft]) from well D has an age range of Gelasian–Aquitanian (1.81–23 Ma). Because the age range is an uncertainty, and considering the correlation of this interval with well A, we interpret the age of this interval as Miocene (23.3–5.33 Ma). The ages of the correlatable surface formations in the region (Cantil Costero and Rosarito Beach) also support this age assignment.
The biostratigraphic data described here allow for the recognition of several hiatuses, or unconformities. However, without information on the structure of the strata, we cannot determine whether these are angular unconformities. The oldest hiatus is represented by either the nonconformable contact between upper Albian conglomeratic sandstones and the Cretaceous granitic rocks, or the disconformity between these conglomerates and the Albian limestones and volcaniclastic rocks of the Alisitos Group. Subsequently, the lack of upper Turonian strata in most wells, except in F and G, indicates a paraconformity or disconformity between middle and upper Turonian strata. In wells F and G, the limit between lower and upper Turonian coincides with a deepening of the depositional environments from transitional–neritic and an increase of siltstone and mudstone lithofacies (Figure 3B). Next, the unconformity between lower Paleocene and upper Paleocene strata is indicated by a paleobathymetric change from deeper (outer neritic–upper bathyal) to shallower (transitional–inner neritic) environments together with a lithological change from silty shale to sandstone. Finally, the lack of upper Eocene–Oligocene strata in all the wells clearly shows a regional unconformity between middle Eocene and Miocene–Pliocene strata.
The stratigraphic record in the VB represents deposition in continental–neritic marine environments during the Albian and Cenomanian (Figures 3 and 4). Subsequently, neritic–bathyal depth environments prevailed during the Turonian through the Eocene, followed by neritic, transitional, and continental deposits in the Neogene. The depositional environments represented in the studied sections commonly fluctuate from continental to neritic, in which palynological assemblages are varied and contain both continental and marine palynomorphs. Continental strata in the VB consists of conglomerates and sandstones characterized by the exclusive presence of terrestrial palynomorphs. Dinoflagellates appear in transitional–inner neritic deposits and become more abundant and varied in middle neritic–outer neritic environments. The abundance and diversity of the continental fossils diminish with depth, whereas marine taxa tend to reach their highest abundances in outer neritic environments.
Neritic environments in the VB are characterized by sandstones and siltstones with common to abundant benthonic foraminifera and scarce planktonic foraminifera. The most common Cretaceous–Paleogene benthonic foraminiferal genera in inner neritic–middle neritic environments are Eponides and Cibicides, whereas Dentalina, Nodosaria, Lenticulina, Gyroidina, Uvigerina, and Spiroplectammina are common in outer neritic environments.
Almost all wells contain intervals representing upper bathyal depth environments; however, the deepest paleodepths recorded in well H are outer neritic (100–200 m [328–656 ft] deep) although well H is located near the Lagunitas high. Cretaceous and Paleogene upper bathyal depth environments commonly contain Gyroidinoides soldanii, Gyroidina octocamerata, Cibicides coalingensis, Oridorsalis sp., and Dorothia sp. Middle bathyal deep environments are present only in a relatively short (100 m thick [328 ft]) Maastrichtian interval in well B and contain benthonic foraminifera representative of deep water, such as the following: Anomalinoides kennae, Osangularia plummerae, as well as the genera Ammodiscoides sp., Osangularia sp., Melonis sp., and Gyroidinoides sp.
Our paleobathymetric analyses allow the identification of three main transgressive–regressive (T–R) sedimentary cycles or sequences during the late Albian–middle Eocene evolution of the VB (Figures 3 and 4). Each T–R cycle begins with deposition of shallow transitional–inner neritic depth sedimentary rocks, follows with outer neritic–bathyal depth environments, and ends again with transitional–neritic depth sedimentary rocks. The three bathymetric cycles overlie either upper Albian volcaniclastic or calcareous deposits, or Cretaceous granitic rocks. Most of the strata filling the VB were deposited in neritic environments along a continental margin with maximum depths in each cycle commonly reaching upper bathyal depths. Green arrows in Figures 3 and 4 show the approximate locations of the deepest sedimentary environments that in turn assist in correlating individual sequences. The blue arrow in Figure 3 marks a middle bathyal interval in well B that represents the maximum depth within a cycle of sequence 3. These three T–R cycles underlie a thin Neogene sedimentary unit and a regional unconformity that separates middle Eocene from middle–lower Miocene strata.
As mentioned before, the basement in wells B and H is a granitic intrusion that was drilled and dated in wells B (135±9 Ma K/Ar, feldspar) and H (102±10 Ma K/Ar, feldspar, unpublished data from Pemex). However, basement rocks in wells D and E are volcaniclastic deposits topped by calcareous and volcaniclastic strata similar to the Alisitos Group. These latter rocks were deposited in the late Albian, as shown in well D, and have intraoceanic arc affinities, thus we interpret these strata to represent part of the arc-apron that locally constituted the basement of the fore-arc basin.
Bathymetric and Depositional Cycles
This cycle records the initial filling of the VB during the Albian. Basal sedimentary rocks of this cycle are mostly nonmarine–shallow marine deposits and are chronologically poorly defined. Probably, tectonically induced subsidence caused the basin seafloor to reach upper bathyal depths in the early–middle Turonian. The latter sedimentary rocks were locally eroded during deposition of the basal conglomerates of cycle 2. Wells D and G do not contain the maximum depths of this transgression. However, the transgression in well H is represented by outer neritic deposits that reached a maximum depth in the Cenomanian. The age of the lower boundary of this cycle does not match the age of any eustatic cycle, but the age of the maximum depths in this sequence roughly coincides with the early Turonian maximum transgression of the Cretaceous at circa 94 Ma (Hardenbol et al., 1998; Haq, 2014).
Initial deposition of this cycle includes conglomerate in wells B, C, D, and E (Figure 3A, B) that changed to fine-grained epiclastic deposits in the Maastrichtian. The transgression producing this cycle correlates with the late Turonian and commonly reached upper bathyal depths in the early Maastrichtian. In this cycle, well B reached middle bathyal depths, whereas wells H and D only reached neritic environments. The age of the lower boundary of this sequence matches with the base of the Tu3 cycle at circa 90 Ma, whereas the maximum depths in this sequence correspond to the late Maastrichtian transgression at circa 69 Ma (Hardenbol et al., 1998; Haq, 2014).
The uppermost bathymetric–depositional cycle in the VB began in the early Paleocene with deposition of neritic sandstones with subsequent deposition of bathyal shales during the late Paleocene–early Eocene. In wells A, B, D, E, and F, this cycle 3 reached upper bathyal environments in the middle Eocene, whereas in wells C, G, and H, cycle 3 reached only middle–outer neritic depths. The age of the lower boundary of this cycle matches with the base of the Da 4 or Sel 1 cycles at circa 62 Ma, whereas the age of the maximum water depths interpreted in this cycle coincide with the early Eocene transgression at circa 53 Ma (Hardenbol et al., 1998; Haq, 2014).
The youngest bathymetric–depositional cycle drilled comprises a thin (<200 m [<656 ft]), sandstone–siltstone unit representing neritic conditions that unconformably overlies the Paleogene cycle 3. Sequence 4 is a Neogene unit found only in wells A and D and was deposited in the early–middle Miocene as indicated in well A, in which it represents middle neritic environments. Neither the limits of, nor the transgression represented by this cycle can be accurately correlated with any recognized Neogene eustatic event. Thus, this transgression likely represents a response to tectonically induced subsidence.
We recognize the three main depositional cycles in the VB through our paleobathymetric and biochronologic analyses. These cycles accumulated three major sequences separated by significant depositional hiatuses that we interpret to be unconformities. Therefore, we propose that these deposits represent three distinctive sequence stratigraphic units that filled the VB fore-arc basin from the late Albian through middle Eocene. Seismic lines in the VB (Ramos-García, 1976; García-Serratos, 2013) indicate the presence of stratigraphic units correlative throughout the basin (see figures 23 and 24 in García-Serratos, 2013) that roughly correspond to the three stratigraphic sequences described here. A poor quality seismic image from the onshore area, north of Guerrero Negro (figures 4, 6, and 7 in Ramos-García, 1976), shows thicker stratigraphic sequences near well C along the basin axis approximately connecting wells A–C to well F (Figure 1).
The chronostratigraphic correlation of unconformity-bound units and sedimentary cycles described here with Cretaceous–Cenozoic regional stratigraphic units are the basis for naming these subsurface sequences as Valle, Rosario, and Sepultura sequences as parts of the VB (Figure 5). The Alisitos arc interval forms part of the basement, and the overlying post–fore-arc Neogene transgressive cycle is here named the Tortugas sequence in reference to coeval marine deposits cropping out in the Pacific shoreline in the Vizcaino Peninsula (Helenes-Escamilla, 1980).
The VB is underlain by volcanic rocks representing an island arc setting, or alternatively, the VB overlies granitic rocks on the east, representing exhumation of arc-related intrusives. The boundary between the Alisitos rocks and rocks from the subduction complex on the west is not well defined, but onshore outcrops suggest a tectonic origin (Sedlock, 1988). Our data indicate that volcaniclastic and calcareous strata similar to those described for the Alisitos Formation in the type locality (Allison, 1974) were deposited in the late Albian in the southern part of the VB (see wells D and E). These basement rocks, as well as the stratigraphic continuity shown by the seismic data, suggest that the strata forming the fill of the VB overlies either Cretaceous granitic rocks of the Peninsular batholith or Lower Cretaceous volcaniclastic and calcareous strata correlative with the Alisitos Group. Following Noda (2016), we postulate that the basement for the VB in the west is represented by rocks of the subduction complex that include ophiolite rocks and rocks from the arc complexes exposed in the Vizcaino Peninsula and that, overall, represent the accretionary wedge. Sequences 1, 2, and 3 contain sedimentary rocks derived from the batholith and Lower Cretaceous volcanic rocks (Kimbrough et al., 2001), and their seismic continuity indicates that they constitute the inner wedge of a convergent margin (Noda, 2016) that developed a fore-arc basin during this time interval (Busby, 2004).
The four sequence boundaries represent changes in the sedimentation in the VB and are most likely related to regional plate-tectonic events. The characteristics of these boundaries are as follows. Initially, the lower boundary of sequence 1 is indicated by the unconformity between the Valle Group and the subduction complex that represents the Albian–Cenomanian onset of the fore-arc sedimentation. Subsequently, the lower boundary of sequence 2 is distinguished by an intermediate Turonian–Coniacian hiatus underlying the Rosario Group. Afterward, the lower boundary of sequence 3 is distinguished by a Maastrichtian–Paleogene local unconformity and shallowing that likely represents a lowstand sea level that exposed parts of the local fore-arc basin. Finally, the lower boundary of sequence 4 overlies a Paleogene–Neogene unconformity that represents the inversion of the fore-arc basin and partial erosion of sequence 3.
The geologic significance of these boundaries is probably related to the following tectonic conditions. Initially, sequence 1 represents the beginning of subsidence and sedimentation in the fore-arc basin in the late Albian (ca. 105 Ma). At this time, subsidence took place after the emplacement of the eastern PRB (Gastil, 1975) and developed into a flat-bottomed basin with neritic depths along a northwest–southeast axis extending from well A to H. The basin reached upper bathyal depths in the Turonian, coincident with the widespread Cretaceous eustatic transgression (Haq, 2014). The subduction complex in the Vizcaino Peninsula was already attached to the southwestern part of the basin as indicated by the presence of the plutonic clasts in conglomerates in the Valle Group at this time (Kimbrough et al., 2001).
The absence of sequence 1 in outcrops from San Diego to El Rosario and in wells D and E is indicated by the unconformity of sequence 2 with the underlying Alisitos Group. This unconformity represents an erosional episode that was probably caused by a compressional event hypothetically related to the influence of the Kula–Farallon spreading ridge as it moved northward along western North America in the Late Cretaceous (Engebretson et al., 1985; Umhoefer, 1987). Coincidentally, a paleogeographic reconstruction of western North America locates the Kula–Farallon ridge near the southern part of the VB at 85 Ma (Blakey, 2003).
Subsequently, deposition of sequence 2 started in the Turonian (ca. 92 Ma), as indicated in wells C (Figure 3) and F (Figure 4). However, in most wells in the north, south, and west of the basin, the lower limit of sequence 2 is Santonian (ca. 86 Ma) and indicates renewed sedimentation after the compressive regime ended.
Afterward, sequence 3 initiated in the early Paleocene (ca. 62 Ma) after a regressive event that caused a widespread hiatus or allowed deposition of sandy intervals in shallow marine environments. Regionally, this hiatus is expressed in outcrops from San Diego to El Rosario and the southern Vizcaino Peninsula, where there is a Maastrichtian–Paleogene unconformity. Both, the unconformity and the shallowing conditions were likely produced by a regional response to the Laramide flat subduction beneath southwestern North America (Lovera et al., 1999; Kimbrough et al., 2001; English and Johnston, 2010).
Finally, sequence 4 lies above the regional Eocene–Miocene unconformity. The scarcity of the upper Paleogene and Neogene sedimentary record is also observed in northwestern Baja California and was created during a compressive event and basin uplift related to the consumption of the Farallon plate and the first collision of the Pacific–Farallon spreading ridge against western North America at circa 37 Ma (Atwater, 1989; Lonsdale, 1991).
Sequence 1 (Valle)
The oldest fore-arc sequence (Figure 5) comprises upper Albian–lower Turonian coarse to fine-grained continental–shallow marine deposits and correlates with the upper part of the Valle Group in Cedros Island and the Vizcaino Peninsula. In Cedros, the Morro Redondo (Kilmer, 1984) and Pinos (Smith et al., 1993b) Formations and the Malarrimo Formation in the eastern side of the Vizcaino Peninsula (Berry and Miller, 1984; Patterson, 1984b) are also correlative with our sequence 1. The lower part of the Valle Group in Cedros and Vizcaino (Kimbrough et al., 2001) could be interpreted as the distal deposition in the outer wedge of the trench–slope setting of the convergent margin. The sedimentary environments represented by the sequence 1 in the VB indicate a regional transgressive event that gradually changed from continental and transitional in the Albian–Cenomanian interval to neritic and upper bathyal in the Turonian. Our interpretation of these depositional environments is consistent with environments indicated by the benthonic foraminifera reported by Berry and Miller (1984) in strata from the Vizcaino Peninsula. These authors noted the presence of several planktonic species together with the benthonic taxa Globorotalites, Gavelinella, Gyroidinioides, and Spiroplectammina that are indicators of outer neritic–upper bathyal environments (Sliter and Baker, 1972). The presence of Marssonella oxycona in one of the samples from the Los Pinos Formation in Cedros Island suggests that the upper strata were deposited at middle bathyal depths of more than 500 m (>1640 ft) (Smith et al., 1993a). Sequence 1 in wells D and E in the southern part of the VB overlie volcaniclastic and shallow-water calcareous strata that correlate with the Aptian–Albian Alisitos Formation. Two samples from the Malarrimo Formation in the northern Vizcaino Peninsula (Robinson, 1975) contain benthonic foraminifera of Coniacian–Santonian age. In general, the facies represented in the VB indicate deeper environments toward the Vizcaino Peninsula to the southwest.
Sequence 2 (Rosario)
This sequence includes terrigenous deposits from late Turonian–late Maastrichtian and is correlative with the Rosario Group (Figure 5) exposed along the peninsular margin from San Diego County in southern California to the El Rosario area in western Baja California. Sequence 2 is also partially correlative with the older strata from the Upper Cretaceous–Paleogene Valle Group from the southern Vizcaino Peninsula (Patterson, 1984b; Kimbrough et al., 2001).
The sedimentary environments represented by sequence 2 range from continental and transitional marine in the middle–upper Turonian, upper bathyal in the Maastrichtian of most wells and reach middle bathyal depths in well B. Paleontological data from sections cropping out at the type locality of the Rosario Formation (Kilmer, 1963; Acosta, 1966; Patterson, 1978; Yeo, 1982; Boehlke and Abbott, 1986; Maestas et al., 2003) and in correlative strata in San Diego County (Gastil and Higley, 1977; Yeo, 1982) also indicate continental–upper bathyal depositional settings. Deep-water structures have been interpreted in Maastrichtian strata cropping out near San Fernando, south of El Rosario (Morris and Busby-Spera, 1988; Busby et al., 1998; Dykstra and Kneller, 2007). Foraminifera (Dykstra and Kneller, 2007) and ichnofossil assemblages (Callow et al., 2013) reported from the San Fernando outcrops indicate middle bathyal paleoenvironments during deposition for these rocks. These paleobathymetric interpretations correspond to the deepest paleoenvironments interpreted for the Maastrichtian interval in well B. However, sequence 2 biofacies observed in the wells studied are mostly neritic, similar to those reported in strata from outcrops of the Rosario Group from Punta Canoas to San Diego County.
Sequence 3 (Sepultura)
This sequence spans lower Paleocene–middle Eocene (Figure 5) and represents the final stage of deposition in the fore-arc basin. The lower part of this sequence correlates with the Paleocene Sepultura Formation in El Rosario area where the upper part correlates with the Delicias and Buenos Aires Formations described near Tijuana (Flynn, 1970) and the La Jolla Group of the San Diego area (Kennedy and Moore, 1971; Gastil and Higley, 1977; Yeo, 1982). To the south, the sequence 3 correlates with the Paleocene–Eocene Bahía Ballenas Formation from the southern part of the Sebastian Vizcaino Peninsula (Abbott et al., 1995) and the Bateque Formation reported near San Ignacio in southern Baja California (Mina, 1957; Smith et al., 1993a). The facies described here are similar to those outcrops of correlative strata in the El Rosario area where a local angular unconformity is documented (Abbott et al., 1993).
Sequence 4 (Tortugas)
Finally, the youngest sequence is Neogene (Figure 5) and unconformably overlies the Paleogene Sepultura sequence. Sequence 4 is widely represented by the Tortugas (Mina, 1957; Kilmer, 1979, Helenes-Escamilla, 1980) and Almejas Formations (Mina, 1957; Smith, 1984) in Cedros Island and the northern Vizcaino Peninsula. However, sequence 4 is scarce in northwestern Baja California, where it is represented by the Rosarito Beach Formation reported from San Diego to Ensenada (Minch, 1967; Gastil and Higley, 1977; Yeo, 1982) and by the Cantil Costero Formation (Santillán and Barrera, 1930), which crops out near El Rosario. South of the VB, the San Ignacio and San Isidro Formations (Mina, 1957) also represent Neogene marine sedimentation. Thus, sequence 4 presents shallow marine facies on the continental shelf that correlate with reported shallow marine and volcanic deposits cropping out in northwestern Baja California. Outcrop facies in the south represent neritic environments, whereas correlative deposits in the Vizcaino Peninsula reach bathyal depths (Helenes-Escamilla, 1980). Subsidence of the continental shelf in the Neogene is probably related to the westward tilting of the Baja Peninsula because of continental rupture in the Gulf of California (Mark et al., 2014). Rift-related asymmetric flexure of the crust probably promoted subsidence of the continental shelf to the west and marine flooding. However, isostatic sea-level changes may also have been a factor in promoting shelf sedimentation during highstand sea-level conditions; however, the lack of complete stratigraphic record for sequence 4 prevents accurate correlation with the Neogene sea-level curve.
Evolution of the Fore-Arc Basin
The stratigraphic record of western Baja California has been described in terms of tectonostratigraphic terranes to emphasize their distinct origin (Coney et al., 1980; Campa and Coney, 1983; Gastil, 1985; Sedlock, 1988; González-León, 1989; Sedlock, 2003). Alternatively, they have been related to an arc system (Busby-Spera, 1988; Morris and Busby, 1996; Busby et al., 1998; Busby, 2004) or even to allochthonous terranes coming from southern Mexico and Central America (Hagstrum et al., 1985; Howell et al., 1985; Beck, 1986; Morris et al., 1986; Lund et al., 1991; Sedlock, 2003). However, stratigraphic data from the Rosario Formation (Helenes and Téllez-Duarte, 2002; Téllez-Duarte et al., 2006) as well as new paleomagnetic data from the PRB plutons (Vaughn et al., 2005) and the Valle Group (Kimbrough et al., 2006) indicate that both the Peninsular Ranges and the Vizcaino terrane have remained in place with respect to cratons in North America since the Cretaceous.
The units described here reflect different tectonic regimes. The Alisitos sequence is the oldest unit recognized here and represents deposition of volcanic rocks with oceanic arc affinities in an extensional fringing arc (Busby, 2004). The uppermost part of this unit is characterized by the biohermal limestones that were exposed along the western side of the PRB (Suárez-Vidal, 1987) and that were drilled in wells D and E. Subsequently, sedimentation in the fore arc initiated in mostly transitional–neritic environments over a continental margin formed by the fringes of the PRB and the Alisitos arc strata as indicated by the basement units sampled by Pemex drilling.
Regional volcanic units with oceanic affinity indicate a tectonic regime that represents an earlier phase of an extensional island arc (Busby et al., 1998; Busby, 2004), so they are excluded from the fore-arc basin stage. These volcanic units, which underlie the Valle and Rosario Groups as used here, are included in the Lower Cretaceous Alisitos volcaniclastic sedimentary group. We also exclude most rock units exposed in the Cedros Island–northern Vizcaino Peninsula area. These latter units are interpreted to be part of the subduction complex developed at the trench slope and progressively exhumed during the termination of the regional subduction. These rocks include the Triassic–Jurassic ophiolitic complex, a Jurassic tectonic mélange, the Jurassic volcaniclastic rocks, and the Albian lower part of the Valle Group (Kilmer, 1984), including the Valle and Vargas Formations in Cedros Island (Mina, 1957; Smith et al., 1993b) and Los Chapunes Formation in northern Vizcaino Peninsula (Patterson, 1984b).
The boundaries of the sequences described here represent major changes in the sedimentation of the VB basin. These boundaries represent the onset of the fore-arc sedimentation (sequence 1), two fore-arc events—first, a Turonian–Coniacian unconformity marking the base of sequence 2 and later a Maastrichtian–Paleogene unconformity separating sequences 2 and 3—and finally, the post fore-arc sedimentation (sequence 4).
Onset of Fore-Arc Sedimentation
Onset of fore-arc sedimentation began after the synbatholithic crustal shortening noted in the Sierra San Pedro Martir region (Johnson et al., 1999). A rapid exhumation of cretaceous intrusives and prominent influx of coarse-grained clastic deposits. This requires steep topographic gradients in the region as already noted by Kimbrough et al. (2001) in the Vizcaíno Peninsula.
Intra Fore-Arc Events
The older event caused a widespread Coniacian gap in the stratigraphic record in the VB that corresponds to the upper boundary of sequence 1 (Figure 5). This unconformity spans the entire sequence 1 (Cenomanian–Santonian) in outcrops from San Diego to El Rosario and in wells D and E. This hiatus is hypothetically linked to the compression of the Kula–Farallon ridge against the North American plate. The younger Maastrichtian–Paleogene hiatus constitutes the base of sequence 3 (Figure 5) and represents a significant regressive event. We interpret this regression as a response to Laramide flat subduction with subsidence and sedimentation in the fore-arc basin following the Laramide orogenic event, as indicated by upper bathyal depths recorded in wells A, B, E, and F (Figures 3 and 4). Finally, an angular unconformity separates the depositional record of the fore arc and the development of the transtensional plate boundary in both margins of the Baja California continental block (Martín et al., 2015). The youngest sequence 4 (Figure 5) is related to margin subsidence after late Oligocene collision of the Pacific–Farallon ridge (Atwater, 1989). This collision produced a compressive event and uplift of the subduction complex and inversion of the fore-arc basin. The regional unconformity separating sequences 3 and 4 and the scarcity of the Neogene sedimentary record in northwestern Baja California collectively support our interpretation. The consumption of the Farallon plate and the first collision of the Pacific–Farallon spreading ridge against western North America at circa 37 Ma (Atwater, 1989; Lonsdale, 1991) likely caused this compression and inversion of the Vizcaino basin.
Our micropaleontologic and lithologic data from eight exploratory wells in the VB provide a chronostratigraphic and paleoenvironmental framework to analyze the evolution of the continental margin in the context of the regional geologic history of southwestern North America.
Our analyses document that the subsiding fore-arc basin experienced three major transgressive–regressive depositional cycles from the Albian through middle Eocene forming three first-order unconformity-bound stratigraphic sequences. Each sequence displays evidence of different environments and correlates with stratigraphic units cropping out along coastal southern California and Baja California. Basal sequence 1 (Valle) marks initial sedimentation in the basin and correlates with the Cretaceous Valle Group, sequence 2 (Rosario) correlates with the Upper Cretaceous parts of the Rosario Group, and sequence 3 (Sepultura) correlates with the Paleogene Sepultura and Bateque Formations. Post fore-arc Neogene marine sedimentary rocks unconformably overlie the fore-arc basin fill, comprise sequence 4 (Tortugas), and correlate with the Tortugas Formation and other wedge-shaped marine and volcanic deposits on the continental shelf of Baja California.
Basement rocks that underlie the basin-fill in the exploratory wells include volcaniclastic deposits cap by bioclastic limestones representing the late Early Cretaceous (Albian) Alisitos arc-apron on the south. Cretaceous granitic rocks of the PRB underlie the basin along the eastern margin. Triassic–Jurassic ophiolitic rocks and tectonic mélange exposed on the Vizcaino Peninsula represent an additional basement unit to the west but only reported in the Lagunitas structural high (Martín et al., 2015).
Basin sedimentation began in the late Albian because of continental margin subsidence, with sequences 1, 2, and 3 subsequently deposited in a convergent continental margin setting. Initial deposition of shallow marine sedimentary rocks of sequence 3 signals the end of a regressive event probably linked to local influence of the Laramide orogeny followed by renewed Paleogene basin subsidence.
Deepest paleobathymetries in the evolving fore-arc basin were reached during the early Turonian, Maastrichtian, and late Paleocene–early Eocene and were correlative with peaks in global maximum flooding surfaces recorded at 98.3 Ma, 72 Ma, and 54 Ma by Hardenbol et al. (1998).
A significant Coniacian unconformity indicates basin inversion because of compression and is hypothetically related to the collision of the Kula–Farallon ridge against North America. The youngest major unconformity separates sequences 3 and 4 encompassing the late Eocene through Oligocene. This angular unconformity records inversion of the VB and subsequent partial erosion of sequence 3 strata as a function of (1) the collision of the Pacific–Farallon ridge with the continental margin at 37 Ma, (2) cessation of subduction of the Farallon plate beneath the continental margin, and (3) the initiation of a translational–transtensional tectonic regime in this region that is related to progressive capture of the Baja California microplate by the Pacific plate. The Vizcaino basin records an important event in the evolution and growth of the continental margin of southwestern North America.
Petróleos Mexicanos Exploración y Producción financed this study and provided samples and well log data for the analyses, and the Centro Nacional de Información de Hidrocarburos of the Comisión Nacional de Hidrocarburos (CNH) of México granted permission to publish this manuscript (CNH consent letter no. 271.071 /2016). We thank technician E. Collins (CICESE) for processing samples and technician V. Frias (CICESE) for assistance with figures. We also thank reviewers whose comments and suggestions improved our report.
Javier Helenes obtained a geological engineering degree from Instituto Politécnico Nacional, Mexico, and an M.S. degree and Ph.D. from Stanford University, California. He has worked as a biostratigrapher (foraminifera and dinoflagellates) in Switzerland, Canada, and Venezuela, and since 1995, he has been a researcher at the Departamento de Geología at CICESE, where he has studied Cretaceous–Holocene dinoflagellates from tropical areas. Javier Helenes is the corresponding author of this paper.
Arturo Martin-Barajas obtained a geological engineering degree from Universidad Nacional Autónoma de México (UNAM), and an M.S. degree and Doctorate from the Université de Paris-Sud, France. He worked as a geologist in the Consejo de Recursos Minerales, and since 1990, he has been a researcher at the Departamento de Geología at CICESE, where he has studied volcanism, tectonics, and stratigraphy of basins around Baja California.
Juan Gabriel Flores-Trujillo obtained a Ph.D. at CICESE, studying taxonomy, paleoecology of dinoflagellates, and their relation to paleo-oceanography. Currently, Flores-Trujillo is working at the UNACAR, Campeche, México. Flores-Trujillo is interested in dynamics of coastal processes and environments as well as palynology and climatic change and is a member of the Red Temática sobre Florecimientos Algales Nocivos (Network on Harmful Algal Blooms) in México.
Iraida Paredes obtained a B.S. degree and an M.S. degree in biological sciences in Venezuela. She also received extensive training in palynology at ETH-Zürich in Switzerland, Amsterdam University, Robertson Research, and Petrobras (CENPES) in Rio de Janeiro. From 1979 to 2003, she worked as a palynologist and stratigrapher at Petróleos de Venezuela-Instituto de Tecnología Venezolana para el Petróleo (PDVSA-INTEVEP), and she is currently working as a palynological consultant in Venezuela.
Maritza Canache has a geological engineering degree from Venezuela, and she has received several training courses at PDVSA-INTEVEP, where she worked from 1996 to 2003 as a stratigrapher and micropaleontologist (foraminifera) studying all the basins in Venezuela. She has also worked material from Guatemala and México, and she is currently a consultant in Venezuela.
Ana-Luisa Carreño obtained a Ph.D. in micropaleontology and is working at the Instituto de Geología, UNAM. She is interested in Cenozoic biostratigraphy using foraminifera, ostracoda, and calcareous nannoplankton in sequences related to the opening of the Gulf of California. She has published over 100 technical and scientific papers, reviews, miscellaneous publications, and book chapters and teaches and directs undergraduate and graduate theses.
Adriana Miranda is interested in biostratigraphy of foraminifera and dinoflagellates and is currently finishing a Ph.D. on the age of marine deposits related to the evolution of the Gulf of California in the Neogene. She is in charge of the micropaleontology database at the National Museum of Paleontology, and she teaches earth sciences and paleobiology at UNAM.