Geology of La Reforma caldera complex, Baja California, Mexico

A new geological map at 1:50,000 scale of La Reforma Caldera Complex has been produced applying modern survey methodologies to volcanic areas. This map aims to represent a reliable and objective tool to understand the geological evolution of the region. La Reforma Caldera Complex is a Pleistocene nested caldera located in the central part of the Baja California peninsula, Mexico. The twelve formations de ﬁ ned within the Quaternary volcanic record were grouped into three phases (pre-caldera, caldera, and post-caldera). The pre-caldera phase (>1.35 Ma) is characterized by scattered eruptions, mostly occurred in submarine environment. The caldera phase (1.35 – 0.96 Ma) groups several distinct explosive and e ﬀ usive eruptions that formed the present-day caldera depression. The post caldera phase includes scattered e ﬀ usive eruptions (ended at 0.28 Ma) and resurgence, characterized by several hundred meters of uplift of the central block within the caldera depression.


Introduction and previous studies
Geological maps have to be objective and detailed because they represent a fundamental tool to understand the geological evolution of a given area, and, when mapping is conducted at relatively high scale, they provide a sound basis for detailed studies aimed at assessment of natural hazards, and exploitation of geothermal potential and/or ore deposits. For this reason, within the project CEMIE GEO (Centro Mexicano de Innovación en Energía Geotérmica), which involves different sites in Mexican Quaternary volcanic districts, we surveyed La Refoma Caldera Complex area and produced a new geological map at 1:50,000 scale Main Map.
La Reforma Caldera Complex is located in the central part of the Baja California peninsula, 35 km to the NW of the town of Santa Rosalía (Demant, 1984). Along with the Aguajito Caldera (Garduño-Monroy, Vargas-Ledezma, & Campos-Enriquez, 1993) and the Tres Vírgenes volcanic complex (Avellán et al., 2018) La Reforma caldera complex represents a manifestation of the Quaternary volcanism in the central part of the Baja California (Figure 1). The first geological contributions in the region focused on the Plio-Pleistocene sedimentary rocks of the Santa Rosalía Basin (Ortlieb, 1978;Ortlieb & Colleta, 1984;Wilson, 1948;Wilson & Rocha, 1955) because of the exploitation of copper and manganese minerals. Wilson and Rocha (1955) provided a detail geological map of the Santa Rosalía area, but did not include La Reforma range, located to the northwest. Schmidt (1975) published the first geological map of the Sierra La Reforma that was interpreted as a complex system of uplifted tectonic blocks. One of these blocks was made of crystalline granodioritic rocks dated by the Author at 91.2 ± 2.1 Ma by K-Ar method. Demant and Ortlieb (1981) recognized the Sierra La Reforma as a resurgent caldera, and Demant (1984) presented a regional geological map of Santa Rosalía with a mineral and petrological characterization that included La Reforma caldera. In 1982, the National Power Company (Comisión Federal de Electricidad = CFE) began geological and geophysical prospecting in the Tres Vírgenes area (southwest of La Reforma) for geothermal exploration (e.g. Lira, González, & Arellano, 1997;Lira, Ramirez, Herrera, & Vargas, 1983) followed by its exploitation in 1986. The CFE survey involved other prospecting areas as El Aguajito (Garduño-Monroy et al., 1993) and set the basis for new studies of Tres Vírgenes (Capra, Macıas, Espındola, & Siebe, 1998;Sawlan et al., 1981;Schmitt, Stockli, & Hausback, 2006;Schmitt, Stockli, Niedermann, Lovera, & Hausback, 2010) with the updated geology of the Tres Vírgenes volcanic complex (Avellán et al., 2018;Macías et al., 2012). Schmitt et al. (2006) dated the age of La Reforma (1.3 Ma) and Aguajito ignimbrites (1.2 Ma) with the U-Pb method in zircon, with the aim of understanding the general evolution of the Quaternary volcanic centres northwest of Santa Rosalia.
Despite these efforts, no detailed studies of La Refoma were carried out to produce a comprehensive volcanological map and evolution of the caldera. The CEMIE GEO project, a Mexican initiative to carry out modern geothermal studies, allowed to perform new investigations of La Reforma caldera, aimed at understanding its geothermal potential.
Here, we present the first contribution of this project with the revised geology and stratigraphy of La Reforma caldera complex that was assisted by 40 Ar/ 39 Ar and U-Pb geochronology (Table 1) from previous studies and new data from the PhD work of García-Sánchez (2019). This new map will set the basis for modern studies in the area, which will be helpful to future prospecting for ore deposits and geothermal potential areas, as well as paleogeographic reconstructions of marine depositional systems and mineralization.

Methods
The geological map, here presented at 1:50,000 scale, has been surveyed at 1:25,000 scale in a logistically difficult area due to the absence of any roads and access feasible only by the coastline and a few mountain pathways ( Figure 2).
The fieldwork was preceded by critical review of existing literature for the studied area, along with analyses of satellite images (including ESRI and Google images) and of a 25 m pixel-sized digital elevation model (DEM) and its derived morphological features (slope aspect, shaded relief, 3D visualization, etc.).
The geological survey, carried out during 5 field campaigns (from 2014 to 2017), has been realized using INEGI topographic map and iGIS® software on iPad mini equipped with Google Earth images. Geological survey was carried out on lithostratigraphic basis, which allowed for the definition of various formations. Indeed, lithostratigraphic units are the only ones easily recognizable in the field because of their lithology and stratigraphic position (Groppelli & Martì-Molist, 2013;Groppelli & Viereck-Goette, 2010; Martí, Groppelli, & da Silveira, 2018; Salvador, 1994). In addition to the stratigraphic position, we used abrupt change in lithology, presence of palaeosols, or intercalation of marine sediment to define formations.
In order to summarize the volcanic succession, we have identified three main phases (pre-caldera, caldera and post-caldera) on the basis of the relationships of the deposits with the caldera evolution. Moreover, we have grouped all the pre-Quaternary rocks into a 'sedimentary and igneous basement' unit.
During field mapping, we have collected samples for petrographic and geochemical analyses and age determination of rocks. Petrographic data allowed for a better characterization of the lithostratigraphic units, whereas radiometric data allowed constraining formations and lithosomes, thus helping reconstruct the evolutionary phases of La Reforma caldera.
All the field data have been stored in a geodatabase, using iGIS software in the field and then ArcGis® platform during the preparation of the geological map.
The colour scale chosen for representation of lithostratigraphic units on the map follows the three evolutionary phases: pre-(brown shaded), syn-(red shaded) and post-(green shaded) caldera phases. A scale of light blue was used to represent the sedimentary and igneous basement units, whereas blue colour was used for the Cretaceous basement (granitoids) in order to highlight its location. This was important in order to highlight the resurgence of the central part of the caldera. Regarding symbols, we have marked crater rims, pit crater rims, caldera rim, bedding attitude, marine terraces and faults. Dykes have been marked by lines too. Insets at 1:20,000 scale highlight the detailed stratigraphic succession in two deep fluvial valleys to the west and south-west.
In addition to the legend, we have included the following elements into the map: (1) an inset showing the geographic location of the mapped area, (2) a stratigraphic scheme to illustrate the relationships among The analyses were performed at: (1) Laboratory of Isotopic Studies of the Centro de Geociencias of the UNAM, Juriquilla, and (2) Geochronology Laboratory at the University of Alaska, Fairbanks. Further details on age determinations are given in García-Sánchez (2019). Figure 2. Geological map on a shaded topography (INEGI data). Numbers refer to the lithostratigraphic units described in Table 2, except (16) Recent alluvial deposits.  (Figures 4(c) and 5(c)). Lava flows are usually thin (2-5 m) and alternated to scoriae. Dikes and small sub-intrusive bodies are also present ( Figure 6(a and b)). Lavas display large variability in texture (from aphyric to porphyritic, with phenocrysts of pl, px, and Fe-Ti oxides).
The formation crops out both inside and outside the caldera Mainly basaltic-andesitic and andesitic lavas.

Locally rhyolitic lavas
This formation includes all the deposits formed after the caldera phase. The volcanic activity is scattered and low volume respect the previous phase. Inside the caldera depression usually the centres are located along the ring faults. It is possible to identify several lithosomes (see Table 3). Arroyo Grande member 15b Sands to gravels deposits made of lavas and plutonic rocks with interbedded three thin (<1 m) white fallout deposits ( Figure 6 (c)). The deposits are matrix supported, partially loose, and coarsely stratified (0.5-1 m) with parallel and lenticular shapes. The total thickness is up to 50 m.
Western side of the caldera depression The deposits form alluvial fans within the western part of the caldera depression probably due to the dismantling of the resurgent block.  (Schmitt et al., 2006).

Mesa El Yaqui formation 12
Blocky lava flow succession, dark-grey in colour, associated with few scoria layers. Aphyric to porphyritic lavas with phenocrysts of pl, ol, px and Fe-Ti oxides. The thickness varies from 3 to 20 m.
These lava flows cover most of the external slopes of the caldera depression, and locally also of its inner walls.
The lavas are basalticandesite in composition.
This formation represents an effusive period during the caldera-forming phase, when most of the caldera depression was already formed. Age: 1.18 ± 0.46 Ma. La Reforma ignimbrite 11 Pyroclastic deposits consisting of basal welded, reddish bed overlain by various welded brownish to dark-grey deposits ( Figure 5(b)). Welded deposits contain black fiamme pumice and accidental lithics (lavas). Loose crystals of pl and px occur in ashy matrix. The thickness is up to 40 m.
This formation crops out in limited areas inside the caldera depression and widely outside the caldera.

Rhyolite
This ignimbrite represents one of the main events caldera-forming and responsible of most present caldera morphology. Age: 1.29 ± 0.02 Ma.
Cerro La Reforma formation 10 Succession of thick lava flows (up to 10 m) associated with minor pyroclastic deposits ( Figure 5(d)). The formation comprises also lava domes along caldera rims (in map n. 10a). Lavas are porphyritic with phenocrysts of pl, px and Fe-Ti oxides.
Xenocrysts of qtz and sedimentary xenoliths are also present. The thickness is more than 400 m because the base is always not visible.
This formation crops out only in the central part of the caldera depression (resurgence block). The original depositional surface appears tilted to SE.
The lavas are dacitic in composition.
This formation represents the first lava flow filling of the caldera depression, later tilted during the resurgence. Age: 1.27 ± 0.02 Ma.

Los Balcones ignimbrite 9
Grey welded pyroclastic deposit made of light-grey pumice and dark grey fiamme, with abundant lithics (grey, black and white lavas) (Figures 4(c) and 5(a and b)). Abundant loose crystals of pl and minor px in greyish ash-rich matrix. The thickness is up to 30 m.  Hyaloclastites, pillow lavas and lava domes with columnar jointing (Figure 4(a and b)). Lava texture varies from aphyric to porphyritic with phenocrysts of pl and px.
This formation crops out widely in the NW and SE sectors of the caldera depression, both inside and outside.
Lava composition varies from basaltic-andesite to andesite with minor trachy-dacite.
The formation includes effusive episodes, mainly submarine, affecting the area immediately before the onset of the caldera phase. Age: 1.42 ± 0.05 Ma (pillow lavas); 1.36 ± 0.06 Ma (lava dome). Contrabando formation 7 Stratified, light-yellow pyroclastic deposit made of white and grey pumices (Figure 4(a)). Loose crystals of pl and px occur in yellowish ash-rich matrix. The thickness is up to 15 m.
This formation crops out only in the SE area along the external slopes of the caldera depression.

ND
The formation represents one of the scattered explosive events affecting the area before the caldera phase. The source area is unknown due the scarce outcrops. Age: 1.47 ± 0.08 Ma. Carrizo ignimbrite (not mappable) Red welded pyroclastic deposit containing yellow and grey pumices and abundant lithics (green or grey lavas and black scoriae) (Figures 4(c) and 5(a)). Abundant loose crystals of pl in reddish ash-rich matrix. The thickness is up to 10 m.
This ignimbrite crops out only in limited areas along deep valleys all around the caldera depression.

Rhyolite
The formation represents one of the scattered explosive events affecting the area before the caldera phase. The source area is unknown due the scarce outcrops. Due to the reduce thickness and few outcrops, this formation is not mappable at the present scale. Age: 1.89 ± 0.27 Ma. Cueva Amarilla ignimbrite 6 Greenish grey pyroclastic deposit with white pumice at bottom and black scoriae at top (Figure 4(a and c)). Loose crystals of pl and px occur in greenish ash-rich matrix. The thickness is up to 30 m.
This ignimbrite crops out in the S and N sector along the external slopes of the caldera depression.

ND
The formation represents one of the scattered explosive events affecting the area before the caldera phase. The source area is unknown due the scarce outcrops. Age: 2.4 ± 1.5 Ma. Mesa de Enmedio ignimbrite 5 Light-grey pyroclastic deposit made of pumices (10-50 cm long). Loose crystals of pl and px occur in yellow ashy matrix. The thickness is up to 5 m.
This formation crops out only close to the Punta Arena

ND
The formation represents one of the scattered explosive events affecting the area before the caldera phase. The source area is unknown due the scarce outcrops.
Brownish siltstones (up to 30 m thick) at base, overlain by fossiliferous sandstones and conglomerates (5-20 m thick). Fossils mainly consist of bivalves (Pectinidae) and well-preserved oysters and sea urchins.
This formation crops out mainly along the coast and deep valleys south of the caldera depression. Inside the caldera, it crops out in the resurgent block and locally at the base of the caldera inner walls.
This formation, known also as Formación sedimentaria de la cuenca de Santa Rosalia, Auct. (Ortlieb & Colleta, 1984;Wilson, 1948), represents the marine sedimentation, during which coeval scattered volcanic activities happened. The Santa Rosalia basin is a tectonic depression due to the opening of the California Gulf. Age: Pliocene-Middle Pleistocene (Ortlieb & Colleta, 1984). Sedimentary and igneous basement Santa Lucia formation 3 Lava flows and domes, associated with dikes. Lavas with aphyric to porphyritic texture and phenocrysts of pl and px and amph. The formation, interlayered with the Comondú Group (see n. 2), is up to 80 m thick.
This formation crops out only in the south margin of the map.
Lavas are andesitic to basaltic in composition.
Andesite-dominated arc lava suites erupted when the Gulf region was affected by oblique-subduction of the Farallon-Guadalupe plate under the North America plate along the western margin of Baja California (Conly et al., 2005). Age: 19.25 ± 0.08 Ma. Comondú Group 2 Red sandstones, siltstones and conglomerates interlayered with lavas (Santa Lucia Formation, see n. 3) (Figure 3(b)). The thickness is up to 100 m, but the base is always not visible.
This group crops out only in the south margin of the map and some deep valleys.
Volcanic and sedimentary unit deposited in an arc-forearc context linked to the Farallon-Guadalupe and North America plates subduction, possibly extending through the Gulf early rifting phases (Ferrari et al., 2013(Ferrari et al., , 2018Umhoefer et al., 2001). Age: 30-12 Ma (Umhoefer et al., 2001). Plutonic rocks 1 Several crystalline intrusive bodies (Figure 3(a)). Holocrystalline rock with 3-4 mm sized phenocrysts of Kfs, pl, qz, amph and bio. Some associated aplitic dikes are present. The thickness is variable (from 8 to more than 50 m) and the base is always not visible.
These rocks crop out inside the caldera depression (western margin of the resurgent block) and outside along Yaqui canyon.
The intrusive rocks are granodioritic in composition.
Also known as Batholites Peninsulares, (Gastil, 1975;McLean, 1988;Schmidt, 1975) these rocks represent a voluminous batholith intruded into the Early Cretaceous supra-crustal volcanic and sedimentary sequence. Intrusions occur as small sheet and diapirs or as a combination of large nested intrusive centres and smaller isolated intrusions (Kimbrough et al., 2001). Age: 97.8 ± 1.5 Ma.
lithostratigraphic units, fundamental to understand the volcanic succession and the evolution of the caldera complex, and (3) a 3D view of the mapped area reporting a simplified geology in order to visualize the relationships between morphology of the caldera complex, resurgence phenomena and the volcanological evolution.
The pre-caldera volcanic activity developed in a shallow water, marine basin, as suggested by the occurrence of siltstones and fossiliferous sandstones (Sedimentary Formation of the Santa Rosalia basin) interbedded with volcanic units. The pre-caldera activity is characterized by both explosive and effusive eruptions, with emplacement of pyroclastic density currents (PDCs; Mesa de Enmedio and Cueva Amarilla ignimbrites), volcaniclastics (Contrabando formation), pillow lavas and domes (Punta Candeleros formation). Only one subaerial ignimbrite is exposed at El Carrizo canyon, SW of the caldera rim (Carrizo ignimbrite). The pre-caldera activity probably built up a stratovolcano, whose remnants are today exposed on the SE and NE parts of the caldera complex, and along the sea cliffs in the Punta Gorda area.
The onset of La Reforma caldera started with the eruption of Los Balcones ignimbrite, nowadays not  exposed within the caldera, but only visible in the Arroyo Grande and El Carrizo canyons, to the SW of the caldera rim, and La Palma canyon to the north. The caldera depression was filled by lava flows alternating with scoriae, with a thickness of more than 400 m (Cerro La Reforma formation). La Reforma ignimbrite marks the second caldera-forming eruption, which enlarged the initial caldera depression. The deposits of La Reforma ignimbrite are extensively exposed outside the caldera rims to the S and SW, whereas they crop out sporadically within the caldera in the northern, eastern and southern parts. The eruptive activity resumed with the emplacement of widespread, thin lava flows, probably erupted along fissures parallel to the caldera-forming ring faults, and composing the Mesa El Yaqui formation. The geographical distribution of these lava flows (bordering almost all the caldera rims), suggests they were accompanied by resurgence of the central block of the caldera, which prevented most of the intrusion of feeding dykes within the caldera depression. After the emplacement of Mesa El Yaqui formation, La Reforma stratigraphic succession is characterized by the occurrence of Aguajito ignimbrite, likely deriving from the nearby Aguajito volcanic complex (Garduño-Monroy et al., 1993;Schmitt et al., 2006). The Aguajto ignimbrite deposits are well exposed to the west, but they have been never reported within La Reforma caldera. The Punta Arena ignimbrite is the last eruption of the caldera phase, and it extensively crops out only within the caldera, where it fills up topographic depressions with thickness of up to 200 m. Thin horizontal stratification and the occurrence of marine shells within the deposits testify for the subaqueous deposition of this formation. The Cueva del Diablo formation contains the postcaldera deposits (Table 3), dominated by the evidence of effusive activity along the ring faults and faults bordering the central resurgent block (Las Minitas dome, Morro de las Palmas dome, Punta Gorda stratocone, Punta El Gato stratocone, Punta Prieta scoria cones). Only a few eruptive centres occur outside the caldera rims, and are located along NE-SW tectonic alignments (Cerro Colorado scoria cone). It is during this phase that the resurgence culminated to the present-day status, with the exposure of the plutonic rocks of the Peninsular Ranges Batholith in the SW part of the resurgent block. The uplift of the central part of the resurgent block determined the closure of the drainages to the E, with the formation of a small sedimentary basin in the western part of the caldera. This morphological barrier was likely responsible for the deposition of the volcaniclastic sediments of the Arroyo Grande member.
The magmatic driven resurgence of the central block of the caldera was accompanied by a regional uplift of the entire complex, in the order of more than 100 m, as testified by several marine terraces visible on the marine cliffs to the east (Figure 7). This regional uplift has been also responsible of the emersion of the whole caldera complex, as it is visible today.

Conclusions
The new geological map of La Reforma caldera complex represents an accurate and updated basis for understanding the geological evolution of this part of   Baja California (Figure 2). The map has relevance for the comprehension of the Pleistocene volcanism in the area as it provides new insights into the inception of volcanism in this part of the peninsula. The content of the map includes a new stratigraphic reconstruction of La Reforma pre-caldera, syn-caldera, and post-caldera activity and their timing. This information will provide support to understand the evolution of the Santa Rosalia Basin and the interaction between marine sedimentation and submarine and subaerial volcanism. The geological map of La Reforma caldera complex also represents an invaluable contribution towards the knowledge of the territory and the exploitation of natural resources.

Software
Geological boundaries, structural features and polygons were digitized from field paper maps using Arc-GIS ESRI (10.2.1 for Desktop, 10.2.1.3497 version), often with the aid of Google Earth Pro. The regional setting scheme (on the upper left corner of the map page) was constructed and edited by using CorelDraw X6. The 3D scene (on the bottom of the map page) was edited using ArcScene (ArcGIS 10.2.1). The map was produced by using ArcGIS ESRI and the final editing page was performed in Adobe Illustrator 6.0.