ABSTRACT The Gulf of Mexico is best understood as a subsidiary basin to the Atlantic, resulting from breakup of Pangea. The rifting process and stratigraphy preceding opening of the gulf are, however, not fully understood. We present new stratigraphic, sedimentologic, and provenance data for the Todos Santos Formation (now Todos Santos Group) in southern Mexico. The new data support a two-stage model for rifting in the Gulf of Mexico. Field and analytical evidence demonstrate that strata assigned to the Todos Santos Group in Mexico belong to two unrelated successions that were juxtaposed after rotation of the Yucatán block. An Upper Triassic fluvial siliciclastic succession in the western Veracruz basin is intruded by the San Juan del Río pluton (194 Ma, U-Pb) along the Valle Nacional fault. We refer to this succession as the Valle Nacional formation (informal) of the Todos Santos Group, and correlate it with El Alamar Formation of northeast Mexico and the Eagle Mills Formation of the northern Gulf of Mexico. Triassic red beds register an early rifting phase in western equatorial Pangea. Sandstone composition indicates that the Valle Nacional formation is mostly arkoses derived from multiple sources. Paleocurrent indicators in fluvial strata of the Valle Nacional formation are S-SW directed, but restoration of paleomagnetically determined counterclockwise rotation indicates a W-SW–flowing fluvial system. Triassic rifting in the Valle Nacional formation and the Central Cordillera of Colombia Triassic extensional event, the record of which is preserved in mid-crustal levels, may represent conjugate margins. The Early–Middle Jurassic Nazas continental volcanic arc predated the Jurassic rifting phase that led to opening of the gulf. A record of arc magmatism is present in eastern Mexico underlying Middle Jurassic synrift successions, and it is present in La Boca and Cahuasas formations in the Sierra Madre Oriental and La Silla Formation north of the Chiapas Massif. These units have a similar age range between ca. 195 and 170 Ma. Arc magmatism in eastern Mexico is correlated with the Jurassic Cordilleran arc of Sonora, California, and Arizona, as well as the Jurassic arc of the Central Cordillera of Colombia. La Boca and La Silla units record intra-arc extension driven by slab rollback. The Jurassic rifting phase is recorded in the Jiquipilas formation of the Todos Santos Group and is younger than ca. 170 Ma, based on young zircon ages at multiple locations. The informal El Diamante member of the Jiquipilas formation records the maximum displacement rift stage (rift climax). Coarse-grained, pebbly, arkosic sandstones with thin siltstone intercalations and thick conglomerate packages of the Jericó member of the Jiquipilas formation are interpreted as deposits of a high-gradient, axial rift fluvial system fed by transverse alluvial fans. These rivers flowed north to northeast (restored for ~35° rotation of Yucatán). The Concordia member of the Jiquipilas formation records the postrift stage. Thick synrift successions are preserved in the subsurface in the Tampico-Misantla basin, but they cannot be easily assigned to the Triassic or the Jurassic rifting stages because of insufficient study. The Todos Santos Group at its type locality in Guatemala marks the base of the Lower Cretaceous transgression. Overall, three regional extensional events are recognized in the western Gulf of Mexico Mesozoic margin. These include Upper Triassic early rifting, an extensional continental arc, and Middle Jurassic main rifting events that culminated with rotation of Yucatán and formation of oceanic crust in the gulf.

ABSTRACT We redefine the “Chontal arc” of the southern Isthmus of Tehuantepec, Mexico, as the Chontal allochthon. The Chontal assemblage is composed of Upper Cretaceous low-grade metavolcanic and metasedimentary rocks included in the Chivela lithodeme. By means of field observations, laser-ablation detrital zircon geochronology, and trace-element geochemistry, we constrained the provenance and tectonic setting of these rocks. We concluded that they form an allochthon emplaced during a Paleogene transpressive event. Basement structure in the greater Oaxaca-Chiapas area was assessed by qualitative interpretation of Mexican State aeromagnetic maps. Chivela lithodeme sediments include a contribution from Albian–Turonian volcanic arc rocks no longer present in the region, likely sourced from the Chortís block or from the Greater Antilles Arc as it collided with southern Yucatan. Maastrichtian basic intrusive units, basalt flows, and pillow lavas with pelagic sediments in the Chontal are subalkaline, plotting in the normal mid-ocean-ridge basalt (N-MORB) field of discrimination diagrams. The igneous rocks are interpreted as pertaining either to the inception of the paleo–Motagua fault zone (left step in the fault trace), or to local backarc extension behind the Chortís block just before it began to migrate eastward, in a basin we call the Chontal basin. The Chontal allochthon was thrust northward onto parautochthonous strata flanking the Mixtequita and Chiapas Massif basements. Chontal allochthon rocks were later intruded by Miocene granitoids related to the inception of Cocos plate subduction arc magmatism. Rocks of the Chontal allochthon have been previously linked to the Cuicateco belt of eastern Oaxaca, but this is challenged here on the basis of lithologic type, chronology, tectonic associations, structural styles, and discontinuous anomaly trends in aeromagnetic maps.

Abstract The Miocene Nanchital conglomerate of the western Chiapas Foldbelt is the coarsest terrigenous clastic depositional Cenozoic unit of the region, probably comprising more proximal sections of hydrocarbon-rich slope-fan reservoirs found in the more distal Sureste Basin of the southern Gulf of Mexico fringe. Traditionally, the felsic igneous and metamorphic components of the conglomerate were assumed to derive from the Permian basement of the nearby Chiapas Massif. However, zircon U–Pb dating of five Nanchital conglomerate clasts from the Chiapas Foldbelt as well as several igneous exposures in SW Tehuantepec indicates that the Nanchital conglomerate's catchment area included the western Isthmus of Tehuantepec for late Middle Miocene and possibly early Late Miocene time, after which the more proximal Chiapas Massif and Chiapas Foldbelt likely became dominant. This study suggests that traditional concerns over the limited extent of quartz-rich clastic source areas feeding terrigenous clastic reservoirs in the Sureste Basin might be overly pessimistic. We propose a temporal framework for viewing Neogene and Quaternary clastic supply to the southern Gulf of Mexico.

Abstract Tectonostratigraphic data derived from ongoing biostratigraphic, chronostratigraphic, paleobathymetric, paleobiogeographic, and lithostratigraphic investigations in west-central and east-central Mexico suggest that the Gulf of Mexico formed in two phases: Phase 1: Rifting and subsequent sea-floor spreading during the Late Jurassic (middle Oxfordian). All but the southwestern portion of the Gulf of Mexico formed during Phase 1. Phase 2: Northwest-to-southeast tectonic transport of allochthonous San Pedro del Gallo terrane remnants along the west side of Walper Megashear during the Middle Jurassic to Early Cretaceous. Where the stratigraphic successions are complete, megafossil data indicates that the San Pedro del Gallo terrane was situated at Southern Boreal paleolatitudes (>30° N) in the Nevadan back arc domain during the Middle Jurassic (late Bathonian to early Callovian) and was subsequently carried to lower paleolatitudes during the Late Jurassic and Early Cretaceous. For example, in the Huayacocotla remnant, the Boreal ammonite Kepplerites was recovered in the subsurface from the Palo Blanco Formation by Cantú-Chapa. In North America, Kepplerites is known from the Izee terrane (east-central Oregon), Western Interior (Montana and Saskatchewan), and northward to southern Alaska. Radiolarian, calpionellid, ammonite, and bivalve faunal data indicate that the Huayacocotla remnant had been transported to Northern Tethyan paleolatitudes (23° N to 29° N) during the Kimmeridgian and Tithonian and to Central Tethyan paleolatitudes (<23° N) by the beginning of the Early Cretaceous.