The initial peopling of the Americas is a contested and evolving topic 1 , with the exact timing of the first arrivals still unknown. Historically, Mexico's understudied and controversial archaeological record has remained on the periphery of First Americans research 2 . However, in recent years, investigations have shown reliable evidence of a late-Pleistocene and early-Holocene human presence in the northwest region 3,4 , the Chiapas Highlands 5 , Central Mexico 6 , and the Caribbean coast [7][8][9] . Evidence of human presence at Chiquihuite Cave extends this antiquity and attests to the cultural variability of older-than-Clovis sites [10][11][12][13][14][15][16][17] and the earliest humans on the continent. Site settings and excavation context.
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 defined 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 effusive eruptions that formed the present-day caldera depression. The post caldera phase includes scattered effusive eruptions (ended at 0.28 Ma) and resurgence, characterized by several hundred meters of uplift of the central block within the caldera depression.
This field trip highlights evidence of late Pliocene–early Pleistocene submarine and subaerial volcanism coeval with marine and marginal-marine sedimentation in the Santa Rosalía Basin in Baja California. The best exposures of these rocks occur at the El Álamo Canyon, which exhibits outcrops of the Tirabuzón and Infierno formations interbedded with submarine and subaerial volcanic and volcaniclastic deposits that are covered by the subaerial ignimbrites of Reforma and Aguajito calderas. Extensive field mapping and stratigraphy carried out in this canyon, aided with 40Ar/39Ar, and U-Pb geochronology, allowed us to divide the stratigraphy into three main sequences that, from base to top, are: (1) Santa Rosalía succession, (2) the Reforma caldera complex, and (3) El Aguajito caldera. This refined stratigraphy indicates that eight episodes of volcanism occurred between 2.5 and 1.36 Ma, during marine sedimentation in an internal continental shelf and in estuaries, coastal lagoons, or protected bays as supported by the fossil record. This sedimentary and volcanic interbedding suggests transgressions and regressions of the sea level, as well as tectonic uplifting. After the inception of volcanism in the Reforma caldera complex (1.29 Ma), the area emerged from these shallow seas followed by the formation of the Aguajito caldera (1.1 Ma), and then later on by the Tres Vírgenes volcanic complex (0.3 Ma). The last complex erupted a lava flow ca. 22 ka that so far stands as the youngest activity in the region. The magmatic evolution in the region is characterized by post-subduction calc-alkaline magmatism. Such magmatism is expressed as pure calc-alkaline rocks from El Aguajito–Reforma calderas, and as hybrid transitional magmas formed by adakitic rocks from the Tres Vírgenes volcanic complex.
The Joya Honda maar (JHm) is located in central Mexico, 35 km NNE of the city of San Luis Potosí. It lies in the Plio-Quaternary alkaline Ventura-Espíritu Santo Volcanic Field located in the eastern part of Mesa Central province. The JHm eruption occurred at 311±19 ka (40Ar/39Ar) along a fissure that formed an elliptical crater (c. 1.3×0.9 km wide and c. 270 m deep) with a major axis oriented to the ENE–WSW. The eruption generated pyroclastic surge deposits that preferentially extended up to a distance of 7 km to the NW–NE of the crater, with a very limited distribution to the south. At the crater rim, the sequence is 60–80 m thick on the NE–NW wall and 1–15 m thick on the south–SW rim. The JHm sequence is divided into five units with different structures, textures, granulometry and components. The juvenile basanite clasts of these units display differences in vesicularity, density and morphology under scanning electron microscopy. These units correspond to the same number of eruptive phases as follows: Phase 1 occurred as a series of alternating strombolian and phreatomagmatic explosions that dispersed fall deposits and base surges; Phase 2 began with strombolian activity that emplaced basanite scoria with low contents of mantle xenoliths; Phase 3 continued with phreatomagmatic explosions that emplaced wet and dry pyroclastic surges; Phase 4 generated strombolian explosions rich in mantle xenoliths; and Phase 5 produced a violent strombolian phase that dispersed fallouts rich in mantle xenoliths and intermixed with discrete phreatomagmatic explosions that emplaced pyroclastic surges. These eruptive fluctuations during the genesis of JHm are a response to the relative proportions of magma–water interaction through time and complex faulting of the calcareous rocks underneath the volcano. The distribution and textural characteristics of the deposits suggest that simultaneous or alternating vents were active during the eruption, possibly following a fissure. These variations may have been subordinated to factors such as the availability of groundwater, the velocity of magma ascent, the discharge rate and degassing.
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