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Grain size, heavy minerals, mineralogical, and geochemical analyses were undertaken on the sand samples from Huatabampo (HB) and Altata (AA) beaches to infer palaeoweathering conditions and source rock characteristics. The mean grain size of HB and AA sands is fine‐grained and fine‐ to medium‐grained, respectively. Magnetite and hypersthene are the common heavy minerals identified in HB sands, whereas magnetite and ilmenite are the dominant heavy minerals in AA sands. HB sands have a high proportion of quartz and plagioclase followed by K‐feldspar and zeolite. Quartz is the dominant mineral in the AA sands, followed by plagioclase and K‐feldspar. Geochemically, the sand samples from HB are classified as wacke, whereas AA sands are classified as wacke and arkose. In chondrite‐normalised REE plot, HB and AA sands show LREE enriched and HREE depleted pattern with negative Eu anomaly. Chemical index of alteration (CIA), plagioclase index of alteration (PIA) values and Th/U ratios of HB and AA sands suggest a low intensity of chemical weathering in the source area. The tectonic discriminant function diagram suggestsan arc setting for HB sands, whereas a rift setting for AA sands. The chondrite‐normalised REE patterns, Eu anomaly, Al2O3/TiO2 ratio, trace elemental ratios, discriminant function, and ternary diagrams suggest that the HB and AA sands were dominantly derived from felsic source rocks. The comparison of REE patterns of HB and AA sands with the source rocks exposed near to the study area suggest that the central granites of Laramide plutonic rocks exposed in Sonora were contributing sediments to the HB beach, whereas AA sands received sediments mainly from the felsic Francisco gneiss in the western Sonobari Complex of Sinaloa. [ABSTRACT FROM AUTHOR]

Nine units of carbonate rocks are recognized, of which the highest five units are examined here under biostratigraphical and chemostratigraphical aspects in order to infer mid‐Cretaceous OAE intervals in Punta China section, Baja California. The biota in Units 5, 6, 7, and 8 show low diversity, while Unit 9 is characterized by a high diversity of microfossils. Benthic foraminiferal assemblage and rudist genera identified in Unit 9 indicate a lower Albian age. Lithofacies and fossil assemblages suggest that the depositional environment was a shallow marine shelf or platform adjacent to caprinid buildups. The limestones of the Alisitos Formation show both negative and positive δ13C values that range between −0.9 and + 7.5‰. Large variations in δ18O values are noticed in the limestone units (−7.9 to −2.2‰). The 87Sr/86Sr ratios of the bulk limestone samples vary between 0.706662 and 0.707305. Most of the limestones have lower 87Sr/86Sr ratios that may be the result of inclusion of less radiogenic strontium from the volcanic rocks. One limestone sample from Unit 7 has a relatively pristine 87Sr/86Sr ratio (0.707305) whose numerical age is 111.61 Ma. The carbon‐isotope curve reveals the presence of two episodes of oceanic anoxic subevents in the studied section. The lower part of Unit 9 has significant negative carbon isotope excursion (CIE) that correlated with the Paquier subevent. Similarly, another negative CIE is also observed in the upper part of Unit 9, which suggests a correlation with Leenhardt subevent. [ABSTRACT FROM AUTHOR]

Microtextures, trace element concentrations, and U–Pb geochronology of detrital zircon grains recovered from the Chachalacas beach in the western Gulf of Mexico were investigated to infer palaeoenvironmental conditions. The mineralogy of bulk sediments revealed the presence of both chemically stable (titano‐magnetite, zircon, and quartz) and unstable (amphibole and K‐feldspar) minerals. The zircon grain microtextures or surface features were grouped as two types: (a) mechanical origin and (b) mechanical and chemical origin. The mechanical surface features such as V‐marks, arc shaped, and meandering steps indicate a high‐energy littoral and sub‐aqueous marine environment. The chemical surface features like solution pits, precipitation, solution crevasses, adhered particles, and silica pellicle revealed a silica saturated marine environment. The well‐rounded zircon grains with bulbous edges are characterized by long transport and aeolian origin. The Th/U ratio, a negative Eu (Eu/Eu* = ~0.08–0.72) and positive Ce (Ce/Ce* = >1) anomalies in the chondrite‐normalized REE patterns of all zircon grains suggested that they primarily formed from magmatic processes. The detrital zircon grain U–Pb ages showed a wide variation from Cenozoic to Proterozoic with an abundance of Cenozoic ages (~1.59–64.3 Ma; n = 44), interpreted as mixing of beach sediments by a longshore current. The comparison of zircon grain ages of this study with ages reported from the source terranes revealed that the likely source for Cenozoic zircon grains was the Eastern Alkaline Province of Mexico. Similarly, the Mesozoic (~68.5–241.9 Ma) and Palaeozoic (~253.2–284.8 Ma) zircon grains in the Chachalacas beach were possibly derived from the Valle de Huizachal and Mesa Central provinces. Proterozoic ages from ~574 to 1846 Ma indicated a probable contribution from the Oaxacan and Chiapas Massif complexes, which are dominated by Grenvillian basement rocks. [ABSTRACT FROM AUTHOR]

The northwestern part of Mexico, southern Arizona, and southern California witnessed extensive arc magmatism during Late Triassic to Early Cretaceous. Here, we present results from petrography, whole‐rock geochemistry (major and trace including rare‐earth elements), and detrital zircon geochronological studies that were carried out on sandstones and shales of the Mural Limestone exposed in the Rancho Bufalo area. Based on the petrography, sandstones are classified as sublitharenite, litharenite, feldspathic litharenite, and lithic arkose. The sandstones contain higher proportion of monocrystalline quartz than polycrystalline quartz. Sandstones and shales from the various members of the Mural Limestone show large variations in major and trace elements concentrations. The average concentrations of ΣREE (total ΣREE) are lower in sandstones than in shales. The chondrite‐normalized REE patterns of the sandstones and shales are moderately fractionated with light REE (LREE) enrichment relative to heavy REE (HREE) and with flat to slightly depleted HREE patterns. The chemical index of alteration values and A–CN–K relationships of sandstones indicate low to moderate intensity of weathering in the source region; however, shale samples suggesting the source rocks were subjected to moderate intensity of chemical weathering. The chondrite‐normalized REE patterns, Al2O3/TiO2 ratio, elemental ratios like Cr/V, Y/Ni, and Eu‐anomaly (Eu/Eu*), and bivariate plots suggest that the sandstones and shales were derived mainly from felsic source rocks. Detrital zircon U–Pb geochronology of sandstones from Fronteras, Rancho Bufalo, and Cerro La Ceja members of the Mural Limestone show five main groups of detrital zircon age populations: (a) Proterozoic, (b) Palaeozoic, (c) Triassic, (d) Jurassic, and (e) Early Cretaceous. Our study suggests that continental volcanic arcs in northwestern Mexico, southern Arizona, and southern California served as the provenance for the Late Triassic‐Jurassic detritus, whereas the Early Cretaceous zircon grains were derived mainly from the active Alisitos volcanic arc along the northwestern coast of Mexico. [ABSTRACT FROM AUTHOR]