Northeastern Sonora Research Articles

Distribution and Habitat of the Jaguar (Panthera onca) in the Madrean Archipelago in Northeastern Sonora, Mexico

Distribution and Habitat of the Jaguar (Panthera onca) in the Madrean Archipelago in Northeastern Sonora, Mexico

Trilobites from the upper middle Cambrian (Miaolingian, Guzhangian) of the Sierra Los Ajos, northern Sonora, Mexico: Biostratigraphic and paleogeographic considerations

A Cambrian sedimentary sequence with a predominant lithology of limestone, sandy limestone, and quartzite pertaining to the Bolsa and Abrigo formations is exposed in the Sierra Los Ajos to the east of Cananea city (northeastern Sonora, Mexico). Two fossil assemblages from Guzhangian layers of the Abrigo Formation are described herein. The trilobites identified comprise Tricrepicephalus texanus (Shumard), Crepicephalus sp., Coosia ariston (Walcott), Coosia sp., Cedaria cf. eurycheilos Palmer, Coosella sp., Llanoaspis modesta Lochman, and Llanoaspis? sp., representing the Cedaria and Crepicephalus zones (Miaolingian, Guzhangian). The Cambrian rocks of northeastern Sonora (Sierra Los Ajos, El Tule, and Mesteñas hills), southern Arizona, and Texas, were deposited as part of a thick sedimentary sequence on the southwestern margin of Laurentia, associated with an inner shelf marine environment. The distribution of the trilobites identified confirms the existence of a widespread Cambrian faunal province that includes the southeastern United States of America and northwestern Mexico.

Provenance and tectonic setting of sandstones of the Lomas Coloradas Formation, Cabullona Group, Sonora, México: Constraints on petrography and geochemistry

The sedimentary rocks of the Cabullona Group are well exposed in the Cabullona Basin of northeastern Sonora, México, which represent syntectonic sedimentation associated with Laramide-style deformation during the Campanian−early Maastrichtian. The sandstones of the Lomas Coloradas Formation deposited at the upper part of the Cabullona Group. Petrography and geochemical (major, trace, and rare earth elements) studies were carried out on the sandstones to interpret the palaeoweathering, provenance, and tectonic setting of the Lomas Coloradas Formation sandstones. Most of the sandstone modal analysis data plotted in the QFL and QmFLt ternary diagrams fall into the fields of dissected arcs and recycled orogens, respectively. Geochemically the sandstones are classified as litharenites while few samples plot into the arkosic field. The chondrite-normalized REE diagrams for sandstones have LREE-enriched, relatively flat HREE patterns with negative Eu anomalies. CIA values and the A‒CN‒K diagram indicate a low intensity of chemical weathering in the source area. The tectonic setting diagram suggests both arc and rift settings for the studied sandstones. The bivariate and ternary diagrams and elemental ratios suggest a felsic source for the Lomas Coloradas Formation. The average REE patterns of the studied sandstones are comparable with REE patterns of the granite from the Caborca block, the rhyolite and granite of the Tarahumara Formation, and the Mesozoic felsic volcanic rocks. The modeled mixture of 40% Proterozoic granite, 30% Triassic arc, 10% Jurassic arc, and 20% Laramide arc represents the probable source rocks for the Lomas Coloradas Formation.

Distribution and Habitat of the Jaguar (Panthera onca) in the Madrean Archipelago in Northeastern Sonora, Mexico

Distribution and Habitat of the Jaguar (Panthera onca) in the Madrean Archipelago in Northeastern Sonora, Mexico

U–Pb zircon geochronology and geochemistry of the Jurassic magmatic rocks from the region of Cananea and Nacozari, northeastern Sonora, Mexico: timing and composition of the southernmost edge of the Jurassic continental arc

Igneous rocks of the Jurassic continental arc of southwest North American Cordillera crop out in the region between Cananea and Nacozari, in northeastern Sonora, Mexico. Contemporaneous igneous rocks occur farther south in the Nazas rift province; however, their tectonic affinity is debated between the southern extension of the Cordilleran arc and an origin associated with decompressive mantle melting during the Pangea breakup. We consider that the studied rocks represent the southernmost expression of the Jurassic continental arc. Four magmatic units are distinguished based on geology, U–Pb zircon geochronology, and geochemical composition. They comprise the Buenos Aires granite dated at 183.8 ± 1.1 Ma, the Mababi granite with ages of 175.5 ± 1.8, 173.5 ± 1.8, and 171.7 ± 1.0 Ma, the Elenita Formation volcanic rocks dated at 169.7 ± 1.7 Ma, and the Cerro Blanco granite dated at 162.3 ± 2.4 Ma. The studied samples correspond to highly differentiated rocks with high-silica contents (69–76 wt.%), low Mg# values (mostly <45), and alkali–calcic and peraluminous compositions suggesting crustal contamination. LREE-enriched normalized slopes with negative Eu anomalies and multielement plots characterized by well-developed negative anomalies of Nb–Ta, P, and Ti provide an unequivocal signal of the continental arc nature. In contrast to the Nazas province, where the Jurassic magmatism consists of discreet volcaniclastic sequences deposited in extensional basins with no evidence of plutonic rocks, the Jurassic continental arc of Arizona and northern Sonora forms a regionally coherent geological feature characterized by large exposures of volcanic and plutonic rocks.

First record of Odocoileus virginianus (Artiodactyla: Cervidae) from the Blancan of Sonora, Northwestern Mexico

ABSTRACT Blancan III or late early Blancan–early late Blancan (3.9–2.6 Ma, Pliocene) cervid remains are described from alluvial deposits of the locality Los Hornitos, northeastern Sonora, northwestern Mexico. These remains include antlers, teeth, a mandible fragment, and other several postcranial elements. These Sonoran Blancan fossil cervid elements constitute the second most abundant group of the mammalian assemblage after the equids in the studied area. The morphometric comparative study with Pliocene and Pleistocene cervid material from United States and Mexico together with statistical analyses (bivariate and multivariate), indicate that herein described material correspond to the species Odocoileus virginianus. This is the first record of this medium-sized deer from the Blancan of Sonora and the earliest unequivocal record to Mexico.

Stratigraphy of the Cambrian (Miaolingian to Furongian) and Ordovician (Tremadocian) west of Arivechi, eastern Sonora, Mexico: Implications on the Cambrian-Ordovician boundary

The Cambrian-Ordovician transition in Mexico has been poorly documented. According to the literature published until now, the most complete middle-upper Cambrian (Miaolingian to Furongian)-Lower Ordovician (Tremadocian) (509-477 Ma) succession is exposed near the town of Arivechi, in northeastern Sonora. This Cambrian-Ordovician sedimentary succession is approximately 900 m thick and is divided into seven lithostratigraphic units. The Cambrian portion of the section consists of El Gavilán, El Mogallón, Milpillas, and La Huerta formations (∼600 m) and range from the upper Wuliuan to Paiban stages (509-494 Ma). El Gavilán Formation consists of limestone and shale with chancelloriids and trilobites of the Altiocculus Subzone of the upper Ehmaniella Zone (Miaolingian, Wuliuan). El Mogallón formation (informal name) is characterized by shale, siltstone, and limestone with abundant poriferans and agnostids of the Ptychagnostus atavus/Bolaspidella Zone (Miaolingian, Drumian). Milpillas formation (informal name) consists of limestone and sandy limestone with trilobites of the Crepicephalus Zone (Miaolingian, Guzhangian). La Huerta formation (informal name) is a limestone and shale intercalations with abundant brachiopods, including Angulotreta triangularis, indicative from the Aphelaspis to the lower Dunderbergia Zones (Furongian, Paibian). The Lower Ordovician (Tremadocian) portion of the section consists of El Santísimo formation herein informaly divided into three members: El Ranchito, El Encinal and La Palma (∼300 m). El Ranchito member consists of limestone, shale, and siltstone with dendroid graptolites of Anisograptus matanensis Zone (Early Ordovician, Tremadocian). El Encinal member consists of siltstone and sandstone with cross-stratification and graptolites of Anisograptus matanensis Zone (Early Ordovician, Tremadocian). La Palma member contains in the lower non-fossiliferous part, a limestone breccia and siltstone, and the upper part contains limestone with conodonts from the Rossodus manitouensis to the Scolopodus subrex Zone (Early Ordovician, Tremadocian). The Cambrian deposits of eastern Arivechi represent open marine platform facies with an abundant fauna of poriferans, brachiopods, trilobites, and echinoderms, typical of Laurentia. In contrast, the Ordovician deposits represent open ocean deposits dominated by graptolites and conodonts that indicate a shallow marine to a shelf environment.

Distribution and habitat of Ocelot (Leopardus pardalis) in Sonora, México

The ocelot (Leopardus pardalis) ranges from northeastern Argentina to southern Texas and southeastern Arizona. It is listed as Endangered in Mexico and the United States. Previous works on ocelots in Sonora, found them occupying different habitats, including tropical deciduous forest, thornscrub, desert grassland, oak woodland, and pine-oak forest, avoiding the driest areas of the Sonoran Desert. The majority of our records are from camera traps, with indirect records from tracks. We analyzed images and videos of more than 100 camera traps used to monitor wildlife in ranches and natural protected areas of Sonora. Cameras were set at an altitude of 70 to 100 cm above ground, some were baited with a sardine-tomato mixture, others were not, the majority were set in areas where other species were the main objective. We obtained 147 recent records from 2015 to 2021 and 28 previous records of ocelots in Sonora. Ocelots occur in the eastern half of Sonora, avoiding areas with less than 400 mm of annual rainfall in the eastern and northwestern Sonoran Desert. Ocelots have been recorded from 53 to 2,151 m elevation (av. 996 m). 21 images in nine localities show females with kittens, indicating breeding populations. In southern and east-central Sonora, ocelots live in tropical deciduous forest, and foothills thornscrub. At Maycoba east of Yécora, in Sierra Huachinera and in the Sky Islands Mountain ranges in northeastern Sonora, ocelots live in temperate vegetation, including desert grassland, oak woodland, and pine-oak forest. Ocelots have been recorded in riparian habitats, transecting other vegetation types in the principal rivers of the state: Ríos Bavispe-Yaqui, Mayo, and Sonora, which drain the Sierra Madre Occidental, the Sky Island Region and southeastern Arizona to the Gulf of California, providing dispersal corridors. Other medium sized rivers provide corridors for the dispersal of ocelots, such as Río Mátape in central Sonora, and Ríos Cocóspera, San Pedro, and Santa Cruz in the north. The nearest Sonoran populations to Arizona are in the Sierra El Alacrán and Río Cocóspera on Rancho El Aribabi.

Vegetación de la Sierra Madre Occidental, México: una síntesis

La Sierra Madre Occidental (SMO) es el complejo montañoso más grande de México, extendiéndose por casi 1200 km desde el noreste de Sonora (30°35' N) hasta el norte de Jalisco (21°00' N). A pesar de su gran importancia ambiental y económica, es todavía deficientemente conocida. Se presenta una sinopsis de los tipos de vegetación, con datos sobre la superficie que ocupan las comunidades reconocidas, intervalos de elevación y climas donde se localizan, así como un mapa. En la zona confluyen tres regiones, albergando ecosistemas templados y semifríos (región Madrense), semiáridos (Madrense-Xerófila) y cálido-secos (Tropical), respectivamente. La región Madrense, en el macizo de la sierra, incluye bosque de pino, bosque mixto de coníferas, bosque de pino-encino, bosque de encino y manchones de bosque mesófilo de montaña, además de comunidades como chaparral (primario y secundario) y vegetación de claros de bosque. La región Madrense-Xerófila, en el piedemonte norte y oriental y estribaciones al oriente de la sierra, presenta bosque bajo abierto (de encino o de pino-encino), matorral perennifolio y zonas de ecotonía hacia pastizal y matorral xerófilo, además de ecotonías con matorral subtropical hacia el sur del Archipiélago Madrense. En la región Tropical, a lo largo de la base occidental de la sierra e ingresando a esta a través de las barrancas y quebradas, predomina bosque tropical caducifolio y matorral subtropical, con pequeñas áreas de bosque tropical subcaducifolio. La alta diversidad de la SMO se ejemplifica con los tres géneros dominantes fisonómicamente en la vegetación: 24 especies de Pinus (46% del total nacional), 54 de Quercus (34%) y 7 de Arbutus (100%).

Spatial and Temporal Limits of the Casas Grandes Tradition: A View from the Fronteras Valley

This article revises the spatial and temporal boundaries of the Casas Grandes tradition associated with northwest Chihuahua, Mexico, based on new data collected in neighboring northeastern Sonora. The Casas Grandes tradition attained its greatest extent during the Medio period (AD 1200–1450/1500) followed by a dramatic demographic and political collapse. Hunter-gatherer groups subsequently occupied most of northwest Chihuahua. Data from the Fronteras Valley, Sonora, presents an alternative scenario, with a clear pattern of cultural continuity from the eleventh century to the colonial period in which sedentary farmers occupied the same landscapes and occasionally the same villages. These observations contribute to our understanding of the spread and subsequent demise of the Casas Grandes tradition in hinterland regions. For the Fronteras Valley, we infer that immigrant groups originally introduced Casas Grandes traditions and that uneven participation in a suite of shared religious beliefs and practices was common to all the hinterlands.

Systematics, ecology, and biochronology of blancan horses from Sonora, northwestern Mexico

We describe a set of dental and postcranial horse material recovered from the locality Los Hornitos, northeastern Sonora, northwestern Mexico. The fossil-bearing unit consists of brown silt with intercalation of fine-grained sandstone and bioturbated incipient soils, set in an alluvial environment and considered Pliocene in age. The comparative study of the available sample indicates the presence of two species: the small-sized hipparionine Nannippus peninsulatus and the medium-sized and primitive equine Equus simplicidens. The isotope composition of δ13C and δ18O in dental enamel tooth indicates a C3–C4 mixed diet for these species and suggests the presence of woodland and open areas in Los Hornitos during the Pliocene. The less negative δ13C values observed in E. simplicidens are related to a more C4 diet, preferring to roam open habitats. The coexistence and optimal resource partitioning between these species are explained, considering their differences in dietary behavior, body size, home range, and/or vagility. The recognized horse species are typical of the Blancan North American Land Mammal Age, and their biochronological information from the southwestern United States and central Mexico establishes the probable age of the Los Hornitos at about 3.9–2.6 Ma, corresponding to the Blancan III or late early Blancan – early late Blancan.

Trilobites from the Crepicephalus Zone (upper Guzhangian Stage, Miaolingian Series, Cambrian) from northern Sonora, Mexico, and its correlation to Arizona and Texas, USA

In the Cananea region, northeastern Sonora, there is a sedimentary sequence of the lower Paleozoic outcrops that includes the Cambrian Bolsa Quartzite and Abrigo Formation. This work describes and illustrates the upper middle Cambrian ptychopariid trilobite fauna from the Abrigo Formation for the first time for Mexico. Trilobites from the inner shelf deposits of the El Tule and Mesteña hills include Crepicephalus sp., Tricrepicephalus texanus, Llanoaspis modesta?, Coosella perplexa, Coosia ariston, and Coosia sp., which represent the Crepicephalus Zone (upper Guzhangian Stage, Miaolingian Series). The distribution of these species denotes a strong affinity with North American fauna, specifically with Arizona and Texas.

Wildlife Use of Drainage Structures Under 2 Sections of Federal Highway 2 in the Sky Island Region of Northeastern Sonora, Mexico

Roads and highways are 1 of the most significant obstacles affecting wildlife movement by fragmenting habitat, altering wildlife migration and use of habitat, while also being a danger to wildlife and humans caused by wildlife-vehicle interactions. To mitigate wildlife mortality on highway sections and to minimize death and injury to motorists as well, road ecologists have proposed structures adapted for the safe passage of wildlife across roads. In this study, photographic sampling was conducted using trail cameras to quantify wildlife activity and use of existing culverts, bridges, and drainages within 2 separate sections of Mexico Federal Highway 2 where previous field assessment had observed high levels of activity. These sections are important areas for the conservation of wildlife, and they are known to be biological corridors for rare species of concern such as jaguar, black bear, and ocelot. The trail cameras were operated for 1 year to document the annual cycle of wildlife movement through the area. With the photographs obtained, a database was created containing the information from each wildlife-culvert interaction. Prior to sampling, an inventory of existing culverts was conducted that measured height, width, volume, and surrounding habitat to assign a hypothesized use quality index. After testing for significant differences in use index among culverts, we recognized that all culverts were equally important for moving wildlife, and that there were no significant differences in the use of culverts by the quality index.

Chemostratigraphy of the lower Cretaceous Mural Limestone, Rancho Bufalo section, Sonora, Mexico: Implications for OAE 1b

A multidisciplinary approach of the Aptian-Albian Mural Limestone including petrography, geochemistry and stable isotope data is undertaken for the 660-m thick Rancho Bufalo section of northeastern Sonora to characterize source of rare earth elements and to infer paleo-redox conditions. The lithofacies of limestones range from mudstone to boundstone. The unaltered signatures of the δ13C values are confirmed by using Mn and Sr concentrations and their ratios along with the absence of correlation between δ13C and δ18O values. The limestones have large variations in ΣREE and Al2O3 contents (8.0 − 95.8 ppm; 0.45% − 7.10 ppm; respectively). Most limestones show non-seawater-like REE + Y patterns with positive Eu anomalies and negative Ce anomalies. The limestones have positive Ba anomalies in six intervals and most of them were resulted due to weathering and depositional processes and not due to sea level variations. The redox proxies of U, V/Cr and V/(V + Ni) indicate that the bottom water conditions fluctuated from oxic to anoxic. The high-resolution carbon isotope curve reveals four OAE 1 b sub-events: Jacob, Kilian, Paquier, and Leenhardt. The carbon isotope curve of the Rancho Bufalo section is comparable to that of the Cerro Pimas and Peregrina Canyon sections of Mexico.

La Caridad Vieja: Vestiges of a removed HS-epithermal system in the La caridad porphyry Cu–Mo system, northeastern sonora, Mexico, based on XRD, SEM, SWIR spectral analysis and U–Pb zircon dating

The present work contributes with new data to better understand the geology and spatial distribution of the alteration and mineralization events, as well as new U–Pb zircon ages, that allow elucidating the magmatic history of the La Caridad Vieja mineral deposit, which is part of the porphyry Cu–Mo system from La Caridad mine, northwest Mexico. Mineralization is hosted in a sequence of volcanic rocks from the Late Cretaceous-Paleocene Tarahumara Formation and is associated with the emplacement of quartz monzonite stocks, which intruded to volcanic rocks. Detailed studies based on petrography, XRD, SEM and SWIR, indicate that the La Caridad Vieja is characterized by a zone of phyllic alteration dominated by qz-ser in the deepest parts, and it is associated with molybdenite + chalcopyrite + pyrite + galena + sphalerite, with values ranging from 0.1 to 0.9% Cu and <0.02% Mo. Above the phyllic alteration zone, there is an intermediate argillic alteration zone (quartz + kaolinite + halloysite + smectite + illite), mainly with pyrite. In specific areas, advanced argillic alteration is observed (alunite + pyrophyllite + dickite + kaolinite + halloysite) accompanied by pyrite + enargite + famantinite + barite. The new U–Pb radiometric ages in zircons indicate an age of 54.71 ± 0.39 Ma for the upper part of the Tarahumara Formation, while a sample of the mineralizing stock yielded 51.95 ± 0.48 Ma. The results of this investigation suggest that La Caridad Vieja corresponds to a portion of the upper part of the Cu–Mo porphyry of La Caridad, which was downthrown by the La Caridad fault, during the Basin and Range extensional event in the Miocene. The reconstruction of the geological model suggests that La Caridad Vieja corresponds to the transitional zone between the Cu–Mo porphyry and an epithermal high sulfidation mineralization zone, which was eroded. Prior to the dislocation caused by the La Caridad fault, the eroded area was covered by a ferruginous conglomerate overlain by rhyolite flows dated at ~24 Ma. On the other hand, geochronological data suggests that the mineralizing event at La Caridad Vieja, although it is contemporaneous with the porphyry system of the La Caridad mine, it continued in this area at least until ~52 Ma.

Stratigraphy, detrital zircon geochronology and provenance of the Morita formation (Bisbee Group) in northeastern Sonora, Mexico

The Morita Formation is a Lower Cretaceous unit of the Bisbee Group that crops out in northern Sonora and southeastern Arizona where it was deposited within the Altar-Cucurpe, and Huachuca sub-basins of the Bisbee basin, respectively. In northern Sonora it either overlies strata of the Upper Jurassic Cucurpe Formation, the Glance Conglomerate, or strata of the Lower Cretaceous Cerro de Oro/Rancho La Colgada formations, and it grades upward into the Mural Limestone. In this contribution we characterize the stratigraphy, petrography and detrital zircon geochronology of the Morita Formation along a ca. 60 km-long northwest-southeast transect that includes the complete columns of the Mule Mountains in southeastern Arizona and the sierras San José, Anibacachi and Rancho Búfalo area in northeastern Sonora. Thickness of these sections varies from a minimum of 555 m in Sierra Anibacachi to a maximum of 855 m in Rancho Búfalo. In most sections, the lower half of the Morita consists of reddish siltstone and mudstone with pedogenic calcareous nodules with interbeds of mostly single-storey sandstone bodies, which changes to isolated multi-storey sandstone bodies in its upper half. In contrast, the lower part of the Rancho Búfalo column consists of reddish mudstone/siltstone with isolated, fining-upward, clast-supported, fine-pebble conglomerate beds that grades into reddish to purple, brown and green mudstone/siltstone with isolated, single-storey to multi-storey sandstone beds of its upper part. Lithofacies association in the Morita Formation suggest it is of fluvial origin but ichnofossils and local herringbone- and flaser-cross bedding in its upper part indicate intermitent marginal marine sedimentation preceding the transgression that deposited the overlying Mural shelf.Sandstone composition of the Mule and San José mountains is dominantly feldspatho-quartzose indicating dominant provenance from basement uplift and transitional continental areas, while composition at Sierra Anibacachi and Rancho Búfalo ranges from feldspatho litho-quartzose to litho-quartzose, indicating provenance from recycled orogen. Detrital zircon grains dated from samples of all the studied sections share very similar populations of Proterozoic to Mesozoic ages, but the Mesoproteozoic and Triassic grains compose ca. 64% of the total grains. Detrital zircon grains of Lower Cretaceous age that are only present in samples from the bottom of the Mule and San José Mountains help to constrain a maximum depositional age of 125.5 ± 0.8 Ma for inception of sedimentation of the Morita Formation in this region. According to this age and to observed stratigraphic relationships, we interpret that the lower contact of the Morita Formation is a disconformity with the underlying Glance Conglomerate, or with the Cerro de Oro Formation in northeastern Sonora. Similarly, according to regional stratigraphic relationships and age data known from the Cucurpe-Tuape region, in north-central Sonora, the age of this unit may be constrained to the Aptian stage, in Sonora, from ca. 125 to 115 Ma. Regional isopach curves confirm merging of the sub-basins in northeastern Sonora, continuous sedimentation of the Morita Formation, and presence of the Cananea high separating them.

The oldest record of North American Lychnothamnus (northeastern Sonora, Mexico): Implications for the evolution, ecology, and paleogeographic distribution of the genus

A large population of Late Cretaceous Lychnothamnus tenuis holding small sized gyrogonites are described after systematically sampling the Fronteras section in the Cabullona Group (northeastern Sonora, Mexico). This discovery represents the oldest record of Lychnothamnus in North America and the first record of this genus in Mexico. The sedimentological analyses of the section suggest that Mexican L. tenuis grew in fluvial and floodplain ephemeral ponds such as their South American counterparts, Lychnothamnus barbosai (oldest known world-wide occurrence of Lychnothamnus) and L. tenuis, from the Turonian–Santonian and Santonian–lower Campanian respectively. The small sized gyrogonites (calcified fructifications of charophytes) of L. tenuis from South America was considered to represent an adaptation to stressful growth conditions since most of the fossil and extant Lychnothamnus species are mainly found in freshwater lacustrine deposits. However, this floodplain-type environment has also been described for a small population of Lychnothamnus sp. recently recovered from the lower Eocene Claron Formation (Utah, USA) and for extant Lychnothamnus barbatus found in Australia, both populations having medium to large gyrogonites. This suggests that rather than being a response to ecological parameters, the gyrogonite size of Lychnothamnus is related to the evolutionary trend of the genus, producing smaller gyrogonites when first appeared in the Upper Cretaceous. Finally, the presence of L. tenuis in Sonora (North America) suggests a south to north colonization route for L. tenuis during the Late Cretaceous.

A new species of Aphonopelma (Araneae: Mygalomorphae: Theraphosidae) from the Madrean pine-oak woodlands of northeastern Sonora, Mexico

The tarantula spider genus Aphonopelma Pocock, 1901 has received considerable attention in recent years but the group's diversity remains poorly understood in Mexico, particularly in the pine-oak woodlands of the Sierra Madre Occidental and associated Madrean "Sky Islands". A pair of tarantulas discovered from an unsampled region in the Sierra de Bacadéhuachi (the westernmost range of the Sierra Madre Occidental) in northeastern Sonora was found to be closely related to four species from the Madrean "Sky Islands" in Arizona and New Mexico. An integrative approach for delimiting species (incorporating data from molecular phylogenetics, morphology, distributions, and breeding periods) suggests that the specimens from Sierra de Bacadéhuachi belong to an undescribed species that is herein named Aphonopelma bacadehuachi sp. nov. This new species adds to our knowledge of an increasingly diverse assemblage of Aphonopelma from the Madrean Pine-Oak Woodlands Hotspot. Collaborations between Mexican and American researchers are needed to accelerate discovery and description of the group's remaining diversity, particularly in light of the many threats facing the ecoregion including habitat degradation and climate change.

Stratigraphy, geochronology and regional tectonic setting of the Late Cretaceous (ca. 82-70 Ma) Cabullona basin, Sonora, Mexico

The Cabullona basin in northeastern Sonora is a continental depocenter whose origin is related to the adjacent Sierra Anibacachi uplift that bounds its tectonic eastern flank. Its exposed, mostly fluvial and lacustrine sedimentary fill, the Cabullona Group, was deposited between 81.9 ± 0.7 and 69.8 ± 0.7 Ma and its outcrops extends for 70 km from north to south. The oldest measured stratigraphic column of the Cabullona Group is the Los Atolillos column of the southern part of the basin, but its base is not exposed. A basal conglomerate in the younger El Malacate (ca. 80 Ma), Cuauhtémoc (ca. 75 Ma) and San Joaquín (ca. 70 Ma) columns onlaps deformed basement rocks. The type section in which the Cabullona Group was previously named is herein referred as the Naco section and is dated ∼73-72 Ma. The younger strata of the Cabullona Group correspond to the fluvial San Joaquín column that onlaps the eastern tectonic boundary of the basin and to the lacustrine Esqueda column. These columns are dated at ca. 70 Ma and may represent the late evolution of the Cabullona basin.Sandstone petrography and detrital zircon geochronology are used to infer provenance of sediments of the Cabullona Group. Sandstones consist of lithic arkose to feldespathic litharenite, indicating provenance from dissected to transitional volcanic arc, but samples of the El Malacate column classify as arkose and lithic arkose with possible provenance from basement uplift of Sierra Los Ajos; litharenite from the Esqueda column indicate arc provenance. Detrital zircons yielded mostly Proterozoic and Mesozoic ages with age peaks at ca. 1568, 167, 100, 80 and 73 Ma indicating possible provenance from the Precambrian basement rocks and the Jurassic continental magmatic arc that underlie the region, the Alisitos arc and La Posta plutons in Baja California, and from the Laramide magmatic arc of Sonora.The Cabullona basin developed nearly contemporaneous to the early, eastwards migrating Laramide magmatic arc that located to the west of the basin, and to a tectonic shortening that occurred in northern Sonora during Late Cretaceous time. In the older columns of the Cabullona Group and in columns of the northern part, the early arc had a distal influence during sedimentation as shown by interbedded ash fall tuffs and minor rhyolitic flows, but sections in the southern part of the basin record more abundant rhyolitic ash-fall tuffs and flows indicating the arc proximity. An important regional flare-up of the arc at ca. 74 Ma is recorded by the Ejido Ruiz Cortines column, while the upper part of the Cabullona Group was interdigitating with rhyolitic rocks by 70 Ma. The Cabullona basin started to form during the shortening event whose age is constrained between ca. 93 and 76 Ma according to U-Pb ages of the syntectonic Cocóspera Formation of northern Sonora and from Laramide arc rocks that overlie it. Ages and correlation of the Cocóspera and the Altar formations may indicate that a Laramide tectonic front extended from north-central Sonora to the Caborca region and whose trace may correspond to a westward extension of the San Antonio fault.

The El Horror uranium anomaly in northeastern Sonora, Mexico: Constraints from geochemical and mineralogical approaches

This work reviews the characteristics of the El Horror uranium prospect in northeastern Sonora, Mexico. It was formerly detected by a radiometric anomaly after airborne gamma ray exploration carried out in the 70's by the Mexican government. As a promising site to contain important uranium resources, the El Horror was re-evaluated by CFE (Federal Electricity Commission) by in situ gamma ray surveys. The study also incorporates rock and stream sediment ICP-MS geochemistry, X-ray diffraction, X-ray fluorescence, Raman spectrometry and Scanning Electron Microscopy (SEM) to provide a better understanding of the radiometric anomaly. The results show that, instead of a single anomaly, it comprises at least five individual anomalies hosted in hydrothermally altered Laramide (80-40 Ma) andesitic volcanic rocks of the Tarahumara Formation. Concentrations for elemental uranium and uranium calculated from gamma ray surveys (i.e., equivalent uranium) are not spatially coincident within the anomaly, but, at least at some degree, they do so in specific sites. X-ray diffraction and Raman spectrometry revealed the presence of rutile/anatase, uvite, bukouvskyte and allanite as the more likely mineral phases to contain uranium. SEM studies revealed a process of iron-rich concretion formation, suggesting that uranium was initially incorporated to the system by adsorption, but was largely removed later during incorporation of Fe+3 ions. Stream sediment geochemistry reveals that the highest uranium concentrations are derived from the southern part of the Sierra La Madera batholith (∼63 Ma), and decrease toward the El Horror anomaly.