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Rancho La Barranca
New age constraints for the Salamanca Formation and lower Río Chico Group in the western San Jorge Basin, Patagonia, Argentina: Implications for Cretaceous-Paleogene extinction recovery and land mammal age correlations
Lower Cretaceous strata of the Sonora Bisbee Basin: A record of the tectonomagmatic evolution of northwestern Mexico
The evolution of Popocatépetl volcano was determined through the definition of rock units following morphostratigraphic criteria and detailed geological sections. The primitive volcano, named Nexpayantla, probably contemporaneous with Pies volcano, a part of the Iztaccíhuatl volcanic complex to the north, grew beneath the site of today's cone. This volcano produced mainly andesitic to dacitic lava flows, presented flank activity in the form of several large dacitic lava domes, and was intruded by dacitic to rhyolitic dikes. The evolution of Nexpayantla volcano finished with a large collapse to the south that produced the Lower Tlayecac avalanche deposit. A new cone, Ventorrillo volcano, was built on the remains of Nexpayantla and was formed mostly by andesitic lava flows, but did not present any recognizable flank activity. Ventorrillo volcano collapsed in a large Bezymianny-type eruption toward the southwest, producing the Upper Tlayecac avalanche deposit and the Tochimilco pumice. The Calpan fan was derived from collapse and eruptions of Pies volcano. The present-day cone grew through the emission of many andesitic to dacitic lava flows, which were grouped into eroded or covered lava slopes (Malpaís, Las Mesas, Metepec, and San Pedro Benito Juarez lava flows), and glaciated (Fraile lava flows) and nonglaciated (Las Cruces, Buenavista, Quimichule, Atlimiyaya, Chiquipixle, and Nealtican lava flows) lava slopes with marked features, both from the central vent and from flank eruptions, mainly to the northeast and southwest of the cone. The Ecatzingo and Ombligo-Xalipilcáyatl flank vents formed two well-defined lineaments. The relative ages of the lava flows were determined through morphology, stratigraphic relations, and tephra cover. Two stages of growth were separated by a large Plinian eruption, which emplaced the Black and White (B&W) and Pumice with Andesite (PWA) fall deposits, which were used as stratigraphic markers. Another twelve Plinian pumice deposits are interstratified with the lava flows. Four large volcaniclastic fans and five valley fill deposits form the volcano's piedmont, and have resulted from the successive emplacement of pyroclastic flows, lahars, and fluvial deposits along several gullies that mark the lower slopes of the volcano. Glacier melting coincident with several of the Plinian eruptions could have been responsible for some of the extensive lahar deposits.
Ordovician stratigraphy and biota of Mexico
Abstract In Mexico, Ordovician sedimentary rocks are exposed in the states of Baja California, Sonora, Chihuahua and Oaxaca, comprising approximately 30 stratigraphic successions ranging from Lower to Upper Ordovician. The ages of the sequences have been established primarily by utilizing conodonts and graptolites, which have also allowed us to differentiate between platform and oceanic basin environments. The State of Sonora has the most complete Ordovician stratigraphic sequences, ranging from Tremadocian to Hirnantian. The deposits in Baja California are Floian in age, while the sequences of Chihuahua range from Sandbian to Katian, and the deposits in Oaxaca are Tremadocian. The Ordovician deposits of northern Mexico (Baja California, Sonora, and Chihuahua) present a palaeogeographic relationship to the North American craton, mainly owing to faunal interspecific affinities, while the southern deposits (Oaxaca) are controversial owing to the high degree of endemism of the faunas; however, they show affinity with Gondwana, Baltica and Avalonia, with a possible insular origin. The biotic assemblages of the Ordovician of Mexico include a variety of taxa, including algae, poriferans, corals, bryozoans, brachiopods, molluscs, trilobites, echinoderms, graptolites and conodonts as predominant elements. Despite many years of field studies in Mexican Ordovician localities, biostratigraphic correlations are as yet insufficient and incomplete or are based on limited interpretations. Thus, the Ordovician biostratigraphic data from Mexico compiled in the present paper have great potential and significant value. The advancement in the knowledge of the Ordovician biostratigraphy of Mexico will contribute to a major understanding of the relationships with the Ordovician System to a continental scale. Future advances will come mainly through increasing the amount and quality of data as well as improving biocorrelations among the Ordovician sequences of Mexico.
Geology of Maracaibo Basin, Venezuela: PART 1
Pleistocene mammals from Extinction Cave, Belize
Increased xenarthran diversity of the Great American Biotic Interchange: a new genus and species of ground sloth (Mammalia, Xenarthra, Megalonychidae) from the Hemphillian (late Miocene) of Jalisco, Mexico
A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the North American Precambrian basement
New records of Neogene Xenarthra (Mammalia) from eastern Puna (Argentina): diversity and biochronology
Synextensional Pliocene–Pleistocene eruptive activity in the Camargo volcanic field, Chihuahua, México
The Mojave-Sonora megashear: The hypothesis, the controversy, and the current state of knowledge
The Mojave-Sonora megashear model, which implies left-lateral strike-slip motion of northern México in Jurassic time, remains one of the most influential ideas concerning the geology of México. A comprehensive review of the literature related to this topic does not yet allow resolution of the controversy over the validity of this hypothesis. A clear conclusion is that the original hypothesis was based on a relatively simplistic model of the geology of Sonora, as the basement of the Caborca terrane is not simply a fragment of the Mojave Precambrian basement province of eastern California. Attempts to use quantitative techniques in testing the model have yielded results contrary to the hypothesis, such as clockwise rotations indicated by paleomagnetic data, and the diversity and complexity of the basement of Caborca indicated by geochemical and geochronological data. Other quantitative methods such as zircon provenance studies in quartzites of the sedimentary cover yield inconclusive results. The main conclusion of the studies of detrital zircons is that Grenvillean zircons are relatively abundant, but that their presence cannot be attributed solely to sources in the Grenville province in a fixist model. Stratigraphic correlations of upper Paleozoic and Mesozoic rocks in Caborca with similar sequences in California and Nevada do not provide convincing arguments of large displacement, but should be evaluated in more detail. Elements that have the potential to test the hypothesis with greater certainty include detailed studies of basement rocks, a refined stratigraphy of the Jurassic volcanic and volcaniclastic arc rocks south of the inferred fault trace, and an increased understanding of depositional trends in the miogeoclinal sequence. Structural studies are sparse in this region. It is particularly important to gain a better understanding of the effects in time and space of Late Cretaceous–Tertiary contractional deformation. A tectonic evolution model that does not conflict with the existing data is the proposal that displacement of a para-autochthonous Caborca terrane may have occurred in the late Paleozoic. Nonetheless, available data and geologic relations in the Caborca region do not require Late Jurassic slip of several hundred kilometers. El modelo de la megacizalla Mojave-Sonora, el cual implica desplazamiento lateral izquierdo en el norte de México durante el Jurásico, permanece como una de las ideas más influyentes en la geología del país. Una revisión general de la literatura relacionada con el tema no permite aún resolver la controversia sobre la validez de la hipótesis, pero una conclusión clara es que la hipótesis original estaba basada en un modelo relativamente simplista de la geología de Sonora, ya que el basamento del terreno Caborca no es un simple fragmento de la corteza Mojave del este de California. Intentos de utilizar métodos cuantitativos han dado resultados contrarios a la hipótesis, como el de las rotaciones horarias indicadas por el paleomagnetismo y la diversidad de basamentos en Caborca que sugieren la geocronología y geoquímica; otros métodos producen resultados indeterminados, como la proveniencia de circones en las cuarcitas de la cobertura del terreno Caborca. La conclusión más relevante de esos estudios es la abundancia de circones de edad Grenvilleana, pero su presencia no puede simplemente atribuirse a fuentes en la Provincia Grenville en un modelo fijista. Las correlaciones estratigráficas entre secuencias Paleozoico tardío y Mesozoico en Caborca y secuencias similares en California y Nevada no producen argumentos convincentes a favor de grandes desplazamientos, pero deben considerarse con datos más detallados. Elementos que podrían evaluar la hipótesis con mayor contundencia son estudios más detallados del basamento, una estratigrafía fina del arco volcánico Jurásico y de las rocas volcanoclásticas al sur de la traza inferida de la falla y un mejor conocimiento de la secuencia miogeosinclinal. Son pocos los estudios estructurales en la región y en particular un problema importante es resolver en tiempo y espacio los efectos de la deformación compresional Cretácico-Terciario. Un modelo que no entra en conflicto con la evidencia existente es la propuesta de que el desplazamiento del terreno parautóctono Caborca haya ocurrido en el Paleozoico tardío. Sin embargo, los datos existentes y las relaciones geológicas en la región de Caborca, no requieren de un desplazamiento de cientos de kilómetros en el Jurásico Tardío.
Geology of the Southwestern Sierra Madre Oriental Fold-and-thrust Belt, East-central Mexico: A Review
Abstract This study presents a regional structural analysis and a stratigraphic study in Hidalgo and Querétaro states, east-central Mexico, where strata of the Sierra Madre Oriental foreland fold-and-thrust belt are structurally juxtaposed against Middle (?) and Upper Jurassic back-arc sequences. Rocks of the San Juan de la Rosa Formation were apparently affected by major compressive deformation during the Early Cretaceous (Nevadian?) and Laramide orogenies. Toward the east, the style of deformation is that of a typical foreland fold-and-thrust belt, whose major thrust faults are located at the Zimapán Basin margins flanked by the El Doctor and Valles-San Luis Potosí carbonate platforms. Below the El Doctor thrust, at least seven thrust faults are exposed that crosscut the middle Cretaceous rocks and the enveloping Upper Jurassic-Lower Cretaceous and Upper Cretaceous marly shaly beds in the form of a duplex. The style of deformation between the two carbonate platforms is probably thin-skinned. Speculative, semibalanced cross sections indicate an average shortening between the Zimapán Basin margins of 39.4% of its initial width. Depth to the sole thrust is estimated to be about 2000 m below the present sea level.
Abstract Middle to Late Triassic turbidite sequences are exposed in the states of Zacatecas and San Luís Potosí in central Mexico. These strata, assigned mostly to the Zacatecas Formation, accumulated in continental slope, toe-of-slope, and basin-plain environments along the passive continental margin of western Pangea. Strata of the Zacatecas Formation are age equivalent to rocks of the Antimonio Formation and Barranca Group in Sonora, the La Boca Formation in Tamaulipas and Nuevo León, and unnamed strata in Baja California. Based on their age, the Zacatecas turbidites correlate with a drop in sea level during the Permian-Triassic assembly of Pangea. The Triassic paleogeographic setting of Mexico is complex and poorly understood, because only dispersed Triassic outcrops exist across Mexico. However, the biogeographic affinities of the faunas from the Zacatecas Formation in central Mexico with those from equivalent strata in Baja California and Sonora suggest that these three regions were connected through the eastern Pacific, and that the Atlantic Ocean did not exist during the Ladinian-Carnian. The Zacatecas sequences underwent three periods of compressive deformation: one during their obduction onto the continental margin at some time during the latest Triassic-earliest Jurassic (?); a second during the Middle to Late Jurassic (Oxfordian) (?), apparently related to transpression; and a third during the Late Cretaceous to Tertiary Laramide orogeny.
An Endemic Cephalopod Assemblage from the Lower Campanian (Late Cretaceous) Parras Shale, Western Coahuila, Mexico
Geology of Venezuela and Its Oil Fields
SEG Newsletter 34 (July)
ABSTRACT Jurassic northward migration of Mexico, which lay on the southern part of the North America plate, resulted in temporal evolution of climate-sensitive depositional environments. Lower–Middle Jurassic rocks in central Mexico contain a record of warm-humid conditions, indicated by coal, plant fossils, and compositionally mature sandstone deposited in continental environments. Paleomagnetic data for central Oaxaca and other regions of central and eastern Mexico indicate that Lower and Middle Jurassic rocks were deposited at near-equatorial paleolatitudes. In the Late Jurassic, the Gulf of Mexico formed as a subsidiary basin of the Atlantic Ocean when the Pangea supercontinent ruptured. Upper Jurassic strata across Mexico, including eolianite and widespread evaporite deposits, indicate dry-arid conditions. Available paleomagnetic data (compaction-corrected) from southern and northeast Mexico for Upper Jurassic strata indicate deposition at ~15°N–20°N. As North America moved northward during Jurassic opening of the Atlantic Ocean, different latitudinal regions experienced coeval Middle–Late Jurassic climatic shifts. Climate transitions have been widely recognized in the Colorado Plateau region. The plateau left the horse latitudes in the late Middle Jurassic to reach temperate humid climates at ~40°N in the latest Jurassic. Affected by the same northward drift, the southern end of the North America plate represented by central Mexico gradually reached the arid horse latitudes in the late Middle Jurassic as the Colorado Plateau was leaving them. As a result, Late Jurassic epeiric platforms developed in the circum–Gulf of Mexico region after a long period of margin extension and were surrounded by arid land masses. We propose that hydrocarbon source-rock deposition was facilitated by arid conditions and wind-induced coastal upwelling.
Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.
ABSTRACT We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.
SEG Newsletter 32 (January)
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