A NEW EARLY CAMPANIAN RUDIST FAUNA FROM SAN LUIS POTOSI IN MEXICO AND ITS TAXONOMIC AND STRATIGRAPHIC SIGNIFICANCE

up to now it and Santonian and Turonian units have been mapped as the El Abra Formation in most available geological maps. Recognition of the successive carbonate platform intervals, between El Abra and Cardenas formations, improves the geological mapping and the understanding of the Mexican Gulf western margin evolution during the Late Cretaceous

Global evolution and paleogeographic distribution of mid-Cretaceous orbitolinids

Members of the Larger Benthic Foraminiferal (LBF) family Orbitolinidae occured from the Cretaceous to the Paleogene, however, they were most diverse during the mid-Cretaceous, and dominated the agglutinated LBF assemblages described from limestones of that period. Various orbitolinid species have been used to zone and date lithologies formed in the shallow, warm waters of the Aptian to the Early Cenomanian, and many, sometimes inaccurate, generic and sub-generic nomenclatures have been proposed to differentiate the often subtle morphological changes that orbitolinids exhibit over time. Until now, it has not been possible to develop an effective global overview of their evolution and environmental development because descriptions of specimens from Asia have been relatively rare. Following our recent study of over 1800 orbitolinid-rich thin sections of material from 13 outcrops of the Langshan Limestone, from the Southern Tibetan Plateau, and from the Barito Basin, South Kalimantan, Indonesia, it has been possible to compare the stratigraphic ranges of these orbitolinids with previously described Tethyan and American forms, based on the use of a planktonic zonal (PZ) scheme, itself tied to the most recent chronostratigraphic scale. This has allowed the reconstruction of the phylogenetic and paleogeographic evolution of the orbitolinids from their Valanginian origin in the Tethys. Although Tethys remained the paleogeographic focus for the orbitolinids, it is inferred here for the first time that a bi-directional paleogeographic migration of some orbitolinid genera occurred from Tethys to the Americas and also to the Western Pacific region. Our observations and dating confirm that global marine regressions in the Aptian were coincident with, and may well have facilitated, these orbitolinid transoceanic migrations. However, migration stopped after rising sea-level in the Early Albian appears to have again isolated these provinces from each other. Tectonic forces associated with the subduction of the Farallon Plate and further sea-level raises led to the opening of the Western Interior Seaway in the North America, which correlates with, and may have been the cause of, the Middle Albian (top of PZ Albian 2) extinction of the American orbitolinids. The extinction of the orbitolinids revealed that the Western Pacific province was split into two sub-provinces, with extinction occurring at the end of the Early Albian (top of PZ Albian 1) in the Northwest Pacific sub-province, and at the end of the Albian (top of PZ Albian 4) in the sub-province that is today South East Asia (on the margins and west of the Wallace Line). The final virtual extinction of the orbitolinids occurred at the end of the Cenomanian in the Tethyan province, which coincides with, and may have been caused by, global anoxic oceanic events that correlate with a near-peak Mesozoic eustatic sea-level high-stand that led to the overall global collapse of the paleotropical reef ecosystem at that time

Magmatism and tectonics of the Sierra Madre Occidental and its relation with the evolution of the western margin of North America

The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of fi ve main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene, and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the Sierra Madre Occidental. The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the fi nal stages of this deformation cycle, during the Paleocene and Early Eocene, ~E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California. Geochemical data show that the Sierra Madre Occidental rocks form a typical calc-alkaline rhyolite suite with intermediate to high K and relatively low Fe contents. Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87Sr/86Sr ratios mostly range between 0.7041 and 0.7070, and initial εNd values are generally intermediate between crust and mantle values (+2.3 and −3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafi c magmas with little (<15%) or no crustal involvement. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that Sierra Madre Occidental ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts. Geophysical data indicate that the crust in the unextended core of the northern Sierra Madre Occidental is ~55 km-thick, but thins to ~40 km to the east. The anomalous thickness in the core of the Sierra Madre Occidental suggests that the lower crust was largely intruded by mafi c magmas. In the westernmost Sierra Madre Occidental adjacent to the Gulf of California, crustal thickness is ~25 km, implying over 100% of extension. However, structures at the surface indicate no more than ~50% extension. The upper mantle beneath the Sierra Madre Occidental is characterized by a low velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range Province of the western United States. The review of the magmatic and tectonic history presented in this work suggests that the Sierra Madre Occidental has been strongly infl uenced by the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene Sierra Madre Occidental is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during and immediately following the final stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, resulted in extension of the continental margin, eventually leading to direct interaction between the Pacific and North American plates