Antimonio Formation Research Articles

Geochemistry of the sedimentary rocks from the Antimonio and Río Asunción formations, Sonora, Mexico: Implications for weathering, provenance and chemostratigraphy

Geochemical (major oxides, trace and rare earth elements) studies were carried out on the siliciclastic rocks of the Antimonio and Río Asunción formations to interpret the paleoweathering conditions and provenance. The weathering indices like the Chemical Index of Alteration (CIA) and Plagioclase Index of Alterations (PIA) indicate a low to moderate intensity of chemical weathering. The chondrite normalized REE patterns of the Antimonio and Río Asunción formations have LREE enriched, flat HREE patterns with a negative Eu anomaly. However, a few samples from these formations show a positive or an absence of the Eu anomaly, suggesting that the sediments were dominantly derived from felsic source rocks with a minor contribution from mafic source rocks. The vertical distribution of trace elements and their ratios indicate that sequence I of the Antimonio Formation presents a signature for felsic source rocks, however, in sequence II the sediment source changed, i.e. mafic rocks were recorded (SAAF06 and SAAF07). Sequences III to VI indicate a felsic source rock. The basal and upper parts of sequence VII (samples SARAF02 and SARAF10) of the Río Asunción Formation indicate mafic source rocks, whereas the remaining samples were largely contributed by felsic source rocks. The lower values of ferromagnesian trace elements and their ratios in sequences VIII and IX reveal a felsic source for the upper part of the Río Asunción Formation. The Antimonio and Río Asunción formations received sediments from two different sources: Permo-Triassic granitoids of northwestern Sonora and Precambrian granitoids from Caborca and North American blocks that contributed felsic rocks. Whereas mafic elements were derived from Precambrian amphibolites and Cambrian basalts located north and south of the study area.

A new Triassic sponge from the Antimonio terrane, Sonora, Mexico

A new Upper Triassic (Norian) chambered sponge, Fanthalamia glomerata n. sp., from the Antimonio Formation (Antimonio terrane) of northwestern Sonora, Mexico, is described. Recrystallized limestone containing the new sponge, together with other marine invertebrates, is interpreted to represent tropical, shallow-water carbonate settings characterized by local biostromal and biohermal buildups. The new species increases understanding of the ancient depositional environment and paleobiogeography of the Antimonio Formation.

Ammonoid discoveries in the Antimonio Formation, Sonora, Mexico: new constraints on the Triassic–Jurassic boundary

The Triassic–Jurassic systemic boundary was recently reported in the middle part of the Antimonio Formation, northwestern Sonora, where five informal sedimentary packages were delineated and characteristic ammonoid faunas were used to establish age control within the succession. The boundary was suggested to lie within the middle part of the 24m-thick package 4, in relatively unfossiliferous and organic-rich, laminated clay-silt mudstone. Despite the absence of diagnostic Hettangian fossils above the postulated boundary interval, its existence was predicted on characteristic uppermost Triassic Crickmayi Zone Choristoceras ammonoids occurring below in package 3 and upper Hettangian to lower Sinemurian (Badouxia Zone) ammonoids found above in package 5. Recent field investigations yielded new ammonoids of the uppermost Triassic Crickmayi Zone, which are described herein. They are assigned to Choristoceras cf. C. nobile Mojsisovics and Rhabdoceras cf. R. suessi Hauer. These characteristic ammonoids occur within the middle and top of package 4. Their discovery along with other stratigraphic evidence necessitates a revision of the boundary and recognition of a previously unrealized unconformity at the Triassic–Jurassic boundary in Sonora. A revised sea-level curve is necessary to account for these new stratigraphic and paleontological findings.

La structuration de la marge pacifique nord-américaine et du ≪ terrane ≫ Caborca : apports de la découverte d'une faune du Jurassique inférieur et moyen dans la série de Pozos de Serna (Sonora, Mexique)

We document an Early and Middle Jurassic age for the lower part of Jurassic rocks of the Pozos de Serna area, on the basis of new datings of ammonites. These rocks are correlated with the Lower Jurassic levels of the Antimonio Formation. The angular unconformity between the Pozos de Serna unit and the Proterozoic and Palaeozoic basement of the Caborca terrane allows us to propose that, in its present position, the whole Caborca terrane, including its Proterozoic basement, is accreted to the North American craton.

The Antimonio Formation in Sonora, Mexico, and the Triassic – Jurassic boundary

The Antimonio Formation furnishes a record of sedimentation across the Triassic–Jurassic system boundary and is one of a few stratigraphic sections globally that preserves latest Triassic to Hettangian ammonoids in stratigraphic succession. The boundary falls near the middle of the formation, within a 155 m thick stratigraphic section, which is divided into five distinct sedimentary packages. The laminated shales and siltstones in the middle of package 4 represent deposition in an anoxic or disaerobic setting. Although shales of package 4 themselves are poorly fossiliferous, they are bounded below and above by Triassic and Jurassic biotas, respectively. The Triassic–Jurassic system boundary should fall within or stratigraphically close to the laminated beds. The transgressive–regressive signature from the Antimonio Formation corresponds closely to that of the Gabbs and Sunrise formations in Nevada and jointly shows eustatic regressive events at or near the beginning of the latest Triassic Crickmayi Zone and another near the top of the Hettangian. The beds from package 4 indicate a transgression closely associated with the Triassic–Jurassic system boundary.

Timing of deformation and accretion of the Antimonio terrane, Sonora, from paleomagnetic data

At present, few paleomagnetic data exist to test tectonic models that link northern Mexico with the evolution of the southern Cordillera of North America. We present new paleomagnetic data for the Antimonio terrane in northwest Mexico. A moderate-inclination, dual-polarity yet secondary magnetization in Triassic-Jurassic strata of the Antimonio Formation and a primary remanence in Cretaceous igneous rocks (combined result: D = 338.9°, I = 57.4° α 95 = 5.6°, k = 34.8; N = 20 sites) are interpreted to indicate stability of the Antimonio terrane with respect to the craton since the Late Cretaceous. A more important result, however, is a shallow-inclination, dual-polarity magnetization in Antimonio Formation strata that fails a fold test ( D = 186.7°, I = −28.4°; α 95 = 7.6°, k = 28.2; N = 14 sites). We interpret the shallow-inclination magnetization as a secondary remanence acquired during accretion of the Antimonio terrane to the North American margin in the Middle or Late Jurassic, or during deformation within an essentially autochthonous Jurassic continental arc.

Upper Triassic Invertebrates from the Antimonio Formation, Sonora, Mexico

A diverse Upper Triassic tropical marine fauna from northwestern Sonora, Mexico, includes 31 taxa of tropical invertebrates including scleractinian corals, spongiomorphs, disjectoporoids, “hydrozoans,” thalamid and nonthalamid sponges, spiriferid and terebratulid brachiopods, gastropods, bivalves, coleoids, and anomuran microcoprolites. They occur within the late Karnian to Norian part of the Antimonio Formation (Antimonio terrane), which is juxtaposed against a fragmented portion of the North American craton. Most of the fauna is also known from the Tethys region. Sixteen Sonoran taxa co-occur in the western Tethys and five have never been known outside this region. Four additional taxa (one identified only at genus level) are geographically widespread. Some taxa occur in displaced terranes of North America, especially in west-central Nevada (Luning Formation). A weak link exists with the California Eastern Klamath terrane but stronger ties exist with Peru. Among Sonoran sponges,Nevadathalamia polystomawas previously recognized only from the Luning Formation, western Nevada. SpongesCinnabaria expansa, Nevadathalamia cylindrica, and a coral,Astraeomorpha sonorensisn. sp., are also known from Nevada. The coralsDistichomeandra austriaca, Chondrocoenia waltheri, Pamiroseris rectilamellosa, andAlpinophyllia flexuosaco-occur in central Europe. Two new taxa, a spongiomorph hydrozoan,Stromatoporidium lamellatumn. sp., and a disjectoporoid,Pamiropora sonorensisn. sp., have distinct affinities with the Tethys. The geographically widespread North American brachiopod,Spondylospira lewesensis, andPseudorhaetina antimoniensisn. gen. and sp. are among the Sonoran fauna. The Sonoran coleoid (aulacocerid)Dictyoconites(Dictyoconites) cf.D. reticulatumoccurs in the Tethys realm andCalliconitescf.C. drakeiis comparable with a species from the Eastern Klamath terrane.Calliconites millerin. sp. is the first occurrence of the genus outside Sicily. The bivalvesMyophorigonia jaworskii, M. salasi, andPalaeocardita peruvianaare known from Sonora and Peru. Eight gastropod taxa includeGuidoniacf.G. intermediaandG.cf.G. parvula, both previously known from Peru, andEucycloscala subbisertusfrom the western Tethys. The gastropods are unlike those already known from other North American terranes.

Paleobiogeography of Mesozoic articulate brachiopods from the Western Cordillera of North America and their potential for paleogeographic studies

Mesozoic brachiopods were, at times, significant elements of marine invertebrate faunas. Current investigations suggest that Mesozoic brachiopods are more common in Mesozoic marine sequences from North America than has generally been assumed. Their neglect is no doubt in part due to the greater utility of other invertebrate and microfossil groups for biostratigraphy. Brachiopods may be preserved in original shell material or silicified. It is therefore necessary to consider which is the most appropriate method of extraction, depending on type of preservation.Lacking planktotrophic larval stages, living articulate brachiopods are limited in their dispersal potential by virtue of their sessile, benthic mode of life. If, in addition, all post-Paleozoic articulate brachiopods possessed a non-planktotrophic larval stage endemism would be likely to develop if gene-flow became severed. This would mean that taxonomic investigation of articulate brachiopods has the potential to provide useful paleobiogeographic and paleogeographic information. Recent investigations have concentrated on making a preliminary survey of some brachiopod occurrences in the Western Cordillera of North America with these goals in mind.The Upper Triassic brachiopod fauna from the Luning Formation of the Pilot and Shoshone Mountains, Nevada, is the most diverse known for the Mesozoic of North America in terms of number of brachiopod species (manuscript submitted with George D. Stanley). This is probably a reflection of how little detailed collecting and systematic study Mesozoic representatives of the phylum have received in North America. The fauna comprises both Tethyan and endemic species. The brachiopods are from the Paradise terrane, probably close to the North American craton in the Late Triassic. One Upper Triassic brachiopod fauna from the Antimonio Formation, Sonora, is by comparison with the Nevada faunas, depauperate, but they do share one common species. Additional time-equivalent brachiopod faunas from outboard terranes of North America and the “classic” European faunas monographed in the nineteenth and early twentieth centuries require investigation to determine their paleobiogeography and their contribution to paleogeography.Jurassic brachiopods from North America have not been subjected to any major revision but they are present at certain horizons. Cretaceous faunas from the southern United States and Mexico contain genera known from Tethys in Europe. Mid-Cretaceous faunas from the Queen Charlotte Islands (Wrangellia terrane) and the Canadian Arctic Islands contain forms that are more typical of mid-latitude to Boreal regions, repectively, of Europe. This suggests a broad correspondence between brachiopod distributions and paleolatitude across considerable paleolongitudinal distances, an observation of relevance to interpreting Early Mesozoic paleobiogeographic distributions.The current work is only scratching the surface of the Phylum's distribution in the Western Cordillera of North America. The aim is to provide a better understanding of brachiopod paleobiogeography, paleogeography, and the evolutionary history of the Brachiopoda during the post-Paleozoic, which does not appear to be their swansong.

Shastasaurus altispinus(Ichthyosauria, Shastasauridae) from the Upper Triassic of the El Antimonio district, northwestern Sonora, Mexico

New specimens ofShastasaurus altispinusMerriam are recognized from the Upper Triassic Antimonio Formation in the El Antimonio district of northwestern Sonora, Mexico. This represents the first occurrence of the species outside of its type locality in Shasta County, California, and the first known occurrence of Triassic ichthyosaurs from Mexico. Some remains are well preserved, but in general they are fragmented, structurally distorted, and incomplete; however, they enhance the current understanding of the morphology of a species that has been known hitherto only from an even more incomplete holotype. The material is the first from anyShastasaurusthat preserves the anterior portions of the skull and lower jaw and implanted teeth. Revised diagnoses for the genus and species are given on the basis of the new material. Biostratigraphic zonation furnishes an early Late Carnian age for the specimens, which is in accord with that for the holotype.

A paleomagnetic test of the proposed Mojave-Sonora megashear in Northwestern Mexico

Three Upper Triassic-Lower Jurassic formations in southwestern United States and northwestern Mexico have been investigated paleomagnetically as a means of testing for displacement along the proposed Mojave-Sonora megashear. Luning Formation of west-central Nevada and Sil Nakya Formation of southern Arizona lie north of the megashear whereas, in northwestern Sonora, Antimonio Formation lies south of the megashear. Previously published results from the Nazas Formation of presumed Lower Jurassic age from north-central Mexico were incorporated into the interpretations. These four units compose two sets of correlative formations on opposite sides of the proposed megashear. Restoration of 800 km of left-lateral displacement along the megashear about the pole of rotation determined by Anderson and Schmidt (1983), improves the grouping of the two sets of paleomagnetic poles noticeably, with the improvement having statistical significance at the 95% confidence level. We feel that the data support, but do not prove, the postulated displacement along the megashear. All four poles are displaced from Irving's (1979) smoothed apparent polar wander path for cratonic North America in the intervals 190 and 200 Ma. The displacement needed to bring all of these poles into coincidence is a pure rotation of about 8° counterclockwise about the sampling area. Two probable Cretaceous poles were obtained from the Sand Wells Formation of southern Arizona and from unnamed volcanics in northwest Sonora. These poles are essentially identical and suggest that displacement along the megashear had ceased by the Cretaceous. Data from the Early Jurassic Mulberry Wash and Pitoikam formations in southern Arizona appear to have been remagnetized in a later Cretaceous event and were not used in this study.