Keith B. Ketner

Cordilleran-margin quartzites in Baja California – implications for tectonic transport

Abstract One of the current controversies in Cordilleran tectonics concerns the position of Baja California prior to ∼300 km of opening of the Gulf of California. Geologic arguments, together with paleomagnetic results from Lower Cretaceous volcanic rocks, suggest that the rocks of the Baja Peninsula formed and evolved along the coast of northwestern Mexico prior to opening of the Gulf. In contrast, paleomagnetic data from Cretaceous–early Tertiary plutonic rocks and clastic strata have been interpreted by some workers to suggest that Baja was located near southern Mexico at approximately 80 Ma. The presence of similar detrital zircon ages in lower Paleozoic quartzites of northeast Baja and in lower Paleozoic strata east of the Gulf, in northwestern Mexico and southwestern US, provides strong support for the northern paleoposition, suggesting that Baja has been transported northward by only ∼300 km.

Permian and Triassic rocks near Quinn River Crossing, Humboldt County, Nevada

Permian and Triassic rocks near Quinn River Crossing, Humboldt County, Nevada, consist of four structural blocks: (1) a Lower Permian volcanic block; (2) a Permian(?) chert-arenite block; (3) a Lower Permian limestone block; and (4) a Permian and Triassic block. The contacts between the Permian volcanic block and the others are interpreted as thrust faults or glide surfaces. None of these rocks are metamorphosed, in contrast to those of the surrounding mountain ranges. Each of the blocks is lithically similar in some respects to rocks of the Osgood Mountains area 80 km to the southeast. The fusulinid and brachiopod faunas of two of the blocks display affinities to those of the McCloud Limestone of northern California and the Coyote Butte Limestone of central Oregon, and the fauna of another block has elements in common with autochthonous rocks of eastern Nevada and Utah. All four blocks probably are allochthonous with respect to the rocks exposed in the surrounding mountain ranges, but their points of origin remain obscure. The rocks at Quinn River Crossing provide a link among the Permian rocks of north-central Nevada, northern California, and central Oregon and a possible key to their original relations, but more comparative data are needed.

Eureka Quartzite in Mexico?—Tectonic implications

At Cerro Cobachi, 90 km east of Hermosillo, Sonora, an Ordovician to Permian miogeoclinal assemblage and an Ordovician to Permian siliceous deep-water assemblage were juxtaposed by thrust faulting between mid-Permian and latest Cretaceous time. Both assemblages resemble counterparts in the Great Basin. One formation, an ultramature quartzite unit in the miogeoclinal assemblage, closely resembles the Middle Ordovician Eureka Quartzite. In the southern Great Basin, isopach lines of the Eureka trend south-southwestward. From a maximum thickness of 134 m near Owens Lake, California, the Eureka thins and splays northward in the southern Inyo Mountains and thins southeastward in the Nopah Range. But south-southwestward, parallel with the isopach lines, it apparently ends abruptly as if faulted. Because the Paleozoic stratigraphy of the western Great Basin and that of west Texas have elements in common, it is quite possible that the southwest-trending facies belts of the Great Basin originally wrapped around the southern border of the continent through northern Mexico and joined corresponding belts in Texas. Two hypotheses are suggested: (1) the Cerro Cobachi terrane, of which the quartzite is a part, is indigenous to northern Mexico, and (2) the Cerro Cobachi terrane is indigenous to California and was displaced tectonically to northern Mexico. The second hypothesis is favored by the apparently abrupt termination of the Eureka Quartzite near Owens Lake, the nearly identical thickness of the two quartzites, and their nearly identical lithic composition and texture.

Recent studies indicate that major structures in northeastern Nevada and the Golconda thrust in north-central Nevada are of Jurassic or Cretaceous age

Geologic mapping recently completed in four areas of northeastern Nevada indicates that major folds and thrusts are of post-Early Triassic age and probably are Jurassic or Cretaceous. Previously published data for northeastern Nevada lead to, or permit, the same conclusion. Basinal deposits of Early Triassic age in the northern Adobe Range are easterly derived clay and carbonate. The apparent lack of westerly derived siliceous orogenic sediments of Sonoma age (Late Permian to Early Triassic) suggests that the Sonoma orogeny took place at some unknown location far from northeastern Nevada and that rocks deformed then, the Golconda allochthon, were emplaced in northern Nevada at a later date.

Mid-Paleozoic age of the Roberts thrust unsettled by new data from northern Nevada

The Roberts thrust is a major thrust in Nevada on which Ordovician to Devonian siliceous facies rocks were carried more than 80 km eastward over contemporaneous carbonate facies. For more than two decades, a mid-Paleozoic age for this structure has been widely accepted. The bases for dating the thrust are (1) the assumption that the Roberts thrust formed at the time of the Antler orogeny—a reliably dated mid-Paleozoic event, and (2) the concept of an overlapping assemblage consisting of sedimentary strata deposited on both the allochthon and autochthon. The first assumption is unproved, and the second relies on another assumption: that the overlap assemblage was not cut and telescoped by the Roberts thrust. In the years since a mid-Paleozoic age for the Roberts thrust became widely accepted, much new information on the stratigraphy and facies relations of Mississippian rocks in northern Nevada has accumulated. One interpretation of these new data suggests that in the Pinon Range, where a mid-Paleozoic age for the thrust was most convincingly displayed, the overlap assemblage actually may have been cut by a major thrust that juxtaposed contrasting facies of Mississippian rocks. If so, the principal evidence used to date the Roberts thrust is compromised, and the time has come for an agonizing reappraisal of all evidence bearing on the question. A mid-Paleozoic age can no longer be taken for granted, and a post-Paleozoic age cannot be ruled out.

Ordovician graptolites from the northern Sierra de Cobachi, Sonora, Mexico

Two Ordovician graptolite assemblages are recognised and described for the first time in the allochthonous siliceous assemblage or succession forming the Guayacan Group of central Sonora in northwestern Mexico. A Middle Ordovician assemblage referable to the Nemagraptus gracilis-Climacograptus wilsoni zonal interval occurs in the upper siliceous shale of unit 1 of the Guayacan Group and an Upper Ordovician assemblage representative of the Dicellograptus ornatus Zone of the Pacific faunal province has been collected from shaly partings in the bedded radiolarian chert constituting unit 2. The stratigraphical make-up of the siliceous assemblage bears a striking similarity to the better known—and correlative—Lower Palaeozoic siliceous succession of central and northern Nevada, giving credence to the hypothesis that the Palaeozoic siliceous assemblage in northwestern Mexico was originally the extension of the Nevadan assemblage into California, which has been transposed to its present position along a left-lateral megashear of the magnitude of at least 1000 km. A review of the proximal development of the spinose species remaining in the form genus Climacograptus Hall has led to the proposal to restrict the genus to Climacograptus bicornis (Hall), its type species, and to two other septate species of early Middle Ordovician age (Llandeilo-early Caradoc) with a primitive type of proximal-end development. The genus Euclimacograptus , with Climacograptus hastatus T. S. Hall as the type species, is proposed for spinose Upper Ordovician climacograptids with an advanced proximal-end development. The genus Ensigraptus , with Climacograptus caudatus Lapworth as the type species, is also proposed for species homeomorphic with those included in Normalograptus Legrand but with a less advanced proximal-end development. Five species are described.