Forrest G. Poole

Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins

Stratiform (bedded) Paleozoic barite occurs as large conformable beds within organic- and chert-rich sediments; the beds lack major sulfide minerals and are the largest and most economically significant barite deposits in the geologic record. Existing models for the origin of bedded barite fail to explain all their characteristics: the deposits display properties consistent with an exhalative origin involving fluid ascent to the seafloor, but they lack appreciable polymetallic sulfide minerals and the corresponding strontium isotopic composition to support a hydrothermal vent source. A new mechanism of barite formation, along structurally controlled sites of cold fluid seepage in continental margins, involves barite remobilization in organic-rich, highly reducing sediments, transport of barium-rich fluids, and barite precipitation at cold methane seeps. The lithologic and depositional framework of Paleozoic and cold seep barite, as well as morphological, textural, and chemical characteristics of the deposits, and associations with chemosymbiotic fauna, all support a cold seep origin for stratiform Paleozoic barite. This understanding is highly relevant to paleoceanographic and paleotectonic studies, as well as to economic geology.

Summary of Regional Evidence for Right-Lateral Displacement in the Western Great Basin: Reply

Right-lateral displacement of 80 to 120 miles across the western Great Basin is indicated by the consistent disruption of sedimentary facies and thickness trends of formations ranging in age from late Precambrian to Mesozoic. Some of this displacement occurs as fault slip and some as a more pervasive large-scale drag (oroflexural bending).

Implications of Latest Pennsylvanian to Middle Permian Paleontological and U-Pb SHRIMP Data from the Tecomate Formation to Re-dating Tectonothermal Events in the Acatlán Complex, Southern Mexico

Limestones in the highly deformed Tecomate Formation, uppermost unit of the Acatlán Complex, are latest Pennsylvanian—earliest Middle Permian in age rather than Devonian, the latter based on less diagnostic fossils. Conodont collections from two marble horizons now constrain its age to range from latest Pennsylvanian to latest Early Permian or early Middle Permian. The older collection contains Gondolella sp., Neostreptognathodus sp., and Streptognathodus sp., suggesting an oldest age limit close to the Pennsylvanian—Permian time boundary. The other collection contains Sweetognathus subsymmetricus, a short-lived species ranging only from Kungurian (latest Leonardian) to Wordian (earliest Guadelupian: 272 ± 4 to 264 ± 2 Ma). A fusilinid, Parafusulina c.f. P. antimonioensis Dunbar, in a third Tecomate marble horizon is probably Wordian (early Guadelupian, early Middle Permian). Furthermore, granite pebbles in a Tecomate conglomerate have yielded ~320-264 Ma U-Pb SHRIMP ages probably derived from the ~288 Ma, arc-related Totoltepec pluton. Collectively, these data suggest a correlation with two nearby units: (1) the Missourian—Leonardian carbonate horizons separated by a Wolfcampian(?) conglomerate in the upper part of the less deformed San Salvador Patlanoaya Formation; and (2) the clastic, Westphalian—Leonardian Matzitzi Formation. This requires that deformation in the Tecomate Formation be of Early—Middle Permian age rather than Devonian. These three formations are re-interpreted as periarc deposits with deformation related to oblique subduction. The revised dating of the Tecomate Formation is consistent with new data, which indicates that the unconformity between the Tecomate and the Piaxtla Group is mid-Carboniferous and corresponds to a tectonothermal event.

Extension of the Cordilleran Miogeosynclinal Belt to the San Andreas Fault, Southern California

Correlations of marine sedimentary rocks in the San Bernardino Mountains and western Mojave Desert area with known uppermost Precambrian and some Paleozoic rocks of the southern Great Basin suggest that a nearly complete succession of Cordilleran miogeosynclinal rocks extends into southern California. Pre-Mesozoic rocks in the San Bernardino Mountains consist of (1) gneiss and schist of Precambrian age (in part older than 1,750 m.y.); (2) the Saragossa Quartzite, here considered to be latest Precambrian and Early Cambrian in age; and (3) the Furnace Limestone, the upper part of which is locally dated by megafossils as Mississippian and possibly Pennsylvanian or Permian but considered here to contain strata ranging in age from Early Cambrian to Permian(?). The Saragossa Quartzite contains distinct units that can be lithologically correlated confidently with parts of the uppermost Precambrian and Lower Cambrian sequence containing the Johnnie, Stirling, Wood Canyon, and Zabriskie Formations of the eastern Mojave Desert and the southern Great Basin. Some rock units on Quartzite Mountain in the western Mojave Desert are lithologically correlated with the uppermost Precambrian and Cambrian Wood Canyon, Zabriskie, Carrara, and Bonanza King Formations of the eastern Mojave Desert and southern Great Basin. The Cordilleran miogeosynclinal belt is obliquely truncated by the San Andreas fault in southern California; a displaced segment may occur in the Salinian block 450 km to the northwest. Cordilleran miogeosynclinal rocks also occur in northwestern Mexico east of the San Andreas fault, indicating a major change in trend of the geosyncline between the Great Basin and Mexico.

Reinterpretation of the stratigraphy and structure of the Rancho Las Norias area, central Sonora, Mexico

Abstract New geologic mapping and fossil data in the vicinity of Rancho Las Norias, 30 km east of Hermosillo, Sonora, Mexico, show that rocks previously mapped as Precambrian instead are Paleozoic. Previous geologic maps of the Rancho Las Norias area show northeast-directed, southwest-dipping reverse or thrust faults deforming both Precambrian and Paleozoic rocks. The revised stratigraphy requires reinterpretation of some of these faults as high-angle normal or oblique-slip faults and the elimination of other faults. We agree with earlier geologic map interpretations that compressional structures have affected the Paleozoic rocks in the area, but our mapping suggests that the direction of compression is from southeast to northwest.

A problematic early tetrapod from the Mississippian of Nevada

ABSTRACT We report here the discovery of a new taxon of Paleozoic tetrapod from the Late Mississippian of Nevada (330–340 Ma). It has a unique vertebral column with principal centra having vertical anterior and posterior faces, ventrally incomplete accessory centra located antero-dorsally in each centrum, and enlarged presacral/sacral vertebrae. The head and pectoral girdle were not preserved but the large femur, robust pelvic girdle and enlarged sacral vertebrae possibly indicate a terrestrial mode of life. This new form significantly extends the western geographic range of known Mississippian tetrapods. It presents a mosaic of primitive and derived features, indicating that continued revision of traditional accounts of vertebral homology and the early diversifications of Paleozoic tetrapods will be necessary.