Geology topics

Benita L. Murchey

Publications and source records attributed to Benita L. Murchey.

5 recordsLinked to original sources

USGS2011

The Cannery Formation: Devonian to Early Permian arc-marginal deposits within the Alexander Terrane, southeastern Alaska

The Cannery Formation consists of green, red, and gray ribbon chert, siliceous siltstone, graywacke-chert turbidites, and volcaniclastic sandstone. Because it contains early Permian fossils at and near its type area in Cannery Cove, on Admiralty Island in southeastern Alaska, the formation was originally defined as a Permian stratigraphic unit. Similar rocks exposed in Windfall Harbor on Admiralty Island contain early Permian bryozoans and brachiopods, as well as Mississippian through Permian radiolarians. Black and green bedded chert with subordinate lenses of limestone, basalt, and graywacke near Kake on Kupreanof Island was initially correlated with the Cannery Formation on the basis of similar lithology but was later determined to contain Late Devonian conodonts. Permian conglomerate in Keku Strait contains chert cobbles inferred to be derived from the Cannery Formation that yielded Devonian and Mississippian radiolarians. On the basis of fossils recovered from a limestone lens near Kake and chert cobbles in the Keku Strait area, the age of the Cannery Formation was revised to Devonian and Mississippian, but this revision excludes rocks in the type locality, in addition to excluding bedded chert on Kupreanof Island east of Kake that contains radiolarians of Late Pennsylvanian and early Permian age. The black chert near Kake that yielded Late Devonian conodonts is nearly contemporaneous with black chert interbedded with limestone that also contains Late Devonian conodonts in the Saginaw Bay Formation on Kuiu Island. The chert cobbles in the conglomerate in Keku Strait may be derived from either the Cannery Formation or the Saginaw Bay Formation and need not restrict the age of the Cannery Formation, regardless of their source. The minimum age of the Cannery Formation on both Admiralty Island and Kupreanof Island is constrained by the stratigraphically overlying fossiliferous Pybus Formation, of late early and early late Permian age. Because bedded radiolarian cherts on both Admiralty and Kupreanof Islands contain radiolarians as young as Permian, the age of the Cannery Formation is herein extended to Late Devonian through early Permian, to include the early Permian rocks exposed in its type locality. The Cannery Formation is folded and faulted, and its stratigraphic thickness is unknown but inferred to be several hundred meters. The Cannery Formation represents an extended period of marine deposition in moderately deep water, with slow rates of deposition and limited clastic input during Devonian through Pennsylvanian time and increasing argillaceous, volcaniclastic, and bioclastic input during the Permian. The Cannery Formation comprises upper Paleozoic rocks in the Alexander terrane of southeastern Alaska. In the pre-Permian upper Paleozoic, the tectonic setting of the Alexander terrane consisted of two or more evolved oceanic arcs. The lower Permian section is represented by a distinctive suite of rocks in the Alexander terrane, which includes sedimentary and volcanic rocks containing early Permian fossils, metamorphosed rocks with early Permian cooling ages, and intrusive rocks with early Permian cooling ages, that form discrete northwest-trending belts. After restoration of 180 km of dextral displacement of the Chilkat-Chichagof block on the Chatham Strait Fault, these belts consist, from northeast to southwest, of (1) bedded chert, siliceous argillite, volcaniclastic turbidites, pillow basalt, and limestone of the Cannery Formation and the Porcupine Slate of Gilbert and others (1987); (2) greenschist-facies Paleozoic metasedimentary and metavolcanic rocks that have Permian cooling ages; (3) silty limestone and calcareous argillite interbedded with pillow basalt and volcaniclastic rocks of the Halleck Formation and the William Henry Bay area; and (4) intermediate-composition and syenitic plutons. These belts correspond to components of an accretionary complex, contemporary metamorphic rocks, forearc-basin deposits, and the roots of a volcanic arc, respectively. The similar early Permian sedimentary, metamorphic, and igneous ages are inferred to represent an arc complex that resulted from juxtaposition of a structural lower plate consisting of metamorphosed Paleozoic arc rocks of the Alexander terrane exposed on Admiralty Island, and a structural upper plate consisting of stratigraphically distinct, unmetamorphosed Paleozoic arc rocks representing another component of the Alexander terrane exposed on Chichagof, Kuiu, and Prince of Wales Islands. The Cannery Formation is associated with the lower-plate package. A volcanic arc with magmatic ages ranging from 293 to 278 Ma formed during subduction of the basin between these two (or more) components of the Alexander terrane in southeastern Alaska. Metamorphic-mineral-cooling ages ranging from 273 to 260 Ma are interpreted to date an early Permian orogenic event. Both the early Permian lower- and upper-plate rocks are unconformably overlain by late early and early late Permian limestone, dolostone, and conglomerate of the Pybus Formation that provide a minimum age for this collision.

Alaska↗

USGS2005

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlit-pyroxenite, quartz diorite), a mafic to felsic dike complex, and mafic-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are mélange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO mélange blocks below the SFVC are similar (166–172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6–6 of Baumgartner et al., 1995a ) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8–10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small sized, polytaxic radiolarian faunas were replaced by very robust, oligo-taxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172–180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfigured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a backarc origin for the CRO.

California↗

USGS2004

Regional analysis of spiculite faunas in the permian phosphoria basin: Implications for paleoceanography

The sponge spiculites of the Permian Phosphoria basin, Antler high, and eastern Havallah basin were the southernmost expression of one of the largest spiculite belts in the Earth's history. This spiculite belt extended from Nevada to the Barents Sea. In Idaho and Nevada, the spicule populations of this belt are dominated by demosponge spicules and are distinctive for their abundant rhax microscleres, large monaxons, and lithistid desmas. They form an Eastern Belt of spiculites that interfingers with spicule assemblages derived from choristid demosponges and hexactinellids that lived along the eastern margin of the deeper Havallah basin. The Havallah basin assemblages are similar to those in Permian arc terranes to the west, and together the sponge populations in this domain constitute a dis- tinct Central Belt. Radiolarians are virtually absent in the siliceous microfossil populations of the Eastern Belt, abundant in the populations of the Central Belt, and dominant in the populations of a Western Belt confined to Mesozoic accretionary complexes in the Pacific Coast States. The scattered sponge spicules in the Western Belt radiolarites were derived from hexactinellids. During the Permian, the relative abundance and apparent diversity of siliceous sponges expanded over a wide range of depths in the basins from Nevada and Idaho to the open ocean. Radiolarian preservation and apparent diversity increased in the deeper Cordilleran basins as well. In the Arctic regions, significant sponge spiculites were deposited in epicratonic basins. At the same time that siliceous sponge populations expanded along the northwestern margin of Pangea, warm-water carbonate producers disappeared. Suppression of carbonate-producing organisms along the margin was critical to the accu- mulation and preservation of both the demosponge spiculites in the Eastern Belt and the spicule-rich argillites of the Central Belt. Vigorous thermohaline circulation was the major control on the paleobiogeography of the late Early, Middle, and early Late Permian along northwest Pangea. It was driven by cold, nutrient- and oxygen-rich northern waters and it produced a coastal current that swept down the margin of the supercontinent. The upwelling associated with deposition of world-class phosphorites in the Phosphoria basin was a part of this larger oceanographic system.

Handbook of Exploration and Environmental Geochemi↗

USGS1990

Age and depositional setting of siliceous sediments in the upper Paleozoic Havallah sequence near Battle Mountain, Nevada; Implications for the paleogeography and structural evolution of the western margin of North America

The upper Paleozoic Havallah sequence of central Nevada is a folded and thrust-faulted association of greenstone, siliceous marine sedimentary rocks, and deep-water clastic rocks. Microfossil assemblages (radiolarians, sponge spicules, and conodonts) are used as tools to unravel the stratigraphy and to interpret the paleoenvironments of the siliceous sedimentary rocks. Nine radiolarian assemblages (Osagean to Guadalupian) are described and used for delineation and correlation of fault-bounded lithotectonic units. The biostratigraphic zonation reveals that the oldest rocks in each lithotectonic unit are progressively younger from the structurally highest to the lowest units, suggesting progressive west-to-east upsection stepping of the Golconda sole thrust with accretion of each unit. Analyses of the radiolarian and sponge spicule faunas permit lateral and temporal comparisons of depositional environments. The lower structural units are coarsening-upward sequences of hemipelagic slope deposits overlain by sponge spicule-rich turbidites derived from a shallow source. The uppermost structural unit is a coarsening-upward basinal sequence. Permian sponge spicules in turbidites of the slope sequences and redeposited fusulinids in the basin sequence are similar to those in adjacent autochthonous (North American) regions. Permian radiolarians and sponge spicules in hemipelagic siliceous argillite of the slope sequences are similar to those in the Northern Sierra terrane to the west; the Havallah basin and the Northern Sierra arc terrane were overlain, therefore, by a similar water mass and may have been in proximity during the Permian. Clastic dikes and sills containing volcanic, metamorphic, and sedimentary rock clasts are Leonardian or younger.

Nevada↗

USGS1990

Biostratigraphic, tectonic, and paleogeographic ties between upper Paleozoic volcanic and basinal rocks in the northern Sierra terrane, California, and the Havallah sequence, Nevada

Biostratigraphic data, based mostly on radiolarian assemblages, establish synchronous deposition in the northern Sierra terrane and the Havallah basin beginning in the Late Devonian and extending into the early Late Permian. Lower Mississippian and mid-Permian arc-derived volcaniclastic debris was deposited in parts of the Havallah basin during episodes of arc volcanism in the northern Sierra terrane. Between these episodes of arc volcanism, from late Early Mississippian to at least Middle Pennsylvanian, the northern Sierra terrane collected siliceous pelagic deposits that correlate with dominantly chert-argillite sections in the Havallah sequence. These intermixed lithic assemblages suggest shared stratigraphic evolution and geographic proximity between the Sierran arc terrane and the Havallah basin during the late Paleozoic. During Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane, lower Paleozoic rocks of the Roberts Mountains allochthon were thrust over coeval deposits on the North American shelf. Chert-quartz-rich siliciclastic debris, derived from the Antler orogenic belt, is interbedded with Upper Devonian and Lower Mississippian distal volcanic rocks in the northern Sierra terrane and with Kinderhookian volcaniclastic rocks and chert in the Schoonover sequence. These quartzose-clastic deposits not only provide an independent lithologic link between the Sierran arc terrane and the Havallah basin, they also tie the arc terrane and basin to North America at the time of the Antler orogeny. Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane occurred in an extensional regime. Extensional tectonism began locally in the Havallah basin during the Famennian and continued into the early Meramecian. Contemporaneous extension in the arc and basin during emplacement of the Roberts Mountains allochthon is difficult to reconcile with existing arc-continent collision models for the Antler orogeny.

California, Nevada↗