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At least 163 records · Page 9Linked to original sources
USGS1975

Chino Valley formation (Cambrian?) in northwestern Arizona

A thin persistent unit (maximum thickness 13 m) of probable Cambrian age in the Chino Valley region of northwestern Arizona consists of three laterally equivalent, mutually exclusive lithofacies. These are, from west to east, lithic sandstone, pebble to boulder conglomerate, and dolomite. The unit, named the Chino Valley Formation, is younger than middle Middle Cambrian and older than Devonian in age. A Late Cambrian age is probable, although no fossil evidence supports this conclusion. The lithic sandstone and conglomerate facies were derived from two apparently active source areas and were probably deposited in shallow marine water. Uplift to the south or southwest of the area almost reversed the regionally westward-dipping Cambrian paleoslope and exposed sedimentary rocks to erosion. Sediment derived from this terrain formed the lithic sandstone facies. Synchronous movement initiated erosion of the Mazatzal Quartzite (Precambrian) from which the conglomerate facies was derived. The dolomite facies is inferred to have accumulated on a mud flat in the supratidal zone.

Arizona↗
USGS1976

Plate tectonic model for the evolution of the eastern Bering Sea Basin

The eastern Bering Sea Basin, composed of the Aleutian and Bowers Basins, is flanked to the north by Mesozoic foldbelts that probably represent zones of plate subduction in Mesozoic time. Present plate subduction occurs 400 to 1,000 km farther south, at the Aleutian Trench. North-south magnetic lineations that formed at an oceanic spreading ridge, probably in Mesozoic time (117 to 132 m.y. ago), have been identified in the Aleutian Basin. The orientation and age of those anomalies can be explained by reconstructing Kula-Farallon Pacific plate motions during late Mesozoic–early Tertiary time. In Mesozoic time, subduction of the Kula plate occurred north of the Aleutian Trench near the present location of the Bering Sea continental margin. At about 70 m.y. B.P. (Late Cretaceous), the zone of subduction shifted south to the present location of the Aleutian Trench, thereby trapping a fragment of oceanic plate imprinted with north-south magnetic lineations within the eastern Bering Sea Basin. A stable basin framework has prevailed behind the Aleutian arc since early Tertiary time.

GSA Bulletin↗
USGS1976

Trace-element variations at Summer Coon volcano, San Juan Mountains, Colorado, and the origin of continental-interior andesite

The Oligocene Summer Coon center, an eroded continental-interior volcano of the eastern San Juan Mountains, Colorado, was the source of magmas ranging in composition from basaltic andesite to rhyolite. Previous Pb and Sr isotope studies indicate derivation of the magmas from an isotopically homogeneous source. This study presents new data for rare-earth elements (REE), U, Th, Ba, Sr, Rb, and Ni from 10 samples of the Summer Coon sequence. Alkali elements are high in all rocks; as SiO 2 increases, Ba increases from 900 to 2,000 ppm, Rb increases from 35 to 90 ppm, Sr decreases from 900 to 350 ppm, K/Rb decreases slightly, Ba/Sr increases, U increases from 0.5 to 2.5 ppm, and Th increases from 2 to 7 ppm. Chondrite-normalized REE patterns are strongly fractionated in comparison with oceanic-arc andesite-dacite sequences. La is 80 to 120 times chondritic abundance, but Yb and Lu are less than 10 times chondritic abundance. Small negative Eu anomalies characterize the rhyolites. Nickel in the andesites is 40 to 70 ppm. The origin of the andesite is interpreted in terms of nonmodal partial melting of a trace-element—enriched garnet-bearing source, possibly subducted crust that has converted to eclogite. Rhyodacite and rhyolite are interpreted as low-pressure crystal-fractionation products of silicic andesite, in which crystallizing phases are hornblende rich in REE and plagioclase.

Colorado↗
USGS1976

Obsidian hydration dating and correlation of Bull Lake and Pinedale Glaciations near West Yellowstone, Montana

The ages of the last two glaciations near West Yellowstone, Montana, can be calculated by obsidian hydration techniques that are calibrated by K-Ar dating of obsidian-bearing lava flows. The average age of glacial abrasion of obsidian in the Pinedale terminal moraines is about 30,000 yr, with most age measurements between 20,000 and 35,000 yr. For the Bull Lake moraines, it is about 140,000 yr, with most measurements between 130,000 and 155,000 yr. This age for the Bull Lake moraines is also supported by geologic relations that show that the moraines are older than a rhyolite flow dated by K-Ar as 114,500 ± 7,300 yr old (lσ). These obsidian hydration ages of the Pinedale and Bull Lake Glaciations correlate well with the last two cold intervals of the marine record. The age determined for the Bull Lake Glaciation near West Yellowstone antedates the last interglaciation of the marine record, which is commonly correlated with the Sangamon Interglaciation. Our results suggest correlation of the Pinedale Glaciation near West Yellowstone with much or all of the Wisconsin Glaciation and of the Bull Lake with the late Illinoian. This differs with the commonly accepted correlation of the Pinedale with the late (“classical”) Wisconsin and of the Bull Lake with the early Wisconsin. The correlation of Bull Lake with late Illinoian appears equally or more compatible with traditional criteria for correlation, namely comparative soil development and degree of preservation of morainal morphology.

Wyoming, Montana, Idaho↗
USGS1976

Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar results

An orogenic event, correlated with the Pan-African event in eastern Africa, affected the Arabian Peninsula between 510 and 610 m.y. ago and is well-recorded geochronologically. The event probably included two thermal pulses or maxima, the first occurring between 560 and 610 m.y. ago and the second between 510 and 540 m.y. ago. The earlier pulse, the more severe one, included the majority of the igneous activity and metamorphism. During the last part of the 510- to 610-m.y. period, left-lateral strike-slip faulting occurred along a set of northwest-trending en echelon fracture zones, whose composite displacement may be as large as 240 km. At least one and probably more orogenic events affected the Arabian Peninsula before the Pan-African event, but only minimum ages can be assigned to these, because thermal effects of the 510- to 610-m.y. event have reset K-Ar ages. Major diorite-granite batholiths, however, formed before 760 m.y. ago.

Arabian Shield↗
USGS1977

Deposition of the Tapeats Sandstone (Cambrian) in central Arizona

Grain size, bedding thickness, dispersion of cross-stratification azimuths, and assemblages of sedimentary structures and trace fossils vary across central Arizona; they form the basis for recognizing six facies (A through F) in the Tapeats Sandstone. Five of these (A through E), present in western central Arizona, are marine deposits containing the trace fossil Corophioides ; several intertidal environments are represented. The association of large-scale cross-bedding (50 to 300 cm) that is characterized by compound cross-stratification, numerous reactivation surfaces, and herringbone patterns is typical of facies A and generally typical of the finer-grained, thinner-bedded facies B. The sedimentary structures and polymodal distribution of foreset azimuths common to facies A and B probably formed on intertidal sand bars during emergence and late-stage tidal runoff. Facies C consists of well-sorted sandstone, gently cross stratified or with continuous parallel stratification, and foresets tangential to the lower bedding surface. This facies generally occurs where the gradient of the depositional surface increases; it apparently was deposited on a beach by shoaling waves. Facies D and, to a lesser extent, the coarser-grained facies E are sandstones with trough cross-stratification, fining-upward cycles, abundant intercalated thin shale and sandstone, rare flaser bedding, and local bipolar distribution of foreset azimuths. Both facies are tidal flat deposits; facies D was probably produced by meandering tidal channels, whereas facies E was likely produced by migration of braided tidal channels. The sixth facies (F), present in eastern central Arizona, is an arkosic small-pebble conglomerate that lacks trace fossils; low dispersion of foreset azimuths and large-scale (1 to 11-m wide) cut-and-fill structure are typical. Facies F was deposited by bedload streams that transported coarse, poorly sorted sand and gravel westward to the intertidal flats.

Arizona↗
USGS1977

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

GSA Bulletin↗
USGS1977

Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat

The electron microprobe was used to determine the bulk composition of immiscible sulfide globules trapped in the glass phase of 25 fresh submarine basalt samples from the Mid-Atlantic Ridge. Twenty-three samples represent a spectrum of primitive through differentiated tholeiites from the FAMOUS dive area; two are differentiated basalts from the Reykjanes Ridge. The analyzed globules range in diameter from 11 to 233 µm. On the average, they constitute only 0.0022 volume percent of the rocks and contain less than 1.5 percent of the sulfur. Compositions of the globules change with differentiation as measured by Fe/(Fe+Mg) or TiO 2 content of the host glass. Globules in glass containing 0.66 to 1.0 wt percent TiO 2 typically contain 20 to 26 wt percent Ni + Cu and have an average atomic Ni/Cu of 1.6. With differentiation toward 1.6 wt percent TiO 2 , Ni + Cu content of the globules falls to less than 10 wt percent and atomic Ni/Cu falls to 0.4. Sulfur content of the host glasses shows a strong correlation with FeO content, increasing from 840 ppm to 1,370 ppm as FeO content increases from 8.0 to 12.6 wt percent. Reference to experimental studies shows that this relationship is consistent with sulfur saturation of the host glass at liquidus temperatures. Crystal fractionation is considered to be the dominant factor in keeping the differentiating melt at sulfur saturation. The sulfide globules may have persisted in the basaltic melt from its place of formation by partial melting in the mantle, or they may have exsolved from the melt as it became sulfur-saturated in a high-level magma chamber. Globule abundance and composition indicate adjustment to the composition of the melt in which they were trapped. Material balance calculations suggest that one-third of the Cu and commensurate amounts of S, Ni, and Fe have settled from the magma as immiscible globules. The sulfide globules contain less than 4 wt percent magnetite, compatible with low f o2 in the magma. Three sulfide phases coexisted in the globules at about 600 °C: monosulfide solid solution, intermediate solid solution, and pentlandite. At lower temperatures, the intermediate solid solution has broken down, and the monosulfide solid solution has exsolved a second generation of pentlandite.

GSA Bulletin↗
USGS1978

A large landslide on Mars

A large landslide deposit on the south wall of Gangis Chasma contains at least 100 billion m 3 of material that moved 60 km across the trough floor at a speed of more than 100 km/hr. The deposit consists of slump blocks at the head, hummocky material farther out, and a vast apron of longitudinally ridged material extending to the toe. The landslide deposit resembles many terrestrial ones but is much larger, and differs from most in having longitudinal rather than transverse ridges on its surface. However, some terrestrial landslides also have longitudinal ridges, particularly those in Alaska that traveled long distances over glacial ice and thus were highly efficient. In order to explain this high efficiency, two possible mechanisms of emplacement are singled out. The first involves a sliding motion on a cushion of air; on Mars, this implies a once much denser atmosphere, or the release of a gas, possibly steam. The second mechanism involves a flow motion of debris that is lubricated by some water; on Mars, the water may have come from ice in the source rock. An analysis of the possible causes for the Martian slide shows that the Martian trough walls are highly susceptible to sliding, and that the landslide may have been triggered by a Mars quake.

GSA Bulletin↗
USGS1978

Uranium abundances and distribution in associated glassy and crystalline rhyolites of the western United States

The abundance and distribution of uranium have been determined in 11 units of rhyolitic lava and ash-flow tuff of calc-alkaline and transitional composition from the western United States in order to further evaluate the potential of rhyolitic glass as a source of uranium ores. Samples consist of coexisting obsidians, perlites, and felsites that range in age from Pleistocene to Oligocene. Uranium abundances in analyzed obsidians are 5 to 46 ppm. Obsidians and coexisting perlites have identical (±5%) uranium concentrations, which confirms that little or no uranium is lost during hydration. Felsites show uranium depletions as high as 80% relative to coexisting obsidians and perlites. Combination of this data with the results of earlier work on peralkaline rhyolites indicates that uranium depletion seems to increase with age, with different rates of depletion for calc-alkaline (slowest) and peralkaline (fastest) compositions. Uranium distribution is homogeneous in obsidians, perlites, and spherulites, but inhomogeneous in felsites. Electron microprobe analyses of the least-depleted felsites indicate that uranium is associated with concentrations of Fe-Ti-Mn oxides or is in accompanying accessory minerals. Secondary Fe-Mn oxides in older, depleted felsites are uranium-bearing, especially along fractures or flow layers. Uranium loss from felsites seems to be largely controlled by low-temperature solution over long time periods, with some precipitation in secondary phases.

GSA Bulletin↗
USGS1979

Modern biogenic gas-generated craters (sea-floor “pockmarks”) on the Bering Shelf, Alaska

As many as 1,340 small craters per square kilometre cover the sea floor of Norton Sound in the northeastern Bering Sea. The craters are circular pits, 1 to 10 m in diameter and less than 1 m deep, observed on sonographs over 20,000 km 2 of northern Norton Sound sea floor. Craters typically are associated with acoustic anomalies, near-surface peaty mud, and gas-charged sediment. The peaty mud is a thick (>1.5 m), nonmarine pre-Holocene deposit that is now covered by a 1- to 3-m-thick layer of Holocene marine mud in the area of the craters. The peaty mud (2% to 8% organic carbon) contains abundant biogenic methane [C 1 /(C 2 + C 3 ) = 256 to 7,669] with carbon isotope (δ 13 C) values of −69‰ to −75‰- Decomposition of organic debris in the peaty mud apparently charges the mud with gas. The peaty and gassy zones attenuate sound waves and cause acoustic anomalies on high-resolution seismic profiles in the area with craters. The craters are forming now, as shown by the disruption of modern ice gouges by the craters. In the absence of storms, the gas apparently is trapped in the peaty mud in a saturated state by the cover of Holocene mud. Periodically, possibly during storms, the gas escapes through the thin Holocene cover and forms craters.

Alaska↗
USGS1979

Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques

Unsorted and unstratified deposits of volcanic rock debris typically flank recently active stratovolcanoes. It is often difficult, using standard geologic procedures, to establish whether a particular deposit was emplaced by a pyroclastic flow, lahar, rock avalanche, or glacier. Determination of the emplacement temperatures of clasts contained in the deposit aids in discriminating among some of these possibilities. The emplacement temperature of a clast can be estimated by analyzing its thermoremanent magnetization. To do this, oriented samples of the clasts are submitted to progressive thermal demagnetization; the directions and magnitudes of the resulting residual remanent vectors provide the information necessary to estimate the temperatures at which the clasts were emplaced. Studies of samples that were given artificial emplacement temperatures reveal that estimates are within plus or minus 25 degree C of the actual values. The temperature range within which estimates are possible depends on the thermoremanent magnetization acquisition curve of a clast. Data obtained from deposits of volcanic rock debris from Mount St. Helens, Washington, indicate that for andesitic and dacitic rocks, the range is roughly 100 to 550 degree C. The procedure probably can be applied to other geologic problems that require temperature information.

Washington↗
USGS1979

The determination of snow avalanche frequency through tree-ring analysis and historical records at Ophir, Colorado

Tree-ring analysis can be a reliable method of determining past snow avalanche events when good historical records are lacking. Characteristic features in the tree-ring record indicative of disturbance include: (1) the occurrence of reaction wood, (2) abrupt changes in growth rate, (3) age of scars caused by avalanche impact, (4) age of trees in reforested tracks, and (5) examination of a “new leader.” This study was conducted at the town of Ophir, a small community in southwestern Colorado with a severe snow avalanche hazard. Historical records indicate that snow avalanches reached the town, or its vicinity, four times during this century (1918, 1951, 1958, and 1959). Tree-ring analysis substantiated this record in part and produced evidence of at least one additional event (middle to late 1880s). From this combined information, the recurrence interval for major snow avalanches capable of endangering the town is thought to be at least as frequent as once in 20 yr. The limitations of tree-ring analysis encountered in this study are also discussed.

Colorado↗
USGS1980

Allochthonous Jurassic ophiolite in northwest Washington

Fragments of Jurassic ophiolite having U-Pb zircon ages narrowly grouped at 160 to 170 m.y. are widespread over parts of northwest Washington. The Haystack thrust fault is inferred to mark the base of the ophiolite in the San Juan Islands and adjacent Cascade foothills; other bodies of mafic and ultramafic rock in the western Cascades may be klippen of the Haystack thrust plate. The Haystack thrust fault is probably the structurally highest and possibly most extensive thrust yet recognized within a family of Late Cretaceous thrust faults in northwest Washington. The ophiolite and its time of emplacement (bracketed between about 100 and 88 m.y.) suggest a similarity with the Coast Range thrust of California which thrust Upper Jurassic ophiolite and the Great Valley sedimentary sequence over the Franciscan assemblage. However, relations in the Cascades are complicated by the extraordinarily diverse character of lower plate rocks, of which very few resemble the Franciscan. We conclude that an original subduction system was modified by later tectonic activity so that a variety of terranes was juxtaposed as a family of rootless thrusts, with the ophiolite forming, at least in some areas, the uppermost structural unit. Perhaps the emplacement of Wrangellia, an allochthonous microcontinent west of the San Juan Islands, caused the thrusting.

Washington↗
USGS1980

Paleomagnetic evidence for oroclinal bending of the southern Antarctic Peninsula

Two classes of fracture zones are distinguished on the basis of their orientations with respect to a spreading axis and length of associated ridge-ridge transform offset. (1) Minor fracture zones associated with transform faults of short offset (<30 km; minitransforms); the minor fracture zones may exhibit an asymmetric V-shaped configuration with respect to a spreading axis at variance with small circles about poles of plate rotation. (2) Major fracture zones associated with transform faults of long offset (>50 km); the major fracture zones exhibit a symmetric configuration with respect to a spreading axis following small circles about poles of plate rotation. Recognition of the coexistence of the two classes of fracture zones with different orientations on a slow-spreading oceanic ridge raises questions regarding the significance of the different orientations and nature of the intervening structural transition. A systematic narrow-beam bathymetric and magnetic investigation was performed to answer these questions in an area encompassing the change from minor fracture zones asymmetric with respect to the axis of the rift valley of the Mid-Atlantic Ridge at lat 26 degree N between major fracture zones, and a major fracture zone, the Kane, symmetric with respect to the axis of the rift valley at lat 24 degree N. The investigation delineated an intervening transitional region where structural features have continuously undergone geometric adjustments that have accommodated the discrepancy in orientation between the two classes of fracture zones for at least the past 6 m.y. A hypothesis of differential structural stability determined by thickness of lithosphere within transform offsets is advanced to explain the observed differences in behavior of the two classes of fracture zones. The orientation of a major fracture zone is constrained to follow small circles along a trajectory of relative plate motion by a long section of thick lithosphere in the associated transform fault. The orientation of a minor fracture zone is susceptible to reorientation in the short section of thin lithosphere in the associated transform fault; the reorientation reflects response to intra-plate and interplate stresses. The geometric adjustments that occur as a consequence of differential structural stability continuously accommodate any discrepancy in orientation that may develop between coexistent major and minor fracture zones, so that an ocean basin such as the Atlantic can open symmetrically.

GSA Bulletin↗
USGS1981

A geologic reconnaissance of the Cycladic blueschist belt, Greece

The Cycladic blueschist belt consists of two distinctive segments separated by a broad zone of superposed granitic and high-temperature metamorphic rocks. The northern segment contains early metamorphic fold axes and parallel glaucophane lineations that trend ∼060° with a progressive increase in metamorphism toward the southeast. The southern segment contains similar fold axes and glaucophane lineations that trend ∼010° with an apparent increase in metamorphism toward the northwest. Radiometric dating of metamorphic minerals from both segments give apparent ages of about 40 to 80 m.y. These data suggest the existence of late Mesozoic or early Cenozoic subduction zones in the Aegean region that subsequently collided.

GSA Bulletin↗
USGS1981

Multichannel seismic evidence bearing on the origin of Bowers Ridge, Bering Sea

Bowers Ridge is a large, arcuate sub-marine ridge that extends north and west from the Aleutian Ridge and separates the abyssal Aleutian and Bowers Basins in the Bering Sea. Two multichannel seismic-reflection lines recorded in 1976 over Bowers Ridge and the adjacent basins confirm the existence of 8- to 10-km-thick sediment wedges on the north side of Bowers Ridge and at the base of the Bering continental margin. Deformed sediment within the Bowers wedge indicates that subduction of the adjacent ocean crust beneath the ridge probably occurred prior to middle Cenozoic time. Flat-lying reflectors near the bottom of the trench suggest that a bathymetric trough and large ridge existed in Mesozoic time. The major period of underthrusting, subsidence, and in-filling of the trench probably occurred from Mesozoic to early Tertiary time. Small amounts of underthrusting may have continued after the early Tertiary development of the Aleutian Ridge; however, by middle Miocene time, the formerly subaerial Bowers Ridge had subsided below sea level. The multichannel seismic data do not show evidence for a buried spreading center within the eastern Aleutian Basin. Consequently, the sediment wedges (trenches) at both Bowers Ridge and the Bering continental margin are believed to be the consequence of subduction that occurred during the convergence of the ridge and the margin. The large size of Bowers Ridge suggests that a large amount of convergence has occurred since Mesozoic time. If Bowers Ridge was a large feature in Mesozoic time, as suggested by the apparent bathymetric trough, then the ridge may be as old or older than the Aleutian Ridge to which it connects.

GSA Bulletin↗
USGS1981

Early evolution of the Bering Sea by collision of oceanic rises and North Pacific subduction zones

Three major bathymetric features exist in the Bering Sea: Shirshov Ridge, Bowers Ridge, and Umnak Plateau. New refraction data over Umnak Plateau and previous geophysical data across Bowers Ridge indicate that a thickened welt of crustal material is present beneath both features. The crustal structure is transitional between oceanic and continental types. Various models for the origin of these features have been investigated. One that has not been proposed previously assumes that the protostructures of Bowers Ridge and Umnak Plateau could have formed outside of the present Bering Sea. According to this model, before formation of the Aleutian Ridge in late Mesozoic or earliest Tertiary time, these protostructures moved into their present Bering Sea positions. Prior to the arrival of these two structures in the Bering Sea, oceanic crust was subducted along the Bering continental margin connecting Alaska and Siberia. The collision of the Umnak Plateau protostructure with the southeastern edge of the margin may have caused subduction to terminate here and move southward. The new southerly position of subduction beneath the Aleutian Ridge was therefore controlled by late Mesozoic or early Tertiary locations of Umnak Plateau, Bowers Ridge, and possibly, the north-trending Shirshov Ridge farther to the west.

GSA Bulletin↗