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At least 307 records · Page 17Linked to original sources
USGS1970

Boulder Creek batholith, Colorado part I: Allanite and its bearing upon age patterns

Allanite is abundant and commonly attains unusually large size as a late-replacement mineral in: (1) the comagnatic rocks of the Precambrian Boulder Creek batholith; (2) associated amphibolite xenoliths and related hybrid rocks; and (3) distinctly younger intrusions of Silver Plume Granite that cut the complex. Allanite porphyroblasts develop by replacement of biotite, probably in the presence of emanations from the rare earth-rich and thorium-rich Silver Plume Granite. The largest allanite crystals are made up of nearly isotropic (metamict) cores and birefringent (recrystallized) rims. Smaller crystals are made up exclusively of birefringent material. The maximum birefringence is shown to be that expectable in allanite of Late Cretaceous to early Tertiary age. As plotted on maps, the birefringence increases, and the thorium and uranium contents of the allanite decrease toward a Laramide stock. The variation in birefringence is, therefore, largely relatable to variations in the post-Laramide radiation dosage brought about by differences in the amounts of uranium and thorium lost during recrystallization. The recrystallized allanite is itself partly replaced by epidote which characteristically occurs as a border between allanite and biotite. Total rare-earth oxides for the eight samples of allanite analyzed range from 17.5 to 21.3 percent by weight. In 13 samples, thorium ranged from 0.50 to 1.14 percent by weight, and uranium from 54 to 158 parts pparts per million. Ranges in optical measurements for 20 samples using the spindle stage are: Nα = 1.719–1.759, Nβ = 1.731-1.774, N γ = 1.741–1.784, birefringence = 0.020–0.032, 2V X (calc.) 70°–84°. Ranges for unit-cell data obtained on 8 samples are: a = 8.948–8.985Å, b = 5.721-5.763Å, c = 10.184–10.240Å, β = 115°7.50′–115°25.89′ and volume = 473.02–478.43Å 3 . The average value for the ratio a:b:c = 1.561:1:1.778.

Colorado↗
USGS1970

Radiometric ages and stratigraphic sequence of volcanic and plutonic rocks, southern Nye and western Lincoln Counties, Nevada

The geochronology of Tertiary igneous events at the Nevada Test Site and adjacent area is outlined by 36 recently determined K-Ar ages, together with other published K-Ar ages. The first evidence of Tertiary igneous activity is the ash-fall bedded tuffs in the Horse Spring Formation. One such tuff has been dated as 29 m.y. old (late Oligocene). Other ash-flow tuffs and lavas formed during the Miocene and Pliocene, according to radiometric age determinations. The youngest ash-flow tuff in this area is about 6 m.y. old. Great volumes of ash and lava were spewed forth 13 to 11 m.y. ago to form the Paintbrush and Timber Mountain Tuffs. Sixteen replicate age determinations on minerals from four densely welded ash-flow tuffs from these formations gave a pooled standard deviation of about ± 2 percent error, provided anomalous ages were rejected on the basis of rock alteration or analytical difficulties. In the Air Force Gunnery Range, just north of the test site, K-Ar ages suggest that the oldest ash flows, the Monotony Tuff, were emplaced 27.6 m.y. ago (late Oligocene) and were followed by outpourings of lava and ash throughout most of the Miocene. Youngest dated lava is about 13 m.y. old. In the southern Egan and northern Seaman Ranges of central Nevada, the Needles Range (?) Formation has an averaged K-Ar age of about 30 m.y., which compares closely with 29.2 m.y., the average of four earlier K-Ar ages determined by other investigators on known Needles Range Formation in eastern Nevada and western Utah. K-Ar ages given by micas from two exposed plutons in the Nevada Test Site suggest emplacement of these plutons at about 93 m.y. ago (early Late Cretaceous), although earlier emplacement in the Mesozoic would be more consistent with Pb-α ages

Nevada↗
USGS1970

Sea-floor spreading at the junction between Gorda Rise and Mendocino Ridge

A Quaternary deformation pattern revealed by new sub-bottom acoustic profiles supplements previous knowledge derived chiefly from magnetic and seismic evidence on the contemporary tectonics off northern California. An inferred age for the sedimentary cover along the axis of the southern part of Gorda Rise suggests that no volcanism has occurred along that part of the rise for more than 100,000 years. Sea-floor spreading at the rise crest has been accommodated by sinking of a keystone block that forms the floor of Escanaba Trough, the median valley of the rise. Differential movement between Pacific and American tectonic plates could have caused the deformation pattern, and during this movement, the displacement offshore beyond the northwest end of the San Andreas fault is inferred to have divided at Cape Mendocino between a fault along Mendocino Ridge and a fault segment that connects with the Blanco fault farther northwest.

California↗
USGS1970

A re-evaluation of basalt-obsidian relations at East Lake Fissure, Newberry Caldera, Oregon

Andesite scoria, agglutinate, and small flows formed by thin lava gushes that erupted from East Lake Fissure on the north wall of Newberry Caldera carry numerous inclusions of platy rhyolite, partly melted platy rhyolite, and frothy obsidian. This association of obsidian and “basalt” has been interpreted as the result of intermingling of mafic and siliceous magmas. The locality has been repeatedly cited as an example of a mixed intrusion of the “basalt-rhyolite association.” Field, petrographic, chemical, and experimental evidence suggest, however, that the inclusions of frothy and massive obsidian are melted fragments of platy rhyolite which were ripped from a rhyolite unit forming part of the caldera wall by uncontaminated andesite magma which rose and fountained from the fissure.

Oregon↗
USGS1970

Earthquake recurrence intervals on the San Andreas fault

Possible recurrence intervals between earth- quakes of different magnitude that may be generated along the San Andreas fault are derived by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates. The recurrence interval for earthquakes of different magnitude at a given point on the fault is believed to follow the relation: (1) R x = D / (S - C) where: R x = recurrence interval at a point on the fault, D = displacement accompanying an earthquake of given magnitude (related empirically to Richter magnitude), S = long-term strain rate (from offset of geo- logic units), C = tectonic creep rate. The recurrence interval for earthquakes of different magnitudes for the total length of the fault is then derived by weighting equation (1) according to the number of break lengths in the total length as follows: (2) R t = DL / (S - C)L t where: R t = recurrence interval for entire fault, L = length of break (related empirically to Richter magnitude), L t = total length of fault. Tectonic creep is believed to be related to Richter magnitude, for example, small for segments of the fault characterized by earthquakes of large magnitude, and large for segments characterized by small earthquakes; and equations (1) and (2) can be weighted according to this relationship

California↗
USGS1970

Nature and origin of early and late cherts in the Leadville Limestone, Colorado

Two generations of chert have been observed in the Mississippian Leadville Limestone of west-central Colorado: (1) an early chert inferred to have precipitated from hypersaline marine waters of high pH after those waters seeped into carbonate muds prior to final burial and lithification of the carbonate; and (2) a late chert that appears to have precipitated from ground waters as amorphous silica after initial lithification but prior to or during karst erosion of the formation in Late Mississippian (?) and Early Pennsylvanian time. A third type of microcrystalline quartz, hydrothermal jasperoid, is associated with ore deposits of Laramide age in the Leadville, but it is not discussed here. Conditions in west-central Colorado at the time that the early cherts formed in the Leadville were probably similar to those associated with the formation of early diagenetic chert in modern sediments, and the early chert in the Leadville is believed to have formed in a manner similar to the modern early cherts. The silica of the late chert appears to have come from Paleozoic sandstones below the Leadville. It was carried by slightly acid artesian waters into the formation, probably in concentrations of less than 60 ppm SiO 2 Super-saturation of amorphous silica occurred within the Leadville and probably was obtained by evaporative concentration at the water-air interface during dry seasons. Dissolution of the limestone by the slightly acid waters may account for the removal of the calcite that is replaced by chert.

Colorado↗
USGS1970

Sr87/Sr86 ratios of Quaternary lavas of the Cascade Range, northern California

Quaternary lavas from Mount Lassen, Mount Shasta, and Medicine Lake, which range in composition from olivine basalt to rhyolite, have Sr 87 /Sr 86 values that fall between 0.7030 and 0.7043, corrected to 0.7080 for the Eimcr and Amend (MIT) SrCO 3 standard. These values are very close to average (corrected) strontium isotopic values of some other circum-Pacific volcanic suites and of lavas of oceanic islands. The low ratios suggest a mantle origin. The notably high strontium content (1400 ppm) in the andesites from Mount Shasta also indicates a mantle origin at depths where plagioclase is absent as a stable phase in the residual material.

California↗
USGS1970

Binary coefficients and clustering in biostratigraphy

The large data arrays common in biostratigraphy make subjective groupings difficult. Because of this, biostratigraphers have commonly based conclusions on the occurrences of relatively few species. The use of binary similarity coefficients, cluster analysis techniques, and digital computers allows a polythetic approach to biostratigraphy. The approach is free of circular and a priori reasoning in that it is established that assemblage zones (major clusters in a dendrogram) are present before the biostratigraphic utility of various taxa for the recognition of the zones is determined. A measure for determining the biostratigraphic fidelity of a species for established zones is proposed. Published data on the distribution of ostracodes and foraminifers from the Eocene and Oligocene of Mississippi and Alabama are used to demonstrate a method of using coefficients and clustering in establishing a zonation.

Alabama, Mississippi↗
USGS1970

A Permian disturbance of K-Ar radiometric ages in New England: Its occurrence and cause

Approximately 200 K-Ar mineral and whole rock ages from New England, half of which are previously unpublished, are used to delineate an area of Permian thermal disturbance. The disturbed area, as outlined by K-Ar mica ages, forms a north-northeast-trending belt 60–80 mi wide that extends from the coast of Long Island Sound in southern Connecticut to southwestern Maine, where it terminates against rocks displaying older radiometric ages. Several possible mechanisms that may have affected the radio-metric systems of pre-existing rocks are examined: (1) contact metamorphism related to contemporaneous igneous activity, (2) alteration associated with major faulting, (3) regional metamorphism in late Paleozoic time, and (4) burial followed by uplift and erosion. Evidence is given that each of these mechanisms was operative locally, especially in the southern portion of the belt. The general lack of late Paleozoic tectonism in New Hampshire and Maine suggests that only burial is a likely cause of the disturbance there.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS1970

Use of Ar36 to Evaluate the Incorporation of Air by Ash Flows

The Ar 36 content of densely welded glasses from ash-flow units provides a means by which the amount of air entrapped and subsequently resorbed by the glasses during compaction and welding may be calculated. The amount of air measured in glasses from nine upper Tertiary ash-flow sheets in the western United States ranges from 0.033 to 13 ppm; median is about 1 ppm. These values are very small compared with the total amounts of volatiles which probably were incorporated during welding. The data strongly suggest that large volumes of air are not incorporated by ash flows during their eruption and lateral movement.

California, Idaho, Nevada↗
USGS1970

Eocene age of the Adak ‘Paleozoic (?)’ rocks, Aleutian Islands, Alaska

In 1948, several specimens identified as the plant genus Annularia, a primitive horsetail of Pennsylvanian or Permian age, were found in tuffaceous sandstone exposed near the northern end of Adak Island, Alaska. These beds form the basal part of the Andrew Lake Formation, a newly named sequence of marine sedimentary rocks that is more than 850 m thick, and, in the main, consists of northwest-dipping tuffaceous sandstone, siltstone, shale, and siliceous siltstone and shale interbedded with basaltic flows or penecontemporaneous(?) sills (or both) a few tens of meters thick. This formation rests depositionally(?) on the Finger Bay Volcanics, the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks that form the bulk of Adak Island. Mollusks, foraminifers, sponge spicules, and fish scales and skeletal remains occur in the lower 350 m of the section immediately overlying the basal “Annularia”-bearing beds. Included in this fauna is the pecten Pro-peamussium (cf. P. stanfordensis Arnold), of probable Eocene age; the associated foraminiferal fauna is provincially considered to be of late Eocene (Narizian) age, and the fish scales are similar to those found in the Narizian and Refugian (Eocene and Oligocene) of California. Examination of the matrix surrounding specimens of “Annularia” revealed a substantial dinoflagellate flora—establishing that the “Annularia”- bearing beds are themselves marine units of middle or late Eocene age. The Andrew Lake Formation probably accumulated in a perched basin along the crestal region of an early Tertiary Aleutian ridge. Accordingly, there is no evidence for a Paleozoic Aleutian ridge. There is only scant evidence that the ridge existed in Mesozoic time.

Alaska↗
USGS1970

Quartz gabbro and anorthositic gabbro: Markers of offset along the San Andreas fault in the California Coast Ranges

Large-scale lateral movement on the San Andreas fault zone is suggested by the distribution of gabbroic rocks that may be slivered remnants of oceanic crust. Distinctive and unusual hornblende quartz gabbro and anorthositic gabbro that are virtually identical both petrographically and chemically are exposed at Logan and Gold Hill in the Coast Ranges along the San Andreas fault. The hornblende quartz gabbro is made up of labradorite to bytownite plagioclase, hornblende, and quartz, with very minor biotite and pyroxene, and accessory metallic opaques and apatite. The coarse-grained anorthositic gabbro with anorthositic layers and associated gneiss is made up mainly of labradorite to bytownite plagioclase, hornblende, lesser clinopyroxene, and, locally, orthopyroxene. The present 100 mi of separation between these two gabbro bodies is probably due to displacement along the San Andreas fault of what was originally one gabbro mass. Somewhat similar quartz gabbro and anorthositic gabbro associated with ultramafic rocks near Eagle Rest Peak in the San Emigdio Mountains may represent a source for the Logan and Gold Hill slivers. This suggests a minimum right-lateral movement of about 200 mi on the San Andreas fault zone. Anorthositic gabbro clasts from Cretaceous conglomerate in the Gualala area have strong resemblance to gabbroic rocks at Logan, Gold Hill, and Eagle Rest Peak and speculatively suggest 350 mi of right-lateral movement on the fault zone. It is tentatively suggested that the gabbro clasts of Gualala may have been shed from a large area of exposed gabbroic oceanic crust, pieces of which occur as fault slivers at Gold Hill and Logan. It is further suggested that the Eagle Rest Peak locality may be a relatively in-place exposure of this gabbroic oceanic crust that lies near a continental-oceanic interface. This interface, thought to represent a fossil subduction zone between Franciscan and Sierran basements, appears to be overlain depositionally by Eocene sedimentary rocks. If these Eocene deposits do “seal over” the possible subduction zone, it suggests that movement on such a zone ceased before the Eocene deposition, and that the new and different pattern of right-lateral movement on the San Andreas fault zone probably began sometime later.

California↗
USGS1970

Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota

Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Minnesota↗
USGS1970

Distribution of the Toquima-Table Head (Middle Ordovician Whiterock) Faunal Realm in the Northern Hemisphere

Discovery of a Whiterock trilobite assemblage in the Albany Mudstone, Girvan District, southwestern Scotland, led to an assessment of the distribution of Middle Ordovician brachiopod and trilobite faunas previously assigned to the White-rock Stage of Cooper (1956). These faunas lie within a belt designated as the Toquima-Table Head Faunal Realm. This realm is closely related to the position of the transition from miogeosynclmal to eugeosynclinal facies, presumed to indicate the position of Ordovician continental margins. In Middle Ordovician time North America, parts of Ireland, Scotland, Norway, Sweden, and northeastern Asia may have constituted a single continental mass.

Ayrshire County↗
USGS1970

Variations of major chemical constituents across the central Sierra Nevada batholith

A study of 193 chemical analyses of plutonic rocks from 132 localities in the central Sierra Nevada shows convincingly that K 2 O decreases systematically westward and suggests that Fe 2 O 3 and TiO 2 may also decrease westward and that FeO, MgO, and CaO may increase. The ratio K 2 O/SiO 2 obviously decreases westward across six of eight provisionally established sequences of granitic rocks. Plots of analyses of rocks from each sequence form discrete fields that are strongly elongate toward zero K 2 O at 40 to 45 percent SiO 2 . The boundaries between fields on these plots and between fields on plots of normative minerals on triangular diagrams are sharp. Compositional trends within sequences are different than the compositional changes that take place across the batholith—rocks in the western Sierra Nevada probably are not compositionally identical with rocks that are present at depth beneath the eastern Sierra Nevada. Progressive decrease of K 2 O in the Paleozoic and Mesozoic country rocks westward across the batholith is consistent with the anatectic model for its origin. However, it also is consistent with the hypothesis developed to explain chemical patterns in volcanic island arcs—that K 2 O increases toward continental land masses because of increasing depth of magma generation along landward-dipping seismic (Benioff) zones. The seismic-zone hypothesis encounters several difficulties, but it cannot be ruled out.

California↗