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282 records · Page 16Linked to original sources
USGS2026

Long-term intermittent connection between the western Snake River Plain and Columbia basin: A two-phased incision history of Hells Canyon

For more than a century, researchers have debated the route of the Snake River across the northern Cordillera and U.S. Pacific Northwest, including the associated incision history of Hells Canyon. Here, we use detrital zircon U-Pb provenance analysis of Miocene strata upstream and downstream of Hells Canyon to constrain the evolution in fluvial pathways over time. Downstream of Hells Canyon, we show that the ca. 10−6 Ma Clarkston Heights gravel was dominantly sourced from the nearby Salmon and Clearwater rivers but also with substantial input from drainages that now flow into the western Snake River Plain. These same western Snake River Plain drainages were contributing to time-equivalent Lake Idaho strata, upstream of Hells Canyon, indicating a Hells Canyon fluvial connection between the western Snake River Plain and Columbia basin before 6 Ma. Results from Lake Idaho strata suggest a two-phased lacustrine history in the western Snake River Plain: Before ca. 6.0 Ma, lake strata were derived from local tributaries, suggesting that the western Snake River Plain was isolated from the modern upper Snake River system. Between ca. 4.3 Ma and 2.2 Ma, the source area expanded to include eastern tributaries in conjunction with migration of high-standing topography of the Yellowstone hotspot. Our results challenge the long-held hypothesis of “capture” of the Snake River through Hells Canyon. Instead, we infer that Hells Canyon was a long-established route for outflow of lakes occupying the western Snake River Plain, possibly intermittently, during a ca. 10−2 Ma phase of slow regional incision, followed by rapid incision in Hells Canyon starting ca. 2 Ma in conjunction with erosion of the topographic barrier impounding Lake Idaho.

Idaho, Oregon↗
USGS1900

Tertiary granite in the northern Cascades

The area to be discussed in this paper is in the immediate vicinity of Snoqualmie pass in the northern Cascades. Snoqualmie pass is in central Washington, about 10 miles north of where the Northern Pacific railroad crosses the range. It is a low pass, less than 3,100 feet above sealevel, and is the only point in the state where the range is crossed by a wagon road. On all sides of the pass, however, rugged peaks rise to elevations of from 5,700 to 6,300 feet. Although the geologist finds that this bold topography prevents rapid progress, while the luxuriant vegetation of the western slope at times presents almost impassable obstacles, yet the rock exposures in the higher parts of the range are such as to furnish conclusive evidence as to their character and relations.

GSA Bulletin↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part II. The Extrusive Rocks

Early eruptions of quartz latites, rather rich in potash, built up a volcanic mountain over 30 miles across on an irregular surface of late Cretaceous sediments. Erosion then removed much of the quartz latite. Renewed volcanism formed a volcano made up of basaltic rocks (mafic phonolite) that covered the quartz latites. The phonolites vary in texture and in the kind and proportion of the felsic minerals present. All contain abundant phenocrysts of augite and some of olivine; some have phenocrysts of leucite, others analcime, and many have pseudoleucite. Some have phenocrysts of biotite and barium sanidine. The groundmasses contain about the same minerals as the phenocrysts with more felsic minerals and more biotite in the coarser-grained groundmasses.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part V. Contact Metamorphism

Very near the contacts of the stocks the sediments have been replaced by sanidine and diopside, through magmatic reaction. An irregular zone of indurated sediments, produced largely by hydrothermal agents, extends outward from the stocks for as much as half a mile. Locally, more intense hydrothermal metamorphism has formed orthoclase and fibrous amphibole with some diopside and locally garnet, phlogopite, spinel, and cordierite. Along the dikes metamorphism extends out from the contacts for a distance of a few inches up to 5 feet. Thermal metamorphism produced recrystallization of the fine interstitial material of the sandstone. Aegirite, calcite, hematite, and pyrite were deposited by hydrothermal solutions. Magmatic reaction produced diopside. Reaction between sandstone and the magma of the headed dike caused a progressive replacement of plagioclase and quartz by potash feldspar. Granitic inclusions in the biotite phonolite dikes have a marginal shell in which plagioclase has been replaced by orthoclase. Surrounding each inclusion is a zone of mixed rock which contains quartz grains much embayed and surrounded by 0.1 mm. prisms of pyroxene.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part VI. Mineralogy

The minerals of the igneous rocks of the Highwood Mountains are described. The primary hornblende of the quartz latites is basaltic and it has been partly replaced by a common green hornblende. Hornblende is rare in the alkalic rocks. Augite is an abundant mineral of the alkalic rocks; in the syenites it contains some acmite, especially on the borders of the crystals. Aegirite is a common metamorphic mineral. The olivine of the shonkinites contains from 12 to 22 mol per cent of fayalite, that of the syenite from Square Butte 50 per cent. Monticellite twinned into trillings on (031) is an abundant mineral in one small intrusive body. In the alkalic rocks barium sanidine is abundant and plagioclase rare; leucite, primary analcime, and pseudoleucite are common and sodalite rare. Natrolite, thomsonite, and other zeolites are widespread and very abundant.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part VII. Petrology

In the shonkinite series olivine, leucite, and analcime crystallized only from magmas with over 20 per cent of mafites. At this stage the leucite and analcime inverted to pseudoleucite. Pyroxene crystallized over the whole range of rocks and changed little in composition until the magma reached the composition of nepheline syenite when it became richer in aegirite. A little pale biotite crystallized early, but most of the biotite is dark and crystallized late. A little barium rich sanidine crystallized early. The abundant late feldspar is poor in barium. Four petrographic subprovinces are represented in the area. The older quartz latites are near lime-alkalic rocks, the syenites and monzonites of Highwood Peak are somewhat farther removed, and the shonkinite series are potash-rich alkalic rocks. The alnoites are very rich in lime. The parent mafic magmas of the various subprovinces formed in depth by differentiation of a primary basaltic magma. These parent magmas were erupted toward the surface and there differentiated in a different way. The differentiation of the shonkinite magma was chiefly by settling of augite, olivine, and some biotite.

Montana↗
USGS1981

Tectonic elements of the southern part of the Gulf of California

Closely spaced seismic and bathymetric surveys in the southern province of the Gulf of California reveal a tectonically active and structurally complex plate boundary. While heat-flow measurements, seismicity, large sediment accumulations on continental margins, and the presence of oceanic crust in deep basins strongly suggest divergent plate motion, magnetic anomaly patterns are conspicuously absent in the Gulf as a whole. The rate of sediment accumulation (range: 6-1,000 m/m.y.), however, provides an alternative for the differentiation of two types of fault traces and suggests a process by which recently formed plate boundary zones evolve. Active faults manifest themselves as steep, elongate scarps delineating the active plate boundary zone. These fault traces appear to converge and diverge, for example in the Pescadero Basin complex, in a manner similar to the Southern California borderland. The floors of the intervening basins are flat and relatively sediment free. Relict faults, also elongate fracture zones, are distinguished from active traces in that they are completely filled with hemipelagic muds. The relict faults are located along both sides of the active plate boundary zone and subparallel to it. These features infer that plate motion is relatively rapid, thus explaining the paucity of sediment accumulation in actively spreading basins in spite of high sedimentation rates. Substantial plate boundary readjustment in short time periods is suggested by the periodic abandoment and subsequent infilling and attachment of formerly active transform faults to the more stable margins of the plates involved. The Gulf of California, therefore, represents a working model of the evolution of a plate boundary and its sedimentological consequences in the early tectonic developmental stage of a passive margin ocean basin such as the Atlantic.

southern Gulf of California↗
USGS1981

Location of a Caldera Source for the Soldier Meadow Tuff, Northwestern Nevada, Indicated by Gravity and Aeromagnetic Data

The Soldier Meadow Tuff in north-western Nevada is part of a sequence of peralkaline and subalkaline rhyolitic rocks of Miocene age that occupies an area of about 1,600 km 2 and has a volume of about 50 km 3 . The formation is mostly welded tuff containing abundant alkali feldspar and quartz phenocrysts in devitrified groundmass containing minute needles of blue-green alkali amphibole. The rhyolite of Badger Mountain is similar in mineralogy and chemistry, but textures suggest that it is a flow rhyolite. The underlying rhyolite of Catnip Mountain and tuff of Trough Mountain are phenocryst-poor but apparently also peralkaline. Gravity and aeromagnetic anomaly maps show 16- by 24-km anomalies near Badger Mountain. An analysis of these maps suggests that observed anomalies could be caused by a concealed caldera of 2,700-m thickness filled with rocks 0.2 g/cm 3 less dense and 0***002 emu/cm 3 lower magnetic susceptibility than the surrounding rocks. Distribution of the Soldier Meadow Tuff and the occurrence of numerous vents in the rhyolite of Badger Mountain suggest that the source for both units is associated with the indicated caldera. Possible alternative sources are other calderas whose locations are suggested by a 7- by 11-km gravity minimum near Rock Spring Table and a 12- by 24-km magnetic minimum west of Soldier Meadow.

Nevada↗
USGS1968

Cenozoic volcanism and sedimentation, silver peak region, western Nevada and adjacent California

Cenozoic deposits of the Silver Peak region, western Nevada, and adjacent California consist principally of continental sedimentary and pyroclastic rocks of the Esmeralda Formation and lavas and tuffs of the Silver Peak volcanic center. The sedimentary rocks comprise several thick sequences of tuffaceous volcanic sandstone and siltstone and interbedded air-fall tuff. These rocks were deposited in basins that coincide in a general way with the present valleys. Thick wedges of conglomerate and sandstone occur along the basin margins and reflect source areas. Most of the sedimentary rocks were deposited under fluctuating fluviatile and lacustrine conditions, but paludal conditions prevailed locally. Abrupt facies changes and numerous local unconformities indicate that deposition was not uniform within a given basin. The sedimentary rocks range in age from late Miocene to late Pliocene. The oldest reliably dated rocks in the Esmeralda Formation are 13.1 m.y. by K-Ar (Barstovian), and all sedimentary strata are younger than an ash-flow sheet dated at 21.5 m.y. by K-Ar. An air-fall tuff in the upper part of the section has a K-Ar age of 4.3 m.y. Rocks of the Silver Peak volcanic center, 4.8 to 6.1 m.y. by K-Ar, crop out principally in the central part of the Silver Peak Range. Along the margins of the range, they overlie and interfinger with the various sedimentary sequences. The volcanic rocks are mostly rhyolite and trachyandesite tuffs and flows with subordinate basalts and andesites. Basin and Range faulting began in late early to middle Miocene time and has continued intermittently to the present, but present topography was largely established by late Pliocene time.

California, Nevada↗
USGS1974

Configuration of Precambrian rocks in southeastern New York and adjacent New England from aeromagnetic data

Two aeromagnetic anomalies of regional extent outline two previously unknown buried masses of highly magnetic, probably Precambrian, rocks in southeastern New York and adjacent Vermont, Massachusetts, and Connecticut. The northern mass extends northeastward from Albany, New York, to Bennington, Vermont, where it appears to be buried beneath weakly magnetic Precambrian rocks of the Green Mountains. The southern mass extends north-northeastward from Beacon, New York, through Stissing Mountain, to Copake, New York, and appears to be the buried northeast extension of the Reading Prong. The shape of the Beacon-Copake magnetic anomaly indicates that the source is near the surface and has a sharp boundary, probably a fault, on its northwestern side; the shape also indicates that the source becomes deeply buried to the southeast and thus supports a parautochthonous interpretation for the northern part of the Reading Prong. In southwestern Massachusetts, the highly magnetic Beacon-Copake mass appears to be overstepped on the east by a buried slice of weakly magnetic Precambrian rocks which, in turn, is overstepped on the east by imbricate slices of weakly magnetic Precambrian rocks exposed along the western front of the Berkshire Highlands. Precambrian rocks exposed in the Green Mountains, the Berkshire and Housatonic Highlands, the eastern part of the Hudson Highlands, and the Manhattan Prong have a remarkably lower amplitude magnetic pattern than those in the Adirondack Mountains and the Reading Prong. This difference in magnetic character appears to represent more than different thicknesses of Precambrian rocks and may reflect a different sequence of Precambrian rocks to the east, a lower grade of Precambrian metamorphism to the east, or possibly a reduction in magnetite content in the eastern Precambrian rocks because of Paleozoic metamorphism.

Connecticut, Massachusetts, New York, Vermont↗