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Structural geologic interpretations from radar imagery
Certain structural geologic features may be more readily recognized on sidelooking airborne radar (SLAR) images than on conventional aerial photographs, other remote sensor imagery, or by ground observations. SLAR systems look obliquely to one or both sides and their images resemble aerial photographs taken at low sun angle with the sun directly behind the camera. They differ from air photos in geometry, resolution, and information content. Radar operates at much lower frequencies than the human eye, camera, or infrared sensors, and thus "sees" differently. The lower frequency enables it to penetrate most clouds and some precipitation, haze, dust, and some vegetation. Radar provides its own illumination, which can be closely controlled in intensity and frequency. It is narrow band, or essentially monochromatic. Low relief and subdued features are accentuated when viewed from the proper direction. Runs over the same area in significantly different directions (more than 45° from each other), show that images taken in one direction may emphasize features that are not emphasized on those taken in the other direction; optimum direction is determined by those features which need to be emphasized for study purposes. Lineaments interpreted as faults stand out on radar imagery of central and western Nevada; folded sedimentary rocks cut by faults can be clearly seen on radar imagery of northern Alabama. In these areas, certain structural and stratigraphic features are more pronounced on radar images than on conventional photographs; thus radar imagery materially aids structural interpretation.
Fission-track ages of accessory minerals from granitic rocks of the central Sierra Nevada batholith, California
Ages of apatite, sphene, allanite, epidote, and garnet from plutonic rocks of the central Sierra Nevada and Inyo Mountains have been determined by the fission-track method. Ages of 44 specimens of apatite range from 54 to 128 m.y. Oldest apatites generally occur in rocks from the western portion of the batholith; youngest are from granitic rocks along the eastern slope of the Sierra Nevada. Thirty-four specimens of sphene have been dated and range in age from 71 to 118 m.y. Within experimental error, all sphenes are as old as, or older than, coexisting apatites. The oldest sphene is from granitic rock of the Inyo Mountains; however, sampled rocks from the western Sierra Nevada do not contain sphene. The youngest sphenes are generally from rocks slightly west of the Sierran crest.
Experiments on formation of contorted structures in mud
Contorted structures can be formed in mud or sand as a result of differential loading. Fifteen sets of experiments were conducted in water tanks to test various factors of possible significance in the contortion of mud by loading. Of six factors tested, the most significant was distribution of load, but others affecting the type of structure under certain conditions were (1) the manner of depositing the mud, (2) the form of the underlying surface, (3) the direction of loading, and (4) the movement or lack of movement of water during loading. Organic material was shown to be unneccessary in forming conical structure or convolute bedding. Strength of base had little or no influence on convolute-structure development. Contortions ranged from the simple anticlinal type with vertical axial plane, commonly referred to as convolute, to structures with gently dipping axial planes, to others with lateral extensions or “flames” from the apexes, and, finally, to those with complex overturned folds. Causes of these variations were determined in terms of the factors listed above. Some additional forms of contorted bedding result from other types of penecontemporaneous deformation such as slumping from undermining or from oversteepening, differential lateral movement, and surface drag; these forms differ from those structures formed by loading.
Glaciation on the continental margin off New England
The Pleistocene glacial limit in the marine environment off New England can be traced by plotting the seaward limit of abundant sandy gravel and the position of shoals. Maximum limit of the last glaciation was probably along an irregular line extending through Nantucket Shoals, across Great South Channel, northern Georges Bank, and at least to the edge of the Scotian Shelf. If, as we assume, glaciers lowered sea level approximately 130 m, the ice margin was probably a subaerial one on Nantucket Shoals and Georges Bank, and it was bordered by outwash and meltwater channels leading away from the ice front. On the Scotian Shelf, the margin may have bordered directly on the ocean, to judge by the lack of shoals and the widespread dispersion of gravel out to the shelf edge. The glaciofluvial nature of the original deposits and marine reworking during the eustatic rise in sea level have made it difficult to recognize ice-contact deposits near the limit of maximum glacial advance. The gravel on shallow banks and ledges is in a bimodal mixture with sand. Association of coarse gravel and sand suggests postdepositional reworking of till by marine processes and removal of silt and clay. Gravel in the Gulf of Maine is mixed with sand, silt, and clay, a mixture characteristic of till.
Mesozoic California and the underflow of Pacific mantle
The Mesozoic evolution of California is interpreted as dominated by the underflow of oceanic mantle beneath the continental margin. Underflow during part of Late Cretaceous time of more than 2000 km of the eastern Pacific plate seems required by the marine magnetic data. Correspondingly, varied oceanic environments—abyssal hill, island arc, trench, oceanic crust, and upper mantle, perhaps also continental rise and abyssal plain—appear to be represented in the eugeosynclinal terranes of California. The rock juxtapositions accord with the concept that these materials were scraped off against the continent as the oceanic plate slid beneath it along Mesozoic Benioff seismic zones, which are now seen as serpentine belts separating profoundly different rock assemblages. The chaotic Franciscan Formation of coastal California consists of deep-ocean Late Jurassic to Late Cretaceous sedimentary, volcanic, crustal, and mantle materials. As open-ocean abyssal oozes and the oceanic crust beneath them were swept into the Benioff-zone trench at the continental margin, they were covered by terrigenous clastic sediments, and the entire complex was carried beneath the correlative continental-shelf and continental-slope deposits (Great Valley sequence) and the older Mesozoic complexes. The other eugeosynclinal terranes of California can be interpreted, albeit with less confidence, in similar terms of underflow of Pacific mantle. In the Klamath Mountains and northern Sierra Nevada, for example, Ordovician and Silurian ocean-floor materials, overlain by or juxtaposed against an Upper Silurian to Permian island arc, were swept in first to the continent, along with a large fragment of oceanic crust and mantle and another fragment of an old orogenic belt. This debris was followed by Permian and Triassic ocean-floor deposits. Late Triassic and Jurassic volcanic products from stocks and batholiths forming in the welded complexes lapped across both landward and oceanward sides of the region. Reversal of Cenozoic extension, strike-slip faulting, and volcanic crustal growth in the western United States reveals a Cretaceous tectonic pattern strikingly like the modern pattern of the Andes, so the paleotectonic setting of North America can be inferred from the South American present. The Mesozoic batholiths of North America, like the late Cenozoic volcanic belt of the central Andes, are products of the same rapid motion of oceanic plates that carried oceanic sediments against the continent to form eugeosynclinal terranes. Magmas generated in the Benioff zones formed the batholiths and the volcanic fields which initially capped them.
U-Th-Pb chronology of zircons from the St. Kevin Granite, northern Sawatch Range, Colorado
Three samples of zircon from the St. Kevin Granite, northern Sawatch Range, Colorado, were analyzed for uranium, thorium, and lead content and for lead isotopic composition; the concentrated HNO 3 leaches of the zircons were similarly analyzed. The concordia age on the zircons was interpreted to be 1420 ± 40 m.y., an age in good agreement with a Rb-Sr whole-rock isochron age of 1470 m.y. (λ 87 Rb = 1.39 × 10 −11 yr −1 ). The concordia age of the leaches was found to be greater by about 5 percent than that of the zircons, perhaps reflecting loss of intermediate daughters in the 238 U decay chain over an extended period of time. Geologic evidence indicates that the St. Kevin Granite formed in large part by local melting of crustal rocks similar to the present wall rocks. If so, xenocrysts of zircon may be present in the granite. Isotopic evidence that the zircons were derived from older rocks is not convincing; however, possible evidence for a xenocrystic component is found in the feet that Pb-Pb ages of two nearly concordant zircons differ by 1.9 percent, an amount that exceeds analytical uncertainty. In addition, the zircon sample that has the greatest Pb-Pb age (1440 m.y.) has a 208 Pb/ 232 Th age of 1615 m.y. 208 Pb/ 232 Th ages greater than Pb-Pb ages are unusual and may suggest a complicated history for the sample. This sample is not the one suspected as the most likely to contain xenocrysts of zircon because xenoliths are not abundant at the sample locality. The zircons, if they are xenocrysts, apparently were almost entirely recrystallized or else lost nearly all their lead to the melt by diffusion.
A seismic-refraction survey of crustal structure in central Arizona
The U.S. Geological Survey conducted a seismic-refraction study of the earth's crust and upper mantle near the Tonto Forest Seismological Observatory (TFO), located 10miles south of the Mogollon Rim near Payson in central Arizona. Two recording lines 400 km long intersect in the approximate form of a cross at TFO; one line trends southeast and the other northeast. The sedimentary layer at most places southwest of the rim is less than 1 km thick, but north of the rim it is 2 to 3 km thick. The velocity in this uppermost layer ranges from 2.6 to 4.7 km/sec, with the higher limit measured near or north of the rim. Arrivals refracted in the upper crust (P g ) can be attributed to two layers for all the shot points south of the rim. The velocity in the upper layer is about 5.9 km/sec with thickness ranging from 2 to 8 km; beneath the upper layer the velocity is about 6.1 km/sec. The upper layer seems to be absent northeast of the rim, where two shot points generated P g arrivals that show only a velocity of 6.2 km/sec. A Poisson ratio of 0.22 for the upper crustal layers was measured from shear and compressional arrivals. The lower crust could not be identified from the first and later refraction arrivals; however, minimum depths to the intermediate layer were determined. An average crustal velocity of 6.2 km/sec was measured from wide-angle reflection alignments. A thin intermediate layer would explain the seismic measurements. A delay-time method was used to map the configuration of the M-discontinuity. The depth below sea level is about 36 km along the northwest-trending line. The northeast-trending line shows a shallow depth of 21 km near Gila Bend, increasing depth to about 34 km under TFO, and a flat M-discontinuity at 40 km depth under the Mogollon Mesa northeast to Sunrise Springs. There is evidence of an abrupt depth change of about 4 km on the M-discontinuity in the vicinity of TFO. The velocity in the upper mantle is 7.85 km/sec. The relation of topographic elevation to crustal thickness suggests an approach to isostatic equilibrium, which is deduced from a near-zero regional free-air gravity anomaly. However, lateral density change in the upper mantle is required to make the crustal-refraction model fit the observed gravity-anomaly values, provided that velocity and density are linearly related.
Potassium-argon ages on lavas of Kohala Volcano, Hawaii
Kohala Mountain is regarded as the oldest volcano on the island of Hawaii. Potassium-argon ages on three lavas of the Hawi Volcanic Series, the youngest sequence of lavas on Kohala, range from 0.14 to 0.20m.y. and indicate that volcanism ceased in the very late Pleistocene.
Lower Llandovery of the northern Appalachians and adjacent regions
Rocks of clearly dated early Llandovery age, as well as rocks that can logically be classed as early Llandovery from their regional relationships, appear to be more widespread than recognized, heretofore, in the northern Appalachians and adjacent regions. Their areal distribution and lithology permit a generalized reconstruction of the paleogeography, which consisted, in general, of three source areas alternating from east to west with three belts of clastic sedimentation. The westernmost clastic belt grades laterally westward into the carbonate rocks of the North American platform. The Central Clastic Belt encloses a belt containing impure carbonates with clastic detritus and clastic interbeds, and, locally, relatively clean carbonate deposits. Llandovery age rocks of the platform include the Manitoulin Dolomite and the Ellis Bay Formation. In the deposits to the east, coeval rocks occur, in part or in whole, within the limy and clastic deposits of the Carys Mills Formation and the Matapedia Group, as well as in the following clastic rock formations: Grimsby, Shawangunk, Tuscarora, Massanutten, Clinch, Smyrna Mills, Perham, Cabano, Weir, Beechhill Cove, Ross Brook, and White Rock.
Rate of sulfuric acid formation in Yellowstone National Park
Sulfuric acid forms near sulfurous hot springs as the result of oxidation of hydrogen sulfide exhalations by atmospheric oxygen. This strong acid rapidly alters the surrounding rocks and can destroy man-made structures and contaminate streams. Four tracts of acid-altered ground in Yellowstone National Park were studied in order to determine the rate at which sulfuric acid is forming. Although the size of the hot-spring areas varied by as much as a factor of 19, acid production was nearly uniform at about 10 grams per square meter of area per day. The near constancy of acid production per unit area implies that the area of land surface is a major control of the oxidation reaction of sulfide to sulfate. This is consistent with a biological origin for the acid by aerobic sulfur-oxidizing bacteria living close to the land surface. Laboratory rates of acid production for sulfur-oxidizing bacteria are as much as 200 times greater than the rates measured in Yellowstone National Park. A strictly biological origin for the acid is, therefore, quantitatively feasible. The data gathered in this study, however, do not rule out the possibility of the formation of natural sulfuric acid in hot springs by inorganic processes.
Role of cohesive strength in the mechanics of overthrust faulting and of landsliding: Discussion
No abstract available.
A revision of stratigraphic nomenclature for middle precambrian rocks in Northern Michigan
The name Marquette Range Supergroup is proposed to supplant the term Animikie Series for middle Precambrian strata of the Northern Peninsula of Michigan and adjacent areas of Wisconsin. The Marquette Range Supergroup consists of the Chocolay, Menominee, Baraga, and Paint River Groups, as defined in previous literature. We feel that this new name to apply to Northern Peninsula rock units is appropriate, as continued investigations have failed to show unequivocal correlation between middle Precambrian rocks of Michigan and the Huronian Supergroup of Ontario. Although the equivalence of the Animikie Group in Ontario and Minnesota with parts of the Michigan rocks is likely, the Stratigraphic complexity of the Michigan sequence requires supergroup rank. The inherent confusion of an Animikie Group in Ontario and Minnesota, and an Animikie Supergroup in Michigan, makes a local name such as Marquette Range Supergroup preferable to Animikie for the middle Precambrian rocks of Michigan. © 1970, The Geological Society of America, Inc.
Geochemical balance of a small watershed and its geomorphic implications
A detailed input-output study of a small forested watershed draining the Wissahickon Formation in the Piedmont of Maryland revealed that chemical solution is five times as effective in removing material as is mechanical erosion. Solution weathering removes 16.9 tons/sq mi/yr of material compared with 3.2 tons/sq mi/yr by mechanical erosion. Plant activity during the growing season increased the concentration of silica, bicarbonate, calcium, and potassium, thus increasing total dissolved solids by one-third. Autumn leaf fall also caused a short-term increase of these ions. Rainfall does not simply dilute floodwaters as the concentration of sulfate, potassium, and calcium increases whereas silica and bicarbonate decrease in concentration during a flood cycle. Our data suggest that during the first half of a flood cycle, both the flood water and the dissolved solids in it come from an area in and immediately adjacent to the flood plain. The weathering model derived from our study suggests that on a long-term basis approximately one-half of the erosion of the Pond Branch watershed is caused by chemical solution of the silicate minerals kaolinite, vermiculite, biotite, and oligoclase. This contrasts to short-term ratio of solutional to mechanical weathering of five to one.
Correlation of aeromagnetics and aeroradioactivity with lithology in the Spotsylvania area, Virginia
The U.S. Geological Survey made a detailed aeromagnetic and aeroradioactivity survey of 1050 sq mi of the Spotsylvania area in the Piedmont province of eastern Virginia. The study area consists of sixteen 7 1/2-minute quadrangles in Spotsylvania and adjoining counties, Virginia. East-west traverses were flown at a spacing of 1/2 mi and a flight elevation of 500 ft above ground. Geophysical data were compiled at a scale of 1:24,000, with a contour interval of 25 gammas for the magnetics and 25 counts per sec for the aeroradioactivity. The geophysical data were checked in the field, and a lithologic map was made. Correlation is excellent between geophysical data and lithologic units, and it is evident that good detailed aeromagnetic and aeroradioactivity maps can provide appreciable shortcuts in mapping an area such as the Piedmont, where lithologies are complex and outcrops sparse. Radioactivity highs are generally associated with magnetic lows, and more importantly, for purposes of geologic interpretation where the magnetic expression is flat and featureless, there is considerable radioactivity detail for geologic guidance. By using the two geophysical maps in conjunction, the geology between widely spaced outcrops can be worked out. In the study area, high radioactivity and low magnetic intensity characterize granitic rocks, many of which were not previously mapped. Mafic bodies are characterized by high magnetic values and low radioactivity. Much of the area is underlain by a sequence of metamorphosed sedimentary and volcanic rocks having alternating quartz-muscovite and magnetiferous chlorite-actinolite assemblages. © 1970, The Geological Society of America, Inc.
On the nature of the Boulder Batholith of Montana
In a recent review of the nature of batholiths, Hamilton and Myers (1967) interpreted the Boulder batholith of western Montana to be "in effect a gigantic mantled lava flow .... only a few kilometers thick," that flowed, under a crust of its own ejecta, across a broad structural basin. Such an interpretation is inconsistent with abundant geologic and geophysical data. The main mass of the batholith, the Butte Quartz Monzonite, does not have the characteristics of a lava flow or a laterally emplaced sheet. Its volcanic cover was not a floating cap but a laterally stable roof that was part of a volcanic plateau which occupied at least twice the area of the batholith. It does not thin toward its edges but is generally steep sided. Its flow structures are predominantly steep rather than near horizontal. It is separated from two smaller flanking plutons by thin vertical septa kilometers long. Its emplacement required more than 4 m.y., a rate orders of magnitude too slow for a single sheet only a few kilometers thick, however extensive. The batholith is more than a few kilometers thick. Recent gravity studies (Burfeind, 1967; Bonini, 1969) suggest a maximum thickness of 9 to 15 km to their authors, but the calculations are based on (1) assumed lateral and vertical homogeneity of the batholith, whereas in reality the Butte Quartz Monzonite core is discontinuously rimmed by more mafic, denser plutons; and (2) inappropriate densities, leading to excessive apparent density contrasts. The gravity data suggest to us that the batholith is more than 15 km thick. Heat flow, cooling rate, and seismic data also are compatible with a thickness of at least 15 km, but are difficult to reconcile with a thickness of only a few kilometers. Convincing examples of extrusive or quasi-extrusive thin batholiths must be sought elsewhere. © 1971, The Geological Society of America, Inc.
Aeromagnetic study of the midcontinent gravity high of central United States
A composite map of detailed aeromagnetic surveys over the midcontinent gravity high provides coverage of the 600-mi-long buried belt of mafic rocks of the Keweenawan Series from their outcrop localities in Minnesota and Wisconsin through Iowa and Nebraska. A map of the subsurface extent of the mafic rocks, based on the intricate magnetic patterns, shows that the rocks form a long, semicontinuous block, averaging 40 mi wide and consisting mainly of a sequence of layered flows. This sequence is probably fault-bounded and has been tilted up along the margins, where the linearity of the anomalies indicates steeper dips. The associated clastic rocks, indicated by a smoother magnetic pattern, occur in basins along both sides of the mafic belt and in grabens and a series of axial basins on the upper surface of the block. The well-defined outliers of flows marginal to the main block and the truncation of some of the outermost flow units along a diagonal boundary striking at an angle to them suggest that the present boundaries of the block are postdepositional structural features. The basins and the edges of the block appear to have controlled later, largely vertical movement in the overlying Paleozoic and younger sedimentary cover. Calculated models based on coincident magnetic and detailed gravity profiles along typical cross sections of the midcontinent gravity high show that the block of mafic rocks is steep-sided and as much as several miles thick. The free-air gravity anomaly, which consists of a large positive maximum flanked by minima, averages very close to zero, indicating that this major crustal feature is regionally compensated, although locally each of its components shows a large departure from equilibrium. Remanent magnetization is a primary factor in the interpretation of the magnetic data. Magnetic property studies of Keweenawan mafic rocks in the Lake Superior region show that remanent magnetization may be five times the magnetization induced by the present Earth's field and differs from it radically in direction. This magnetization was acquired before the flows were tilted into their present positions. A computed magnetic profile shows that a trough of flows with such a magnetization and inward-dipping limbs can account for the observed persistent lows along the western edge of the block, the relatively low magnetic values along the axis of the block, and the large positive anomaly along the eastern side of the block. Flows as much as 1 mi thick near the base of the sequence have a remanent magnetization with a nearly opposite polarity. This reverse polarity has been measured on both sides of Lake Superior and is probably also present farther south, particularly in Iowa where the outer units of the block in an area north of Des Moines give rise to a prominent magnetic low. The axis of this long belt of Keweenawan mafic rocks cuts discordantly through the prevailing east-west-trending fabric of the older Precambrian terrane from southern Kansas to Lake Superior. This belt has several major left-lateral offsets, one of which produces a complete hiatus in the vicinity of the 40th parallel where an east-west transcontinental rift or fracture zone has been proposed. The axial basins of clastic rocks are outlined by linear magnetic anomalies and show a concordant relation to the structure of the mafic flows. These basins are oriented at an angle to the main axis, suggesting that the entire feature originated as a major rift composed of a series of short, linear, en echelon segments with offsets similar to the transform faults characterizing the present mid-ocean rift system. This midcontinent rift may well have been part of a Keweenawan global rift system with initial offsets consisting of transform faults along pre-existing fractures, but apparently it never fully developed laterally into an ocean basin, and the upwelling mafic material was localized along a relatively narrow belt.