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At least 343 records · Page 19Linked to original sources
USGS1971

Lunar metallic particle ("mini-moon"): An interpretation

A troilite-rich nickel-iron particle ("mini-moon") recovered from the moon may be a mound detached from a sphere of silicate glass. Erosion and pitting of the particle may have been caused by passage through a cloud of hot gas and particulate matter formed by meteorite impact on the lunar surface. This explanation is in contrast to the theory that the particle was meteoritically derived molten material that was furrowed during solidification after lunar impact, subsequently pitted by high-velocity particles, and then abraded and polished by drifting dust while on the lunar surface.

Science↗
USGS1971

Evolving subduction zones in the western United States, as interpreted from igneous rocks

Variations in the ratio of K 2 O to SiO 2 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

western United States↗
USGS1974

Mercury's surface: Preliminary description and interpretation from Mariner 10 pictures

The surface morphology and optical properties of Mercury resemble those of the moon in remarkable detail and record a very similar sequence of events. Chemical and mineralogical similarity of the outer layers of Mercury and the moon is implied; Mercury is probably a differentiated planet with a large iron-rich core. Differentiation is inferred to have occurred very early. No evidence of atmospheric modification of landforms has been found. Large-scale scarps and ridges unlike lunar or Martian features may reflect a unique period of planetary compression near the end of heavy bombardment by small planetesimals.

Science↗
USGS2002

Eagle collapse center: Interpretation of evidence for late Cenozoic evaporite-related deformation in the Eagle River basin, Colorado

Evaporite tectonism resulted in deformation and collapse over an area of ~2500 km2 that is referred to as the Eagle collapse center. The collapse center includes much of the Eagle and Colorado River drainage basins between Vail, Dotsero, and McCoy, Colorado. The volume loss of evaporitic rocks by dissolution in the collapse center is estimated to be nearly 1700 km3 3 . Before ca. 10 Ma, Miocene basaltic flows partly covered an extensive, nearly horizontal, low-relief surface. Parts of this surface collapsed 1.3 km near the present-day Eagle and Colorado Rivers. Remnants of this surface outside the area of collapse, such as highlands of the White River uplift, the flank of the Gore Range, and Basalt Mountain, stand at elevations of 2.9–3.6 km. The high-standing Castle Peak basaltic cap, situated near the center of the Eagle collapse center, may not have collapsed, or collapsed little. The areas of collapse lie within or nearby known and inferred limits of the Pennsylvanian Eagle Valley Evaporite (mostly halite, gypsum, and anhydrite) that was deposited in the Central Colorado trough. Our geologic mapping and research in the Eagle collapse center delineate synclinal sags in the basaltic flows with amplitudes of 0.5–1 km, sinuous and discontinuous high-angle faults that cut basaltic flows, elongate grabens, evaporite-cored anticlines, and an ellipsoidal fault system that drops a 30 km X 10 km mountain block of younger strata into evaporite. Collapse as far as 20 km from the Colorado and Eagle Rivers suggests that the greater load on evaporite beneath surrounding highlands causes lateral flow of evaporite toward anticlinal crests in river valleys. Thus, gravity-driven evaporite flow and removal of evaporite by dissolution in groundwater and by subsequent discharge to surface waters combine to produce large-scale collapse. Although most evaporite tectonism post dates the basaltic flow capped surface, local angular unconformities under this surface record earlier, possibly Laramide evaporite tectonism, and overthickened post-evaporite red beds record some late Paleozoic evaporite deformation

Colorado↗
USGS1961

Use of water-well data in interpreting occurrence of aquifers in northeastern Lyon County, Minnesota

In northeastern Lyon County the areal distribution of aquifers of Cretaceous age determined from well-bottom altitudes suggests a series of interbedded sandstones striking northwestward and overlapping one another to the northeast. Probably the numerous thin sandstone aquifers in the area were deposited near the flanks of a Precambrian granite “high” area by a transgressing Late Cretaceous sea.

Minnesota↗
USGS1971

Interpretation of an aeromagnetic strip across the northwestern United States

This report discusses the results of aeromagnetic survey bounded by latitudes 45°30′ N. and 47°00′ N. and extending from the Rocky Mountains, to approximately 120 mi offshore in the Pacific Ocean. East of the Rocky Mountains, a larger area has been surveyed in the Great Plains bounded approximately by latitudes 44°50′ N. and 48°10′ N. and by longitudes 104°W. and 110°W. Throughout the area of the survey, the magnetic map is marked by conspicuous northeast and northwest anomaly trends, lineaments, and breaks in the anomaly pattern. Their regional distribution, over-all magnetic character, and geologic evidence suggest that they are major structural features in the basement rocks. The close correspondence of structural and geologic features in younger rocks with these basement magnetic and structural trends suggest that basement trends controlled or at least greatly influenced intrusion, deposition, and structural history of younger rocks. In some cases, evidence suggests that basement structures have been reactivated during later tectonic activity. Perhaps even more striking than the northeast- and northwest-trending features are large east-west magnetic discontinuities which, in some cases, extend completely across the strip to the edge of the shelf, and which, in some cases, can be correlated with large-scale discontinuities dating back to the Precambrian.

Idaho, Montana, North Dakota, Oregon, South Dakota↗
USGS1973

Interpretation of a high-grade Precambrian terrane in northern Idaho

A terrane of high-grade metamorphic rocks in northern Idaho and northeastern Washington is almost completely surrounded by low-grade rocks of the Precambrian Belt Supergroup. The high-grade terrane includes both Belt and pre-Belt rocks. Four events of folding and metamorphism occurred in the high-grade terrane. The first three events may have been associated with the Late Cretaceous emplacement of quartz monzonite of the Kaniksu batholith; the fourth may have been associated with a slightly later emplacement of granodiorite or with a Tertiary plutonic and volcanic episode. A much older event of plutonism in the high-grade terrane is recorded by zircon, which was dated by the Pb-U method at 1,500 m.y. from pre-Belt meta-igneous augen gneiss. Evidence of regional events intermediate in age between 1,500 and 100 m.y. has been found in the surrounding low-grade rocks but not in the high-grade terrane.

Idaho↗
USGS1982

Formation and interpretation of dilatant echelon cracks

The relative displacements of the walls of many veins, joints, and dikes demonstrate that these structures are dilatant cracks. We infer that dilatant cracks propagate in a principal stress plane, normal to the maximum tensile or least compressive stress. Arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks. Breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction. Near the parent-crack tip, a rotation of the local principal stresses is induced in the same sense, but not necessarily through the same angle. Incipient echelon cracks form at the parent-crack tip normal to the local maximum tensile stress. Further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane. The walls of these twisted cracks may be idealized as helicoidal surfaces. An array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle. Thus, many echelon cracks grow from a single parent because the work done in creating the array, as measured by its surface area, decreases as the number of cracks increases. In cross sections perpendicular to the propagation direction, echelon cracks grow laterally, each crack overlapping its neighbors, until the mechanical interaction of adjacent cracks limits this growth. Dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving, sigmoidal path. Sigmoidal echelon cracks may link at tip-to-plane intersections, leaving a step in the through-going crack wall. The geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the Earth.

Geological Society of America Bulletin↗
USGS1987

Implications of the northwestwardly younger age of the volcanic rocks of west-central California: Alternative Interpretation

Fox and others (1985) have made an important contribution to our understanding of iihe evolution of the Mendocino triple junction and the San Andreas transform. They have summarized a large amount of data on the ages and distribution of volcanic centers along the central California coast; their summary clearly shows that the locus of volcanism migrated northwestward along the coast during at least the past 15 m.y. As Fox and others (1985) poin ted out, however, several of the volcanic centers that are older than 15 m.y. do not fit a simple model of northwestward-migrating volcanism. In the following comments, I offer additional information on the ages and locations of volcanic rocks in west-central California and then briefly discuss the revised data set and its implications for the late Oligocene and early Miocene history of the San Andreas transform. My purpose is to suggest that the sequential northwestward-migrating volcanism documented by Fox a nd others (1985) was preceded by an episode of nearly synchronous volcanism that occurred throughout west-central California about 22-24 m.y. ago.

GSA Bulletin↗
USGS1976

Sclerochronology: a tool for interpreting past environments

X-radiographs of stony coral slabs reveal two types of annual density bands. Detailed studies of these bands in relation to known variations in air temperatures indicate that sclerochronology is a valid tool for documenting time sequences and changing environmental conditions on a coral reef.

Geology↗
USGS2009

Paleontological interpretations of crater processes and infilling of synimpact sediments from the Chesapeake Bay impact structure

Biostratigraphic analysis of sedimentary breccias and diamictons in the Chesapeake Bay impact structure provides information regarding the timing and processes of late-stage gravitational crater collapse and ocean resurge. Studies of calcareous nannofossil and palynomorph assemblages in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville A and B cores show the mixed-age, mixed-preservation microfossil assemblages that are typical of deposits from the upper part of the Chesapeake Bay impact structure. Sparse, poorly preserved, possibly thermally altered pollen is present within a gravelly sand interval below the granite slab at 1392 m in Eyreville core B, an interval that is otherwise barren of calcareous nannofossils and dinocysts. Gravitational collapse of watersaturated sediments from the transient crater wall resulted in the deposition of sediment clasts primarily derived from the nonmarine Cretaceous Potomac Formation. Collapse occurred before the arrival of resurge. Low pollen Thermal Alteration Index (TAI) values suggest that these sediments were not thermally altered by contact with the melt sheet. The arrival of resurge sedimentation is identified based on the presence of diamicton zones and stringers rich in glauconite and marine microfossils at 866.7 m. This horizon can be traced across the crater and can be used to identify gravitational collapse versus ocean-resurge sedimentation. Glauconitic quartz sand diamicton dominates the sediments above 618.2 m. Calcareous nannofossil and dinoflagellate data from this interval suggest that the earliest arriving resurge from the west contained little or no Cretaceous marine input, but later resurge pulses mined Cretaceous sediments east of the Watkins core in the annular trough. Additionally, the increased distance traveled by resurge to the central crater in turbulent flow conditions resulted in the disaggregation of Paleogene unconsolidated sediments. As a result, intact Paleogene clasts in Eyreville cores are rare, but clasts of semilithified Potomac Formation silts and clays are common.

Chesapeake Bay↗
USGS2010

Age, genesis, and paleoclimatic interpretation of the Sangamon/Loveland complex in the Lower Mississippi Valley, USA

For more than a century, the Sangamon paleosol (the Sangamon) has been an integral part of geologic and pedologic investigations in the central United States, including the Upper Mississippi and Lower Missouri River Valleys. Compositional, pedologic, micromorphologic, stratigraphic, and age data indicate that the prominent reddish paleosol developed in silt-rich deposits of the Lower Mississippi Valley, from southernmost Illinois to northwestern Mississippi, represents multiple periods of soil formation, and is wholly or in part time equivalent to the Sangamon of the central United States. Thermoluminescence data, for localities where the Sangamon developed in loess, indicate that the primary period of loess deposition was from 190 to 130 ka (oxygen isotope stage, OIS6), that loess deposition continued intermittently from 130 to 74 ka (OIS5), and that deposition was wholly or in part coeval with Loveland loess deposition in the central United States. Beryllium-10, chemical, and pedologic data indicate that in the Lower Mississippi Valley: (1) the Sangamon represents a minimum time period of 60-80 k.y.; (2) there were at least two periods of soil formation, ca. 130-90 ka and 74-58 ka (OIS4); and (3) rates of weathering and pedogenesis equaled or exceeded the net loess-accumulation rate until at least 46 ka (OIS3) and resulted in development of a paleosol in the overlying basal Roxana Silt. Along a N-S transect from southern Illinois to western Mississippi, Sangamon macroscopic characteristics as well asthe micro-morphology, chemistry, and mineralogy, suggest a regional paleoclimate during periods of soil formation that: (1) was warm to hot, with a wider range in temperature, precipitation, and evapotranspiration than present; (2) had seasonal to decadal or longer periods of drought; and (3) had down-valley (southward) trends of increasing temperature and precipitation and decreasing seasonality and variation in annualto decadal precipitation. ?? 2011 Geological Society of America.

Geological Society of America Bulletin↗
USGS2013

Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa

Aeromagnetic and gravity data sets obtained for Alabama (United States) have been digitally merged and filtered to enhance upper-crustal anomalies. Beneath the Appalachian Basin in northwestern Alabama, broad deep-crustal anomalies of the continental interior include the Grenville front and New York–Alabama lineament (dextral fault). Toward the east and south, high-angle discordance between the northeast-trending Appalachians and the east-west–trending wedge of overlapping Mesozoic and Cenozoic Gulf Coastal Plain sediments reveals how bedrock geophysical signatures progressively change with deeper burial. High-frequency magnetic anomalies in the Appalachian deformed domain (ADD) correspond to amphibolites and mylonites outlining terranes, while broader, lower-amplitude domains include Paleozoic intrusive bodies and Grenville basement gneiss. Fundamental ADD structures (e.g., the Alexander City, Towaliga, and Goat Rock–Bartletts Ferry faults) can be traced southward beneath the Gulf Coastal Plain to the suture with Gondwanan crust of the Suwannee terrane. Within the ADD, there is clear magnetic distinction between Laurentian crust and the strongly linear, high-frequency magnetic highs of peri-Gondwanan (Carolina-Uchee) arc terranes. The contact (Central Piedmont suture) corresponds to surface exposures of the Bartletts Ferry fault. ADD magnetic and gravity signatures are truncated by the east-west–trending Altamaha magnetic low associated with the Suwannee suture. Arcuate northeast-trending magnetic linears of the Suwannee terrane reflect internal structure and Mesozoic failed-rift trends. Geophysical data can be used to make inferences on surface and subsurface geology and vice versa, which has applicability anywhere that bedrock is exposed or concealed beneath essentially non-magnetic sedimentary cover.

Alabama↗
USGS2000

Prolonged winter undernutrition and the interpretation of urinary allantoin:creatinine ratios in white-tailed deer

The urinary allantoin:creatinine (A:C) ratio (expressed in micromoles of allantoin to micromoles of creatinine) has shown potential as an index of recent winter energy intake in preliminary controlled studies of elk ( Cervus elaphus ) involving mild condition deterioration (up to 11% loss of body mass). To ensure reliable nutritional assessments of free-ranging cervids by measuring A:C ratios of urine in snow, it is essential to extend this work. We assessed the effect of moderate and severe winter nutritional restriction on urinary A:C ratios of captive white-tailed deer ( Odocoileus virginianus ) that lost up to 32% body mass and related these ratios to metabolizable energy intake (MEI), body-mass loss, and other reported nutritional indicators. Deer in the control group were fed a low-protein, low-energy diet ad libitum, whereas deer in the treatment group were fed restricted amounts of the same diet. MEI was below the winter maintenance requirement for all deer, but was lower ( P = 0.029) in treatment deer than in control deer. Percent body-mass loss differed between the two groups as the study progressed, and represented the full range of physiological tolerance (0-32% loss). Mean A:C ratios of control deer, which lost up to 17.4% body mass, showed a slight increasing ( P = 0.086) trend, whereas initially similar A:C ratios of severely restricted deer increased ( P = 0.0002) markedly by the eighth week (0.52 vs. 0.09 µmol:µmol). The urinary A:C ratio was not related ( P = 0.839) to recent (2 days prior to urine sampling) MEI, but there was a marginally significant relation ( r 2 = 0.42, P = 0.110) between the A:C ratio and cumulative percent mass loss. The urinary A:C ratio was directly related to urinary urea nitrogen:creatinine ( r 2 = 0.59, P < 0.0001) and 3-methylhistidine:creatinine ( r 2 = 0.43, P < 0.0001) ratios. This study confirms that elevated and increasing A:C ratios may be due either to increasing energy intake or to accelerated tissue catabolism and increased endogenous contributions to urinary allantoin excretion.

Canadian Journal of Zoology↗