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At least 595 records · Page 33Linked to original sources
USGS1973

Geology of a system of submarine canyons south of Puerto Rico

A strongly dendritic submarine canyon system with four major canyons occupies a 30-km indentation in the insular shelf off the south coast of Puerto Rico between Guanica and Ponce. Each canyon has several headward branches at depths of 100 to 1,100 m. Each of the five major rivers that reach the coast between Guanica and Ponce is opposite a canyon head, and off each of the rivers a channel 20 to 30 m deep is incised into the insular shelf. The entire canyon system is 40 km long and terminates at a depth of about 3.3 km; no fan or other constructional feature appears to have been formed. A 410-km continuous seismic profiling survey shows that three unconformable stratigraphic units underlie the canyon area. One sample of indurated clay of probable Miocene age was dredged from a canyon-wall outcrop of the middle stratigraphic unit. The three stratigraphic units may be roughly correlative with (in ascending order) the Juana Diaz Formation, the Ponce Limestone, and an unnamed upper section found in a nearby drill hole ashore. Canyon-axis sediment at depths of 700 to 2,200 m is entirely of pelagic origin, indicating that sediment is not now being transported down the canyons.

Guanica, Ponce↗
USGS1976

Geology and Rb-Sr ages of reactivated Precambrian gneisses and granite in the Marenisco-Watersmeet area, northern Michigan

Rb-Sr dating of Precambrian W (lower Precambrian) gneisses and granitic rocks that form the cores of antiforms or domes mantled by metamorphosed Precambrian X (middle Precambrian) bedded rocks in the Marenisco-Watersmeet area, northern Michigan, shows that the rock systems have been disturbed to different degrees by complex tectonic and thermal events subsequent to primary crystallization. The disturbance is indicated by a lowering of the whole-rock Rb-Sr ages of many samples of gneiss from 2,600 m.y. or older to about 1,800 m.y. and by the scattering of data points, although some of the points can be fit to secondary isochrons; probably the major mechanism was redistribution of previously accumulated radiogenic Sr, which is indicated by the high initial 87 Sr/ 86 Sr of the resultant isochrons. The disturbance is interpreted as having resulted from internal cataclasis and recrystallization accompanied by metamorphic segregation and (or) partial melting during reactivation of the gneisses about 1,800 m.y. ago. Variations in the degree of reconstitution and mobility of individual bodies of basement rocks are indicated by differences in structural reconstitution of the basement rocks and in the metamorphic grade of the Precambrian X bedded rocks.

Michigan↗
USGS1978

Hypothesis: Many earthquakes in the central and southeastern United States are causally related to mafic intrusive bodies

Assessment of earthquake hazards in the United States is based largely upon knowledge acquired in the seismically active parts of the western United States. Earthquakes in the central and southeastern United States are seismologically and geologically very anomalous, however, compared with those in the western United States. For example, shallow earthquakes of magnitude 7-f in the western United States in general have tectonic surface faulting associated with them. No tectonic surface faulting, however, has been found to be associated with the New Madrid, Mo., earthquakes of 1811 and 1812, several of which had estimated magnitudes of 7-f. The lack of surface faulting is, in part, responsible for the fact that the New Madrid earthquakes have not been related with certainty to any particular geologic structural element, which makes assessment of the seismic hazards associated with possible similar future earthquakes very difficult. Geologic and seismologic data from the central and southeastern United States, however, suggest a spatial correlation between mafic intrusive rocks and seismic activity. An additional supportive correlation is that the trend of many dikes coincides with the trend of nodal planes in many fault-plane solutions. These correlations differ greatly from the direct correlation of earthquakes with faults in the western United States. Consideration of different concepts of earthquake sources in the central and southeastern. United States seems necessary- The preferred concept is that local high-stress concentrations, which may result in earthquakes, occur at or close to the contact of mafic intrusives and their felsic host rocks. Ancient rift zones may have been the primary control of the location of the mafic intrusives. Implications of a causal relation between mafic intrusives and seismicity are that more useful seismo-tectonic maps may be possible and that the source dimensions of the earthquakes would be small. Because of low attenuation, however, effects of earthquakes in the central and southeastern United States, even if relatively small because of smaller source dimensions, must be considered hazardous.

Journal of Research of the U.S. Geological Survey↗
USGS1976

Mississippi carbonate shelf margins, western United States

Regional linear carbonate shelf margins, or stratigraphic reefs, are postulated to have developed during Mississippian time along the eastern flank of the Cordilleran miogeosyncline in the Western United States. These shelf margins are analogous to well-documented ancient and modern geologic counterparts, such as the Guadalupian reef of the West Texas Permian basin, the Lower Cretaceous Edwards shelf margin of the Gulf Coast, and the modern Florida reef tract-Straits of Florida province. Two Mississippian shelf margins are believed to have existed: The lower one developed as an integral part of a widespread carbonate depositional complex of Kinderhookian through early Meramecian age; the upper shelf margin developed west of the earlier stratigraphic reef as part of a regional carbonate depositional complex of middle Meramecian to late Chesterian age. Evidence for both shelf margins consists of (1) a linear physical barrier, (2) restricted sediments in the shelf interior, (3) abrupt basinward thinning of sediments and basin-starvation just seaward of the shelf edge, (4) profound fades-changes coincident with the basinward thinning from light-colored, skeletal, shelf carbonate rocks to dark, fine-grained, silty, basinal carbonate rocks, and (5) the consistent regional occurrence of the first four patterns. Seaward topographic relief along the front edge of the lower shelf margin was probably about 200-400 m, and maximum relief along the central sector of the upper shelf margin may have approached 1,000 m.

Arizona, California, Colorado, Idaho, Montana, Nev↗
USGS1976

The Placer River Silt — An intertidal deposit caused by the 1964 Alaska earthquake

At the head of Turnagain Arm near Anchorage, the major lasting geologic consequence of the Alaska earthquake of 1964 was deposition of the Placer River Silt, an intertidal deposit covering an area of more than 18 km 2 and containing more than 20X10 6 m 3 of sediment. This formation, which was derived from erosion of intertidal bars in Turnagain Arm, averages 1.5 m thick seaward of the Seward Highway and 0.9 m thick landward. Its distribution is controlled both, by the arrangement of the major streams that enter tidewater and by manmade features such as the highway and railroad embankments. The Placer River Silt is still being deposited. The contemporary lowland sedimentation system of the Portage area includes a number of depositional settings: (1) In the Placer River Silt upper tidal flats, levees, and channelway fans, (2) elsewhere in the intertidal zone tidal stream channels, lower tidal flats, and saltwater marsh, and (3) in the supratidal zone gravel levees, freshwater marsh, bogs, and lakes. Since 1964, the critical environmental results of the earthquake-caused deposition have been abandonment of the settlement of Portage, formation of an unsightly blanket of silt, destruction of natural plant communities, localized erosion, and creation of quicksand hazards.

Alaska↗
USGS1977

Cauldron subsidence near Mount Lewis, Nevada: A misconception

The map and interpretation of the geology of the Mount Lewis quadrangle, northern Shoshone Range, Nev., contained in U.S. Geological Survey Professional Paper 465 (1965) have recently been criticized and reinterpreted by C. T. Wrucke and M. L. Silberman in Professional Paper 876 (1975). I contend, however, that the reinterpretation ignores much cogent evidence and is quite inconsistent with the geometric disposition of the rocks. No justification exists for postulating a caldera. The faults considered by Wrucke and Silberman to constitute the bounding faults of a caldera hence normal faults downthrown toward the "caldera" are in part nonexistent, in part a strike-slip fault formed concurrently with the Roberts thrust fault in Devonian and Mississippian time, and in part a listric thrust branching into the hanging wall of the main thrust. In short, the two real faults of Wrucke and Silberman's postulated caldera ring are about 200 million years older than the Oligocene volcanic rocks dated by Wrucke and Silberman. The paucity of dikes associated with the faults is thus readily understood, whereas it would be highly anomalous were the faults defining a caldera. The evidence for our thrust-fault interpretation was presented in the text, map, and sections of Professional Paper 465; further detail is included here. The evidence is clear and unequivocal: no caldera is here present.

Nevada↗
USGS1974

Paleozoic tectonics in the Edna Mountain quadrangle, Nevada

Geologic mapping at scale 1:24,000 of the Edna Mountain 15-minute quadrangle, Humboldt County, Nev., revealed two episodes of pre-Mesozoic deformation that are difficult to reconcile with either the Antler or the Sonoma orogeny. We believe that the older episode predated the Antler orogeny and may be as old as Late Cambrian. The younger episode may have been more localized, predated the Sonoma orogeny, and was probably Late Pennsylvanian to Permian in age. Deformation related to Antler and Sonoma orogenies also occurred. These four episodes suggest that cycles of uplift, folding, faulting, and erosion began early in the development of the southern Cordillera and continued intermittently throughout Paleozoic time. West of the continental shelf was a broad subsiding basin marked by narrow troughs and elongate structural highs which emerged, matured, and diminished at different times. Waxing and waning deformation in various parts of the geosyncline acting upon local "highs" and troughs can explain the seemingly erratic distribution in north-central Nevada of different fades of time-correlative stratigraphic units and structural blocks of Paleozoic age. Telescoping of facies by thrust faulting certainly took place, but large displacements within brief periods are not essential to the validity of the explanation.

Nevada↗
USGS2011

Comment on “An unconfined groundwater model of the Death Valley Regional Flow System and a comparison to its confined predecessor” by R.W.H. Carroll, G.M. Pohll and R.L. Hershey [Journal of Hydrology 373/3–4, pp. 316–328]

Carroll et al. (2009) state that the United States Geological Survey (USGS) Death Valley Regional Flow System (DVRFS) model, which is based on MODFLOW, is “conceptually inaccurate in that it models an unconfined aquifer as a confined system and does not simulate unconfined drawdown in transient pumping simulations.” Carroll et al. (2009) claim that “more realistic estimates of water availability” can be produced by a SURFACT-based model of the DVRFS that simulates unconfined groundwater flow and limits withdrawals from wells to avoid excessive drawdown. Differences in results from the original MODFLOW-based model and the SURFACT-based model stem primarily from application by Carroll et al. (2009) of head limits that can also be applied using the existing MODLOW model and not from any substantial difference in the accuracy with which the unconfined aquifer is represented in the two models. In a hypothetical 50-year predictive simulation presented by Carroll et al. (2009), large differences between the models are shown when simulating pumping from the lower clastic confining unit, where the transmissivity is nearly two orders of magnitude less than in an alluvial aquifer. Yet even for this extreme example, drawdowns and pumping rates from the MODFLOW and SURFACT models are similar when the head-limit capabilities of the MODFLOW MNW Package are applied. These similarities persist despite possible discrepancies between assigned hydraulic properties. The resulting comparison between the MODFLOW and SURFACT models of the DVRFS suggests that approximating the unconfined system in the DVRFS as a constant-saturated-thickness system (called a “confined system” by Carroll et al., 2009) performs very well.

Death Valley↗
USGS1977

Land use and land cover changes: A framework for monitoring

Information on the rate and kind of change in the use of land resources is essential to-the proper planning, management, and regulation of the use of such resources. Starting in 1975, the U.S. Geological Survey has been engaged in mapping and inventory of land use and land cover at a scale of 1:250000 with plans to change to new 1:100000 base maps as they become available. Such scales are-appropriate for acquiring benchmark or baseline data on land use and land cover on a nationwide basis with a reasonable degree of standardization and accuracy, within a practical time frame of 6 to 7 years, and at a reasonable level of detail in categorization. Changes in land use and land cover occur at highly variable rates from place to place. Thus variations, in the intervals between updates of land use and land cover data should be carefully evaluated in order to provide an appropriate frame of reference for the systematic updating of such data. A methodological framework is needed for monitoring changes in land use and land cover that will be timely, relatively inexpensive, and appropriate for widely varying needs at national, interstate-regional, state, multicounty, county, and city levels. In this article a partial framework for monitoring land use and land cover changes is provided with opportunity to extend its use to greater levels of detail in categorization and shorter intervals of frequency when needed for selected areas.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Geology, geochemistry, and fluid-inclusion petrography of the Sapo Alegre porphyry copper prospect and its metavolcanic wallrocks, west-central Puerto Rico

The Sapo Alegre prospect, a small porphyry copper-molybdenum occurrence in west-central Puerto Rico, is characterized by distinct zones of alteration and mineralization of quartz diorite porphyry. A biotite-chlorite zone in the porphyry near its contact with surrounding metavolcanic rocks contains copper, molybdenum, gold, silver, selenium, and tellurium. A quartz-sericite-pyrite zone within the porphyry contains abundant sulfur and traces of selenium, but metals of the biotite-chlorite zone are nearly absent and zinc, manganese, nickel, sodium, calcium, and magnesium contents are very low. Quartz grains in the biotite-chlorite zone contain abundant fluid inclusions in which halite crystals are common, whereas in the quartz-sericite-pyrite zone inclusions are less abundant and rarely contain halite inclusions. Metavolcanic rocks northwest of the mineralized porphyry are altered to amphibolitic hornfels near the contact. Biotitic alteration is strong outward from the hornfels and grades into chlorite alteration. The hornfels has very low sulfide content, and the biotite zone has moderate amounts of copper and sulfur, decreasing outward. The chlorite zone is characterized by an abrupt increase in zinc and manganese content. Fluid-inclusion data suggest temperatures between 300° and 400°C for hydrothermal fluids in the biotite-chlorite zone of the porphyry and 250° to 300°C for fluids in the quartz-sericite-pyrite zone. Pressures equivalent to about 1.5 km of burial are indicated.

Río Víví district↗
USGS1973

Geologic implications of the Apollo 14 Fra Mauro breccias and comparison with ejecta from the Ries Crater, Germany

On the basis of petrographic and laboratory and active seismic data for the Fra Mauro breccias, and by comparison with the nature and distribution of the ejecta from the Ries crater, Germany, some tentative conclusions regarding the geologic significance of the Fra Mauro Formation on the moon can be drawn. The Fra Mauro Formation, as a whole, consists of unwcldcd, porous ejecta, slightly less porous than the regolith. It contains hand-specimen and larger size clasts of strongly annealed complex breccias, partly to slightly annealed breccias, basalts, and perhaps spherule-rich breccias. These clasts are embedded in a matrix of porous aggregate dominated by mineral and breccia fragments and probably largely free of undevitrified glass. All strongly annealed hand-specimen-size breccias are clasts in the Fra Mauro Formation. To account for the porous, unwelded state of the Fra Mauro Formation, the ejecta must have been deposited at a temperature below that required for welding and annealing. Large boulders probably compacted by the Cone crater event occur near the rim of the crater. They probably consist of a similar suite of fragments, but are probably less porous than the formation. The geochronologic clocks of fragments in the Fra Mauro Formation, with textures ranging from unannealed to strongly annealed, were not reset or strongly modified by the Imbrian event. Strongly annealed breccia clasts and basalt clasts are pre-Imbrian, and probably existed as ejecta mixed with basalt flows in the Imbrium Basin prior to the Imbrian event. The Imbrian event probably occurred between 3.90 or 3.88 and 3.65 b.y. ago.

Ries Crater↗
USGS1977

Geology of the gabbroic complex along the northern border of the Josephine Peridotite, Vulcan Peak area, southwestern Oregon

The terrane bordering the alpine-type Josephine Peridotite on the north in the Vulcan Peak area, southwestern Oregon, is composed of intrusive hornblende gabbro (Middle Jurassic) and scattered remnants of clinopyroxene-bearing ultramafic rocks and amphibolite. The amphibolite, which preliminary analyses suggest is of andesitic composition, has undergone regional metamorphism to the amphibolite facies and three episodes of plastic folding. The ultramafic rocks overlie the amphibolite with a possible magmatic sedimentary contact, although the contact is not entirely clear and a fault cannot be ruled out. The ultramafic rocks are partially recrystallized owing to the gabbro intrusion and later partially serpentinized; they have undergone two episodes of plastic folding, the second of which correlates with the third in the amphibolite. The intrusion of the gabbro began as early as the second folding episode in the amphibolite and probably continued at least intermittently throughout the third episode of folding and possibly later. After the high-temperature deformation, the Josephine Peridotite was thrust northward over the gabbroic complex along an east-striking south-dipping thrust fault, after which both of these terranes were thrust westward over the Upper Jurassic Dothan Formation along a major north-striking east-dipping thrust fault. The nature of the ultramaflc rocks, their association with an extensive gabbro terrane, and the proximity of a large alpine-type peridotite suggest that the gabbroic and ultramafic complexes are part of an ophiolite sequence. However, if the ultramaflc rocks are cumulates and were deposited on the amphibolite terrane, then the gabbroic complex is somewhat different from the ideal ophiolite model.

Oregon↗
USGS1976

Geology of central Costa Rica and its implications in the geologic history of the region

The oldest rocks in Costa Rica, the Nicoya Complex, crop out in an arcuate belt along the Pacific coast and outside the mapped area. The complex is an ophiolite assemblage and indicates that a Late Cretaceous arc-trench system, probably a southerly extension of the Middle America Trench, lay along and west of the present Pacific coast. Polarity of the system is indicated by the presence of a volcanic olivine basalt arc east of the ophiolite belt and by a younger magmatic arc farther east. Relative lack of metamorphism of the ophiolite assemblage suggests that the rocks were obducted rather than subducted. Fossil evidence indicates that the complex was emplaced no later than the late Campanian. By the late Eocene both the ophiolite assemblage and the olivine basalt arc had been established as major sediment sources for basins that lay to the east, and they continued as such through the Oligocene and possibly into the middle Miocene. Widespread deposition of trachyandesite lava and volcaniclastic sediments, local marine planation, uplift, and renewed volcanism mark what is presumed to be the end of the Miocene. Extreme south-to-north coarsening of the unconformably superposed Doan Formation and the presence of corals in the shaly facies of the Doan in the southern part of the mapped areas suggest that marine sedimentation there was undisturbed by orogeny and that intrusion of the Talamancan magmatic rocks must have followed deposition of the Doan. Volcaniclastic block breccia, characteristic of northern exposures of the Doan, indicates that volcanism in the proto- Cordillera Central had begun by at least Doan time. Volcanism has been a dominant process in central Costa Rica since that time.

Journal of Research of the U.S. Geological Survey↗
USGS1976

Geology of the Brysch uranium mine, Karnes County, Texas

Approximately 13,700 tons (12,400 tonnes) of oxidized uranium ore, averaging about 0.1 percent U 2 O 8 , was mined during 1966 and 1967 from the lower unit of the Deweesville Sandstone Member of the upper Eocene Whitsett Formation, from depths of 75 to 90 feet (23-27 m). The mine is in the Karnes County uranium area, 3 miles (5 km) east of Falls City, Tex. Meta-autunite, Ca(UO 2 ) 2 (PO 4 ) 2 2-6H 2 O, and meta-tyuyamunite, Ca(UO 2 ) 2 (PO 4 ) 2 3-5H 2 O, were identified in samples of the ore. The host rock is light-colored, medium-grained, well-sorted feldspathic sandstone that contains fossil wood and Ophiomorpha . It also contains clinoptilolite, cristobalite, and montmorillonite that probably formed as alteration products of volcanic grains in the original sediment. The host rock was deposited in a beach environment near or on a delta formed at the mouth of a stream channel oriented roughly normal to the beach. The Deweesville can be traced for many miles on the surface and represents in general the point of maximum transgression in a transgressive-regressive cycle. The upper unit of the Deweesville, which overlies the host rock, is now exposed in the mine and contains, in ascending order, storm-beach, shoreface, beach, and tidal-flat facies. The shape of the ore body suggests that it originated as an unoxidized ore roll. The ore-bearing fluids may have entered through porous fluvial rock extending updip. Plant material, abundant in the original host sediment, probably supplied reductant necessary for the roll formation.

Texas↗
USGS1978

Uranium in waters and aquifer rocks at the Nevada Test Site, Nye County, Nevada

Previous chemical, geological, and hydrological information describing the physical and chemical environment of the Nevada Test Site (a Federal reserve for the testing of nuclear explosive devices) has been combined with new radiochemical and isotope data for water and rock samples in order to explain the behavior of uranium during alteration of thick sequences of rhyolitic volcanic rocks and associated volcaniclastic sediments. A model is proposed in which uranium mobility is controlled by two competing processes. Uranium is liberated from the volcanic rocks through dissolution of the glassy constituents and is carried in solution as a uranyl carbonate complex. Uranium is subsequently removed from solution by adsorption on secondary oxides of iron, titanium, and manganese, as observed in fission-track maps of aquifer rocks. The model explains the poor correlation of dissolved uranium with depth within tuffaceous sequences in which percolation of ground water is predominantly downward. Good positive correlation of dissolved uranium with dissolved Na, total dissolved solids, and total carbonate supports the glass dissolution model, while inverse correlation of dissolved uranium with U 234 /U 238 ratios of waters implies uranium is being absorbed by a relatively insoluble, surficial phase. Alpha radioactivity of Test Site water is primarily caused by high U 234 contents, and beta activity is highly correlated with dissolved K (K 40 ). Small amounts of dissolved radium, Pb 210 , and Po 210 are present but no evidence was found for alpha activity sources related to nuclear testing (Pu, U 235 ). A filtered but unacidified carbonate solution of uranium was found to be stable (± 10 percent of original U concentration) for years when stored in acid-washed polyethylene bottles.

Nevada↗
USGS1963

Applications of geohydrologic concepts in geology

Subsurface water, an active agent in many geologic proceses, must be considered in interpreting geologic phenomena. Principles of the occurrence, distribution, and movement of subsurface waters are well established and readily applicable. In many interpretations in geologic literature, geohydrologic principles have been employed realistically, but in many others these principles have been either ignored or violated. Explanations of genesis of underclays and associated deposits afford some examples wherein principles of movement and activity of vadose and ground water have been ignored and others in which they have been used advantageously. Postulates stating that waters percolate downward from swamp areas do not allow for the usual movement of subsurface water in such environments. The idea that sediments were leached by vadose water after uplift satisfies the geohydrologic requirements. Weathering and solution form porous and permeable zones subjacent to unconformities in dense rocks such as carbonates and granites; this illustrates the geohydrologic and economic significance of unconformities. Examples are Mohawkian carbonate aquifers of northern Illinois and oil-bearing limestones of Mississippian age of eastern Montana. The flushing effects of meteoric water and other hydrodynamic factors active during erosion periods are important elements in the genesis and concentration of brines. Explanation of the origin and occurrence of brines must include consideration of the geohydrologic environments throughout their geologic history.

Journal of Hydrology↗
USGS1973

Stress changes ahead of an advancing tunnel

Instrumentation placed ahead of three model tunnels in the laboratory and ahead of a crosscut driven in a metamorphic rock mass detected stress changes several tunnel diameters ahead of the tunnel face. Stress changes were detected 4 diameters ahead of a model tunnel drilled into nearly elastic acrylic, 2·50 diameters ahead of a model tunnel drilled into concrete, and 2 diameters ahead of a model tunnel drilled into Silver Plume Granite. Stress changes were detected 7·50 diameters ahead of a crosscut driven in jointed, closely foliated gneisses and gneissic granites in an experimental mine at Idaho Springs, Colorado. These results contrast markedly with a theoretical elastic estimate of the onset of detectable stress changes at 1 tunnel diameter ahead of the tunnel face. A small compressive stress concentration was detected 2 diameters ahead of the model tunnel in acrylic, 1.25 diameters ahead of the model tunnel in concrete, and 1 diameter ahead of the model tunnel in granite. A similar stress peak was detected about 6 diameters ahead of the crosscut. No such stress peak is predicted from elastic theory. The 3-dimensional in situ stress determined in the field demonstrate that geologic structure controls stress orientations in the metamorphic rock mass. Two of the computed principal stresses are parallel to the foliation and the other principal stress is normal to it. The principal stress orientations vary approximately as the foliation attitude varies. The average horizontal stress components and the average vertical stress component are three times and twice as large, respectively, as those predicted from the overburden load. An understanding of the measured stress field appears to require the application of either tectonic or residual stress components, or both. Laboratory studies indicate the presence of proportionately large residual stresses. Mining may have triggered the release of strain energy, which is controlled by geologic structure.

Colorado↗