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Results for “Geological Society of America Bulletin, Part I”

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At least 73 records · Page 4Linked to original sources
USGS1967

Stratigraphy and correlation of the precambrian belt supergroup of the southern Lewis and Clark Range, Montana

Several well-exposed and little-deformed Belt Supergroup sections have been studied in the southern Lewis and Clark Range . In the area studied, the Belt thins eastward or northeastward due both to primary sedimentation and to pre-Middle Cambrian erosion. These rocks can now be more precisely correlated with the well-known sections near Bonner, Helena, and Glacier National Park. In the western part of the area, the Missoula Group is thickest and lithologically intermediate between the sections at Bonner and Marias Pass. Formation names from these two sections are applied in the southern Lewis and Clark Range . The thin Belt sequence in the eastern part of the area includes the lower part of the Missoula Group and older formations that may be traced southward into the Helena, Empire, and Spokane Formations of the Helena area. Consequently, the Helena Dolomite of the Helena area, the Siyeh Formation of the Marias Pass area, and the "Newland Limestone" of the Bonner area are probably lateral equivalents.

Montana↗
USGS2009

Processes active in mafic magma chambers: The example of Kilauea Iki Lava Lake, Hawaii

Kilauea Iki lava lake formed in 1959 as a closed chamber of 40 million m 3 of picritic magma. Repeated drilling and sampling of the lake allows recognition of processes of magmatic differentiation, and places time restrictions on the periods when they operated. This paper focuses on evidence for the occurrence of lateral convection in the olivine-depleted layer, and constraints on the timing of this process, as documented by chemical, petrographic and thermal data on drill core from the lake. Lateral convection appears to have occurred in two distinct layers within the most olivine-poor part of the lake, created a slightly olivine-enriched septum in the center of the olivine-depleted section. A critical marker for this process is the occurrence of loose clusters of augite microphenocrysts, which are confined to the upper half of the olivine-poor zone. This process, which took place between late 1962 and mid-1964, is inferred to be double-diffusive convection. Both this convection and a process of buoyant upwelling of minimum-density liquid from deep within the lake (Helz, R.T., Kirschenbaum H. and Marinenko, J.W., 1989. Diapiric melt transfer: a quick, efficient process of igneous differentiation: Geological Society of America Bulletin, v. 101, 578–594) result from the fact that melt density in Kilauea Iki compositions decreases as olivine and augite crystallize, above the incoming of plagioclase. The resulting density vs. depth profile creates (1) a region of gravitationally stable melt at the top of the chamber (the locus of double-diffusive convection) and (2) a region of gravitationally unstable melt at the base of the melt column (the source of upwelling minimum-density melt, Helz, R.T., Kirschenbaum H. and Marinenko, J.W., 1989. Diapiric melt transfer: a quick, efficient process of igneous differentiation: Geological Society of America Bulletin, v. 101, 578–594). By contrast the variation of melt density with temperature for the 1965 Makaopuhi lava lake does not show a decrease in density as temperature decreases, so neither process should have occurred in that lava lake. Because many mafic magmas crystallize significant olivine and/or pyroxene before they begin to crystallize plagioclase, the density relations observed for Kilauea Iki, and the processes that result from them, may be relevant to crystallization in other mafic magma chambers. The results for the 1965 Makaopuhi lava lake emphasize the role of bulk composition as a critical control on magmatic processes.

Hawai'i↗
USGS1960

Geophysical investigation of Mono Basin, California

Gravity and seismic studies in Mono Basin , Mono County, California , completed during the summer of 1957 revealed a large, roughly triangular block that had subsided about 18,000 ± 5000 feet and received an accumulation of about 300 ± 100 cubic miles of light clastic sediments and volcanic material of Cenozoic age. The seemingly near-vertical faults that bound this great block are displaced toward the center of the basin from the surrounding mountain masses, but in general they are parallel to well-defined Basin and Range trends. The gravity minimum anomaly associated with the Mono Basin structure has a residual gravity relief of about 50 mgals, and the lowest gravity readings (on Paoha Island) yield a complete Bouguer gravity value of about - 260 mgals with respect to the International Ellipsoid. The computed depth of subsidence is based on a density of 2.3 gms/ cm 3 for the basin fill and 2.7 gms/cm 3 for the basement rocks. Seismic-refraction profiles at several places in the basin demonstrate that the Cenozoic deposits are thick where the gravity is low and relatively thin where the gravity is higher. Along common seismic and gravity profiles steep seismic dips coincide with steep gravity gradients. Numerous seismic reflections are present within the basin fill. Anomalies on four aeromagnetic profiles are related in part to volcanic material within the Cenozoic section. It is concluded that Mono Basin may be a volcano-tectonic depression caused by subsidence along faults, following extrusion of magma from a magma chamber at depth. Volcanic rocks of Pliocene(?) and Pleistocene ages are exceptionally abundant in this area.

California↗
USGS1968

The formation of columnar joints in the upper part of Kilauean lava lakes, Hawaii

Cracks were observed forming at the surface of Makaopuhi lava lake during the March 1965 Kilauea eruption, and were studied by repeated mapping and observations of this lake ; the 1963 Alae lava lake was similarly studied. Cracks open within a minute after molten lava is exposed at the surface, and form either random or oriented orthogonal networks which outline large plates of unjointed crust. Within a few hours, additional cracks subdivide the plates into polygons averaging 15 ft in width. The accumulation of gases trapped beneath the crust near centers of polygons, and the escape of gases from marginal parts, cause upbowing of polygon centers and downsagging of margins. As the crust cools and increases in thickness following stagnation of the lake , existing cracks extend downward and new cracks open. Some new cracks subdivide pre-existing polygons, and short cracks of shallow depth form near polygon centers. Still other cracks apparently open at depths of tens of feet within the crust, propagate upward, and finally feather out near the surface into short cracks which are concentrated in long swarms that cross several polygons. Cracks initiate at temperatures ranging from ambient to about 900°C, and propagate downward into parts of the crust near 1000°C. Cracking results from stresses induced largely by thermal contraction, but also by differential subsidence of the crust. Seismic recordings of shock vibrations due to cracking of Kilauea Iki lava lake indicate a diurnal variation in frequency, with a maximum around midnight and a minimum around noon.

Hawaii↗
USGS1980

Use of altered volcanic ash falls in stratigraphic studies of coal-bearing sequences: an example from the Upper Cretaceous Ferron Sandstone Member of the Mancos Shale in central Utah.

The Ferron consists of 5 delta cycles, each of which includes one coal zone which contains at least one, and usually several, laterally persistent kaolinitic claystone partings. Laboratory study of the partings demonstrates that they represent altered volcanic ash falls. These partings have proven particularly useful in reconstructing the depositional history of the C coal bed of the Emery coal field which accumulated in a basin that developed concurrently with subsidence of the delta plain during both the constructive and destructive phases of the third delta cycle.-from Authors

Geological Society of America Bulletin↗
USGS2017

Varve formation during the past three centuries in three large proglacial lakes in south-central Alaska

The sediments stored in the large, deep proglacial lakes of south-central Alaska are largely unstudied. We analyzed sediments in 20 cores, up to 160 cm long, from Eklutna, Kenai, and Skilak Lakes, using a combination of repeated lamination counting, radionuclide dating, event stratigraphy, and tephrochronology. We show that the characteristically rhythmic layers were deposited annually. Most of these glacial varves consist of one coarse-grained base and a fine-grained top, but varves composed of multiple coarse-grained turbidite pulses are common too. They are likely related to successive episodes of high sediment discharge during flooding, and they become more frequent in all three lakes, along with increased sedimentation rates, during the nineteenth century late phase of the Little Ice Age. These flood turbidites were generated by rain events and intense melting of snow and ice. Other (mega) turbidites are a result of earthquake-triggered slope collapses (e.g., A.D. 1964). Some event layers are present in all three lakes. In addition, the annual time series of varve thickness (normalized annual sedimentation rate) are significantly correlated among the three lakes (ρ > 0.27; p < 0.001). Differences between the varve thickness records can be attributed partly to the dam construction at Eklutna Lake and outbursts from an ice-dammed lake at Skilak Lake. Geomorphologic differences among the catchments result in further differences in sedimentation patterns in the three lakes.

Alaska↗
USGS1996

Waters associated with an active basaltic volcano, Kilauea, Hawaii: Variation in solute sources, 1973-1991

Chemical and isotopic analyses of samples collected from a December 1962-m-deep research borehole at the summit of Kilauea Volcano provide unique time-series data for composition of waters in the uppermost part of its hydrothermal system. These waters have a distinctive geochemical signature: a very low proportion of chloride relative to other anions compared with other Hawaiian waters—thermal (•30°C) or nonthermal (<30°C)—and with most thermal waters of the world. Isotope data demonstrate that the borehole waters are of essentially meteoric origin, with minimal magmatic input.

Hawaii↗
USGS1991

Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau

A large alkaline, saline lake, Lake T'oo'dichi', occupied the entire eastern part of the Colorado Plateau region during deposition of the Brushy Basin Member of the Upper Jurassic Morrison Formation. The lake extended from near the site of Albuquerque, New Mexico, to near the site of Grand Junction, Colorado, and occupied a region that encompassed the San Juan and ancestral Paradox basins, making it the largest ancient alkaline, saline lake known.

Geological Society of America Bulletin↗
USGS2001

Origin of late Quaternary dune fields on the Southern High Plains of Texas and New Mexico

Mostly stabilized late Holocene eolian sands on the Southern High Plains of the United States were studied to determine their origins and to assess whether present dune stability depends more strongly on sediment supply, sediment availability, or transport limitations. Geomorphic, sedimentological, and geochemical trends indicate that late Holocene dunes formed under westerly paleowinds, broadly similar to those of today. Mineralogical and geochemical data indicate that the most likely source for the sands is not the Pecos River valley, but the Pleistocene Blackwater Draw Formation, an older, extensive eolian deposit in the region. These observations suggest that new sand is supplied whenever vegetation cover is diminished to the extent that the Blackwater Draw Formation can be eroded, in agreement with modern observations of wind erosion in the region. We conclude, therefore, that Southern High Plains dunes are stabilized primarily due to a vegetation cover. The dunes are thus sediment-availability limited. This conclusion is consistent with the observation that, in the warmest, driest part of the region (where vegetation cover is minimal), dunes are currently active over a large area. Geochemical data indicate that Southern High Plains dunes are the most mineralogically mature (quartz rich) sands yet studied in the Great Plains, which suggests a long history of eolian activity, either in the dune fields or during deposition of the Blackwater Draw Formation.

New Mexico, Texas↗
USGS1969

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.

Maine, New Brunswick, Quebec↗
USGS1998

Lithological and hydrological influences on ground-water composition in a heterogeneous carbonate-clay aquifer system

The influence of clay units on ground-water composition was investigated in a heterogeneous carbonate aquifer system of Miocene age in southwest Florida, known as the Intermediate aquifer system. Regionally, the ground water is recharged inland, flows laterally and to greater depths in the aquifer systems, and is discharged vertically upward at the saltwater interface along the coast. A depth profile of water composition was obtained by sampling ground water from discrete intervals within the permeable carbonate units during coring and by squeezing pore water from a core of the less-permeable clay layers. A normative salt analysis of solute compositions in the water indicated a marine origin for both types of water and an evolutionary pathway for the clay water that involves clay diagenesis. The chemical composition of the ground water in the carbonate bedrock is significantly different from that of the pore water in the clay layers. Dissolution of clays and opaline silica results in high silica concentrations relative to water in other parts of the Intermediate aquifer system. Water enriched in chloride relative to the overlying and underlying ground water recharges the aquifer inland where the confining clay layer is absent, and it dissolves carbonate and silicate minerals and reacts with clays along its flow path, eventually reaching this coastal site and resulting in the high chloride and silica concentrations observed in the middle part of the Intermediate aquifer system. Reaction-path modeling suggests that the recharging surficial water mixes with sulfate-rich water upwelling from the Upper Floridan aquifer, and carbonate mineral dissolution and precipitation, weathering and exchange reactions, clay mineral diagenesis, clay and silica dissolution, organic carbon oxidation, and iron and sulfate reduction result in the observed water compositions.

Geological Society of America Bulletin↗
USGS1985

Carboniferous U-Pb age of the Sebago batholith, southwestern Maine: Metamorphic and tectonic implications

Two phases (pink and white granite) of the Sebago batholith of southwestern Maine have been dated by the U-Pb zircon method. Identical upper concordia intercepts of both rocks indicate an intrusive age of 325 ± 3 m.y. for the batholith. The lower intercept of the pink-phase sample, 114 ± 13 m.y., is inferred to represent episodic lead loss due to the intrusion of the nearby Cretaceous Pleasant Mountain stock. The lower intercept of the white-phase sample, 18 ± 21 m.y., suggests only modern dilatancy lead loss. Monazites have ages of 272 m.y. (pink) and 282 m.y. (white) which are thought to be cooling ages. Rb-Sr whole-rock data have low initial 87 Sr/ 86 Sr ratios of 0.7031 (pink) and 0.7053 (white). These data, in conjunction with published 40 Ar/ 39 Ar, Rb-Sr, K-Ar, and fission-track ages, suggest that little or no uplift occurred in this part of New England until the Permian and that the uplift rate from 275 m.y. to 225 m.y. was ∼3 times as rapid as was the rate for 225 m.y. to the present. The Carboniferous age of the Sebago batholith suggests that currently accepted metamorphic and tectonic interpretations for southwestern Maine and for east-central New Hampshire require revision.

Maine↗
USGS1988

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic mélange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values (<1.5). An incomplete sampling of cherts in tectonic blocks and from Holocene gravels derived from the Esk Head subterrane yields radiolarian-based ages of Late Triassic, Early Jurassic, Middle Jurassic, and Late Jurassic. Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Geological Society of America Bulletin↗
USGS1953

Limestone walls of Okinawa

Wall-like ridges of limestone that stand well above the surrounding terrain are an interesting phenomenon on the island of Okinawa. These ridges rim a variety of topographic features, but all are believed to represent the same formative processes. Rimming ridges or walls occur along the banks of streams crossing areas of limestone, along the upthrown sides of faults, around a sink hole, and along terrace edges. The limestone walls of Okinawa are believed to have formed by the cementation, or casehardening, of steep exposures of poorly consolidated limestone, followed by differential erosion which brings the cemented zones into relief. Cementation occurs wherever poorly consolidated limestone is exposed to alternate wetting and drying. It is best developed, however, on steeply sloping exposures where no soil accumulates to hinder drying. Differential erosion results in part from slower solution of cemented limestone, due to its greater density, and in part from accelerated solution in the surrounding soil-covered areas. Rampart walls along the seaward edges of terraces have been described from many Pacific islands and are present on many others. They are believed to be comparable to the walls of Okinawa and all are believed to have been formed under similar conditions.

Geological Society of America Bulletin↗
USGS1967

Rocks of eocene age on fippennies ledge, Gulf of Maine

In August 1965, a scallop dredge from R/V Albatross brought up many pieces of fossiliferous opaline chert or porcellanite of Eocene age from Fippennies Ledge , a bank 70 m deep in the central Gulf of Maine . Their presence in this area supports the idea that part of the Gulf is underlain by sedimentary rocks of Tertiary age . Occurrence in the porcellanite of two identifiable species of Bryozoa (ascophoran Cheilostomata), Kleidionella lobata Canu and Bassler and "Ocheto-sella" robusta Canu and Bassler, suggests (1) correlation with the Castle Hayne Limestone of Claiborne to Jackson age ; (2) a temperate zone, distinctly American biogeographic provenance; and (3) accumulation in quiet water.

Maine↗
USGS1969

Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado and northern New Mexico

Upper Cenozoic basaltic rocks in and near the northern Rio Grande depression, a major intracontinental tension-rift structure, vary systematically in petrology and chemistry with distance from the depression. Basalts and basaltic andesites of alkalic affinities, commonly showing evidence of crustal contamination, were erupted east and west of the depression concurrently with its formation, whereas little-contaminated tholeiitic basalts filled parts of the depression late in its history. Eruption of the contrasting basalt types was in part concurrent. The lateral change from alkalic to tholeiitic basaltic volcanism may reflect different conditions of magma generation in the mantle that are related to changes in crustal thickness and thermal gradient across the rift. Recent experimental studies suggest that the variations in magma composition may be due to differing depths of magma fractionation, the tholeiitic basalts originating at shallower depths than the alkalic basalts.

Colorado, New Mexico↗
USGS2025

Seismic response of rock towers at the Trona Pinnacles (U.S.A.) to the 2019 Ridgecrest earthquake sequence: Theory, observations, and models

We analyze the seismic response of a class of fragile geologic features (FGFs), referred to as rock towers (RTs) at the Trona Pinnacles, a group of RTs in southern California that suffered strong shaking during the 2019 Ridgecrest earthquake sequence. FGFs, including RTs, may provide maximum constraints on past earthquake shaking intensity, and thereby support probabilistic seismic hazard assessments (PSHAs). In a rare case study, we explore the hypothesis that RT structural integrity is time dependent, as damage accumulates progressively. We develop finite‐element method (FEM) models of the RTs using photogrammetric shape models. We validate the models by comparing numerical simulations of their response to broadband ground shaking with low‐intensity seismic recordings obtained at the Pinnacles. Results of our simulations are in good agreement with the seismic recordings of actual earthquake aftershocks. We next use the results of the FEM models to analyze the response and evolution of RTs. Our analyses elucidate the influence of geometry over their seismic response, providing a rationale that may explain the rarity of slender RTs at Trona: high‐aspect‐ratio structures that respond in bending develop detrimental tensile stresses that crack the rock, whereas low‐aspect‐ratio ones’ response also includes shearing, which does not compromise material integrity as much as tension. Field measurements with a rebound hammer support this finding, suggesting that the material around the base of slender rocks has been weakened relative to other parts of the RT. We also study how to define simplified mechanical models (“archetypes”) to predict the natural frequencies of RTs. Results from our work illuminate the fundamental mechanisms of seismic response and progressive failure of RTs, and open new avenues of research to potentially incorporate these geologic features as long‐return period constraints on PSHA, in ways analogous to those of the widely used precariously balanced rocks.

California↗
USGS1957

Military geology in the United States sector of the European theater of operations during World War II

Geology, which was of far-reaching importance on the Western Front of World War I, played a less spectacular role during World War II in so far as the United States armies in Europe were concerned. The U. S. Army in the European Theater of Operations (ETO) used geologists in two capacities: (1) to make staff studies at the level of Theater Headquarters, and (2) as officer personnel in a water-supply unit. In the first category, only one group of seven geologists was employed, forming part of the Information Section, Intelligence Division, Office of the Chief Engineer, ETO. The products of this group consisted largely of regional and localized terrain (trafficability) studies that ranged geographically from the Normandy invasion beaches to Czechoslovakia. Problems of water supply, sources of road material, and many other questions of a geologic nature also arose. French geologists collaborated closely with the work of the section. As the campaign progressed, the Military Geology Unit of the U. S. Geological Survey made important contributions to the geologic intelligence of Germany.

Geological Society of America Bulletin↗