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

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At least 235 records · Page 13Linked to original sources
USGS2000

Transient groundwater-lake interactions in a continental rift: Sea of Galilee, Israel

The Sea of Galilee, located in the northern part of the Dead Sea rift, is currently an intermediate fresh-water lake. It is postulated that during a short highstand phase of former Lake Lisan in the late Pleistocene, saline water percolated into the subsurface. Since its recession from the Kinarot basin and the instantaneous formation of the fresh-water lake (the Sea of Galilee), the previously intruded brine has been flushed backward toward the lake. Numerical simulations solving the coupled equations of fluid flow and of solute and heat transport are applied to examine the feasibility of this hypothesis. A sensitivity analysis shows that the major parameters controlling basin hydrodynamics are lake-water salinity, aquifer permeability, and aquifer anisotropy. Results show that a highstand period of 3000 yr in Lake Lisan was sufficient for saline water to percolate deep into the subsurface. Because of different aquifer permeabilities on both sides of the rift, brine percolated into a aquifers on the western margin, whereas percolation was negligible on the eastern side. In the simulation, after the occupation of the basin by the Sea of Galilee, the invading saline water was leached backward by a topography-driven flow. It is suggested that the percolating brine on the western side reacted with limestone at depth to form epigenetic dolomite at elevated temperatures. Therefore, groundwater discharging along the western shores of the Sea of Galilee has a higher calcium to magnesium ratio than groundwater on the eastern side.

Lake Tiberias, Sea of Galilee↗
USGS2008

Ground-motion modeling of the 1906 San Francisco earthquake, part I: Validation using the 1989 Loma Prieta earthquake

We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli intensity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave-propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agreement with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area.

Bulletin of the Seismological Society of America↗
USGS1986

Migration of volcanism in the San Francisco volcanic field, Arizona

The remanent magnetization of volcanic rocks has been determined at 650 sites in the San Francisco volcanic field in the southern part of the Colorado Plateau. The polarity of remanent magnetization—combined with K-Ar age determinations, spatial and petrographic associations, stratigraphic relations, and state of preservation of the cinder cones—provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and >100 intermediate to silicic flows, flow sequences, and vents. The age assignments for basaltic rocks include 243 Brunhes (<0.73 Ma) vents, 220 Matuyama (0.73 to 2.48 Ma) vents, and 147 pre-Matuyama (2.48 to about 5.0 Ma) vents. Basaltic volcanism migrated northeastward before Matuyama time at a rate of ∼1.2 cm/yr and eastward (S87° ± 5°E) over the past 2.5 m.y. at a rate of 2.9 ± 0.3 cm/yr. Concomitant acceleration in total magma production (from 75 to 1,400 × 10 −6 km 3 /yr) and frequency of basaltic eruptions (from 1 per 17,000 yr to 1 per 3,000 yr) occurred between 5 and 0.25 Ma. For the past 0.25 m.y., magma production (∼180 × 10 −6 km 3 /yr) and perhaps eruption frequency have decreased. This evolutionary sequence, coupled with the lead and strontium-isotopic composition of the rocks, can be explained by magmatism caused by shear heating at the base of the lithosphere. We propose that this eastward drift of volcanic activity represents absolute westward motion of the North American plate. Our model is in agreement with a model in which the African plate is fixed to the deep mantle.

Arizona↗
USGS2010

Magnitude and timing of downstream channel aggradation and degradation in response to a dome-building eruption at Mount Hood, Oregon

A dome-building eruption at Mount Hood, Oregon, starting in A.D. 1781 and lasting until ca. 1793, produced dome-collapse lithic pyroclastic flows that triggered lahars and intermittently fed 10 8 m 3 of coarse volcaniclastic sediment to sediment reservoirs in headwater canyons of the Sandy River. Mobilization of dominantly sandy sediment from these reservoirs by lahars and seasonal floods initiated downstream migration of a sediment wave that resulted in a profound cycle of aggradation and degradation in the lowermost reach of the river (depositional reach), 61-87 km from the source. Stratigraphic and sedimentologic relations in the alluvial fill, together with dendrochronologic dating of degradation terraces, demonstrate that (1) channel aggradation in response to sediment loading in the headwater canyons raised the river bed in this reach at least 23 m in a decade or less; (2) the transition from aggradation to degradation in the upper part of this reach roughly coincided with the end of the dome-building eruption; (3) fluvial sediment transport and deposition, augmented by one lahar, achieved a minimum average aggradation rate of ~2 m/yr; (4) the degradation phase of the cycle was more prolonged than the aggradation phase, requiring more than half a century for the river to reach its present bed elevation; and (5) the present longitudinal profile of the Sandy River in this reach is at least 3 m above the pre-eruption profile. The pattern and rate of channel response and recovery in the Sandy River following heavy sediment loading resemble those of other rivers similarly subjected to very large sediment inputs. The magnitude of channel aggradation in the lower Sandy River, greater than that achieved at other volcanoes following much larger eruptions, was likely enhanced by lateral confinement of the channel within a narrow incised valley. A combination of at least one lahar and winter floods from frequent moderate-magnitude rainstorms and infrequent very large storms was responsible for flushing large volumes of sediment to the depositional reach. These conditions permitted a sedimentation response in the Sandy River that approached the magnitude of channel aggradation resulting elsewhere from large explosive eruptions and high-intensity rainfall regimes, despite the fact that the Sandy River aggradation was in response to an unremarkable dome-building eruption in a climate dominated by low to moderate rainfall intensities.

Oregon↗
USGS2006

Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range

Eocene-middle Miocene volcanic rocks in the northern Pine Forest Range, Nevada, are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the Basin and Range province. A conformable sequence of Cenozoic volcanic and sedimentary strata in the northern Pine Forest Range dips ???30??W, and 11 new 40Ar/39Ar ages from this sequence define 3 major episodes of volcanic activity. Pre-Tertiary basement and older (ca. 38 Ma) Tertiary intrusive rocks are overlain unconformably by Oligocene (ca. 30-23 Ma) basalt flows and dacitic to rhyolitic ash-flow tuffs interbedded with fine-grained tuffaceous sedimentary rocks. Oligocene rocks are overlain by ???550 m of ca. 17-16 Ma basalt flows equivalent to the Steens Basalt in southern Oregon, and basalt flows are capped by a thin 16.3 Ma ignimbrite that likely is correlative with either the Idaho Canyon Tuff or the Tuff of Oregon Canyon. The northern Pine Forest Range is bounded to the east by a major down-to-the-east normal fault that dips ???40??E with well-developed fault striations indicative of dip-slip motion. This fault initiated at an angle of ???70?? and was rotated ???30?? during uplift of the range. A suite of 17 apatite fission-track ages from the Pine Forest footwall block demonstrates that exhumation, uplift, and slip on the range-bounding fault began ca. 12-11 Ma and continued until at least 7 Ma, with moderate slip since then. The Pine Forest Range did not undergo significant extension before or during peak Oligocene and Miocene volcanism, and similar geologic relationships in nearby ranges suggest that a larger region of northwestern Nevada was also little extended during this interval. Basin and Range faulting in northwestern Nevada appears to have begun no earlier than 12 Ma, making it distinctly younger than deformation in much of central and southern Nevada, where peak extension occurred in the middle Miocene or earlier. ?? 2006 Geological Society of America.

Geological Society of America Bulletin↗
USGS1999

Structural and petrologic evolution of the Lihue basin and eastern Kauai, Hawaii

The topography of the eastern part of the Hawaiian island of Kauai is dominated by the Lihue basin, a large (∼110 km 2 ) semicircular depression bounded by steep cliffs and partly filled by late rejuvenated-stage (or posterosional stage) volcanic material. As with other large, semicircular basins on ocean-island volcanoes, the subsurface geology and origin (e.g., structural collapse vs. fluvial erosion) of the Lihue basin are poorly understood. New analyses of samples collected from eastern Kauai and drill holes within the basin document several important features of the late-stage geologic evolution of Kauai. First, thick (>300 m) sequences of rejuvenated-stage Koloa Volcanics in the Lihue basin show systematic, basin-wide geochemical trends of increasingly incompatible elements with time, indicating a gradual decrease in the extent of partial melting of mantle sources with time. Second, beneath the rejuvenated-stage volcanics in the basin, a thin layer of postshield alkalic stage lavas (e.g., hawaiites and mugearites) overlies older shield-stage tholeiitic lavas of the Napali Member, indicating that the Lihue basin formed by structural collapse, not fluvial erosion. Third, a large (∼2–5 km 3 ) matrix-supported breccia, interpreted as deposits of one or more debris flows, is within the rejuvenated-stage volcanics throughout the basin, and correlates with surficial exposures of the Palikea Breccia west of the basin. Isotopic compositions of the bulk breccia are similar to those of tholeiites from the east side of Kauai, and distinct from those of west Kauai tholeiites. Clasts within the breccia are dominantly hawaiite and alkali gabbro. The source region of the breccia in the steep cliffs and highlands of the central massif to the west of the basin must contain magmatic products of an extensive postshield alkalic stage, including hawaiite flows and one or more large intrusive bodies or ponded sequences of alkali gabbro.

Hawaii↗
USGS1987

Playa-lake basins on the Southern High Plains of Texas and New Mexico: Part II. A hydrologic model and mass-balance arguments for their development.

Hydrologic, geologic, geomorphic, and mass-balance data suggest that most of the ∼30,000 playa lake basins on the Southern High Plains have developed by a combination of dissolution of caliche and piping of surface material into the unsaturated zone rather than by eolian processes as has generally been stated. A conceptual model suggests that particulate organic material, much of which is sorbed on smectite clays, is carried downward from the surface into the unsaturated zone by recharging water. The organic material is oxidized to CO 2 , which dissolves in the water, forms carbonic acid, and dissolves lithologic carbonates. Because organic material is transported and oxidized deep in the unsaturated zone, CO 2 concentrations are much higher at depth than in the soil zone, and recharging water remains thermodynamically subsaturated with respect to carbonates and thus able to dissolve them throughout the unsaturated zone. Dissolution promotes lithologic instability, leading to piping and eluviation of material within the unsaturated zone. Playa basins expand laterally as recharge is concentrated at the edge of the playa floor because of lowered permeability in the center that results from accumulation of clays and other fine sediment. Mass-balance calculations of gas, liquid, and solid fluxes beneath a playa basin suggest that sufficient mass is transported to account for the volume of the depression. Particulate flux is estimated by relating it to the CO 2 flux out of the unsaturated zone. Solute flux is estimated from the difference between input values from the playa lake water and that observed in ground water. Gas flux is measured directly from gas samples at specific depths below the: surface.

Geological Society of America Bulletin↗
USGS1992

Utility of radiocarbon-dated stratigraphy in determining late Holocene earthquake recurrence intervals, upper Cook Inlet region, Alaska

During the great 1964 earthquake, parts of coastal southern Alaska subsided tectonically as much as 2 m, and this led to burial of high-intertidal organic-rich marshes by low-intertidal and tidal silt. In the tectonically active part of upper Cook Inlet, the presence of stratigraphic sections containing numerous prehistoric interbedded layers of peat and silt suggests that such stratigraphy resulted when marshes and forests were similarly inundated and buried by intertidal and tidal sediment as a result of great, prehistoric earthquakes. This study tests the feasibility of using buried, radiocarbon-dated, late Holocene peat layers that are exposed in the intertidal zone of upper Cook Inlet to determine earthquake recurrence intervals, because estimates of the recurrence intervals of past earthquakes are needed for evaluation of the potential for future earthquakes. In a reconnaissance study of interbedded peat and silt, 65 conventional radiocarbon dates from peat and other organic material in 25 measured sections in the intertidal zone and one drillhole were used. Radiocarbon ages from the tops of peat beds cluster weakly but may indicate that regional subsidence events recurred at irregular intervals between about 200 to 800 radiocarbon yr within the past 3,200 radiocarbon yr. Conversion to calibrated ages does not alter this range substantially but may extend both ends of the age range. Coeval and correlative stratigraphy and radiocarbon data in the buried peat layers of upper Cook Inlet strongly suggest sudden, subsidence-induced layering. Because of problems associated with conventional radiocarbon dating, the complex stratigraphy of the study area, the tectonic setting, and regional changes in sea level, conclusions from the study do not permit precise identification of the timing and recurrence of paleoseismic events.

Alaska↗
USGS2019

Holocene earthquake history and slip rate of the southern Teton fault, Wyoming, USA

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, ∼6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4−4.6 ka) slip rate of ∼1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4−9.9 ka) slip rate exceeds the Holocene (9.9−4.6 ka) rate by a factor of ∼2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The ∼5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of ∼4−5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles.

Wyoming↗
USGS2015

Earthquake geology of the Bulnay Fault (Mongolia)

The Bulnay earthquake of July 23, 1905 (M w 8.3-8.5), in north-central Mongolia, is one of the world's largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the 20th century. The 375-km-long surface rupture of the left-lateral, strike-slip, N095°E trending Bulnay Fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike-slip surface ruptures worldwide. Along the 80 km stretch, between 97.18°E and 98.33°E, the fault zone is characterized by several meters width and the mean left-lateral 1905 offset is 8.9 ± 0.6 m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault, allow us to estimate a mean horizontal slip rate of 3.1 ± 1.7 mm/yr over the Late Pleistocene-Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event with recurrence intervals of ~2700-4000 years.

Bulnay Fault↗
USGS2004

The offshore Palos Verdes fault zone near San Pedro, Southern California

High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wave-cut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment.

California↗
USGS1983

Process and rate of dedolomitization: Mass transfer and C14 dating in a regional carbonate aquifer

Regional dedolomitization is the major process that controls the chemical character of water in the Mississippian Pahasapa Limestone (Madison equivalent) surrounding the Black Hills, South Dakota and Wyoming. The process of dedolomitization consists of dolomite dissolution and concurrent precipitation of calcite; it is driven by dissolution of gypsum. Deuterium and oxygen isotopic data from the ground water, coupled with regional potentiometric maps, show that recharge occurs on the western slope of the Black Hills and that the water flows northward and westward toward the Powder River Basin. A significant part flows around the southern end of the Black Hills to replenish the aquifer to the east of the Hills. Depth of flow was inferred from interpretation of the silica geothermometer based on the temperature-dependent solubilities of quartz and chalcedony in water. Chemical effects of warm water in the Pahasapa Limestone include changes in the solubility products of minerals, conversion of gypsum to anhydrite, solution and precipitation of minerals, and increases in the tendency for outgassing of carbon dioxide. Where sulfate reduction is not important, sulfur isotope data show that (1) in the Mississippian aquifer, most of the sulfate is from dissolution of gypsum and (2) some wells and springs have a hydrologic connection with overlying Permian and Pennsylvanian evaporites. Sulfate ion concentration, a progress variable, shows a strong correlation with pH as a result of the combined effects of the dedolomitization reactions. Mass-balance and mass-transfer calculations were used to adjust 14 C values to determine a range of ground-water flow velocities between 2 and 20 m/yr. These velocities are characteristic of carbonate aquifers. The average rates of dolomite and gypsum dissolution are 1.7 × 10 −4 and 3.4 × 10 −4 mmol/kg of H 2 O/yr, respectively. The precipitation of calcite is occurring at the rate of 3.4 × 10 −4 mmol/kg of H 2 O/yr. The close agreement among the model results demonstrates that dedolomitization is controlling water-rock interactions in this regional carbonate aquifer system.

Geological Society of America Bulletin↗
USGS1973

Potassium-argon ages and paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth' magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth's rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth's center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.

Hawai'i↗
USGS2022

Earthquake early warning for estimating floor shaking levels of tall buildings

This article investigates methods to improve earthquake early warning (EEW) predictions of shaking levels for residents of tall buildings. In the current U.S. Geological Survey ShakeAlert EEW system, regions far from an epicenter will not receive alerts due to low predicted ground‐shaking intensities. However, residents of tall buildings in those areas may still experience significant shaking due to the acceleration amplification caused by tall buildings’ dynamic behavior, as recently experienced by residents of the 52‐story building in downtown Los Angeles (DTLA) during the 2019 M 7.1 Ridgecrest earthquake. Using more than 400 recorded response data acquired from 77 instrumented buildings in California, here we compare the Federal Emergency Management Agency (FEMA) P‐58 and American Society of Civil Engineers (ASCE) 7‐16 simplified equations for peak floor acceleration (PFA), finding that the ASCE estimation is close to the median of data recorded in large and long‐distance events, whereas the current FEMA estimation is not suitable. In the second part of this article, four instrumented tall buildings in DTLA are extensively studied, and the performance of the simplified and response spectrum (RS) methods giving both an estimation of the free‐field horizontal peak ground acceleration (PGA) and pseudospectral acceleration is evaluated. The results show that the RS method is as accurate as the response history analysis as long as the ground‐motion RS is accurate, whereas the ASCE 7‐16 prediction is conservative. However, when ground‐motion RS or PGA is estimated for DTLA using a ground‐motion model (GMM), the performance of the RS method significantly degrades due to underestimation by the GMM at long periods. The results of this study imply that a nonergodic GMM, which may give more accurate prediction in Los Angeles, could improve the results for PFA when the building’s behavior is dominated by a few long‐period fundamental modes, as is the case for the 52‐story building in DTLA.

California↗
USGS2021

New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

British Columbia, Northwest Territories↗
USGS1973

Mantle convection and volcanic periodicity in the pacific; Evidence from Hawaii

The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en échelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.

Hawaii↗
USGS1951

Iron formation and associated rocks in the Iron River district, Michigan

The iron formation of the Iron River district is part of a Precambrian sequence of strata characterized by a high iron content and varied mineralogy. The iron formation, where unoxidized, consists largely of interlaminated chert and siderite. It is underlain by a graphitic slate containing about 20 per cent iron in the form of very finely disseminated pyrite. Graywacke overlies the iron formation with at least local disconformity; this rock consists of clastic grains in a matrix of siderite or iron-rich chlorite. Above the gray-wacke is a magnetic ironstone, a laminated rock that now consists of iron-rich chlorite, magnetite, chert, and siderite, each layer being an intermixture of two or more of these constituents. The average iron content of the entire sequence, including the iron formation, is approximately 20 per cent. The rocks are only slightly metamorphosed, although the area is one of intense structural deformation. The origin of the rocks is discussed, and it is concluded that the high iron content is primary. The rocks are believed to be the products of an era of iron-rich sedimentation in which the specific iron mineral formed —sulfide, carbonate, or silicate—depended upon the immediate depositional environment. Inasmuch as the formation of iron-rich minerals continued despite extensive changes in both the basin of deposition and the adjacent land areas, the ultimate cause for such an epoch is one that transcended such factors. Evidence is presented to show that the climate of the era, when linked with certain other factors, is entirely adequate to explain the formation of these iron-rich rocks.

Michigan↗
USGS1981

Search for high-calcium limestone in Silurian reefs of northern Indiana

During Silurian time, the Indiana part of the Wabash Platform was a shallow-water area between the proto-lllinois and pro-to-Michigan Basins and a site of growth of hundreds, or perhaps thousands, of reefs. Today, most reefs of northern Indiana are dolomite, but some are dolomitic limestone, and a few are limestone of high purity in deposits that can be mined by open-pit methods. Four of the five generations of reefs of Silurian age in the Great Lakes area have been recognized in northern Indiana. All known oldest-generation reefs in Indiana (second-generation in the Great Lakes area) are dolomitic. A reef in Carroll County containing commercial amounts of high-calcium limestone is the next oldest (third generation), with roots in basal Louisville limestone, and a limestone reef of similar size in Grant County is of a younger (fourth) generation, with its roots in upper Louisville limestone and dolomite. Numerous slightly younger reefs with mostly mixed dolomitic-calcitic compositions are also of this generation, but all known examples are too small to be commercially important. One small limestone reef has been found in the youngest (fifth) generation, with roots in limestone of the Kokomo Limestone Member of the Salina Formation. All known limestone reefs are restricted to an area of six counties in north-central Indiana, although no apparent depositional environment as revealed from study of surrounding inter-reef rocks has been found to account for any restriction. Dolomitization is more likely related to the textures and lithologies of the individual reefs.

Geological Society of America Bulletin↗