R-3 Repository :: Browsing by Author "Clark, Howard C."

Browsing by Author "Clark, Howard C."

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A model for the formation of the Earth's core
(1976) Walraven, David; Clark, Howard C.
A computer model was developed to investigate the formation of the core by the sinking of iron spheres in a silicate matrix. The model was evaluated for two different initial temperature distributions for three different iron sphere masses. It was found that core formation would be substantially complete within 2 million years since the onset of the process. An early formation of the core, producing subsequent degassing of the mantle, is consistent with presently available geologic data.
A paleomagnetic study of early tertiary basalts in West Texas
(1967) Winters, Martha Diane; Clark, Howard C.
The natural remanent magnetization of thirty-one early Tertiary lava flows from West Texas was measured. Stratigraphic controls and some potassium-argon dates were used to determine the relative age of the flows. Stability of magnetization was established by ac demagnetization. Because of large secondary magnetic components and chemical alteration it was not possible to obtain reliable remanent magnetic directions or pole positions for sixteen flows. Of the fifteen reliable flows, six are normally magnetized, seven are reversely magnetized, and two are intermediate. Seven of the reliable sites spanning a period of time from Eocene to Miocene yield remanent magnetic directions which are not significantly different. From these seven virtual geomagnetic pole positions an early Tertiary pole position for North America of 14°E and 75°N is deduced. The published European and North American pole positions have been examined on the basis of rigorous reliability criteria. No North American results were considered reliable, and a mean pole position of 145°E and 74°N was obtained from the reliable European early Tertiary results. The data is consistent with the hypotheses that the early Tertiary magnetic field was not aligned with the present spin axis, that North America has moved west relative to Europe a distance of 4° since early Tertiary time, and that the rate of sea floor spreading at present may be low relative to what it has been.
Atmospheric and deep seated sources of unsupported lead-210 in soil profiles and the use of lead-210 and polonium-210 as indicators of uranium mineralization at depth
(1983) Ageli, Hadi S.; Adams, John A. S.; Clark, Howard C.; Class, C. M.
The increase of supported lead-21/polonium-21 with depth in soil profiles over buried uranium ore has been found to be a very cost effective method of uranium prospecting. Lead-21/polonium-21 soil profiles of the uppermost meter are superior to radon-222 track etch cups because the flux of radon-222 is highly variable from time to time depending on many factors, such as changes in wind velocity, pressure, and temperature. On the other hand, lead-21 has a half-life of 22.3 years and it is a time-integrated measurement of radon-222; the method integrates over six half-lives of lead-21 (about 133 years). At Felder Mine, Texas, the lead-21/polonium-21 soil profile increases with depth across a fault zone since the faulting facilitates the upward migration of radon-222. Reversed profiles have been observed where radon-222 accumulates under a semi-impermeable caliche layer. Also at Felder, polonium-21 from the radon-222 mobilized and released to the atmosphere during strip mining operations was detected where the soil had been removed.
Glacial marine sedimentation in the Weddell Sector of the East Antarctic Rise
(1981) Harlan, Janis G.; Anderson, John B.; Casey, Richard E.; Clark, Howard C.
Seventeen cores were taken by John Anderson and Dennis Kurtz along two shelf-to-rise transects in the northeastern Weddell Sea during January-February 1978 as part of Islas Orcadas cruise 1578. These cores were examined by Wright (198) for mass flow characteristics and by Harlan for paleooceanography. The initial study showed that both shelf cores were comprised of basal till, twelve out of the fifteen slope-to-rise cores were mass flow deposits, and only three represented normal hemipelagic sedimentation. In this study, the three deepest rise cores were extensively analyzed to obtain sediment types and foraminifera for paleooceanography. All three cores were found to be composed predominantly of silt and sandy silt. Sediment types in the eastern Weddell Sea were analyzed as to size frequency distribution. These types were utilized to determine textural variation in each core. Each sediment type was further examined to determine paleocurrent velocities in this area. Bottom water activity was found to be persistently low in the eastern Weddell Sea. Morphological glauconite occurs throughout the cores and indicates slow sedimentation. Only two foraminlferal assemblages were found in these cores, a Uviqerina assemblage, believed to represent somewhat warmer water than present, and a Globocassidulina assemblage. Both faunas are restricted to Brunhes deposits. The overall depositional environment in this area appears to be characterized by low current activity and normal continental rise sedimentation, as evidenced by fine-grained, fairly sorted material Interspersed with Infrequent sediment slumping from the upper slope and intermittent ice-rafting of sediment. This contrasts sharply with the paleocirculation record of the western Weddell Sea, which is indicative of periodic bottom water activity.
Gravity study of the Big Bend region, Brewster County, Texas
(1980) Metcalfe, Cynthia Watson; Clark, Howard C.; Anderson, John B.; Nettleton, L. L.
The main object of this study was to model gravity data in the Big Bend area in order to determine the underlying crustal structure and its possible relation to the Rio Grande rift. Existing data, along with some stations measured for this study, were contoured to produce free air and Bouguer anomaly maps. These maps were compared to those that exist for the region of the rift. In addition, a profile was taken perpendicular to structural trends in the Big Bend area for comparison to profiles modeled across the rift. The same process was used to model this profile as in the studies of the rift to which the profile was compared. A positive Bouguer gravity ridge was found in the region that is similar to that found along the Rio Grande rift by other workers. Since gravity models are not unique solutions, it was necessary to compare other features of the two areas to determine if they were genetically linked. An examination of the structure, geologic history, mineralization, and geophysical characteristics of the two regions revealed certain similarities but many differences. Although the early history of the Big Bend area is similar to that of the Rio Grande rift, the Quaternary history appears to be unrelated.
K♯°/AR♯° age dating on glauconites and paleontologic interpretation near the Cretaceous-Tertiary transition in Texas.
(1984) Gammill, Laura M.; Adams, John A. S.; Casey, Richard E.; Clark, Howard C.
Several exposures of the Cretaceous-Tertiary contact in Texas were searched for material suitable for radiometric age dating. Glauconites from Littig Pit, Travis County, Texas were sufficient in quality and quantity. K4/Ar4 dating on glauconites from Littig Pit indicates that 4 to 6 Ma of lowermost Tertiary section are missing. At least one nannofossil zone and at least two foraminiferal zones missing from the lowermost Tertiary limit the unconformity to 2.5 to 5 Ma. The concordant radiometric and paleontologic dates refute the view of Odin that all low potassium glauconites are suspect. The uppermost Cretaceous represents an undetermined amount of time. It is possible that the Midway Group was deposited rapidly during the last major Paleocene transgression of Vail et al (1977). The data supports the conclusion of Berggren and Aubert (1975) that the Midway fauna are correlatable worldwide. The iridium layer of Alvarez et al (1982) is missing at the Littig Pit, confirming missing section and illustrating how this iridium layer may be used as a worldwide time marker.
Marine geology of the western Ross Sea: implications for Antarctic glacial history
(1982) Meyers, Nathan Cebren; Anderson, John B.; Valley, John W.; Clark, Howard C.
The stability of the Antarctic Ice Sheet has long been the subject of debate. Recently, basal till has been identified in the Ross Sea, yielding evidence for the expansion of the Antarctic Ice Sheet and its grounding in the Ross Sea. However, the sedimentologic criteria used to identify basal till may be inconclusive. Therefore, one purpose of this study is to support the validity of these sedimentologic criteria. Another goal is to determine if basal tills are present in the western Ross Sea. Marine sedimentologic processes on high latitude continental shelves are not well known. Therefore, surface sediment distributions can be used to infer marine processes active in the western Ross Sea. Three major sediment types, siliceous ooze and mud, sands, and diamictons were identified. Sands include graded volcanic sands, granitic sands, and calcareous shell hash sands. Diamictons include compound glacial marine sediment, basal till, and mass flow diamictons. Siliceous sediment distribution is controlled both by bottom currents and surface currents. Sands are found in near-coastal or shallow waters, or in areas of high sediment supply. The distribution of surface sediments in the western Ross Sea is controlled by thermohaline bottom currents, by surface currents, by the presence of polynyas, and by sediment gravity flow processes. Basal tills were identified as far north as Coulman Island, and as far south as Ross Island, but no basal tills were found in McMurdo Sound. A petrologic analysis of these basal tills reveals that three petrologic provinces can be recognized in the western Ross Sea and four petrologic provinces can be recognized in the central Ross Sea. The boundaries between petrologic provinces can be correlated with geologic provinces in Victoria Land and Marie Byrd Land. Paleo-ice flow paths are reconstructed based on petrologic province boundaries and bathymetry, and indicate that during the last glacial maximum ice flowed into the Ross Sea from both East and West Antarctica. The petrologic data also indicate that sedimentologic criteria used to identify basal tills are indeed valid. The marine sedimentologic record in the western Ross Sea indicates that the ice sheet which grounded there initially eroded the sea floor, and later deposited basal till except in Mcmurdo Sound where flow was restricted. Sharp contacts between basal till and compound glacial marine sediment indicate that decoupling of the Ice Sheet from its base was rapid; gradational contacts between compound glacial marine sediment and siliceous sediments indicate that ice-shelf retreat was slow allowing siliceous sediments to slowly dominate sedimentation.
Numerical models of subduction dip angle with variable viscosity
(1981) Wong, Peter Kin; DeBremaecker, Jean-Claude; Lallemant, Hans G. Avé; Clark, Howard C.
Many models have been used to examine subduction dip angle but none of which is both, dynamic and time dependent. Numerical models are developed to follow the thermal evolution of the subduction zone. The energy equation includes advection, the adiabatic gradient, viscous dissipation, radiogenic sources and variable diffusivity. The pressure is eliminated by combining the two momentum equations into a fourth, order stream-function equation. Both the energy and stream-function equations are solved numerically on a special non-uniform offset grid using the Altemating-Directon Implicit method. In a convection cell with Herring-Nabarro viscosity, circulation will be restricted to the left side of the cell while the descending limh is on the right. Altering the aspect ratio by changing the depth, has a much more pronounced effect on the total circulation and velocities than by changing the width. A convection cell will reorganize its circulation pattern to achieve steady-state when a highly viscous block, is "inserted” into the cell to disrupt normal circulation. Finally, the convective pattern in a system in which the boundary conditions are perpetually changing is determined by both its present boundary conditions and also by those preceding it by several millions of years.
Pleistocene seismic stratigraphy of the Galveston South addition, offshore Texas
(1984) Lewis, Dana Lynn; Clark, Howard C.; Bally, Albert W.; Anderson, John B.
Seven seismic sequences were identified on the outer shelf and upper slope of the northwestern Gulf of Mexico from a grid of high resolution seismic sections. The sequences were deposited during eustatic fluctuations of sea level during Wisconsin glacial and interglacial stages. The sequences include deposits of an early Wisconsin low stand, middle Wisconsin high stand, and late Wisconsin low stand of sea level. Growth faults trending parallel to the shelf break expanded the secton from 6% to 6%. The faults resulted from salt flowage at depth in response to loading by thick deltaic deposits, which prograded to the shelf edge during low stands of sea level. Smaller, local fault systems are associated with diapiric intrusions. Diapirs exerted little control over Wisconsin sedimentation in the study area. Diapirs on the inner shelf are stock-like in appearance while those on the outer shelf and upper slope are broad swells and ridges.
Preliminary determination of the Radon-222 baseline in Houston, Tex.
(1979) Moed, Barbara A.; Adams, John A. S.; Gesell, Thomas F.; Clark, Howard C.
Radon-222 is a naturally occurring radionuclide that is present in rocks, sediments, and soils in detectable quantities. The migration of radon-222 is promoted by nonreactive chemical properties and by a. 3.82 day half life. A large fraction of the natural radioactivity in surface air is comprised of radon-222 and its short-lived progeny. The activity of radon-222 in soil gas and in surface air exhibits great variability spatially and temporally. This study investigated the temporal variation of radon-222 activities in soil gas and in the surface layer of the atmosphere. The alpha activity in soil gas was monitored at one site for a period of six months. Integrated countrates were recorded daily. Atmospheric radon-222 activities were monitored continuously at an adjacent site during four months of the same period. This difference in the span of monitoring periods was caused by equipment downtime in the air monitoring system. Meteorological data were concurrently tabulated. Univariate graphs were constructed to show the variation of daily mean atmospheric radon-222 activities and of soil gas activities versus mean temperature, mean barometric pressure, average wind speed, and resultant wind direction. The effects of rainfall on these activities were also assessed. The mean of daily soil gas activities is 4.2 counts/ minute; the range of these observations is 1. - 11. counts/ minute. The mean of the atmospheric radon activities is .39 pCi/1; the range of daily mean values is .22 - .79 pC/1. Atmospheric radon activities exhibited distinct diurnal variations. Composite soil samples from the vicinity of the monitoring site were collected and analyzed for uranium-238 and thorium-232 concentrations. The radon-222 and radon 22 emanating powers of these samples were also determined.
Radiolaria in the Holocene sediment of the Gulf of Mexico and the basins off southern California: assemblage changes with water depth and eutrophism
(1982) Wigley, Cynthia R.; Casey, Richard E.; Anderson, John B.; Clark, Howard C.
Radiolarian assemblages in Holocene sediments from the Gulf of Mexico and Caribbean and the southern California borderland reflect overlying oceanographic conditions, such as general productivity, upwelling, and preservational parameters. A reconnaissance investigation of the lateral variations in the radiolarian assemblage with increasing water depth indicates that the changes are not consistent enough in either region for the establishment of a definite depth zonation; however a number of radiolarian species and higher taxa show some general trends with depth. Symbiotic taxa, and spumellaria in general, are more abundant in the thanatocoenosis of the Gulf region than off California and are indicative of oligotrophic conditions. Low oxygen content and high dissolved silica concentration in the bottom water of the Gulf of Mexico's Orca Basin lead to good preservation in this locality. Diversity, up to the family level, is greater in the California assemblages due to a mixing of radiolarian faunas and better preservation. There are more deep-water radiolarian taxa present at shallower depths, and all depths, in the California assemblages as compared to those of the Gulf due to more dynamic upwelling conditions which enhance these species both in the biocoenosis and thanatocoenosis.
Radiolarian densities, diversities, and taxonomic composition in recent sediment and plankton of the southern California continental borderland: relationship to water circulation and depositional environments
(1985) Cleveland, Michael N.; Casey, Richard E.; Anderson, John B.; Clark, Howard C.
The California Current, the eastern limb of the North Pacific gyre, exhibits the following characteristics‘common to eastern boundary currents: wide, shallow, slow, diffuse boundaries, common upwelling, great seasonal variation, invasions of water masses from outside the system, and cold, low salinity waters. Studies on plankton tows and Holocene sediments have correlated components of the siliceous microplankton (radiolarians and some diatoms) with a number of those characteristics such as: the main directions of movement of the invading waters, the provenance of these waters, the presence and degree of upwelling, seasonality and its impact on the underlying sediments. Certain types of radiolarians have been found to be potentially useful in determining fossil anoxic and oxic conditions as well as paleodepth. This study involved analysis of box core sediment and plankton tow samples from the southern California continental borderland for radiolarian density, diversity, taxonomic makeup, and other features which were related to oceanographic and environmental conditions. Depositional environments were defined for the sediment samples and radiolarian indicators useful for paleoenvironmental interpretation were defined. A number of borderland environments were identified and the anoxic nearshore basin was found to have the best preservational qualities for radiolarians and thus the most representative radiolarian biocoenosis.
Radiolarian responses to the 1982-83 California El Nino and their implications
(1985) Carson, Thomas; Casey, Richard E.; Clark, Howard C.; Dunbar, Robert B.
Radiolarian responses to the 1982-83 California El Nino event were unusually high standing crops of both warm and cold water radiolarians, high diversity, and the appearance of distinctly central gyre and eastern tropical Pacific faunas. These and other observations suggest that anomalously strong transport from the west and south occurred through at least November 1953 and that the California Current had shifted eastward over the Southern California Eorderland. A generic group of radiolarians, the spongasterids, was used effectively in monitoring these water mass influences. Radiolarian distributions in 3.-3.5 Ma circuoi-northeastern Pacific sediments indicate a warm North Facific at this time. Marked breaks in distributional trends off southern Baja California suggest an oceanic frontal zone in this area, To the south radiolarian assemblages exhibit a distinctly eastern tropical Pacific character. Spongaster pentas occurs as far north as Deep Sea Prilling Site 173. This suggests periodic breakdowns of this frontal zone (El Ninos?) may have occurred.
Sediment gravity transport on the Weddell Sea continental margin
(1980) Wright, Robyn; Anderson, John B.; Casey, Richard E.; Clark, Howard C.
Understanding sediment gravity transport and continental slope processes has become increasingly important, especially with the current emphasis upon oil and gas exploration in the deep marine environment. Core x-radiography and textural and mineralogic analysis of sediment samples from Weddell Sea piston cores has enabled identification of several mechanisms of sediment mass transport which significantly influence sedimentation on the continental slope, abyssal plain, and continental shelf. End member processes such as debris flows and turbidity currents are found to occur throughout the eastern Weddell Sea, between °W and 45°W; however textural and visual evidence also substantiates the presence of sediment gravity flow processes which are transitional between end-member mechanisms. Downslope transition in sediment character between slump, debris flow, and turbidite deposits is related to a corresponding transition in the mechanics of grain support and transport. Because most material transported to the marine environment in the Weddell Sea is of poorly sorted glacial origin, analysis of the competence of the marine transport agents is augmented. Textural and mineralogic sorting within the turbidity current mechanism, for example, is found to be extremely efficient. Generation of well sorted quartz-rich sands from poorly sorted lithic source material occurs over relatively short distances, and proves to be a mechanism by which potentially large amounts of quartz sands are produced in the Weddell Sea. The intimate association of these sands with glacial sediments offers an alternative explanation for the quartzite/diamictite associations in ancient sequences which have been cited as evidence against a glacial origin for these deposits.
Sedimentation in the north San Clemente basin, California continental borderland
(1979) Pratt, David E.; Warme, John E.; Casey, Richard E.; Clark, Howard C.
Three broad sedimentary environments within the north San Clemente Basin have been defined. The south slope of San Clemente Island and the northernmost part of the San Clemente Basin receive predominantly siliciclastic sediments shed from San Clemente Island. Clastic sediments deposited in San Clemente Canyon are sorted and transported by the action of tidally generated currents. Currents measured in the canyon show upcanyon and downcanyon reversals of flow nearly every 12 hours, and currents reached velocities of 18 cm/second during periods of peak flow. The currents have winnowed silt and clay from sediments at the head of the canyon, leaving a sandy lag deposit. Pine-grained sediment is transported into the basin and slowly deppsited as the currents lose velocity. Three piston cores from the northern part of the basin penetrated the Holocene-Plelstocene boundary. Slightly more than a meter of Holocene sediment was measured in each core, as determined by mlcropaleontologic methods. The central part of the north San Clemente Basin is dominated by pelagic sedimentation. Basin physiography effectively blocks transport of clastic sediments from San Clemente Island and mainland California to the area. Hemipelagic sediments and distal turbidites are transported through the Navy Fan of the south San Clemente Basin and into the southern part of the study area. The observed distal turbidites may be a result of Early Holocene sea-level regressions. Sediments within the north San Clemente Basin are in a reducing environment, as indicated by their color and the presence of authigenic sulfides. Micropaleontologic data from this study indicate that kummerform Globlgerina pachyderma tests are a result of displacement into unfavorable environments.
Seismic stratigraphy of the eastern continental shelf of the Wedell Sea, Antarctica
(1983) Roqueplo, Christine M.; Clark, Howard C.; Anderson, John B.; Wegner, R.
Interpretation of a 1 miles of seismic data, covering the eastern part of the Weddell Sea continental shelf, in Antarctica, reveals a passive margin environment adjacent to the East Antarctic craton. Two major seismic stratigraphic sequences bounded by regional unconformities onlap a highly eroded and possibly faulted acoustic basement. The basement terranes which exhibit high magnetic susceptibilities were tentatively correlated with Mesozoic igneous rocks identified in the Transantarctic Mountains. The older sequence is formed of nearshore deposits which do not contradict the presence of ice on the mainland. The younger sequence, however, exhibits the seismic response expected from glacially derived sediments. The angular unconformity separating both units was probably related to a major (late Oligocene?) drop of sea level, while the erosional event which followed the deposition of the glacial series corresponds to a peak in glacial activity (Pliocene?). Structural framework of the area can only be approached through broad scale studies involving the complex relationships between the East and West Antarctic subcontinents.

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