Geological Society of America Bulletin

Published by Geological Society of America
Online ISSN: 1943-2674
Publications
Article
The upper part of the New Albany Shale is divided into three members. In ascending order, these are 1) the Morgan Trail Member, a laminated brownish-black shale; 2) the Camp Run Member, an interbedded brownish-black and greenish-gray shale; and 3) the Clegg Creek Member, also a laminated brownish-black shale. The Morgan Trail and Camp Run Members contain 5% to 6% total organic carbon (TOC) and 2% sulfide sulfur. Isotopic composition of sulfide in these members ranges from -5.0‰ to -20.0‰. C/S plots indicate linear relationships between abundances of these elements, with a zero intercept characteristics of sediments deposited in a non-euxinic marine environment. Formation of diagenetic pyrite was carbon limited in these members. The Clegg Creek Member contains 10% to 15% TOC and 2% to 6% sulfide sulfur. Isotopic compositions of sulfide range from -5.0‰ to -40.0‰. The most negative values occur in the upper Clegg Creek Member and are characteristic of syngenetic pyrite, formed within an anoxic water column. During euxinic deposition, phosphate and trace metals accumulated below the chemocline because of limited vertical circulation in the water column. Increased productivity would have resulted in an increased flux of particulate organic matter to the sediment, providing an effective sink for trace metals in the water column. Phosphate and trace metals released from organic matter during early diagenesis resulted in precipitation of metal-rich phosphate nodules. -from Authors
 
Article
The thick, richly fossiliferous succession of the upper Windermere Supergroup, Mackenzie Mountains, northwestern Canada, provides a test of integrated biostratigraphic and chemostratigraphic frameworks in terminal Proterozoic correlation. The C- and Sr-isotopic abundances of lower Keele Formation carbonates approximate those for other pre-Varanger samples, confirming that the simple disc-like fossils of the underlying Twitya Formation predate all known diverse Ediacaran faunas. "Tepee" and Sheepbed carbonates record strong post-glacial isotopic excursions; in contrast, delta13C values for Gametrail through Risky carbonates vary only within the narrow range of about +l% to +2%. A second negative excursion occurs in Ingta Formation carbonates that immediately underlie the paleontologically determined Precambrian-Cambrian boundary. The upper Windermere profile as a whole compares closely with curves determined for other terminal Proterozoic successions. The lowermost diverse Ediacaran assemblages in the Sheepbed Formation correlate chemostratigraphically with the oldest fauna in Namibia, but the two assemblages differ in taxonomic composition. Blueflower assemblages correlate both chemostratigraphically and taxonomically with faunas from Australia, China, Siberia, and elsewhere. Increasing data support the hypothesis that paleontological and geochemical data together provide a reliable means of correlating terminal Proterozoic sedimentary rocks throughout the world.
 
Article
The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.
 
Article
On the basis of composition, it is possible to distinguish three major groups of Apollo 12 basaltic rocks: olivine-pigeonite basalts and gabbros, ilmenite-bearing basalts and gabbros, and feldspathic basalts. Two major groups of Apollo 11 basalts are also distinguishable: ophitic ilmenite basalts and intersertal ilmenite basalts. Compositional variations between samples within groups are generally dominated by MgO variations, whereas differences between groups are primarily inverse variations of TiO2 and SiO2 or Al2O3 and FeO. Results of fractionation calculations indicate that the MgO variation trends are explained principally by low-pressure fractionation of early-crystallized olivine ± pigeonite ± chrome spinel. The Al2O3 versus FeO trend in the basalts might possibly be explained by near-surface fractionation, but the TiO2 versus SiO2 trend is not explainable in this way. Investigations of the latter trend in terms of possible processes of high-pressure fractional melting or fractional crystallization indicate that the compositional variations cannot be the products of simple variations in depth or degree of fractionation. Our data are consistent with the view that the mafic magmas formed by partial melting in the lunar interior, and that near-surface fractionation, with the exception of removal or addition of olivine, has not been extensive.
 
Article
Lunar surface erosion due to Apollo 11 descent engine
 
Article
Experimental crystallization of a lunar picrite composition (sample 12002) at controlled linear cooling rates produces systematic changes in the temperature at which crystalline phases appear, in the texture, and in crystal morphology as a function of cooling rate. Phases crystallize in the order olivine, chromium spinel, pyroxene, plagioclase, and ilmenite during equilibrium crystallization, but ilmenite and plagioclase reverse their order of appearance and silica crystallizes in the groundmass during controlled cooling experiments. The partition of iron and magnesium between olivine and liquid is independent of cooling rate, temperature, and pressure. Comparison of the olivine nucleation densities in the lunar sample and in the experiments indicates that the sample began cooling at about 1 deg C/hr. Pyroxene size, chemistry, and growth instability spacings, as well as groundmass coarseness, all suggest that the cooling rate subsequently decreased by as much as a factor of 10 or more. The porphyritic texture of this sample, then, is produced at a decreasing, rather than a discontinuously increasing, cooling rate.
 
Article
Pyroxene phenocrysts in Apollo 12 sample 12052, a porphyritic basalt, have a weak, planar preferred orientation and a lineation as determined by crystal-elongation measurements and universal-stage measurements in orthogonal thin sections. The structures are probably the result of laminar flow. Numerous vugs and the flow foliation in sample 12052 suggest crystallization as a surface flow or a near-surface sill. Flowage appears to have ceased by the time of crystallization of the variolitic groundmass of the sample. The fabric data suggest a two-stage crystallization of sample 12052.
 
Article
The geological setting, petrography and history of this Apollo 15 lunar rock sample are discussed, characterizing the sample as coarse-grained anorthosite composed largely of calcic plagioclase with small amounts of three pyroxene phases. The presence of shattered and granulated minerals in the texture of the rock is traced to two or more fragmentation events, and the presence of irregular bands of coarsely recrystallized plagioclase and minor pyroxene crossing larger plagioclase grains is traced to an earlier thermal metamorphic event. It is pointed out that any of these events may have affected apparent radiometric ages of elements in this rock. A comparative summarization of data suggests that this rock is the least-deformed member of a suite of similar rocks ejected from beneath the regolith at Spur crater.
 
Article
Major and trace element abundances in volcanic rocks of island arcs vary regularly in both the lateral and in the stratigraphic sequence. The earliest manifestations of island arc evolution are tholeiitic rocks occurring on the oceanic side of recent island arcs. These tholeiites are characterized by chemical features such as low K2O content and relatively high iron enrichment, that is, high FeO/MgO in medium SiO2 (59 percent) rocks. The tholeiites are followed stratigraphically or laterally by calc-alkaline rocks and finally by shoshonites or alkaline rocks, showing progressive increase in K2O content, increase in K2O/Na2O ratio, and decreasing iron enrichment. Trace elements of the potassium type are the most sensitive indicators of compositional variation showing differences between tholeiites and shoshonites by two orders of magnitude. Trace elements and major elements suggest a continuous gradational sequence from tholeiites through calc-alkaline rocks to shoshonites. Rare-earth elements show two diverse abundance patterns: (1) a primitive chondritic-like pattern is characteristic for island arc tholeiitic association; and (2) a fractionated light rare-earth elements enriched pattern is typical for calc-alkaline and shoshonitic and alkaline associations.
 
Article
Extremely high atmospheric wind velocities are needed to erode particulate matter on Mars. Settling velocities are roughly equivalent to terrestrial settling velocities for clay to fine sand-size particles; suspension transport may be dominant for fine particles on Mars. Yellow clouds suggest that required threshold erosion velocities are reached and that a great deal of fine-grained material is carried in suspension. Yellow cloud origins are concentrated over the southern latitudes and areas of major topographic relief. The cloud distribution pattern suggests that high threshold velocities are attained by transient atmospheric disturbances such as slope winds and dust devils.-
 
Article
A comparison of side-looking airborne radar (SLAR) images and black and white aerial photos of similar scale and illumination of an area in the Mojave Desert of California shows that aerial photos yield far more information about geology than do SLAR images because of greater resolution, tonal range, and geometric fidelity, and easier use in stereo. Nevertheless, radar can differentiate some materials or surfaces that aerial photos cannot; thus, they should be considered as complementary, rather than competing tools in geologic investigations. The most significant advantage of SLAR, however, is its freedom from the stringent conditions of weather, date, and time that are required by small-scale aerial photos taken with a specified direction and angle of illumination. Indeed, in low latitudes, SLAR is the only way to obtain small-scale images with low illumination from certain directions; moreover, in areas of nearly continuous cloudiness, radar may be the only practical source of small-scale images.
 
Article
Progressive changes in volcanic chemistry along a traverse across the Arabo-Ethiopian swell suggest a shallowing of the zone of melting with concomitant crustal thinning from plateau to rift. The gradational nature of this thinning seems to exclude a predrift fitting of the Red Sea coastal escarpments and accords with geological evidence for extension of continental crust beyond the escarpments into the rift zone. Na/K and, to a lesser extent, Ti are the most sensitive discriminants in relating volcanic chemistry to tectonic site. A strong coupling of crust and upper mantle, despite horizontal plate movement, is inferred.
 
Article
Each near-side lunar mare displays a range of geomorphic indices on its surface. This is interpreted to be due to the presence of effusions of different ages on the surface of each mare. This interpretation contradicts the theory that mare surfaces were formed contemporaneously. The landing sites of Apollo 11 and 12 have geomorphic indices of 10.3 and 8.4, respectively; the radiometric ages of the rocks are approximately 3.6 × 109 and 3.4 × 109 yrs. Geomorphic indices on mare surfaces range from less than 5 to more than 14, indicating that the two landing sites were formed approximately in the middle of the time span required for mare surface formation. Using four possible relationships between geomorphic index and age, the length of time between the beginning and end of mare effusion is inferred to be at least 1 × 109 yrs, and possibly twice as much. Some evidence is presented that effusive activity may have reached two maxima.
 
Article
Mathematical proof establishes identity of hypsometric integral and elevation-relief ratio, two quantitative topographic descriptors developed independently of one another for entirely different purposes. Operationally, values of both measures are in excellent agreement for arbitrarily bounded topographic samples, as well as for low-order fluvial watersheds. By using a point-sampling technique rather than planimetry, elevation-relief ratio (defined as mean elevation minus minimum elevation divided by relief) is calculated manually in about a third of the time required for the hypsometric integral.
 
Article
The Amaro horst is a 90-km by 25-km block of sialic crust uplifted 1.5 km above the axis of the main Ethiopian rift, near its southern termination. The Amaro horst resembles the famous Ruwenzori horst of the Western rift, but additionally the former has had a Neogene-Quaternary volcanic history that reveals episodic uplift of the horst from the axis of a bilaterally downwarping-downtilting rift floor. Vertical forces that produced normal faults uplifted the horst to stratigraphic elevations higher than the plateaus outside the rift.
 
Article
North American feldspathoidal and silica unsaturated rocks progression in space and time
 
Article
Lake covered ancient meteorite impact crater in Canada, noting eroded rim and microscopic shock criteria
 
Article
Large numbers of calcic anorthosite inclusions varying in size from centimetres to several hundred metres occur in the Beaver River gabbro unit of the Beaver Bay Complex exposed along the North Shore of Lake Superior. Lesser numbers of similar inclusions occur in the Duluth Complex. Both host rocks crystallized at about 1.13 b.y. B.P. and are of Keweenawan age. The inclusions have been divided into four groups, based on outcrop and petrographic data, as follows: (1) Tectonite inclusions that were sheared and recrystallized. The tectonites consist of plagioclase (An76-78), minor to accessory olivine (Fo78), and accessory magnetite-ilmenite. (2) Igneous inclusions that have igneous structures and textures. Plagioclase ranges in composition from An54 to An80. Principal mafics are augite (Wo40En45Fs15) and orthopyroxene (Wo2En80Fs18), but more iron-rich pyroxenes (Wo38En40Fs22), including pigeonite (Wo10En55Fs35), occur as much smaller grains in most igneous inclusions and are comparable to pyroxenes in host rocks. Textural evidence and mineral chemistry suggest that the Fe-rich pyroxenes, as well as the more albitic plagioclase, represent contamination. (3) Intermediate inclusions whose textures are intermediate between the tectonites and igneous rocks. Some of these appear to have been incompletely recrystallized, and some were modified to more igneous-appearing rocks by reaction with host rocks. (4) Cataclastic and brecciated inclusions having brittle deformation that is not apparent in the host rocks. Representatives of each group were analyzed for rare-earth elements (REE) and Sm-Nd, Rb-Sr isotopes. The chemical data correlate with the textural groups. For example, tectonites have low REE abundances with Nd about one times chondrites, whereas those in the igneous-textured group have higher REE abundances with Nd up to as much as seven times chondrites. Isotopic data show that the isotopic systems of the inclusions were disturbed and that contamination was introduced. Tectonites have 87Sr/86Sr and 143Nd/144Nd ratios that are less than those of the Beaver River gabbro when recalculated to 1.13 b.y. Igneous-textured inclusions have 143Nd/144Nd ratios that are lower than tectonite values at 1.13 b.y., but 87Sr/86Sr ratios vary widely, with some being greater than ratios in the host Beaver River gabbro. None of the inclusions has an isotope ratio that would allow a co-magmatic relationship with either the Duluth Complex or the Beaver River gabbro. Sm-Nd data from an incompletely equilibrated inclusion suggest recrystallization between 1.14 b.y. B.P. and 1.9 b.y. B.P., perhaps as a result of the 1.8 b.y. B.P. Penokean event. Calculations of a crystallization age for the inclusions are speculative but suggest crystallization at or before 1.9 b.y. B.P.
 
Article
Ventifacts occurring on extensive wind-deflated surfaces throughout the ice-free Wright Valley are the product of complex evolutionary processes. The wind produces a lag gravel which continues to evolve at a reduced rate as coarser granule and gravel fractions are removed. The morphology of ventifacts forming the lag gravels suggests that the distribution of wind-polished faces is determined largely by the shape of the original unpolished rock fragments. In the early stages ventifacts tend to be oriented either transverse or parallel to the wind direction. Salt weathering is also a major factor in determining the morphology of the Wright Valley ventifacts.
 
Article
Approximately 30 reversing barchan dunes occur along the northern side of lower Victoria Valley, Antarctica. Laminae which dip steeply upwind suggest that much of the internal structure of the dunes develops during periods when the wind reverses itself-possibly during winter. This wind reversal and the fact that the dune cores are frozen suggest that the migration of the dunes is small. Grain-size analyses of the dune sands indicate that the dunes are similar in texture to some barchans of more temperate climates.
 
Article
A petrographic study of eleven samples of clastic rock returned from the moon by Apollo 15 suggests that two lithologies are present. The distinction between the two lithologies is based on the glass content of the rock matrices and the morphology of the detrital particles. Group I rocks have abundant, glass-rich, porous matrices and glass particles with morphologies comparable to those of glass particles in the lunar soil. The group I rocks were probably formed by welding or sintering of surficial soil deposits by impact-generated base surges of limited extent. Group II rocks have an essentially mineralic matrix and have an abundance of rounded mineral grains. Sample 15455 is the only Apollo 15 sample assigned to this group. In its general textural features, sample 15455 is comparable with the group II rocks from the Fra Mauro Formation at the Apollo 14 site. Textural features such as shock modification and rounding of mineral grains suggest that this sample is the product of a large-scale impact-generated base surge which possibly resulted from the Imbrian event.
 
Article
The origin of glazed rock fragments and glassy splatter in the bottom of fresh, 0.5- to 3.0-m-diameter lunar craters has been the subject of much debate. Evidence presented from Apollo 15 data indicates that one such crater, with an associated glass-coated projectile, is of low velocity, 'secondary' origin. It is shown that, for the crater examined, the glassy material cannot have been formed in situ by impact fusion due to the low energy involved. It is suggested that the glass in many such craters may be carried as a semi-liquid or solidified coating on incoming rock projectiles ejected from local primary impact craters of larger size.
 
Conference Paper
The Equatorial Layered Deposits in the Firsoff crater area have been interpreted as the product of different processes and depositional environments: fluid escape and evaporite precipitation and aeolian reworking, transport, and deposition.
 
Article
New U-Pb isotopic data for detrital zircon populations from the Upper Devonian(?) and lower Carboniferous Bragdon Formation add constraints to the nature of source areas for epiclastic rocks in the ensimatic eastern Klamath arc terrane. Bragdon clastic units include lithic-rich sandstone containing quartz, chert, and sedimentary-lithic clasts, volcanic-lithic-rich sandstone, and crystal-rich tuffaceous sand- stone. These three compositions reflect a composite source that contained sedimentary and low-grade metasedimentary rocks, volcanic rocks, and penecontemporaneous volcanic debris. U-Pb isotopic data from detrital zircon indicate an ultimate Precambrian continental source for components of the Bragdon Formation and corroborate the previous suggestion that lower Paleozoic rocks of the Shoo Fly Complex, Yreka terrane, or related rocks may have been the immediate (last-cycle) sources. In modern convergent margins, a complex and ongoing relationship between ensimatic arc volcanism and continental sediment sources may be expected from the pattern of oceanic subduction zones that generally dip toward continents and intervening marginal basin systems, providing a context for understanding the presence of continent-derived detritus in an ancient ensimatic arc.
 
Article
Skeletal and dendritic olivine crystals in the Archean volcanic ultramafic spinifex rocks and in harrisitic ultramafic layers of the Rhum pluton are classified as (1) plate, (2) randomly oriented, (3) porphyritic, or (4) branching type. Volcanic and plutonic examples are remarkably similar. Each type records the degree of pre-nucleation supersaturation with olivine that was attained by the parent melt. The rapid induction of supersaturation necessary to form skeletons and dendrites in a plutonic environment is attributed to changing water content or adiabatic expansion of the magma. The olivine crystals in harrisitic and spinifex rocks are not quench crystals; they grew rapidly from olivine-rich melts as the result of extreme supersaturation induced by slow cooling and slight supercooling below a liquidus with shallow slope in temperature-composition space. Skeletal and dendritic olivine crystals grow readily in magma and are poor indices of cooling rate and crystallization environment.
 
Article
Lunar mare ridges and arches in Mare Serenitatis were mapped to understand better their mode of formation. Maps of these features indicate that several pre-mare impacts in the Serenitatis area may be responsible for the localization of the circular ridge systems and that the subsurface, pre-mare topography is more complex than previously recognized. Apollo Lunar Sounder cross sections of ridge systems in southern Serenitatis indicate 50 to 100 m of local relief on these features. Small-scale features of ridges, such as medial lineations and lobate margins, do not conclusively define the origin of the ridges. However, estimates of crustal shortening from Lunar Sounder data and the coincidence of the major ridge system with the Serenitatis mascon suggest that ridges and arches were formed by gravitational readjustments of the mare fill along four probable impact structures and along a north-trending fracture pattern.
 
Article
Individual ash-flow sheets distributed over wide areas in the Mogollon-Datil volcanic province can be delineated and related by flow direction techniques to specific source cauldrons. Two major mid-Tertiary ash flows in the Mogollon Plateau have measurable microscopic directional fabric indicative of primary flow direction imprinted in the ash-flow sheets during late-stage laminar flow. Regional stratigraphic relationships and flow patterns of the ash-flow sheets indicate a late Tertiary origin of the Mogollon Plateau depression. They also show that Basin-Range faulting in southwestern New Mexico was not initiated until after emplacement of the younger ash flow (23 m.y. B.P.). Directional fabric is an inherent property of many calc-alkalic ash-flow sheets and measurement of preferred orientation provides a powerful tool in unravelling the geologic history of complex volcanic terrane.
 
Article
Study of volcanic ash samples collected from a variety of recent eruptions using petrography, chemical analyses, and scanning electron microscopy to characterize each type and to relate ash morphology to magma composition and the type of eruption. The ashes are placed in the broad genetic categories of magmatic and phreatomagmatic. The morphology of ash particles from magmatic eruptions of high viscosity magma is governed primarily by vesicle density and shape. Ash particles from eruptions of low viscosity magmas are mostly droplets. The morphology of ash particles from phreatomagmatic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal glass ash particles.
 
Article
Mass-balance interpretation of a soil chronosequence provides a means of quantifying elemental addition, removal, and transformation that occur in soils from a flight of marine terraces in northern California. Six soil profiles that range in age from several to 240,000 yr are developed in unconsolidated, sandy-marine, and eolian parent material deposited on bedrock marine platforms. Soil evolution is dominated by (1) open-system depletion of Si, Ca, Mg, K, and Na; (2) open-system enrichment of P in surface soil horizons; (3) relative immobility of Fe and Al; and (4) transformation of Fe, Si, and Al in the parent material to secondary clay minerals and sesquioxides. Net mass losses of bases and Si are generally uniform with depth and substantial, in some cases approaching 100 percent; however, the rate of loss of each element differs markedly, causing the ranking of each by relative abundance to shift with time. Loss of Si from the sand fraction by dissolution and particle-size diminution, from about 100 percent to less than 35 percent over 240 ky, mirrors a similar gain in the silt and clay size fractions. The Fe originally present in the sand fraction decreases from greater than 80 percent to less than 10 percent, whereas the amount of Fe present in the clay and crystalline oxyhydroxide fractions increases to 25 percent and 70 percent, respectively.
 
Article
Basaltic rocks, hyaloclastites, and fossil fragments incorporated in volcanic material, dredged from the flanks of Northeast Bank on the Southern California Borderland, show trace element abundances typical of eastern Pacific basin alkali basalts but enriched (880 ppm) Ba. The fossil fragments, incorporated in breccia, hyaloclastite, and agglomerate, include a fauna which lived in less than 50 m of water, as well as forms from the intertidal zone. Bathymetry of the bank and the depth range of the dredged zone indicate that there has been at least 300 or as much as 500 m subsidence since the volcanism which incorporated the fauna in volcanic material. Isostatic adjustments due to crustal load of the bank can account for this subsidence.
 
Article
The Cave Basalt, a high-alumina pahoehoe flow containing numerous lava tubes, originated at the southeast flank of Mount St. Helens, southwestern Washington, and flowed down a stream valley incised in older pyroclastic flow deposits. In situ charcoal samples from two localities within lava tubes yield C14 dates of 1,860 ± 250 years B.P. and 1,925 ± 95 years B.P. Detailed survey of 9,125 m of lava tubes, correlated with surface geologic mapping, yields several geomorphic relations of basalt flows. Most of the lava tubes apparently formed between shear planes in laminar lava flow, although some tube sections show evidence that the tube roof formed by accretion of spattered lava in turbulent flow. Partial collapse of tube interiors reveals: (1) The wall separating the tube interior from the preflow country rock may be thinner than 25 cm. (2) Lava flows can erode the surface over which they flow. (3) Collapse of the tube interior can occur immediately after the tube has been drained of molten lava and before the walls cool completely. (4) Lava tubes may represent the thickest part of the lava flow, occupying topographic lows (stream channels). (5) Tubes can be modified extensively through accretion and erosion by later lava flows. Many surface features of basalt flows are directly related to lava tubes. Pressure within the closed lava-tube system caused by out-gassing and hydrostatic pressure and overflow of lava from ruptured roof sections (or from channel overflow prior to roof formation) result in formation of a topographic high along many sections of the lava-tube axis. If roof rupture does not occur, tumuli may develop in weak areas of the roof, forming positive features (may be solid or hollow) 40 to 50 m in diameter and several meters high. Most of the hollow tumuli of the Cave Basalt collapsed, probably as a result of withdrawal of supporting lava during drainage of the lava tubes. Raised-rim craters found in many parts of the flow are associated with lava tubes and were probably formed by collapse of hollow tumuli.
 
Article
Deep dredging in the Tonga Trench (Southwest Pacific Ocean) at a depth of 9150 to 9400 m yielded fresh to granulated and serpentinized peridotite and dunite. Other rocks recovered there and at three stations deeper than 7000 m include basalts, tuffs, and tuffaceous agglomerates. Chemical analyses of the fresh peridotite, with combined H2O < 0.10 weight percent, indicate that the rock consists of Si, Mg, Fe (6 percent), and Cr + Ni about 0.7 percent. Mineralogically, the peridotite contains forsteritic olivine and enstatite with minor spinels. The ultramafic mass exposed at 9400 m probably is an accumulate exposed by faulting.
 
Article
An evaluation has been made of the relative roles of fractional crystallization and crustal contamination in the genesis of basaltic magmas from the Black Rock Desert region in Utah. As a result, geochemical variations of these basalts have been defined as a function of their ages of eruption.
 
Article
The Pajarito fault zone forms the W border of the Velarde graben, the presently active, central subbasin of the Espanola basin section of the Rio Grande rift in N-central New Mexico. The fault zone is a NNE-trending zone of predominantly down-to-the-E faults that cut Miocene to Pliocene volcanic rocks along the E flank of the Jemez Mountains. The total displacement across the fault zone during its 5Ma history is between 200-600m. Rates of displacement range from 0.02 to 0.136mm/yr. Abrupt facies changes between older volcanics and volcaniclastic sediments of the Jemez Mountains appear to have controlled the local position, trend, and character of the Pajarito fault zone. Evidence presented suggests that the Pajarito fault zone has underdone extension in two directions during the past 1.1Ma, approximately parallel and perpendicular to the local trend of the fault zone. These directions indicate that the Pajarito fault zone has reoriented the regional minimum and intermediate stress directions to perpendicular and parallel, respectively, to the local trend of the fault zone, and that both minimum and intermediate stress directions are tensional. A tectonic history for the Pajarito fault zone area is traced. -from Author
 
Article
The Sudbury Basin in the Canadian Shield has been proposed as a meteorite impact site subsequently deformed by endogenic tectonism. Detailed study of the structural geology strengthens this hypothesis and strongly suggests that the deformation of the basin was coeval with major folding and flattening (2.0 to 1.6 b.y. ago) of the rocks of the eastern Southern province and the northwesternmost Grenville province. The structural geometry indicates that most of these rocks have a similar strain history. Finite-strain analysis of deformed concretions within the Sudbury Basin suggests that originally it was almost circular in outline.
 
Article
Acoustic and seismic outputs of booming sands and singing (squeaking) sands in response to shearing are investigated, with samples of silent sands studied for controls. A vertical-axis geophone buried at shallow depth and an air microphone were used in the studies. The frequency spectra of the acoustic and seismic responses, propagation delays, comparison of acoustic and seismic traces, grain size and grain surface texture, particle morphology, coherent behavior of grains in assemblages, and relation to prevalent local winds were studied. Mechanisms are still obscure and disputed; slumping and avalanches were induced artificially in some studies. Existence of booming dune phenomena on Mars or on the moon is conjectured.
 
Article
Regional metamorphism of upper-greenschist to epidote-amphibolite facies produced variations of olivine and chrome spinel textures and compositions within the East Dover ultramafic bodies that can be attributed to differing intensities of recrystallization of the ultramafic rocks. Olivine shows a progressive change from highly strained grains in the least recrystallized rocks to relatively strain-free grains in the most recrystallized rocks. Fo content of olivine bears a direct relationship to texture: with increase in degree of recrystallization, olivine changes composition from Fo90 to Fo97. Chrome spinel grains are ubiquitously rimmed by opaque spinels. Variations in thickness of these rims and shape of the translucent chrome spinel cores occur in a regular manner. Thickest rims occur around the most anhedral cores, and these are found in the most strongly recrystallized rocks. Euhedral chrome spinels with thin opaque spinel rims are found in the least recrystallized rocks. With increasing recrystallization, Mg/(Mg + Fe) and A1/(A1 + Cr) ratios increase in the spinel cores. Zones of intensity of recrystallization indicated by both olivine and spinel textures and compositions are geographically concentric with the map outlines of the ultramafic bodies. Observed textural and compositional variations suggest a reaction involving simultaneous serpentinization and recrystallization of olivine in the presence of an oxidizing fluid. Partial buffering of this fluid (which originated in the country rocks) and subsequent development of a substantial fo2 gradient between the margins and cores of the larger ultramafic bodies resulted in a more nearly complete reaction at the margins than in the cores of the bodies. This reaction need not have occurred as a single event, and equilibrium was only maintained on the scale of hand specimens (fo2, olivine composition) to fractions of mineral grains (spinel composition).
 
Article
Two main types of deformational breccia occur in the Sierra Madera cryptoexplosion structure: monolithologic breccias composed of shattered rock of a single lithology and mixed breccias composed of rocks of several lithologies. Monolithologic breccias generally show no mineralogic signs of shock deformation, but a few samples are shatter-coned in a manner suggesting simultaneous formation of breccias and shatter cones. Mixed breccias, forming irregular, cross-cutting bodies, consistently contain moderately to highly shocked material, with mineralogic evidence of shock pressures of 50 kb to more than 200 kb, which, with evidence from the structural geometry of Sierra Madera and orientation of shatter cones, indicate an impact origin of the breccias. The mode of occurrence of the breccias, petrographic characteristics, and association with shock features are shared by breccias in many other cryptoexplosion structures in both carbonate and crystalline rock terranes, suggesting that such breccias have a common origin.
 
Article
Very silicic, peraluminous rhyolite lavas from Glass Mountain, Mono County, east-central California, contain 2 to 3 ppm Sr - approximately two orders of magnitude less than the average for silicic igneous rocks - and less than 20 ppm Ba. On the basis of available data on the partition of Sr between coexisting feldspar and melt phases, the rhyolite is found to represent about 25 percent of a hypothetical "parent" magma of silicic composition having a "normal" Sr content of 150 ppm. Available strontium and lead isotopic data on similar nearby Pleistocene rhyolites suggest that the initial magmas from which the rhyolite lavas of Glass Mountain and other Quaternary silicic volcanic rocks of the area were differentiated were derived from mafic or ultramafic source material.
 
Article
Stromatolitic domes having apparent growth reliefs up to 100 m or more characterize the lower member of the Noonday Dolomite, late Proterozoic, Death Valley region, eastern California. Long to short, subparallel, initially subvertical tubes penetrate the bedding of these structures at angles that range from 90° where stratification was initially horizontal to downslope angles up to 110° (and acute upslope angles) as initial dips increase. Tubes generally are missing where initial dips exceed 20°. They are common in the larger buildups and most abundant in the upper and central parts of these. They may be open or filled with coarsely sparry secondary dolomite, medium-gray ultra microgranular dolomite, silty brown dolomite, or drusy quartz. They pinch and swell irregularly along their length from a mean diameter of 1.5 cm and are subcircular to irregular in horizontal cross section. Secondary crystalline growths similar to those in many tubes are common along lamination surfaces and as irregular vug fillings. Weathering along intersecting fracture sets locally produces a subvertical lineation in the same orientation as associated tubes. Possible origins considered for the tubes and found improbable are: (1) metazoan burrows, (2) interspaces of columnar stromatolites, (3) columnar stromatolites, (4) solution pipes, and (5) root casts. Instead the spaces now represented by filled vugs, sparry lamination surfaces, and tubes are interpreted as having, in general, a common origin caused by the upward movement of fluids through the stromatolite mounds.
 
Article
The two main hypotheses concerning the origin of compositional heterogeneities in magma chambers are discussed: (1) models in which the development of compositional zonation is simultaneous with the birth and growth of the magma body and (2) models in which zonation develops within an initially homogeneous batch of magma. The paper presents an overview of the geological possibilities and evaluates them on the basis of current research. Calculations are presented for boundary-layer flow in isothermal ternary component systems, and it is demonstrated that multicomponent diffusion effects may be very significant, as was earlier suggested by Trial and Spera (1988).
 
Article
Sharp et al. (1971) define chaotic terrain as an irregular jumble of topographic forms covering a certain area within Pyrrhae Regio and adjacent regions centered at about 10 deg S., 35 deg W. This area is covered by Mariner 6 television imagery. An analysis of fracture patterns in the Martian surface from high-resolution Mariner 6 imagery suggests that the lineaments observed in both the chaotic terrain and the cratered plateau areas in Pyrrhae Regio are tectonic fractures resulting from stresses within the Martian crust.
 
Article
Mineralogy and chemistry of earth upper mantle, using partial fusion-partial crystallization model
 
Article
The thermodynamic meteorologic model of Adem is used to trace the evolution of climate from Triassic to present time by applying it to changing geography as described by continental drift and polar wandering. Results show that the gross changes of climate in the Northern Hemisphere can be fully explained by the strong cooling in high latitudes as continents moved poleward. High-latitude mean temperatures in the Northern Hemisphere dropped below the freezing point 10 to 15 m.y. ago, thereby accounting for the late Cenozoic glacial age. Computed meridional temperature gradients for the Northern Hemisphere steepened from 20 to 40 C over the 200-m.y. period, an effect caused primarily by the high-latitude temperature decrease. The primary result of the work is that the cooling that has occurred since the warm Mesozoic period and has culminated in glaciation is explainable wholly by terrestrial processes.
 
Article
Electron microprobe measurements of olivine and metal in about 85 percent of the known pallasites were made. The olivines are unzoned, and coexisting crystals within most pallasites have identical compositions. This indicates a high degree of internal equilibrium. Relative to terrestrial olivines, pallasitic olivines are depleted in Ni. Thermochemical calculations demonstrate that the low Ni contents of pallasitic olivines are consistent with metal-olivine equilibrium at 1000° C or less. Cooling rate measurements show that the pallasites cooled more slowly (0.5° to 2.0° C per million years) than the bulk of the iron meteorites and that the pallasites form a homogeneous cooling rate group. Application of a modified Prior's rule to the pallasites suggests that olivine and metal equilibrated in the presence of other silicates. The observed compositional distribution of olivine from pallasites is compatible with that predicted from a chondritic parent melt. The pallasites were probably derived from deep within their parent body, either at or near the core. Most iron meteorites were either formed from different parent bodies or came from isolated metal pools within the silicate mantle.
 
Article
An extremely well-preserved Holocene fringing coral reef occurs at an average elevation of 5 m below sea level around the margins of the central Enriquillo Valley, Dominican Republic. The reef records the latest marine incursion from the east into an 85-km-long, 12-km-wide tectonic depression and appears to represent a unique preservation. Excellent cross sections of the reef exposed in erosional gullies reveal a composition and zonation typical of modern Caribbean reefs that are found in offshore low-energy environments. Radiocarbon age determinations (2) indicate that reef growth coincided with sea-level rise following the last ice age (5,930 + or - 100 to 4,760 + or - 90 yr B.P.). Deltaic deposition and possible vertical movements on active fault scarps dammed the eastern mouth of the valley and created Lago Enriquillo, the level of which was then rapidly lowered by evaporation in an arid climate to produce a saline lake approximately 40 m below sea level. Stratigraphic studies of rocks along the valley edge and data from drill holes in the basin center indicate that there were earlier post-Miocene marine incursions similar to that described here.
 
B. Apollo X photograph looking obliquely inside the lunar crater Neper, 145 km wide. The prominent concentric structure of the central peak suggests uplifted layered rocks as at Sierra Madera.
Article
The central peaks of Copernicus or a similar lunar crater have been considered as a target for manned exploration, partly on the supposition that the peaks expose rock uplifted from beneath the crater floor. This supposition is based on an analogy with central uplifts of terrestrial cryptoexplosion structures. Sierra Madera in west Texas is one of these structures. The structure of its central uplift is described as a proposed analog of the central peaks of large lunar impact craters. Setting and stratigraphy of the Sierra Madera are discussed together with structural framework, the central uplift, fold patterns, and fault patterns. It is concluded that the central uplifts of Sierra Madera and similar cryptoexplosion structures appear to be analogous to central peaks of large lunar impact craters.
 
Article
The flow direction technique, previously applied to ash-flow sheets, can be used to determine direction of movement and locate eruptive centers for lava flows. The method provides statistically stronger and more consistent flow direction data for lava than ash-flow tuff. The accuracy and reliability of the technique was established on the porphyritic basaltic andesite of Mount Taylor, New Mexico, which erupted from a known center, the Mount Taylor Amphitheater. The technique was then applied to volcanic units with unknown sources: the John Kerr Peak Quartz Latite and mid-Tertiary andesite flows in the Mogollon Mountains, both in southwestern New Mexico. The flow direction technique indicated flow patterns and suggested source areas for each rock unit. In the Mogollon Mountains flow direction measurements were supported by independent directional criteria such as dips of cross beds, stratigraphic thickening, facies changes, and megascopic textures.-
 
Article
Cleavage in graywacke, siltstone, and pelite of the Michigamme Slate appears to have been produced by tectonic dewatering. Clastic dikes, from paper-thin dikelets to zones 10 m wide, are intruded parallel to cleavage in many outcrops. In other places, liquefication and mobilization have homogenized the sediments into massive, structureless rocks. The planar cleavage which pervades all rock types is produced by pelitic folia formed during tectonic dewatering. Ellipsoidal concretions with the same ductility as the host siltstone indicate large plane strains. Maximum shortening is perpendicular to cleavage with greater than threefold extension in the cleavage. Lengths parallel to fold axes are essentially unchanged, but stratigraphic thicknesses may be grossly exaggerated by deformation. Tectonic dewatering of the Michigamme Slate may have occurred after tectonic dewatering of the underlying Siamo Slate.
 
Top-cited authors
Eric Eslinger
  • The College of Saint Rose
John Tchalenko
  • University of the Arts London
John F. Dewey
  • University College Oxford, British Museum of Natural History, UC Davis, UT Austin
Arthur Palmer
  • State University of New York, Oneonta
Jay Quade
  • The University of Arizona