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At least 127 records · Page 7Linked to original sources
USGS1970

Green River oil shale—concept of origin extended: An interdisciplinary problem being attacked from both ends

A much fuller understanding of the Green River oil shale and its organic chemistry will emerge when the geologists, paleontologists, organic chemists, biologists, paleolimnologists, and biogeochemists, who are now working on it, integrate their findings with those of the others. We know from the geology, paleontology, and paleolimnology that the biologic progenitors of the organic substance in the Green River oil shale could only have been microscopic algae, and other micro-organisms, that grew and accumulated in the central parts of large, shallow lakes that existed under a subtropical climate. The only nonlacustrine organic components were wind-blown, or water borne, pollens and waxy spores. These, however, made up a large and important part of the organic-rich sediment. The geology of the Green River Formation shows that as the algal and pollen-rich sediment was buried deeper and deeper, progressively more of its pore water and dissolved constituents were expressed. Static pressures may have reached as much as 210 kg cm −2 , and the ambient temperature rose, with depth, to somewhere within the range between 90° and 125° C. Beneath the ancient lakes a tectonically quiescent environment persisted for tens of millions of years after their organic sediments had been deeply buried. The organic material of the Green River oil shale can be divided into three fractions—a small bitumen fraction that is extractable with common organic solvents, a major fraction called koerogen that consists of insoluble pyrobitumens, and a somewhat smaller inert fraction that is neither soluble nor does it yield oil on pyrolysis. As all three fractions originated in the same algal, pollen-rich sediment, an explanation for their marked differences must be sought in their geochemical history or from a study of the modern analogues of their progenitors. The components of the bitumen fraction consisted of “biological markers” that were inherited from the Eocene plants and animals in which they originally formed. Diagenesis has changed these hydrogen-rich compounds, but not enough to obscure their provenance. Kerogen presumably became insoluble because its hydrogen-rich components polymerized. My speculation is that the inert fraction was derived from a polyphenolic substance produced in the original algal ooze by “non-enzymatic browning.” Only three Classes of non-marine algae need be considered as progenitors of the Green River oil shale; the Xanthophyceae, the Chlorophyceae, and the Cyanophyceae. Only the Cyanophyceae (the blue-green algae) meet the biologic and paleontologic requirements to have served as the dominant precursors of the Green River oil shale. Several other oil shales clearly were derived from the Xanthophyceae, specifically Botryococcus. The blue-green algal ooze now forming, and accumulating, in Mud Lake, Florida, has been studied biologically and chemically as a possible present-day analogue of the Green River oil shale precursor. In this small lake we have established the fact that a bacterial inhibitor is produced, which inhibits decay of the algae and thereby permits the accumulation of energy-rich organic compounds. We infer that a similar indigenous inhibitor must have acted in the Eocene lakes to permit them to become the huge energy sinks they were. Studies of the organic chemistry of living blue-green algae show that they contain appreciable percentages of fatty acids, hydrocarbons, and very large percentages of proteins. These promising, energy-rich compounds could serve as source materials for potential conversion into oil shale in the geologic future. Certain marine anaerobic bacteria convert fatty acids into aliphatic hydrocarbons. Fresh-water obligate anaerobes should be investigated to see if they also convert fatty acids into hydrocarbons. The part played by aquatic animals that live in, or on, freshwater algal ooze in synthesizing hydrocarbons has not been investigated, but deserves attention. Pollen grains, of course, must be considered an important precursor of hydrocarbons produced on pyrolysis. They contain far higher percentages of long chain hydrocarbons and alcohols than most plant materials. The major problem ahead is to account for the progressive hydrogenation and subsequent polymerization of the relatively oxygen-rich constituents of algae such as the polysaccharides, amino acids, ammo sugars, and fatty acids into the insoluble pyrobitumens that constitute, particularly, the kerogen fraction of the Green River oil shale.

Wyoming↗
USGS1971

Impact breccias in carbonate rocks, Sierra Madera, Texas

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.

Texas↗
USGS1971

Paleozoic metasediments in the northern Ruby Mountains, Nevada

New evidence indicates that high-grade regionally metamorphosed marble and quartzite in the northern Ruby Mountains are Paleozoic strata. Correlation is based on a match of the lithologic sequence to strata of Cambrian to Devonian age in nearby areas; particularly diagnostic is a brown dolomite at the base of the Cambrian carbonate sequence. The metamorphic complex contrasts with unmetamorphosed terranes exposed in nearby areas, and apparently formed in the Mesozoic as part of a metamorphic infrastructure.

Nevada↗
USGS1971

Structural implications of an offset Early Cretaceous shoreline in northern California

Recognition of a nonmarine to marine transition in sedimentary rocks at Glade Creek and Big Bar in the southern Klamath Mountains permits reconstruction of the approximate position of a north-trending Early Cretaceous (Valanginian) shoreline. At the southern end of the Klamath Mountains, the shoreline is displaced 60 mi or more to the east by a west-northwest-trending fault zone. South of this fault zone the shoreline is buried at a much lower level beneath late Cenozoic rocks in the Great Valley. This large displacement probably is the result of differential movement along a system of left-lateral tear faults in the upper plate of the Coast Range thrust. The westward bulge of the Klamath arc also may have resulted from this faulting, as the amount and direction of the bulge is comparable with the displacement of the Valanginian shoreline. Basal clastic strata at both Glade Creek and Big Bar contain abundant fresh-water or brackish-water clams, many of which consist of unabraded paired valves. These are conformably overlain by Valanginian marine strata containing Buchia crassicollis solida. The position of the Valanginian shoreline beneath the Great Valley cannot be directly observed because it is buried by thick late Cenozoic deposits. However, its approximate westernmost limit must lie between the outcrop belt of marine strata on the west side of the valley and drill holes to basement on the east side, in which equivalent strata are absent. Franciscan rocks containing Valanginian fossils occur 10 mi southwest of Glade Creek, but these are deep-water marine eugeosynclinal rocks that were deposited far to the west of the shoreline. The deformation responsible for the displacement of the Valanginian shoreline and juxtaposition of the Franciscan rocks and Klamath Mountain basement rocks involved eastward under-thrusting of the Franciscan beneath the Coast Range thrust contemporaneous with differential movement along tear faults within the upper plate.

California↗
USGS1972

Paleomagnetic correlations and Potassium-Argon dating of Middle Tertiary ash-flow sheets in the eastern Great Basin, Nevada and Utah

Directions of natural remanent magnetization are used to identify and correlate individual cooling units in the middle Tertiary ash-flow province in central and eastern Nevada and western Utah. Potassium-argon dating indicates that the minimum time between eruptions of individual but genetically related ash-flow cooling units is on the order of 0.8 m.y. As this interval is long in comparison with the secular variation of the direction of the geomagnetic field, in a given volcanic province the direction of natural thermoremanent magnetization is a unique characteristic of each cooling unit. Ash-flow sheets investigated include the Stone Cabin Formation, the tuff of Pancake Summit, the Windous Butte Formation, the Needles Range Formation, the Bates Mountain Tuff, and the tuff of Clipper Gap, of Oliogcene to early Miocene age. The original areas of individual ash-flow cooling units are as great as 8,000 km 2 ; the volumes, 1,300 km 3 . The paleomagnetic correlations, made over distances up to 200 km, confirm most of the previously made lithologic correlations and allow more accurate delineation of single cooling units. These correlations are particularly useful in those parts of the Basin and Range province where the Tertiary stratigraphic section consists mainly of ash-flow sheets, and the outcrops are confined to mountain ranges that are separated by alluvium-filled valleys.

Nevada, Utah↗
USGS1972

Paleomagnetism and potassium-argon ages of the Sonoma Volcanics, California

Paleomagnetic data and potassium-argon ages indicate that the Sonoma Volcanics was erupted during the Pliocene Gilbert reversed and Gauss normal polarity epochs. The Gilbert reversed epoch is represented in the Howell Mountains east of Napa and east of St. Helena, in the mountains immediately east of the Valley of the Moon, and on the hill just north of Santa Rosa. The Gauss normal epoch is represented by the rocks from Mount St. Helena and possibly by the flows from the upper part of Sonoma Mountain and the rhyolite flows north of Sonoma. The age of the volcanic rocks ranges from 5.3 m.y. to about 2.9 m.y. Volcanic rocks approximately 11.8 m.y. old occur at Burdell Mountain northwest of Novato. This volcanic sequence is probably part of the Tolay Volcanics of the Petaluma area.

California↗
USGS1972

Apollo 11 and 12 mare basalts and gabbros: Classification, compositional variations, and possible petrogenetic relations

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 TiO 2 and SiO 2 or Al 2 O 3 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 Al 2 O 3 versus FeO trend in the basalts might possibly be explained by near-surface fractionation, but the TiO 2 versus SiO 2 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.

GSA Bulletin↗
USGS1972

Structure of Sierra Madera, Texas, as a guide to central peaks of lunar craters

Like hundreds of other lunar craters of probable impact origin, Copernicus contains central peaks presumed to expose rocks uplifted from beneath the crater floor. A possible analog of these peaks on Earth is the central uplift of the Sierra Madera cryptoexplosion structure, a probable impact scar (astrobleme) in stratified Permian and Cretaceous rocks of west Texas. The most conspicuous part of this 12-km-wide structure is its central uplift, 6 to 8 km across, in which the oldest rocks have been raised 1,200 m above their normal position. The uplift is surrounded by a structural depression, beyond which is a concentric rim containing some folds and circumferential normal faults with downthrow toward the center. The intensity of folding and faulting increases inward from the flanks of the central uplift toward a central zone about 2 km across where dips and fold plunges are near vertical or overturned. As in salt domes, the beds moved inward as well as upward to occupy their present positions, causing faulting and radial folding due to crowding in the center. Repetition by folding due to inward movement of upper beds over lower appears to have been followed by upward movement of the core of the uplift pulling away from the flanks by faulting. Minor outward-directed thrusting may have been caused by gravitational spreading of the rising dome. The structural depression surrounding the uplift resulted from tectonic thinning accompanying the inward movement of rocks in the uplift. Analogy with experimental craters and with other cryptoexplosion structures indicates that the uplift at Sierra Madera protruded into a crater (since destroyed by erosion) that was about 12 km across. Inasmuch as the rocks forming such uplifts are derived from below the crater floors, analogous peaks in lunar craters such as Copernicus may offer samples of lunar crust uplifted from distances below the crater floor on the order of one-tenth of the crater diameter.

Texas↗
USGS1973

Oil shale formed in desert environment: Green River Formation, Wyoming

The oil shale beds of the Wilkins Peak Member of the Green River Formation differ from most of the Green River oil shale of Colorado and Utah because very few of them have varves; they have numerous mud cracks and, locally, desiccation breccias. Many have a copropelic microstructure and contain labial plates and skin fragments of chironomid larvae. Microfossils of some of these oil shale beds include such normally perishable things as first instar midge larvae, a single spiral chloroplast of a green alga, and sporangia of aquatic fungi. Taken together, these features indicate that the oil shale originated from algal ooze that formed on the bottom of very shallow, spring-fed lakes. The algae (dominantly blue-greens) were not planktonic but grew on, and in, the flocculent ooze. At intervals, perhaps tens to hundreds of years, the lake levels fell enough to expose the algal ooze to the air and partial drying. In the resulting rubbery organic gel, the most delicate microorganisms were heat-fixed and thereby preserved as “mummified” tissue.

Wyoming↗
USGS1973

Origin of Comb Layering and Orbicular Structure, Sierra Nevada Batholith, California

A new descriptive term, comb layering , is proposed to replace the informal term Willow Lake-type layering , first introduced by Poldervaart and Taubeneck (1959) to describe layering in granitoid rocks in which constituent crystals are oriented approximately perpendicular to individual layers. The term schlieren layering is proposed to describe the “normal” layering of granitic rocks defined by alternating layers enriched or depleted in the normal mafic minerals. In such layers, elongate minerals commonly lie in the plane of the layering. Comb layering is widespread in plutonic rocks of California and is commonly associated with orbicular diorites. Evidence from a detailed study of three localities in the Sierra Nevada indicates that comb layering forms chiefly in overturned troughs along overhanging walls of plutons or along walls of dikes or pipes that cut country rocks adjoining plutons. Orbicular rocks associated with the comb layering are formed by a nucleus surrounded by multiple comb layers. The growth direction in comb layers can be determined by the branching and widening of plagioclase and hornblende crystals and is invariably toward the parent pluton. Field data indicate that comb layering cannot have formed from silicate magma, and further suggest that the layers have been deposited by large volumes of aqueous fluids that migrated upward along contacts between magma and wallrock or along the interface between magma and previously solidified melt. Comb layering and orbicules are largely restricted to structural traps into which upwardly migrating, solute-rich water was channeled owing to its low density. The comb layers grew on the solid walls of fluid-filled channels, whereas orbicules formed by precipitation of comb layers on hobbling inclusions suspended within the upward-flowing fluid.

California↗
USGS1973

Basement ages and basement depths in the eastern equatorial pacific from Deep Sea Drilling Project Legs 5, 8, 9, and 16

Recent literature contains numerous references to basement ages and basement depths determined by the Deep Sea Drilling Project. The data are derived from a variety of sources, many of them inadequately documented or preliminary, and are not uncommonly inaccurate or conflicting. In this paper we present tabulations of basement ages and depths from DSDP Legs 5, 8, 9, and 16 in the eastern equatorial Pacific, refer them to the latest biostratigraphic time scale, and document and discuss their error limits. We recommend that in future use of this type of data a similar practice be adopted and that the precise source of data, time scale used, and procedures for determining ages and errors be clearly identified in order to avoid confusion. Based on the data presented here, we also give the relations between basement age, distance from the spreading center, and basement depth. The errors inherent in the data cause these relations to be very general and to have less resolution than ascribed to them by some previous investigators.

GSA Bulletin↗
USGS1973

Alaska-Aleutian range batholith: Geochronology, chemistry, and relation to circum-Pacific plutonism

Potassium-argon mineral ages and reconnaissance mapping of approximately 65,000 sq km in south-central Alaska indicate that the Mesozoic and Cenozoic plutonic rocks in the region were emplaced during three discrete intrusive epochs. Most of the plutonic rocks are part of the Alaska-Aleutian Range batholith; the remainder appear as outcrops in isolated plutons northeast of the main batholith. Maximum and minimum concordant mineral ages on coexisting biotite and hornblende are used to mark the beginning and ending, respectively, of each intrusive epoch. The oldest intrusive epoch, Early and Middle Jurassic, contains plutonic rocks emplaced between about 176 and 154 m.y. ago. Jurassic plutonism occurred along a magmatic arc at least 1,300 km long, extending from about 480 km southwest of Becharof Lake northeast to the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which represents the roots of the arc portion of an early Mesozoic arc-trench system, also continues southwest into the Bering Shelf. The associated trench is thought to be represented by an imbricated mélange of ophiolite and submarine lava with associated chert and argillite that occupies a belt 140 km southeast of the magmatic arc. Clastic sediments more than 4.5 km thick occupy the 140-km-wide arc-trench gap. Late Cretaceous and early Tertiary plutonic rocks, emplaced between about 83 and 58 m.y. ago, are found mainly in the northern part of the batholith and in isolated plutons to the northeast toward Mount McKinley. Middle Tertiary plutonic rocks ranging in age from 38 to 26 m.y. occur in two areas within the batholith and also in the Mount McKinley area. Although some of the Late Cretaceous and Tertiary plutons are associated with extrusive rocks, these plutons are largely post-tectonic, and magma was generated beneath both stable platform areas as well as beneath former eugeosynclinal regions in which deformation had essentially ceased. Magma for the Jurassic plutonic rocks appears to have been generated along or above an early Mesozoic subduction zone. The region southeast of the zone is considered to represent a classic example of continental accretion of eugeosynclinal sediments caused by under-thrusting of oceanic crust beneath an island arc. Magma generation for the Cretaceous and Tertiary plutons in the northern part of the batholith does not appear related to the early Mesozoic subduction zone, for it would require that the zone shift inland, or toward the continent, from its position during the Jurassic and then shift away from the continent once again to its present position.

GSA Bulletin↗
USGS1973

Chemical variation related to the stratigraphy of the Columbia River basalt

Study of major element chemical analyses of Columbia River basalt leads to a grouping of most of the analyses into 11 chemical types which are distinguished with little overlap on a SiO 2 -MgO variation diagram. Other diagnostic variation diagrams are total iron (‘FeO’)-MgO, K 2 O-MgO, and TiO 2 -MgO. A four-unit informal stratigraphy has been adopted in order to define the relations between chemical composition and stratigraphic position. From oldest to youngest, the four stratigraphic units are (1) lower basalt of Bond (1963) and Picture Gorge basalt, (2) lower Yakima basalt, (3) middle Yakima basalt, and (4) upper Yakima basalt. Most of the Picture Gorge and lower basalt flows are relatively rich in MgO (approximately 4.5 to 7.1 percent) and are distinguished by intermediate SiO 2 relative to MgO. Furthermore, the Picture Gorge basalt generally has low K 2 O relative to MgO. The lower Yakima basalt consists almost entirely of flows with relatively low MgO content (approximately 3.0 to 5.5 percent) and with the highest SiO 2 relative to MgO of any flows of the Columbia River basalt. The middle Yakima basalt contains flows of three distinct chemical types, which together cover the same MgO range as the lower Yakima flows but which have considerably lower SiO 2 and higher ‘FeO’ and TiO 2 relative to MgO. Flows in the upper Yakima basalt are of diverse composition; two of the youngest flows are distinguished by having the lowest SiO 2 and highest ‘FeO’, TiO 2 , and P 2 O 5 relative to MgO of any analyzed Columbia River basalt. Flows of one or more chemical types may form the dominant lithology in a stratigraphic unit, but single flows of the same chemical types may occur in any stratigraphic unit. Some lava sampled in the eastern part of the plateau has more TiO 2 than does lava of otherwise similar composition sampled in the western part of the plateau. This is tentatively interpreted as reflecting a heterogeneous composition for the mantle beneath the Columbia Plateau.

Oregon, Washington↗
USGS1973

Timing of mesozoic and cenozoic plutonic events in circum-Pacific North America

Evaluation of isotopic ages of granitic intrusive rocks of large batholiths in circum-Pacific North America indicates that Mesozoic and Cenozoic plutonism was episodic but not periodic. Three intrusive epochs have been defined in the Alaska-Aleutian Range batholith of Alaska on the basis of concordant potassium-argon ages of coexisting biotite and hornblende or muscovite. Only two intrusive epochs based on concordant ages of mineral pairs and an older plutonic episode can be recognized in the Sierra Nevada and southern California batholiths. Detailed intrusive histories for other parts of Alaska, British Columbia, and Yukon Territory cannot be inferred from available data. The intrusive epochs in the western United States do not correlate with the epochs in the Alaska-Aleutian Range batholith. The spacing, duration, and synchroneity of intrusive epochs in the circum-Pacific batholiths of North America are not yet well established.

GSA Bulletin↗
USGS1973

Shallow structure and geologic development of the Southern Red Sea

A series of 34 shallow-penetration seismic-reflection profiles made across the Red Sea show that it developed in two main stages. Initially, an early or pre-Miocene uplift and lateral extension resulted in crustal thinning and eventual formation of the main Red Sea Basin. During Miocene time, the Red Sea was isolated from the Indian Ocean but possibly connected with the Mediterranean Sea, which, like the Red Sea, was an evaporite basin at that time. A distinct acoustic reflector (reflector S) in the Red Sea marks the top of the Miocene evaporite sequence and is correlative with reflector M in the Mediterranean, which is similarly identified with termination of evaporite conditions. In Pliocene time, connection with the Indian Ocean was re-established, the opening to the Mediterranean was closed, and normal marine conditions were resumed in the Red Sea. Sea-floor spreading first started in Pliocene-Pleistocene time, and resulted in the formation of the axial zone of the Red Sea.

GSA Bulletin↗
USGS1973

Boulder Batholith, Montana: A product of two contemporaneous but chemically distinct magma series

Rocks of the Late Cretaceous composite Boulder batholith, though successively emplaced in a relatively small segment of the Earth's crust within a very brief time span (78 to 68 m.y.), can be grouped chemically into two magma series: (1) the main series , defined principally by plutons in the central and northern parts of the batholith; and (2) the sodic series , defined mostly by plutons in the southern part. For any given SiO 2 content, the rocks of the main series tend to be higher in K 2 O and lower in Na 2 O than rocks of the sodic series. The chemical distinction between the two series proposed is also expressed by variation patterns for U, Th, Rb, and Sr abundances, by lead isotope compositions, but not by strontium isotope compositions. The prebatholith Elkhorn Mountains Volcanics (Late Cretaceous), especially the mafic members, are chemically and isotopically similar to the rocks of the main series, confirming geologic evidence of the genetic association between them. The postbatholith Lowland Creek Volcanics (early Eocene), though chemically more closely related to the sodic series, isotopically are more akin to, but slightly more radiogenic than, the main series. Post–Lowland Creek volcanic rocks (Miocene or Pliocene) are compositionally similar to the sodic series rocks. Spatial distribution of the batholith and the volcanic rocks exhibits a very crude chemical zonation of the region: for a given silica content, relatively more potassic rocks (main series and prebatholith volcanic rocks) tend to occur mainly in the north and east, whereas relatively more sodic rocks (sodic series and postbatholith volcanic rocks) predominate in the south and west. Available field, chemical, and isotopic evidence collectively suggests that the observed compositional variations for the Boulder batholith are most reasonably interpreted in terms of a model involving two magma series derived from two or more magma sources within the lower crust or upper mantle. These source regions are interred to vary chemically and isotopically, either laterally or vertically; in view of the rather small areal extent of the Boulder batholith, however, a vertically zoned source region is more probable.

Montana↗