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At least 127 records · Page 7Linked to original sources
USGS2003

Extensive deposits on the Pacific plate from Late Pleistocene North American glacial lake outbursts

One of the major unresolved issues of the Late Pleistocene catastrophic-flood events in the northwestern United States (e.g., from glacial Lake Missoula) has been what happened when the flood discharge reached the ocean. This study compiles available 3.5-kHz high-resolution and airgun seismic reflection data, long-range sidescan sonar images, and sediment core data to define the distribution of flood sediment in deepwater areas of the Pacific Ocean. Upon reaching the ocean at the mouth of the Columbia River near the present-day upper continental slope, sediment from the catastrophic floods continued flowing downslope as hyperpycnally generated turbidity currents. The turbidity currents resulting from the Lake Missoula and other latest Pleistocene floods followed the Cascadia Channel into and through the Blanco Fracture Zone and then flowed west to the Tufts Abyssal Plain. A small part of the flood sediment, which was stripped off the main flow at a bend in the Cascadia Channel at its exit point from the Blanco Fracture Zone, continued flowing more than 400 km to the south and reached the Escanaba Trough, a rift valley of the southern Gorda Ridge. Understanding the development of the pathway for the Late Pleistocene flood sediment reaching Escanaba Trough provides insight for understanding the extent of catastrophic flood deposits on the Pacific plate .

Journal of Geology↗
USGS2004

Geochemical discrimination of five pleistocene lava-dam outburst-flood deposits, western Grand Canyon, Arizona

Pleistocene basaltic lava dams and outburst‐flood deposits in the western Grand Canyon, Arizona, have been correlated by means of cosmogenic 3 He ( 3 He c ) ages and concentrations of SiO 2 , Na 2 O, K 2 O, and rare earth elements. These data indicate that basalt clasts and vitroclasts in a given outburst‐flood deposit came from a common source, a lava dam. With these data, it is possible to distinguish individual dam‐flood events and improve our understanding of the interrelations of volcanism and river processes. At least five lava dams on the Colorado River failed catastrophically between 100 and 525 ka; subsequent outburst floods emplaced basalt‐rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites. Chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long‐lasting dam complexes. These chemical data, in combination with age constraints, increase our ability to correlate lava dams and outburst‐flood deposits and increase our understanding of the longevity of lava dams. Bases of correlated lava dams and flood deposits approximate the elevation of the ancestral river during each flood event. Water surface profiles are reconstructed and can be used in future hydraulic models to estimate the magnitude of these large‐scale floods.

Arizona↗
USGS2004

Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

Journal of Geology↗
USGS2005

Paleoproterozoic metamorphism in the northern Wyoming province: Implications for the assembly of Laurentia

U-Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high-grade metamorphism at ???1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ???3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 ??m) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well-defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ???100 m.yr. younger than the proposed time of Wyoming-Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ???1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia. ?? 2005 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS2006

Timing of magmatism following initial convergence at a passive margin, southwestern U.S. Cordillera, and ages of lower crustal magma sources

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali-calcic Fe- and Sr-enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal-miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo-Triassic intrusion ages, and a prolonged postemplacement thermal history within the long-lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc. ?? 2006 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS2006

Calcite-graphite thermometry of the Franklin Marble, New Jersey Highlands

We present new stable-isotope data for the Mesoproterozoic Franklin Marble from outcrops along an 80-km traverse parallel to and across strike of the structural grain of the western New Jersey Highlands. Calcite and dolomite from marble have an average ??13C of 0.35??? ?? 0.73??? PDB (n = 46) and a more limited range than other Mesoproterozoic marbles from the Adirondacks and the Canadian Grenville Province. The small range of ??13C values from the New Jersey samples is consistent with the preservation of a primary marine isotopic signature and limited postdepositional isotopic modification, except proximal to Zn or Fe ore deposits and fault zones. Fractionations between calcite and well-formed graphite (??13C[Cal-Gr]) for analyzed Franklin Marble samples average 3.31???. ?? 0.25??? (n = 34), and dolomite-graphite fractionations average 3.07??? ?? 0.30??? (n = 6). Taken together, these indicate an average temperature of 769?? ?? 43??C during metamorphism associated with the Ottawan Orogeny in the New Jersey Highlands. Thus, carbon isotope fractionations demonstrate that the Franklin Marble was metamorphosed at granulite facies conditions. Metamorphic temperatures are relatively constant for the area sampled and overprint the metamorphosed carbonatehosted Zn-Fe-Mn ore deposits. The results of this study support recent work proposing that pressure and temperature conditions during Ottawan orogenesis did not vary greatly across faults that partition the Highlands into structural blocks. ?? 2006 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS1997

The Bishop Tuff: New insights from eruptive stratigraphy

The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain‐derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less‐welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of dense‐welded massive ignimbrite, equivalent to ≥2.5 mm/s of non‐welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south‐central part of the caldera (F1‐8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre‐eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.

California↗
USGS1998

Paleoslope analysis of slump folds in the devonian flysch of Maine

Ancient submarine slide and slump deposits in the Devonian flysch of central and northern Maine show considerable variation in fold style, from symmetric to asymmetric to sheath geometries. Building on earlier work by Farrell and Eaton, we suggest that the spectrum of fold styles reflects the degree of simple shear within each slump deposit. We present a stereographic approach to paleoslope analysis that exploits fold hinge attitudes, axial surface attitudes, sheath axes, vergence, S‐ and Z‐asymmetry—depending on the style of slump folding. Our paleoslope determinations from widely scattered locations across the Devonian foreland basin in Maine show a regionally consistent pattern of westerly to northwesterly slopes.

Journal of Geology↗
USGS2007

Differentiating pedogenesis from diagenesis in early terrestrial paleoweathering surfaces formed on granitic composition parent materials

Unconformable surfaces separating Precambrian crystalline basement and overlying Proterozoic to Cambrian sedimentary rocks provide an exceptional opportunity to examine the role of primitive soil ecosystems in weathering and resultant formation of saprolite (weathered rock retaining rock structure) and regolith (weathered rock without rock structure), but many appear to have been affected by burial diagenesis and hydrothermal fluid flow, leading some researchers to discount their suitability for such studies. We examine one modern weathering profile (Cecil series), four Cambrian paleoweathering profiles from the North American craton (Squaw Creek, Franklin Mountains, Core SQ-8, and Core 4), one Neoproterozoic profile (Sheigra), and one late Paleoproterozoic profile (Baraboo), to test the hypothesis that these paleoweathering profiles do provide evidence of primitive terrestrial weathering despite their diagenetic and hydrothermal overprinting, especially additions of potassium. We employ an integrated approach using (1) detailed thin-section investigations to identify characteristic pedogenic features associated with saprolitization and formation of well-drained regoliths, (2) electron microprobe analysis to identify specific weathered and new mineral phases, and (3) geochemical mass balance techniques to characterize volume changes during weathering and elemental gains and losses of major and minor elements relative to the inferred parent materials. There is strong pedogenic evidence of paleoweathering, such as clay illuviation, sepic-plasmic fabrics, redoximorphic features, and dissolution and alteration of feldspars and mafic minerals to kaolinite, gibbsite, and Fe oxides, as well as geochemical evidence, such as whole-rock losses of Na, Ca, Mg, Si, Sr, Fe, and Mn greater than in modern profiles. Evidence of diagenesis includes net additions of K, Ba, and Rb determined through geochemical mass balance, K-feldspar overgrowths in overlying sandstone sections, and K-feldspars with reaction rims in weathered basement. The sub-Cambrian paleoweathering profiles formed on granite are remarkably similar to modern weathering profiles formed on granite, in spite of overprinting by potassium diagenesis. ?? 2007 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS2007

Eruption and deposition of the Fisher Tuff (Alaska)--Evidence for the evolution of pyroclastic flows

Recognition that the Fisher Tuff (Unimak Island, Alaska) was deposited on the leeside of an &sim;500&ndash;700‐m‐high mountain range (Tugamak Range) more than 10 km away from its source played a major role in defining pyroclastic flows as momentum‐driven currents. We reexamined the Fisher Tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features. We studied the tuff at 89 sites and sieved bulk samples from 27 of those sites. We find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume (22 km) that deposited &sim;0.2 km 3 of dacite magma as a pyroclastic fall layer to erupting &sim;10&ndash;100 km 3 of andesitic magma as Scoria‐rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera, including those in valleys within the Tugamak Range. The distribution of the flow deposits and their welding, internal stratification, and the occurrence of lithic breccia all suggest that the pyroclastic flows were fed from a fountaining column that vented from an inclined conduit, the first time such a conduit has been recognized during a large‐volume caldera eruption. Pyroclastic flow deposits before and after the mountain range and thin veneer deposits high in the range are best explained by a flow that was stratified into a dense undercurrent and an overriding dilute turbulent cloud, from which deposition before the range was mainly from the undercurrent. When the flow ran into the mountain range, however, the undercurrent was blocked, but the turbulent cloud continued on. As the flow continued north, it restratified, forming another undercurrent. The Fisher Tuff thus records the passing of a flow that was significantly higher (800&ndash;1100 m thick) than the mountain range and thus did not require excessive momentum.

Alaska↗
USGS2008

Book review of Avalanche Dynamics by Shiva P. Pudasaini and Kolumban Hutter. Springer: Berlin-Heidelberg, 2007. 602 pages, 225 figures, 15 tables

This highly specialized book is interesting not only because of its important subject matter but also because of its egocentric perspective. The majority of the book provides a nearly exhaustive retrospective of the authors’ many contributions to the shallow‐flow theory of granular avalanches, and it also critiques contributions by others. Indeed, some readers (including this reviewer) might be dismayed to find their own contributions critiqued inaccurately! This brief review will not attempt to correct inaccuracies, however, but will emphasize the book’s main features.

Journal of Geology↗
USGS2009

Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California

The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time. ?? 2009 by The University of Chicago.

Journal of Geology↗
USGS1952

Origin of the Mima Mounds, Thurston County region, Washington

There has been recent favorable consideration of the idea that the Mima mounds were made by gophers. The writer believes the evidence indicates that gophers function only in the reworking of the mound material, not in the primary construction. The plausibility of the earlier glacial or periglacial theory has been increased by recent knowledge of permafrost and of the deposits made by combined water and ice in cold climates. The gopher theory, as it has been applied to the Mima mounds, contains internal disharmonies and ignores significant field evidence supporting the earlier idea.

Journal of Geology↗
USGS1957

On the postglacial history of the Devils Lake Region, North Dakota

Devils and Stump lakes in eastern North Dakota have been diminishing in area more or less continuously since the land around them was settled in the 1880's. Desiccations similar to the current one have occurred at least once and possibly two or more times in the past and are indicated directly and indirectly by tree stumps recently uncovered as the lake water receded and by lacustrine deposits containing buried soils and vertebrate remains. The lake levels seem to respond in a very sensitive manner to slight climatic changes. Probably the present desiccation and certainly the ones in the past are the result of changes toward a drier and warmer climate. The first may have been synchronous with the Thermal Maximum. A high abandoned strand line and associated lacustrine deposits containing buried soils and vertebrate remains indicate at least one period in the postglacial past, possibly following the Thermal Maximum, that was wetter and cooler than the present. A second desiccation, more definitely established and based on the radiocarbon dating of a tree stump, occurred less than 700 years ago, possibly during the sixteenth century.

North Dakota↗
USGS1981

Eolian features produced by the December 1977 windstorm, southern San Joaquin Valley, California

High wind velocities during a windstorm in December of 1977 in the southern San Joaquin Valley, California, created eolian features and effects of a magnitude not previously reported in the literature. Three principal mechanisms of sediment movement by wind were studied by examining the size distribution of deposits in telephone poles (saltation and suspension), feeder pens and abandoned buildings (saltation and suspension), and gravel ripples (surface creep). The maximum grain sizes of rocks lodged in telephone poles at heights of 240, 160, and 80 cm are 7 mm X 11 mm X 6 mm, 10 mm X 15 mm X 6 mm, and 20 mm X 23 mm X 5 mm, respectively. In some areas with rocky soils, lag materials formed gravel ripples.

Journal of Geology↗
USGS1981

Boulder deposits and the retreat of mountain slopes, or ' gully gravure' revisited

Observations on mountains composed chiefly of shale and capped with Tuscarora Sandstone in the Valley and Ridge province of southwest Virginia suggest that slopes retreat by a process similar to but different from Bryan's (1940) "gully gravure." The process appears to operate as follows: Bouldery alluvium protects the floors of hollows on the mountain flanks. In each hollow, the junction between the boulder-armored floor and the base of the unprotected shale side slope is the locus of greatest erosion. Runoff from rainstorms that have recurrence intervals of the order of 10 to 10² years incises gullies along the margins of the bouldery deposits, but lacks the competence to disturb the deposits. Periodically, however, runoff from catastrophic rainstorms that have recurrence intervals of the order of perhaps 10³ years mobilizes the bouldery alluvium and deposits it in the gullies. The armored floor thus shifts laterally. Subsequently, new gullies are incised adjacent to the old ones. The result is the lateral migration of one or both walls of the hollow. Eventually, ridges between hollows are destroyed, and valley floors covered with bouldery alluvium become new interfluves in an unending process that lowers mountain flanks. Evidence of this process includes the distribution of surficial deposits that show past topographic inversion of hollows and noses, correlation of surficial clast size with cross-slope variations in topography, and topographic comparisons between mountain flanks mantled and unmantled by Tuscarora boulders. This process probably is common wherever large resistant clasts are transported downslope over less resistant bedrock. Lateral migration of stream valleys has been shown to take place on mountain piedmonts under similar circumstances, but this study suggests that such migration may also take place on the upper slopes of mountains in hollows where water flow may be significant only a few times per century.

Journal of Geology↗
USGS1982

Sources, sinks and storage of river sediments in the Atlantic drainage of the United States

The history of sediment and its movement in the Atlantic drainage demonstrate some of the difficulties of modeling sediment on a river-basin scale. Soil erosion was accelerated by a factor of at least 10 when European settlers cleared forests and planted crops. Although increasing soil-conservation practice and decreasing crop farming have since reduced the rates of erosion, large quantities of eroded material are still stored on hillslopes and in stream valleys where they continue to augment the sediment loads of the rivers. The sediment from this episode of erosion that is largely past can be expected to emerge from storage for many decades and perhaps even several centuries to come. The reservoirs that have been built on many of the major rivers trap significant portions of the moving sediment which, in some places, may be remobilized by large floods. Essentially all the river sediment that reaches the Atlantic coastal zone is trapped in estuaries and coastal marshlands. Probably less than 5% is deposited on the floor of the continental shelf or the deep sea.

Journal of Geology↗