Imnaha Basalt, Columbia River Basalt Group

The Columbia River volcanic episode began with the eruptionof the coarsely porphyritic Imnaha Basalt between 17.0 and 16.5m.y. B.P. Lava poured from NNW trending vertical fissures andlocal vents north and south of the Seven Devils-Wallowa Mountainsdivide, covering a deeply dissected topographic surface of morethan 30, 000 km², with an estimated volume of 6000 km³. A minimumof 26 flows or flow units are represented in 14 or 15 members.These include 11 chemical types and are exposed in sectionsranging to 577 m in thickness. All flows have normal polaritywith the exception of the youngest and oldest whose polarityis either reversed or transitional. The petrologic and majorelement chemical features of the Imnaha Basalt have much incommon with those of the Picture Gorge Basalt exposed in theJohn Day Basin of north-central Oregon, but the latter is younger,equivalent in age to part of the Grande Ronde Basalt formation. Using major and trace elements, the flows of Imnaha Basalt areclearly distinguished from those of all other formations ofthe Columbia River Basalt Group. Imnaha Basalt has lower SiO₂,K₂O, Ba and Rb than does Grande Ronde Basalt and differs frommost Wanapum and Saddle Mountains Basalt flows in its lowerTiO₂ and P₂O₅ contents. The 11 Imnaha chemical types fall into two subgroups, the AmericanBar (AB) and Rock Creek (RC) subgroups, which differ in thecoarseness of their groundmass, the abundance of olivine, theirphenocryst assemblages, their SiO₂ contents, CaO/Al₂O₃ ratio,and in their Sc, V, Sr, and Ni contents. Flows of the two subgroupsinterdigitate, but AB flows are predominant at the base of thesequence and RC flows at the top. One flow is a hybrid of thetwo magma types. Neither subgroup displays a significant variation in SiO₂ content,but each does show systematic variation in K₂O, P₂O₅, TiO₂,Ba, Zr, Rb, and the REE, all of which vary inversely with MgO/(MgO+ FeO + Fe₂O₃). AB flows show a systematic increase in contentsof the incompatible elements upward in the succession, whileRC flows show a less obvious decrease upwards. Modelling of major and trace elements indicates that the chemicalvariations within each subgroup can be explained by simultaneouscrystal fractionation and assimilation of lower crustal material,in which the mass assimilated is only slightly less than thatlost by crystal fractionation; the mass fractionated varyingup to 50 per cent and the mass assimilated up to 42 per centof the original magma mass. These processes alone cannot explainthe relationships between the two Imnaha subgroups, nor thatbetween either subgroup and the overlying aphyric Grande Rondebasalt. The value of more complex quantitative models, in whichrecharge by more primitive magma, a variable composition forthe lower crustal contaminant, and the partial melting of aheterogeneous source, is limited by lack of data. Some suchprocess, or combination of processes in addition to a combinationof crystal fractionation and lower crustal assimilation, wouldseem to be required to account for the diversity in the earliestColumbia River basalts.     

Crystal Fractionation and Recharge (RFC) in the American Bar Flows of the Imnaha Basalt, Columbia River Basalt Group

Variable isotope and incompatible element ratios require multiplesources for the Columbia River Basalt Group (CRBG) as a whole,but smaller, stratigraphically coherent, groups of flows withinthe CRBG show much smaller variations in these ratios and appearto be derived from a single source. One such group of flows,the American Bar (AB) subgroup of the Imnaha Basalt, is examinedin detail. Sixty-nine samples from the six major AB flows havebeen reanalyzed for 23 major and trace elements in order tofurther constrain the processes responsible for the variationsbetween their compositions. It is shown that the compositional differences between the ABflows include the compatible behavior of Sr, which requiresthe participation of plagioclase, and can be accommodated ina model which fractionates a crystal assemblage of plagioclase,clinopyroxene, and orthopyroxene (16:10:4). Apparent differencesin the degree of fractionation between major elements ({smalltilde}30%) and trace elements ({small tilde}50%) are mainlyexplicable in terms of recharge accompanying the crystal fractionation(RFC), but some ambiguity in the enrichment factors for Y, P,and Zr (but not for Nb) remains unexplained and permits thepossibility of some crustal assimilation. The fractionation assemblage (plagioclase+augite?orthopyroxene)differs from the observed phenocryst suite (plagioclase+olivine+augite)and, in the absence of critical experimental data, is assumedto represent a higher pressure assemblage stable near the crust/mantleboundary. The physical model which has been developed to explainthe AB eruption entails a large magma reservoir in the lowercrust immediately above the crust/mantle boundary in which fractionationand recharge accompanied periodic eruption, probably withoutsignificant crustal assimilation either within the magma reservoiror during the rapid expulsion of magma to the surface. Chemical,isotopic, and mineralogic differences between coherent flowsubgroups (Rock Creek, American Bar, and the lower flows ofGrande Ronde Basalt, for example) are abrupt. They include changesin isotope and incompatible element ratios as well as changesin SiO₂ and K concentrations which require magma derivationfrom a different combination of sources.     

Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, ⁸⁷Sr/⁸⁶Sr=0.70307–0.70371, _Nd=+7.7-+4.8, ¹⁸O=+5.6±6.1, and ²⁰⁶Pb/²⁰⁴Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low ⁸⁷Sr/⁸⁶Sr, high _Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like ¹⁸O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher ⁸⁷Sr/ ⁸⁶Sr and ¹⁸O, lower _Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K₂O/TiO₂, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased     

A case of simple magma mixing in the Columbia River Basalt Group: The Wilbur Creek,Lapwai, and Asotin flows,Saddle Mountains Formation

Three small sequential Saddle Mountains flows, occupying similar areas on the Columbia Plateau, were erupted over a short interval of time. In the Lewiston Basin area the middle flow of the trio (Lapwai) has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty-five elements analyzed. Systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting. The Lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous. This implies efficient mixing between equal amounts of Asotin and Wilbur Creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption. The Wilbur Creek and Asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source (Carlson 1984). The mixing processes described here cannot be related to that earlier mantle/crust mixing process.     

Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks,South-Central Washington

More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger:

1. a)
   the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene).     
   
   

Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada

Ages of channels of the anastomosing upper Columbia River, south‐eastern British Columbia, Canada, were investigated in a cross‐valley transect by ¹⁴C dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long‐lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross‐valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long‐term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155–168). A long‐term (4550 years) average of 1·75 mm year^?1 (after compaction) was based on ¹⁴C dates, while a short‐term sedimentation rate of 0·8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short‐term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super‐ imposition) of active channels on recent channel‐fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.     

Isotopic and Geochemical Constraints on the Origin and Evolution of the Columbia River Basalt

New major and trace element data on over 70 samples are combinedwith a wider knowledge of the regional stratigraphy, and ofthe tectonic evolution of the boundary between the ColumbiaPlateau and the northern margin of the Basin and Range province,to distinguish three subgroups within the Columbia River BasaltGroup (CRBG): the Picture Gorge Basalt; the main sequence ofColumbia River flood basalts, here named the Clarkston Basalt;and the Saddle Mountains Basalt. The subgroups are characterizedby different incompatible element and Sr-, Nd-, and Pb-isotoperatios, and they are interpreted in terms of different sourceregions mobilized under different tectonic conditions. The majordifferences between the subgroups are consistent with partialmelting processes in the upper mantle, and it is argued thatthey reflect previous partial melting episodes which resultedin source regions that were variably enriched and depleted inincompatible elements. The major variations within the PictureGorge and Clarkston Basalt subgroups include increases in theabundances of large ion lithophile elements (LILE) and increasesin the ratios of LILE/high field strength elements (HFSE) whichare interpreted as the addition of a lithospheric/subduction-relatedcomponent. The Picture Gorge Basalt has a depleted isotopic and chemicalsignature on which is superimposed an enrichment of LILE toproduce a trace element pattern similar to that of other 17–0-Mabasalts erupted south of the Olympic Wallowa Lineament. Thispattern is characteristic of volcanism associated with the Basinand Range extensional province, and others have attributed itto a source component derived from an enriched subcontinentallithospheric mantle (SCLM). Of the Clarkston Basalts, the Imnaha and Grande Ronde Basaltsform chemical and isotopic arrays which indicate mixing of componentsfrom two distinct source regions. One had high ratios of LILE/HFSEand light rare earth elements (LREE)/HFSE, and as these arenot common in oceanic basalts, this component is thought tohave been derived from the continental mantle lithosphere. Itsisotope ratios are more enriched (older?) than those of thePicture Gorge Basalt, and its Rb/Sr ratios are much higher thanthose in its source rocks, consistent with preferential mobilizationof LILE at the time of magmatism. The second component was derivedfrom an asthenospheric source similar to that of Hawaii basaltsand is most obviously attributed to mantle plume activity. Basaltsof the Eckler Mountain and Wanapum Formations (smaller, separateformations of the Clarkston Basalt as redefined in this paper)fit this mixing model less well and may represent mixing betweenmore than two components. Flows of the third CRBG subgroup,the Saddle Mountains Basalt, also carry a lithospheric geochemicalsignature and have long been recognized as having more radiogenicisotopic signatures than the other two subgroups. Thus, SaddleMountains flows appear to require a lithospheric source enrichedin LILE at an even earlier time, and we concur with other workersthat the isotopic and chemical evidence implies their derivationfrom subcontinental lithospheric mantle enriched at {small tilde}2000Ma. Within each subgroup, the chemical effects of partial melting,fractional crystallization, and magma mixing processes can allbe distinguished within particular flow sequences. In the ImnahaBasalt variable degrees of partial melting during the generationof the CRBG magmas, and gabbro fractionation within the lowercrust, played major roles in their evolution. In the GrandeRonde Basalt fractional crystallization appears to be restrictedto >10%. The chemical and isotopic data for each CRBG subgroup, and thedifferent sources which those data imply, can be accommodatedin a tectonic model which includes the passing of the Yellowstonehotspot south of the center of the CRBG eruption before significantBasin and Range extension had moved north of the Brothers Faultzone at 15 Ma.     

Holocene landscape evolution and geoarcheology of low-order streams in the Rio Grande basin,San Juan Mountains,Colorado, USA

This geoarcheological study investigates soil stratigraphy and geochronology of alluvial deposits to determine Holocene landscape evolution within the Hot Creek, La Jara Creek, and Alamosa River drainage basins in the San Juan Mountains of Colorado. Geomorphic mapping and radiocarbon dating indicate synchronicity in patterns of erosion, deposition, and stability between drainage basins. In all three basins, the maximum age of mapped alluvial terraces and fans is ~ 3300 cal yr BP. A depositional period seen at both Hot Creek and the Alamosa River begins ~ 3300 to 3200 cal yr BP. Based on soil development, short periods of stability followed by alluvial fan aggradation occur in the Alamosa River basin ~ 2200 cal yr BP. A period of landscape stability at Hot Creek before ~ 1100 cal yr BP is followed by a period of rapid aggradation within all three drainages between ~ 1100 and 850 cal yr BP. A final aggradation event occurred between ~ 630 and 520 cal yr BP at La Jara Creek. These patterns of landscape evolution over the past ~ 3300 yr provide the framework for an archeological model that predicts the potential for buried and surficial cultural materials in the research area.     

Origin and differentiation of Triassic dolerite magmas,North Carolina,USA

Ten dolerite dikes intruded into Triassic fault troughs in the Piedmont area of North Carolina have been analyzed for the contents of major elements plus selected trace elements. The average composition of the initial magma, as indicated by four chill margins for major elements and three for trace elements, is: SiO₂, 48.6%; Al₂O₃, 16.9%; TiO₂, 0.57%; Fe₂O₃, 3.30%; FeO, 6.72%; MgO, 10.59; CaO, 10.42%; Na₂O, 2.03%; K₂O, 0.20%; MnO, 0.20%; Rb, 2.6 ppm; Sr, 133 ppm; Zr, 46 ppm; Th, 0.4 ppm; and U, below detection limit of approximately 0.1 ppm. One large dike (BP) exhibits a Palisades-type of differentiation by crystal settling of olivine, and the comparatively thick JY dike shows development of micropegmatite toward the center; the smaller dikes, however, are relatively homogeneous across their width. Study of the relationship between SiO₂ content and the ratio FeO+Fe₂O₃/MgO+ FeO+Fe₂O₃ indicates that most dikes crystallize under conditions of decreasing oxygen pressure, but the differentiation trend of the JY, RD, and RS dikes indicates either constant or increasing oxygen pressure during their evolution.Statistical comparison of the composition of the initial dolerite magmas with a variety of basalt types around the world suggests that the North Carolina dolerites are far more similar to oceanic or oceanic margin tholeiites than to continental tholeiites. The North Carolina rocks are distinctly different from plateau basalts but are similar to the chill zones of the Precambrian Bushveld and Stillwater lopoliths. The comparatively low contents of Th, U, and Sr, plus the relatively high K/Rb ratio all support the possibility that the magmas for the North Carolina dolerites evolved in a dominantly oceanic environment. It seems distinctly possible that continental-type crust and mantle did not exist in the Appalachian Piedmont area in Triassic time, even after major orogeny and the concurrent formation of granitic intrusions.     

The new Wallula CO<Subscript>2</Subscript> project may revive the old Columbia River Basalt (western USA) nuclear-waste repository project

A novel CO₂ sequestration project at Wallula, Washington, USA, makes ample use of the geoscientific data collection of the old nuclear waste repository project at the Hanford Site nearby. Both projects target the Columbia River Basalt (CRB). The new publicity for the old project comes at a time when the approach to high-level nuclear waste disposal has undergone fundamental changes. The emphasis now is on a technical barrier that is chemically compatible with the host rock. In the ideal case, the waste container is in thermodynamic equilibrium with the host-rock groundwater regime. The CRB groundwater has what it takes to represent the ideal case.     

Crystal clots,amphibole fractionation and the evolution of calc-alkaline magmas

Petrographic and microprobe investigations of calc-alkaline (CA) rocks from the High Cascade Range (i.e., Mt. St. Helens, Mt. Jefferson, Crater Lake and Mt. Shasta) of western North America show that crystal clots represent primary igneous phase assemblages and are not products of amphibole reactions with melt. For each eruptive complex, crystal clots display diverse modal proportions even within a single eruptive unit. Nevertheless, in all cases the crystal-clot minerals are also represented in the rock as phenocrysts or microphenocrysts. Basalts contain clots of ol+plag+mgt, ol+mgt, cpx+ plag+mgt, cpx+mgt and plag+mgt; andesites, clots of cpx+mgt, opx+mgt, cpx+opx+plag+mgt, cpx+plag+mgt, opx+plag+mgt and plag±mgt; and dacites, clots of opx+mgt, cpx+opx+plag+ mgt, opx+plag+mgt, amph+plag+mgt±ilm, amph+mgt±ilm and plag±mgt. The bulk compositions of most of these clot assemblages could not have been derived from amphibole percursors. Although some amphiboles in dacitic rocks display a breakdown reaction of amph=plag+cpx+opx +mag, these mineral clusters, unlike those of clots, typically have a relict amphibole crystal outline and a fine-grained metamorphic texture. Plagioclase grains in the mineral clusters lack oscillatory zoning which is typical of crystal clot plagioclase grains. The euhedral to subhedral shapes of most clot minerals and the oscillatory zoning present in most clot plagioclase grains are not likely to have formed from the breakdown of amphibole. Crystal clots are also observed in Hawaiian and ocean floor basalts, although amphibole fractionation has not been proposed for those lavas. Magnetite fractionation may be the controlling process limiting iron enrichment in CA magmas rather than amphibole fractionation. Textural evidence indicates that magnetite is an early-forming phase in CA magmas. V, which is concentrated in magnetite, shows a strong decrease with increasing silica in many CA rocks, supporting a magnetite fractionation model.Hawaii Institute of Geophysics Contrib. No. 969     

Reconstructing the ancestral (Plio-Pleistocene) Rio Grande in its active tectonic setting, southern Rio Grande rift, New Mexico, USA

Deposits of the ancestral Rio Grande (aRG) belonging to the Camp Rice Formation are preserved and exposed in the uplifted southern portion of the Robledo Mountains horst of the southern Rio Grande rift. The sediments are dated palaeomagnetically to the Gauss chron (upper Pliocene). The lower part of the succession lies in a newly discovered palaeocanyon cut into underlying Eocene rocks whose margins are progressively onlapped by the upper part. Detailed sedimentological studies reveal the presence of numerous river channel and floodplain lithofacies, indicative of varied deposition in channel bar complexes of low‐sinuosity, pebbly sandbed channels that traversed generally dryland floodplains and shifted in and out of the study area five times over the 1 Myr or so recorded by the succession. Notable discoveries in the deposits are: (1) complexes of initial avulsion breakout channels at the base of major sandstone storeys; (2) common low‐angle bedsets ascribed to deposition over low‐angle dunes in active channels; (3) palaeocanyon floodplain environments with evidence of fluctuating near‐surface water tables. Sand‐body architecture is generally multistorey, with palaeocurrents indicative of funnelling of initial avulsive and main fluvial discharge from the neighbouring Mesilla basin through a narrow topographic gap into the palaeocanyon and out over the study area. An avulsion node was evidently located at the stationary southern tip to the East Robledo fault during Gauss times, with aRG channels to the north flowing close to the fault and preventing fan progradation. Subsequent Matuyama growth of the fault caused (1) deposition to cease as the whole succession was uplifted in its footwall, (2) development of a thick petrocalcic horizon, and (3) fan progradation into the Mesilla basin. Parameters for the whole aRG fluvial system are estimated as: active single channels 2 m deep and 25 m wide; valley slope 0·24–0·065°; maximum mean aggradation rate 0·05 mm year^–1; major channel belt avulsion interval 200 ky; individual channel recurrence interval 100 ky; minimum bankfull mean flow velocity 1·54 m s^–1, minimum single‐channel discharge 77 m³ s^–1, bed shear stress 22·3 N m^–2; and stream power 34·3 W m^–2.     

Physical and chemical conditions of the formation and evolution of late pleistocene-holocene magmas of the Gorely and Mutnovsky volcanoes, southern Kamchatka

A detailed mineralogical and geochemical study of basic volcanic rocks from the modern edifices of the Gorely (Q₃⁴-Q₄⁴) and Mutnovsky (Q₃²-Q₄) volcanoes, as well as the results of numerical modeling with the COMAGMAT program, made it possible to estimate the role of fractional crystallization, the fluid regime, and geodynamic conditions in the petrogenesis of the studied basaltoids. The specific features of the evolution of magmas of the two volcanoes give grounds to suggest that beginning from the Late Pleistocene (Q₃⁴), all of the considered territory experienced a change in geodynamic regime, with an increasing role of extending strains in its evolution.     

Upstream control of river anastomosis by sediment overloading,upper Columbia River,British Columbia,Canada

Anastomosing rivers, systems of multiple interconnected channels that enclose floodbasins, constitute a major category of rivers for which various sedimentary facies models have been developed. While the sedimentary products of anastomosing rivers are relatively well‐known, their genesis is still debated. A rapidly growing number of ancient alluvial successions being interpreted as of anastomosing river origin, including important hydrocarbon reservoirs, urge the development of robust models for the genesis of anastomosis, to facilitate better interpretation of ancient depositional settings and controls. The upper Columbia River, British Columbia, Canada, is the most‐studied anastomosing river and has played a key role in the development of an anastomosing river facies model. Two hypotheses for the origin of upper Columbia River anastomosis include the following: (i) downstream control by aggrading cross‐valley alluvial fans; and (ii) upstream control by excessive bedload input from tributaries. Both upstream and downstream control may force aggradation and avulsions in the upper Columbia River. In order to test both hypotheses, long‐term (millennia‐scale) floodplain sedimentation rates and avulsion frequencies are calculated using ¹⁴C‐dated deeply buried organic floodplain material from cross‐valley borehole transects. The results indicate a downstream decrease in floodplain sedimentation rate and avulsion frequency along the anastomosed reach, which is consistent with dominant upstream control by sediment overloading. The data here link recent avulsion activity to increased sediment supply during the Little Ice Age (ca 1100 to 1950 ad ). This link is supported by data showing that sediment supply to the upper Columbia study reach fluctuated in response to Holocene glacial advances and retreats in the hinterland. Upstream control of anastomosis has considerable implications for the reconstruction of the setting of interpreted ancient anastomosing systems. The present research underscores that anastomosing systems typically occur in relatively proximal settings with abundant sediment supplied to low‐gradient floodplains, a situation commonly found in intermontane and foreland basins.     

Debris-flow simulations on Cheekye River, British Columbia

Cheekye River fan is the best-studied fan complex in Canada. The desire to develop portions of the fan with urban housing triggered a series of studies to estimate debris-flow risk to future residents. A recent study (Jakob and Friele 2010) provided debris-flow frequency-volume and frequency-discharge data, spanning 20-year to 10,000-year return periods that form the basis for modeling of debris flows on Cheekye River. The numerical computer model FLO-2D was chosen as a modelling tool to predict likely flow paths and to estimate debris-flow intensities for a spectrum of debris-flow return periods. The model is calibrated with the so-called Garbage Dump debris flow that occurred some 900  years ago. Field evidence suggests that the Garbage Dump debris flow has a viscous flow phase that deposited a steep-sided debris plug high in organics in centre fan, which then deflected a low-viscosity afterflow that travelled to Squamish River with slowly diminishing flow depths. The realization of a two-phase flow led to a modelling approach in which the debris-flow hydrograph was split into a high viscosity and low viscosity phase that were modelled in chronologic sequence as two separate and independent modelling runs. A perfect simulation of the Garbage Dump debris flow with modelling is not possible because the exact topography at the time of the event is, to some degree, speculative. However, runout distance, debris deposition and deposit thickness are well known and serve as a good basis for calibration. Predictive analyses using the calibrated model parameters suggest that, under existing conditions, debris flows exceeding a 50-year return period are likely to avulse onto the southern fan sector, thereby damaging existing development and infrastructure. Debris flows of several thousand years return period would inundate large portions of the fan, sever Highway 99, CN Rail, and the Squamish Valley road and would impact existing housing development on the fan. These observations suggest a need for debris-flow mitigation for existing and future development alike.     

Paragenetic evolution in Peejay Field,British Columbia,Canada

Triassic sediments from the Halfway Formation of Peejay Field in British Columbia, Canada, are described here with special reference to the mineral alterations during diagenesis. These sediments were deposited along the irregular coastline during the middle Triassic transgression. The vertical lithologic succession is complex and considerably altered. The major lithologies concerned are silty sandstones, dolarenite, dololutite, sporadic coquinoid, and accumulated shell debris largely composed of pelecypods, which have been diagenetically altered to dolomite. Evaporites were either introduced through solution and precipitation or by replacement. In certain sandstone units, intense corrosion to complete replacement of clastic quartz as well as matrix by dolomite is observed. The introduction or removal of minerals in the sediments during diagenesis may result from physico-chemical changes in the equilibrium between sediments and the interstitial fluid caused by burial. Hence, the study of diagenetic minerals and paragenesis may be an important tool for interpreting the environment of deposition and the postdepositional history.

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Zusammenfassung Triassische Sedimente der Halfway-Formation des Peejay Fields in British-Columbia, Canada, werden unter Berücksichtigung der Mineralumwandlungen während der Diagenese beschrieben. Diese Sedimente wurden während der Mittel-Triassischen Transgression entlang der unregelmäßigen Küstenlinie abgelagert. Die lithologische Folge ist komplex und beträchtlich verändert. Die Hauptgesteinsfolge besteht aus tonigen Sandsteinen, Dolareniten, Dololutiten, sporadischen Koguinoiden und Schalentrümmern, die meistens von Pelecypoden stammen und diagenetisch in Dolomit umgewandelt worden sind. Evaporite entstanden entweder durch Lösungszufuhr und Ablagerung oder Verdrängung. In einigen Sandsteinhorizonten weisen sowohl klastische Quarzkörner als auch die Grundmasse intensive Korrosionserscheinungen auf, die bis zur vollständigen Verdrängung durch Dolomit führen können. Das Eindringen oder Verschwinden von Mineralien in den Sedimenten während der Diagenese kann durch physikochemische Änderungen des Gleichgewichts zwischen Sedimenten und der miteingeschlossenen Porenlösung durch Sedimentüberdeckung verursacht werden. Somit kann die Untersuchung diagenetischer Mineralien und ihrer Paragenese ein wichtiges Mittel zur Deutung des Ablagerungsmilieus und der Geschichte nach der Ablagerung sein.

     

Controls on natural levée development in the Columbia River, British Columbia, Canada

Natural levées of the Columbia River near Golden, British Columbia, were investigated to identify the mechanisms that control levée development and morphology. Topographic profiles of 12 levée pairs were surveyed, and measurements of water-surface elevation, flow velocity, flow direction and turbidity were obtained during an average magnitude flood (1·2 years recurrence interval). Sedimentation rates and grain-size distributions were measured from sediment traps placed along levée-to-floodbasin transects. Results show that water and sediment exchange between the channel and floodbasin was mainly by advection. During flooding, local floodbasins behave more as efficient water pathways than water storage features, resulting in down-valley floodbasin flows capable of limiting basinward growth of levées. Levée shape results primarily from two independent factors: (1) maximum channel water stage, which limits levée height; and (2) floodbasin hydraulics, which control width. In the Columbia River, the competence of floodbasin flows results in relatively narrow and steep levées. Natural levées grow under two general conditions of deposition as governed by flood-stage elevation relative to levée-crest elevation: front loading and back loading. During large floods when crests are inundated, front loading preferentially aggrades the proximal portions of levées with sediment directly from the channel, thus increasing levée slope. During average or below-average floods when many levée crests are not overtopped, back loading preferentially aggrades the distal levée areas and floodbasin floor, reducing levée slope. In the study area, a balance between front and back loading sustains these narrow and steep levée shapes for long periods, reflecting an equilibrium between hydraulic regime, floodplain morphology and deposition.     

Basic magmas in the early evolution of the Southernmost Patagonian Batholith,Chile

Basic magmas played an important role in the early evolution of a segment of the Patagonian Batholith (141 to 34 m.y.) exposed south of Tierra del Fuego, Chile. The basic magmatism lasted 40 m.y. to possibly 60 m.y. (141 to 84 m.y.). It is represented by early gabbros and hornblendites (141 to 103 m.y.) and by synplutonic dikes and some of the numerous basic inclusions in the granitoids of a Cretaceous (113 to 84 m.y.) plutonic group. Some of the inclusions are interpreted as the disrupted portions of synplutonic dikes.Available data indicate that basaltic to rhyolitic volcanism which took place in the area during the Early Cretaceous was contemporaneous with the basic magmatism reported here and during its latest stage, with granitoid magmas. Therefore, gabbros apparently formed part of the roots of the island arc during the early stages of its evolution, whilst granitoids may have had a greater participation at a later stage in the root system.

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Zusammenfassung Basische Magmen spielten eine bedeutende Rolle bei der frühen Entwicklung eines Abschnittes des Patagonischen Batholithen (141-34 Mio. J.), der südlich Tierra del Fuego, Chile, aufgeschlossen ist. Der basische Magmatismus dauerte 40 bis etwa 60 Millionen Jahre (141-84 Mio. J.). Er besteht aus frühen Gabbros und Hornblenditen (141-103 Mio. J.) und aus synplutonischen Gängen und einigen der zahlreichen basischen Einschlüsse in dem Granitkomplex der kretazischen Plutonit-Gruppe (113-84 Mio. J.). Einige der Einschlüsse werden als isolierte Anteile von synplutonischen Gängen gedeutet.Die zur Verfügung stehenden Daten belegen, daß der basaltische bis rhyolitische Vulkanismus, der sich in diesem Gebiet während der frühen Kreide ereignete, gleichzeitig mit dem hier vorgestellten basischen Magmatismus und in dessen letztem Stadium zusammen mit granitischen Magmen ereignete. Daher waren offensichtlich Gabbros an den Wurzeln des Inselbogens während der frühen Entwicklungsstadien beteiligt, während Granotioide mit größerem Anteil an dem späterem Stadium des Wurzelsystems beteiligt waren.

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Resumen Magmas básicos tuvieron un importante rol en la historia temprana del segmento del Batolito Patagónico (141 a 34 M a) que se expone al sur de Tierra del Fuego, Chile. El magmatismo básico tuvo una duración de unos 40 Ma y hasta unos 60 Ma (141 a 84 Ma). Este magmatismo está representado por gabros y hornblenditas (141 a 103 Ma), por diques sinplutónicos y por algunas de las numerosas inclusiones básicas en los granitoides pertenecientes a un grupo plutónico del Cretácico (113 a 84 Ma). Algunas de las inclusiones se interpretan como partes de diques sinplutónicos.La información existente permite indicar que el volcanismo basáltico y riolítico que tuvo lugar en la región, durante el Cretácico Inferior, fue contemporáneo con el magmatismo básico del que se informa en este artículo. Aparentemente, la etapa final de este volcanismo habría sido coetáneo con un importante magmatismo granítico. En consecuencia, aparentemente, en las etapas iniciales de la evolución de la cadena volcánica, los gabros habrían constituído parte de su raíz, mientras que posteriormente habría habido mayor participación de granitoides.

     

Late Quaternary activity of the Pajarito fault, Rio Grande rift of northern New Mexico, USA

The Pajarito fault forms the western margin of the Rio Grande rift in north-central New Mexico, and lies adjacent to Los Alamos National Laboratory, a major Federal research facility. Vertical displacement on this normal fault over the past 1.2 Ma has created a 50- to 120-m-high fault scarp on Bandelier Tuff (1.2 Ma), yielding a long-term average slip rate of ca. 0.1 mm/yr. In support of a Laboratory-wide seismic hazards assessment, we excavated 14 trenches in the Pajarito fault zone to determine the age of the most recent displacement event, the recurrence interval between events, the displacement per event, and the variability in slip rate and recurrence through time. The large number of trenches was required by the large height of the fault scarp and the complexity of the fault zone. Only about half the trenches contained significant thicknesses of Holocene deposits, but in those trenches there was clear evidence for an early-to-mid-Holocene displacement event. The previous event was at least 20–40 ka, and the average recurrence interval over the past ca. 300 ka was about 20–40 kyr. We infer that much of the structural relief across this fault developed soon after eruption of the Bandelier Tuff between 1.0 and 1.2 Ma, and that slip rate slowed considerably after that time.     

Crystallization conditions of leucite-bearing magmas and their implications on the magmatological evolution of ultrapotassic magmas: the Vico Volcano, Central Italy

Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kd_ol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation. In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kd_ol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity. Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%) in-equilibrium (Kd_ol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure. Received November 28, 2000; revised version accepted September 27, 2001