the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

the bonneville estates rockshelter rodent fauna and changes in late pleistocene middle holocene climates and biogeography in the northern bonneville basin usa abstract excavations at bonneville estates rockshelter nevada recovered rodent remains from stratified deposits spanning the past ca 12 500 14c yr bp 14 800 cal yr bp specimens from horizons dating to the late pleistocene and early holocene include species adapted to montane and moist and cool habitats including yellow bellied marmot marmota flaviventris and bushy tailed woodrat neotoma cinerea shortly after 9000 14c bp 10 200 cal yr bp these mammals became locally extinct or nearly so taxonomic diversity declined and the region became dominated by desert woodrats neotoma lepida and other species well adapted to xeric low elevation settings the timing and nature of changes in the bonneville estates rodent fauna are similar to records reported from nearby homestead and camels back caves and provide corroborative data on terminal pleistocene early holocene environments and mammalian responses to middle holocene desertification moreover the presence of northern pocket gopher thomomys talpoides at bonneville estates adds to a sparse regional record for that species and similar to homestead cave it appears that the ca 9500 14c yr bp 10 800 cal yr bp replacement of the northern pocket gopher by botta s pocket gopher in the great salt lake desert vicinity was also in response to climate change

fluorapatite in the paleoproterozoic thelon basin structural stratigraphic context in situ ion microprobe u pb ages and fluid flow history fluorapatite cements have been documented at more than 18 localities in the thelon basin canada occupying stratigraphic positions from the paleosol below the unconformity to well within the upper parts of depositional sequence of the fluvial siliciclastic thelon formation ion microprobe analyses targeted using cathodoluminescence images to areas of higher uranium content on petrographically characterized cements provide the most precise pin on the timing of mesodiagenesis in the northeastern part of the thelon basin at 1667 5 ma expulsion of phosphate rich basinal brines was associated with development of faults and local breccias in the lower strata where extensive quartz cementation took place before the apatite event the new data do not support previous interpretations for multiple episodes of fluorapatite cementation within the basin despite the wide range of stratigraphic positions and variations in style of occurrence the tectonic cause remains speculative as does the relationship of the fluorapatite cements to the unconformity style uranium mineralization thelon fluorapatite cements are older than those reported in the athabasca basin suggesting distinct fluid flow histories occurred within the two basins the difference in timing indicates that the basinwide events that generated the fluids with the chemistry capable of producing fluorapatite cement took place independently but under similar conditions within the thelon and athabasca basins

geochemistry of an ophiolitic complex from new caledonia the ophiolites of new caledonia are composed of ultramafics overlain by mafic rocks all of which were affected by low p metamorphism the mafic rocks studied gabbroic cumulates and basaltic flows and dikes from montagne des sources are similar to recent mid ocean ridge rocks they are olivine normative with mg mg fe2 ratios ranging from 0 69 in lavas to 0 90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of ti and v with the mg fe ratio the lavas have a flat ree pattern with a slight depletion of light ree and a la yb ratio 2 the dikes have three different types of ree patterns the first type is nearly parallel to that of lavas the second one is enriched in lree la yb 4 and the third type with the lowest ree contents and a distinct lree depletion is similar to that of cumulitic pyroxene gabbro the variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of langmuir et al 1977 in agreement with structural and tectonic observations the geochemical data suggests that the ophiolites were formed during the spreading of a mid ocean ridge with a spreading half rate of about 1 cm year

timing of emplacement of the muskox intrusion constraints from coppermine homocline cover strata the muskox intrusion is a middle proterozoic shallow level mafic ultramafic layered intrusion in the northwest corner of the bear province of the canadian shield the intrusion was emplaced into the lower proterozoic akaitcho group and hepburn metamorphic plutonic complex of the wopmay orogen and overlying middle proterozoic coppermine homocline strata to date radiometric paleomagnetic and geochemical evidence has produced only equivocal results concerning the age of the muskox intrusion relative to overlying coppermine river group basalts and mackenzie diabase dykes stratigraphic redefinition of and structural relationships to the cover sequence of the intrusion provide the best available evidence for the age of the muskox intrusion although previously inferred to be a pre dismal lakes group structure the canoe lake fault is here shown to cross cut dismal lakes strata with west side down displacement equivalent to that documented to the south where it offsets the intrusion this relationship suppo

geochronology of the narakay volcanic complex implications for the age of the coppermine homocline and mackenzie igneous events u pb isotope analysis of four zircon separates from a rhyolite porphyry that is part of the bimodal narakay volcanic complex dease arm great bear lake yields an age of 1663 8 ma east trending feldspar phyric dykes that cut the complex yield zircons that are within analytical error the same age as the porphyry the narakay volcanic complex occurs in the upper part of the hornby bay group the lowermost group of the coppermine homocline and the stratigraphic position of the dated rocks indicates that 1663 ma is the depositional age of the upper hornby bay group this is the first u pb zircon age determined from rocks of the coppermine homocline and has important implications for previous geochronology and extrabasinal correlation earlier rb sr dates from the conformably overlying coppermine river group basalts of ca 1200 ma when taken together with the new data suggest that the dismal lakes group which conformably overlies the hornby bay group was deposited during a period of more than 400 ma

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

mid continent tectonic inversions northwest territories canada abstract four types of tectonic inversion are recorded on reflection seismic data from the area northwest of great bear lake in the northwest territories canada the strata involved span about 1 7 ga of proterozoic and phanerozoic history and have been subjected to at least five tectonic events two extensional two compressional and one transpressional two types of positive inversion one type of negative inversion and one type of double inversion positive following negative are documented one of the positive inversions is atypical in that it involves no reversal in tectonic polarity it was effected by a renewal of compression during the entire 1 7 ga time span the successive tectonic phases occurred in a mid continental setting far from any known active plate margin s

petrology and u pb geochronology of mafic high pressure metamorphic coronites from the tshenukutish domain eastern grenville province abstract a suite of mafic and ultramafic rocks from the tshenukutish domain eastern grenville province have been the subject of a detailed petrographic and geochemical study combined with u pb geochronology the tshenukutish domain is the highest tectonic level in the recently discovered eclogite bearing manicouagan imbricate zone the mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain the lac espadon segment and the baie du nord segment in the lac espadon segment cumulate textures and geochemical characteristics suggest that the units represent a layered mafic and ultramafic complex and an unrelated apatite rich fe ti mafic pegmatite nelsonite body in contrast the samples from the baie du nord segment represent fairly homogeneous fe ti gabbros and anorthosites the samples studied show abundant petrographic evidence for high pressure metamorphism for instance the widespread development of garnet clinopyroxene and kyanite bearing metamorphic assemblages estimates of the conditions of high pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750 900 c and 1600 1800 mpa the gabbroic end members from both segments suggest within plate tholeiite petrotectonic signatures ree patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source cumulate textures fractionation and inherited zircons suggest high crustal level emplacement of the magmatic protoliths in the tshenukutish domain in the lac espadon segment u pb ages of igneous zircon and baddeleyite give an age range of 1643 1629 ma for the emplacement of the layered complex the nelsonite in the les gives a poorly constrained igneous zircon age of 1596 126 99 ma in the baie du nord segment baddeleyite gives an age of 1170 5 ma for emplacement of the fe ti gabbros this is the youngest intrusion in the td and thus gives a maximum limit for the onset of grenvillian metamorphism ages of high p metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated fe ti gabbro margins 1046 3 ma overgrowth of igneous zircon 1042 22 28 ma in the nelsonite saccharoidal zircon replacing baddeleyite 1030 10 7 ma and 1030 12 ma and new growth of equant zircon 1012 12 ma from the undeformed interior of the gabbros these ages indicate that the high p metamorphism of the tshenukutish domain was late grenvillian ottawan retrogression to amphibolite facies is constrained by the ages of titanite 1006 997 ma in deformed gabbro margins rutile ages range between 960 and 929 ma and are interpreted as cooling ages the difference between the ages of titanite and rutile suggest slow cooling of the tshenukutish domain after initial high pressure and temperature metamorphism and exhumation

early proterozoic orogeny and exhumation of wernecke supergroup revealed by vent facies of wernecke breccia yukon canada in the yukon the oldest known supracrustal succession the wernecke supergroup was deposited in a marine basin before 1 71 ga the earliest orogenic event to disturb these strata was the racklan orogeny which produced folds and fabrics at peak temperatures of 450 550 c these features and those of the correlative forward orogeny are recognized at the surface and in the subsurface throughout much of northwestern canada zones of wernecke breccia hydrothermal breccias 1 60 ga were emplaced into the wernecke supergroup after racklan deformation and metamorphism two main types of breccia are recognized grey sodic breccias and colourful potassic breccias in the slab mountain area a belt of grey breccias contains abundant megaclasts of country rock including blocks of a subaerial lava succession the slab volcanics these grey breccias are interpreted as a vent facies of wernecke breccia and their emplacement into the stratigraphically lowest unit of the wernecke supergroup infers that at least 9 km of exhumation occurred in the core of a major racklan anticline prior to brecciation the slab volcanics are preserved only as clasts in wernecke breccia and are interpreted as fragments of a former valley filling basalt succession which overlay deformed and deeply incised strata of the wernecke supergroup resume au yukon la plus ancienne succession supracrustale connue le supergroupe de wernecke a ete deposee dans un bassin marin avant 1 71 ga l evenement orogenique le plus precoce pour distribuer ces strates a ete l orogene racklan qui a produit des plis et des textures a des temperatures de pointe de 450 550 c ces caracteristiques et celles de l orogene correlative forward sont reconnues a la surface et sous la surface dans une grande partie du nord ouest du canada les zones de breches de wernecke des breches hydrothermales 1 60 ga ont ete mises en place dans le super groupe de wernecke apres la deformation et le metamorphisme de l orogene racklan deux principaux types de breches sont reconnus des breches sodiques grises et des breches potassiques colorees dans la region du mont slab une ceinture de breches grises contient beaucoup de megaclastes de la roche encaissante incluant des blocs d une succession de laves subaeriennes les volcaniques slab ces breches grises sont interpretees comme etant un facies d event des breches de wernecke et leur mise en place dans l unite stratigraphique la plus basse du supergroupe de wernecke porte a croire qu au moins 9 km d exhumation a eu lieu dans le centre d un grand anticlinal de l orogene racklan avant la brechification les volcaniques slab sont preservees uniquement en tant que clastes dans les breches de wernecke et elles sont interpretees comme etant des fragments d une succession anterieure de basalte qui remplissait les vallees et qui recouvrait les strates deformees et fortement incisees du supergroupe de wernecke

a middle holocene vegetation record from the mojave desert of north america and its paleoclimatic significance abstract packrat middens from the mccullough range in the southeastern mojave desert of nevada record vegetation conditions during the periods 6800 to 5060 yr b p and 1250 yr b p to present their age and location are ideal to test for middle holocene aridity the period between 6800 and 5060 yr b p does appear to have been more arid than during the late holocene period of record this agrees with other evidence for middle holocene aridity from the southwest arguments for strengthened middle holocene monsoons in the region are based chiefly on packrat midden samples older than 7500 yr b p or on samples that are younger than 5500 yr b p simulations of 6000 yr b p climate that fail to show strong subtropical flow into the southwest are therefore supported by evidence for enhanced aridity in that region

diagenetic fluorapatite and aluminum phosphate sulphate in the paleoproterozoic thelon formation and hornby bay group northwestern canadian shield in the northwestern part of the canadian shield fluorapatite and a rare earth element bearing hydrated aluminum phosphate sulphate mineral aps occur as cements in continental successions near the base of the paleoproterozoic thelon formation thelon basin and hornby bay group hornby bay basin these minerals occupy interstitial sites form euhedral crystals display micro scale zonation make up part of an unmetamorphosed paragenetic assemblage and are distributed in correlative units across thousands of square kilometres suggesting a diagenetic origin stratigraphy geochronology and other lines of evidence suggest that the thelon formation and hornby bay group containing these phosphatic cements as well as the ellice formation and athabasca group are correlative and may have been originally interconnected the evidence suggests that the basal thelon formation and the hornby bay group underwent similar and approximately coeval diagenetic mineral paragenesis furthermore the diagenetic fluid

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

fault reconstructions using aeromagnetic data in the great bear magmatic zone northwest territories canada the great bear magmatic zone located in wopmay orogen is a 1 875 1 84 ga belt 450 km long by 100 km wide of volcanic and allied plutonic rocks interpreted as a paleoproterozoic magmatic arc the

el nino controls holocene rabbit and hare populations in baja california abstract the el nino southern oscillation enso is a major source of climatic variation worldwide with significant impacts on modern human and animal populations however few detailed records exist on the long term effects of enso on prehistoric vertebrate populations here we examine how lagomorph rabbit and hare deposition rate population age structure and taxonomic composition from abrigo de los escorpiones a well dated trans holocene vertebrate fauna from northern baja california mexico vary as a function of the frequency of wet el nino events and eastern pacific sea surface temperatures ssts derived from eastern pacific geological records faunal indices vary significantly in response to el nino based precipitation and sst with substantial moisture driven variability in the middle and late holocene the late holocene moisture pulse is coincident with previously documented changes in the population dynamics of other vertebrates including humans as the frequency and intensity of enso is anticipated to vary in the future these results have important implications for change in future vertebrate populations

proterozoic flood basalts from the coppermine river area northwest territories isotope and trace element geochemistry the voluminous proterozoic continental tholeiites of the coppermine river province northwest territories which are coeval with the mackenzie magmatic event 1 27 ga old and were emplaced over a short period of time 5 ma have trace element and sr nd pb isotopic characteristics comparable to those of phanerozoic flood basalts the variations of the compositional parameters are inferred to be due to a mixing of at least two components mantle and crust in addition the mantle component probably includes two end members the first is a mantle plume whereas the other represents the base of the continental lithosphere the crustal component reflects contamination of the magma by precambrian basement during its ascent to the surface

reconstructing sea level change from the internal architecture of stromatolite reefs an example from the mesoproterozoic sulky formation dismal lakes group arctic canada the mesoproterozoic dismal lakes group arctic canada contains a relatively thin yet regionally extensive stromatolitic reef complex that developed subtidally during a major transgression shoaled to sea level and was overlain by intertidal to supratidal carbonate and evaporite strata the september lake reef complex exhibits a complex internal architecture that records the interaction between stromatolite growth and changes in accommodation space derived from both higher order 4th or 5th order parasequence scale changes in sea level and the variable bathymetry of the sea floor reef growth which was initiated during three sea level cycles records progressive marine transgression over depositional lows that were formed during pre reef subaerial exposure and erosion of the underlying strata a fourth sea level cycle represented by spectacular coniform stromatolites with 10 m of synoptic relief marks a more dramatic rise in sea level and establishment of the main reef complex aggradation and ev

erratum superimposed low grade metamorphism in the mount fisher area southeastern british columbia implications for the east kootenay orogeny middle proterozoic 1500 1350 ma belt purcell strata exposed in the purcell and southwestern rocky mountains were affected by at least three distinct episodes of deformation and regional metamorphism the oldest episode 1300 1350 ma apparently terminated belt purcell sedimentation and involved foiding regional metamorphism and granitic intrusion the second episode 800 900 ma occurred during deposition of the windermere supergroup and involved uplift block faulting and low grade regional metamorphism mesozoic cenozoic metamorphism deformation and plutonism overprinted the results of the earlier deformation and metamorphism illite crystallinity and muscovite polymorph ratios indicate that purcell strata in the mount fisher area are in the lower greenschist to prehnite pumpellyite facies of regional metamorphism in the steeples and fisher blocks this metamorphism is related to structures that formed during the late cretaceous paleocene deformation however in the sand creek block the regional metamorphism is related to the development of a spaced cleavage that is folded by a late cretaceous pdeocene nappe regional considerations suggest that this cleavage formed during the 1300 1350 ma episode of deformation and metamorphism the east kootenay orogeny as currently defined embraces the two older episodes of tectonism it is proposed that the term east kootenay orogeny be restricted to designate the 1300 1350 ma episode and that the term goat river orogeny designate the 800 900 ma episode of tectonism the east kootenay and goat rivet orogenies appearto be correlative with the racklan and hayhook orogenies recognized in the northern canadian cordillera

isotope and trace element geochemistry of proterozoic natkusiak flood basalts from the northwestern canadian shield abstract the proterozoic continental tholeiites of natkusiak province 723 ma old of the northwestern canadian shield are part of the franklin magmatic event they resemble phanerozoic flood basalts in their incompatible trace element and sr nd pb isotopic variations as well as in their large volumes and a relatively short period of eruption the wide variations in sr nd pb isotopic and incompatible trace element ratios e g initial nd 5 6 to 5 7 in the natkusiak basalts together with the correlations between the isotopic and trace element ratios imply the involvement of a crustal component in their genesis the natkusiak basalts are inferred to be derived from the subcontinental upper mantle and contaminated by precambrian basement during their ascent

mechanical models for dikes a third school of thought abstract geological and geophysical data from continental volcanic centers and giant radial swarms and from oceanic shield volcanoes and rift zones indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height what traps dikes within the shallow lithosphere and promotes these highly eccentric shapes gravity induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms pressure gradients due to magma flow are secondary to insignificant because the flow direction is dominantly horizontal this configuration of vertical blade shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes in school i a dike is disconnected from its source and ascends under the influence of buoyancy in school ii a dike is connected to a magma reservoir and is driven upward by magma flux from the source we review the geological and geophysical data supporting the vertical dike horizontal flow propagation configuration and suggest the abundance and veracity of these data in many different geological settings and the modeling results that address this physical process warrant adding this as a third school of thought a new analytical solution for the boundary value problem of a homogeneous isotropic and linear elastic solid with a vertical fluid filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution dike tip stress intensity and stable height model results indicate that in a homogeneous crust dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness k ic 100 mpa m 1 2 however density stratification of the crust is an effective mechanism for trapping kilometer scale dikes even if the host rock is very weak k ic 0 this analysis may explain why vertical dikes propagate laterally for great distances but reside within a narrow range of depths in the crust

late wisconsin early holocene vegetation in the bonneville basin abstract woodrat middens from the northern bonneville basin allow a reconstruction of vegetation changes from 14 000 to 9000 yr ago cold montane steppe dominated by sagebrush covered much of the western bonneville basin prior to 13 000 yr ago from 13 000 10 800 yr ago the region was vegetated by limber pine woodlands in lower montane settings and a mosaic of limber pine and sagebrush steppe along basin flour margins these low elevation limber pine woodlands began to retreat upslope after about 11 000 yr ago due to increasingly drier climatic conditions and were replaced by relatively more xeric desert scrub dominated by sagebrush and shadscale the growth of limber pine at low elevations suggests that summer temperatures were as much as 6 c lower than at present this evidence is in apparent conflict with the currently accepted post provo lake bonneville chronology especially the magnitude of the postulated near dessication of lake bonneville from ca 13 000 12 200 yr ago

holocene bison in the great basin western usa bison bison bison were widespread in the eastern and northern parts of the great basin the area of internal drainage in the arid western usa during the late holocene particularly after 1600 14

decimeter scale mapping of carbonate controlled trace element distribution in neoarchean cuspate stromatolites abstract ancient stromatolites can provide key insights into the early evolution of life on earth neoarchean fenestrate stromatolites from the 2520 ma upper nauga formation transvaal supergroup south africa preserve cuspate morphologies they possess clearly delineated support and drape structures interpreted as dolomitized microbial mat material petrographic observations show that the biogenic structures are composed of planar s to non planar ferroan dolomite encased in ferroan calcite including herringbone calcite textures the cuspate stromatolites were analyzed using synchrotron rapid scanning x ray fluorescence srs xrf and more conventional techniques to determine i whether element distributions could be distinguished in ancient stromatolites at both cm to dm scales ii whether element distributions show variation between biogenic and abiogenic textures and iii the sample s paragenesis the distributions of ca fe mn pb cu as br al si p and s directly correspond to dolomitized stromatolitic structures and show trace element distributions are principally controlled by calcite and dolomite occurrence dolomite formation was mainly driven by seawater derived fluids given the high concentrations of fe and retention of marine shale normalized rare earth element and yttrium reysn patterns however the spatial association of dolomite to stromatolite structures may reflect microbially influenced mineral nucleation given the complexity of this sample s paragenetic evolution trace metal distributions cannot be conclusively tied to specific metabolic pathways bioaccumulation or passive binding however the results show srs xrf can be used for quantifiable spatial in situ investigation of ancient microbialites

paleobiogeographic changes at the pleistocene holocene boundary near pintwater cave southern nevada abstract in 1996 approximately 70 000 mammal and lizard bones were recovered from pintwater cave in the northern mojave desert of southern nevada these bones date between 32 000 and 7350 14c yr b p between 32 000 and 10 100 14c yr b p the local fauna consisted of a mix of xeric and cool mesic adapted species ochotona princeps and thomomys talpoides then occupied the region although these animals were extirpated by the onset of the middle holocene sauromalus obesus and dipodomys deserti probably migrated to the region during the latest pleistocene dipsosaurus dorsalis entered the pintwater cave record after 8000 14c yr b p consistent with climatic interpretations for the northern great basin these data suggest a cool and moist latest pleistocene climate for the northern mojave desert in contrast to the northern great basin however this region experienced predictable summer precipitation coupled with increasingly warmer winters by 10 100 14c yr b p in both regions the warm middle holocene began ca 8300 14c yr b p however whereas the northern great basin probably experienced warm and dry conditions at that time the northern mojave desert remained warm with relatively predictable summer precipitation the modern northern mojave desert biota probably was not established until after 8300 14c yr b p

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

structural morphology and associated strain in the central cordillera british columbia and washington evidence of oblique tectonics the central cordillera developed during early jurassic to late cretaceous time as the continental edge was subducted under the exotic terranes of quesnellia and stikinia the earliest jurassic structures formed during prograde metamorphism and include cylindrical and double verging folds that trend subparallel to the orogen these structures are associated with mineral lineations and stretch values between 120 and 400 parallel to the fold axes later disharmonic structures formed during the emergence of the crust after the metamorphic peak and in response to decreasing p t conditions present tectonic models do not incorporate the kinematics capable of forming the early structures and thereby only address the emergent phase a scenario is offered which involves oblique subduction of the craton edge and which is kinematically compatible with the early structural morphology and associated strains

unroofing incision and uplift history of the southwestern colorado plateau from apatite u th he thermochronometry the source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics the 1 9 km uplift of the colorado plateau since the late cretaceous is a prime example of this problem we used apatite u th he thermochronometry 230 analyses 36 samples to provide the fi rst single system regional scale proxy for the unroofi ng history of the southwestern quadrant of the plateau the results confi rm overall southwest to northeast unroofi ng from plateau margin to plateau interior a single phase of unroofi ng along the plateau margin in late cretaceous to early tertiary sevierlaramide time contrasts with multiphase unroofi ng of the southwestern plateau interior in early and mid to late tertiary time the early cretaceous was characterized by northeastward tilting and regional erosion followed by aggradation of 1500 m of upper cretaceous sediments along the eroded plateau margin sevier laramide denudation affected the entire southwestern plateau was concentrated along the plateau margin and migrated from northwest to southeast following a period of relative stability of the landscape from ca 50 30 ma signifi cant unroofi ng of the southwestern plateau interior occurred between ca 28 and 16 ma additional denudation north of the grand canyon took place in latest tertiary time mid tertiary dates from the grand canyon basement at the bottom of the upper granite gorge limit signifi cant incision of the modern grand canyon below the kaibab surface to 1500 m difference in vertical structural position if these models are correct they indicate that a proto grand canyon of kilometer scale depth had incised post paleozoic strata by the early eocene evidence for kilometerscale mid tertiary relief in northeast fl owing drainages along the plateau margin as well as the mid tertiary episode of plateau interior unroofi ng imply that the southwestern plateau interior had attained substantial elevation by at least 25 20 ma if not much earlier these observations are inconsistent with any model calling for exclusively late tertiary uplift of the southwestern plateau sevier laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere expanding the cordillera s orogenic highlands into its lowstanding cratonic foreland the onset of the laramide slab s demise at ca 40 ma and the major pulse of extension in the basin and range from ca 16 10 ma appear to have had little infl uence on the denudation history of the southwestern plateau in contrast the post laramide unroofi ng episodes may be explained by drainage adjustments induced by rift related lowering of regions adjacent to the plateau without the need to otherwise modify the plateau lithosphere our data do not preclude a large component of post early eocene elevation gain or the geodynamic mechanisms it may imply but they do point toward laramide age buoyancy sources as the initial cause of signifi cant surface uplift ending more than 500 m y of residence near sea level

terminal pleistocene change in mammal communities in southeastern washington state usa abstract small mammal communities in western north america experienced declines in taxonomic richness across the late pleistocene to holocene transition pht a recent natural global warming event one community also experienced a decline in evenness and others replaced one species with a congener variability in response of small mammal communities to pht warming is apparent at the presently arid and xeric marmes site in the columbia basin of southeastern washington state megafauna were absent by about 13 000 cal yr bp evenness of small mammals declined about 11 700 cal yr bp and again about 11 400 cal yr bp whereas richness declined about 11 400 cal bp regional faunal turnover was however minimal among small bodied taxa local mammal communities are depauperate as a result of megafaunal extinctions and subsequent decreases in small mammal richness and evenness the latter chronologically corresponds with a decrease in primary productivity driven by increasing warmth and aridity more faunas must be studied in order to fully document the range of variability in the responses of mammalian communities to pht warming documentation of patterns in those responses will facilitate understanding and enhance predictive accuracy with respect to responses of mammalian communities to modern global warming

early proterozoic geomagnetic field in western laurentia implications for paleolatitudes local rotations and stratigraphy abstract the east west trending post orogenic subvertical cleaver dykes from the great bear magmatic arc are dated by the u pb baddeleyite method to be 1740 5 4 ma they have a mean direction of magnetization of d i 136 5 and 57 4 k 64 95 4 5 based on samples from 17 diabase dykes a mean location 65 7 n 118 03 w and a corresponding paleopole at 19 4 n 83 3 w a95 6 1 studies of baked contacts country rocks distant from dykes and later intrusions that cut them show that the dykes became magnetized soon after intrusion the paleopole is not significantly different from that obtained from the 1759 1 ma post orogenic jan lake granite from the hudsonian orogen in southeast saskatchewan these paleopoles fall among a cluster previously determined from post orogenic overprints in a wide variety of rocks and geological settings across laurentia their mean is 20 8 n 94 5 w a95 5 2 collectively they record a widespread remagnetization episode following the hudsonian orogeny during which the polarity of the field remained constant this single polarity interval the cleaver superchron has a mean age of approximately 1747 ma and its duration is at least 13 million years but is probably much longer with further refinement it could become a global statigraphical marker there are 18 paleopoles from red beds and igneous rocks which record the paleofield in western laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 ma during the hudsonian orogeny over this interval western laurentia remained at a low and essentially constant latitude reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency there are numerous magnetochrons which could in future assist in regional stratigraphic correlations on the basis of six studies we estimate that the reference paleopole for this interval is at 1 1 s 71 7 w a95 12 1 n 6 paleopoles from the other 12 studies are spread over a 100 arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering archean slave and superior cratons during the hudsonian orogeny these reference paleopoles show that during the hudsonian orogeny western laurentia was in paleolatitudes of 15 30 during post hudsonian time ca 1750 ma paleolatitudes were about 20 higher paleowind directions inferred by others indicate that laurentia during these intervals was in the northern hemisphere

timing of colorado plateau uplift initial constraints from vesicular basalt derived paleoelevations we have developed a technique for measuring paleoelevation on the basis of the vesicularity of basaltic lava flows we are currently applying this technique to determine the timing and extent of cenozoic uplift of the colorado plateau because the technique measures paleoatmospheric pressure it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness the modal size of the vesicle bubble population is larger at the top than at the bottom this leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field and the vesicle sizes can be measured in the laboratory on the colorado plateau volcanic fields are generally found around the margins of the plateau basaltic lavas range in age from 0 to 23 ma present elevations range from 1 7 km to 3 3 km samples were collected from lava flows in the areas of marysvale springerville grand mesa and the san juan mountains and analyzed to determine the timing and extent of uplift the analysis of the samples using x ray computed tomography imaging and our numerical techniques for determining vesicle population statistics indicate uplift of 350 2200 m depending on age there is a clear relationship between age and amount of uplift original elevation subtracted from present elevation the results indicate slow uplift of 40 m m y between 25 ma and 5 ma only 800 m of uplift during that time and rapid uplift of 220 m m y since 5 ma 1100 m during that time these results reconcile the long standing controversy between interpretations of ancient versus recent uplift by providing an uplift history curve for the colorado plateau

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

a sedimentary overlap assemblage links australia to northwestern laurentia at 1 6 ga abstract the columbia nuna supercontinent existed from approximately 1 9 ga to 1 3 ga laurentia was part of columbia and the western edge of laurentia current coordinates was likely proximal to a large landmass during parts of this interval here we present detrital zircon ages of a paleoproterozoic sedimentary succession in northern yukon canada that bear on the evolution of columbia the sedimentary succession is preserved as clasts within 1 60 ga hydrothermal megabreccias analyses of detrital zircon reveal abundant 1 78 1 68 ga zircon with evolved hf isotope values 16 1

palaeovolcanology and tectonic setting of a proterozoic metatholeiitic sequence near the baltic shield margin northern norway abstract the kvenvik greenstone formation 1500 m thick is a very well preserved 2 0 1 8 ga old sequence of morb type tholeiitic metabasaltic lavas and volcaniclastic rocks they were deposited as cyclically repeated couplets in a continental shallow water to terrestrial environment near the margin of the baltic shield the effusive facies comprise massive and amygdaloidal lava pillow lava pillow breccia and hyaloclastite volcaniclastic facies are ash and accretionary lapilli tuff lapilli tuff and tuffaceous sedimentary rocks lithofacies associations and primary structures in the tuffs indicate air fall base surge and pyroclastic flow deposits with little input of epiclastic material the prevalence of shallow water to subaerial emplacement throughout the kvenvik greenstone formation and correlative sequences to the south indicates deposition in a spasmodically subsiding basin almost certainly formed by taphrogenic tectonism the rifting may have been incidental to coeval plate convergence tectonism evidenced by calc alkalic magmatism in the repparfjord komagfjord area 50 100 km northeast of the kvenvik area

a proposed laramide proto grand canyon abstract the absence of rim gravels north of grand canyon and of canaan peak type gravels south of grand canyon suggests that a paleocanyon which intersected the transport of these gravels north and south may have begun forming in the laramide in approximately the same position as today s central grand canyon this laramide age canyon is envisioned as having flowed generally from the sw to ne from the peach springs canyon area to mile 197 where it was captured by karst then along a n 60 e joint system to the kanab point area where it converged with drainage coming off the west side of the kaibab arch from there it flowed north along the west flank of the kaibab arch to paleogene lake claron the critical idea suggested by this proposed model is that the modern colorado river utilized laramide paleotopography in establishing its course through the central grand canyon with younger sections of the canyon integrating with it later in the middle to late miocene this paleocanyon route in association with headward erosion from the grand wash cliffs toward the kaibab arch after 16 17 ma helps account for the total volume of rock eroded from grand canyon which cannot be explained by present day incision rates

origin of thick first cycle quartz arenite successions evidence from the 1 7 ga ortega group northern new mexico abstract the 1700 ma old ortega group in northern new mexico represents a 1000 m thick succession of metamorphosed quartz arenites and subordinate pelites published ages of detrital zircon grains from the ortega group indicate that the sediments were derived mainly from 1760 1700 ma old volcano plutonic basement rocks and not from the archean wyoming province to the north evidence for recycling within the ortega group also is lacking consequently the ortega group quartz arenites are considered to be first cycle sediments the compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering abnormally high al contents of pelites within the ortega group imply intense chemical weathering in the provenance similar to present day low relief tropical regions quartz arenites in the ortega group were deposited exclusively within a shallow marine shelf environment influenced by tide storm and wave processes prolonged sediment transport especially on the tide dominated inner shelf in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains although the maturity of sediments in the ortega group can be accounted for the mode of origin of 850 m of quartz arenites remains enigmatic thick quartz arenite sequences of global distribution in the phanerozoic coincide with first order eustatic sea level rises world wide occurrences of 1700 ma old thick quartz arenite shelf successions suggest that the ortega group may have accumulated in response to a similar major eustatic sea level rise

magmatic evolution of the southern great bear continental arc northwestern canadian shield geochronological constraints the calc alkaline great bear continental arc in the wopmay orogen developed after a collision ca 1890 ma of the archean slave craton with the paleoproterozoic hottah terrane to the west u pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development i intrusion of a few small sodic leucogranite plutons at 1873 2 ma into a previously folded metasedimentary sequence ii abundant calc alkaline felsic volcanism and subvolcanic intrusions during the period 1870 1866 ma bracketed by five ages iii intrusion of large calc alkaline granitic plutons including the marian river batholith dated by zircon and titanite from two samples at 1866 2 3 ma and iv emplacement of the potassic faber lake rapakivi granite at 1856 2 3 ma the arc was developed on the hottah terrane due to easterly subduction of an oceanic plate under the amalgamated slave craton hottah terrane the oldest exposed rocks in the southern part of the arc are remnants

seismic reflection trace synthesized from proterozoic outcrop and its correlation to seismic profiles in northwestern canada abstract geological descriptions of the proterozoic dismal lakes and hornby bay group strata northeast of great bear lake in canada s northwest territories were used to compute a synthetic seismic reflection trace that was then correlated with reflection data recorded in the colville hills region to the west reflectivity of the synthetic trace shows a striking similarity to reflections on the recorded profiles at travel times appropriate for the stratigraphic level of the dismal lakes and hornby bay groups the similarity suggests that the method of synthesizing traces from outcrop information can be a valuable tool in areas where well control or other direct ties are lacking calibration of the colville hills seismic profiles with the synthetic trace indicates that the reflections are best interpreted in terms of the observed regional stratigraphy and along with geometries observed on more structurally complex reflection profiles immediately west of the colville hills affirms an interpretation of proterozoic layer parallel thrust faulting in the area from this information it follows that a deep detachment may occur near the top of the hornby bay group in either the kaertok or lady nye formations and that the autochthonous basement probably 1 8 2 0 ga hudsonian is situated about 1 0 1 5 s about 3 0 4 5 km beneath the lowest observed detachment in the colville hills region

the late quaternary biogeographic histories of some great basin mammals western usa the great basin of arid western north america provides one of the most detailed late pleistocene and holocene mammal records available for any part of the world though the record is by far strongest for small mammals of the 35 genera of now extinct north american pleistocene mammals 19 are known to have occurred in the great basin a list that is likely to be complete or nearly so of these 19 seven can be shown to have survived beyond 12 000 radiocarbon years ago a proportion similar to that for north america as a whole horses camels mammoth and helmeted musk oxen appear to have been the most abundant of these genera pygmy rabbits brachylagus idahoensis yellow bellied marmots marmota flaviventris and bushy tailed woodrats neotoma cinerea declined in abundance at the end of the pleistocene at about the same time as populations south of their current arid western distributional boundary were extirpated subsequent declines occurred during the hot dry middle holocene pygmy rabbits also declined as modern pinyon juniper woodlands developed across the great basin the snake range of eastern nevada has seen the late pleistocene or holocene extinction of both northern pocket gophers thomomys talpoides and pikas ochotona princeps coupled with the rarity of yellow bellied marmots here these histories make the snake range a biogeographic oddity these and other great basin mammal histories provide significant insights into the possible responses of great basin small mammals to global warming

microfossils from the early proterozoic hornby bay group district of mackenzie northwest territories canada spheroidal and filamentous organic walled microfossils have been detected in ca 1 7 ga old cherts of the hornby bay group northwest territories canada the majority of the spheroidal forms range from 1 to 4 m in diameter are referable to the genus sphaerophycus and probably represent the preserved sheaths of chroococcacean cyanophytes a single robust walled 27 m diameter spheroidal microfossil of undetermined affinity is also present the filamentous forms are tubular unbranched and range from 1 to 8 m in diameter they appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria the filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic walled microfossils where they occur in silica the filaments are preserved as three dimensional tubular micro structures which are readily recognized as microfossils in contrast where they extend from silica into adjacent dolomite they are highly compressed and not read

geochemistry and petrogenesis of basaltic rocks from coppermine river area northwest territories proterozoic basaltic flows 2000 m thick and associated dykes and sills from the coppermine river area northwest territories have chemical compositions typical of continental tholeiites the low mg fe ratio and abundances of ni and cr indicate that the lavas were extensively fractionated prior to extrusion the variations of incompatible elements such as k rb ree y zr nb and th suggest that the rocks were affected by interaction with continental crust the samples least affected by contamination have trace element compositions very similar to those of p type mid ocean ridge basalts it is suggested that continental tholeiites have been generated from the same source as p type oceanic tholeiites and geochemical features such as the enrichment of some lithophile elements in many of these rocks may be related to crustal contamination the variations within the volcanic pile of the coppermine river area are related to those of an exposed part of the muskox layered intrusion

early proterozoic magmatism in yukon canada constraints on the evolution of northwestern laurentia northwestern laurentia after cratonization at about 1 85 ga underwent a series of tectonic and magmatic events during the proterozoic that were followed by separation of laurentia from another landmass probably australia the oldest magmatic event produced the bonnet plume river intrusions bpri which intruded the wernecke supergroup as short dikes and small stocks the bpri are hydrothermally altered tholeiitic diorites gabbros and subordinate anorthositic and syenitic rocks with trace element signatures consistent with a rift origin depleted mantle model ages range from 2 29 to 2 57 ga and end values range from 0 7 to 1 7 an increasing crustal component is apparent in rocks with more evolved compositions four u pb zircon ages 1705 9 0 7 1709 4 1 4 1711 1 5 1 and 1713 6 12 7 ma indicate a paleoproterozoic age for the bpri these dates constrain the age of the wernecke supergroup to ca 1710 ma making it the oldest supracrustal succession in western laurentia e g 240 ma older than the belt supergroup of southeastern british columbia and the northwestern united states the wernecke supergroup was deposited in the first rift basin to open along the western margin of laurentia but was later inverted by the pre 1 6 ga racklan orogeny an event possibly influenced by transmission of compression from the yavapai and mazatzal orogenies in southern laurentia the neoproterozoic southwestern united states east antarctica sweat reconstruction which places australia next to northwestern laurentia is supported by linkages between paleoproterozoic and mesoproterozoic geological features in northwestern canada and australia

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

paleomagnetism and the evolution of fluids in the proterozoic athabasca basin northern saskatchewan canada in the athabasca basin diagenetic hematite of variable paragenesis occurs throughout the sandstones and underlying paleoregolith this hematite carries three distinct single component magnetizations a d 158 i 62 95 5 n 21 b d 11 i 36 95 7 n 6 and c d 18 i 79 95 3 n 27 in some areas of the sandstones such as near reactivated fault zones the diagenetic hematite has been altered to goethite which yields a very low intensity incoherent d magnetization ages for the a b and c magnetizations inferred from comparisons with paleomagnetic directions in precambrian rocks whose ages are known approximately are 1750 1600 1600 1450 and about 900 ma respectively the a magnetization is carried by the earliest formed hematite and its estimated age compares well with u pb ages of 1650 1700 ma for early diagenetic apatite u pb and rb sr ages of approximately 1500 and 900 ma for uraninite and illite coeval with hematite that carries the b and c magnetiza

geothermometry and geobarometry in pelitic rocks of north central wopmay orogen early proterozoic northwest territories canada pressure and temperature estimates for the pelitic schists and gneisses along the east flank of the hepburn batholith early proterozoic have been determined by means of an electron microprobe study of coexisting garnet biotite plagioclase and cordierite garnet biotite geothermometry using recent calibrations yields results between 560 and 765 c consistent in spatial distribution with andalusite sillimanite and sillimanite k feldspar metamorphic zones calibrations of the garnet cordierite equilibrium give temperature estimates on average 23 c lower both sets of temperature estimates for the garnet cordierite bearing assemblages are consistent with the theoretical stability field of these rocks geobarometry based on the 3 caal 2 si 2 o 8 ca 3 al 2 si 3 o 12 2 al 2 sio 5 sio 2 equilibrium outlines a broad pressure culmination with variation from 2 6 to 5 2 kbar the p culmination coincides with a structural plunge culmination as outlined by the change in plunge azimuth of folds of bedding by a re entrant in the trace half window of a major premetamorphic thrust fault and by a narrowing of the synmetamorphic funnel shaped hepburn batholith garnet cordierite geobarometric calibrations generally give pressure determinations in the same range as the grossular anorthite results the spread in the garnet cordierite pressure estimates from various calibrations however points to the need for considering the role of water when using the garnet cordierite geobarometer

shuswap terrane of british columbia a mesozoic core complex the shuswap terrane of the northwest cordillera owes its distinctive characteristics to deformation and high grade regional metamorphism that occurred primarily during the mesozoic within the terrane the monashee complex the overlying selkirk allochthon and the intervening monashee decollement record strain and metamorphism that occurred in middle to late jurassic time during and subsequent to the accretion of a western allochthonous terrane the selkirk allochthon moved eastward across the monashee complex after emplacement of the western allochthonous terrane but before late mesozoic to early cenozoic telescoping of the rocky mountain belt in this latter event the monashee complex and overlying allochthonous slices were transported eastward relative to the north american craton on a sole fault that developed during listric thrusting of the rocky mountain foreland synmetamorphic to late metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the monashee decollement zone were generated by shear strain during the middle jurassic emplacement of the selkirk allochthon and are not related to any upper crustal extension that occurred in the tertiary uplift normal faulting brittle reactivation of the mylonitic decollement zone widespread resetting of k ar and rb sr mineral dates and arching of the terrane are events that culminated in the eocene

petrogenesis of the 723 ma coronation sills amundsen basin arctic canada implications for the break up of rodinia the coronation mafic sills and associated natkusiak flood basalts in arctic canada are part of the 723 ma franklin igneous events a flood basalt and dyke swarm province that extends across the southern arctic of north america and greenland two representative sills 70 and 25 m thick respectively outcrop near coronation gulf in nunavut they are composed of clinopyroxene plagioclase olivine and fe ti oxides and have tholeiitic composition similar to the natkusiak basalts their end values range between 4 2 and 5 9 and have la yb n ratios of 1 5 2 these rocks are mantle derived and were affected by minor crustal contamination compositional variations indicate that the coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from west to east across the province a feature similar to the mackenzie dyke swarm however the mackenzie dyke swarm and the franklin igneous events have distinct chemical composition the franklin rocks are related to a mantle plume located near victoria island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break up of rodinia the absence of such dykes and sills in siberia argues against locating siberia against northern laurentia in a rodinia reconstruction 2003 elsevier b v all rights reserved

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

small mammal data on early and middle holocene climates and biotic communities in the bonneville basin usa abstract archaeological investigations in camels back cave western utah recovered a series of small mammal bone assemblages from stratified deposits dating between ca 12 000 and 500 14c yr b p the cave s early holocene fauna includes a number of species adapted to montane or mesic habitats containing grasses and or sagebrush e g lepus townsendii marmota flaviventris reithrodontomys megalotis and brachylagus idahoensis which suggest that the region was relatively cool and moist until after 8800 14c yr b p between ca 8600 and 8100 14c yr b p these mammals became locally extinct taxonomic diversity declined and there was an increase in species well adapted to xeric low elevation habitats including ground squirrels lepus californicus and neotoma lepida the early small mammal record from camels back cave is similar to the 11 300 6000 14c yr b p mammalian sequence from homestead cave northwestern utah and provides corroborative data on bonneville basin paleoenvironments and mammalian responses to middle holocene desertification

naturally weathered feldspar surfaces in the navajo sandstone aquifer black mesa arizona electron microscopic characterization abstract naturally weathered feldspar surfaces in the jurassic navajo sandstone at black mesa arizona was characterized with high resolution transmission and analytical electron microscope hrtem aem and field emission gun scanning electron microscope feg sem here we report the first hrtem observation of a 10 nm thick amorphous layer on naturally weathered k feldspar in currently slightly alkaline groundwater the amorphous layer is probably deficient in k and enriched in si in addition to the amorphous layer the feldspar surfaces are also partially coated with tightly adhered kaolin platelets outside of the kaolin coatings feldspar grains are covered with a continuous 3 5 m thick layer of authigenic smectite which also coats quartz and other sediment grains authigenic k feldspar overgrowth and etch pits were also found on feldspar grains these characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems and may partially contribute to the commonly observed apparent laboratory field dissolution rate discrepancy at black mesa feldspars in the navajo sandstone are dissolving at 10 5 times slower than laboratory rate at comparable temperature and ph under far from equilibrium condition the tightly adhered kaolin platelets reduce the feldspar reactive surface area and the authigenic k feldspar overgrowth reduces the feldspar reactivity however the continuous smectite coating layer does not appear to constitute a diffusion barrier the exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer leached layer versus re precipitated silica which is uncertain at present however the nanometer thin layer can be detected only with hrtem and thus our study raises the possibility of its wide occurrence in geological systems rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface

the muscovite melt bathograd and low p isograd suites in north central wopmay orogen northwest territories canada in north central wopmay orogen syntectonic low p buchan type suites of mineral isograds outline regional metamorphic temperature culminations that are associated at the higher structural levels with emplacement of early proterozoic plutons in the west part of a deformed and eastward transported continental margin prism the mapped isograds mark the first occurrence of biotite staurolite andalusite sillimanite sillimanite k feldspar and k feldspar plagioclase quartz muscovite granitic pods in metapelites with increasing proximity to the plutons microprobe analyses and field observations have resulted in the formulation of reactions for the ideal pelitic system k2o na2o feo mgo al2o3 sio2 h2o al2o3 sio2 h2o to account for the various mineral assemblages of each metamorphic zone a p t petrogenetic grid showing erosion surface p t curves for the northern wopmay orogen pelites compiled on the basis of the mapped isograds and the inferred reaction s for each metamorphic zone documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar the configuration of a new bathograd based on the invariant model reaction sillimanite k feldspar plagioclase biotite quartz vapor muscovite liquid and interpolated across three metamorphic suites is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite grossular quartz al2sio5 geobarometry the positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure dependence of anatectic granitic pod mineral associations

comparing 1800 1600 ma accretionary and basin processes in australia and laurentia possible geographic connections in columbia abstract we focus on potential pinning points on the southern and eastern margins of australia and the southern and western margins of laurentia and interpret a convergent plate boundary along the southern margin of the proto australian continent during the palaeo to mesoproterozoic 1800 1600 ma specific geological events within the southern australian margin can be correlated with events preserved along the medicine bow yavapai mazatzal belt in southwestern laurentia these belts were characterised by southward continental growth with varying degrees of crustal reworking and accretion of juvenile material in australia crustal reworking appears to have been the dominant process from 1800 to 1670 ma with more juvenile material accreted at 1620 ma in contrast the southern laurentian terranes were dominated by accretion of juvenile material from 1800 to 1620 ma although reworked continental crust was also accreted to the margin in the interior of both continents basins developed with comparable histories from 1800 to 1670 ma thereafter the evolution of the interior basins diverged with sag phase processes dominant in northern australia and a protracted depositional hiatus in laurentia the geologic evolution is consistent with palaeomagnetic data and provides further evidence for geographic links between eastern australia and western laurentia from 1800 to 1600 ma

rift zone magmatism petrology of basaltic rocks transitional from cfb to morb southeastern brazil margin compositions of basaltic samples from the southeastern brazil passive margin 18 24 s depict the change from continental to oceanic lithosphere during the opening of the south atlantic ocean samples studied range from 138 to 105 m y old and are from 12 petrobras drill cores recovered from the coastline to about 150 km offshore in the espirito santo campos and santos basins compositions vary ranging for example from 49 54 wt sio2 0 5 3 0 wt tio2 0 6 5 0 feo mgo and 1 6 la yb n but can be grouped i basalts enriched in incompatible elements such as k some k2o 2 0 wt rb 18 ppm zr 120 ppm and lree some feo 16 wt most with sio2 51 54 wt and resembling serra geral continental flood basalts sg cfb of southern brazil ii basalts less enriched or transitional in incompatible elements having k2o 30

limited basement involvement in caledonian deformation hinnoy nnorth norway and tectonic implications abstract structural and petrographic relationships on east hinnoy north norway indicate that the caledonian nappes which are exposed directly to the east were emplaced over an older precambrian crystalline terrane lofoten terrane while at amphibolite facies metamorphic conditions caledonian structural and metamorphic effects disappear structurally downward away from the basal thrust of the nappe stack the limited involvement of the basement in caledonian deformation is explained by the limited availability of water which was introduced from external sources the external water source is hypothetically the autochthonous and allochthonous cover and the precambrian austerfjord group metasedimentary rocks within the basement which underwent prograde metamorphic dehydration during caledonian orogenesis the mechanisms by which addition of water concentrated strain in the basement probably include reaction induced ductility and hydrolytic weakening of the constituent silicate minerals there are two main tectonic implications 1 in collisional metamorphic belts the lithosphere of the underthrusted plate may remain rigid below 15 25 km depth and 2 the common phenomenon of detachment of crystalline thrust sheets at mid crustal levels may be controlled in some cases by the limited access of water to pre existing crystalline rocks rather than by thermal structure

the bear river dykes 1265 1269 ma westward continuation of the mackenzie dyke swarm into yukon canada abstract the 1 27 ga mackenzie dyke swarm the largest on earth radiates from a point in the western canadian arctic and extends across much of the canadian shield possible western extensions of the swarm are largely obscured by younger sedimentary cover paleoproterozoic inliers in northern yukon host the bear river dykes brd herein dated by u pb baddeleyite and zircon methods at 1268 5 1 5 ma and 1264 6 1 2 ma the brd share similar geochemical and nd isotopic characteristics with the mackenzie dykes and the coeval coppermine river basalts and are regarded as products of the same plume hydrothermal activity at 1270 ma in breccia of the nor mineral occurrence northwest of the brd was probably generated by brd at depth the brd generally strike northwest approximately 90 to the orientation predicted by the radiating dyke model this difference may be explained by an anomalous local stress field at the time of brd emplacement or reorientation of brd during subsequent events of cordilleran deformation using the nor mineral occurrence as the westernmost locale of mackenzie dyking and assuming a model of uniform dyke radiation the arc of the mackenzie swarm is at least 50 greater than previously recognized bringing the total arc of dyke radiation to 150

evolution of regional metamorphism during back arc stretching and subsequent crustal shortening in the 1 9 ga wopmay orogen canada the stratigraphic units structural elements and metamorphic mineral assemblages of a regional metamorphic culmination in the 1 9 ga wopmay orogen are exposed over greater than 30 km of composite structural depth in a series of oblique sections produced by cross folding regional metamorphism developed continuously in three sequential rapidly changing thermo tectonic regimes within an evolving continental magmatic arc at ca 1900 ma stretching of intra arc crust resulted in the accumulation of clastic sediment and bimodal volcanic rift fill deposits the onset first stage of regional metamorphism is marked by high t low p mineral assemblages condensed metamorphic zonal sequences and extensive areas of high grade gneisses devoid of associated plutons these features are interpreted in terms of a high thermal gradient related to stretching and thinning of the continental lithosphere five to ten million years after stretching following deposition of a west facing sedimentary prism a suite of 1896 1878 ma plutons was emplaced into the rift and margin deposits as they underwent subhorizontal shortening and deformation during the calderian orogeny thrusted and folded syn orogenic foredeep deposits are also intruded by the syn tectonic plutons at high and intermediate structural levels syn tectonic metamorphic mineral growth and metamorphic zonal sequences which are spatially related to the plutons document heat advection into the deforming marginal prism and mark a second stage of regional metamorphism related to the emplacement of the plutonic bodies inverted mineral isograds in autochthonous proterozoic units beneath a basal decollement record downward thermal relaxation of isotherms following east directed calderian transport of the deformed thickened and still hot marginal prism over a relatively cold basement derivation of multi point p t trajectories from post tectonic poikiloblastic garnets charts metamorphic mineral growth during uplift and erosion of the internal zone documenting the third final stage of regional metamorphism in wopmay orogen the short erosional time interval less than 11 ma between tectonic thickening and the end of uplift constrains the heat required for this last metamorphic stage to be inherited from the two preceding thermo tectonic regimes epicontinental stretching and the emplacement of the syn tectonic plutonic suite

normal and inverted metamorphic isograds and their relation to syntectonic proterozoic batholiths in the wopmay orogen northwest territories canada abstract three progressive metamorphic suites are developed in pelitic rocks of the northern wopmay orogen two suites are related to the hepburn batholith and one to the wentzel batholith all three suites are cut by post metamorphic wrench faults some of which have significant vertical displacement the structural relief so provided reveals that medium and high grade isograds associated with the hepburn batholith dip inward towards the batholith and are thus hot side up isograds associated with the wentzel batholith dip away from the batholith and are thus hot side down it is concluded that hepburn batholith has the form of the flattened funnel fed from depth and that wentzel batholith is the arched roof of an intrusive complex of unknown shape at depth

a gis model for predicting wetland habitat in the great basin at the pleistocene holocene transition and implications for paleoindian archaeology the great basin s earliest inhabitants relied heavily on wetland systems that underwent a great decline culminating in the early holocene in this paper we derive expectations for regional paleoindian archaeology by modeling the distribution and disappearance of lake margin wetland habitats basin geomorphology and hydrology are used to predict change through time in the occurrence of these habitats fifty two pluvial lakes are drained from their pleistocene highstands by computer simulation using gis and the changing extent of associated wetlands is quantified we find that as these lakes declined to shallow depths many experienced corresponding increases in wetland habitat prior to complete desiccation thus while the total area of wetland habitat in the region could have maintained some stability through this process the number of available basin wetlands would have dropped relatively rapidly as smaller basins completely dried up the model therefore implies that those lakes that disappeared earliest should hold the archaeological signatures most restricted to early paleoindian times assemblages associated with these lakes should also reflect higher forager mobility corresponding with the greater number of basin wetlands available we find preliminary support for the model in several archaeological cases but emphasize the need for further basin specific research to clarify the nature and timing of wetland habitat loss and the extent to which this was a constraint for paleoindian populations

continental intra arc depressions a nonextensional model for their origin with a proterozoic example from wopmay orogen the 1 875 1 86 ga great bear magmatic zone is a linear belt 100 km wide by 800 km long of low tio 2 high al 2 o 3 volcanic rocks ranging continuously from basalt to rhyolite cut by allied hornblende biotite bearing plutonic rocks comprehensive study of the facies volcanic styles chemistry and petrography indicates that it is similar to cenozoic continental arcs preservation of high level volcanic rocks suggests that the zone never had an unusually thick crust and therefore is similar to some intra arc depressions such as the mesozoic paleogene depression of chile common in younger continental arcs these long linear depressions are the sites for voluminous pyroclastic volcanism contain sections tens of kilometres thick and are zones of subsidence whose surface remains close to sea level they originate when the mass of basaltic andesite arriving at the base of the crust equals the mass of vitric ash removed from the areas by high level atmospheric transport during ash flow eruptions

the tule springs local fauna rancholabrean vertebrates from the las vegas formation nevada abstract a middle to late pleistocene sedimentary sequence in the upper las vegas wash north of las vegas nevada has yielded the largest open site rancholabrean vertebrate fossil assemblage in the southern great basin and mojave deserts recent paleontologic field studies have led to the discovery of hundreds of fossil localities and specimens greatly extending the geographic and temporal footprint of original investigations in the early 1960s the significance of the deposits and their entombed fossils led to the preservation of 22 650 acres of the upper las vegas wash as tule springs fossil beds national monument these discoveries also warrant designation of the assemblage as a local fauna named for the site of the original paleontologic studies at tule springs the large mammal component of the tule springs local fauna is dominated by remains of mammuthus columbi as well as camelops hesternus along with less common remains of equus including e scotti and bison large carnivorans including canis dirus smilodon fatalis and panthera atrox are also recorded micromammals amphibians lizards snakes birds invertebrates plant macrofossils and pollen also occur in the deposits and provide important and complementary paleoenvironmental information the fauna occurs within the las vegas formation an extensive and stratigraphically complex sequence of groundwater discharge deposits that represent a mosaic of desert wetland environments radiometric and luminescence dating indicates the sequence spans the last 570 ka and records hydrologic changes in a dynamic and temporally congruent response to northern hemispheric abrupt climatic oscillations the vertebrate fauna occurs in multiple stratigraphic horizons in this sequence with ages of the fossils spanning from 100 to 12 5 ka

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

an uplift history of the colorado plateau and its surroundings from inverse modeling of longitudinal river profiles 1 it is generally agreed that a region encompassing the colorado plateau has been uplifted by sub crustal processes admittance calculations tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes notwithstanding advances in our understanding of present day setting uplift rate histories are poorly constrained and debated an improved history will aid discrimination between proposed models here we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles we assume that the shape of a river profile is controlled by uplift rate and moderated by erosion in our model uplift rate is allowed to vary smoothly as a function of space and time upstream drainage area is invariant with time simultaneous inversion of river profiles from the colorado rio grande columbia and mississippi catchments shows that three phases of regional uplift occurred the first phase occurred between 80 and 50 myrs when 1 km of uplift was generated at a rate of 0 03 mm yr a second phase occurred between 35 and 15 myrs when 1 5 km of uplift was generated at a faster rate of 0 06 mm yr a final phase of uplift commenced 5 myrs ago these distinct phases of late cretaceous and oligocene uplift are corroborated by stratigraphic considerations by thermochronometric data and by stratigraphic evidence of periodic clastic efflux delivered into the gulf of mexico an episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region which is currently centered on yellowstone

the late pleistocene of death valley a climatic reconstruction based on macrofossil data abstract wood rat neotoma macrofossil deposits from death valley california northern mohave dessert dating from the wisconsinan glacial epoch provide a basis for reconstruction of the glacial age climate at low elevations of the area elevational depression of the woodland zone 1200 1500 m relative to today presence of a woodland semidesert ecotone at approximately 450 m and species composition of the low elevation semidesert which included a yucca species yucca whipplei chaparral yucca now locally extinct point toward a pleistocene climate which was less arid and more equable than today precipitation is estimated to have been three to four times the present values and summers 8 14 c cooler the increase in precipitation is inferred to be part of a generally wetter regional in the northern mohave desert and the central great basin during the last glacial presence of warm desert species at 10 200 yr b p indicates that climatic conditions similar to those of the present were established at the lower elevations by this time

characterization of the paleoproterozoic hottah terrane wopmay orogen using multi isotopic u pb hf and o detrital zircon analyses an evaluation of linkages to northwest laurentian paleoproterozoic domains abstract the paleoproterozoic wopmay orogen is modeled as a simple arc continent collision involving the archean slave craton and an enigmatic continental arc terrane termed the hottah terrane hottah is a composite terrane that includes a juvenile 2 6 2 1 ga basement and a younger 1 93 1 90 ga continental arc the extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin and eventual role in the development of the orogen multi isotopic u pb hf o analyses of detrital zircon from rocks of the hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics the data confirm that ca 2 6 2 1 ga crust is an important component of the terrane but hf isotopic data indicate mostly non juvenile compositions consistent with recycled neoarchean crust sources ages of detrital zircon are distinct from those expected of the slave craton but are similar to adjacent 2 0 1 90 ga paleoproterozoic terranes of northwestern laurentia the similarity to terranes such as the ksituan buffalo head and taltson thelon magmatic zones south and east of the wopmay orogen indicate that hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the nuna continent between 2 0 and 1 9 ga and was rifted prior to the collision with the slave margin that produced the calderian orogeny

eruptive style and construction of shallow marine mafic tuff cones in the narakay volcanic complex proterozoic hornby bay group northwest territories canada abstract the early proterozoic 1663 ma narakay volcanic complex exposed in great bear lake northwest territories canada is a bimodal suite of basalt and rhyolite erupted in a continental setting and consisting largely of pyroclastic rocks interlayered with shallow marine sedimentary rocks of the hornby bay group mafic pyroclastic rocks consist of lapilli tuff tuff tuff breccia and agglomerate that represent the remnants of small subaerial tuff cones 0 5 to 2 km in diameter that in most cases have subsided into the volcanic conduit stratification styles sedimentary structures and grain morphologies in pyroclastic rocks reflect variations in the water magma ratio during eruptions and have been used to help elucidate eruptive mechanisms and reconstruct volcanic edifices basaltic pyroclasts are commonly bounded by fracture surfaces and are morphologically similar to modern pyroclasts produced by thermal quench fragmentation or steam blast disruption of magma most fragments have low vesicularity and scoria is only locally abundant which indicates that eruptive energy was supplied mostly by water melt interaction rather than exsolution of magmatic gases cored bombs and lapilli fusiform bombs and pyroclasts similar in texture to those of strombolian cinder and agglutinate spatter are uncommon but are stratigraphically widespread and imply the occurrence of strombolian eruptions presumably when water access to the vent was impeded massive bedding is typical of the tuffs and in addition to the poorly sorted ash rich nature of the tuffs implies deposition from water and or steam rich hydrovolcanic eruption clouds and cypressoid jets by airfall and dense pyroclastic flows uncommon well stratified and sorted ash and lapilli tuff record airfall and pyroclastic flow deposition from eruption clouds rich in magmatic gases base surge deposits are uncommon and occur only in the subaerial portion of a sequence of tuffs inferred to record the progradation of a cone margin surge platform into standing water few of the tuff cone deposits display a systematic vertical sequence of stratification styles structures and grain morphologies this indicates that either the eruptive style varied irregularly between hydrovolcanic and strombolian and or that pyroclasts of different origin were mixed during eruptions

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

sequence stratigraphy provenance c and o isotopic composition and correlation of the late paleoproterozoic early mesoproterozoic upper hornby bay and lower dismal lakes groups nwt and nunavut abstract late paleoproterozoic basins of canada s north including the thelon athabasca and hornby bay basins can be broadly correlated based on similarities in age sequence stratigraphy paleocurrents and detrital zircon provenance but their detailed evolution is poorly understood and their age and lateral equivalencies are poorly constrained this study contributes to the understanding of sedimentary basins during a critical interval of time representing the latest stages of amalgamation of the supercontinent laurentia nuna through a detailed study of the hornby bay basin the hornby bay basin strata have been divided into four unconformity bounded subsequences a1 a2 a3 a4 that belong to the first of three unconformity bounded post hudsonian proterozoic stratigraphic packages in northwestern canada subsequences a2 1600 ma and a3 analysis of sm nd isotopes on mudrocks yielded isotopic compositions in the range 8 2 to 2 3 nd 1600 ma in subsequence a2 and 9 8 to 5 0 nd 1400 ma in subsequence a3 an nd versus time graph illustrates two distinct groupings that plot within known isotopic evolution fields of the taltson thelon tectonic zone and the great bear magmatic zone sediment provenance was strongly influenced by the paleoproterozoic forward orogeny but there are no significant changes in sedimentary provenance across the a2 a3 sequence boundary stable isotope analyses of c and o in the dolomitic east river formation of subsequence a2 reveal a relatively monotonous signal with an average of 0 5 13 c and 8 76 18 o which is consistent with global values of other late paleoproterozoic carbonate successions enriched mn values and depleted sr values however suggest that the east river carbonate rocks experienced some degree of meteoric alteration it is unknown how much this alteration affected stable isotope compositions based on similarities in sedimentary provenance age and sequence stratigraphy subsequence a2 of the hornby bay group is correlated with the wernecke supergroup yukon the latter is interpreted to represent the deep water distal equivalent of the upper hornby bay group in a westward deepening basin deposition of subsequence a2 is recorded across north america and is mirrored by deep water sedimentation along the eastern margin of the north australian craton the tectonic evolution of subsequence a2 in laurentia is therefore remarkably similar to that of the north australian craton up to 1600 ma the age of the forward orogeny and this has implications for interpretation of the latest stages of accretion of the supercontinent nuna

fluvial lacustrine and volcanic sedimentation in the angikuni sub basin and initiation of 1 84 1 79 ga baker lake basin western churchill province nunavut canada abstract continental siliciclastic and volcanogenic deposits of the baker lake group accumulated in numerous sub basins in the interior of the western churchill province between 1 84 and 1 79 ga in the angikuni sub basin on the southeast flank of greater baker lake basin baker lake group rocks outcrop in two segments that extend northeast from angikuni lake they are also exposed in scattered outliers throughout the region at northern angikuni lake in the northern segment conglomerates pebbly sandstones and mudrocks of the angikuni formation unconformably overlie archean basement and are unconformably overlain by ultrapotassic volcanic and siliciclastic rocks of the christopher island formation these rocks record alluvial fan fluvial and sand flat playa deposition in a fault bounded trough formed adjacent to a wedge shaped basement uplift although the angikuni formation was tilted before principal christopher island formation volcanism at northern angikuni lake geochemical and nd isotopic data from mudrocks indicate derivation from earlier or coeval christopher island volcanic like sources the outliers demonstrate that faulting produced significant changes in the structural level of archean basement before christopher island formation volcanism near rack lake in the southern segment angikuni formation conglomerates sandstones and mudrocks define 100 m scale upward fining and upward coarsening to upward fining sequences relative to the northern segment these rocks were deposited in a more distal fine grained sand flat to semi perennial fresh water evaporitic lake setting the angikuni formation at rack lake records deposition between and likely during periods of volcanism as indicated by a conformable angikuni christopher island contact abundant volcanic detritus and local lacustrine chemical sediments that contain magnesite strontianite barite and apatite which reflect the chemistry of the volcanic rocks baker lake basin may have originated during regional uplift and extension within the western churchill province due to terminal collision and post collision processes in trans hudson orogen to the south while the western margin of ancestral north america was a free face

the intracratonic paleoproterozoic forward orogeny and implications for regional correlations northwest territories canada reflection seismic data from the colville hills and anderson plain document an intracratonic compressional event herein named the forward orogeny which affected strata considered equivalent to the 1663 ma hornby bay group outcropping on coppermine homocline forward orogeny structures were peneplaned and unconformably overlain by strata considered equivalent to the 1267 ma dismal lakes group a comparable tectonic history is recorded in the exposed rocks of coppermine homocline to the east structural orientations indicate a general northwest southeast direction of maximum compression during an early phase and a more west east direction in a later phase regional sequence a of g m young and co workers is subdivided into preorogenic sequence a1 hornby bay and equivalents and postorogenic sequences a2 dismal lakes and equivalents and a3 coppermine river group and equivalents the forward orogeny dated as approximately 1663 ma the age of the syntectonic kaertok formation on coppermine homocline

timing of colorado plateau uplift initial constraints from vesicular basalt derived paleoelevations comment and reply sahagian et al 2002 1 provide an interesting look at the uplift history of the colorado plateau through an analysis of paleoelevations estimated using a study of basalt vesicularity they conclude that the colorado plateau experienced slow uplift beginning ca 25 ma with rapid uplift since 5

black mats spring fed streams and late glacial age recharge in the southern great basin black mats are prominent features of the late pleistocene and holocene stratigraphic record in the southern great basin faunal geochemical and sedimentological evidence shows that the black mats formed in several microenvironments related to spring discharge ranging from wet meadows to shallow ponds small land snails such asgastrocopta tappanianaandvertigo berryiare the most common mollusk taxa present semiaquatic and aquatic taxa are less abundant and include catinellids fossaria parva gyraulus parvus and others living today in and around perennial seeps and ponds the ostracodescypridopsis okeechobiandscottia tumida typical of seeps and low discharge springs today as well as other taxa typical of springs and wetlands are common in the black mats several new species that lived in the saturated subsurface also are present but lacustrine ostracodes are absent the 13c values of organic matter in the black mats range from 12 to 26 reflecting contributions of tissue from both c3 sedges most shrubs and trees and c4 saltbush saltgrass plants carbon 14 dates on the humate fraction of 55 black mats fall between 11 800 to 6300 and 230014c yr b p to modern the total absence of mats in our sample between 6300 and 230014c yr b p likely reflects increased aridity associated with the mid holocene altithermal the oldest black mats date to 11 800 11 60014c yr b p and the peak in the14c black mat distribution falls at 10 00014c yr b p as the formation of black mats is spring related their abundance reflects refilling of valley aquifers starting no later than 11 800 and peaking after 11 00014c yr b p reactivation of spring fed channels shortly before 11 20014c yr b p is also apparent in the stratigraphic records from the las vegas and pahrump valleys this age distribution suggests that black mats and related spring fed channels in part may have formed in response to younger dryas yd age recharge in the region however the inception of black mat formation precedes that of the yd by at least 40014c yr and hydrological change is gradual not rapid