bluuebunny/crossref_metadata_2025 huggingface dataset:
Mise en évidence d'un cisaillement ductile dextre d'âge crétacé moyen dans la région de Tête Jaune Cache (nord-est du complexe métamorphique Shuswap, Colombie-Britannique)
Authors: Driessche Jean Van Den , Maluski Henri
Published: April 2011

The boundary between the external zones (Rocky Mountains) and the internal zones of the eastern Canadian Cordillera is marked by a Tertiary half graben, the Rocky Mountains Trench (RMT). In the south Cordillera, east of the Shuswap metamorphic complex, the fault limiting the trench is superimposed on an early major thrust, the Late Jurassic Purceli thrust. On approaching this discontinuity, the ductile deformation of the Miette Group, a detrital Precambrian suite, is characterized by a subvertical foliation and a subhorizontal stretching lineation parallel to the fold axes. The deformation intensity, its noncoaxial characters, and its geographic extension are interpreted as resulting from a dextral crustal shear, parallel to the mapped trace of the Purcell thrust and RMT. The dextral slip is deduced from a microtectonic analysis of the observed rotational criteria and is consistent with the small angle occurring between the directions of the linear structure (stretching lineations and fold axes) and those of adjacent discontinuities. The Middle Cretaceous age (100–78 Ma) attributed to this deformation is based on the age of syn- to late-tectonic metamorphic minerals as dated by the 39Ar–40Ar method. A kinematic model involving vectorial decomposition of an oblique convergence is proposed, suggesting the simultaneous occurrence, in the Middle Cretaceous, of two suborthogonal conjugated movement directions, respectively parallel and normal to the general Cordilleran trend. [Journal Translation]
Stratigraphy, paleoecology, and glacial history of the Gillam area, Manitoba
Authors: Nielsen Erik , Morgan Alan V. , Morgan Anne , Mott R. J. , Rutter N. W. , Causse C.
Published: April 2011

Sections along the Nelson River in northern Manitoba, outcropping upstream and downstream from Limestone Dam, record a long succession of late Quaternary events. The oldest sediment exposed consists of sandy, nonfossiliferous Sundance till of northwestern provenance and related to a Kansan or Illinoian glaciation. The paleosol developed in the Sundance till is assigned to the Yarmouthian or Sangamon interglacial on its stratigraphic position and depth of weathering. Fossiliferous, clayey Amery till of eastern provenance overlies the Sundance till and underlies the nonglacial Nelson River sediments. Aspartic acid D/L ratios of wood fragments from the Nelson River sediments correlate with an aspartic acid D/L ratio of similar wood from the Missinaibi Formation in Ontario. Beetle analysis indicates the Nelson River sediments were deposited north of the tree line under conditions more severe than those found in the area today. The deposits are believed to be of latest Sangamon or possibly Mid-Wisconsinan age. The Wisconsinan Stage is represented by the Long Spruce and Sky Pilot tills deposited by ice from the east. These tills are texturally and compositionally similar but are different colours. The overlying Henday sediments record glaciofiuvial deposition and mass wasting along the eastward retreating ice margin. Varves indicate the area was covered by glacial Lake Agassiz for less than 100 years after the ice retreated. The breakup of the Laurentide Ice Sheet in Hudson Bay and the final drainage of Lake Agassiz took place 7800–8000 years ago when the Hudson Bay Lowland was inundated by the marine water of the Tyrrell Sea. The area emerged from the Tyrrell Sea about 6500 years BP.
Paleomagnetism of the Late Precambrian Fourchu Group, Cape Breton Island, Nova Scotia
Authors: Johnson Rex J. E. , Voo Rob Van der
Published: April 2011

Volcanogenic sediments of the Fourchu Group and a gabbroic intrusion, which are found in the Avalonian terrane of south-eastern Cape Breton Island, have been sampled for paleomagnetic analysis. Upon detailed thermal and alternating-field demagnetization, three often-superimposed components of magnetization are obtained. One of these is aligned with the present-day geomagnetic field direction in Nova Scotia and is assumed to be of recent origin. The second group of directions is south-southeasterly and shallow, is postfolding in age, and is inferred to represent a Carboniferous overprint. The third direction, carried almost always by hematite, is also postfolding and yields a dual-polarity mean direction to the northwest or south-east, with a fairly steep inclination (D = 132°, I = −63°). This last direction is not seen in Avalonian or other North American rocks of Devonian or younger age; it is, therefore, bracketed in age between the earliest folding of the rocks and the latest Silurian. Given that Taconic folding has not been reported for this area, we assume that this magnetization was introduced in the rocks during uplift and oxidation after an Avalonian folding phase. For the Avalon terrane of Nova Scotia, the available paleomagnetic data reveal a set of moderately high paleolatitudes for the Late Precambrian and early Paleozoic, in contrast to the near-equatorial values predicted for the area under the assumption that it remained fixed with respect to the craton. On the other hand, strong similarities exist between Avalonian paleolatitudes and those for Armorica and Gondwana; a tentative reconstruction is proposed in which Avalon is adjacent to Armorica and Gondwana in the Late Precambrian and early Paleozoic.
The Cheticamp pluton: a Cambrian granodioritic intrusion in the western Cape Breton Highlands, Nova Scotia
Authors: Barr Sandra M. , Macdonald Alan S. , Blenkinsop John
Published: April 2011

The Cheticamp pluton consists of biotite granodiorite (locally megacrystic) in the north and museovite–biotite granodiorite in the south, in probable faulted contact. These two rock types, especially the biotite granodiorite, show a broad range in modal and chemical compositions. They are interpreted to be cogenetic, with the museovite–biotite grandiorite derived from the biotite granodiorite by crystal fractionation involving mafic minerals, plagioclase, and sphene. The overall peraluminous composition of the suite resulted from the fractionation process, probably enhanced by alteration, rather than from derivation from peraluminous source rocks.A seven-point, whole-rock, Rb–Sr isochron indicates an age of 525 ± 40 Ma. The pluton intruded dioritic rocks and quartzo-feldspathic gneisses, thus indicating Precambrian ages for these units. It probably also postdates the Western Highlands volcanic–sedimentary complex, a major undated stratigraphic unit in the Cape Breton Highlands. Although the age overlaps the range of Rb–Sr ages from plutons of the Avalon Terrane of the Appalachian orogen, the geological setting of the Cheticamp pluton differs from that of true Avalonian plutons, such as those in southeastern Cape Breton Island.
A synrhabdosome of Saetograptus fritschi cf. linearis (Bouček) from Cornwallis Island, Arctic Canada
Authors: Lenz A. C. , Melchin M. J.
Published: April 2011

A synrhabdosome of Saetograptus fritschi cf. linearis (Bouček) is described from mid-Ludlow strata of northwestern Cornwallis Island. The species is characterized by a sicula with a flaring aperture and dorsally projecting antivirgellar spine and by thecae with paired(?) spines or lappets.
The origin of the chrysolite asbestos veins in southeastern Quebec
Authors: O'Hanley David S.
Published: April 2011

The chrysotile asbestos deposits of southeastern Quebec occur in serpentinized peridotites of Ordovician ophiolites. The asbestos is localized in veins within blocks of partly serpentinized peridotite bounded by zones containing schistose serpentinite and granitic masses. It is thought that these zones localized shear and provided the channelway s by which water entered the peridotite. Different generations of chrysotile veins can be identified locally by cross-cutting relationships, but these observations cannot be generalized regionally. The asbestos was produced late during the deformation of the peridotite, filling extensional fractures in the peridotite induced by stresses resulting from motion along the shear zones.The factors that control the formation of the chrysotile veins are the degree of serpentinization of the peridotite and the size of the peridotite mass. Other factors being equal, the smaller the peridotite mass is, the shorter the time necessary to serpentinize it. Smaller masses of peridotite, once serpentinized, weaken, develop a foliation, and will not develop cross-fiber veins. Larger peridotite masses will be less serpentinized and stronger and will develop cross-fiber veins.
Brachiopod biostratigraphy of the middle Ordovician Cow Head and Table Head groups, western Newfoundland
Authors: Ross Jr Reuben J. , James Noel P.
Published: April 2011

Brachiopods from shallow-water limestone boulders in beds 12, 13, and 14 of the allochthonous Cow Head Group belong in the earliest Whiterockian Orthidiella Zone. Brachiopods of the autochthonous intertidal to deep-water Table Head Group are equivalent to those of the younger Whiterockian Anomalorthis Zone. The Cow Head is older than the Table Head.Integration of the brachiopod data from western Newfoundland with those from the type Whiterockian in Nevada, combined with the evidence of graptolites from both areas, indicates that the Orthidiella Zone spans the Australian Castlemainian, Yapeenian, and lower Darriwilian Dal zones.
A stratigraphical and faunal revision of the Ordovician–Silurian strata of the Percé area, Quebec
Authors: Lespérance Pierre J. , Malo Michel , Sheehan Peter M. , Skidmore W. B.
Published: April 2011

The Ordovician–Silurian strata of the Percé area are assigned to the Matapédia Group. They occur in a southwestern monoclinal sequence, unconformably overlying Cambrian strata, and are assigned to the Pabos and White Head formations. The Pabos Formation is preponderantly a terrigenous sequence, whereas the White Head is preponderantly a carbonate sequence. The Pabos strata of this area are included in the new Rouge Member, in which four brachiopod-dominated communities are recognized: the Dalmanella, Catazyga, Sowerbyella, and Epitomyonia communities, within which trilobites occur sporadically. The White Head Formation is divided into three new limestone members and a new mudstone member. The basal Burmingham Member has yielded a Catazyga Community and a Sowerbyella-like Community. The Côte de la Surprise Member is composed of mudstones with the previously described Hirnantia Community. The Rouge, Burmingham, and Côte de la Surprise members are Ashgillian (Upper Ordovician), but the uppermost two members of the White Head Formation, the L'Irlande and Des Jean members, are Llandoverian. They yield an Acernaspis Community, assigned a paleoecological position intermediate between those of the Clorinda and graptolite communities. The Matapédia Group limestones and shales in the structurally complex northeastern sequence are informally termed the Grande Coupe beds. These beds are partly or wholly time-correlative with the Rouge and Burmingham members but were deposited in deeper water. A Stenopareia Community includes the highly fossiliferous Grande Coupe beds, with a local development of the Foliomena Community. The Percé area is unique within the Quebec Appalachians because the strata of the Matapédia Group are highly fossiliferous, with distinct European affinities in the Ordovician, and because the monoclinal sequence is a deepening-upward sequence, probably to the north and west of deeper water clastics.
Postglacial geomorphic development of the Dinosaur Provincial Park badlands, Alberta
Authors: Bryan Rorke B. , Campbell Ian A. , Yair Aaron
Published: April 2011

Experimental studies concerning current geomorphic processes and erosion rates in the badlands of Dinosaur Provincial Park, Alberta, have not explained the unusual extent of badland development or prominent nonstructural near-horizontal surfaces that occur in the park. Two of these surfaces result from spillway development associated with Wisconsin deglaciation, and the extent of badland development is associated with major spillway concentration and exposure of highly erodible Cretaceous strata. A third surface is associated with erosion caused by locally generated runoff. All surfaces are blanketed with aeolian sands and silts deposited around 5500 BP, which profoundly affected the hydrology of the area and water and sediment discharge from the badlands to the Red Deer River. Subsequent stripping of the aeolian cover by streams, along with piping and tunnel erosion, has reexposed vulnerable Cretaceous strata and restored the high erosion rates now observed in these badlands.
Quaternary stratigraphy and history, Williams Lake, British Columbia
Authors: Clague John J.
Published: April 2011

Thick valley-fill sediments in the vicinity of Williams Lake, British Columbia, provide a detailed record of the late Quaternary history of an area near the centre of the former Cordilleran Ice Sheet. Stratigraphic units assigned to the late Wisconsinan Fraser Glaciation, the preceding (penultimate) glaciation, and the present interglaciation are described. Especially noteworthy are (1) thick units of sand and gravel deposited by braided streams, perhaps during periods of ice-sheet growth; and (2) complex glaciolacustrine sediments that accumulated in ice-dammed lakes during periods of deglaciation.Glaciers from the Coast and Cariboo mountains coalesced and flowed north over central British Columbia during late Wisconsinan time. Fraser Glaciation advance sediments and older Pleistocene deposits were partially removed by this ice sheet, and the eroded remnants were mantled with till. At the end of the Fraser Glaciation, the Cordilleran Ice Sheet downwasted and retreated southward along an irregular front across the study area. Parts of the ice sheet stagnated and disintegrated into tongues confined to valleys. Sediment carried by melt streams flowing from decaying ice masses was deposited in glacial lakes, in stream channels, and on floodplains.
Verendrye valley and the Glidden esker, Saskatchewan: subglacial and ice-walled features in southwestern Saskatchewan, Canada
Authors: Christiansen E. A.
Published: April 2011

Verendrye valley is more than 100 km long, about 1 km wide, and at least 225 m deep. Southward, the valley grades into the Glidden esker, the surface of which rises about 280 m above the lowest point in the valley. The sediments in the valley and esker are 205 and 75 m thick, respectively, and form a fining-upward sequence from sand and gravel at the base to silt with sand interbeds in the upper part of the sequence, which, in turn, is covered by a blanket of glacial lake clay.The concave-upward longitudinal profile of Verendrye valley, the genetic relationship between Verendrye valley and the Glidden esker, and the age of the features indicate the valley and esker were formed by a subglacial stream during the last deglaciation 15 500 – 14 000 years ago. The fining-upward sequence of sediments in the valley and esker suggests the sub-glacial valley formed time transgressively by headward erosion while the glacier margin remained stationary at the Glidden esker.
Holocene and latest Pleistocene glacial chronology, Glacier National Park, Montana
Authors: Carrara Paul E.
Published: April 2011

Moraines of two different age groups have been identified fronting the present-day glaciers and snowfields in Glacier National Park, Montana. The subdued, vegetated moraines of the older group have been found at 25 sites, mainly in the central part of the Lewis Range. These older moraines are in places overlain by the Mazama ash. Although the exact age of the moraines has not been determined by radiocarbon dating, vegetative evidence and correlation with other pre-altithermal age moraines in the Rocky Mountains suggest that these older moraines date from 10 000 BP or earlier. Whether these moraines are the product of a separate advance after the end of the Wisconsin glaciation or are simply the product of the last advance or stillstand of Wisconsin glaciers before final deglaciation is not known.Moraines of the younger group, consisting of fresh bouldery rubble, are common throughout Glacier Park. Tree-ring analyses indicate that some of these younger moraines were deposited by advances that culminated during the mid-19th century. At that time there were more than 150 glaciers in Glacier Park. This episode of mid-19th century climatic cooling resulted in the most extensive glacial advance in this region since the end of the Wisconsin glaciation.Present-day glaciers have shrunk drastically from their mid-19th century positions; more than half the glaciers present during that time no longer exist. Much of this retreat occurred between 1920 and the mid-1940's, corresponding to a period of above-average summer temperatures and below-average annual precipitation in this region. Between 1966 and 1979, several of the larger glaciers in the Mount Jackson area of Glacier Park advanced as much as 100 m.
U–Pb geochronology in the Trans-Hudson Orogen, northern Saskatchewan, Canada
Authors: Schmus W. R. Van , Bickford M. E. , Lewry J. F. , Macdonald R.
Published: April 2011

We have obtained U–Pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern Trans-Hudson Orogen in northern Saskatchewan. These data constrain the timing of early Proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the Trans-Hudson Orogen to other early Proterozoic orogens in North America.With one exception, all units studied so far yield zircon ages of 1890–1835 Ma, most of which are systematically earlier than previously reported Rb–Sr isochron ages on the same or similar units, suggesting open-system behavior in the Rb–Sr systems. Five metarhyolites, from volcanic sequences in the La Ronge domain, Glennie domain, and Hanson Lake block, give ages ranging from 1888 to 1876 Ma. Most of the plutons we dated, ranging from gneissic syntectonic tonalites and granodiorites to less-deformed late intrusions such as the Wathaman batholith and other smaller bodies, yield ages of 1870–1850 Ma, apparently constraining peak plutonic activity to about 1860 ± 10 Ma ago. The youngest unit found is a small discordant pluton with an age of 1836 ± 7 Ma. The concordance of ages of volcanics on the one hand and of plutons on the other suggests that domainal distinctions are mainly lithotectonic rather than temporal.Zircons from the Sahli charnockitic granite in the Hanson Lake block yield equivocal results. Discordia upper and lower intercepts for the Sahli granite suggest that granitic rocks at least 2500 Ma old were subjected to high-grade metamorphism about 1800–1900 Ma ago, with substantial resetting of zircons. Reworked Archean basement is thus present in this domain, supporting previously reported Rb–Sr isochron data from the Sahli granite. No other indications of Archean basement in the Trans-Hudson Orogen are documented, although one sample from the adjacent Peter Lake domain shows that it consists of Archean continental crust.Zircon ages in the range 1890–1835 Ma from this part of the Trans-Hudson Orogen are similar to those obtained from igneous units of the Penokean and Wopmay orogens, in North America, and from the Svecofennian Orogen, suggesting essential synchroneity of igneous and tectonic events in these four major orogens during major Proterozoic continental assembly.
First report of Polyplacophora (Mollusca) from the Silurian of North America
Authors: Kluessendorf Joanne
Published: April 2011

Polyplacophorans, previously unreported from the Silurian of North America, have been recovered from trilobite–cephalopod accumulations in Wenlockian–Ludlovian carbonate buildups at five localities in the central United States. The specimens, which occur as isolated valves, are preserved as impressions, primarily of the ventral surface, in dolomitic matrix. Poor preservation precludes precise taxonomic determination of most specimens; these are described as nine morphotypes. However, two new taxa are erected (Chelodes raaschi sp. nov.; Hawthorneachiton lowenstami gen. et sp. nov.). Although rare, polyplacophorans apparently are a widespread and persistent faunal element of Silurian carbonate buildups.
Fracture mechanics and deformation processes in natural quartz: a combined Vickers indentation, SEM, and TEM study
Authors: Ferguson Colin C. , Lloyd Geoffrey E. , Knipe Robert J.
Published: April 2011

Conventional Vickers microhardness testing can be used to determine mode I fracture toughness, KIc, as well as hardness, H. The crack length versus indenter load formulation previously used in geology indicates that KIc for quartz increases with decreasing load. This analysis is unreliable because it neglects residual stresses, which depend on the modulus to hardness ratio (E/H). A recent analysis incorporating this effect gives results compatible with published bend-test determinations. Earlier results systematically under- or overestimate KIc depending on whether (E/H)1/2 is greater or less than 4.15.An extensive SEM study to determine three-dimensional indent–fracture morphology reveals a region of high back-scattered electron contrast immediately adjacent to the indenter contact. TEM analysis shows this is an intensely deformed region with very high fracture density and evidence of partial melting. A submicrometre-scale blocky structure, perhaps resulting from cataclasis, occurs between fractures. The analysis shows that "plastic" deformation associated with indentation in quartz is controlled largely by fracturing, although the absence of voids suggests some crystal–plastic accommodation process may also operate.
Illemocrinus amphiatus, a new cladid inadunate crinoid from the Middle Ordovician of Ontario
Authors: Eckert James D.
Published: April 2011

A new cladid inadunate crinoid, Illemocrinus amphiatus n. gen., n. sp., is described from the Middle Ordovician Bobcaygeon Formation of Ontario. Illemocrinus, the oldest known member of the Thalamocrinidae, is characterized by an obconical cup containing two anal plates including quadrangular radianal, narrow radial facets, isotomously branching arms, and an erect anal tube composed of vertical rows of nodose plates. Illemocrinus probably evolved from Palaeocrinus, indicating a North American origin for the Thalamocrinidae.
Paleoenvironmental geochemistry of the Claggett marine cyclothem in south-central Saskatchewan
Authors: Whittaker S. G. , Kyser T. K. , Caldwell W. G. E.
Published: April 2011

The Upper Cretaceous (Campanian) Lea Park Formation of south-central Saskatchewan, a product of continuous deposition in the Claggett sea of the Western Interior basin, contains unusually well-preserved fossil shells ideally suited for analysis of their chemical and isotopic compositions.Although both ammonites and bivalves secreted shells in isotopic equilibrium with the waters in which they lived, ammonites usually have higher δ18O and lower δ13C values than coexisting bivalves. These values point to the ammonites and bivalves having inhabited isotopically distinct waters within the Claggett sea. The benthonic bivalve Inoceramus is inferred to have lived in a thin bottom layer, the isotopic composition of which was modified by that of pore waters expelled at the sediment–water interface following compaction of sediments. The nekto-benthonic ammonites Scaphites and Baculites are inferred to have lived mostly at successively higher levels in the water column, the isotopic composition of which was modified by an influx of meteoric waters.Variations in trace-element/Ca ratios in molluscan shells are attributed to changes in the chemical composition of the Claggett sea, with essentially constant 87Sr/86Sr ratios suggesting that most chemical variations resulted from processes operating within the basin rather than at mid-ocean ridges. Sr concentrations in the sea decreased during Lea Park deposition, presumably as a result of weathering of increased proportions of igneous rocks relative to carbonate rocks in the western sourcelands. Anomalous Mg/Ca ratios are present in molluscan shells at a biostratigraphically correlative level. These ratios are primary and attributed to weathering of Mg-rich igneous rocks on the western flank of the basin. Probably produced in less than 40 000 years, the anomalous Mg/Ca ratios may form a chemostratigraphic and chronostratigraphic marker for the correlation of upper Campanian rocks.
U–Pb zircon ages from the Lynn Lake and Rusty Lake metavolcanic belts, Manitoba: two ages of Proterozoic magmatism
Authors: Baldwin D. A. , Syme E. C. , Zwanzig H. V. , Gordon T. M. , Hunt P. A. , Stevens R. D.
Published: April 2011

Two ages of magmatism have been determined from zircon in felsic flows and plutons in the Churchill Province of Manitoba. A rhyolite flow from the Lynn Lake metavolcanic belt has a U–Pb age of [Formula: see text], and a rhyolite flow from the adjacent Rusty Lake metavolcanic belt has an age of [Formula: see text]. Tonalite and quartz diorite from two composite plutons emplaced into the volcanic rocks at Lynn Lake have ages of [Formula: see text] and [Formula: see text], indistinguishable from the age of the Rusty Lake belt rhyolite. The arcuate domain of metavolcanic rocks that includes the Rusty Lake belt in the southeast, the Lynn Lake belt in the north, and the La Ronge belt (Saskatchewan) in the southwest has previously been considered a single structural sub-province with similar ages throughout. Our results and published U–Pb ages from Saskatchewan indicate that an older magmatism is represented by volcanic rocks in the Lynn Lake belt; a younger magmatism, by volcanic rocks in the Rusty Lake and La Ronge belts and plutons in the Lynn Lake belt. At Lynn Lake the older magmatism (1910 Ma) produced mafic, intermediate, and felsic volcanic rocks and synvolcanic plutons. The volcanic rocks are geochemically similar to Cenozoic island-arc magmatic sequences. These rocks were isoclinally folded and subsequently intruded by the 1876 Ma plutons. The younger, dominantly subaerial, volcanism (1878 Ma) at Rusty Lake was predominantly felsic, and the coeval plutons were granitoid. The distribution of ages and the 8 km thickness of the younger volcanic sequence suggest that the older rock served as basement during the younger magmatism.
A discussion of pyrrhotite fabrics
Authors: Simigian Sandra , Starkey John
Published: April 2011

The classification of pyrrhotite fabrics proposed by Lusk and Ostwald identifies E-type pyrrhotite, which has no dimensional or crystallographic preferred orientation and an equant grain shape, and F-type pyrrhotite, which has a distinct dimensional and crystallographic preferred orientation and flattened, elongate grain shapes. The results of an analysis of pyrrhotite fabrics studied for Ducktown, Tennessee, do not conform with this classification. The data suggest that the three parameters, crystallographic orientation, dimensional orientation, and grain shape, are independent. Therefore, the E- and F-type classification of pyrrhotite fabrics appears not to have universal applicability.
Lead isotope ratios in Niagara Escarpment rocks and galenas: implications for primary and secondary sulphide deposition
Authors: Farquhar R. M. , Haynes S. J. , Mostaghel M. A. , Tworo A. G. , Macqueen R. W. , Fletcher I. R.
Published: April 2011

Galenas occurring in vugs and veins in the dolomites of the Eramosa, Goat Island, and Gasport members of the Middle Silurian Lockport Formation in the Niagara Escarpment have a small but significant range in lead isotopic composition. These differences are recorded in specimens from exposures in the Niagara Gorge in the east to the Guelph quarry, 100 km to the northwest. The galena isotopic data fit linear trends on 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb plots within experimental error. The most radiogenic ratios are anomalous, giving model ages (based on the growth curves of Zartman and Doe) up to 200 Ma younger than the most likely age of primary deposition of ~ 300 Ma. Sulphides currently being deposited from springs at the floor of the Vinemount quarry have a lead isotopic composition similar to that of some of the galena in the overlying dolomites. Rocks from several of the formations exposed at this site contain lead only slightly more radiogenic than the most radiogenic galenas. These results are consistent with the hypothesis of Haynes and Mostaghel that the modern sulphides result from local solution and redeposition of older sulphides. Previously published isotopic ratios for lead in the potassium feldspar constituents of ~ 300 Ma plutonic intrusives in the Appalachians cluster around the least radiogenic galena ratios and suggest that the lead in the Niagara Peninsula galenas has been remobilized from rock units in the Appalachian Basin. Grenville crystalline basement rocks do not appear to have contributed significantly to this lead. A single sample of galena from the Eramosa Member, exposed in the Ebel quarry in the Michigan Basin at the northwestern extension of the escarpment, has 207Pb/204Pb and 206Pb/204Pb ratios that are substantially more radiogenic than those in any of the galenas from the Niagara region of the Appalachian Basin and a 208Pb/204Pb ratio that lies significantly below the linear distribution defined by the Niagara region data set. We tentatively interpret this difference as reflecting derivation of the lead in the two basins from different crustal blocks.
The genesis of the Deer Trail Zn–Pb–Ag vein deposits, northeast Washington, U.S.A.: evidence from fluid-inclusion and stable-isotope studies
Authors: Fluet D. W. , Changkakoti A. , Morton R. D. , Gray J. , Krouse H. R.
Published: April 2011

The Zn–Pb–Ag-bearing vein deposits of the Deer Trail mine, northeastern Washington, U.S.A., are hosted by Beltian metasediments that have been intruded by the Jurassic–Cretaceous Loon Lake granitoid batholith.Vein infilling took place in three stages: pre-ore, ore, and post-ore. Fluid-inclusion studies revealed homogenization temperatures and salinities of approximately 300 °C and 6.5–8.5 wt. % NaCl equivalent, respectively, for the pre-ore stage; 250–150 °C and 4–7.5 wt. % NaCl equivalent for the ore stage; and less than 150 °C and 3 wt. % NaCl equivlent for the post-ore stage. The calculated δ18O values of the hydrothermal fluids and δD values of fluid inclusion waters were +8 to +11‰ (SMOW) and −128 to −134‰ (SMOW), respectively, for the pre-ore stage; 0 to +10‰ and −89 to −143‰ for the ore stage; and 0 to −5‰ and −139‰ for the post-ore stage. The δ34S(ΣS) of the hydrothermal fluids was approximately +10‰(CDT).The results provide insight into the evolution of a geothermal system dominated by meteoric waters and driven thermally by the cooling batholith. The chemical and isotopic compositions of the veins were influenced by chemical reactions and isotope exchange of the hydrothermal fluids with the Precambrian metasedimentary rocks.
Brunhes–Matuyama paleomagnetism in three lava sections in Iceland
Authors: Kristjánsson L. , Jóhannesson H. , Eiríksson J. , Gudmundsson A. I.
Published: April 2011

This paper reviews previous field research on the paleomagnetism of volcanics in Iceland at localities whose age is presumed to span the Brunhes–Matuyama chron boundary. Additional mapping and laboratory measurements are presented from three localities of late Quaternary reverse-to-normal transitions. At two, Tjörnes in north Iceland and Eyjafjöll in south Iceland, the identity of the transition is confirmed by K–Ar dating. At Tjörnes and at Ingólfsfjall in southwest Iceland, short-period variations of the paleomagnetic field are demonstrated to be useful in stratigraphic mapping.
Tectonic interpretation of west-verging folds in the Selkirk Allochthon of the southern Canadian Cordillera
Authors: Brown Richard L. , Lane Larry S.
Published: April 2011

The Selkirk Allochthon, a composite tectonic slice composed of North American paleocontinental-margin deposits and more distal, possibly marginal-basin "suspect terrane," was displaced eastward toward the craton in the Late Jurassic and Late Cretaceous.The Carnes Nappe, a major west-verging recumbent anticline within the Selkirk Allochthon, is considered the southern continuation of Scrip Nappe, which in the Monashee Mountains has an inverted limb length of 50 km. The west-verging nappe and associated structures are interpreted as having originated in the Early to Middle Jurassic during accretion of western allochthonous terranes and prior to eastward displacement of the Selkirk Allochthon.The reversal from westward vergence away from the North American craton to eastward vergence is considered as marking a fundamental change in the evolution of the orogenic belt and may reflect a transition from underthrusting of western allochthonous terranes on blind-shear zones to east-directed breakthrough thrusts.
Stratigraphy of the Steep Rock Group, northwest Ontario: a major Archaean unconformity and Archaean stromatolites
Authors: Wilks M. E. , Nisbet E. G.
Published: April 2011

The Archaean Steep Rock Group of northwest Ontario, situated in the Wabigoon Subprovince of the Superior Province, Canada, comprises five formations: Wagita Formation (clastics), Mosher Carbonate, Jolliffe Ore Zone, Dismal Ashrock, and Witch Bay Formation (metavolcanics). Reinvestigation of the geology of the group has shown that the basal clastics of the Wagita Formation (0–150 m) unconformably overlie the Marmion Complex (a massive tonalite – tonalite gneiss terrane, 3 Ga old). Overlying the basal elastics is the Mosher Carbonate (0–500 m), containing diverse stromatolite morphologies. Extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes. Stratigraphically above the Mosher Carbonate is the Jolliffe Ore Zone (100–400 m), which is divided into a lower Manganiferous Paint Rock Member and an upper Goethite Member. Within the Jolliffe Ore Zone thin layers of "Buckshot Ore" occur. These are horizons of haematitic pisolites and fragments, set in a lighter ferruginous matrix of kaolinite and gibbsite. Overlying the Jolliffe Ore Zone is the Dismal Ashrock, a dominantly high-Mg pyroclastic rock (22% MgO) with minor interbedded lava flows (15% MgO). In contact with the Dismal Ashrock are the metavolcanics of the Witch Bay Formation. This juxtaposition is not exposed in the Steep Rock mine section, and the Witch Bay Formation may be separated from the Dismal Ashrock by a structural break. The Witch Bay Formation is only provisionally included in the Steep Rock Group.The group is interpreted as a sequence deposited in an extensional or rifting environment. The unconformity has regional significance, and it may be possible to define an extensive cratonic nucleus of 3 Ga or older age in northwest Ontario.
Petrographic constraints on modelling the crystallization of basalt magma, Cow Lakes, southeast Oregon
Authors: Russell J. K. , Nixon G. T. , Pearce T. H.
Published: April 2011

Thermodynamic calculations and models of olivine zoning profiles are used to estimate the crystallization history of a basaltic magma from Cow Lakes, southeast Oregon. The lava is an alkali olivine basalt containing olivine and plagioclase phenocrysts and microphenocrysts. The geometry and range of chemical zoning in the olivine phenocrysts have been delineated by laser interference microscopy and electron microprobe analysis. The olivine phenocrysts are characterized by homogeneous cores and rims that exhibit strong, continuous, normal zoning (ΔFo = 7–19 mol%).Thermodynamic modelling has been used to estimate the magmatic crystallization path of the Cow Lakes basalt on the basis of the phenocryst assemblage and mineral compositions. The calculated crystallization path begins at 1290 °C and 0.5 GPa ([Formula: see text]) with equilibrium crystallization of the olivine to 1265 °C. Plagioclase appears at 1225 °C, followed by clinopyroxene at 1205 °C. Intratelluric crystallization was terminated prior to crystallization of the clinopyroxene, which is seen in the groundmass but not as phenocrysts.The thermodynamic modelling provides a means to numerically simulate the zoning patterns in olivine defined by the laser interference microscopy. Simulated and observed zoning patterns both have compositionally flat cores and strongly zoned rims. The extent of zoning observed in the olivine phenocrysts is, however, approximately twice the predicted extent, and it appears that a significant proportion of olivine phenocrysts crystallized during ascent or upon eruption.
Geochemistry and petrogenesis of the early Proterozoic Hemlock volcanic rocks and the Kiernan sills, southern Lake Superior region
Authors: Ueng W. C. , Fox T. P. , Larue D. K. , Wilband J. T.
Published: April 2011

During the early Proterozoic, the 2 km thick differentiated gabbroic Kiernan sills were emplaced into a thick accumulation of pillow basalt and associated deep-water strata, the Hemlock Formation, in the southern Lake Superior region. On the basis of major elements and trace elements (including rare-earth-element data), the Kiernan sills and the hosting volcanic rocks of the Hemlock Formation were determined to be comagmatic in origin, and both evolved from assimilation – crystal fractionation processes. The major assimilated components in these igneous rocks are identified as terrigenous sedimentary rocks. Assimilation affected the abundance of Nb, Ta, light rare-earth elements, and most likely P, Rb, Th, and K in the magma. The effect of chemical contamination from wall-rock assimilation accumulates with increasing differentiation.With wall-rock contamination carefully evaluated, a series of tectonic discriminating methods utilizing immobile trace elements indicates that the source magma was a high-Ti tholeiitic basalt similar to present-day mid-ocean-ridge basalts (MORB). It is suggested from this study that most of the enriched large-ion lithophile elements and LREE of the magma were not inherited from the mantle but from assimilation of supracrustal rocks. Chemical signatures of these rocks are distinctively different from those of arc-related volcanics. A rifting tectonic regime analogous to the opening of the North Atlantic Ocean and extrusion of North Atlantic Tertiary volcanics best fits the criteria revealed by this study.
Stable-isotope pattern predicted in surge-type glaciers
Authors: Waddington E. D. , Clarke G. K. C.
Published: April 2011

The distribution pattern of stable-isotope ratio δ18O in cold glaciers, ice streams, and ice sheets has the potential to reveal past changes in flow rate, for example those associated with surges. In this study, we use a time-dependent numerical model of ice flow to establish that each surge creates a stratigraphic horizon across which δ18O is discontinuous. Two plausible relations between glacier geometry and δ18O in snowfall allow us to bracket the expected magnitude of this isotopic signal. These stratigraphic markers could be located by δ18O analysis of a longitudinal series of ice cores or by detailed longitudinal sampling of exposed ice.Calculations for a model with characteristics resembling those of Steele Glacier, Yukon Territory, showed that at most three stratigraphic markers could be detected at any one time. The discontinuities in δ18O were as large as 0.8‰. This is an order of magnitude larger than mass spectrometer precision but comparable to observed background noise at Steele Glacier.
Foraminiferal inventory and lithologic description of the Lower Cretaceous (Albian) Hulcross Shale, Monkman area, northeastern British Columbia
Authors: Stelck C. R. , Leckie Dale
Published: April 2011

Correlation of an arenaceous microfauna of 41 species (44 subspecies) from the Hulcross Formation of the Monkman Pass area with that found in the Hulcross Shale on the Peace River has a further corroboration in matching spectra of volcanic ash beds at both localities. The Monkman Pass equivalents of both the basalmost portion and the upper part of the Hulcross beds on the Peace River are in sandy to continental facies. Diatoms are plentiful in the Monkman Pass section in the shaly facies.The foraminiferal genera include Bathysiphon, Thuramminoides, Psammosphaera, Hippocrepina, Saccammina, Reophax, Glomospira, Ammodiscus, Psamminopelta, Haplophragmoides, Ammobaculites, Trochammina, Textulariopsis, Pseudobolivina, Verneuilinoides, Uvigerinammina, Gaudryina, Eggerella, and Dorothia. Only Thuramminoides, Haplophragmoides, and Trochammina occur in abundance, indicating water of depth around 100 m at time of deposition and of salinity close to that of normal seawater. The low degree of bioturbation and the presence of Chondrites in the lower Hulcross suggest that poorly oxygenated bottom waters prevailed. Upwards, the diversity and abundance of bioturbation increase, indicating higher oxygenation contents caused by better circulation. As such, the Hulcross basin had a stratified water column.
Plutonic and hydrothermal events in the Ackley Granite, southeast Newfoundland, as indicated by total-fusion 40Ar/39Ar geochronology
Authors: Kontak D. J. , Tuach J. , Strong D. F. , Archibald D. A. , Farrar E.
Published: April 2011

Results of total-fusion 40Ar/39Ar dating (eight biotites, four muscovites, and one hornblende) of magmatic and hydrothermal stages of the high-silica, chemically zoned Ackley Granite indicate three distinct episodes of magmatic activity, viz. ≥ 410, 378–374, and 355 Ma, and that the metallogenic system (W–Sn, Mo) evolved synchronously with only one of the magmatic phases.The oldest suite is indicated by two biotite dates at 410.4 ± 4.4 and 392.5 ± 7.6 Ma, which are themselves considered as representing partial resetting related to a tectonothermal event of Acadian age. Thus, the 410 Ma age offers the best estimate of intrusion, although it must be considered a minimum. The main magmatic pulse for the Ackley Granite is indicated by dates between 378 and 374 Ma, with most of these phases occurring in the southern part of the complex. Concordant hornblende–biotite ages (374.8 ± 3.8 and 372.3 ± 4.8 Ma, respectively) for one of the southern phases (Rencontre Lake granite) suggests rapid cooling for this part of the complex. In contrast, the data for a contemporaneous intrusion (Kepenkeck granite) in the northern part of the Ackley Granite are discordant, with a muscovite–biotite pair yielding ages of 378.4 ± 4.8 and 367.7 ± 4.3 Ma, respectively. Four remaining biotite dates, widely distributed within the southern part of the Ackley Granite, gave similar ages of ca. 368 Ma.Three hydrothermal muscovites from mineralized greisen zones along the soumern margin of the granite are dated at 371.3 ± 4.5, 371.3 ± 5.4, and 373.5 ± 4.0 Ma, essentially coeval with the main magmatic event of the Ackley Granite. Since these paragenetically late muscovites give slightly older ages than the magmatic biotites of the second pulse, it is suggested that the ambient temperature for a large part of the Ackley Granite remained in excess of the 250–300 °C biotite blocking temperature for several million years after initial intrusion.A third and presumably magmatic event, presently of unknown dimensions, occurred at 355 Ma. The coincidence of this age with previously published Rb–Sr whole-rock isochron dates of 355 Ma for the Ackley Granite might indicate that this younger magmatic event was in part responsible for resetting the Rb–Sr systematics.
Paleomagnetism of Cretaceous volcanic rocks of the Sverdrup Basin—magnetostratigraphy, paleolatitudes, and rotations
Authors: Wynne P. J. , Irving E. , Osadetz K. G.
Published: April 2011

The principal magnetization of lavas of the Isachsen and Strand Fiord formations on Axel Heiberg Island is shown to predate the Eocene Eurekan Orogeny. Basalt flows of the Strand Fiord Formation, volcanigenic sandstone from the Christopher Formation, and the uppermost flows of the Isachsen Formation are normally magnetized. Reversed magnetizations are found only in the Isachsen Formation, occurring at two horizons, which, we suggest, correspond to M0 and M1 of the M sequence of marine magnetic anomalies (118–123 Ma). It is possible, therefore, that we have located, at least approximately, the base of the Cretaceous normal polarity superchron in these sections. Because inclinations are steep, the analysis of directions of magnetization is not straightforward and has been done by two methods. Method I assumes that no relative rotations have occurred amongst sample localities, and calculations on this basis show a 33 ± 24 °(P = 0.05) counterclockwise rotation with no paleolatitudinal displacement relative to North America. The rotation is in agreement with the rotation of 36 ± 8 °(P = 0.05) determined earlier from the Permian Esayoo Formation on Ellesmere Island. Analysis by method I assumes that the Esayoo and the Isachsen – Strand Fiord sampling localities on Axel Heiberg and Ellesmere Island are contained within what is essentially one large structural domain. The agreement (using method I) of paleolatitude with that of North America is consistent with standard plate reconstructions in which there is a gap of about 300 km between Greenland and Ellesmere Island. However, the dispersion of site-mean directions is greater than that expected for paleosecular variation during the Cretaceous, and therefore some of the dispersion may be attributable to relative motions amongst collecting localities. Therefore, by method II, relative rotations amongst localities are assumed to have occurred, and inclinations and declinations are analysed separately. As with method I, declinations are predominantly counterclockwise from that expected, but by method II the mean inclination (74 ± 2 °standard error) is significantly shallower than that expected (79 ± 1 °standard error). This apparent flattening is consistent with the idea that the Arctic Islands were close to Greenland in the Cretaceous and that there was no gap along Nares Strait. Hence both methods of calculation yield similar counter clockwise rotation, but each gives slightly different paleolatitudes. The latter difference cannot at present be resolved.
Palynology and geochronology of Eocene beds from Cheslatta Falls and Nazko areas, central British Columbia
Authors: Rouse G. E. , Mathews W. H.
Published: April 2011

Early Tertiary lavas and sediments were collected from two areas west and south of Prince George, British Columbia, and processed for K/Ar dating and palynoassemblages.Samples from Cheslatta Falls gave K/Ar dates of 36.5 and 37.7 Ma and yielded a palynoassemblage very similar to that from the Jackson Group in Alabama and Mississippi. This Late Eocene assemblage is interpreted as having developed in a humid subtropical environment, prior to a climatic cooling that, we believe, should be assigned to Early Oligocene time.The Tertiary lavas and sediments from the Nazko area, west of Quesnel, gave three Middle Eocene dates and over 60 species of palynomorphs. These correlate with other rocks and palynoassemblages of Kamloops Group equivalents that occur southward across the International Boundary and north to latitude 55°N. The paleoclimate was wet and in the range of very warm temperate to humid subtropical.