bluuebunny/crossref_metadata_2025 huggingface dataset:
Séquence de la transition Illinoien–Sangamonien : forage IAC-91 de l'île aux Coudres, estuaire moyen du Saint-Laurent, Québec
Authors: Occhietti S. , Long B. , Clet M. , Boespflug X. , Sabeur N.
Published: April 2011

A series of Pleistocene deposits extending over 155 m below sea level was drilled at Aux Coudres Island, in the middle St. Lawrence Estuary, Quebec. The series, which basis is unknown, is divided into two sedimentary units: the lower Baie-Saint-Paul Glacial Complex facies (−155 to −125 m), which is correlated with the Illinoian (isotopic stage 6), and a stratified sequence referred to as the île aux Coudres Formation. The latter is subdivided into four zones: a very compact lower clay (−125 to −102 m), rhythmites with Paleozoic schist debris (−102 to −71 m), and prodeltaic silts and deltaic sandy silts with brackish water benthic foraminifera (−71 to −2 m). The spore and pollen content indicates a schrub tundra followed by an afforestation sequence of a boreal forest that changes to an Abies forest and then to an interglacial mixed forest with Betula, Jugions, Carpinus or Ostrya, Carya, and, at the top, Betula, Tsuga, Quercus, and Ulmus. The accumulation of the sediments of the Île aux Coudres Formation required approximately 3500 years, beginning with a deep marine environment (about 300 m) followed by shallowing waters during the subsequent glacioisostatic rebound phase of the regression. The sedimentation is assigned to a main postglacial marine invasion, referred to as the Guettard Sea, which occurred prior to two regional glacial episodes and was partly contemporaneous with Bell Sea invasion in the Hudson Bay lowlands. A major postglacial sedimentary influx in the Atlantic Ocean, during the Illinoian-Sangamonian transition and at the beginning of the Sangamonian (transition 6–5 and early substage 5e) is inferred from this marine event.
The origin of a tephra-like bed near Mount Cayley volcano, southwestern British Columbia, Canada
Authors: Brooks Gregory R. , Hicki Edward J.
Published: April 2011

A distinct white bed 1–2 cm thick is present within backwater deposits at the upstream end of the Turbid Creek debris fan, southwestern British Columbia, Canada. This white bed and the enclosing backwater deposits accumulated within an impoundment of Squamish River caused by a large ca. 4800 BP debris avalanche originating from Mount Cayley, a Pliocene–Pleistocene volcano. The white bed is composed of silt and clay detritus and resembles tephra. About 5% of the grains exhibit optical characteristics consistent with volcanic glass. The remainder, however, are mineral and lithic particles (mainly cristobalite and Na-feldspar, with minor quartz, and trace amphibolite, mica group, and chlorite). The white bed is interpreted to be dust derived from the debris avalanche and washed into the impoundment, based upon (i) the high proportion of mineral and lithic grains within the bed, (ii) the contemporaneity of the bed and the debris avalanche, (iii) the lack of Holocene volcanic activity at Mount Cayley, and (iv) the significant age difference between the bed and known tephras in southwestern British Columbia.
Multiple pre-Wisconsinan glaciations along the northwestern edge of the Allegheny Plateau in Ohio and Pennsylvania
Authors: Szabo John P. , Totten Stanley M.
Published: April 2011

Stratigraphic studies of the glacial geology of the northwestern margin of the Allegheny Plateau of northeastern Ohio and northwestern Pennsylvania include evidence for multiple pre-Wisconsinan glaciations. Near the former eastern margin of the Scioto lobe at Mount Gilead, Ohio, Millbrook, Gahanna, and Chesterville tills have an inferred Illinoian age based on thermoluminescence ages of 146 000 ± 25 000 and 124 000 ± 16 000 BP from an overlying silt interpreted as loess. Northampton Till overlies Millbrook Till of the Killbuck lobe and its correlative Mogadore Till of the Cuyahoga lobe, but does not overlie the correlative Titusville Till of the Grand River lobe. There are signficant differences in carbonate contents among these tills. Millbrook Till and its equivalents contain more kaolinite than other tills, and the Northampton Till contains more clay. Millbrook Till and its correlatives represent the most extensive Illinoian glaciation of the plateau, whereas the other Illinoian tills were deposited by ice that flowed a shorter distance onto the Allegheny Plateau. Butler, Mapledale, and Slippery Rock tills, formerly assigned to an Illinoian glaciation, are pre-Illinoian in age. Scarcity of materials for the dating and the lack of recognizable paleosols separating the tills from Wisconsinan deposits inhibits the reliability of the chronological classification of these tills.
A varve-based calibration of the Bridge River tephra fall
Authors: Leonard Eric M.
Published: April 2011

A 1 cm thick layer of Bridge River tephra is preserved in a core of varved lake sediments recovered from Hector Lake, Alberta. The varve sequence extends from the core top to well below the tephra layer and has bseen calibrated to calendar years. A varve count yields a calendar age of 2332 BP for the ash layer, with a potential error of about ±50 years. This age is about 30 years younger than the current best radiocarbon-based estimate of the time of the eruption. However, in light of the potential errors in both the varve count and the radiocarbon age, the two ages are not inconsistent. The tephra contains no biotite. In the past, biotite-free Bridge River tephra has been associated with a possible late (ca. 2000 BP) tephra fall, but the tephra from Hector Lake is clearly associated with the main (2300–2400 BP) eruptive event.
Allerød–Younger Dryas Coleoptera from western Cape Breton Island, Nova Scotia, Canada
Authors: Miller Randall F.
Published: April 2011

Studies of Coleoptera remains from two late-glacial sites on Cape Breton Island, Nova Scotia, Canada, present a picture of the paleoenvironment and paleoclimate during the Allerød–Younger Dryas transition in the Maritimes region. They provide evidence for the Younger Dryas event in northeastern North America. Between 11 300 and 10 800 BP, the beetle assemblages at the Campbell site are typical of faunas of the modern middle to northern boreal forest. The West Mabou site contains beetle fossils younger than 10 900 BP, possibly as young as 10 500 BP, extending into the time period of the Younger Dryas, dated from 10 800 to 10 000 BP in the Maritimes. A "cold climate" indicator recognizable in the beetle fauna, Olophrum boreale, occurs in relative abundance and provides an interesting comparison with sites in Europe where the same northern boreo-montane species is evident at the beginning of the Younger Dryas.
A large earthquake occurring 700–800 years ago in Aialik Bay, southern coastal Alaska
Authors: Mann Daniel H. , Crowell Aron L.
Published: April 2011

A conifer forest on the shore of Verdant Cove, an inlet of Aialik Bay on the southeast coast of the Kenai Peninsula, Alaska, was buried by high-energy beach sediments shortly after 860 ± 50 I4C years BP. The switch from ocean-distal forest to cobble beach indicates a radical change in depositional environment suggestive of rapid subsidence of 1–3.5 m. The presence of hemlock, a tree taxon sensitive to salt-water exposure, and the preserved cast of a tree trunk suggest that subsidence and burial occurred rapidly. By 690 ± 60 14C years BP, forest peat was accumulating atop the beach sediments burying the forest and rapid spit progradation was underway. Spit progradation implies land emergence or stable sea level, especially in this case where the spit has a limited sediment supply. We infer that subsidence of the spit ca. 860 ± 50 14C years BP was followed by slow land emergence up to the time of the 1964 AD great earthquake, when the area subsided coseismically 1.4 m. The sudden drowning of the buried forest in Verdant Cove was probably caused by coseismic subsidence and later emergence by gradual, interseismic uplift. Comparison of the Verdant Cove record with previously reported data from south-central Alaska suggests that the penultimate great earthquake in the rupture zone of the 1964 Alaska earthquake occurred between 700 and 800 calendric years BP.
On the discovery of an oviraptorid skeleton on a nest of eggs at Bayan Mandahu, Inner Mongolia, People's Republic of China
Authors: Dong Zhi-Ming , Currie Philip J.
Published: April 2011

A partial skeleton of Oviraptor (which means egg thief), collected at Bayan Mandahu (Inner Mongolia, People's Republic of China) in 1990 was lying on top of a nest of eggs. Of the six known skeletons of this genus from Upper Cretaceous Djadokhtan sediments, this is the second occurrence in which the theropods were interacting with the eggs when they were buried by sand and dust during sandstorms. Two explanations for the association of Oviraptor with eggs are that the theropod may have been eating the eggs, or it may have been incubating and protecting them. Evidence presented suggests that the latter hypothesis is more likely. It is also conceivable that the female oviraptorid was in the process of laying eggs when she died.
Un diatrème phréatomagmatique montérégien dans les Appalaches du Québec
Authors: Morin David , Jébrak Michel , Marquis Robert
Published: April 2011

A subcircular positive magnetic anomaly and breccias affecting a basanite and its country-rock metasedimentary rocks reveal the presence of a diatreme with a diameter of approximately 420 m, at Eastman, in the Quebec Appalachians. The post-Middle Devonian age, the position in the line of the Monteregian plutons, and the basanite composition, which is comparable to that of the Cretaceous Monteregian alkaline lamprophyres, suggest that the diatreme is related to the Monteregian magmatism. It is located at the junction of two orthogonal tectonic corridors: the north-north-east Baie Verte – Brompton line and an east−west fault network along the prolongation of the Ottawa−Bonnechère Graben. These structures are zones of weakness that probably served as a conduit for the ascending magma and near-surface water to trigger phreatomagmatic eruptions.
Archerpeton anthracos from the Joggins Formation of Nova Scotia: a microsaur, not a reptile
Authors: Reisz R. R. , Modesto S. P.
Published: April 2011

New anatomical information, provided by reinterpretation of the pectoral girdle and exposure of previously unprepared portions of the holotype, reveals that the early tetrapod Archerpeton anthracos (Westphalian A of Joggins, Nova Scotia) is a microsaurian lepospondyl, rather than a reptile, as originally described. Archerpeton anthracos is similar to another Joggins microsaur, Asaphestera intermedia, from which it is distinguished by autapomorphies of the appendicular skeleton. The discovery that A. anthracos is a microsaur increases the number of lepospondyls at Joggins to six species and reduces the number of amniotes to two species, Hylonomus lyelli and Protoclepsydrops haplous.
Seismic properties of rock samples from the Pikwitonei granulite belt – God's Lake domain crustal cross section, Manitoba
Authors: Fountain David M. , Salisbury Matthew H.
Published: April 2011

Laboratory measurements of compressional and shear wave velocity to confining pressures of 600 MPa for a suite of representative samples collected from the Pikwitonei granulite belt and God's Lake domain, an Archean crustal cross section in the northwestern Superior Province, provide the basis of comparison of these terranes with the seismic characteristics of Archean lower crust. We found that felsic rocks in the Pikwitonei granulite belt and God's Lake domain, which make up the bulk of these terranes, have a similar average compressional wave velocity of 6.5 km/s at 600 MPa, indicating that felsic rocks show little velocity change across the amphibolite–granulite facies transition. Compressional wave velocities for mafic rocks from each terrane are between 7.1 and 7.3 km/s. Apparent Poisson's ratio ranges from 0.24 to 0.26 and 0.26 to 0.28 for felsic and mafic rocks, respectively. These velocity data compare favorably with data for similar lithologies from the Kapuskasing uplift. Using the relative abundances of the constituent lithologies, the weighted average compressional wave velocities of the God's Lake domain and Pikwitonei granulite belt at 600 MPa are 6.56 and 6.63 km/s, respectively. These values, coupled with velocity distribution functions based on the population statistics and relative abundance for each lithology, show that there is no correspondence between the seismic characteristics of the Pikwitonei granulite belt and typical Archean and Proterozoic lower crust. The average properties of the Pikwitonei granulite belt and God's Lake domain, however, correspond well with typical Archean and Proterozoic middle crust. This suggests that either the Pikwitonei granulite belt represents an extreme felsic end member of Archean lower crust or that the deepest levels of the Superior Province crust are not exposed in the Pikwitonei granulite belt. Similar distribution function diagrams for acoustic impedance show that the Pikwitonei granulite belt is characterized by high acoustic impedance contrasts, but the high-impedance component is low in abundance. If the strong reflections observed under the Pikwitonei granulite belt in recent Lithoprobe surveys are not due to other causes, such as favorably oriented bodies of metamorphosed banded iron formation, diabase, or rock units not exposed in this region but present at depth, then they are caused by surprisingly small volumes of mafic metavolcanic rocks.
Pressure – temperature and tectonic evolution of Triassic lawsonite – aragonite blueschists from Pinchi Lake, British Columbia
Authors: Ghent Edward D. , Erdmer Philippe , Archibald Douglas A. , Stout Mavis Z.
Published: April 2011

A blueschist and eclogite terrane is associated with one of the largest faults in the Canadian Cordilleran Orogen, the Pinchi fault. Blueschists (in situ) and retrogressed eclogite blocks occur along the Pinchi fault zone near 54°30'N and 124°W. Critical blueschist facies mineral assemblages include lawsonite–glaucophane, jadeite–lawsonite–glaucophane–quartz, and aragonite. White mica 40Ar/39Ar spectra on blueschist and eclogite yield ages in the range 221.8 ± 1.9 to 223.5 ± 1.7 Ma, establishing a direct link between the blueschists and eclogites. Preservation of aragonite sets rigid constraints on the pressure–temperature–fluid–time conditions of unroofing. K–Ar dates indicate that this is some of the oldest documented metamorphic aragonite. Comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200–300 °C, with pressures greater than 8–10 kbar (1 kbar = 100 MPa). Unroofing likely occurred during collision of the Cache Creek terrane with Quesnellia in the Late Triassic to Middle Jurassic. The fault was initiated as a plate boundary and was active as late as Eocene time as a strike-slip zone. The Pinchi blueschist terrane is similar to others in the North American Cordillera and highlights a tectonic regime of repeated blueschist metamorphism and rapid unroofing along many parts of the western margin of North America in the early Mesozoic.
Whiterockian (Ordovician) crinoid fauna from the Table Head Group, western Newfoundland, Canada
Authors: Ausich William I , Bolton Thomas E , Cumming Leslie M
Published: April 2011

Three crinoids are described from the Table Point Formation, Table Head Group (Whiterockian-Llanvirn) from Aguathuna quarry, Port au Port Peninsula, western Newfoundland, including Trichinocrinus terranovicus Moore and Laudon, Isotomocrinus apheles n.sp., and Brechmocrinus eos n.gen. and n.sp. Trichinocrinus terranovicus is one of the oldest camerate crinoids known. It was previously described from a single specimen, and the numerous additional specimens confirm that this crinoid had few fixed calyx plates, demonstrate that the small "pinnulets" are spines, and reveal that this crinoid had a very unusual posterior. The posterior interradius may indicate that all crinoids had homologous plating in the posterior interray rather than the camerate posterior interray being distinct from other crinoids. Isotomocrinus apheles n.sp. is the oldest species of this Ordovician genus, and Brechmocrinus eos n.gen. and n.sp. is the third known Arenig to Llanvirn cladid with a compound radianal.
Interpretation of the satellite magnetic anomaly of the Nova Scotia marginal basin
Authors: Ghods A , Arkani-Hamed J
Published: April 2011

Satellite magnetic anomaly maps show well-defined negative anomalies over some deep sedimentary marginal basins, such as the Nova Scotia marginal basin. A possible explanation would be the thermal demagnetization of the oceanic upper crust due to thermal blanketing by the sediments and the oceanic lower crust and uppermost mantle due to subsidence into hotter regions beneath. We examine this possibility by computing the thermoviscous remanent magnetization of the oceanic lithosphere beneath the Nova Scotia basin using a detailed thermal evolution model which takes into account the continental rifting, sea-floor spreading, and subsequent subsidence. It is concluded that the thermal demagnetization is not sufficient to explain the entire observed negative magnetic anomaly over the basin; it contributes ~40% to the anomaly. We suggest that a major part, ~60%, of the anomaly arises from the particular location of the early Mesozoic oceanic lithosphere beneath the basin, which has a relatively weaker bulk remanent magnetization compared with a highly magnetic continental crust in the west and north and the strong magnetic oceanic lithosphere of the Cretaceous Quiet Zone in the east.
The 1870s avulsion of the Saskatchewan River
Authors: Smith Norman D , Slingerland Rudy L , Pérez-Arlucea Marta , Morozova Galina S
Published: April 2011

Several major avulsions of the Saskatchewan River have occurred in the Cumberland Marshes (east-central Saskatchewan) during the past few thousand years. The most recent avulsion occurred in about the 1870s, converting over 500 km2 of floodplain into a belt of anastomosing channels, splay complexes, and small lakes, a region that is still evolving today. The avulsion began near the tip of a large meander bend by following a small outflowing creek (Sturgeon River) which in turn followed an abandoned former channel of the Saskatchewan River. Flow began to permanently divert out of the Saskatchewan when a narrow strip of floodplain separating the Sturgeon River from the nearby Torch River became breached. Diversion into the connected Sturgeon-Torch began to increase sometime in the 1870s and probably culminated around 1882. The triggering event for the avulsion may have been a chute cutoff of the meander bend, shown by numerical modeling experiments to have significantly raised water-surface elevations at the avulsion site. Increasing flow diversion soon overwhelmed the smaller Sturgeon-Torch channel (now known as the New Channel), and several crevasse splays formed to help accommodate avulsive discharge. Sixteen kilometres downstream, most of the avulsive flow spilled out of the New Channel to form a shallow (~1 m), marshy floodplain lake which flowed eastward down the regional floodplain gradient to the basin presently occupied by Cumberland Lake. Since its inception, the avulsion-generated lake has become gradually infilled by prograding splay complexes fed by networks of anastomosing channels to characterize most of the present-day avulsion belt.
Detrital zircon geochronology and regional correlation of metasedimentary rocks in the Coast Mountains, southeastern Alaska
Authors: Gehrels G E , Kapp P A
Published: April 2011

U-Pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the Coast Mountains in southeastern Alaska. These rocks belong to the Port Houghton assemblage, which consists of upper Paleozoic pelitic and psammitic schist, metaconglomerate, metabasalt, and marble. The Port Houghton assemblage rests unconformably(?) on metamorphosed and deformed mid-Paleozoic arc-type volcanics (Endicott Arm assemblage), which gradationally overlie upper Proterozoic(?) - lower Paleozoic continental margin strata (Tracy Arm assemblage). Three main clusters of ages are present: 330-365 Ma (19 grains), 1710-2000 Ma (27 grains), and 2450-2680 Ma (6 grains). Additional grains are approximately 2334, 2364, and 3324 Ma. Comparison of these ages with detrital zircon ages in other Cordilleran assemblages supports previous interpretations that metasedimentary rocks in the Coast Mountains (i) form a southwestern continuation of the Yukon-Tanana terrane of eastern Alaska and Yukon, (ii) are not correlative with strata of the Alexander terrane, and (iii) contain detritus that was probably shed from cratonal rocks in the Canadian Shield to the east. Several scenarios exist to explain the occurrence of these continental margin rocks west (outboard) of arc-type and ocean-floor assemblages such as the Stikine, Cache Creek, Quesnel, and Slide Mountain terranes.
The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: northern Siberia data
Authors: Brasier M D , Sukhov S S
Published: April 2011

Carbon and oxygen isotopes have been studied in Lower to Middle Cambrian fossiliferous carbonates from sections along the Malaya Kuonamka, Olenek, and Muna rivers of the Yudoma-Olenek facies belt, northern Siberian Platform. Marked temporal oscillations in delta 13C are recorded from shallow-water Nemakit-Daldynian to Atdabanian carbonates, as seen elsewhere on the platform. Rapid sea-level rise during the so-called "Sinsk event" led to starved-basin conditions from the middle Botomian (Bergeroniellus micmacciformis Zone) to late Amgan (Tomagnostus fissus Zone) and to deposition of organic-rich argillaceous carbonates of the Kuonamka Formation. Mayan carbonates were laid down during a phase of renewed and more rapid carbonate sedimentation, and relatively uniform delta 13C signatures fall from approx. +1.5omicron in the Anomocarioides limbataeformis Zone to near zero throughout the Lejopyge laevigata Zone. These results confirm that a long-term decline took place in maximal delta 13C values between the late Neoproterozoic and Middle Cambrian, accompanied by a dampening in the amplitude of shorter term oscillations. This was accompanied by a change from rift to drift patterns of crustal subsidence; from low to high sea levels; from an interior basin to outer platform locus for hydrocarbon source rocks; from rare to abundant metazoans in the biosphere; by the first major extinction in the Phanerozoic fossil record; and from "icehouse" to "greenhouse" conditions. We infer that factors leading to changes in the rate of carbon burial were amplified at times of low sea level, high climatic extremes, and a primitive biosphere (late Neoproterozoic), and dampened at times of high sea level, lower climatic variation, and a more advanced biosphere (Middle Cambrian).
Structural evolution of the eastern Amisk collage, Trans-Hudson Orogen, Manitoba
Authors: Ryan James J , Williams Paul F
Published: April 2011

Deformation recorded in the Amisk collage in the central part of the Paleoproterozoic Flin Flon Belt (southeastern Trans-Hudson Orogen) is divided into pre-, early, late, and post-Hudsonian orogeny, distinguished by significant changes in metamorphic conditions and the orientation of structures. Detailed structural analysis, petrography, and high-precision geochronology, combined with previous mapping and geochemical studies, indicate a structural history spanning 180 Ma in the Amisk collage, and the database provides an excellent opportunity to study the structural evolution of Precambrian greenstone belts. Accretion of the 1.92-1.88 Ga tectono-stratigraphic assemblages in the Amisk collage began prior to 1.868 Ga. The deformational history records six generations of ductile structures (F1-F6), followed by development of brittle-ductile and brittle structures (F7), which may have continued as late as 1.690 Ga, during exhumation of the collage. The steep, generally north-northeast macroscopic structural grain is dominated by two regional foliations (S2 and S5), and contrasts strongly with the less steeply inclined, east-west grain in the adjacent Kisseynew Domain. Maximum displacements between tectono-stratigraphic assemblages occurred along early rather than late shear zones. Vertical extension was important in post-D1 deformations, even in the later stages. Postorogenic, low-angle extensional features that are common to many mountain belts appear to be absent, possibly indicating that erosion was the dominant unroofing mechanism.
Crustal structures of Grenville, Makkovik, and southern Nain provinces along the Lithoprobe ECSOOT Transect: regional seismic refraction and gravity models and their tectonic implications
Authors: Louden Keith E , Fan Jianming
Published: April 2011

Crustal structures of the eastern Grenville, Makkovik, and southern Nain provinces are determined using seismic reflection-refraction and gravity data along the Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT). Within the Grenville Province, the velocity model contains a 5 km thick upper crust and a variable-thickness middle to lower crust. The total crustal thickness varies from 25 to 43 km, with the thickest crust in the south and thinnest crust in the north. A high-velocity, lower crustal wedge is coincident with a strong band of northward-dipping reflectors. The two-dimensional velocity structure is compatible with modelling of a 60 mGal gravity high over the Hawke River terrane. In the Makkovik Province, the thickness of upper crustal velocities increases to 17 km. The velocity decrease in the upper to middle crust from the Grenville Province to the Makkovik Province is similar to that of refraction models across the Grenville Front in Ontario and Quebec. It is possibly related to a decrease in metamorphic grade from south to north and (or) a larger volume of unmetamorphosed plutons in the Makkovik Province. A lower crustal layer is coincident with a region of increased reflectivity in the lower crust. There are no major crustal discontinuities associated with terrane boundaries within the Makkovik Province. The base of the crust is consistent with a change from north- to south-dipping reflectors beneath the Cape Harrison domain. Alternatively, it may consist of a thick zone of complex velocity variations, consistent with a zone of diffusive reflectivity observed to the north of the Allik domain.
New 40Ar/39Ar isotopic dates from Miocene volcanic rocks in the Lake Mead area and southern Las Vegas Range, Nevada
Authors: Harlan Stephen S , Duebendorfer Ernest M , Deibert Jack E
Published: April 2011

New 40Ar/39Ar dates on volcanic rocks interlayered with synextensional Miocene sedimentary rocks in the western Lake Mead area and southern end of the Las Vegas Range provide tight constraints on magmatism, basin formation, and extensional deformation in the Basin and Range province of southern Nevada. Vertical axis rotations associated with movement along the Las Vegas Valley shear zone occurred after 15.67 ± 0.10 Ma (2 sigma ), based on a 40Ar/39Ar date from a tuff in the Gass Peak formation in the southern Las Vegas Range. Basaltic magmatism in the western Lake Mead area began as early as 13.28 ± 0.09 Ma, based on a date from a basalt flow in the Lovell Wash Member of the Horse Spring Formation. Isotopic dating of a basalt from the volcanic rocks of Callville Mesa indicates that these rocks are as old as 11.41 ± 0.14 Ma, suggesting that volcanic activity began shortly after formation of the Boulder basin, the extensional basin in which the informally named red sandstone unit was deposited. The red sandstone unit is at least as old as 11.70 ± 0.08 Ma and contains megabreccia deposits younger than 12.93 ± 0.10 Ma. This result shows that formation of the Boulder basin was associated with development of topographic relief that was probably generated by movement along the Saddle Island low-angle normal fault. Stratal tilting associated with extension occurred both prior to and after 11.5 Ma.
Campanian to Paleocene dinoflagellate assemblages from the Turtle Mountain core hole, Manitoba, western Canada
Authors: McIntyre David J
Published: April 2011

Dinoflagellate assemblages from the Turtle Mountain core hole provide evidence for the Paleocene age of the Turtle Mountain Formation and confirm that the Boissevain Formation and the Coulter Member of the Pierre Formation are Maastrichtian. Evidence from dinoflagellates suggests that the Odonah Member of the Pierre Formation is late Campanian. The dinoflagellate assemblages indicate that nearshore shallow marine conditions were present at times during deposition of all the formations.
Holocene environmental history of a peatland in the Lena River valley, Siberia
Authors: Jasinski JPP , Warner B G , Andreev A A , Aravena R , Gilbert S E , Zeeb B A , Smol J P , Velichko A A
Published: April 2011

A 3.86 m core of peat and organic lake mud from a polygonal peatland in the Lena River valley of Siberia was radiocarbon dated and analyzed for pollen, plant macrofossils, chrysophyte stomatocysts, stable isotopes, and charcoal. At around 7200 BP, a shallow lake or open-water wetland supported diverse aquatic macrophytes. The site had transformed initially into a richer fen with Carex, Comarum palustris, and Drepanocladus and later a poorer fen with Sphagnum which persisted until around 3000 BP. Fire may have been responsible for silt being blown onto the peatland, which changed the hydrological and geochemical conditions for development of the poor fen. Ice accretion led to an increase in the height of the centre of the polygon and expansion of Sphagnum peatland .18O values become progressively more enriched, which reflects more direct input of summer precipitation waters and less groundwater during this period. Finally, the peatland surface was elevated sufficiently to limit water and nutrient supply, thereby allowing Ericaceae and Betula to grow at the coring site. Fire burned the peatland surface and may have exaggerated the extremely slow rate of peat accumulation. Fire may also be a factor in maintaining the open Larix dahurica forest in the region today, while climate may be contributing to reducing postfire regeneration. Fire and climate together may be controlling the character and composition of forests near tree line in the Lena River valley of this part of Siberia.
Character of the Middle Ordovician Sauk-Tippecanoe sequence boundary in the Ottawa Embayment (eastern Ontario): possible evidence for platform-interior, Taconic tectonism
Authors: Dix George R , Molgat Marianne P
Published: April 2011

In eastern Ontario, a Middle Ordovician disconformity that separates predominantly sandstone of the Rockcliffe Formation from underlying dolomite of the Carillon Formation (Beekmantown Group) has, historically, been considered equivalent to the Sauk-Tippecanoe sequence boundary. The character of the formation contact varies considerably, and includes (1) rare, serrated micropinnacles (centimetres in height); (2) smoothed, irregular to undulose erosional bedforms along shallow (<30 cm) incised channels at least ten's of metres in width; (3) a planar boundary with no obvious evidence of erosion; (4) compaction effects across the boundary, as well as within uppermost Carillon and lowermost Rockcliffe strata; and (5) interbedding of Carillon- and Rockcliffe-like lithologies. The latter two observations suggest that, locally, the disconformity represents a relatively minor gap in time. Local, irregular paleotopography is interpreted as some combination of paleokarst, mechanical erosion, and differential sediment loading. The Carillon-to-Rockcliffe transition, from peritidal carbonates to marine or estuarine sandstones, is punctuated by local, anomalous structural and stratigraphic features: ptygmmatically folded fractures, local disconformities, and deposition, then differential erosion of high-energy fluvial-dominated sediments. Differential erosion, fracturing, and paleokarst are local, anomalous attributes of the pre-Carillon paleosurface as well. Combined, these events bracket Carillon deposition, and are interpreted to reflect local platform-interior tectonism coincident with onset of distal Taconic orogenesis. Platform-interior expression of distant, developing changes in plate-boundary dynamics may be linked to incipient basement reactivation along an underlying, shallowly buried Proterozoic fault system, later manifest as the Ottawa-Bonnechere Graben.
Magnetic properties and modelling: Lithoprobe seismic reflection line 7C, southern Kisseynew Gneiss Belt, Trans-Hudson Orogen
Authors: Thomas M D , Tanczyk E I
Published: April 2011

The Missi Suite, comprising metamorphosed, magnetite-bearing molasse deposits, runs west-northwest across the southern part of the Kisseynew Gneiss Belt in the Trans-Hudson Orogen. It flanks an internal structural boundary, north of which lies the migmatitic core of the Kisseynew Gneiss Belt, formed largely of arc-derived metasediments assigned to the Burntwood Suite. Lithoprobe reflection seismic images crossing the boundary indicate north-facing, homoclinal structures in which the Missi and Burntwood suites are structurally interlayered. The strong magnetic signature associated with the Missi Suite allows an independent assessment of the seismic models, through magnetic modelling constrained by rock magnetic properties. Along Lithoprobe line 7C, the pattern of a prominent magnetic high over the Missi Suite and a subdued magnetic field over the Burntwood Suite is reflected in values of magnetic susceptibilities, demonstrating a close link between magnetic anomalies and surface geology. Measurements of natural remanent magnetization provide an additional critical constraint for modelling. In the seismic model, gently folded north-dipping layers become progressively less steep at depth, eventually becoming subhorizontal. The magnetic model indicates that layers dip at approximately 45° to a depth of about 7 km, a picture supported by steep reflectors in the seismic data themselves and in down-plunge structural modelling. It is proposed that the layers, which probably contain layer-parallel imbricate thrusts, are rooted in a subhorizontal décollement, interpreted from the seismic image, at a depth of about 10 km. The magnetic model is consistent with the presence of a ramp-like structure developed along the ancient margin of the Kisseynew basin.
Change from thrusting to syncollisional extension at a mid-crustal level: an example from the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland)
Authors: Manatschal Gianreto , Ulfbeck David , van Gool Jeroen
Published: April 2011

The Palaeoproterozoic Nagssugtoqidian Orogen in West Greenland represents a mid- to deep-crustal section through a collisional orogen with a complex intrusive, tectonic, and metamorphic history. In the northeastern central part of the orogen, in the Ussuit area, Palaeoproterozoic intrusive and supracrustal rocks are sandwiched between slices of Archaean rocks forming a stack of lithotectonic units. Juxtaposition of these units occurred during west- to northwest-vergent thrusting along ductile shear zones (= D1) associated with a foliation formed at upper amphibolite facies conditions. D1 structures were folded (= FA) and then reactivated and locally omitted by localized east- to east-northeast-vergent extensional ductile shear zones (= D2) at near-peak metamorphic conditions. Shallowly east-plunging, transport-parallel upright folds (= FB) affect but are also truncated by D2 extensional shear zones, suggesting an interplay between FB folding and D2 shearing, compatible with a scenario of overall shortening perpendicular to fold axial surface during simultaneous extension parallel to fold axes. Consequently, the structures preserved in the Ussuit area document a deformation history which results from a change from thrusting to syncollisional extension occurring in mid- to lower crustal levels at peak metamorphic conditions.
The early tectono-magmatic evolution of the Southern Province: implications from the Agnew Intrusion, central Ontario, Canada
Authors: Vogel D C , James R S , Keays R R
Published: April 2011

The Palaeoproterozoic Southern Province comprises a thick, continental rift related volcanic-sedimentary sequence along the southern margin of the Archaean Superior Province. The Agnew Intrusion (50 km2), which is a member of the East Bull Lake suite of layered intrusions, occurs adjacent to the Superior Province - Southern Province boundary in central Ontario, Canada, and provides an opportunity to examine the early tectono-magmatic evolution of a Palaeoproterozoic rifting event. The Agnew Intrusion is a well-exposed, 2100 m thick, layered gabbronoritic to leucogabbronoritic pluton. It was the product of at least four recognizable, but chemically similar, high-Al2O3 and low-TiO2 magma pulses. Structural data, coupled with excellent stratigraphic correlations between the Agnew Intrusion and other East Bull Lake suite layered intrusions, suggest that these plutons are erosional remnants of one or more sill-like bodies that may originally have formed an extensive, subhorizontal mafic sheet. We argue on the basis of field evidence that the early evolution of the Southern Province was characterized by a large, mantle plume induced magmatic event that gave rise to a Palaeoproterozoic continental flood basalt province. However, the incompatible trace element characteristics of the Agnew Intrusion parental magma (i.e., large ion lithophile and light rare earth element enrichment and high field strength element depletion) are more typical of modern subduction-modified subcontinental lithospheric mantle. Given that this is a prevailing geochemical signature of mafic rocks in the Archaean-Palaeoproterozoic, we suggest that there was a fundamental difference in both the composition and structure between the ancient and more modern mantle. "Subduction-like" geochemical signatures may have been imparted to the entire developing mantle during early Earth differentiation.
Seismic refraction data in the Gulf of Saint Lawrence: implications for the lower-crustal blocks
Authors: Jackson H R , Marillier F , Hall J
Published: April 2011

A refined model for the wide-angle reflection-refraction profile 88-3 that crosses the foreland basin of the Appalachian orogen is presented. The two major layers of the cratonic crust have velocities of 6.2 and 6.7 km·s-1 with low gradients. Near the exposed North American craton, the 6.7 km·s-1 velocity appears at the shallow depth of 13 km and the M discontinuity at greater than 40 km depth. Towards the orogen, a southeastward-dipping ramp is developed at mid-crustal levels at the top of the 6.7 km·s-1 layer. Near the Appalachian Front lower-crustal velocities typical of the Grenville are replaced with a velocity of 7.2 km·s-1. The collinear reflection profile confirms the southward-dipping ramp, the shallowing M discontinuity, and the lateral changes in the lower crust. Refraction profiles combined with reflection and gravity data are used to distinguish the Grenville lower-crustal block. The Grenville lower-crustal block is defined as a pattern of lower-crustal and upper-mantle reflectivity, with associated velocities, and a more negative gravity anomaly than lower-crustal blocks of the Appalachian terranes. The Grenville block terminates northwest of the coast of Newfoundland near the Appalachian Front. Thus, this lower-crustal province does not underlie the Humber tectono-stratigraphic zone. This is different than the relationship of the edge of the Grenville lower-crustal block to surface zones in New England. A thick-skinned tectonic style with basement-involved deformation appears to be typical of the Newfoundland Appalachians; in contrast, a thin-skinned structural style is observed in New England. The change in structural styles occurs in the bend of the Appalachian orogen across the Gulf of St. Lawrence.
Geology of metasedimentary rocks and Late Cretaceous deformation history in the northern Valhalla complex, British Columbia
Authors: Schaubs Peter M , Carr Sharon D
Published: April 2011

The Valhalla complex, a Cordilleran metamorphic core complex, is a domal culmination made up of gently dipping interlayered sheets of igneous and supracrustal rocks that were deformed and metamorphosed in the Middle Jurassic and Late Cretaceous, and exhumed by extensional faults in the Eocene. Mapping, fabric, and metamorphic studies of predominantly metasedimentary rocks in Valhalla and Passmore domes in the northern part of the complex, together with published geochronological data, reveal a significant Late Cretaceous tectonic history. This includes extensive magmatism, the culmination of upper amphibolite facies metamorphism (approx. 800°C and 8 GPa), migmatization, development of a dominant penetrative transposition foliation, and localization of strain on ductile thrust faults termed the Gwillim Creek shear zones. The Valhalla assemblage, a package of metasedimentary rocks in Valhalla and Passmore domes, comprises a heterogeneous sequence of pelitic schist, marble, calc-silicate gneiss, psammitic gneiss, metaconglomerate, quartzite, amphibolite gneiss, and ultramafic rocks. Based on the presence of distinct laterally continuous marker units and similar lithologic ordering, we propose that the Valhalla assemblage is correlative with part of the Palaeozoic North American stratigraphic succession. If this is correct, then the Valhalla assemblage represents an inverted sequence of strata that has been thinned by as much as 60%; thinning may have occurred during Late Cretaceous transposition foliation development. The Gwillim Creek shear zones, originally mapped in a restricted locality in Gwillim Creek, were found to merge into one broad, ductile shear zone beneath Valhalla dome and extend throughout the entire Valhalla complex. The general style and timing of Late Cretaceous deformation in the Valhalla complex is characteristic of that found throughout the Shuswap complex in a belt of rocks that were at mid-crustal levels during the Cretaceous. This zone is thought to have accommodated Cretaceous - Early Tertiary shortening in the eastern Cordillera, and is the ductile equivalent of the higher level Rocky Mountain thrust belt to the east.
A large arthropod trackway from the Gaspé Sandstone Group (Middle Devonian) of eastern Canada
Authors: Braddy Simon J , Milner Andrew RC
Published: April 2011

A large arthropod trackway from the Cap-aux-Os Member of the Battery Point Formation (Gaspé Sandstone Group, Middle Devonian), from the Baie de Gaspé, eastern Canada, is described and assigned to the ichnotaxon Palmichnium (= Paleohelcura) antarcticum (Gevers et al., 1971). A large stylonurid eurypterid or scorpion is considered the most likely producer. A shallow-water marginal fluvial environment is inferred as the setting, the animal making a transition from walking to swimming along the course of the trackway.
Evidence for catastrophic subglacial meltwater sheetflood events on the Bruce Peninsula, Ontario
Authors: Kor PSG , Cowell D W
Published: April 2011

The Bruce Peninsula, a carbonate bedrock escarpment, lies "downflow" from a sculpted bedrock terrain at the French River. The sculpted forms are attributed to a hypothesis of erosion by regional-scale, subglacial meltwater flooding. This paper presents new data from the Bruce Peninsula that tests the meltwater outburst hypothesis in a downflow direction of the predicted flood path. The bedrock surface of the Bruce Peninsula shows extensive development of sculpted features that bear a striking resemblance to s-forms at the mouth of the French River. They are self-similar and hierarchical in scale, ranging in dimensions from a few centimetres to several kilometres. Remarkable concentrations of potholes are located near the brow of the escarpment. The Bruce Peninsula lacks a pervasive cover of unconsolidated sediment. What little sediment exists has been modified into long, narrow drumlins. The Niagara Escarpment on the peninsula has been back wasted into the edge of the Paleozoic Michigan Basin. Along its east-facing slope, the escarpment is marked by more overdeepened reentrant valleys and intervening promontories than is normal for the rest of the escarpment. Clusters of rounded, percussion-marked boulders of exotic origin are concentrated at the heads of the reentrant valleys. Taken together, these features are inferred to support the hypothesis that subglacial outburst floods beneath the Laurentide ice sheet crossed Georgian Bay and strongly sculpted the Bruce Peninsula. The consistent orientation of the reentrant valleys, aligned with the French River sculpting across the basin to the northeast, and the backwasting of its caprock attest to the power and directional stability of the sheetfloods.
Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville Province, Ontario
Authors: Nadeau Léopold , van Breemen Otto
Published: April 2011

The Central Gneiss Belt comprises parautochthonous gneisses overlain by northwest-transported allochthonous terranes originating from the pre-Grenvillian Laurentian margin or from farther outboard as inferred for the Parry Sound allochthon. In the Huntsville region, orthogneisses of the Algonquin allochthon yielded U-Pb zircon igneous crystallization ages at 1444 +12-8, 1442 +9-8, and 1432 +54-98 Ma. In absence of direct evidence for older intrusions, the association of these plutonic rocks with gneisses giving Nd crustal residence ages of ca. 1.7 Ga sets the Algonquin allochthon apart from the underlying parautochthon, and from the overlying Muskoka allochthon. Orthogneisses from the latter also give zircon igneous ages at 1453 ± 6 Ma and 1427 +16-13 Ma, with no older memory. These ages closely correspond to the Nd model ages of associated gneisses, testifying to the juvenile nature of this terrane. Plutonic ages from the Algonquin and Muskoka allochthons are older than those of the Parry Sound allochthon to the northwest, thereby confirming its exotic nature. The importance, distribution, and nature of 1475-1410 Ma plutonism down structural section from the juvenile Muskoka allochthon, via the more mature Algonquin allochthon, into the parautochthon and Grenville foreland, testify to the development of an ensialic arc along that part of the Laurentian margin tectonically incorporated in the Central Gneiss Belt. Furthermore, the age and structural relationship require Grenvillian break-back thrust reactivation to account for the high structural position of the Muskoka allochthon, which was part of the Laurentian ramp during overthrusting of the younger and farther travelled Parry Sound allochthon.