2016

Permanent URI for this collectionhttps://hdl.handle.net/1807/71215

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A 1300 year reconstruction of paleofloods using oxbow lake sediments in temperate southwestern Québec, Canada
(Canadian Science Publishing, 2016-01-10) Oliva, Frank; Viau, Andre E.; Bjornson, Jean; Desrochers, Nicolas M; Bonneau, Marc-André
The study of paleofloods provides important information on past flood frequency and intensity for regions where there is a paucity of records, it therefore extends our knowledge of flood occurrence beyond the historical record. Many paleoflood reconstructions come from the arid dry climate of southwestern USA and from Europe with few studies being conducted in temperate climates of North America. This study uses sediment cores from oxbow lakes to reconstruct past flood events in a temperate region. Cores extracted from two oxbow lakes along the Désert River in southwestern Québec, Canada were analyzed for magnetic susceptibility, loss-on-ignition, grain-size and were radiocarbon dated (14C). Using a combination of magnetic susceptibility variations, along with changes in grain-size and organic material content, 5 floods were identified within the 220 cm core (1300 years) from the North oxbow lake, and six floods in the 118 cm core (600 years) from the South oxbow lake. This study provides evidence to support the use of oxbow lakes in temperate regions as a proxy of past floods, thus helping us understand hydroclimatic changes at regional scales. Data that spans a longer period of time and in different environments is key to increase flood modelling accuracy to improve mitigation strategies under a changing climate.
40AR/39AR THERMOCHRONOLOGY STUDIES OF THE THOR-ODIN – PINNACLES AREA, SOUTHEASTERN BRITISH COLUMBIA: TECTONIC IMPLICATIONS OF COOLING AND EXHUMATION PATTERNS.
(Canadian Science Publishing, 2016-04-27) van Rooyen, Deanne; Carr, Sharon D.
The Thor-Odin dome is a basement-cored tectonothermal culmination in southern British Columbia containing high-grade metamorphic rocks that were polydeformed during the Cordilleran orogenesis. A north-south 40Ar/39Ar thermochronology transect was carried out throughout a ∼7 km thick tilted section in the Thor-Odin dome and structurally overlying rocks to construct thermochronological histories using existing U–Pb geochronology data with new 40Ar/39Ar data and to determine the nature of the boundary between the dome and overlying rocks at Cariboo Alp. Hornblende cooling dates are ∼62–58 Ma at the highest structural level, ∼57–55 Ma in the middle, and ∼57–53 Ma at Cariboo Alp on the upper boundary of the dome. Muscovite and biotite cooling dates are ∼53–50.5 Ma; identical throughout the dome, margin, and overlying panel. The Cariboo Alp area separating the Thor-Odin dome from overlying rocks did not accommodate major post-cooling extensional deformation; rather, it is a Late Cretaceous to Paleocene compressional shear zone. These domains cooled at different rates from >700 to ca. 300 °C, with upper structural levels cooling at rates of ca. 20 °C/Ma and the lowest levels at rates in excess of 120 °C/Ma. All levels passed through the closure temperature for argon in biotite (here calculated to be 320–330 °C) together at ca. 52–51 Ma. Differential cooling rates are the result of interaction between northeast-directed compressional transport of rocks towards the foreland of the orogen overlapping with activity on the Columbia River fault zone, reflecting crustal-scale extension that reached a peak in the Eocene.
A large onychodontiform (Osteichthyes: Sarcopterygii) apex predator from the Eifelian-aged Dundee Formation of Ontario, Canada.
(Canadian Science Publishing, 2016-12-06) Mann, Arjan; Rudkin, David M.; Evans, David C.; Laflamme, Marc
The Devonian marine strata of southwestern Ontario, Canada have been well documented geologically, but their vertebrate fossils are poorly studied. Here we report a new onychodontiform (Osteichthyes, Sarcopterygii) Onychodus eriensis n. sp. from the Dundee Formation (Eifelian-Givetian boundary, 390-387 Ma) of southwestern Ontario represented by two well-preserved onychodontiform lower jaws. The most complete specimen consists of a large (28cm), well preserved right jaw with most of the dentition present. The dentary has 50 teeth, not including the parasymphysial tusk whorl, which is poorly preserved but consists of at least three tusks. The anteriormost teeth of the dentary are also not complete, but the second dentary tooth is notably procurved. The posterior teeth are conical and approximately equal in size for much of the length of the tooth row. Onychodus eriensis n. sp. differs from the closely related contemporary species Onychodus sigmoides, and all other onychodonts, in that it has a strong dorsal curvature of the anterior dentary ramus, and marked anterior expansion of the dentary. An expanded phylogenetic analysis of Devonian onychodontiforms suggests that O. eriensis is closely related to Onychodus jandemarrai. The new material indicates that Onychodontiformes is more diverse than previously recognized, and that further analysis of vertebrate remains from southwestern Ontario will lead to additional insights into the diversity of Devonian sarcopterygians.
A new hesperornithiform (Aves) specimen from the Late Cretaceous Canadian High Arctic with comments on high latitude hesperornithiform diet
(Canadian Science Publishing, 2016-07-16) Wilson, Laura Elizabeth; Chin, Karen; Cumbaa, Stephen
Here we describe a new hesperornithiform specimen from the Upper Cretaceous Kanguk Formation of Devon Island, Nunavut, Canada. This specimen (NUVF 286) is referred to cf. Hesperornis sp. based on size and shape of femora and teeth preserved with other skeletal elements. Previous osteohistologic analyses indicate a sub-adult ontogenetic stage at the time of death. This new cf. Hesperornis specimen includes the first teeth associated with a high-latitude hesperornithiform, allowing for comments on the trophic behavior of these birds. Paleoecological reconstructions based on current knowledge of polar assemblages and comparisons to modern birds suggest that high-latitude hesperornithiform birds may have had more varied diets than previously assumed.
Age and origin of the Cannon Point syenite, Essex County, New York: Southernmost expression of Monteregian Hills magmatism?
(Canadian Science Publishing, 2016-11-11) Bailey, David G.; Lupulescu, Marian V.; Chiarenzelli, Jeffrey R; Traylor, Jonathan P.
Two syenite sills intrude the local Paleozoic strata of eastern New York State and are exposed along the western shore of Lake Champlain. The sills are fine-grained alkali feldspar syenites and quartz syenites, with phenocrysts of sanidine and albite. The two sills are compositionally distinct, with crossing REE profiles and different incompatible element ratios, prohibiting a simple petrogenetic relationship. Zircon extracted from the upper sill yield a U-Pb age of 131.1 +/- 1.7 Ma, making the sills the youngest known igneous rocks in New York State. This age is similar to the earliest intrusions in the Monteregian Hills of Quebec, over 100 km to the north. Sr and Nd radiogenic isotope ratios are also similar to those observed in some of the syenitic rocks of the eastern Monteregian Hills. The Cannon Point syenites have compositions typical of A-type, within-plate granitoids. They exhibit unusually high Ta and Nb concentrations, resulting in distinct trace element signatures that are similar to silicic rocks of the Valles Caldera, a large, rift-related magmatic system. We suggest that the Cannon Point syenites were melts derived primarily by anatexis of old, primitive, lower crustal material in response to Mesozoic rifting and to the intrusion of mantle-derived magmas. The sills indicate that the effects of continental rifting were spatially and temporally extensive, resulting in the reactivation of basement faults in the Lake Champlain valley hundreds of kilometers west of the active rift boundary, and crustal melting over 50 Ma after the initiation of rifting.
Age and tectonic setting of granitoid plutons in the Chéticamp belt, western Cape Breton Island, Nova Scotia, Canada
(Canadian Science Publishing, 2016-08-21) Slaman, Lisa R.; Barr, Sandra M.; White, Chris E; van Rooyen, Deanne
Geological mapping in the Chéticamp granitoid belt in combination with petrographic and geochemical studies and U-Pb (zircon) dating by laser ablation inductively coupled plasma mass spectrometry have resulted in major reinterpretation of the geology in the western part of the Ganderian Aspy terrane of Cape Breton Island. Nine new U-Pb (zircon) ages show that the former “Chéticamp pluton” consists of 10 separate plutons of 5 different ages: Late Neoproterozoic (ca. 567 Ma), Cambrian-Ordovician (490-482 Ma), Ordovician-Silurian (442-440 Ma), mid-Silurian (ca. 428 Ma), and late Devonian (366 Ma). The three Late Neoproterozoic granodioritic to monzogranitic plutons are older than the adjacent metavolcanic and metasedimentary rocks of the Jumping Brook Metamorphic Suite, whereas the tonalitic to quartz dioritic Cambrian-Ordovician plutons intruded those metamorphic rocks. Petrographic characteristics and approximately 100 whole-rock chemical analyses show that with the exception of the mid-Silurian Grand Falaise alkali-feldspar granite which has A-type within-plate characteristics, the plutonic units have calc-alkaline affinity and were emplaced in a volcanic-arc tectonic setting. These results are evidence that fragments of a long history of episodic subduction-related magmatism and terrane collision is preserved in this small part of Ganderia. Eight new Sm-Nd isotopic analyses are consistent with the Ganderian affinity of the Chéticamp plutonic belt. The ca. 490-482 Ma plutons are the first direct evidence in Cape Breton Island for the Penobscottian event recognized in the Exploits subzone of central Newfoundland and in New Brunswick. However, the structural relationship of the Chéticamp plutonic belt to the rest of the Aspy and Bras d’Or terranes remains enigmatic, as is the apparent absence of effects of Devonian deformation and metamorphism in the older plutonic units.
Alignment of fluvio-tidal point bars in the middle McMurray Formation: implications for structural architecture of the Lower Cretaceous Athabasca Oil Sands Deposit, northern Alberta
(Canadian Science Publishing, 2016-03-18) Broughton, Paul Leonard
The northern Athabasca Oil Sands Deposit accumulated on sub-Cretaceous structure partially configured by multi-stage pre-Cretaceous salt dissolutions in Prairie Evaporite (Middle Devonian) substrate that continued concurrent with deposition of McMurray Formation (Aptian) strata. Dissolution fronts only 250 m below advanced along NW- and NE-oriented fracture-fault lineaments that coalesced into larger salt removal areas. This structural grain was transmitted to the overlying dissected Upper Devonian karst topography draped by lower McMurray braided rivers along a lattice-like channel network. The dominant NW structural grain continued during middle McMurray deposition with fluvial-estuarine point bars aligned along subparallel tidal channels. Regional salt removal fronts concurrent with middle McMurray deposition migrated north of the Bitumount Trough, resulting in the 200 km² central collapse. The northern Athabasca Deposit area was configured as a funnel-shaped lower estuary structure consisting of aligned Upper Devonian-lower McMurray fault block terraces that stepped down northward into the central collapse. Sinuous river channels of the upper estuary, constrained along stable substrate of the main paleovalley, flowed northward onto the unstable floor of this funnel-form lower estuary. The main paleovalley fairway branched into multiple 10s of km long sub-parallel fluvio-estuarine tidal channels aligned parallel to the NW structural grain. Sand transport fairways cascaded over the step down terraces and permitted aggradations of overlying fluvio-tidal point bars to accumulate into giant commercially attractive sand complexes. The internal architecture of these 10s of meters thick sand deposits included deposit-wide erosion surfaces resulting from cycles of collapse-subsidence, stabilized substrate and erosion, and renewed subsidence and aggradation.
An arborescent lycopsid stem fragment from the Palliser Formation (Famennian) carbonate platform, southwestern Alberta, Canada, and its paleogeographic and paleoclimatic significance
(Canadian Science Publishing, 2016-09-17) Pratt, Brian R.; van Heerde, Johan
A partially silicified stem fragment of an arborescent lycopsid, tentatively identified as Leptophloeum rhombicum, is documented from peritidal carbonates in the Palliser Formation (Upper Devonian; Famennian) of southwestern Alberta. An unlikely inhabitant of these tidal flats, the log must have rafted in from a relatively nearby land area. The most probable sources are the Kootenay island arc to the paleo-northwest and hypothetical Montania to the southwest. The specimen is evidence that either or both these equatorial areas had a humid paleoclimate and vegetated coastal marshes and swamps.
Apatite fission-track evidence for regional exhumation in the subtropical Eocene, block faulting, and localized fluid flow in east-central Alaska
(Canadian Science Publishing, 2016-01-10) Dusel-Bacon, Cynthia; Bacon, Charles R.; O'Sullivan, Paul B.; Day, Warren C.
The origin and antiquity of the subdued topography of the Yukon–Tanana Upland (YTU), the physiographic province between the Denali and Tintina faults, are unresolved questions in the geologic history of interior Alaska and adjacent Yukon. We present apatite fission-track (AFT) results for 33 samples from the 2300 km2 western Fortymile District in the YTU in Alaska and propose an exhumation model that is consistent with preservation of volcanic rocks in valleys that requires base level stability of several drainages since latest Cretaceous–Paleocene time. AFT thermochronology indicates widespread cooling below ~110 °C at ~56–47 Ma (early Eocene) and ~44–36 Ma (middle Eocene). Samples with ~33–27, ~19, and ~10 Ma AFT ages, obtained near a major northeast-trending fault zone, apparently reflect hydrothermal fluid flow. Uplift and erosion following ~107 Ma magmatism exposed plutonic rocks to different extents in various crustal blocks by latest Cretaceous time. We interpret the Eocene AFT ages to suggest that higher elevations were eroded during the Paleogene subtropical climate of the subarctic, while base level remained essentially stable. Tertiary basins outboard of the YTU contain sediment that may account for the required >2 km of removed overburden that was not carried to the sea by the ancestral Yukon River system. We consider a climate driven explanation for the Eocene AFT ages to be most consistent with geologic constraints in concert with block faulting related to translation on the Denali and Tintina faults resulting from oblique subduction along the southern margin of Alaska.
Arctosaurus osborni, a Late Triassic archosauromorph reptile from the Canadian Arctic Archipelago
(Canadian Science Publishing, 2016-10-16) Sues, Hans-Dieter
Arctosaurus osborni is known only from an incomplete cervical vertebra from the Upper Triassic Heiberg Formation of Cameron Island, Nunavut, Canada. Re-examination of the unique specimen indicates that it represents an archosauromorph reptile, possibly from the clade Allokotosauria. To date, Arctosaurus osborni represents the sole record of Late Triassic continental tetrapods from Nunavut and the northernmost record of such animals anywhere in the world. Arctosaurus osborni est connu seulement d'une vertèbre cervicale incomplète provenant de la Formation Heiberg du Trias supérieur de l'île Cameron, Nunavut, Canada. Révision de l'exemplaire unique indique qu'il représente un reptile archosauromorphe, peut-être de le groupe Allokotosauria. À ce jour, Arctosaurus osborni représente le seul record des tétrapodes continentaux du Trias supérieur du Nunavut et le record du plus septentrional de ces animaux partout dans le monde.
Biochemostratigraphy of the Eramosa Formation in southwestern Ontario, Canada
(Canadian Science Publishing, 2016-02-12) Bancroft, Alyssa M.; Kleffner, Mark A.; Brunton, Frank R.
The lithostratigraphic term ‘Eramosa’ was introduced in Ontario more than a century ago to include a distinctive package of thin-to-medium bedded, black-to-medium-brown dolostones that make up key cuesta faces and railway road cuts along the Eramosa River in the City of Guelph, southwestern Ontario, Canada. This stratigraphic unit makes up part of a stacked carbonate succession that constitutes one of the most economically significant Paleozoic sedimentary rocks in Ontario. The strata assigned to the Eramosa have a complex history of lithostratigraphic study and the relative age, regional lithostratigraphic relationships, and varied depositional environments of the Eramosa were poorly understood. This research, which combines conodont biostratigraphy and carbonate carbon (δ
Biotic processes in the Okanagan Highlands floras: possible evidence of hybridization in plants adapting to a temperate forest
(Canadian Science Publishing, 2016-01-17) DeVore, Melanie L.; Pigg, Kathleen B.
Hybridization is a key mechanism for increased diversification and speciation among modern plants, and is especially important in certain families such as Rosaceae and Anacardiaceae. This mechanism is found to occur most commonly in areas of ecological disturbance, distribution and migration. Resulting hybrid individuals have characteristic intermediate or mosaic patterns that combine those of the two parents in morphologically distinct and identifiable ways. The diverse latest early Eocene flora of Republic, Washington, USA is among the most collected of the Okanagan Highlands floras. This fossil flora includes relatives of modern taxa that are known to hybridize and produce characteristic patterns of leaf morphology and venation (Rosaceae; Anacardiaceae). In this study we document patterns found in representative fossil leaves that are comparable to those indicative of hybridization in the modern relatives. We use the genera Sorbus L. (Rosaceae) and Rhus L. (Anacardiaceae) as models to demonstrate potential hybridization in the latest early Eocene.
Camel fossils from gravel pits near Edmonton and Vauxhall, and a review of the Quaternary camelid record of Alberta
(Canadian Science Publishing, 2016-03-15) Jass, Christopher N.; Allan, Timothy E.
Camelid remains are known from several Quaternary palaeontological localities in Alberta, yet most specimens are undescribed in the literature. Specimens reported here comprise a large sample of the known camelid record from the province, and provide further insight into the record of Quaternary megafauna of western Canada. Remains from the Edmonton area include specimens pre- and post-dating the Last Glacial Maximum (LGM), whereas remains from the Vauxhall area are post-LGM. A metapodial fragment of a giant camel originally described as Titanotylopus from the Edmonton area is likely from earlier in the Pleistocene or late Pliocene. Camelid remains are not overly abundant in Alberta, but are widely distributed, having been recovered from several sites across the province. A new radiocarbon date of 11,280±40 14C yr BP on a radioulna of Camelops cf. C. hesternus represents only the fourth direct age assessment of a Quaternary camelid from Alberta. Radiocarbon data may suggest linkages to patterns of extirpation observed in camelid populations from northern Canada, followed by re-colonization following deglaciation. Key Words: Pleistocene, Camelops, Camelidae, biogeography, radiocarbon dating
Carbon isotope (δ13Ccarb) and facies variability at the Wenlock-Ludlow boundary (Silurian) of the Midland Platform, UK
(Canadian Science Publishing, 2016-02-09) Blain, John Allan; Ray, David C.; Wheeley, James R.
The Wenlock-Ludlow series boundary (Silurian) has been recognized as a time of pronounced sea level rise and the end of a globally recognised Late Homerian Stage (Mulde) positive carbon isotope excursion (CIE). However, the precise timing and synchronicity of the end of the excursion with respect to the Wenlock-Ludlow boundary is debated. Within the type Wenlock and Ludlow areas (UK), high resolution δ13Ccarb isotope data are presented across the Wenlock-Ludlow boundary, and within a range of carbonate platform settings. Correlation between sections and depositional settings has been based upon the characteristics of high-order sea level fluctuations (parasequences). Comparisons between parasequence bounded δ13Ccarb values reveal clear spatial variations, with lighter values recorded from more distal settings and heavier values from shallower settings. Temporal variations in the δ13Ccarb values are also documented and appear to reflect local variations in carbonate provenance and productivity in response to sea level rise. While δ13Ccarb values converge in all sections towards the Wenlock-Ludlow boundary, the apparent end of the Mulde CIE appears diachronous and is progressively older within more distal settings.
Carbonic fluids in the Hamadi gold deposit, Sudan: Origin and contribution to gold mineralization
(Canadian Science Publishing, 2016-12-02) Cheng, Xihui; Xu, Jiuhua; Wang, Jianxiong; Xue, Qingbo; Zhang, Hui
The Hamadi gold deposit is located in North Sudan, and occurs in the Neoproterozoic metamorphic strata of the Arabian–Nubian Shield. Two types of gold mineralization can be discerned: gold-bearing quartz veins and altered rock ores near ductile shear zones. The gold-bearing quartz veins are composed of white to gray quartz associated with small amounts of pyrite and other polymetallic sulfide minerals. Wall-rock alterations include mainly beresitization, epidotization, chloritization, and carbonatization. CO2-rich inclusions are commonly seen in gold-bearing quartz veins and quartz veinlets from gold-bearing altered rocks; these include mainly one-phase carbonic (CO2 ± CH4 ± N2) inclusions and CO2–H2O inclusions with CO2/H2O volumetric ratios of 30% to ∼80%. Laser Raman analysis does not show the H2O peak in carbonic inclusions. In quartz veins, the melting temperature of solid CO2 (Tm,CO2) of carbonic inclusions has a narrow range of −59.6 to −56.8 °C. Carbonic inclusions also have CO2 partial homogenization temperatures (Th,CO2) of −28.3 to +23.7 °C, with most of the values clustering between +4.0 and +20 °C; all of these inclusions are homogenized into the liquid CO2 state. The densities range from 0.73 to 1.03 g/cm3. XCH4 of carbonic fluid inclusions ranges from 0.004 to 0.14, with most XCH4 around 0.05. In CO2–H2O fluid inclusions, Tm,CO2 values are recorded mostly at around −57.5 °C. The melting temperature of clathrate is 3.8–8.9 °C. It is suggested that the lowest trapping pressures of CO2 fluids would be 100 to ∼400 MPa, on the basis of the Th,CO2 of CO2-bearing one-phase (LCO2) inclusions and the total homogenization temperatures (Th,tot) of paragenetic CO2-bearing two-phase (LCO2–LH2O) inclusions. For altered rocks, the Tm,CO2 of the carbonic inclusions has a narrow range of −58.4 to ∼−57.0 °C, whereas the Th,CO2 varies widely (−19 to ∼+29 °C). Most carbonic inclusions and the carbonic phases in the CO2–H2O inclusions are homogenized to liquid CO2 phases, which correspond to densities of 0.70 to ∼1.00 g/cm3. Fluid inclusions in a single fluid inclusion assemblage (FIA) have narrow Tm,CO2 and Th,CO2 values, but they vary widely in different FIAs and non-FIAs, which indicates that there was a wide range of trapping pressure and temperature (P–T) conditions during the ore-forming process in late retrograde metamorphism after the metamorphism peak period. The carbonic inclusions in the Hamadi gold deposit are interpreted to have resulted from unmixing of an originally homogeneous aqueous–carbonic mixture during retrogress metamorphism caused by decreasing P–T conditions. CO2 contributed to gold mineralization by buffering the pH range and increasing the gold concentration in the fluids.
Chemical weathering east and west of the emerging Caledonides in the Silurian—early Devonian, with implications for climate
(Canadian Science Publishing, 2016-03-01) Kiipli, Enli; Kiipli, Tarmo; Kallaste, Toivo; Märss, Tiiu
The late Llandovery (Silurian)—early Lochkovian (Devonian) climate of the tropical zone is tracked considering orogenesis, global glaciation events and cratonic drift. Mineral and chemical compositions of clay fractions of Canadian (the Franklinian Basin) and Estonian (the Baltoscandian Basin) sedimentary rocks from different sides of the emerging Caledonides were studied, using clay as an index of climatic conditions of the provenance area. Illite is the main clay mineral with addition of orthoclase, muscovite and chlorite in both regions. Authigenic chert is present in some Canadian sections. The similarity of clay minerals and elemental composition in the two regions suggest common chemical weathering conditions. Nevertheless, the Al/Ti ratio of the
Coastal retreat rates and sediment input in the Minas Basin, Nova Scotia
(Canadian Science Publishing, 2016-12-02) Wilson, Erin K.; Hill, Paul S.; van Proosdij, Danika; Ruhl, Monique
The strong tidal currents of the Minas Passage in the Bay of Fundy, Nova Scotia, have made this area an important site for testing and development of tidal power technologies. Understanding sediment processes in this area is essential for determining the impacts that large-scale tidal power extraction would have on the system. Previous estimates of sediment input to the Basin suggest that much more sediment enters the Basin than accumulates within it; therefore, the bottom sediment texture should be in hydrodynamic equilibrium with bottom currents. A recent study, however, showed that sediment texture is generally finer that what was expected based on current speeds. This paper uses geographic information systems methods to provide updated and more highly resolved measurements of the amount of sediment entering the Minas Basin from the dominant source, which is coastal erosion. Volumetric input from coastal erosion is 1.1 × 106 m3·a−1, which is more than two times smaller than previous estimates. This updated value makes input rates comparable to accumulation rates, and agrees with the hypothesis that bottom sediment texture is not in equilibrium with current speeds. Grain-size distributions also support the hypothesis that the Minas Basin acts as a sediment trap.
Crustal Accretion of Thick, Mafic Crust in Iceland: Implications for Volcanic Rifted Margins
(Canadian Science Publishing, 2016-04-28) Karson, Jeffrey A.
Rifting near hotspots results in mantle melting to create thick, mafic igneous crust at Volcanic Rifted Margins (VRMs). This mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward. Seismic velocities, crustal drilling, and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions. The lower crust of these regions is not exposed but is inferred from seismic velocities (Vp>6.5 km/sec) and petrological constraints to be gabbroic to ultramafic in composition. Limited access to crustal sections generated along VRMs have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading. Active processes in Iceland provide a glimpse of subaerial spreading with the creation of a thick (40-25 km) mafic igneous crust that may be analogous to the transitional crust of VRMs. Segmented rift zones that propagate away from the Iceland hotspot, migrating transform fault zones, and rift-parallel strike-slip faults create a complex plate boundary zone in the upper, brittle crust. These structures may be decoupled from underlying lower crustal gabbroic rocks that are capable of along-axis flow that smooths-out crustal thickness variations. Similar processes may be characteristic of the early history of VRMs and volcanic hotspot ridges related to rifting and seafloor spreading proximal to hotspots.
Depositional timing of Neoarchean turbidites of the Slave craton - recommended nomenclature and type localities
(Canadian Science Publishing, 2016-08-01) Haugaard, Rasmus; Ootes, Luke; Heaman, Larry M; Hamilton, Michael A.; Shaulis, Barry J.; Konhauser, Kurt K.O.
Two temporally distinct Neoarchean turbidite packages are known to occur in the Slave craton. The older is a greywacke-mudstone succession that includes the renowned Burwash Formation (ca. 2661 Ma). In this study, a previously undated tuff bed is demonstrated to have crystallized at ca. 2650.5 Âą 1.0 Ma refining the deposition age of these turbidites between ca. 2661 and 2650 Ma. The younger turbidites are locally distinctive as they contain interstratified banded iron formation (BIF). Previous work demonstrated that the younger turbidites were deposited between ca. 2640 to 2615 Ma, based entirely on maximum depositional ages from detrital zircons. A ~3-cm-thick felsic-to-intermediate tuff bed was discovered interbedded with these BIF-bearing turbidites. The tuff bed contains a single age population of zircon with a crystallization age of 2620 Âą 6 Ma defining the depositional timing of these BIF-bearing turbidites. New U-Pb detrital zircon dates from extensive turbidite sequences in the eastern and central part of the Slave craton are also presented. We use the new and previously published results to recommend nomenclature for these extensive sedimentary rocks in the Slave craton. The ca. 2661 to 2650 Ma turbidites remain part of the previously ascribed Duncan Lake Group. The younger ca. 2620 Ma turbidites are assigned to the new Slemon Group. Where robust age-data exist, we recommend formation names and include type localities for each.
Determining the Paleoclimate and Elevation of the Late Eocene Florissant Flora: Support from the Coexistence Approach
(Canadian Science Publishing, 2016-03-01) Zaborac-Reed, Stephanie J.; Leopold, E.B.
Published estimates on the paleoclimate of the Florissant fossil beds site of Colorado (34.1 Ma) have led to extremely different estimates of paleoelevation. The purpose of this paper is to review the paleoclimate methods used, and to add the Coexistence Approach (CA) method to estimate paleotemperature for the Florissant site. The basis of the CA method is the dependence on overlapping climatic requirements for different taxa in the flora and their nearest living relatives. These relationships tie the fossil identities to the actual plant habitats and climatic requirements of close relatives. Based on the CA method, we conclude that the mean annual temperature ranged between 14.3° C and 18.2° C. These results support the conclusions offered by several other studies that the Florissant was a warm-temperate to subtropical flora, and are consistent with recent conclusions by structural geologists as well as other studies indicating that the Florissant site was deposited at a relatively low elevation, probably between 1 to 1.5 km.

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