摘要:

Until now, comparisons between terrestrial and marine proxy climatic records have been achieved successfully only for the last 1.2 Ma or back to oxygen isotope stage 36. The Baoji loess–paleosol section of north-central China possesses 37 field-identifiable paleosols and 37 loess units representing a time span of about 2.5 Ma. The Baoji section is judged by the authors to be the most complete pedostratigraphic sequence in north-central China. This section, coupled with recent success in correlating isotope records back to stage 63, or approximately 1.88 Ma, has enabled a more detailed comparison over a longer time span. Using magnetic reversals as markers, the records are shown to have nearly equal numbers of identifiable warm and cold periods during the last 1.8 Ma. In addition, although there are exceptions, data sets reveal similar patterns during the same time intervals, indicating climatic events on a global scale.

ISSN:0008-4077    

DOI:10.1139/e91-089

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A deformed block of silt incorporated in a push moraine of the Lignite Creek drift of the Nenana River valley in central Alaska contains the 10 cm thick Stampede tephra. The silt block and tephra are folded, sheared, and cut by multiple fault planes and appear to have been entrained by a glacier and deformed by glaciotectonic processes while frozen. The Stampede tephra is also found in a paleosol preserved within a sequence of eolian sediments near the Tanana River some 175 km to the northeast, allowing direct tephrochronologic correlations between the depositional record of Quaternary glaciations in the Alaska Range and eolian sediments in unglaciated central Alaska. The Lignite Creek drift predates deposits of the late Wisconsin Riley Creek and penultimate Healy glaciations but postdates the Stampede tephra, indicating that it dates to the middle Quaternary, and is not part of the Tertiary Nenana Gravel Formation.

ISSN:0008-4077    

DOI:10.1139/e91-090

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Two undeformed biotite granites recovered from exploratory hydrocarbon drill holes that penetrated basement in the Fort Simpson magnetic high were analyzed for U–Pb zircon geochronology and Sm–Nd isotopic composition. Both samples give crystallization ages of 1845 Ma, with errors of less than 5 Ma. This age overlaps with the waning stages of magmatism in the Great Bear magmatic zone of Wopmay Orogen, 200 km to the east. One sample has an initial ϵNdof +1.3 and aTDMof 2.14 Ga, whereas the other yielded an initial ϵNdof −2.1 and aTDMof 2.45 Ga. The latter sample indicates that Early Proterozoic light rare-earth element enriched lithosphere with a crustal history dating back to at least 2.45 Ga is present in the subsurface west of Wopmay Orogen.

ISSN:0008-4077    

DOI:10.1139/e91-091

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The Fort Norman area is marked by a complex structural and depositional history with temporally and spatially variable temperature and maturity gradients. Maturity of the Middle Devonian Canol – Hare Indian unit indicates two terrains of markedly different paleotemperatures and thermal gradients, roughly coinciding with the Palaeozoic Keele Arch (north) and Root Basin (south), with a sharp transition (lateral discontinuity) between them. Maturity of the unconformably overlying Cretaceous to early tertiary section is lower and laterally continuous across the entire study area, showing no relation to the sub-Cretaceous structures. The vertical maturity profile in the southern terrain is unique in that two discontinuous segments record markedly different thermal histories, neither of which is related to present burial conditions. Maturity of the Devonian formations was established some time prior to the sub-Cretaceous erosion, whereas that in the Cretaceous to Tertiary section postdates Palaeocene but predates a major part of the later Tertiary deformation and erosion. In contrast, the continuous maturity profile recorded in the northern terrain reflects a significant effect of the Tertiary thermal conditions, indicating much lower thermal gradients in the Keele Arch than in the Root Basin during the early phase of maturation. Along with the structural pattern and sedimentary history, the striking differences indicated in the thermal gradients between the Root Basin and the Keele Arch suggest a strike-slip-related pull-apart or another stretching mechanism for the Root Basin in contrast to the compression in the Keele Arch. In addition to the characterization of the regional thermal history and its variation with time, the maturity pattern obtained allows some estimate to be made of the thickness of the eroded section and timing of the major erosional phases.

ISSN:0008-4077    

DOI:10.1139/e91-092

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The organic matter (OM) sampled in 15 oil exploration wells and 102 outcrops of Cambrian-Ordovician rocks in the St. Lawrence Lowlands consists of zooclasts (chitinozoans, graptolites, and scolecodonts) and solid bitumen (mainly pyrobitumen). The reflectance of pyrobitumen, transformed in vitrinite-equivalent (Ro-Std-B), indicates that the upper part of the platform sequence is mature (condensate zone) in the Québec area, but overmature in the Montréal area.The platform is divided into three domains based on optical texture of OM and types of bitumen: domain 1, south of Montréal, contains a highly reflecting and coked pyrobitumen showing alteration rims; domain 3, east of Trois-Rivières, contains low-reflecting, late solid bitumen commonly associated with oil impregnations; domain 2, located midway, contains a pyrobitumen with moderate reflectance and, locally, fine mosaic texture. The thermal maturation in the autochthonous sequences of the St. Lawrence Lowlands and in the Appalachian allochthons increases from the north-east toward the south-west and in the direction of the Appalachian belt. In wells, the gradient ofRo-Std-Bwith depth decrease from the autochthonous zone toward the Appalachian belt, and is inversely related with thickness of the sequences. Isoreflectance values parallel the outline of the Chambly–Fortierville Syncline in the central and eastern parts of the basin. Consequently, thermal maturation predates folding. Reflectance jumps observed between the Lowlands and the first Appalachian overthrusts, and observed when crossing Logan's Line, demonstrate that the maximum burial of Appalachian sequences predates the tectonic transport. TheRo-Std-Bin allochthonous zones shows higher values in the St-Francis River cross-section than in structural equivalent units of the Québec area. Therefore the increase of thermal maturation observed from the northeast toward the southwest, in the St. Lawrence Lowlands, is also developed in the Appalachian allochthonous units. A post-Taconic regional thermal event explains this similarity in both autochthonous and allochthonous sequences, with the sequences of the Connecticut Valley – Gaspé Synclinorium being the most thermally mature.Zones of highest thermal maturation, locally observed in the Montréal area, are explained by (i) hydrothermal activity (Ro-Std = 3–4%), accounted for by sulfate and sulfide mineralization (Ba, Zn, Pb) and by (ii) contact metamorphism, related to alkaline intrusions (Ro-Std-B = 13%). The contact metamorphism is restricted to aureoles less than 5 km wide around the Monteregian alkaline intrusions, but the hydrothermal alteration, apparently not related to contact metamorphism, covers an area of 10 km around mineralized domains.

ISSN:0008-4077    

DOI:10.1139/e91-093

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A detailed geochemical study of Middle Ordovician volcanic rocks, undertaken in the northern Miramichi Highlands of New Brunswick, shows that 10 basaltic suites can be distinguished. These suites are assigned to the Tetagouche and Fournier groups. The contact between these two groups is a major thrust zone, marked for over 70 km by a prominent blueschist zone. All the Tetagouche Group volcanic rocks have chemistries consistent with extrusion in a continental rift, but most Fournier Group basalts in the Miramichi Highlands have chemistries suggestive of an oceanic back-arc setting. The chemical signatures, stratigraphic variations, and structural data indicate that the northern Miramichi Highlands preserve a section across a telescoped Middle Ordovician back-arc basin that initially opened as a result of asthenospheric injection near the rear part of a Lower Ordovician ensialic arc.

ISSN:0008-4077    

DOI:10.1139/e91-094

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The Île Cadieux alnöite, a small Cretaceous intrusion, is a member of the Monteregian Hills Petrographic Province. Mineral chemistry was determined for olivine, spinel, clinopyroxene, melilite, and a potassium–iron–nickel sulphide akin to djerfisherite. Whole rocks were analysed for major elements and 24 trace elements (including rare-earth elements). The Île Cadieux alnöitic rocks have greater MgO/Al2O3ratios than most alnöites but are otherwise similar to typical alnöites with respect to major and trace elements. Comparison of spinel and whole-rock chemical compositions from the Île Cadieux and Île Bizard (located 15 km to the northeast) alnöitic intrusions suggests that these magmas were generated under different physicochemical conditions of melting, or alternatively that they are fractions of a single magma which crystallized under different conditions.

ISSN:0008-4077    

DOI:10.1139/e91-095

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The West Moose River pluton is one of a series of latest Devonian – Early Carboniferous plutons of granite and gabbro–diorite in the Cobequid Highlands (southern edge of Avalon zone). Intrusion of minor gabbro preceded emplacement of the granite, which is fractured and cut by later mafic dykes, of which some predate and others postdate a Namurian compressive deformation. Minor hybrid phases reflect mixing of mafic and felsic magmas, which are also represented by basalt and rhyolite in the synchronous Fountain Lake Group. The mafic rocks are continental tholeiites reflecting regional Early Carboniferous extension. Later dykes are more fractionated, with high P2O5, TiO2, and other high-field-strength elements. The dykes are pervasively altered, probably by residual fluids from the granite hydrothermally circulated by the hot mafic rocks. Higher Ga/Al, Hf, and Zn content in basalts than dykes suggests upward increase in halogen complexing in the mafic plumbing system. Late-stage fluids concentrated K, Rb, and Cs in the dykes but not the lavas. Alteration in granite includes mafic partings (biotite and Fe–Ti oxides) along fractures, particularly near dykes, and local albitization. Such alteration hinders determination of granite protolith from geochemistry, but mineral composition and the regional enrichment of the granite in Nb, Y, and Ga/Al is similar to that in A-type granites. Alteration associated with the mafic dykes continued for tens of millions of years after pluton emplacement.

ISSN:0008-4077    

DOI:10.1139/e91-096

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Quartzofeldspathic xenoliths representing the probable structural basement to the northern Dunnage Zone occur in lamprophyre dykes on Alcock Island and vicinity, Notre Dame Bay. The dykes occur close to the boundary between seismically defined lower crustal blocks (LCB) below the Dunnage Zone, so the assignment of the xenoliths to a specific subsurface source is equivocal. The xenoliths include (i) clinopyroxene-bearing meta-(quartz) diorite to tonalite; (ii) plagioclase + quartz-rich rocks; and (iii) garnetiferous gneiss, including granulite. Paragneiss is absent, so the gneissic xenoliths do not appear to be direct analogues of Gander Zone metasediments thought to underlie parts of the Dunnage Zone farther to the southeast. The xenoliths also differ from Grenvillian crust exposed in western Newfoundland: the granulites are mineralogically distinct (contain clinopyroxene + garnet rather than orthopyroxene + garnet) and yield higher metamorphic P (approx. 8–10 kbar versus approx. 3–7 kbar) (1 kbar = 100 MPa), and the granitoid xenoliths are dioritic–tonalitic rather than granitic. Consequently, we propose that the xenoliths are derived from a non-Gander, non-Grenville source that may include Central LCB and (or) exotic basement material. We cannot, however, exclude the possibility that the xenoliths represent deeper structural levels of Grenvillian or Gander Zone crust than are presently exposed in outcrop.

ISSN:0008-4077    

DOI:10.1139/e91-097

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A structural study of the Early Proterozoic Hurwitz Group has been carried out in the Tavani area, approximately 80 km southwest of Rankin Inlet, District of Keewatin, Northwest Territories. The Hurwitz Group, in the Tavani area, consists of orthoquartzites of the Kinga Formation (Whiterock Lake Member) and quartz-feldspar arenites and litharenites of the Tavani Formation. More localized lithologies include siltstone, sandstones, breccia, and shales beneath the Whiterock Lake Member and conglomerate, shales, and shale-clast breccias within the Tavani Formation.Open, northeast-trending folds dominate the Hurwitz Group, although overturned folds and easterly trending structures are developed locally. Later northwest-trending faults complicate the northeast-trending structural pattern. Three structural domains are defined: north of the Wilson River, the Whiterock Syncline, and the Last Lake belt. The interrelationship of folding and faulting creates distinctive patterns in each domain, though overall, folding is demonstrated to be a local response to basement faulting, including reactivation of Archean structures. An overall north–south compressive regime is envisaged in which heterogeneities in the Archean basement are responsible for local peculiarities.

ISSN:0008-4077    

DOI:10.1139/e91-098

出版商:NRC Research Press    

年代:1991

数据来源: NRC