摘要:

Tentative biostratigraphic correlations, based on marine faunas, have been made by various workers between Ufimian and Kazanian sequences in their type areas in the Volga–Urals region of Russia and Roadian and Wordian sequences in their type area in Texas, United States. Unfortunately, palynological correlation between the Russian and United States sequences is not possible, due to lack of data from the latter. However, detailed palynological data are available from rocks of Roadian and Wordian age in the Canadian Arctic Archipelago, and therefore indirect correlations are possible. Palynomorph assemblages from the Canadian Arctic and other circumpolar areas, such as the southern Barents Sea and Greenland, are different from those of the Ufimian and Kazanian Russian sequences in their type areas. This is likely to be the result of variations in the parent flora in response to significant paleoclimatic differences. For example, the climate of the Volga–Urals region in Late Permian times was probably hot and arid, whereas that of the Canadian Arctic, Barents Sea, and Greenland was cooler and probably more humid.

ISSN:0008-4077    

DOI:10.1139/e17-001

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

On 13 August 1978, a lake on the western Arctic coast was artificially drained, in a multidisciplinary experiment on the growth of permafrost on the unfrozen bottom of the drained lake. A bowl-shaped talik (unfrozen basin) with a maximum depth of about 32 m underlay the lake bottom prior to drainage. In the first winter after drainage, downward freezing started on the exposed lake bottom and upward freezing from permafrost beneath the talik. After drainage, the soft lake-bottom sediments hardened from water loss and freeze–thaw consolidation. Gradual thinning of the active layer at many sites was accompanied by ground uplift and the growth of aggradational ice. Downward and upward freezing has resulted in solute rejection, freezing-point depressions, pore-water expulsion from the freezing of the saturated lake-bottom sands, and convective heat transfer from groundwater flow in an open hydrologie system. The increasingly saline intrapermafrost groundwater, flowing at an increasingly negative temperature because of a freezing-point depression, has accelerated the rate of permafrost growth in the interpermafrost zone in the direction of flow. The experiment has demonstrated that the growth of permafrost at the drained lake site, and at other sites with groundwater flow, requires a three-dimensional conductive–convective heat transfer approach.

ISSN:0008-4077    

DOI:10.1139/e17-002

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The rift-related Rapitan Group of the Mackenzie Mountains of northwestern Canada acquired several magnetizations due to pulses of hydrothermal activity. The first pulse, attributed to initiation of Rapitan rifting, produced a widespread overprint (P2) that may be reflected in the basal Mount Berg Formation. Two later pulses produced overprints similar to components found in an earlier study. Development of iron formation and hematite pigment in the overlying Sayunei Formation is attributed to the second pulse, represented by a paleopole (N = 10 sites; 334°E, 01°S; δp, δm = 4°, 9°) that coincides with poles of the Franklin igneous events of northern Canada. The Franklin episode, suggested on geological grounds to be coeval with Sayunei deposition, dates the Sayunei at ca. 725 Ma. This relation implies that rifting in Mackenzie Mountains could be related to rifting in northern Canada. A third pulse, reflected by a pole at 007°E, 16°N (N = 6 sites; δp, δm = 6°, 12°), is attributed to final rifting during deposition of the Shezal Formation at the top of the Rapitan. Overprints attributed to Sayunei and Shezal times indicate regional latitudes of 6 ± 4° and 4 ± 6° during the Sturtian glaciation. During Mount Berg time, the regional latitude could have exceeded 25°. All directions have been tilt corrected and some have been then rotated, based on comparisons with a P2 reference overprint.

ISSN:0008-4077    

DOI:10.1139/e17-003

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Analyses of the Little Dal lavas and the 779 Ma Tsezotene sills, both of the Mackenzie Mountains Supergroup, Northwest Territories, Canada, show them to be continental tholeiites that are geochemically related. The plagioclase- and clinopyroxene-phyric lavas are geochemically evolved and enriched in iron (up to 20.2 wt.% as Fe2O3). Two magmatic lineages are identifiable and may represent different degrees of partial melting in the same source region, but cannot be related by fractional crystallization. Within each lineage, geochemical variation can be explained by fractional crystallization involving up to 60% crystallization of the original magma. The most evolved lavas occur at the base of the pile; less fractionated lavas occur toward the top of the sequence. The Nd isotopic composition of the Little Dal lavas averages ϵ = 1.4 at 780 Ma. Trace element and isotopic compositions are permissive of contamination by continental crust, but do not require a crustal component. The preserved volume of the Little Dal basalts is anomalously low (−100 km3) compared with other Proterozoic continental tholeiites. There is considerable basaltic detritus in the sedimentary rocks of the overlying Coates Lake and Rapitan groups, and much of the original lava sequence may have been eroded. The Little Dal magmatic event is interpreted to be an early manifestation of rifting of North America from Australia.

ISSN:0008-4077    

DOI:10.1139/e17-004

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Metamorphic rocks found at the base of the Jurassic Donqiao ophiolite of northern Tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif. The rocks form a sequence some 8 m thick, varying from high-grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down. The mineral paragenesis is similar to other such aureoles, and indicates that temperatures in excess of 750 °C may have been reached during metamorphism. The lack of high-pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone. Ar–Ar geochronology on amphibole separates provides dates of 175–180 Ma for the displacement of the ophiolite, significantly older than the age of emplacement estimated from stratigraphie relationships. The ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment.

ISSN:0008-4077    

DOI:10.1139/e17-005

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The final advance of the Erie lobe into Ohio during the Port Bruce Stade of the Late Wisconsinan deposited the Ashtabula Till. Wave erosion and mass wasting along the south shore of Lake Erie show that the Ashtabula Till consists of laterally traceable lithofacies, which are used to determine the depositional history of the Ashtabula Till. At each section, lithofacies sequences are divided into two sub-sequences, each consisting of massive, matrix-supported diamicton (Dmm) overlain by stratified, matrix-supported diamicton (Dms). Some Dmm are sheared (Dmm(s)) and are interpreted as lodgement till, whereas other Dmm and Dms were deposited as melt-out till. Some sections contain lenses of fines (Fm and Fl), current-reworked diamictons (Dmm(c) and Dms(c)), and resedimented diamictons (Dmm(r) and Dms(r)). The two sub-sequences represent two advances of Ashtabula ice that deposited the Euclid and Painesville moraines about a kilometre apart. During and after recession of the Ashtabula ice, waves and currents in Lake Maumee and its successors reworked outwash and diamictons to form the lake plain. The texture of Dmm(s) is significantly different from that of most other diamictons, and Dmm has the smallest carbonate content of all diamictons. Analysis of the variations in texture and composition among lithofacies provides additional evidence of the effectiveness of lithofacies logging in interpretation of glacial processes.

ISSN:0008-4077    

DOI:10.1139/e17-006

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The Dirt Hills structure is expressed at the surface as a broad, arcuate moraine, 7.5 km wide, 120 m high, and 40 km long. The structure is composed mainly of bedrock and drift consisting of three slabs stacked to form a single block 215 m thick. The shear zones that bound the slabs are mainly in clays of the Bearpaw Formation. The Dirt Hills structure originated during the last deglaciation when the glacier, under compressive flow, readvanced up the Missouri Coteau escarpment and stacked the subglacial slabs of bedrock and drift. The subparallel, arcuate ice-thrust ridges are formed presumably by imbricate thrust faults originating from the uppermost shear zone.

ISSN:0008-4077    

DOI:10.1139/e17-007

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

New sedimentological, geochemical, and geochronological data from the Cottrells Cove Group in central Newfoundland provide important constraints on the nature of the Notre Dame Subzone, its tectonic setting, and the history of the Laurentian margin during the Early Ordovician. The Cottrells Cove Group forms the eastern extension of the Roberts Arm Group and correlates with the Chanceport Group on New World Island. It is represented by two volcano-sedimentary formations that occur in a complex thrust stack. The Fortune Harbour Formation consists of calc-alkalic, island-arc lavas, followed by a 1250 m thick succession of volcaniclastic deposits, radiolarian cherts, and calc-alkalic, mafic flows, which were deposited in a back-arc, basin-plain setting. The volcaniclastic deposits include felsic tuff, which has a U–Pb zircon age of 484 ± 2 Ma and an inheritance component of 2517 ± 26 Ma. These new U–Pb and Nd-isotope data suggest that the island-arc–back-arc volcanism and sedimentation in the Notre Dame Subzone developed in the vicinity of continental margin and approximately 10 Ma earlier than has previously been established. The Moores Cove Formation is undated but contains boulders of calc-alkalic basalt and is presumed to be at least in part younger than the Fortune Harbour Formation. Tholeiitic lavas, together with associated radiolarian cherts and volcaniclastic deposits, constitute the basal part of the Moores Cove Formation and may have been deposited in a back-arc environment synchronously with some parts of the Fortune Harbour Formation. They are conformably followed by an over 1200 m thick, coarsening-upward succession of lower-slope and submarine-fan deposits. The polymictic flysch, containing clasts of island-arc basalt, accompanied by other volcanic, plutonic, ultramafic, and sedimentary detritus, may record Middle or Upper Ordovician uplift and erosion of obducted arc–back-arc, volcano-sedimentary sequences and their ophiolitic substrate.

ISSN:0008-4077    

DOI:10.1139/e17-008

出版商:NRC Research Press    

年代:1997

数据来源: NRC