Lithospheric Architecture of the Hudson Bay Region
Hudson Bay is a vast inland sea that penetrates deeply into north-central Canada, forming a conspicuous element of the coastline and concealing several fundamental tectonic elements of North America, including most of the Paleoproterozoic Trans Hudson orogen (THO) and the Paleozoic Hudson Bay basin. The THO formed due to a collision between two Archean domains, the Superior and Churchill Provinces of the Canadian Shield, and is similar in scale and tectonic style to the modern Himalayan-Karakorum orogen. Tectonic reconstructions suggest that the lobate shape of the indentor (Superior Province) formed an orogenic template that exerted a persistent influence on the tectonic evolution of the region, resulting in anomalous preservation of juvenile crustal material. Based on analysis of gravity and magnetic data, we propose a model in which juvenile crust in the southeastern part of Hudson Bay formed within an island-arc setting proximal to the Superior Province, in contrast to the Reindeer Zone of Saskatchewan and Manitoba which accreted first to the Churchill Province. Thick, cold and refractory lithosphere that underlies the Bay is well imaged by surface-wave studies and comprises a large component of the cratonic mantle keel that forms the nucleus of the North American continent. The existence of an unusually thick mantle root beneath Hudson Bay indicates that subduction and collision are root-forming (or at least root-preserving) processes. Although the Hudson Bay basin is the largest by surface area of four major intracratonic basins in North America, it is also the shallowest. Available evidence suggests that basin subsidence may have been triggered by eclogitization of crust that was previously thickened during the Trans-Hudson orogeny. Relatively stiff Early Paleozoic lithosphere may have inhibited subsidence of the Hudson Bay basin relative to other basins of similar age in North America.
Geological Setting and Petroleum Potential of the Paleozoic Hudson Platform, Northern Canada
The Hudson Platform covers an area of 600,000 km2 and represents one of the largest Paleozoic sedimentary basins in Canada. The Hudson Platform contains the large Hudson Bay Basin and smaller Moose River Basin. The Hudson Bay and Moose River basins are surrounded and underlain by Precambrian igneous and metamorphic rocks of the Canadian Shield. The Hudson Platform contains Ordovician to Cretaceous sedimentary strata, with a maximum known thickness of about 2500 m in Hudson Bay. The lower Paleozoic succession includes Late Ordovician to Early Devonian shallow marine carbonates and thin mudstones, deposited during widespread early Paleozoic marine inundation of the Canadian Shield, and Early to Late Devonian marine carbonates, evaporates, and mudstones deposited in saucer-shaped, isolated basin depocentres. There is no record of late Paleozoic sedimentation in the region, perhaps related to cratonic uplift accompanying the Alleghenian Orogeny. Lower Paleozoic strata are unconformably overlain by thin, erosional remnants of Middle Jurassic and Early Cretaceous nonmarine sandstones, mudstones and lignite seams (Moose River Basin) and Early Cretaceous marine sandstones and mudstones (Hudson Bay Basin). The Hudson Platform is currently considered a frontier prospect for hydrocarbon exploration. However, the long- held view that the region is underlain by a thin sedimentary succession with no appreciable hydrocarbon source rocks or reservoir intervals is erroneous. Geological and geophysical data indicate the Hudson Bay Basin contains many prospective petroleum reservoir and trap types, potentially including hydrothermal dolomite. Recent studies indicate Upper Ordovician oil shales are widespread and may have generated hydrocarbons in deeper parts of the Hudson Bay Basin. New high resolution bathymetric surveys in northern Hudson Bay have led to the recognition of circular sea-floor depressions similar to fluid or gas-escape pockmarks. A modern re-evaluation of the petroleum systems and energy resource potential of the Hudson Platform is the focus of a new Geological Survey of Canada research initiative.
Sea Level and Paleoenvironment Control on Late Ordovician Source Rocks, Hudson Bay Basin, Canada
Hudson Bay Basin is one of the largest Paleozoic sedimentary basins in North America, with Southampton Island on its north margin. The lower part of the basin succession comprises approximately 180 to 300 m of Upper Ordovician strata including Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation. These units mainly comprise carbonate rocks consisting of alternating fossiliferous limestone, evaporitic and reefal dolostone, and minor shale. Shale units containing extremely high TOC, and interpreted to have potential as petroleum source rocks, were found at three levels in the lower Red Head Rapids Formation on Southampton Island, and were also recognized in exploration wells from the Hudson Bay offshore area. A study of conodonts from 390 conodont-bearing samples from continuous cores and well cuttings from six exploration wells in the Hudson Bay Lowlands and offshore area (Comeault Province No. 1, Kaskattama Province No. 1, Pen Island No. 1, Walrus A-71, Polar Bear C-11 and Narwhal South O-58), and about 250 conodont-bearing samples collected from outcrops on Southampton Island allows recognition of three conodont zones in the Upper Ordovician sequence, namely (in ascendant sequence) Belodina confluens, Amorphognathus ordovicicus, and Rhipidognathus symmetricus zones. The three conodont zones suggest a cycle of sea level changes of rising, reaching the highest level, and then falling during the Late Ordovician. Three intervals of petroleum potential source rock are within the Rhipidognathus symmetricus Zone in Red Head Rapids Formation, and formed in a restricted anoxic and hypersaline condition during a period of sea level falling. This is supported by the following data: 1) The conodont Rhipidognathus symmetricus represents the shallowest Late Ordovician conodont biofacies and very shallow subtidal to intertidal and hypersaline condition. This species has the greatest richness within the three oil shale intervals to compare other parts of Red Head Rapids Formation. 2) Type I kerogen is normally formed in quiet, oxygen-deficient, shallow water environment. Rock-Eval6 data from 40 samples of the three oil shale intervals, collected from outcrops on Southampton Island, demonstrate that the proportion of Type I kerogen gradually increases in the mixed Type I-Type II kerogen from the lower to upper oil shale intervals. 3) Pristane/phytane ratio can be used as a paleoenvironment indicator. The low ratios in the three oil shale intervals range from 0.5 to 0.9 and indicate anoxic and hypersaline conditions. In addition, the presence of isorenieratene derivatives from green phototrophic sulfur bacteria (Chlorobiaceae), with highest relative concentrations in the lower oil shale intervals, points to anoxia reaching into the photic zone of the water column.
Mantle Lithosphere Structures of the Hudson Bay Region as Defined by Seismic Anisotropy
The origins of the Hudson Bay intracontinental basin remain unresolved. Possibilities include eclogitic subsurface loads in the uppermost mantle, dynamic loading related to the subducted Farallon slab deeper in the mantle or more dynamic topography related to larger-scale mantle convection. Inferred causative mantle structure in these models underlies both Hudson Bay as well as neighbouring Proterozoic Canadian shield and its component Archean cratons. The candidate mechanisms represent effects integrated over at least the uppermost 1000 km of the mantle, therefore determining contributions from differing depths, backstripping, is a traditional way forward toward better understanding the whole mantle volume. Crustal studies represent the first such step and significant recent progress via bedrock mapping projects has helped clarify the tectonic history of the region and suggests which surface blocks are most extensive at depth. Regional teleseismic arrays with 200-400 km station spacing are both improving regional velocity (S- and Rayleigh-wave) models and providing previously unavailable, detailed information about structure beneath each station using SKS splitting and receiver function analysis. These structures are defined by associated changes in physical rock properties, by changes in rock fabric that produce seismic anisotropy, or by both. For example, beneath the western shore of Hudson Bay at Churchill, uppermost mantle layers dip due north between 50 and 130 km depths, beneath the northern margin of the 1.82 Ga Trans-Hudson Orogen. Beneath Rankin Inlet and the Hearn-Chesterfield Terrane boundary, 500 km to the north, uppermost mantle layers dip southward toward 170° at 20° dip between 70 and 120 km depths. Northeast another 500 km beneath Repulse Bay and the 2.6 Ga Rae craton, structures dip westward toward 260° at 35° dip between 80 and 130 km. Staying within the Rae craton, but an additional 200 km to the northeast at Igloolik, uppermost mantle structures dip westward toward 260° at 45° dip between 70 and 100 km. Metamorphism ages of 1.85-1.83 Ga suggest that the former two settings are Proterozoic in age, whereas the latter may represent Archean craton assembly. If true, this region has many features in common with the Slave craton in western Canada. Apparently uniform Moho depths of 36-39 km and SKS-splitting polarizations of 050-070° with delays of 0.6- 1.0 s across this region obviously belie much greater structural complexity. Multiple layers, each with distinct dip orientation and anisotropic symmetry axes, may ultimately prove irresolvable with the available back azimuthal coverage from earthquakes. Integrating results between all of the region's stations and with new tectonic models appears promising and may clarify deeper (sublithospheric) mantle structure governing the dynamic topography of the whole region or indicate productive future research.
HuBLE-UK, the Hudson Bay Lithospheric Experiment: Insights into the formation of the Canadian Shield From Seismic Tomography and Shear Wave Splitting
Upper Mantle Structure and Azimuthal Anisotropy Beneath Hudson Bay From Rayleigh Wave Tomography
Hudson Bay is a large intracratonic basin situated within the northern part of the Canadian Shield. The region is surrounded by Archean cratons, notably the Superior to the south and east and the Rae-Hearne to the north and west, and is largely underlain by the Paleoproterozoic Trans-Hudson orogen. Global and continental scale tomographic images of North America suggest that the seismological lithosphere of the Canadian Shield is at its thickest and fastest beneath Hudson Bay, but a regional-scale seismic study is necessary to provide sufficient detail to comprehend the structure and evolution of the region. Broadband seismograph stations were installed around the Hudson Bay region in 2006 and 2007 as part of the HuBLE (Hudson Bay Lithospheric Experiment) project. The combination of these stations with existing permanent and temporary seismograph deployments has resulted in a high-quality teleseismic data set for which detailed multi- disciplinary seismic studies of the region can be carried out. Fundamental-mode Rayleigh wave dispersion curves are calculated for >100 two-station paths. The data span a range of periods ∼15-- 220~seconds, corresponding to depths from the middle crust to the sublithospheric mantle. Comparison of the dispersion curves with those derived from global reference models and from the Canadian Shield average 'CANSD' suggests that the upper mantle beneath Hudson Bay can be characterised by a typical cratonic signature of anomalously high phase velocities. Many Hudson Bay dispersion curves exhibit higher phase velocities than those of 'CANSD', suggesting a seismically faster and thicker lithosphere than average for the Canadian Shield. 1D mantle models estimated from the two-station dispersion measurements show an average lithospheric thickness of ∼225~km, with velocities ∼4--6% above global reference values. A tomographic inversion is carried out to solve simultaneously for isotropic phase velocity heterogeneity and azimuthal anisotropy at a range of periods from 30 to 200~seconds. The resulting phase velocity maps reveal a complex pattern of structural heterogeneity beneath Hudson Bay, as well as multiple layers of anisotropy, likely associated with both 'frozen' lithospheric fabric and present-day sublithospheric mantle flow.