Continental Margin Of Eastern Canada Research Articles

Paleoseismicity of the continental margin of eastern Canada: Rare regional failures and associated turbidites in Orphan Basin

Paleoseismicity of the continental margin of eastern Canada: Rare regional failures and associated turbidites in Orphan Basin

Reconstruction of sea-surface conditions at middle to high latitudes of the Northern Hemisphere during the Last Glacial Maximum (LGM) based on dinoflagellate cyst assemblages

A new calibration database of census counts of organic-walled dinoflagellate cyst (dinocyst) assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the Northern Hemisphere after standardisation of taxonomy and laboratory procedures. The database comprises 940 reference data points from the North Atlantic, Arctic and North Pacific oceans and their adjacent seas, including the Mediterranean Sea, as well as epicontinental environments such as the Estuary and Gulf of St. Lawrence, the Bering Sea and the Hudson Bay. The relative abundance of taxa was analysed to describe the distribution of assemblages. The best analogue technique was used for the reconstruction of Last Glacial Maximum (LGM) sea-surface temperature and salinity during summer and winter, in addition to sea-ice cover extent, at sites from the North Atlantic ( n=63), Mediterranean Sea ( n=1) and eastern North Pacific ( n=1). Three of the North Atlantic cores, from the continental margin of eastern Canada, revealed a barren LGM interval, probably because of quasi-permanent sea ice. Six other cores from the Greenland and Norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation. At the remaining sites ( n= 54), relatively close modern analogues were found for most LGM samples, which allowed reconstructions. The new LGM results are consistent with previous reconstructions based on dinocyst data, which show much cooler conditions than at present along the continental margins of Canada and Europe, but sharp gradients of increasing temperature offshore. The results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea-ice cover, whereas relatively warm conditions may have prevailed offshore in summer. From these data, we hypothesise low thermal inertia in a shallow and low-density surface water layer.

Development of a major polygonal fault system in Upper Cretaceous chalk and Cenozoic mudrocks of the Sable Subbasin, Canadian Atlantic margin

Detailed interpretation of 2D and 3D seismic data from the Sable Subbasin (continental margin of eastern Canada) has revealed an extensive polygonal fault system affecting Upper Cretaceous chalk of the Wyandot Formation (the Wyandot Chalk) and Cenozoic mudrocks of the Banquereau Formation. This paper offers the first detailed study of a basin-scale polygonal fault system affecting a chalk-dominated unit. The Wyandot Chalk is up to 300 m thick and pervasively deformed by small-scale faults that are morphologically and quantitatively similar to those previously described from polygonal fault systems observed in mudrocks of the North Sea, and elsewhere. This suggests that they may have formed due to a common mechanism. The data presented in this paper point towards a lithological control upon the development of the polygonal fault system and argue that the syneresis hypothesis posed by Cartwright and Dewhurst [Geol. Soc. Am. Bull. 110 (1998) 1242] best explains the distribution and initiation of the analysed fault system. 3D seismic attribute maps reveal an orthogonal intersection relationship between polygonal faults and tectonic faults in the study area. This relationship is interpreted to indicate that local stress perturbations around the tectonic faults influenced the propagation of the polygonal faults.

Sediment trap observations from the Gulf of St. Lawrence and the continental margin of eastern Canada

Sediment trap samples have provided the first direct observations of the sinking particles that account for the export of material out of the photic zone in the Gulf of St. Lawrence, as well as their relationship to variations in the trophic regime. Particles were collected at several sites over 24-h periods using 0.03 m 2 (collecting surface) and 0.5 m 2 free-drifting sediment traps at 50 and 150 m. Total mass flux varied widely (80–1500 mg/m 2/d), as did carbon flux (16–300 mg C/m 2/d). Small cylinders consistently oversampled with respect to big cylinders, regardless of depth or drifter design. Also, 6-month time series were obtained with a moored, 0.125 m 2 trap at two sites. In the Anticosti Gyre, time-series fluxes were consistent with those obtained from the big drifting trap (means: 480 mg dry wt/m 2/d; 39 mg C/m 2/d), and with independently measured sediment accumulation rates. Numeric fluxes of phytoplankton cells were similar to moderately productive ocean margins during the April 1994 bloom, but otherwise resembled those from oligotrophic regimes. Fecal pellet numeric fluxes, in contrast, were always high, similar to other continental margins. The composition of the material collected by the small and big traps is a good indicator of the changing trophic regime in the water column. Relative numeric abundances suggest three distinctly different periods. A “bloom” period (represented by April 1994, but including a weaker late-fall bloom over a shelf valley), when a variety of centric and pennate diatom cells made up 70–95% of the particle numbers; a transitional or “post-bloom” period (June 1994), when phytoplankton were less abundant, pennate forms were scarce and a single species dominated the centric diatoms; and a “non-bloom” period (May to December, 1993) when fecal pellets and microzooplankton accounted for greater numbers than the phytoplankton cells, including abundant dinoflagellates. The time-series Anticosti Gyre trap showed continued large-particle settling throughout the winter with total mass and carbon fluxes similar to the ice-free seasons. The most frequent fecal pellets were 50–109 μm diameter compact and loose rods, produced by the dominant calanoid copepods. Large macrozooplankton fecal pellets occurred only sporadically. Many pellets <49 μm were collected in December 1993, probably produced by Microcalanus, which was unusually abundant at this time. Oval pellets occurred over a broad range of diameters, suggesting multiple origins. The bulk of the settling material produced by the pelagic food web in the Gulf appears to be of zooplanktonic origin (mainly fecal pellets and abundant microzooplankton). Diatom frustules were the most frequently encountered particles, numerically, but phytoplankton rarely made up more than 35% of the 2D projected area of all particles. Much of the organic matter produced photosynthetically must thus be transformed by heterotrophs before escaping from the surface and intermediate waters in this region.

Sea-ice cover, sea-surface salinity and halo-/thermocline structure of the northwest North Atlantic: modern versus full glacial conditions

Seasonal sea-ice develops along the eastern continental margins in the northern North Atlantic, where freshwater and/or meltwater outflow are responsible for relatively low salinity in surface waters and very pronounced water mass stratification. Sea-ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea floor. Organic-walled dinoflagellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern North Atlantic, and used to develop transfer functions (best analogue method) for the reconstruction of the seasonal spreading and duration of sea-ice cover, in addition to salinity and temperature of the warmest month of the year. Application of the best analogue approach to cores from the Labrador Sea reveals large variations in sea-ice cover and sea-surface conditions throughout the last glacial stage and during the early Holocene. Isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub-surface water masses. Specific study of the last glacial maximum LGM time slice (16–20 ka on a 14 C time scale) in the northwestern North Atlantic shows much more extensive sea-ice than at present, with perennial sea-ice lying along the continental margins of eastern Canada. Seasonal spreading of the cover of sea-ice offshore was accompanied by large seasonal contrasts in temperature, with very cold winters but relatively warm summers, a pattern linked to strong stratification between a buoyant low saline surface layer having a low thermal inertia, and the underlying intermediate oceanic waters.

High seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern Canada

A deep seismic reflection profile on the continental rise off eastern Canada shows strong dipping reflections, apparently from within lower oceanic crust, over a region of limited lateral extent. The origin of these and similar reflections described elsewhere is uncertain. This paper reports new seismic-refraction results from the region of these reflections. They appear to be associated with a layer of P-wave velocity of about 7.4 km s −1 or higher, while the surrounding crust has a velocity ∼ 7 km s −1. The Moho, with velocity 8.1 km s −1, lies just below the reflectors, and the total crustal thickness is around 8 km. The crust is interpreted to be oceanic rather than continental, and the high velocities of the dipping reflectors are comparable to those of the mafic/ultramafic crust-mantle transition zone in the Bay of Islands ophiolite complex.

Geothermal model of the continental margins of eastern Canada

Abstract Deep temperature distributions on six seismic cross sections on the passive continental margins of eastern Canada have been calculated. Each of the considered profiles of the crustal structure includes sections with continental and oceanic crusts, and also a zone of ocean-continent transition. Present calculations show that on each profile the crust and upper mantle beneath the ocean have temperatures considerably higher than those beneath the continent. The difference between sub-oceanic and sub-continental temperatures at the same depth amounts to 100–150 °K. At the depth of 50 km the temperature in the continental and oceanic regions reaches 500°–700°C and 700°–900°C respectively. In the region with continental crust, the crustal radioactivity and the outflow of heat from the mantle constitute approximately equal heat sources in producing the crustal temperatures. In the area with oceanic crust the mantle heat flow is the dominant source of heating; on these sections of the profiles more than about 75% of the surface heat flow has its origin in the mantle. The thickness of the lithosphere across the margins decreases seawards; in the region with oceanic crust its thickness reduces to 70–75 km, typical of the North Atlantic.

Offshore Geology and Petroleum Potential of Eastern Canada

The continental margin of eastern Canada is underlain by a series of Mesozoic-Cenozoic sedimentary basins that have been targets of active hydrocarbon exploration for over two decades. Data from approximately 170 released exploratory wells and about 600 000 km of multichannel seismic coverage have delineated three major geological/geochemical regions. On the Scotian Shelf, 18 significant (predominantly gas/condensate) discoveries have been made out of 78 wildcats drilled since 1967. Seven of these, including the Venture field, are in an overpressured zone that has been explored extensively only since 1979. The 61 wildcat wells drilled on the Grand Banks through mid- 1985 have yielded 13 significant (predominantly light oil) discoveries, including the giant Hibernia oil field. Labrador-Southeast Baffin Shelf exploration has yielded six gas/condensate discoveries in 24 wildcat wells drilled since 1971. As petroleum exploration is still at an early stage off eastern Canada these results represent an auspicious beginning.

Transects of the ancient and modern continental margins of eastern Canada

Rocks of the west flank of the northern Appalachian Orogen (miogeocline) record the history of the late Precambrian-early Paleozoic passive continental margin of Eastern North America. The ancient margin was destroyed by ophiolite obduction and arc collision during the Ordovician Taconic Orogeny. The present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re-entrants of a previous orthogonal margin bounded by rifts and transforms. Four major terranes are recognized east of the miogeocline in Newfoundland and Nova Scotia. From west to east, these are the Dunnage, Gander, Avalon and Meguma. The Dunnage and Gander terranes were linked to the miogeocline during the Middle Ordovician Taconian Orogeny. The Avalon terrane arrived later, possibly during the mid-Paleozoic Acadian Orogeny. The Meguma terrane of southern Nova Scotia had docked with the Avalon terrane by Carboniferous time. The Dunnage terrane contains arc volcanics which lie above an ophiolitic substrate. The Gander terrane comprises a thick sequence of clastic sedimentary rocks, underlain by basement rocks with continental affinities. It has been interpreted as a continental margin, perhaps once on the eastern side of the Paleozoic Iapetus ocean. The Avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by Grenvillian basement. Its tectonic affinities are unclear. The Meguma terrane comprises a thick sequence of sediments, derived from the south-east. It is found only in southeastern Atlantic Canada. The boundaries between terranes are compressional in the west and steep, transcurrent faults in the east. The surface extent of the geological terranes is grossly correlative with deep structural zones, although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth. A striking feature of the deep crustal structure is a lower, high velocity crustal layer beneath the Dunnage and Gander terranes. The modern margin of Atlantic Canada developed by rifting and by transform motion between adjacent continents. Stretching and thinning of the lithosphere, and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust, are the most likely processes responsible for the evolution of the modern margin. These processes predict the observed deep sedimentary basins along the margin, the thinning of continental crust, and the high seismic velocities found within the ocean-continent transition zones. Rifting adjacent to Nova Scotia began in Late Triassic-Early Jurassic time between the present African and North American plates. These plate motions are also responsible for the major transform margin south of the Grand Banks. Separation between Iberia and the eastern Grand Banks occurred in mid-Cretaceous time, before the Late Cretaceous opening of the Labrador Sea. While the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust, the Grand Banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover. Numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment. Mimicry of Paleozoic promontories and re-entrants by modern rift and transform margin segments, the location of Mesozoic sedimentary basins on ancestral Appalachian structures, and the reactivation and propagation of major Precambrian and Paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins. The rift phase of both the ancient and modern passive margins is characterized by volcanism, mafic dike intrusion and by the development of basins filled with clastic sediments. The drift phase of both the ancient margin and the present Nova Scotia margin is marked by a change in sedimentary environment, such that carbonates replaced the rift phase clastic sediments. Two of the markers used to delineate the ancient ocean-continent transition zone; carbonate banks and steep gravity anomaly gradients, should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone. Furthermore, it is naive to view the ancient transition as simple and narrow, for the modern margins exhibits complex transition zones between 30 and 300 km wide. In general, the evolution of the ancient and modern passive margins appear to be remarkably similar. Predictably, closing the present Atlantic will mimic the evolution of the Appalachian Orogen.

Continental Margin of Eastern Canada--Geologic Framework and Petroleum Potential: ABSTRACT

The Atlantic-type continental margin of eastern Canada is underlain by a series of Mesozoic-Cenozoic sedimentary basins separated by basement highs or areas of thinner sediments. Regional and/or salt tectonics have structured the Mesozoic sequence, which is masked by a less-deformed wedge of prograding uppermost Cretaceous and Cenozoic sediments. The basins have been targets of active hydrocarbon exploration for over 2 decades. Data from 138 exploratory wells and over 680,000 km (420,000 mi) of multichannel seismic coverage have indicated four major geologic/geochemical regions: Scotian Shelf, southern Grand Banks, northeastern Grand Banks, and Labrador-Southeast Baffin Shelf. On the Scotian Shelf, 13 significant gas/condensate discoveries have been made out of 62 wildcats drilled since 1967. Five of the discoveries, including the Venture field, are in an overpressured zone that has been explored only since 1979. No commercial hydrocarbon accumulations have been found in the southern Grand Banks where 28 wildcats were drilled between 1966 and 1975. The northeastern Grand Banks region has been actively explored since 1971. The 22 wildcat wells drilled through late 1983 have yielded six significant light oil discoveries, including the giant Hibernia oil field. Labrador-Southeast Baffin Shelf exploration has yielded six gas/condensate discoveries in 26 exploratory wells drilled since 1971. The Geological Survey of Canada has developed hydrocarbon-generation models to explain the regional variation in oil and gas occurrence and to assess future potential in terms of the nature and thermal maturity of the source rocks, type of organic material, and time of trap formation. These factors are related to the geologic history of the margin, which is characterized regionally by diachronism in major basin inception and in the resultant stratigraphic record. We predict an exciting future for this vast petroleum province. End_of_Article - Last_Page 1203------------

Crustal structure, seismic stratigraphy, and rift processes of the continental margin off eastern Canada: ocean bottom seismic refraction results off Nova Scotia

Two crustal seismic refraction profiles were obtained on the continental margin of eastern Canada. These lines were located on the upper continental rise and on the outer continental shelf off Nova Scotia. A large air gun and explosive charges provided the sound sources, and ocean bottom seismometers were used to receive the signals. The travel time data were analyzed using the tau–p extremal inversion method and ray tracing techniques. This analysis allowed velocity–depth models to be constructed that included low-velocity zones and velocity gradients, as well as abrupt velocity contrasts at layer interfaces.The results show that about 7 km of sediments lies beneath the upper continental rise, beneath which oceanic layers 2 and 3 have been identified. Layer interfaces at depths of 0.39, 1.79, and 2.80 km below the sea floor correspond to strong, regional seismic reflectors, perhaps correlative with horizons Au, A*, and β. The top of oceanic layer 2 is associated with a velocity of 5.3 km s−1 and there is no difficulty in distinguishing between basement and high-velocity sediments in this region.The crust beneath the outer shelf includes 9–16 km of sediments. A layer with compressional wave velocity of 6.3 km s−1 constitutes the main crustal layer, below which the mantle is estimated to lie at total depths of 26–30 km. Within the sediments the velocity–depth model for the upper 5 km agrees well with sonic log velocities measured in deep wells. A seismic stratigraphy for the sediments can be deduced by comparing the refraction result with sonic log velocities and with the regional stratigraphy. The total thickness of the mid-Jurassic and older sediments, not sampled in the wells, is at least 4.5 km. The total thickness of crustal rocks of pre-rift age is between 14 and 21 km, which is significantly thinner than the 35–38 km values measured beneath mainland Nova Scotia in earlier studies. Both the thin crust and the high rate of subsidence during the Early Jurassic are consistent with extention of the lithosphere during initial rifting of this margin in Late Triassic time.

Rifting process and thermal evolution of the continental margin of Eastern Canada determined from subsidence curves

The subsidence history of the Nova Scotia and Labrador shelves, determined from deep exploratory well data, indicates that these regions underwent extension during rifting in the Early Jurassic and Late Cretaceous, respectively, and have since subsided passively due to conductive cooling of the lithosphere. The timing of the extension process is consistent with the ages of oldest sea floor adjacent to these margins. Extension parameters determined from the subsidence history of the Nova Scotian margin indicate both crustal and subcrustal material extended by approximately the same amount. For Labrador, the extension parameters indicate that the mantle lithosphere was extended considerably more than the crust. The magnitudes of extension on both margins are in good agreement with observed crustal thickness determined from seismic refraction data. Profiles depicting the sedimentary stratigraphies and depth to basement were constructed across each margin. The temperature distribution within the sediments was calculated from the extension parameters using a two-dimensional model with sediment units of varying thickness, radioactive heat production, and conductivity. The calculated temperature distribution across the margins is in good agreement with bottom hole temperatures measured in deep exploratory wells, and with surface heat flow measurements where they are available. Similar plots of temperature paleotemperature within the sediments suggest that thermal conditions have been favorable for hydrocarbon generation in some of the older strata.

Oceanic heat flow measurements over the continental margins of eastern Canada

Sixteen heat flow measurements have been obtained on the continental rise and slope off Nova Scotia and off the southern Grand Banks of Newfoundland. The mean of the 14 most reliable values is 48 ± 4 mW m−2 (1.15 ± 0.09 μcal cm−2 s−1), which is in agreement with the mean for previous measurements on ocean floor of age greater than 100 m.y. However, the heat flux from the crust indicated by the new values is significantly lower because of the heat produced in the some 5 km of terrigenous sediments underlying the stations. The low values could represent an edge effect between continent and ocean. Two lines of stations across the Nova Scotia rise show large heat flow variations. Numerical models indicate that the variations could arise from thermal refraction by high thermal conductivity salt, which probably constitutes the 'sedimentary ridge complex' outlined by deep seismic reflection profiles. The salt or evaporites formed in the restricted basin that was parallel to this rifted margin during the first phase of opening of the Atlantic Ocean. The heat flow variations measured on a profile perpendicular to the southern edge of the Grand Banks are smaller. This margin was a transform fault between Newfoundland and northwest Africa during the early ocean opening, so that the seafloor in the region of the profile was produced after the restricted basin phase of evaporite formation.