Gordon B. J. Fader

Gas-related sedimentary features from the eastern Canadian continental shelf

Abstract The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.

Quaternary geology and surficial sediment processes, Browns Bank, Scotian Shelf, based on multibeam bathymetry

Abstract Browns Bank is located on the glaciated continental shelf off southern Nova Scotia. Geological mapping of Browns Bank is based on interpretation of multibeam bathymetric and backscatter data, in conjunction with 220 line km of seismic reflection profiles and sidescan sonograms, sea-floor sediment samples and bottom photographs. The Fundian Moraine, part of the previously identified end moraine system on the continental shelf off Nova Scotia, is a prominent west–east, flat-topped, multi-lobate ridge identifiable in the multibeam bathymetric data and in geophysical records. The Fundian Moraine was subjected to erosion in the surf zone during sea-level rise in depths less than 100 m. A series of north–south, roughly parallel till ridges, continuous in the subsurface with the Fundian Moraine, is interpreted as interlobate moraines suggesting formation by a tidewater glacier. The Browns Bank Moraine, connected to the Fundian Moraine, is evidence of a newly identified grounded ice position farther seaward than the latter. Bedforms, including obstacle marks, comet marks, sand waves and megaripples, are evidence of a vigorous anticyclonic current pattern on Browns Bank. The combination of multibeam bathymetric imagery with high-resolution geoscientific information represents a powerful technique for sea-floor geological investigations.

Evidence for Wisconsinan glaciations in the Verrill Canyon area, Scotian Slope

Abstract A composite thickness of about 25 m of sediment has been cored from the Verrill Canyon on the Scotian Slope. It is interpreted that the majority of this sequence was deposited in a glaciomarine environment during oxygen isotopic stage 2 and the top of stage 3. These sediments, as seen in high-resolution seismic reflection profiles, are well stratified, become thicker upslope, are laterally variable in thickness, and pass upslope into possible outer shelf tills. Three wedge-shaped units of incoherent reflections interfinger with the parallel reflections and terminate in water depths greater than 700 m. These wedge-shaped units are interpreted as slumped diamict and outwash deposits. The age of the uppermost wedge-shaped unit is 26,000–21,000 yr based on extrapolation of radiocarbon dates. This unit documents a late Wisconsinan glacier readvance on the outer Scotian Shelf. The underlying wedge-shaped unit, estimated to be 70,000 yrs old, extends further west along the continental slope, and may represent a more extensive early Wisconsinan ice advance. A third wedge-shaped unit, inferred to have formed during isotopic stage 6, is possibly a remnant of the first glaciation in the study area.

The western Grand Banks of Newfoundland: Transgressive Holocene sedimentation under the combined influence of waves and currents

Abstract The surficial sediments on the western Grand Banks of Newfoundland have been reworked from older deposits (primarily late Wisconsinan glacial and glaciomarine sediments) during the post-glacial sea-level fall and Holocene rise. Winnowed medium to coarse sands and lag gravels that are generally less than 0.5 m thick chracterize the areas above the lowstand elevation at −110 to −120 m. The Holocene sediments which have accumulated in the deeper interbank channels, isolated basins and on the upper continental slope are thicker and consists of the fine sands and muds that were removed from the banks. The deposits below the lowstand elevation coarsen upward because little deposition occured during the sea-level rise. In general, the Holocene succession becomes thicker and finer grained to the south. Active sediment movement occurs at all depths less than about 110 m. Wave ripples composed of sandy gravel form to depths in excess of 100 m in response to long-period (15 s) surface waves, but the predominant bedforms consist of current-generated sand ribbons, megaripples and sandwaves which exhibit unidirectional migration azimuths toward the southwest, even though the storm-current directions are highly variable and the maximum sediment-transport rates are directed toward the east or southeast. The bedform orientation and resultant transport are instead parallel to the southerly and southwesterly storm currents which occur during the prolonged, post-storm spin-down period, and to the weak ( −1 ) Labrador Current and the orbital motion associated with the largestwaaves. Thus, the interaction of the oceanic current and wave-orbital motion with the directionally-variable strom currents is responsible for the consistent orientation of the current-generated bedforms, and for the distribution of Holocene deposits.

A comparison of models of glacial sedimentation along the eastern Canadian margin

Abstract The distribution and thickness of glacial and postglacial sediments of the eastern Gulf of Maine, Scotian Shelf, Grand Banks of Newfoundland, Labrador Shelf and Hudson Strait have been mapped using seismic reflection systems and lithological, chronological and biostratigraphic data from cores and seabed samples. Deposits of till, represented by incoherent seismic reflections, commonly occur as sheets but also as linear ridges, lenses and tongues. Discontinuous multiple till sequences, whose thicknesses range from 1–300 m occur across the shelves and pinch out in water depths of 500–700 m. Till thickness and distribution is in part controlled by erodability and geomorphology of the underlying substrate. Glaciomarine sediments up to 300 m thick, represented by medium to high intensity, parallel, continuous coherent seismic reflections occur as conformable to slightly ponded deposits. Glaciomarine sediments can interfinger, conformably overlie or abruptly terminate against the till surfaces. On the southern shelves, from the Gulf of Maine to northeast Newfoundland Shelf and also in eastern Hudson Strait, the glaciomarine sediments are typically interbedded with the tills at former ice-sheet buoyancy zones where much of the material has been derived from subglacial meltout. On the Labrador Shelf, subglacial meltout did not occur as the till and glaciomarine sediments are of different lithology and do not interfinger. From the Gulf of Maine to the Grand Banks of Newfoundland, acoustically weakly stratified, ponded postglacial muds were eroded from the shallow bank surfaces by a marine transgression and deposited in the basins of the shelf. On the Labrador Shelf, iceberg scouring has reworked the entire bank top surface into an ice-keel turbate up to 10 m thick and released the fines which now occupy the basins. The thick (mid-late Wisconsinan) multiple till sections and associated glaciomarine sediments in the basins of the southern shelves suggest the presence of an ice shelf for 20,000 years, beneath which large volumes of sediment were deposited. In contrast, the relatively thin (mid-late Wisconsinan) glacial sequence of the Labrador Shelf and the lack of subglacial deposits within the glaciomarine sequence may indicate either a rapid ice-sheet retreat for that area or an ice shelf largely devoid of englacial debris.

Seabed processes on the northeastern Grand Banks of Newfoundland; Modern reworking of relict sediments

Abstract On the northeastern edge of the Grand Banks of Newfoundland, between 140 and 70 m depth, a thin Holocene sediment veneer unconfortmably overlies Tertiary siltstones and sandstones. These surficial sediments show a depth-controlled facies distribution. Below 90 m the facies range from continuous fine sand, through lag gravel and sand ribbons with arcuate sand waves and megaripples normal to the axis of the ribbons. Above 90 m depth alternating sand ridges up to 3 m in height and lag gravels occur. Megaripples and arcuate sand waves (400 m wave length) with megaripples on their stoss side are developed on the terrace. The regional geology suggests that many of the surface sediments were inherited from Pleistocene deposits subsequently reworked in coastal environments by an early Holocene transgression. Side-scan sonar surveys, bottom photography and submersible observations demonstrate intermittent sediment mobilization of recent material. In particular, photographs show asymmetrical ripples in well-sorted sand after seasonal storms. Side-scan sonar coverage in the Hibernia area shows fields of two- and three-dimensional megaripples (5 m length) which appear to be migrating across trawl marks left by recent fishing operations. These sedimentary bedforms are attributed to unidirectional storm driven currents, ocean currents and extreme waves. The larger bedforms may reflect paleoceanographic conditions while the small bedforms appear to be modern.

A Late Pleistocene Low Sea-Level Stand of the Southeast Canadian Offshore

A widespread, submarine, low sea-level stand is interpreted to occur at a present depth of 110–120m across the Scotian Shelf and is dated at 15 ka BP. Evidence in support of this position includes: 1) absence of fine-grained muddy sediments above and their abundance below 110m; 2) the distribution of well-sorted sand and rounded gravel clasts above the low sea-level position in contrast to the widespread occurrence of angular clasts below; 3) the occurrence of terraces cut into both bedrock and glacial sediments at 110–120m water depth; 4) the occurrence of unconformities on glacial sediments dated from 40-15 ka BP which exist as erosional remnants above the low sea-level position; 5) the distribution of continuous deposits of till below and its general absence above the low sea-level stand; 6) the relative distribution of relict versus modern iceberg furrows at the seabed above and below the low sea-level stand; and 7) the occurrence of a subaerially desiccated crust above 110m water depth and its absence below. The low sea-level position occurs at approximately the same depth across the outer continental shelf. This suggests that the glaciers had retreated from the shelf before the maximum lowering and that differential warping has not occurred since its formation, otherwise the low stand would be discontinuous. It also supports the idea that for the offshore area of the southeast Canadian continental shelf, glacial isostatic rebound was largely over by the time the low sea-level stand was formed.