- Email: [email protected]
Multibeam bathymetry of glaciated terrain off southwest Newfoundland
Multibeam bathymetry of glaciated terrain off southwest Newfoundland John Shaw *, Robert C. Courtney Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Now Scotia B2 Y 4A2, Canada
Received I October 1996; accepted 6 May 1997
Abstract Inner St. George’s Bay, Newfoundland, was mapped in 1988 using a suite of conventional geophysical and sampling tools. The Quaternary deposits, sea-level history, and Holocene littoral sediments were described in a series of papers. In 1995, part of the previously mapped area was imaged using a multibeam mapping system. The resulting highresolution images of bathymetry and backscatter are interpreted with reference to the previously collected data. The multibeam imagery shows a Late Wisconsinan morainal bank that has been modified by marine processes during and following a -25 m postglacial sea-level lowstand. Two types of terrain occur on the morainal bank. In the extreme west, where morainal till is close to the seabed, the image shows areas with high reflectivity and rough texture that consist of bouldery gravel. Farther east, the flanks of the moraine are buried by a prograded wedge of sediment eroded from the bank during the Holocene transgression and transported towards the adjacent basin by waves and currents. Here, areas of relatively smooth seabed on the multibeam bathymetry image consist of either mobile, rippled, poorly sorted gravel (high reflectivity), or sand (lower reflectivity). The image shows a submarine valley which, according to the previous data, was glacially overdeepened and subsequently partly filled by glacial-marine and postglacial sediments. Shallow areas with high reflectivity on either side of the valley are seafloor exposures of a draped, glacial-marine unit that was deposited when the former ice margin was located close to the present coast (between 13.3 and 10.8 ka). An area of mounded morphology and high backscatter, located below the level of the postglacial sea-level lowstand, is interpreted as till that has not been modified by wave processes. The multibeam bathymetry and backscatter images reveal the inadequacies of the previous map that was compiled using data from conventional survey techniques. 0 1997 Elsevier Science B.V. Keywords: multibeam
bathymetry;
glaciation; Newfoundland;
1. Introduction Inner St. George’s Bay, southwest Newfoundland (Fig. 1 ), was surveyed in 1988 (Forbes and Shaw, 1989). The acoustic systems used were a sparker sub-bottom profile system, a Geopulse Bubble Pulser sub-bottom profile system, and a Klein sidescan sonar system. Twenty-two grab * Corresponding
author.
E-mail: [email protected]
0025-3227/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PZI SOO25-3227(97)00093-S
backscatter
samples and two cores were collected. In addition, six gravity cores and twenty-two grab samples were collected from an adjacent bay (Port au Port Bay) where similar acoustic facies are present. Analysis of these data revealed a complex pattern of subsurface topography and acoustic facies (Forbes and Shaw, 1989; Shaw and Forbes, 1990, 1992). Inner St. George’s Bay contains a basin with a maximum depth of 100 m that is separated from
126
48” 30
-
Former Margin
Ice
Multibeam
/”
@-_ Flat Bay Bmok
0
I
I
I
lllkm
,
1
Fig. 1, Location maps: (a) general location of the study area; (h) St. George’s Bay. showing location of a former Late Wisconsinan ice margin and the position of the seismic profile shown in Fig. 5; (c) inner St. George’s Bay, showing the area of the multibeam bathymetry survey.
the outer bay by a shallow (20-25 m depth) sill. The basin contains two southwest-trending glacially overdeepened valleys, one of which extends to at least 195 m below present sea level. Eight
seismo-stratigraphic units were recognised within the Quaternary sequence, of which three record the presence and retreat of a major Late Wisconsinan ice margin. The near-surface
J. ShaM: R. C. Courtney
/ Marine
distribution of six of the seismo-stratigraphic units (in the area that was subsequently mapped using a multibeam mapping system) is shown on Fig. 2. Two units are absent from the map: Unit 2 (that is buried by thick deposits of Unit 4) and Unit 5 (that occurs west of the outlined area). Ice-contact deposits (i.e. till) of Unit 1 form a morainal bank up to 85 m thick that extends across the bay. Thinner ice-contact deposits occur in parts of the basin, overlying bedrock. Acoustically stratified sediments within the glacially deepened valleys in the basin are interpreted as subaqueous outwash (Unit 2). A draped, acoustically stratified glacial-marine mud (Unit 3) that occurs in many parts of the inner bay was deposited when ice retreated to the position of the present coast. This
127
Geology 143 (19971 125-135
unit has not been cored in St. George’s Bay, but in adjacent Port au Port Bay radiocarbon determinations on core samples (shell) range from 13.3 to 10.8 ka (Forbes et al., 1993). Radiocarbon dates on glacial-marine sediments onshore range back to 13.7 ka (Brookes, 1969, 1974; Grant, 1991). When the former ice margin retreated to the present coast, the crust was isostatically depressed, and the late glacial sea stood at a level of +44 m (Brookes et al., 1985). Relative sea level dropped to -25 m at ca. 9.5 ka (Forbes et al., 1993) because of glacial rebound, and has been rising since. Glacial sediments were reworked, resulting in deposition of further units: acoustically transparent mud (Unit 4) that is found in deeper water
Spillover deposits (sand and gravel) a
Barrier-platform (sand)
•In
Lowstand deposits Postglacial
delta mud
cl #
Draped glacial-manne mud Ice contact sediments (till)
Fig. 2. Near-seabed occurrences of seismo-stratigraphic units based on the 1988 surveys, as reported by Shaw and Forbes (1990). When compared with Figs. 3 and 4 this shows the limitations of conventional mapping or, conversely, the advantages of mapping using multibeam systems.
throughout the basin; sand ( Unit 5). that mainly occurs as a seaward-thickening and fining sheet to the west of the morainal bank; lowstand deltas (Unit 6) that appear as prograded prisms graded to -25 m; barrier-platform sands (Unit 7) that prograded into the basin in the vicinity of the gravel barriers at Flat Island and Stephenville; and spillover deposits (Unit S), comprising gravel and sand that prograded into the basin from the sill. In 1995, part of the submarine moraine extending across inner St. George’s Bay was surveyed using a multibeam bathymetry mapping system. The objective was to collect data in a region previously mapped using conventional techniques, and to use the existing data to interpret the highresolution bathymetry. To our surprise, the multibeam imagery revealed that the seafloor was much more complex than we had envisaged, particularly with regard to the presence of modern features such as pock marks caused by gas venting, submarine fans and slides on the flanks of a barrier platform, and large sandy bedforms on its surfAce: these aspects of the imagery are discussed elsewhere (Shaw et al.. 1997). In this paper we focus on the surface morphology of the glacial deposits in the bay.
2. Data collection, processing, and display Multibeam bathymetry and backscatter data were collected in 1995 using the CSS Fwdwick G. Ctwd, a SWATH (small-waterplane-area twinhull) vessel equipped with a Simrad EM- 1000 multibeam bathymetric system. The sounder is mounted in the starboard pontoon. This system produced 60 beams arrayed over an arc of 1SO degrees. The swath of seafloor covered on each survey line was typically four to five times water depth. To provide sufficient overlap, line spacing was typically 100 m in depths less than 40 m. and 200 m in depths of 40-100 m. Navigation was by a global positioning system. with real-time differential corrections relayed by radio from a shore station or Coast Guard beacon, providing an accuracy of +2 m. The data set was collected over a period of 3 days at speeds ranging from 13 to 16 kts. The coverage is shown in Fig. lc.
Erroneous depth values were removed using the CARIS ‘HIPS’ system (Hydrographic Information Processing System). Data were adjusted for tidal variation using tidal predictions for Stephenville (Canadian Hydrographic Service, 1995). Bathymetric data were gridded in 8-m bins (horizontal ). shaded using artificial illumination, and viewed on a Hewlett Packard workstation using GRASS, a public domain image analysis system. Coloured. three-dimensional shaded relief views were displayed on a Silicon Graphics workstation. Air photographs of Flat Island Spit, taken in 1949, were scanned, rectified using GRASS, and combined with the bathymetric data. The resultant multibeam image is shown in Fig. 3. The numbers in Fig. 3 indicate features discussed in the text; the black lines are seismic profiles also discussed. Backscatter data taken from the EM-1000 raw datagrams were gridded to produce an image ( Fig. 4) complementary to that of the seabed morphology. The Simrad sounder records two separate estimates of backscatter (or reflectivity): a mean \,alue for each beam. and a full time series of estimates that spans the beam footprint. These backscatter estimates are corrected within the sounder for the beam pattern, attenuation in the water column. and Lamberts law (Urick, 1983). .Ahhough the full time series estimates are recorded at a cross-track resolution of I5 cm, the resolution of the data is actually constrained by the alongtrack sampling frequency of 3- 5 m, dependent on water depth and ship’s speed. There is little appreciable difference between the gridded maps 01 mean backscatter and the full time series estimates for regional scale maps with a grid interval in excess of 5 m. The mean backscatter value is plotted in Fig. 4. The average backscatter versus angle of incidence has been applied to remove the mean systematic variation with beam angle. The backscatter intensity was normalised to a mean grazing angle value of 50 degrees. The backscatter intensity of sound returned by the seafloor is a complex function of frequency, angle of incidence, surface and volume roughness. amongst other acoustical parameters (e.g., Jackson ct al.. 1986). so it would be overly simplistic to directly associate a bottom type with a single backscatter estimate at a single incident angle. In
J. Shaw, R.C. Courtney J Marine Geology 143 (1997) 125-135
129
Fig. 3. Digital bathymetric image of inner St. George’s Bay, merged with scanned, rectified air photographs of Flat Island Spit taken in June 1949. The relief data are illuminated from the northeast (upper right). Numbers are referred to in the text. The dashed lines are the axes of bedrock submarine valleys. North is vertically upwards (approx.).
addition, the transducer array on Creed had been damaged in a previous cruise, and the backscatter estimates taken near-nadir were too low. However, some general correlations were observed based on seismic data, sidescan-sonar data, and bottom samples collected in this area (Forbes and Shaw, 1989; Shaw and Forbes, 1990, 1992), and reports of other researchers (deMoustier and HughesClarke, 1995). Gravelly and rough surfaces typically had high backscatter estimates of -20 to - 30 dB, while smooth-surfaced fine-grained sands and muds returned much less energy (- 35 to -40 dB). An interpretation of a sleeve-gun sub-bottom seismic reflection profile collected in 1989 (Josenhans et al., 1989) is shown in Fig. 5. The location of this profile is shown in Fig. 1. Interpretations of sparker seismic-reflection profiles collected in 1988 (Forbes and Shaw, 1989)
are shown in Figs. 6 and 7. Locations of these profiles are shown in Fig. 3. During the multibeam survey we deployed a multitip sparker sub-bottom profiling system, and collected a small amount of data, using features on the multibeam image to plan tracks.
3. Interpretation of the multibeam image The principal element of the bathymetric image is an elongate trough, up to 100 m deep, that trends towards the northeast, with a branch that trends to the southeast (see dashed lines in Fig. 3). On the backscatter image (Fig. 4) the trough has the light tone characteristic of a muddy, nonreflective bottom. To the west of the trough is a flat-topped sill with depths of 20-25 m. This has a predominantly dark tone, indicative of a
Fig. 4. Backscatter
image of the same arca shown in Fig. 3
reflective (gravel ) bottom, with patches of light tone, indicative of less-reflective sand. The eastern margin of the sill is a sinuous, steep slope with a height of 30-45 m. Along the bottom of the image is Flat Island Spit, a 12-km-long barrier composed of gravel beach ridges (Shaw and Forbes, 1992). Offshore from Flat Island is a shallow (<25 m deep) sandy, barrier platform with large transverse bedforms and a shore-parallel bar close to the coast. Submarine fans ( 1) on the flank and at the northeast end of the barrier platform result from sand moving off the platform and into the adjacent basin (Shaw et al., 1997). 3.1. Suhmurinr
vdleys
The central part of the image is a basin with a maximum depth of 100 m. Trending northeast to southwest across the basin is a sinuous, glacially overdeepened submarine valley-the St. George Valley (Shaw and Forbes, 1990)--that extends to
a maximum depth of 195 m below sea level; a tributary bedrock valley ( 155 m) trends southeast towards the coast. The St. George Valley, shown by the long dashed line in Fig. 3, contains thick deposits of acoustically stratified, basin-fill style glacial-marine sediments (Unit 2), overlain by about 20 m of glacial-marine mud (Unit 3). The latter has closely spaced, continuous, coherent internal reflections (cf. Fig. 7). It is present in the submarine valley and is also draped over the underlying terrain in the relatively shallow areas on either side of the valley. The uppermost unit in the basin is postglacial silty mud (Unit 4), characterised by weak acoustic stratification (cf. Fig. 7) that is masked by gas where the sediment is more than 5 m thick. Seafloor depressions (2) are aligned along the axis of the submarine valley in depths of 855100 m. They may have formed by current erosion (cf. Fader and Buckley, 1996), or perhaps they are pockmarks formed by gas venting (Fader, 1991).
131
J. Shaw, R.C. Courtney / Marine Geology 143 (1997) 125-135
Bedrock
NE I 5
0
I
I
10
15
0.3
I 20
DISTANCE (km) Fig. 5. Sleeve-gun
seismic reflection
transect
through
the sill of inner St. George’s
Shaw et al. (1997) show a 3-D image of what they interpret as a plume of gas bubbles venting from one of these depressions. 3.2. The morainal bank The lOO-m-deep basin of inner St. Georges Bay is separated from the Gulf of St. Lawrence by a flat-topped sill with a mean depth of 24 m (Fig. 5). The sill is a morainal bank that formed at a Late Wisconsinan ice margin prior to 13.3 ka, one of a series of end moraines located offshore from embayments on the west coast of Newfoundland (Grant, 1989; Josenhans et al., 1990). The moraine was subaerially exposed during a -25 m sea-level lowstand during the early Holocene (Shaw and Forbes, 1990; Forbes et al., 1993), at which time a peninsula extended southwest from the north coast, and the inner bay was connected to the Gulf of St. Lawrence by a narrow channel. The morainal bank was submerged and partly reworked by transgression. waves during the Holocene Reworked sediment has accumulated on the east flank of the bank as a wedge of prograded sand and gravel, the ‘spillover deposits’ of Shaw and Forbes ( 1990).
Bay. Location
is shown
in Fig.
I,
Sidescan sonar data, sub-bottom profiler data, and grab samples (Shaw and Forbes, 1990, 1992) show that the morainal bank is comprised of two terrain types. At the extreme left of the bathymetric image (3) local relief varies up to a maximum of 5 m. In this zone, ice-contact deposits (on what was formerly the highest part of the moraine) are close to the seabed, and lie just below a lag veneer of bouldery gravel. The largely immobile gravel is interspersed with thicker (l-2 m) lenses of mobile sand and gravel, resulting in the patchy, light and dark appearance of the backscatter image in this area (Fig. 4). Farther east (4) the ice-contact sediments of the morainal bank are overlain by a thick wedge of postglacial sediments, the ‘spillover’ deposits. Here the seafloor is relatively smooth, and consists of extensive sheets of gravel ripples and scattered sand waves. Several grab samples from areas of gravel ripples consisted of poorly sorted polymodal fine gravel and up to 40% sand. The active margin of the spillover deposits, on the flank of the basin, is a sinuous, steep (20 degrees) slope, the top of which ranges from 30 to 45 m above the adjacent basin. A sparker seismic reflection profile (Fig. 6; see also fig. 12 in Shaw and Forbes, 1990) shows
132
Glacial-marine
mud -_
Bedrock
0
‘!
--__
\
2
1
DISTANCE (km) Fig. 6. Sparker seismic reflection dntu xrosb the margin 01‘ thy sp~llo~cr deposits correspond with a lobate area of positive relief on the multibeam image
steeply dipping, prograded internal reflections in the spillover deposits. The finger-like trough (5) that extends several kilometres southwest from the basin is a continuation of the submarine valley. The trough is very narrow because it has been almost completely infilled by barrier-platform sediments on the south side, and by a wedge of spillover that prograded out from the sill on the north side. The trough is completely infilled to the southwest.
To the east of the sill are irregular areas of high relief (6) that appear dark-toned (reflective) on the backscatter image ( Fig. 4). These are seafloor
(lucation shou,n in Fig. 3). The ‘slide deposits.’
exposures of glacial-marine mud (Unit 3) that is the equivalent of the Emerald Silt Formation mapped on the Scotian Shelf and elsewhere by King and Fader ( 19861, Fader et al. ( 1982), and others. The high reflectivity is due to the presence of a thin surficial veneer of fine angular gravel, the result of current winnowing. This unit is also the equivalent of the glacial-marine overlap sediments that have been mapped on nearby coasts of St. George’s Bay (cf. Brookes, 1974), and that consist of shelly marine clays and silts. Published radiocarbon dates on the onshore sediments (Grant, 199 1) range back to an (unadjusted) age of 13.7 ka ( Brookes, 1969). There are no radiocarbon dates from offshore in St. George’s Bay, but in adjacent Port au Port Bay four radiocarbon dates
J. Shaw, R. C. Courtney
1 Marine
Geology
133
143 (1997) 125-135
Glacial-marine mud
0.5
1.0
1.5
Distance (km) Fig. 7. Sparker seismic reflection profile across an area where glacial-marine deposits are exposed on the seafloor (location shown in Fig. 3).
on shell samples in the Emerald Silt equivalent range from 10.8 to 13.3 ka (Forbes et al., 1993). Fig. 7 is a sparker profile across an exposure of the Emerald Silt equivalent. It forms an area of high relief with an average water depth of 42 m, corresponding with a dark (reflective) area on the backscatter image (Fig. 4). The profile shows internal reflections truncated at the seafloor in this area. A submersible investigation of a similar exposure nearby has shown that the high reflectivity of the seafloor is due to veneer of fine, angular gravel (dropstones), winnowed by waves and currents from the glacial-marine unit. However, not all areas of seafloor with high backscatter are exposures of the glacial-marine unit. The seafloor at 7 is smooth on the bathymetry image (Fig. 3), but Fig. 4 shows an irregular area of relatively high backscatter in this vicinity. This is an area of sand and gravel dredge spoils dumped on a smooth, muddy bottom. The spoil heaps
show up clearly on unpublished sidescan sonograms collected during the 1988 survey. 3.4. Seafloor exposures of till below the postglacial sea-level lowstand
Mounds with relief of 15 m (8) are interpreted as till, exposed at the seafloor, below the level of the postglacial sea-level lowstand (-25 m). The till probably retains its primary morphology, and the associated high backscatter is likely due to a surficial veneer of muddy bouldery gravel at the seafloor.
4. Discussion and conclusions High-resolution multibeam systems are relatively new geological mapping tools whose value in understanding the Quaternary history of
glaciated environments is just beginning to be appreciated. From a practical viewpoint. they facilitate efficiently planned geophysical surveys. For example, in 1997 we mapped Bay of Islands, a fiord in western Newfoundland, using a multibeam system, and subsequently used the multibeam data to plan a series of geophysical lines and deployments of a shallow gas sampling system. In the example described, the complex morphology of the seafloor results partly from glacial processes such as overdeepening of St. George Valley, emplacement of the morainal bank, and deposition of a drape of glacial-marine mud in the bay. This glaciated terrain has also been considerably modified by postglacial events, including trimming of the moraine during the sealevel lowstand and in the Holocene transgression. formation of ‘spillover’ deposits, winnowing of glacial-marine muds at the seafloor, deposition of and progradation of barrier mud in basins, platforms. In this respect, inner St. George’s Bay is an unusual example of glaciated terrain; multibeam images from deeper water in other parts of the eastern Canadian shelf show much less modification of glacial features by postglacial marine processes. For example, unpublished images from Placentia Bay (south Newfoundland ) show curvilinear assemblages of crag and tail features, and relict iceberg pits and furrows. Likewise, a submarine moraine on the Scotian Shelf is heavily imprinted with relict iceberg furrows. It has not been substantially modified during the postglacial sea-levellowstand and subsequent Holocene transgression.
Acknowledgements We express our thanks to the masters and crews of the F. G. Creed and to Tony Atkinson and James Currie. Interpretation of the data benefited from discussions with Heiner Josenhans, David Piper, Carl Amos, John Shaw (University of Alberta), and Gordon Fader. This is Geological Survey of Canada contribution number 1996408.
References Brookes. LA., 1969. Late-glacial marine overlap in western Newfoundland. Can. J. Earth Sci., 6: 1397-1404. Brookes. I.A., 1974. Late Wisconsinan glaciation of southwestern Newfoundland (with special reference to the Stephenville map-area). Geol. Surv. Can. Pap. 73-40. Brookes, I.A., Scott. D.B., McAndrews, J.H.. 1985. “Post-glacial relative sea-level change, Port au Port area. west Newfoundland”. Can. J. Earth Sci., 22: 1039-~1047. Canadian Hydrographic Service, 1995. Canadian Tide and Current Tables, Volume I. Atlantic Coast and Bay of Fundy. Department of Fisheries and Oceans Canada, Ottawa. deMoustier. C., Hughes-Clarke. J. 1995. Seafloor classification with swath bathymetry systems. Chapter 31. Coastal Multibeam Training Notes, Ocean Mapping Group, University of New Brunswick. Fader, G.B.J.. 1991. Gas-related sedimentary features from the eastern Canadian continental shelf. Cont. Shelf Res.. I I: 1123 1153. Fader. G.B.J.. Buckley. D.E.. 1996. “Environmental Geology of Halifax Harbour, Nova Scotia”. Geosci. Can. 23 (4) 152 171. Fader. G.B.W., Kmg. L.H., Josenhans, H.W., 1982. Surticial geology of the Laurentian Channel and western Grand Banks of Newfoundland. Geol. Surv. Can. Pap. 81-22: Mar. Sci. Pap. 21. 37 pp. Forbes. D.L.. Shaw. J.. 1989. Navicula operations in southwest Newfoundland coastal waters: Port au Port Bay, St. George’s Bay. La Poile Bay to Barasway Bay and adjacent inner shelf. Cruise Report 88018 (E). Open File 2041, Geological Survey of Canada, Ottawa. 57 pp. I-orbes, D.L., Shaw, J., Eddy. B.G.. 1993. “Late-Quaternar) sedimentation and the postglacial sea-level minimum in Port au Port Bay and vicinity, west Newfoundland”. Atl. Geol.. 39: 1 26. Grant. D.R., 1989. Surficial geology, Sandy Lake Bay 01’ Islands. Newfoundland. Map 1664A. scale 1 : 250,000, Geological Survey of Canada, Ottawa. Grant, D.R., 1991. Surficial geology, Stephenville--Port aux Basques, Newfoundland. Map 1737 A, scale I : 250,000, Geological Survey of Canada, Ottawa. Jackson. D.R.. Winebrenner. D.P.. Ishimuru. A.. 1986. Application of composite roughness model to high-frequency bottom backscattering. J. Acoust. Sot. Am., 79: 1410-1422. Josenhans, H.W., Sanford, B.V., Sparkes, R., Johnston, B.L.. Boyce, A., Nielsen, J.. Belliveau. M., 1989. Baffin 89-008 Cruise Report. Open File 2115, Geological Survey of Canada, Ottawa. Josenhans. H., Zevenhuizen. J., MacLean, B., 1990. Preliminar) stratigraphic interpretations from the Gulf of St. Lawrence. In: Current Research, Part B, Geol. Surv. Can. Pap. 90-lB, pp. 59-75. King, L.H., Fader. G.B.J., 1986. Wisconsinan glaciation of the Atlantic continental shelf of southeast Canada. Geol. Surv. Can.. Bull. 363, 72 pp. Shaw. J.. Forbes. D.L., 1990. “Late Quaternary sedimentation
J. Shaw, R. C. Courlney / Marine Geology 143 ( 1997) 125-135 in St. George’s Bay, southwest Newfoundland: acoustic stratigraphy and seabed deposits”. Can. J. Earth Sci., 27: 9644983. Shaw, J., Forbes, D.L., 1992. “Barriers, barrier platforms, and spillover deposits in St. George’s Bay, Newfoundland: paraglacial sedimentation on the flanks of a deep coastal basin”. Mar. Geol., 105: 119-140.
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Shaw, J., Courtney, R.C., Curry, J.R., 1997. The marine geology of St. George’s Bay, Newfoundland, as interpreted from multibeam bathymetry and back-scatter data. Geomar. Lett. 17, 1888194. Urick, R.J., 1983. Principles of Underwater Sound. McGrawHill, 3rd ed., New York, 384 pp.
Received I October 1996; accepted 6 May 1997
Abstract Inner St. George’s Bay, Newfoundland, was mapped in 1988 using a suite of conventional geophysical and sampling tools. The Quaternary deposits, sea-level history, and Holocene littoral sediments were described in a series of papers. In 1995, part of the previously mapped area was imaged using a multibeam mapping system. The resulting highresolution images of bathymetry and backscatter are interpreted with reference to the previously collected data. The multibeam imagery shows a Late Wisconsinan morainal bank that has been modified by marine processes during and following a -25 m postglacial sea-level lowstand. Two types of terrain occur on the morainal bank. In the extreme west, where morainal till is close to the seabed, the image shows areas with high reflectivity and rough texture that consist of bouldery gravel. Farther east, the flanks of the moraine are buried by a prograded wedge of sediment eroded from the bank during the Holocene transgression and transported towards the adjacent basin by waves and currents. Here, areas of relatively smooth seabed on the multibeam bathymetry image consist of either mobile, rippled, poorly sorted gravel (high reflectivity), or sand (lower reflectivity). The image shows a submarine valley which, according to the previous data, was glacially overdeepened and subsequently partly filled by glacial-marine and postglacial sediments. Shallow areas with high reflectivity on either side of the valley are seafloor exposures of a draped, glacial-marine unit that was deposited when the former ice margin was located close to the present coast (between 13.3 and 10.8 ka). An area of mounded morphology and high backscatter, located below the level of the postglacial sea-level lowstand, is interpreted as till that has not been modified by wave processes. The multibeam bathymetry and backscatter images reveal the inadequacies of the previous map that was compiled using data from conventional survey techniques. 0 1997 Elsevier Science B.V. Keywords: multibeam
bathymetry;
glaciation; Newfoundland;
1. Introduction Inner St. George’s Bay, southwest Newfoundland (Fig. 1 ), was surveyed in 1988 (Forbes and Shaw, 1989). The acoustic systems used were a sparker sub-bottom profile system, a Geopulse Bubble Pulser sub-bottom profile system, and a Klein sidescan sonar system. Twenty-two grab * Corresponding
author.
E-mail: [email protected]
0025-3227/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PZI SOO25-3227(97)00093-S
backscatter
samples and two cores were collected. In addition, six gravity cores and twenty-two grab samples were collected from an adjacent bay (Port au Port Bay) where similar acoustic facies are present. Analysis of these data revealed a complex pattern of subsurface topography and acoustic facies (Forbes and Shaw, 1989; Shaw and Forbes, 1990, 1992). Inner St. George’s Bay contains a basin with a maximum depth of 100 m that is separated from
126
48” 30
-
Former Margin
Ice
Multibeam
/”
@-_ Flat Bay Bmok
0
I
I
I
lllkm
,
1
Fig. 1, Location maps: (a) general location of the study area; (h) St. George’s Bay. showing location of a former Late Wisconsinan ice margin and the position of the seismic profile shown in Fig. 5; (c) inner St. George’s Bay, showing the area of the multibeam bathymetry survey.
the outer bay by a shallow (20-25 m depth) sill. The basin contains two southwest-trending glacially overdeepened valleys, one of which extends to at least 195 m below present sea level. Eight
seismo-stratigraphic units were recognised within the Quaternary sequence, of which three record the presence and retreat of a major Late Wisconsinan ice margin. The near-surface
J. ShaM: R. C. Courtney
/ Marine
distribution of six of the seismo-stratigraphic units (in the area that was subsequently mapped using a multibeam mapping system) is shown on Fig. 2. Two units are absent from the map: Unit 2 (that is buried by thick deposits of Unit 4) and Unit 5 (that occurs west of the outlined area). Ice-contact deposits (i.e. till) of Unit 1 form a morainal bank up to 85 m thick that extends across the bay. Thinner ice-contact deposits occur in parts of the basin, overlying bedrock. Acoustically stratified sediments within the glacially deepened valleys in the basin are interpreted as subaqueous outwash (Unit 2). A draped, acoustically stratified glacial-marine mud (Unit 3) that occurs in many parts of the inner bay was deposited when ice retreated to the position of the present coast. This
127
Geology 143 (19971 125-135
unit has not been cored in St. George’s Bay, but in adjacent Port au Port Bay radiocarbon determinations on core samples (shell) range from 13.3 to 10.8 ka (Forbes et al., 1993). Radiocarbon dates on glacial-marine sediments onshore range back to 13.7 ka (Brookes, 1969, 1974; Grant, 1991). When the former ice margin retreated to the present coast, the crust was isostatically depressed, and the late glacial sea stood at a level of +44 m (Brookes et al., 1985). Relative sea level dropped to -25 m at ca. 9.5 ka (Forbes et al., 1993) because of glacial rebound, and has been rising since. Glacial sediments were reworked, resulting in deposition of further units: acoustically transparent mud (Unit 4) that is found in deeper water
Spillover deposits (sand and gravel) a
Barrier-platform (sand)
•In
Lowstand deposits Postglacial
delta mud
cl #
Draped glacial-manne mud Ice contact sediments (till)
Fig. 2. Near-seabed occurrences of seismo-stratigraphic units based on the 1988 surveys, as reported by Shaw and Forbes (1990). When compared with Figs. 3 and 4 this shows the limitations of conventional mapping or, conversely, the advantages of mapping using multibeam systems.
throughout the basin; sand ( Unit 5). that mainly occurs as a seaward-thickening and fining sheet to the west of the morainal bank; lowstand deltas (Unit 6) that appear as prograded prisms graded to -25 m; barrier-platform sands (Unit 7) that prograded into the basin in the vicinity of the gravel barriers at Flat Island and Stephenville; and spillover deposits (Unit S), comprising gravel and sand that prograded into the basin from the sill. In 1995, part of the submarine moraine extending across inner St. George’s Bay was surveyed using a multibeam bathymetry mapping system. The objective was to collect data in a region previously mapped using conventional techniques, and to use the existing data to interpret the highresolution bathymetry. To our surprise, the multibeam imagery revealed that the seafloor was much more complex than we had envisaged, particularly with regard to the presence of modern features such as pock marks caused by gas venting, submarine fans and slides on the flanks of a barrier platform, and large sandy bedforms on its surfAce: these aspects of the imagery are discussed elsewhere (Shaw et al.. 1997). In this paper we focus on the surface morphology of the glacial deposits in the bay.
2. Data collection, processing, and display Multibeam bathymetry and backscatter data were collected in 1995 using the CSS Fwdwick G. Ctwd, a SWATH (small-waterplane-area twinhull) vessel equipped with a Simrad EM- 1000 multibeam bathymetric system. The sounder is mounted in the starboard pontoon. This system produced 60 beams arrayed over an arc of 1SO degrees. The swath of seafloor covered on each survey line was typically four to five times water depth. To provide sufficient overlap, line spacing was typically 100 m in depths less than 40 m. and 200 m in depths of 40-100 m. Navigation was by a global positioning system. with real-time differential corrections relayed by radio from a shore station or Coast Guard beacon, providing an accuracy of +2 m. The data set was collected over a period of 3 days at speeds ranging from 13 to 16 kts. The coverage is shown in Fig. lc.
Erroneous depth values were removed using the CARIS ‘HIPS’ system (Hydrographic Information Processing System). Data were adjusted for tidal variation using tidal predictions for Stephenville (Canadian Hydrographic Service, 1995). Bathymetric data were gridded in 8-m bins (horizontal ). shaded using artificial illumination, and viewed on a Hewlett Packard workstation using GRASS, a public domain image analysis system. Coloured. three-dimensional shaded relief views were displayed on a Silicon Graphics workstation. Air photographs of Flat Island Spit, taken in 1949, were scanned, rectified using GRASS, and combined with the bathymetric data. The resultant multibeam image is shown in Fig. 3. The numbers in Fig. 3 indicate features discussed in the text; the black lines are seismic profiles also discussed. Backscatter data taken from the EM-1000 raw datagrams were gridded to produce an image ( Fig. 4) complementary to that of the seabed morphology. The Simrad sounder records two separate estimates of backscatter (or reflectivity): a mean \,alue for each beam. and a full time series of estimates that spans the beam footprint. These backscatter estimates are corrected within the sounder for the beam pattern, attenuation in the water column. and Lamberts law (Urick, 1983). .Ahhough the full time series estimates are recorded at a cross-track resolution of I5 cm, the resolution of the data is actually constrained by the alongtrack sampling frequency of 3- 5 m, dependent on water depth and ship’s speed. There is little appreciable difference between the gridded maps 01 mean backscatter and the full time series estimates for regional scale maps with a grid interval in excess of 5 m. The mean backscatter value is plotted in Fig. 4. The average backscatter versus angle of incidence has been applied to remove the mean systematic variation with beam angle. The backscatter intensity was normalised to a mean grazing angle value of 50 degrees. The backscatter intensity of sound returned by the seafloor is a complex function of frequency, angle of incidence, surface and volume roughness. amongst other acoustical parameters (e.g., Jackson ct al.. 1986). so it would be overly simplistic to directly associate a bottom type with a single backscatter estimate at a single incident angle. In
J. Shaw, R.C. Courtney J Marine Geology 143 (1997) 125-135
129
Fig. 3. Digital bathymetric image of inner St. George’s Bay, merged with scanned, rectified air photographs of Flat Island Spit taken in June 1949. The relief data are illuminated from the northeast (upper right). Numbers are referred to in the text. The dashed lines are the axes of bedrock submarine valleys. North is vertically upwards (approx.).
addition, the transducer array on Creed had been damaged in a previous cruise, and the backscatter estimates taken near-nadir were too low. However, some general correlations were observed based on seismic data, sidescan-sonar data, and bottom samples collected in this area (Forbes and Shaw, 1989; Shaw and Forbes, 1990, 1992), and reports of other researchers (deMoustier and HughesClarke, 1995). Gravelly and rough surfaces typically had high backscatter estimates of -20 to - 30 dB, while smooth-surfaced fine-grained sands and muds returned much less energy (- 35 to -40 dB). An interpretation of a sleeve-gun sub-bottom seismic reflection profile collected in 1989 (Josenhans et al., 1989) is shown in Fig. 5. The location of this profile is shown in Fig. 1. Interpretations of sparker seismic-reflection profiles collected in 1988 (Forbes and Shaw, 1989)
are shown in Figs. 6 and 7. Locations of these profiles are shown in Fig. 3. During the multibeam survey we deployed a multitip sparker sub-bottom profiling system, and collected a small amount of data, using features on the multibeam image to plan tracks.
3. Interpretation of the multibeam image The principal element of the bathymetric image is an elongate trough, up to 100 m deep, that trends towards the northeast, with a branch that trends to the southeast (see dashed lines in Fig. 3). On the backscatter image (Fig. 4) the trough has the light tone characteristic of a muddy, nonreflective bottom. To the west of the trough is a flat-topped sill with depths of 20-25 m. This has a predominantly dark tone, indicative of a
Fig. 4. Backscatter
image of the same arca shown in Fig. 3
reflective (gravel ) bottom, with patches of light tone, indicative of less-reflective sand. The eastern margin of the sill is a sinuous, steep slope with a height of 30-45 m. Along the bottom of the image is Flat Island Spit, a 12-km-long barrier composed of gravel beach ridges (Shaw and Forbes, 1992). Offshore from Flat Island is a shallow (<25 m deep) sandy, barrier platform with large transverse bedforms and a shore-parallel bar close to the coast. Submarine fans ( 1) on the flank and at the northeast end of the barrier platform result from sand moving off the platform and into the adjacent basin (Shaw et al., 1997). 3.1. Suhmurinr
vdleys
The central part of the image is a basin with a maximum depth of 100 m. Trending northeast to southwest across the basin is a sinuous, glacially overdeepened submarine valley-the St. George Valley (Shaw and Forbes, 1990)--that extends to
a maximum depth of 195 m below sea level; a tributary bedrock valley ( 155 m) trends southeast towards the coast. The St. George Valley, shown by the long dashed line in Fig. 3, contains thick deposits of acoustically stratified, basin-fill style glacial-marine sediments (Unit 2), overlain by about 20 m of glacial-marine mud (Unit 3). The latter has closely spaced, continuous, coherent internal reflections (cf. Fig. 7). It is present in the submarine valley and is also draped over the underlying terrain in the relatively shallow areas on either side of the valley. The uppermost unit in the basin is postglacial silty mud (Unit 4), characterised by weak acoustic stratification (cf. Fig. 7) that is masked by gas where the sediment is more than 5 m thick. Seafloor depressions (2) are aligned along the axis of the submarine valley in depths of 855100 m. They may have formed by current erosion (cf. Fader and Buckley, 1996), or perhaps they are pockmarks formed by gas venting (Fader, 1991).
131
J. Shaw, R.C. Courtney / Marine Geology 143 (1997) 125-135
Bedrock
NE I 5
0
I
I
10
15
0.3
I 20
DISTANCE (km) Fig. 5. Sleeve-gun
seismic reflection
transect
through
the sill of inner St. George’s
Shaw et al. (1997) show a 3-D image of what they interpret as a plume of gas bubbles venting from one of these depressions. 3.2. The morainal bank The lOO-m-deep basin of inner St. Georges Bay is separated from the Gulf of St. Lawrence by a flat-topped sill with a mean depth of 24 m (Fig. 5). The sill is a morainal bank that formed at a Late Wisconsinan ice margin prior to 13.3 ka, one of a series of end moraines located offshore from embayments on the west coast of Newfoundland (Grant, 1989; Josenhans et al., 1990). The moraine was subaerially exposed during a -25 m sea-level lowstand during the early Holocene (Shaw and Forbes, 1990; Forbes et al., 1993), at which time a peninsula extended southwest from the north coast, and the inner bay was connected to the Gulf of St. Lawrence by a narrow channel. The morainal bank was submerged and partly reworked by transgression. waves during the Holocene Reworked sediment has accumulated on the east flank of the bank as a wedge of prograded sand and gravel, the ‘spillover deposits’ of Shaw and Forbes ( 1990).
Bay. Location
is shown
in Fig.
I,
Sidescan sonar data, sub-bottom profiler data, and grab samples (Shaw and Forbes, 1990, 1992) show that the morainal bank is comprised of two terrain types. At the extreme left of the bathymetric image (3) local relief varies up to a maximum of 5 m. In this zone, ice-contact deposits (on what was formerly the highest part of the moraine) are close to the seabed, and lie just below a lag veneer of bouldery gravel. The largely immobile gravel is interspersed with thicker (l-2 m) lenses of mobile sand and gravel, resulting in the patchy, light and dark appearance of the backscatter image in this area (Fig. 4). Farther east (4) the ice-contact sediments of the morainal bank are overlain by a thick wedge of postglacial sediments, the ‘spillover’ deposits. Here the seafloor is relatively smooth, and consists of extensive sheets of gravel ripples and scattered sand waves. Several grab samples from areas of gravel ripples consisted of poorly sorted polymodal fine gravel and up to 40% sand. The active margin of the spillover deposits, on the flank of the basin, is a sinuous, steep (20 degrees) slope, the top of which ranges from 30 to 45 m above the adjacent basin. A sparker seismic reflection profile (Fig. 6; see also fig. 12 in Shaw and Forbes, 1990) shows
132
Glacial-marine
mud -_
Bedrock
0
‘!
--__
\
2
1
DISTANCE (km) Fig. 6. Sparker seismic reflection dntu xrosb the margin 01‘ thy sp~llo~cr deposits correspond with a lobate area of positive relief on the multibeam image
steeply dipping, prograded internal reflections in the spillover deposits. The finger-like trough (5) that extends several kilometres southwest from the basin is a continuation of the submarine valley. The trough is very narrow because it has been almost completely infilled by barrier-platform sediments on the south side, and by a wedge of spillover that prograded out from the sill on the north side. The trough is completely infilled to the southwest.
To the east of the sill are irregular areas of high relief (6) that appear dark-toned (reflective) on the backscatter image ( Fig. 4). These are seafloor
(lucation shou,n in Fig. 3). The ‘slide deposits.’
exposures of glacial-marine mud (Unit 3) that is the equivalent of the Emerald Silt Formation mapped on the Scotian Shelf and elsewhere by King and Fader ( 19861, Fader et al. ( 1982), and others. The high reflectivity is due to the presence of a thin surficial veneer of fine angular gravel, the result of current winnowing. This unit is also the equivalent of the glacial-marine overlap sediments that have been mapped on nearby coasts of St. George’s Bay (cf. Brookes, 1974), and that consist of shelly marine clays and silts. Published radiocarbon dates on the onshore sediments (Grant, 199 1) range back to an (unadjusted) age of 13.7 ka ( Brookes, 1969). There are no radiocarbon dates from offshore in St. George’s Bay, but in adjacent Port au Port Bay four radiocarbon dates
J. Shaw, R. C. Courtney
1 Marine
Geology
133
143 (1997) 125-135
Glacial-marine mud
0.5
1.0
1.5
Distance (km) Fig. 7. Sparker seismic reflection profile across an area where glacial-marine deposits are exposed on the seafloor (location shown in Fig. 3).
on shell samples in the Emerald Silt equivalent range from 10.8 to 13.3 ka (Forbes et al., 1993). Fig. 7 is a sparker profile across an exposure of the Emerald Silt equivalent. It forms an area of high relief with an average water depth of 42 m, corresponding with a dark (reflective) area on the backscatter image (Fig. 4). The profile shows internal reflections truncated at the seafloor in this area. A submersible investigation of a similar exposure nearby has shown that the high reflectivity of the seafloor is due to veneer of fine, angular gravel (dropstones), winnowed by waves and currents from the glacial-marine unit. However, not all areas of seafloor with high backscatter are exposures of the glacial-marine unit. The seafloor at 7 is smooth on the bathymetry image (Fig. 3), but Fig. 4 shows an irregular area of relatively high backscatter in this vicinity. This is an area of sand and gravel dredge spoils dumped on a smooth, muddy bottom. The spoil heaps
show up clearly on unpublished sidescan sonograms collected during the 1988 survey. 3.4. Seafloor exposures of till below the postglacial sea-level lowstand
Mounds with relief of 15 m (8) are interpreted as till, exposed at the seafloor, below the level of the postglacial sea-level lowstand (-25 m). The till probably retains its primary morphology, and the associated high backscatter is likely due to a surficial veneer of muddy bouldery gravel at the seafloor.
4. Discussion and conclusions High-resolution multibeam systems are relatively new geological mapping tools whose value in understanding the Quaternary history of
glaciated environments is just beginning to be appreciated. From a practical viewpoint. they facilitate efficiently planned geophysical surveys. For example, in 1997 we mapped Bay of Islands, a fiord in western Newfoundland, using a multibeam system, and subsequently used the multibeam data to plan a series of geophysical lines and deployments of a shallow gas sampling system. In the example described, the complex morphology of the seafloor results partly from glacial processes such as overdeepening of St. George Valley, emplacement of the morainal bank, and deposition of a drape of glacial-marine mud in the bay. This glaciated terrain has also been considerably modified by postglacial events, including trimming of the moraine during the sealevel lowstand and in the Holocene transgression. formation of ‘spillover’ deposits, winnowing of glacial-marine muds at the seafloor, deposition of and progradation of barrier mud in basins, platforms. In this respect, inner St. George’s Bay is an unusual example of glaciated terrain; multibeam images from deeper water in other parts of the eastern Canadian shelf show much less modification of glacial features by postglacial marine processes. For example, unpublished images from Placentia Bay (south Newfoundland ) show curvilinear assemblages of crag and tail features, and relict iceberg pits and furrows. Likewise, a submarine moraine on the Scotian Shelf is heavily imprinted with relict iceberg furrows. It has not been substantially modified during the postglacial sea-levellowstand and subsequent Holocene transgression.
Acknowledgements We express our thanks to the masters and crews of the F. G. Creed and to Tony Atkinson and James Currie. Interpretation of the data benefited from discussions with Heiner Josenhans, David Piper, Carl Amos, John Shaw (University of Alberta), and Gordon Fader. This is Geological Survey of Canada contribution number 1996408.
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