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Marine Geology
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Anatomy of the tidal scour system at Minas Passage, Bay of Fundy, Canada
John Shaw a,⁎, Brian J. Todd a, Michael Z. Li a, Yongsheng Wu b
a Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, Dartmouth, NS, Canada B2Y 4A2b Coastal Ocean Science Section, Ocean Science Division, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, Canada B2Y 4A2
Please cite this article as: Shaw, J., et al., Anhttp://dx.doi.org/10.1016/j.margeo.2012.07
a b s t r a c t
a r t i c l e i n f o
Article history:Received 20 February 2012Received in revised form 17 July 2012Accepted 18 July 2012Available online xxxx
Communicated by J.T. Wells
Keywords:scour troughbanner bankBay of Fundytides
Strong currents have eroded thick Quaternary sediments to create a scour trough at Minas Passage, in the Bayof Fundy, Canada, site of Earth's largest tides. We describe this trough in the context of a larger system thatcomprises a range of elements, viz: 1) Scour troughs extending 170 m below mean sea level are incisedinto thick glaciomarine sediments and have exhumed bedrock over wide areas. The flanking uneroded ter-rain has low relief and a winnowed surface. 2) Sets of sand banner banks off Cape D'Or and Cape Chignecto.3) The atypical set of banner banks at Cape Split, consisting of the Scots Bay dune field and its counterpart, alarge gravel bank trapped in the Minas Passage scour trough. 4) Low-relief banks with sand ribbons and bar-chan dunes alongside some banner banks, and termed ‘shadow banks’. 5) A large (0.8 km3) sediment drift atthe entrance to Minas Channel (without large bedforms). The location of troughs and banks can be correlatedwith tidal-current patterns: trenches are located in regions of very strong bi-directional currents; bannerbanks near headland-sited tidal gyres; shadow banks in areas of maximum mean bottom stress asymmetry;and the sediment drift at the entrance to Minas Channel in an area of weak bottom stress at all stages of thetides. Previous work has argued that the scour system formed after 3400 14C yrs BP (radiocarbon years beforepresent) following collapse of a barrier system across Minas Passage. We speculate that formation of thescour trough system may have released vast quantities of sediment that have not been accounted for in pre-vious sediment budgets, and that much of this released sediment has been sequestered in the late-Holocenesalt marshes at the head of the Bay of Fundy.
Minas Passage is a narrow channel connecting Minas Basin withthe rest of the Bay of Fundy (Fig. 1). It was surveyed in the 1960s aspart of studies in support of a proposal to build a barrage across thechannel and generate electricity using tidal power. Results of detailedsurveys published by Huntec Ltd. (1966) show the distribution ofbedrock and the overlying Quaternary sediments. The report authorschristened the deep trough in the passage as the ‘Minas Passage ScourTrench’. Eventually interest in the project waned, particularly sincetidal modeling suggested that construction of the barrage couldalter tidal regimes as far away as New England (Greenberg, 1975).Today Minas Passage is of interest once again, although rather thanconstructing a barrage the idea is to develop fields of in-stream tur-bines placed on the sea floor.
In 2009 the Fundy Ocean Research Centre for Energy (FORCE)established a test site for in-stream tidal energy technology inMinas Passage, and began evaluating the first of several devicesthere. In advance of the selection of a site for pilot deployments, thearea was surveyed with multibeam sonar systems, part of a systemat-ic survey of the entire Bay of Fundy that resulted in the publication of
12 Published by Elsevier B.V. All rig
atomy of the tidal scour syst.007
seventeen Geological Survey of Canada (GSC) 1:50,000 scaleA-series maps in 2011. Sheet 16 covers most of the study area(Todd et al., 2011a) and sheet 15 the remainder (Todd et al.,2011b). These GSC surveys were followed by intensive surveys ofthe selected site, located at a depth of 45 m on a bedrock seafloor (Fig. 2). The surveys revealed for the first time the true com-plexity of Minas Passage and adjacent areas, revealing not only thescour trough but ancillary scour troughs and systems of bedformsof varying types.
A range of studies has been conducted recently in order to modeland ascertain flow conditions and determine the probability that thedeployment of tidal devices might alter environmental conditionssuch as tidal range and sedimentation (e.g., Karsten et al., 2008).However, these efforts have proceeded without a thorough evalua-tion of the nature of the seafloor, and have in some instances reliedon sparse sediment sample data (e.g., Wu et al., 2011). Because theavailable multibeam sonar data have been subject to groundtruthingduring cruises in 2009 (Todd et al., 2010a) and 2011 (Todd et al.,2012), the time is now favourable for an evaluation of the morphology,surficial geology and short‐ and long‐term processes of this area.Accordingly it is the purpose of this paper to:
• Describe the structure and morphology of the system of tidally-generated landforms at Minas Passage.
hts reserved.
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 1. Location of the Bay of Fundy, Canadian Atlantic coast. Labelled black outline box shows the extent of Fig. 2.
2 J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
• To ascertain the relationship between these observations and mod-ern hydrodynamic processes.
• To describe the evolution of the area during the Holocene within thecontext of the expanding tidal amplitude and rising relative sea levelsof the Bay of Fundy.
2. Regional setting
The Bay of Fundy (Fig. 1) is a body of water bounded by a line fromthe southwestern tip of Nova Scotia to Maine. It has theWorld's largestrecorded tides (16.3 m) (Archer and Hubbard, 2003; O'Reilly et al.,2003, 2005). Tidal range decreases towards the mouth of the bay,and on the continental shelf south of the Bay of Fundy, in the Gulf of
Fig. 2. Multibeam sonar topographic imag
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
Maine, is only about 1 m. The high tides are caused by near resonanceof the Bay of Fundy/Gulf of Maine system. However, the extreme tidesnear the head of the bay is a recent phenomenon. The range was rela-tively small in the early Holocene (Shaw et al., 2010) when relativesea level, having fallen from a high of +48 m ca. 13,500 BP, reached alowstand depth of −25 m ca. 7000 BP before rising once more (Amosand Zaitlin, 1984–1985).
Tidal modeling shows how tidal range expanded during the Ho-locene in both the Bay of Fundy and the adjacent Gulf of Maine(Scott and Greenberg, 1983; Gehrels et al., 1995; Shaw et al.,2010). Greenberg (1975) modeled the effects of a tidal barrierplaced across the eastern end of Minas Channel, and demonstratedthat Mean HighWater (MHW) would rise by nearly 0.5 m at various
e of the Minas Passage scour trough.
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
3J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
locations in New England. This is because the Bay of Fundy and theattached Gulf of Maine are one tidal system with a natural periodof about 13 hours, a period close enough to resonate with thesemi-diurnal tides (Garrett, 1972, 1974); any change in the lengthof the system (or an effective change such as a partial blockage)would bring the period closer to resonance with the M2 tide, andthus alter the tides in the entire system (Garrett, 1974; Greenberg,1979).
3. Study area
The northeast extremity of the Bay of Fundy divides into twoarms: Chignecto Bay, and Minas Basin (Fig. 1). The outermost en-trance to Minas Basin is termed Minas Channel. Minas Channel nar-rows eastwards to a 5-km wide channel known as Minas Passage(Fig. 2), which is bounded on its south side by the bold bedrock head-land of Cape Split, the recurved termination of North Mountain. Thebody of water immediately south of Cape Split is Scots Bay. MinasPassage and Minas Channel together constitute the study area.
To the east of Minas Passage lies Minas Basin (Fig. 1), a shallowbody of water in which depths of 20 m below mean sea level are typ-ical. Yet farther east is the most landward part of the Bay of Fundy, thevery shallow and extensively intertidal Cobequid Bay, which containselongate sand bars and sand flats (Dalrymple and Zaitlin, 1994) char-acteristic of a tide-dominated estuary.
Minas Passage deepens to ~170 m below mean sea level and is no-table for tidal currents that approach 5 m s−1 at times. It was investi-gated thoroughly in the 1960s as a potential site for the harnessing oftidal power. In the report by Huntec Ltd. (1966), detailed maps andcross-sections show that the area is underlain by Carboniferous and Tri-assic sediments and Triassic basalts. The isopachmap of unlithified sed-iments shows a large area of exposed bedrock in Minas Passage. Theunlithified sediments attained thicknesses of >60 m west of the pas-sage, and >90 m to the east, south of Parrsboro (see geographic loca-tions in Fig. 5).
Grounded ice was absent from the Gulf of Maine and Bay of Fundyby approximately 14 ka (King and Fader, 1986; Schnitker et al., 2001;Shaw et al., 2006). Swift and Borns (1967) described and mapped thecopious glacial outwash deposits that occur along the north shore ofMinas Basin, and coined the term “Five Islands Formation”. The frontof the retreating ice sheet stood just inland of the modern coast heresome 14,000 years ago. Relative sea-level was dropping rapidly as theice retreated, so that the glacial deposits were incised by streams, andformed lowstand deltas. The formation is most extensive at Parrsboro.Sub-bottom profiling established that the formation extends wellbelow sea level, and that a graded outwash plain extended acrossMinas Basin (ibid.). We have no piston cores and no radiocarbon dateson the glaciomarine sediments at Minas Passage, but a sample of Yoldiasp. from a depth of 61 cm in piston Core 46 collected in 1997 (GSCcruise 97020) at a location just west of Isle Haute has a conventional(δ13C=25‰) age of 13,750±6014C yrs BP (Beta-257497).
A major sand body adjacent to the Minas Passage Scour Trough wasdiscovered during the Huntec Ltd. (1966) feasibility study. Swift et al.(1966) interpreted this “Scots Bay dune field” as having formed as a re-sult of interaction between the tidal stream, the scour trench, and CapeSplit. It was mapped by Miller and Fader (1990) who described largeand very large dunes (according to Ashley, 1990; see also Dalrympleand Rhodes, 1995) composed of well-sorted coarse sand, up to 15 mhigh and resting on a lag gravel surface. They suggested that sandspilled over into the deep water of the scour trough, computed the vol-ume of the dunefield (35×106 m3), and suggested that itmay havemi-grated several hundred metres to the northwest of its 1966 position(although navigational errors were considered possible). They alsomapped an area of sand ribbons to the east of the dune field.
Patterns of tidal currents and sediment transport have beenmodeled by Li et al. (2009) and Wu et al. (2011) who also summarize
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
previous work. Todd et al. (2010b) provided an overview of mappedbedforms in Bay of Fundy and the relationship between bedformfields and model-predicted sediment transport patterns. Very strongtidal currents (up to 5 m s−1) enter and exit Minas Passage. Theresidual tidal flow shows a major anticlockwise gyre off Cape Split,and lesser clockwise gyres in western Minas Basin and west of CapeD'Or, respectively (Fig. 5). Comparison of modelled net transport fluxand geomorphology indicates that banner banks (Dyer and Huntley,1999) form under tidal eddies, and that the morphological asymmetryof the dunes agrees with the direction of net sediment movement pre-dicted by sediment transportmodels (Li et al., 2009; Todd et al., 2010b).
4. Methods
4.1. Multibeam sonar surveys
The Minas Passage area (Fig. 1, box) was mapped in 2007 by theCCGS Matthew, equipped with a Simrad EM-710 multibeam sonar,and hydrographic launches equipped with the Simrad EM-3002 sys-tem. The surveys were undertaken by the Canadian HydrographicService and Natural Resources Canada. The data were processed inthe CARIS HIPS system to remove erroneous soundings, and griddedat 5-m spatial resolution. Tidal variations were removed using datafrom a tide station established at Parrsboro. Bathymetric data werereduced to a common datum of mean sea level and exported to theGlobal Mapper GIS system for viewing and analysis. Backscatterstrength values were extracted from the multibeam data usingUNIX-based software developed by the Geological Survey of Canada.Backscatter strength is the percentage of the outgoing signal returnedto the system from the seafloor, and is commonly assumed to be aproxy for seafloor roughness. In general, smooth sandy and muddyseafloors have low backscatter strength, whilst rugged gravel androck bottom yield high values. The angular variation in backscatterwas corrected to a 45° incidence angle. The backscatter strengthvalues in decibels were reduced to a 0–255 scale and gridded for dis-play. The distribution of backscatter strength in the study area isshown on Fig. 3.
4.2. Groundtruthing surveys
Groundtruthing of the newmultibeam sonar data was undertakenon two occasions. In 2009 the CCGS Hudson was the platform for sur-veys (Todd et al., 2010a). Geophysical data were collected on lineswest of the passage, using a 6 cubic inch sleeve gun seismic reflectionsystem, and a Huntec Deep-Tow-Seismic (DTS) system with internaland external hydrophones. A Klein 3000 sidescan sonar was alsotowed. Bottom photographs and video were collected using theCampod imaging system (Gordon et al., 2007). Grab samples wereobtained with a large-volume IKU grab (capable of collecting 1 m3
of relatively undisturbed sediment) and the smaller (0.02 m3) vanVeen grab. The vessel was again used in 2011 when grab samplesand bottom photographs were collected (Todd et al., 2012). Ships’tracks and sample locations for the 2009 survey are indicated onFig. 4.
4.3. Other data sources
Considerable amounts of previously collected data were availablefor the study area, although unfortunately the excellent Huntechydrosonde seismic reflection profiles collected in 1966 could notbe located, and we had to make do with the small number of profilescontained in the report (Huntec Ltd., 1966). Sidescan sonar andsub-bottom profile data collected by Miller and Fader (1990) atScots Bay (Fig. 2), just south of Minas Passage, were accessed. Thesedata include Bubble Pulser low-resolution seismic reflection dataand Seistec high-resolution seismic reflection data. Analyses of grab
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 3. Backscatter strength in the Minas Passage area. The backscatter strength has been made semi-transparent and is draped over the gray elevation model. Notable featuresare: (A) Sand in a major sediment sink west of Cape Chignecto; (B) The banner banks off Cape D'Or; (C) The Scots Bay dune field. While much of the scour trench has high back-scatter strength due to the presence of coarse gravel at the seafloor, the bedrock area (D) has a mottled appearance, with low backscatter strength. Labelled white box shows theextent of Fig. 11.
4 J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
samples from the Scots Bay dune field were reported by Miller andFader (1990).
At the test site just west of Cape Sharp (location on Fig. 5)high-resolution multibeam sonar and sidescan sonar surveys wereconducted, sidescanmosaicsmade, and numerous grab samples and bot-tom photographs collected in addition to oceanographic data. Summa-ries of these data are available in a series of environmental assessmentdocuments published by FORCE (http://fundyforce.ca/assessment).These data have been particularly useful because they demonstratethat even areas considered to be bedrock may have a high percentagecover of bouldery gravel. In the following description of bedforms thescheme of Ashley (1990) is used, along with further clarifications ofDalrymple and Rhodes (1995) and Reynaud and Dalrymple (2011).
4.4. Tidal current modeling
Various features of the scour trough system were compared withthe model-predicted tidal current patterns to explore the relationshipbetween observed seabed features and the hydrodynamic processes.The near-bed tidal current was computed from the 3-dimensionalFinite-Volume Coastal Ocean Model (FVCOM; Chen, et al., 2003).The FVCOM model used in this study was specifically developed forthe upper Bay of Fundy (Wu et al., 2011) and includes the five largest
Fig. 4. Ships’ tracks and sample locations in the study area. Illustrated seismic reflection prolabelled. (For interpretation of the references to color in this figure legend, the reader is re
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
tidal constituents (M2, N2, S2, K1, O1). The resolution of the unstruc-tured model mesh varies between about 100 m in Minas Passage toapproximately 7500 m in the outer part of the domain. There aretwenty-one levels in the vertical dimension, with enhanced resolu-tion near the surface and bottom. The tidal model was evaluatedagainst independent observational data of tidal elevation and tidalcurrent. The evaluation shows that the model predictions are ingood agreement with the observations (Wu et al., 2011). The magni-tude of the mean difference between model results and observationsof theM2 semi-major axis velocities is 0.06 m s−1, less than 3% of themean. The mean discrepancy of phase is 0.20 degrees.
5. Results
5.1. The scour troughs
The scour troughs are steep-sided depressions excavated belowthe general level of the surrounding sea floor. They occupy an areaof 240 km2 and have a combined volume of 5 km3 (calculated withthe Global Mapper GIS system). They comprise a primary troughand several lesser ones (Figs. 2, 5). The primary trough is incisedinto an area of flat to gently-sloping seabed 20 to 30 m deep, and at-tains a maximum depth of ~170 m below sea level just north of Cape
files are highlighted in blue and labeled. Locations of photographs shown on Fig. 9 areferred to the web version of this article.)
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 5. Schematic map of the Minas Passage area, showing principal elements of the scour trough system. The turbine evaluation site is indicated by the red star. Labelled black boxesshow the extent of Figs. 6, 7 and 8. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
Split. The trough shallows and bifurcates in the east and west. Thetrough edges are predominantly sharp and well-defined (Figs. 6, 7),so that the transition from virtually horizontal slopes to slopes of10° occurs over distances of ~5 m. In the east however, where thetrough bifurcates at the entrance to Minas Basin, slope edges areless clearly defined. In addition to the main scour trough, several less-er scour troughs are evident. A trough extending northeast from CapeD'Or (Fig. 6, C) is about 25 m deeper than the surrounding seafloor. Asecond trough farther south (Figs. 6, E; 8a) has been excavated 25 mbelow the surrounding seafloor, and contains dunes oriented normalto its principal axis. This trough is separated from the primary troughby a ridge of ‘resistant’ glacial sediment, perhaps a moraine. Withinthe troughs, high backscatter strength predominates, and grab sam-ples show that gravel and bouldery gravel predominate at the sea-floor, with one exception (see below).
Several terrain types within the troughs are distinguished by re-lief, gross roughness, backscatter strength, and texture. Eroded Qua-ternary sediments with the acoustic character of glaciomarine mudhave a rough terrain, with relief of the order of several metres acrossdistances of ~30 m. At the western end of the scour trough (Fig. 8b)the seafloor has a scalloped appearance, with 2.5 m‐high ridgesspaced 30 m apart on average, transverse to principal tidal flow direc-tions. Preliminary examination of a video mosaic from the 2011 sur-vey (Todd et al., 2012) reveals that the inter-ridge areas consist ofsemi-indurated mud containing pebbles, while the ridges are boulder
Fig. 6. Western part of the scour trough system showing: (A) the sharply-defined westerntrough marked ‘E’ is adjacent to a morainal ridge, and contains a series of transversely-aligbanks. See Fig. 5 for location.
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
gravel with attached algae and marine organisms, suggesting sedi-ment immobility. At present we do not understand the genesis ofthese features. In general, eroded Quaternary sediments have highbackscatter strength due to the presence of boulder-cobble gravel atthe seafloor. Bottom photographs at sample site 2009039‐32 (Fig. 9a)show poorly sorted sub-rounded gravel, primarily cobble-sized, but in-cluding boulders; colonies of horse mussels also occur at this location.
Smooth Quaternary sediment has high backscatter strength, andmay comprise glaciomarine sediment or till armored with coarsegravel. Morainal ridges up to 10 m high are also composed of till,and probably armoured with bouldery gravel. Sediment drifts occurdue east and west of Black Rock (Fig. 7G). The long ridge to the eastis marked by gravel dunes, and may be depositional or an erosionalremnant. Gravel dunes are present in trains, primarily in the easternpart of Minas Passage. They attain heights up to several metres, andwavelengths up 25 m. Solitary sand dunes up to 8 m high and 500 mlong occur in the small triangular-shaped scour trough (Fig. 8a).They have low backscatter strength, and appear to be trapped in thetrough.
Exhumed bedrock crops out on the seafloor in several areas (Fig. 5),and constitutes about 18% of the trough area. The predominant bed-rock terrain in the largest exposure of bedrock (Fig. 7) has relief of2–5 m, strong linear features (bedding planes), and correspondswith areas mapped as Carboniferous and Triassic sedimentary rockby Huntec Ltd. (1966). A second type of bedrock terrain occurs just
edge of the principal trough; (B), a pair of banner banks; (C) a small scour trough. Thened symmetrical dunes, seen in the inset map. (F) and (G) are shadow or subsidiary
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 7. Terrain in theMinas Passage principal scour trough. (A) Area of basalt outcrop offshore Cape Split; (B) UnerodedQuaternary sediment at south side of trough,with a sharp erosionaledge; (C) Exposed bedrock, with east–west trending fault shown by dashed white line; (D) Uneroded Quaternary sediment covering bedrock; (E) Exposed basalt platform; (F) Graveldune field; (G) Sediment drift in lee of Black Rock. The white box indicates the location of the turbine lease block. See Fig. 5 for location.
6 J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
west of Cape Sharp as a series of platforms up to 10 m above the sur-rounding seafloor, with joint patterns, and relative flat surfaces. Theseare outcrops of the Devonian North Mountain Basalt (Huntec Ltd.,1966). In the turbine test areas (Fig. 5) detailed mapping shows
Fig. 8. Seafloor images of the small scour trough with transverse dunes (a) and thewestern end of the main scour trough (bottom). See Fig. 5 for locations.
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
that the basalt and other bedrock types have patchy veneers of boul-ders and cobbles, with gravel in depressions (joints, bedding planes).Basalt is also exposed on the seafloor at Cape Split.
5.2. Banner banks
Banner banks are pairs of banks located either side of headlands(Dyer and Huntley, 1999) and organized into series of sand dunes.They are common along the north coasts of the Bay of Fundy (Duffyet al., 2004). A set of banner banks off Cape Chignecto (Fig. 2) is sep-arated by a bedrock shoal that extends offshore from the cape. Thesebanks are covered by dunes that are 2–4 m high. A second and moresubstantial set of banner banks off Cape D'Or has dunes up to 12 mhigh (Fig. 6, B). The western bank has bi-lateral asymmetry; thedunes on the north side indicate current flow to the southeast,while those on the south side indicate a northwest flow.
5.3. The Scots Bay dune field
A set of banks is present either side of Cape Split, but they differmarkedly from the more typical banner banks noted above as theycomprise a sandy bank in Scots Bay and an eastern gravel bank. TheScots Bay dune field (Miller and Fader, 1990) is a large sand body(Fig. 10) with superimposed dunes. It is located on the edge of thescour trough upon the flanking platform which is at a depth of 40 min this area. However, the dunes descend to depths of 70 m on thesloping erosional surface of the scour trough. Repetitive seismic sur-veys revealing the asymmetric cross-sectional profiles of the dunesshow that sand migrates to the northeast along the east side of thebank and to the southwest along the west side, resulting in abi-lateral asymmetry of the dune field. Grab samples, IKU samples,and bottom photographs show that the bank comprises well-sortedmedium to coarse sand with a component of comminuted shell de-bris. Small and medium dunes are superimposed on the large andvery large dunes, and analysis of sub-bottom profile data demon-strates that the Scots Bay dune field is located above an unconformitythat separates it from the underlying Quaternary sediments (Millerand Fader, 1990).
5.4. Gravel bank
A counterpart to the Scots Bay dune field exists to the east of CapeSplit. This gravel bank (Fig. 7) is located in Minas Passage in depths of85–120 m. The accumulation is ~20 m thick, 800 m across and 3 kmlong. The north face is marked by dunes. The dunes on the western
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 9. Bottom photographs from expedition Hudson 2009039: (a) gravel in the area of eroding Quaternary sediments within the principal scour trough, station 32, photo 101950,depth 59 m; (b) Boulder cobble gravel in the bedrock area, station 40, photo 200442, depth 60 m: (c) Rounded boulders and cobbles in the gravel dune field, station 44, photo153404, depth 97 m; (d) sandy gravel on the shadow bank southeast of the Scots Bay dune field, station 34, photo 133928, depth 37 m. Photograph locations are indicated onFig. 4. Width of photographs=1.2 m.
7J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
half are up to 5 m high with wavelengths up to 80 m. Those on theeastern half are smaller (1.5 m) with shorter wavelengths (30 m).Bottom photographs (Fig. 9c) show smooth, well-rounded cobblesand pebbles with no biologic attachments.
Fig. 10. Vicinity of Scots Bay dune field, showing the dune field (A) and the shadowbank (B). The latter is covered by curving sand ribbons (C) and a field of small barchandunes whose geomorphology indicates northward movement (indicated by arrows).The dunes extend into the scour trough (E). Location shown on Fig. 5.
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
5.5. 'Shadow' banks
Southeast of the Scots Bay dune field (Figs. 5, 10) and northwest ofthe banner bank off Cape Advocate (Figs. 5, 6F) are very low-reliefbanks. They have distinctive curving planform, leading to a 'comma'shape. Low-relief ridges of sand on these banks correspond with the‘sand ribbons’ mapped by Miller and Fader (1990). Because of theirapparent association with the banner banks we denote them as 'shad-ow' banks. The Scots Bay shadow bank, however, unlike that off CapeAdvocate, also supports barchan dunes (Todd, 2005) commonly200 m across and up to 7 m high. Their orientation suggests a north-ward migration. Bottom photography on the Scots Bay shadow bank(Fig. 9d) shows a mainly sandy sea floor with current ripples, patchesof angular pebbles, some cobbles, occasional boulders; accumulationof sediment in the lee of cobbles and boulders (obstacle marks) indi-cates the predominant northward flow direction of the strong cur-rent. Sub-bottom profile data at both banks (see below) shows thatthey are up to 5 m thick, and are separated from the underlying Qua-ternary sediments by unconformities.
5.6. Other sediment sinks
The western banner bank at Cape Chignecto abuts one of the larg-est sediment sinks in the area, a large accumulation of sand at the en-trance to Minas Channel (Figs. 2, 5). This deposit has an area of~50 km2, attains a maximum thickness of ~30 m, and is devoid ofdunes. This contrasts with adjacent areas where sand either consti-tutes a veneer over glacial landforms, or occurs in dune fields.
5.7. Character and origin of the eroded sediments
The scour troughs are incised into a flat-to‐gently sloping seabedwith predominantly high backscatter strength, interpreted as the
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 11. Area of striped backscatter strength southwest of the Scots Bay dune field.A = Scots Bay dune field; B = scour trough; C= shadow bank. Dotted white line marksthe southern edge of the scour trough. Note different character of backscatter strength tothe north and south of this edge. Location shown on Fig. 3.
8 J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
surface of Quaternary sediments winnowed and eroded by increasingtidal energy in the late Holocene (Shaw et al., 2010). IKU sample2009039‐043 shows that a surface lag of sub-angular pebble-cobblegravel is underlain by reddish sandy silty clay (Todd et al., 2010a,). Anarea of striped backscatter strength (Fig. 11) southwest of the ScotsBay dune field corresponds with subtle low-relief (b0.5 m) ridges ontheflat seafloor, and extends into the scour trough. This patternmay in-dicate more resistant layers in the winnowed and eroded Quaternarysediments; the change in orientation on Fig. 11 reflects draping of theQuaternary sediments over an underlying bedrock ridge seen onFig. 13. The lag surface extends under both the Scots Bay dune fieldand the shadow banks (Figs. 13, 14).
Sub-bottom profiling reveals that acoustically stratified Quaternarysediments underlie the surface lag (Figs. 12, 13, 14). Fig. 12 shows Qua-ternary sediments up to 40 m thick overlying an irregular-surfaced un-conformity on folded sedimentary rock. Some stratification is evident inthe Quaternary sediments, and the lower section appears to be drapedover the underlying terrain. Based on their acoustic character we inter-pret the bulk of the Quaternary sediments as being of glaciomarine or-igin, and likely the muddy, offshore equivalent of the Five IslandFormation at the adjacent coast (Swift and Borns, 1967; Wightman,1970), and were deposited by suspension fallout from turbid plumes.
Fig. 12. Airgun seismic reflection line across Minas Passage, survey Hudson 2009039 (Todd eshow acoustic stratification in the Quaternary sediments, some of which is shown here. Th
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
5.8. Scour trough morphodynamics
Model predictions (Fig. 15) show that during both flood and ebb,very strong currents occur in Minas Passage, and coincide with the lo-cations of the scour troughs. On the incoming tide, eastward-directedflows predominate everywhere. Very soon after the onset of the floodtide, a large clockwise gyre develops east of Cape Split over the gravelbank in the scour trough and this flow pattern persists throughout theflood period. This gyre can be seen on Fig. 15A, which shows flows atpeak flood (i.e., ~3 h before high tide). A small anticlockwise gyre alsodevelops east of Cape D'Or during the flood cycle.
With the onset of ebb, the gyre east of Cape Split dissipates, andstrong tidal flows to the west predominate. Soon after the onset ofebb, a major anticlockwise tidal gyre develops over the Scots Baydune field. This large gyre persists through ebb, but the center ofthe gyre tends to drift northwest as ebb progresses. This gyre isseen on the peak ebb (Fig. 15B). At this time, a weak clockwise gyreis observed in Advocate Bay (Fig. 2). This gyre persists late into theebb cycle, and is well seen on Fig. 15C, two hours after peak ebb.These are the major gyres, but lesser gyres develop elsewhere overthe tidal cycle.
The locations of bedform fields thus correlate with gyre locations.Scots Bay dune field is at the site of a large gyre; the dunes at the westside of the Scots Bay dune field experience a flow reversal, whilstthose on the east are subject to a northeast-directed flow of varyingintensity. The gyre east of Cape Split overlies the gravel bank in thedeepest part of Minas Passage. The gravel bank experiences verystrong flows on the incoming tide, the strongest flows being on itsnorth side where gravel dunes are located. However, the flood-tidegyre produces weak westward flows on the south side of the gravelbank, where dunes are absent. Tidal gyres flanking Cape D'Or coincidewith the location of the set of banner banks in that vicinity.
In both instances, the shadow banks, unlike the adjacent dunefields, have a unidirectional flow regime. On the shadow bank eastof the Scots Bay dune field, for example, the flow is always directedto the northeast parallel to the sand ribbons, irrespective of tide.The barchan dunes on this bank are associated with unidirectionalcurrents and sediment scarcity. Wu et al. (2011, their figure 15)shows that the location of both shadow banks corresponds to thehighest values of the asymmetry indicator for mean bottom stress.
The large sandy sediment sink west of Cape Chignecto at the en-trance to the Minas Channel corresponds to a region of weak tidal cur-rents (Fig. 15) and low mean bottom shear stress (Fig. 12, Wu et al.,2011) during all stages of the tide. The deposit is devoid of largedunes, likely due to weak tidal currents.
t al., 2010a); location shown on Fig. 4. Huntec DTS seismic reflection data from this linee draping style is indicative of glaciomarine sedimentation.
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 13. Bubble-pulser seismic reflection profile just south of the Scots Bay dune field, survey Navicula 89009; location shown on Fig. 4. Interpretation also based on high-resolutionseismic reflection data from the same line. Thick deposits of acoustically-stratified glaciomarine sediments overlie bedrock and are separated from the overlying sand layer by anunconformity. ‘A’ indicates a bedrock ridge.
9J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
6. Discussion
6.1. Evolution of the scour trough system
The evolution of the scour trough system is linked with the evolu-tion of the wider Bay of Fundy during the Holocene, a time of tidal
Fig. 14. Bubble-pulser seismic reflection profile across the Scots Bay dune field, survey Navicureflection data from the same line. Thick deposits of acoustically-stratified glaciomarine sediThe dunes descend into the scour trough.
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
amplification and relative sea-level rise. Using an aggregation of 146sea-level index points, Shaw et al. (2010) argued for the existence of abarrier across Minas Passage that delayed tidal expansion in MinasBasin. With the rapid breakdown of this barrier c. 3400 14C yrs BP,tidal amplitude rapidly expanded in Minas Basin, water temperaturesdropped, tidal currents and turbidity increased, and the form of the
la 89009; location shown on Fig. 4. Interpretation also based on high-resolution seismicments overlie bedrock and are separated from the upper sand layer by an unconformity.
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 15. Model-predicted bottom currents in the Minas Passage region at several stagesin the tidal cycle. The black dots indicate the locations of the modeled currents, and thelines indicate directions of flow. White lines indicate trough boundaries and yellowlines show locations of principal sand banks. A: Peak flood (approximately 3 hours be-fore high water) when flow is uni-directional almost everywhere except for a clock-wise gyre over the gravel bank east of Cape Split; B: Peak ebb, when a majoranticlockwise gyre is developed over the Scots Bay dune field and a weak clockwisegyre exists in Advocate Bay; C: Late in the ebb cycle (two hours after peak ebb)when both the Scots Bay and Advocate Bay gyres are very well developed, but in op-posing directions. Locations on the upper map are Cape Chignecto (1), Advocate Bay(2), Cape D'Or (3), Cape Spencer (4), Greville Bay (5), Cape Sharp (6) and Cape Split(7). (For interpretation of the references to color in this figure legend, the reader is re-ferred to the web version of this article.)
10 J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
inner estuary was changed from lagoonal/mesotidal to macrotidal. Ifthis is scenario is correct, then the various elements of the scour troughsystem have evolved since c. 3400 14C yrs BP.
6.2. Morphodynamics of the scour trough system
There is a good correlation between modern tidal currents and thevarious elements of the scour trough system (Fig. 16). Thus the tidalgyres around headlands appear to be responsible for the formation
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
of banner banks flanking these headlands. The operation of this pro-cess in the Bay of Fundy was evident in acoustic Doppler current pro-filer data from the central bay analyzed by Duffy et al. (2004), whoshowed variations in tidal currents associated with a shifting tidaleddy. Strongest currents on the outgoing tide occurred on the sea-ward side of the bank. At low tide very strong currents existed onthe north side. On the flood strong currents were present everywhere.
Very large dunes on the banner bank off Cape D'Or are best devel-oped on the landward side, where currents are almost unidirectional,whereas smaller dunes exist on the seaward side where currents arebidirectional. The correlation between the clockwise tidal gyre north-east of Cape Split and the location of the gravel bank also suggeststhat this gravel bank trapped in Minas Passage should be classifiedas the Cape Split eastern banner bank in pairing with the bannerbank west of the headland.
6.3. Comparisons with elsewhere
Tidally-scoured troughs analogous to those at Minas Passage havebeen referred to as ‘tidal scour cauldrons’, with some of the bestknown examples being those in restricted Japanese Straits (Moji,1979). More recent work in one of these straits—the Bungo Channel—by Ikehara (1998) reveals some similarities with Minas Passage. Theseabed has been eroded at the narrowest part of the strait, and aroundpromontories and islands; sandbanks with large dunes are locateddownstream of the ‘scour holes’. While Ikehara's figure 4 is an instruc-tional schematic illustration of the Bungo Channel system in relationto tidal currents, it is arguable that the survey methodologies used donot permit a useful comparison with the Minas Passage system, wherecomplete multibeam coverage reveals a very complex seafloor.
It is interesting, however, to note the contrasting sea-level historiessetting of these two examples. The Bungo Channel was closed duringthe last glacial maximum, and it was during the postglacial relativesea-level transgression that tidal currents created scour holes andsand bodies. However, during the highstand phase (after c. 5500 BP),tidal energy decreased. Similarly, tidal scour cauldrons in the Irish Seadescribed by Wingfield (1995) are moribund, having formed over a re-stricted period of early postglacial low relative sea level. By contrast, theMinas Passage scour trough system has evolved recently, against abackground of rising relative sea level, and steadily increasing tidal am-plitude in the late Holocene after destruction of a barrier across MinasPassage c. 3400 14C yrs BP; the system has been moving closer to reso-nance, and scouring remains active today. These controls on the timingof scour in seaways are explored in the mid-Tertiary setting by Anastaset al. (2006).
6.4. Scour-trough evolution and the Bay of Fundy
Various writers have demonstrated that tidal amplitude in the Bayof Fundy has been expanding throughout the Holocene, in concertwith rising sea levels on the continental shelves. The idea that arapid environmental change occurred in Minas Basin c. 3400 14C yrsBP, whereby the marine fauna changed character, was further devel-oped by Shaw et al. (2010) who argued that Minas Passage had beenblocked by a sandy barrier system until this time. With the break-down of the barrier, tidal ranges increased in the basin, and thescour trough developed. It is computed that the volume of sedimentremoved from the scour trough and released into the Bay of Fundysince c. 3400 14C yrs BP is ~5 km3.
While attempts have been made to calculate sediment budgets inthe upper Bay of Fundy (Amos and Joice, 1977), these have not in-cluded components from trough erosion. It is possible that the sedi-ment eroded from troughs was sequestered in the extensive saltmarshes and estuarine muds that fill valleys at the head of the Bayof Fundy (e.g., Noordijk and Pronk, 1981). Published radiocarbondates from the salt marsh sediments rarely exceed 4000 14C yrs BP
em at Minas Passage, Bay of Fundy, Canada, Marine Geology (2012),
Fig. 16. Cartoon illustration generalized relationships between geomorphic elements of the scour trough system and tidal currents during flood and ebb. A = shadow bank; B= bannerbank; C = large sediment drift at Cape Chignecto; S = Scots Bay dune field; G = gravel bank.
11J. Shaw et al. / Marine Geology xxx (2012) xxx–xxx
and are mostly younger than 3500 14C yrs BP (Shaw et al., 2010). Thesalt-marsh sediments commonly overlie sunken forests with rootedtree stumps that are subaerially exposed at the modern low tide level(Dawson, 1856; Goldthwait, 1924; Johnson, 1925; Lyon and Harrison,1960; Harrison and Lyon, 1963; Grant, 1970, 1975; Bleakney andDavis, 1983).
The area of salt marshes in the upper bay is estimated at 590 km2,and assuming a 9 m rise in high water since 3400 BP (Shaw andCeman, 1999; Shaw et al., 2010) then the volume of sediment seques-tered in salt marshes is 5.3 km3. This is comparable to the amounteroded from the Minas Passage scour troughs (although we acknowl-edge that the sand component was likely sequestered in dune fieldsin the study area (which have a combined volume of ~0.9 km3),and in sandy deposits at the head of Cobequid Bay (Dalrymple et al.,1992; Dalrymple and Zaitlin, 1994). We speculate that, as tidal ampli-tude continued to expand after the breakdown of a barrier acrossMinas Passage, scour rough erosion triggered the rapid growth ofsalt marshes in the last 3400 years (Shaw et al., 2010) by supplyingcopious amounts of sediment.
7. Conclusions
• The Minas Passage scour trough sensu stricto is part of a wider sys-tem of troughs and banks that have formed in an area of very strongtidal flows in the Minas Passage area, Nova Scotia.
• The troughs are eroded into a thick succession of lateglacial andearly postglacial sediments, and have exhumed bedrock across awide area. Elsewhere in the troughs, gravel predominates at theseafloor, organized into trains of dunes in some places; large soli-tary dunes are trapped in the small trough south of Cape D'Or.
Please cite this article as: Shaw, J., et al., Anatomy of the tidal scour systhttp://dx.doi.org/10.1016/j.margeo.2012.07.007
• The location of troughs and banks is well explained by model tidalflow parameters. Troughs occur under tidal-flow maxima. Bannerbanks have bi-lateral asymmetry indicative of their location undertidal gyres. ‘Shadow’ banks, so termed because they exist adjacentto the larger banner banks off Cape D'Or and the Scots Bay dunefield, correspond with areas of unidirectional flows and strong bot-tom stress asymmetry.
• We speculate that the growth of scour troughs can be linked withgrowth of salt marshes in the upper bay since 3500 BP.
Acknowledgements
We thank Don Forbes and Calvin Campbell (Geological Survey ofCanada) for reviewing the manuscript. Charles Hannah (Fisheries andOceans Canada) provided useful comments and inspiration. We are es-pecially grateful to Bob Dalrymple (Queen's University, Kingston) andan anonymous external reviewer for comments that permitted us togreatly improve the manuscript. This is Earth Sciences Sector contribu-tion number 20110380.
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