The evolution of Buctouche Spit, New Brunswick, Canada

Marine Geology 124(1995) 215-236

The evolution of Buctouche Spit, New Brunswick, Canada Jeff Ollerheadl, Robin G.D. Davidson-Arnott Department of Geography, University of Guelph, Guelph, Ont. Nl G 2 WI, Canada

Received 21 February 1994; revision accepted 8 June 1994

Abstract Buctouche Spit is an 1l-km long sandy spit on the northeast shore of New Brunswick, Canada. It is a typical flying spit, having a narrow proximal section which is characterized by a single foredune that is prone to overwash and transgression, and a much wider prograding distal section which is characterized by a well-developed foredune backed by a series of relict foredunes. An optical-luminescence dating method in which infrared stimulation is used on potassium feldspars was developed for this study. It is a very promising method for dating Holocene deposits, yielding good resolution and having simplicity of measurement. The luminescence ages obtained indicate that Buctouche Spit extended at a rate of -4 m yr-’ between 715 f45 years ago and 240 +25 years ago. This extension rate equates to a distal end accumulation rate of -56,000 m3 yr-’ for this period. At this rate of accretion, the present Buctouche Spit would have formed during the past 2000 years. The core data indicate that the spit is 22200 years old, which is consistent with the preceding estimate. Since 240 k25 years ago, the rate of sediment accumulation at Buctouche Spit has apparently been falling. The accumulation rate between 1839 and 1945 was -23,000 m3 yr-‘, less than half of the historic rate of - 56,000 m3 yr-' . Since 1945, the distal end of Buctouche Spit has not prograded/extended measurably. This is because Buctouche Spit’s present vertical growth rate is, at most, equal to the rate of relative sea-level rise at the spit and may be as little as half of the rate of relative sea-level rise. Present sediment supply to Buctouche Spit is probably < 16,000 m3 yr- ‘. It is likely that the much larger sediment supply required to sustain an historic accretion rate of - 56,000 m3 yr- ’ was derived from the reworking of a former sandy barrier located seaward, and possibly updrift of, the present spit. Buctouche Spit is now in a limited sediment supply situation and it has become a constrained spit. Sediment is being eroded from the centre section of the spit, transported to the distal end, and then carried away by ebb-tidal currents. The centre section of the spit will likely breach within the next hundred years. If a breach (or breaches) becomes permanently established in the centre section of the spit, the distal end will suffer significant erosion and may be destroyed. After this, the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise. It is concluded that the key to understanding Buctouche Spit’s long-term evolution is recognizing that it is migrating landward both by continuous processes like overwash, and by the more discrete and catastrophic process of overstepping.

IPresent address: Department of Geography, Mount Allison University, Sackville, NB EOA 3C0, Canada. 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0025-3227(95)00042-9

1. Introduction A spit is one type of depositional barrier that forms at the downdrift end of a littoral cell. Spits are typically attached to the mainland at the proximal end and have a “free” distal end. As with other barrier systems, spits have a subaqueous platform (Meistrell, 1966) and a subaerial environment consisting of beach, dune, and marsh deposits. The form and location of spits change rapidly in response to changes in a number of controlling processes such as water level, sediment supply, and wave climate. Barrier spits are an important natural resource on many coastlines because they offer protection for harbours, they provide habitat for fish. waterfowl, and a variety of other biota, and they have significance for a range of recreational activities. Management of barrier spits and related environments requires an understanding of the factors that control their evolution and their responses to changes in variables such as sediment supply and sea level. In order to address some of the deficiencies in our understanding of sand spits, a study of the evolution of Buctouche Spit, a significant but little studied feature in New Brunswick, Canada, was undertaken between 1990 and 1993. The purpose of this project was to characterize the subenvironments of the spit, document both the short- and long-term evolution of this spit using a sedimentbudget approach, assess the relative importance of the major environmental variables which have controlled the geomorphic evolution of this spit, and speculate on how this spit is likely to evolve in the future. 1.1. The study area Buctouche Spit is a Holocene deposit located on the southwest shore of the Northumberland Strait, which separates Prince Edward Island and New Brunswick (Fig. 1). The Strait is relatively shallow, with mean water depths of lo-20 m in the western and central portions and 30-40 m in the eastern portion between Prince Edward Island and Cape Breton. Underlying the Strait is soft Carboniferous bedrock, primarily sandstones, siltstones, and shales of Pennsylvanian age (Gussow.

1953 ). which are covered with late Wisconsinan till and a variety of post-glacial sediments (Kranck, 1971, 1972). The youngest sediments in the Strait. found close to the coasts, are Holocene marine sediments which vary in composition from medium sand to tine silt. The coast of the Strait is dominated by bedrock outcrops, sandy barrier systems, and numerous drowned river valleys (Fig. 1). Most of the drowned river valleys can be matched to channels on the floor of the Northumberland Strait ( Kranck. 1972). Buctouche Spit is the southernmost of a series of barrier systems that stretch along the northeast shore of New Brunswick. It is approximately 11 km long and reaches a maximum subaerial width of about 700 m at its distal end (Fig. 2). It is constructed primarily of reworked glacial sand overlying sandstone bedrock. The spit complex is approximately 5 m thick at the proximal end and 7 ~8m thick at the distal end (measured from mean dune crest to base of present spit platform). The updrift proximal end is characterized by a single dune ridge, l-2 m high, backed by low overwash terraces and fringing lagoon marshes. The distal end is characterized by well-developed dune recurves, 2-4 m high, separated by swales and tidal ponds (Fig. 3). The most seaward recurve is a well-developed, active foredune. The recurves behind the active foredune are relatively stable relict foredunes which exhibit few, if any, blowouts. There is no indication that secondary dune types (e.g., parabolic dunes) are forming on Buctouche Spit. The end of the most seaward recurve is currently prograding into - 5 m of water and is essentially perpendicular to the main seaward shore (Figs. 2 and 3). The present rate of sediment supply to Buctouche Spit is estimated to be - 8000 m3 yr- ’ (New Brunswick Department of Natural Resources, 1975). This figure was established by measuring subaerial changes in shoreline position on vertical aerial photographs and it seems to be a reasonable lower limit. The coast is a mixed micro-tidal area (maximum tides do not exceed 1.3 m) with a semi-diurnal cycle dominating. There is unrestricted access between the bay and Strait for tidal flow. The water around the spit is usually free of ice from

J. Ollerhead, R G.D. Davidson-Arnott/Marine

Fig. 1. Bathymetry of the Northumberland in fathoms.

Geology I24 (1995) 215-236

211

Strait and location of Buctouche Spit, New Brunswick, Canada. Note that depths are

April to November. Prevailing winds are offshore from the west-southwest, while the highest waves are generated by less-frequent winds blowing from the north to northeast sector over a fetch > 90 km. The mean wave approaching the spit during the ice-free season is likely propagating from the north or north-northeast with a significant height of 0.3

m and a period of 3 seconds. The maximum wave approaching the spit during the ice-free season is likely also propagating from the north or northnortheast with a significant height of 2 m and a period of 6-7 seconds. Thus, Buctouche Spit is located in a relatively low-energy coastal environment.

218

J. Ollerheud, R. G. D. Duvidsorz-Amott;Murine

Grology 124 (1995) 215-236

: ---. j Emergent at :..... low tide

q

5 2 metres

0

5 5 metres > 5 m&es

*. *. * Progradatlona . . ‘. . spit platform

Fig. 2. Bathymetry

around

Buctouche

Spit and the subenvironments

Early work by Grant (1970) and Kranck ( 1972), using limited data sets, suggested that the rate of relative sea-level rise at Buctouche Spit for the past 3000 years was on the order of 2-4 m kyr- ’ . More recent work by Scott et al. ( 1981) and Quinlan and Beaumont (1981) suggests that the value is much lower. Scott et al. ( 1981) used marsh foraminiferal

that comprise

f

Lighthouse

the feature.

Note that depths

are in metres

zonations in subsurface sediments, a reasonably accurate indicator of past sea level with a documented resolution of _+O.l m (Scott and Medioli, 1978, 1980), to determine relative sea-level curves for four sites on Prince Edward Island (P.E.I.). They found that for the past 3000 years the average rate of relative sea-level rise at the west end of P.E.I. was 0.8 m kyr-‘.

J OIlerhead, R.G.D. Davidson-ArnottjMarine Geology 124 (1995) 215-236

219

Fig. 3. Oblique aerial photograph of Buctouche Spit viewed from south to north (June, 1992).

Quinlan and Beaumont ( 1981) used the maximum ice model of Peltier and Andrews (1976), the minimum ice model of Grant (1977), and a geophysical earth model based on the work of Peltier and Andrews ( 1976), Farrell and Clark (1976), and Peltier et al. (1978), to calculate theoretical post-glacial relative sea-level curves for six sites in Atlantic Canada, including western P.E.I. Quinlan and Beaumont’s (1981) maximum ice-model results suggest a theoretical rate of relative sea-level rise for western P.E.I. of - 1 m kyr-’ for the past 4000 years. Their minimum ice-model results are dismissed as unreasonable because sealevel data collected from the literature by Quinlan and Beaumont (1981) clearly agree more strongly with the maximum ice model results (for all six sites) and because subsequent work by Scott et al. (1987) also supports the maximum ice-model results. Since the quality of Scott et al.‘s (1981) data appears to be high and in good agreement with Quinlan and Beaumont’s ( 1981) results, it seems reasonable to accept a compromise between these two results and assume that over the past

4000 years the rate of relative sea-level rise at Buctouche Spit has been -0.9 m kyr-‘. 2. Methodology 2.1. Site description and survey The general morphology and sedimentary characteristics of Buctouche Spit and its subenvironments were determined using vertical and oblique aerial photographs, topographic maps, hydrographic charts, ground surveys, and echo sounding. Seven sets of vertical aerial photographs were obtained for the Buctouche Spit area from Energy Mines and Resources Canada ( 1945, 1959, 1965, 1971) and the Maritime Provinces Land Information Corporation (1954, 1963, 1973). These aerial photographs were used to determine the recent stability of various spit subenvironments, to measure rates of shoreline change, and to augment field notes and surveys. Numerous aerial and ground photographs were taken during the 1991 and 1992 field seasons.

J. Oikrheud, R. G. D. Duvidson-Amott:Murinr

220

Geology 124 (1995) 215-236

sounding techniques. The whole survey was referenced to mean high tide as defined by the Canadian Hydrographic Service (C.H.S.) and unless otherwise noted, all elevations in this text are referenced to this datum.

Buctouche Spit was surveyed approximately every 1000 m along lines running perpendicular to the shoreline (Fig. 4). The positions of these lines were established using a Loran C navigation system. The survey was carried out using a level and standard surveying techniques. Wood stakes were placed near the crest of the foredune along each line and at potential coring or trenching sites. All stakes were included in the survey grid. Survey lines were extended offshore using standard echo-

2.2. Determination oj’shoreline change Changes in the shoreline of Buctouche Spit for the past 149 years were quantified by incorporating

-

l

Dating Sample(s)

0

2 km

Line 5

Line 6

,

Y

I

I

9

/

Line 7

Line 12 Fig. 4. Locations

of survey lines and sample collection

sites at Buctouche

Spit (numbered).

J. OIlerhead, R G. D. Davidson-ArnottlMarine Geology 124 (1995) 215-236

an 1839 chart of the spit compiled by Captain Bayfield (a reproduction of the chart, provided by the National Archives of Canada, was used), aerial photographs of the spit for 1945, 1954, 1965, and 1973, and the current (1988) digital map file for the spit in the SPANSTM Geographic Information System (G.I.S.). Twelve unique ground control points (e.g., road intersections) were chosen prior to data input to ensure acceptable data rectification. Maximum locational error in the control points is estimated to be <3 m for the aerial photograph mosaics and ~20 m for the Bayfield chart. Maximum locational error was assessed by comparing the positions of buildings (except for the 1839 chart) and protected sections of sandstone bluff shoreline (behind the spit) which are unlikely to have eroded more than a few metres since 1839. In all cases, the mean high tide line was used to define the shoreline. 2.3. Collection of sediment samples and coring The sedimentary characteristics of Buctouche Spit were determined by collecting surface samples (both on the subaerial spit and underwater using SCUBA), cores, and borehole samples. Few samples were collected in the bay because the distribution of bottom sediments in the bay have already been well documented by Thibault (1978). Samples were collected from a variety of elevations inside five dunes on the spit by digging trenches into the dunes. Four shallow cores were recovered using vibracoring techniques (Lanesky et al., 1979; Smith, 1984). Cores were taken using 0.102 m (4 inch) diameter aluminum irrigation pipe. Because depth of penetration in medium to coarse sand is restricted to 0.5-1.5 m when vibracoring, cores were collected from different depths at multiple sample points to achieve a continuous 2 to 5 m core. For example, a 1.0 m core would be taken, the equipment moved laterally 0.5 m, a 0.9 m hole augered, and then a second 1.0 m core taken. The established survey grid was used to locate each core. All cores were sealed after extraction, transported to a laboratory, cut open with a router, and logged. Samples were collected from each core for grain-size analysis. All results were corrected

221

for compaction by comparing length of core taken to length of core recovered (maximum total error was co.05 m). It was assumed that no core material was lost from the pipes during extraction because core catchers were used. Five boreholes were drilled on the spit by an experienced operator using 0.076 m (3 inch) diameter by 1.524 m (60 inch) long solid augers. Sediment samples were collected from the bottom of each borehole after each auger section was added (i.e., approximately every 1.524 m) and stored in plastic sample bags. Comments by the drill operator about the “feel” of the drilling were recorded and used in conjunction with the sediment samples to log the gross subsurface structure of the spit. Grain size analysis was completed using a combination of standard sieving and pipette techniques and a fall column based on the design of Rigler et al. (1981). The method of moments was used to calculate the mean, standard deviation, skewness, and kurtosis of measured distributions in either phi (sieving) or chi (settling velocity) units. 2.4. Dating procedures Samples for both luminescence and radiocarbon (‘“C) dating were collected from dunes at the sites indicated on Fig. 4, to try to establish the approximate time of dune formation. The luminescence dating samples were extracted under very low light conditions from pits dug into the sides of dunes at sites 1 to 5 and 7 to 10 (referred to by site number). Two or three samples were collected in each pit. Samples of beach sediment were collected at site 6 for experimental purposes (Ollerhead et al., 1994). Additional samples were taken at each site for dosimetry and water-content determination (Ollerhead et al., 1994). Luminescence-dating techniques can be used to determine the length of time that has elapsed since a mineral was last exposed to sunlight (Berger, 1988). These techniques are geomorphically relevant because they can provide a measure of length of time since the most recent disturbance of the environment immediately surrounding the collected sediments. Both optical (Aitken, 1992; Berger, 1993) and thermoluminescence (Wintle

222

J. Ollerheud. R.G.D. Davidson-Arnolt;Muuill~

and Huntley, 1982; Singhvi and Mejdahl, 1985; Berger, 1988; Forman, 1989) dating techniques were pursued as part of this project. Details of the luminescence-dating procedures are provided in Ollerhead ( 1993) and Ollerhead et al. ( 1994). Samples of shell and peat were collected for radiocarbon dating. Shells and shell fragments were obtained from dunes at sites 2, 9, and 10, and several peat samples were collected at site 11 from an outcrop on the beach (Fig. 4). The shells collected at sites 9 and 10 were from the same trench as the associated luminescence samples. The shells collected at site 2, however, came from a trench located on the southeast side of the sampled dune whereas the luminescence samples came from the northwest side of the dune. An important concern with the shell samples was that the circumstances under which the shells were incorporated into the dunes could not be determined. The shell and peat samples were sent to the Environmental Isotope Laboratory at the University of Waterloo (Ontario, Canada) for radiocarbon (i4C) dating. This laboratory uses standard techniques to prepare and measure samples (Drimmie et al., 1992). The results obtained were calibrated using the calibration curves of Stuiver and Pearson (1986) for the peat and those of Stuiver et al. (1986) for the shells. 2.5. Wave rejiaction modeling Wave refraction patterns around Buctouche Spit were computer modeled to gain insight into the distribution of wave energy around the spit (Ollerhead, 1993). A simple wave climate, estimated using the wave hindcast of Hale and Greenwood (1980) and linear wave theory, a simplified spit bathymetry, and the computer model of May ( 1974) were used to predict wave refraction patterns around Buctouche Spit.

3. Results 3. I. Subaerial spit characteristics Buctouche Spit can be divided into five major subenvironments which are both functionally and

Geology 124 i 1995) 215-236

morphologically different: ( 1) the dunes and swales, (2) the beach and nearshore, (3) the progradational spit platform, (4) the lagoon, and (5) the offshore (Figs. 2 and 3). The erosional proximal section of the spit can also be distinguished morphologically from the depositional distal section of the spit. In terms of subaerial features, the distal portion of the spit is characterized by multiple relatively stable recurved dunes. The most seaward recurve is an active foredune while the interior recurves are relatively stable, relict foredunes which exhibit no evidence of disturbance by washover or blowout formation. The Strait-side beach is quite wide in this section (30-70 m) and backed by roughly parallel but unevenly spaced, relatively high dunes (3.5-4.5 m). There are several interior tidal ponds/ marshes and narrow fringing marshes along the bay side of the spit. The central portion of the spit is dominated by a single, moderately high dune (3.5-4.0 m). The Strait-side beach is 20-50 m wide and there is a narrow fringing marsh along the bay side of the spit. There are washover channels in the transition zone and the eastern face of the foredune is scarped in places. The proximal portion of the spit is characterized by a low foredune (1.0-2.0 m) with a scarped eastern face backed by low deposits of reworked dune sediments. Beach width varies from 25 to 60 m. Note that the entire subaerial spit is constructed of well-sorted medium sand (Table 1).

3.2. Shoreline evolution Comparison of the spit shoreline in 1945, 1954, 1965, and 1973, with the 1988 shoreline indicates that very little subaerial extension took place at the distal end of Buctouche Spit between 1945 and 1988. The small differences in shoreline position evident over this period are primarily due to the migration of sandwaves on the Strait side, slight morphologic adjustments of the very distal end of the spit, digitizing error, and some image rectification error. Between 1839 and 1988 the distal end of Buctouche Spit advanced approximately 400 m

J. OIlerhead, R.G.D. Davidson-ArnottlMarine Geology 124 (1995) 215-236

223

Table 1 Summary of Buctouche grain-size analysis results. The Udden-Wentworth grain-size scale is used where: medium sand= 0.250-0.500 mm diameter, fine sand=0.125-0.250 mm diameter, and very fine sand=0.063-0.125 mm diameter (Wentworth, 1922) Spit subenvironment [elevation relative to mean high tide] Offshore [-8 to -6m] Progradational spit platform t-8 to -2m] Beach and nearshore [-2 to 0.5 m] Dunes and swales [0.5 to 5 m] Lagoon [-1.5 to -1 m]

Number of samples

Mean grain size (mm)

Mean grainsize range (mm)

Udden-Wentworth class

Sorting

3

0.20

0.16-0.26

Fine sand

Well sorted

I

0.24

0.19-0.28

Fine sand

Well sorted

12

0.34

0.24-0.48

Medium sand

Well sorted

31

0.37

0.26-0.43

Medium sand

Well sorted

4

0.29

0.10-0.44

Medium sand

Poorly sorted

and the middle section of the spit narrowed (Fig. 5). Although some of the plotted differences are due to error, all sources of error combined are not sufficient to explain the observed differences between these two shorelines (maximum error < 20 m). The data show that a new subaerial lobe developed at the distal end of the spit between 1839 and 1988 (Fig. 5). There are a series of recurved relict foredunes on this lobe along with the presently active foredune (Fig. 3). Since this lobe existed in 1945, the distal end of the spit must have extended approximately 400 m in < 106 years, a subaerial growth rate of - 4 m yr-’ . Comparison of the 1839 and 1988 shorelines (Fig. 5) shows landward migration and narrowing of the central section of Buctouche Spit. This suggests that the centre section may be a source for some of the sediment used to extend the spit. The aerial photographs and ground survey also revealed the presence of longshore sandwaves, similar to those at Long Point, Lake Erie, Ontario (Stewart and Davidson-Arnott, 1988), migrating along the Strait side of Buctouche Spit (Fig. 3). The largest sandwaves at Buctouche Spit are < 500 m long and ~50 m wide. No more than three or four sandwaves are seen at Buctouche Spit in any given year.

3.3. Subsurface stratigraphy Vibracores

Composite cores were recovered from four locations (Fig. 6) using a multiple-entry technique. In all cases, the sequences found in the cores were consistent with the horizontal sequences of sediments, shells, rock fragments, and vegetation types observed on the surface of the spit complex. Core 1 was taken at the distal end of the spit. The upper unit of the core contains homogenous, well-sorted medium sand with roots from Ammophila breviligulata at the very top (Fig. 6). Underlying this unit, is a unit of coarse sand, sandstone fragments, and rounded pebbles, situated just below 0 m elevation, that is interpreted to be beach sediments. The beach sediments grade down into medium sand with shell, which in turn grades into fine sand with shell at a depth of --3m. The core taken at site 2, near the fulcrum point which divides the erosional and depositional portions of the spit (Fig. 5), is very short and contains only well-sorted medium sand and Ammophila breviligulata roots and stems (Fig. 6). The buried roots and stems, centred at -0 m elevation, likely indicate a former dune or swale surface which was

J. Ollerhend. R. G D. Drrvir~.Fon-Amott,, Marine Geo1og.v 124 ( 199.5) 21.7-236

224

-

1839 shoreline

Northumberland

attachment point. exhibits a sequence consisting of very fine sand overlain by fine sand and capped by peat between -2.0 and -2.1 m elevation ( Fig. 6). This sequence is very similar to the nearsurface sequence that currently exists on the bed of the lagoon west of coring site 4. The lowest three units in core 4 are overlain by alternating layers of well-sorted medium sand and peat of various thicknesses, presumably representing a cyclic relationship between overwash/dune growth and the re-establishment of marsh vegetation as sea level rose. Bowholes

Buctouche

O-

lkm

new lobe

Fig. 5. A comparison of the 1839 and 1988 shorelines of Buctouche Spit. Note that transgression has occurred northwest of the fulcrum point, where there is a negative sediment budget. and that progradation has occurred southeast of the fulcrum point, where there is a positive sediment budget.

buried by subsequent accretion. No sediments similar to present Strait-side beach deposits were found. Core 3 was taken near the centre of the spit. The top unit of the core contains well-sorted medium sand capped by Ammophila breviligulutu roots (Fig. 6). Sandstone fragments at _ -0.6 m elevation are evidence of overwash in this area. Two thin layers of peat were found at - 1.0 and - 1.2 m elevation, with well-sorted fine sand between them. The bottom unit is well-sorted medium sand with marine shell and sandstone fragments mixed in. This unit is probably a washover deposit which may explain why further core penetration could not be achieved at this site. The bottom section of core 4, taken near the

Five boreholes were drilled on the spit (Fig. 7). The upper four units in borehole 1 (distal end of the spit) are, from bottom to top: fine sand, medium sand, coarse sand, and medium sand. This sequence is the same as that logged from core 1. Below the beach deposit, centred at w -0.5 m elevation, mean sediment size grades from 0.37 mm down to 0.06 mm (Fig. 7). The unconsolidated sediments appear to be sitting on a layer of rock fragments and/or till which is underlain by bedrock. The sequence logged from borehole 2, drilled near the fulcrum point, is very similar to that of borehole 1. In borehole 2 the rock fragments and/or till layer was not encountered until N - 15 m elevation. Bedrock was reached at < -7.6 m elevation at boreholes 3, 4, and 5 (Fig. 7). The top unit in all three of these boreholes contains well-sorted medium sand. Relatively thin layers of well-sorted fine and very fine sand were encountered at the bottom of boreholes 3, 4, and 5, and a layer of rock fragments and/or shell was encountered in boreholes 4 and 5 above the fine sand layer. These layers may well represent accumulations of shell on former bay bottoms. 3.4. Dating results The results of the luminescence dating program are summarized in Table 2 and the radiocarbon dating results for the shell and peat samples are summarized in Table 3. Note that the thermoluminescence dates presented in Table 2 are consistent with the optical dates but that they have much

J. Ollerhead, R G. D. Davidson-ArnottlMarine Geology 124 (1995) 215-236

Some rounded pebbles with sandstone fragments

q Coarse sand ( 0.500- 1.000 mm) q Medium sand ( 0.250 - 0.500 mm)

Very fine sand (0.063

225

I

’

- 0.125 mm)

/ Fig. 6. Vibracoring results from Buctouche Spit.

greater error bars associated with them. As such, only the optically obtained dates will be discussed further. For a description of how these dates were obtained, see Ollerhead et al. (1994). 3.5. Wave refraction patterns The wave refraction study indicates that waves with intermediate heights (- 1 m) and periods ( -4 s) have the greatest influence on recurve development at the distal end of Buctouche Spit (Ollerhead, 1993). These waves are large enough to do significant work on the spit and yet small enough that they do not shoal and break well offshore. In most cases, energy from waves of this size is distributed evenly along the shoreline. The results indicate that waves approaching from the northeast and east have the greatest effect on shaping the very distal end of the spit. Waves propagating from the south and southeast primarily affect the spit platform.

4. Discussion 4.1. Subaerial spit characteristics

The subenvironments identified at Buctouche Spit are similar to those identified at other flying spits (e.g., Kraft, 1971; Davidson-Arnott and Fisher, 1992). Most flying spits have a pseudostable proximal section backed by marsh, a narrow, erosional middle section prone to overwash, and a wider, more stable, accretional distal section characterized by dune recurves. For example, Hurst Castle spit in Hampshire, England (King, 1972), and Flat Island spit in Newfoundland, Canada (Shaw and Forbes, 1987), are morphologically similar to Buctouche Spit. 4.2. Shoreline evolution Comparison of the 1839 and 1988 Buctouche Spit shorelines (Fig. 5) shows that a new distal

J. Oilerhead, R. G.D. Davidson-Amott;‘Marine Geology 124 (1995) 215-236

226

4

2 H

__--________

0

E0

-2

g

-4

j

-5

B

-8

6 F 5

-10

1 y

-14 -16

Coarse sand ( 0.500 - 1.000 mm)

-18

Madlum sand ( 0.250 - 0.500 mm)

-12 __________-------__________--------

Fine sand ( 0.125 - 0.250 mm)

-20

Vety flns sand (0.063 - 0.125 mm) Rock fragments and/or till .

Rock fragments and/or shall

Fig. 7. Borehole

Table 2 Ages determined using three luminescence Refer to Ollerhead et al. (1994) for details Method

and sample

K-feldspar infrared optical l-l 2-I 3-1 4-l 5-l 8-1 9-l 9-2 IO-1 Quartz thermoluminescence l-l Polyminerai thermoluminescence 1-1

dating

drilling

methods.

Age (years)”

x5*45 660+45 310,25 l65*20 s&Job 18Oi I5 205 & 20 190*25 225 f 20

1100*200

800 f 700

“Ages are rounded to the nearest 5 years. bAn alternative method suggested an age of 12 &2 years.

results from Buctouche

Spit.

lobe developed over this period and that landward migration and narrowing of the centre section of the spit has been occurring since at least 1839. The fact that the centre section of the spit is getting narrower suggests that it is experiencing net losses of sediment. Since the spit grew (extended) approximately 400 m between 1839 and 1945, a subaerial growth rate of -4 m yr-‘, it is possible that the centre section of the spit may have become a sediment source for this growth. The peat sample which was radiocarbon dated was extracted from a pit dug into the beach on the Strait side of the spit at the very proximal end (sample site 11, Fig. 4). It was found to have an age of 935&70 yrs cal B.P. (Table 3). This age equates to a calibrated radiocarbon age of 975 + 70 yrs before 1990 (1950 is 0 yrs cal B.P.). It is assumed that the dated peat was formed in marsh behind Buctouche Spit. It is further assumed that the proximal end of the spit must have migrated landward at least its current width ( - 300 m) since

J. OIlerhead, R G.D. Davidron-ArnottjMarine

Geology 124 (1995) 215-236

221

Table 3 Radiocarbon (“‘C) dating results. Note that 1950 is 0 yrs cal B.P. Sample

Sample site

Laboratory number

% modern carbon

Uncorrected 14C age (yrs B.P.)

Calibrated l“C

type

Peat Shell Shell Shell

11 2 9 10

35445-2648 35448-2650 35449-2653 35450-2654

88.2kO.8 99.8 kO.9 82.3 kO.7 82.lkO.8

1010+70 Modem 1570t70 1590570

935+70 Modem 1210+ 100 1235+ 100

the peat was formed -975 years ago. Thus, the proximal end of the spit has likely been migrating landward at a rate 20.3 m yr-’ for at least the past 1000 years. Given the location of the peat outcrop, the spit attachment point must also be migrating southeast along the coast as it migrates landward. 4.3. Subsurface stratigraphy The data from the vibracores and boreholes offer further information about spit age and migration history. For example, no sediments similar to present Strait-side beach deposits were found in the core taken at site 2 (Fig. 6). This likely means that the spit has not migrated landward more than its present width at this site. This is not surprising because coring site 2 is located at the start of the depositional distal section of the spit (Figs, 2 and 6). The well-sorted medium sands found in core 2 likely represent, from bottom to top: lagoon floor (spit platform), lagoon-side beach, and dune sediments. The adjacent modern lagoon floor, beach, and dune are comprised of well-sorted medium sand (mean size=0.37, 0.38, and 0.36 mm, respectively) which is similar to that found in the lower portion of core 2 (mean size = 0.34 mm). As is the case on the present spit, it is not possible to distinguish between these three subenvironments in this area using sediment characteristics alone. The marram stems located at -0 m elevation in core 2, extend to a depth of -0.3 m (Fig. 6). If one assumes that the marram did not begin to grow until this section of the spit reached an elevation of -0.1 m above spring high tide (0.4 m above mean high tide), one would conclude that this section of the spit was a subaerial environment

age (yrs cal B.P.)

by -890 years ago, given a rate of relative sealevel rise at Buctouche Spit of -0.9 m kyr-’ . The luminescence age for sample 1-1, collected from a dune close to coring site 2 (Figs. 4 and 6), is 765 f 45 years (Table 2), which is reasonably consistent with the preceding rough estimate. The thin peat deposits in core 3 (Fig. 6) indicate that tidal marsh once existed at this location and that there was an alternation between overwash and the re-establishment of marsh vegetation over a period of several hundred years as relative sea level rose. Assuming a rate of relative sea-level rise at Buctouche Spit of -0.9 m kyr - ‘, it is likely that marsh existed in this location - 1100 years ago assuming that the top peat layer, currently located at - 1.0 m elevation, formed near the mean high tide line. At present, peats are forming in locations up to 0.1-0.2 m above mean high tide. Therefore, these peat deposits could be 1300 years old. These observations suggest that the centre section of Buctouche Spit has been migrating landward with rising sea level for at least the past 1100 years. Core 4 contains a layer of peat at -2.0 m elevation (Fig. 6). This means that marsh probably existed in the vicinity of coring site 4 -2200 years ago. This peat could be up to 2400 years old if it formed 0.1-0.2 m above the mean high tide line. From these data, it is concluded that the present Buctouche Spit is at least 2200 years old. The core and borehole data presented in Figs. 6 and 7 provide sufficient information to allow some comparison between Buctouche Spit’s platform and those found at other spits. Most evidence suggests that spit platforms are, as is the case for Buctouche, composed of finer sediments than those overlying them (i.e., there is a coarsening-up

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sequence). For example, Coakley ( 1983. 1992) examined borehole data from Long Point, Lake Erie, Ontario and found well-sorted medium sand overlying a sequence of interlaminated silty sands. uniform post-glacial muds, glacial clay. till, and bedrock. The coring and borehole data from Buctouche Spit are also consistent with the models presented by Kraft et al. ( 1973). The thick layers of very fine and fine sand under the distal end of Buctouche Spit seem to have accumulated over a long period of time. The current bathymetry around the spit and the rate of relative sea-level rise at the spit suggest that they pre-date the present spit platform. The present spit platform is growing into - 5 m of water whereas the thick layers of very fine and fine sand under the distal end of the spit extend from 4-5 m depth down to 14-15 m depth. These layers likely accumulated behind a barrier that preceded the present Buctouche Spit. There is evidence from other areas that barriers can be destroyed and subsequently replaced. For example, De Boer (1964) traced the evolution of Spurn Head. a spit at the mouth of the River Humber in England, and found that this spit has been breached at its proximal end, had its distal end destroyed due to sediment starvation because sediments arriving from updrift were passing through the breach, and been rebuilt three times in the past 700&X00years. 4.4. Accretion rates As noted, Buctouche Spit is currently growing into - 5 m of water (Fig. 2). Thus, for the purpose of calculating volumes of sediment contained in various portions of the present spit, it is estimated that the submarine spit platform is 65 m thick. The proximal and central sections of the spit combined are assumed to be 6700 m long, 1500 m wide, and 6 m thick. This estimate includes both the subaerial and subaqueous parts of the spit. The distal end of the spit is assumed to be 3500 m long, 2000 m wide, and 7 m thick. These dimensions equate to a total volume of - 1.1 x 10’ m3. The mean luminescence age for the dunes at dating sites 1 and 2 is 7 15 f 4.5 years and the mean luminescence age for the dunes at dating sites 3 and 4 is 240+25 years, a difference of 475 years (Table 2). Dating sites 1 and 2 are approximately

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1900 m from dating sites 3 and 4 (Fig. 4). Therefore, Buctouche Spit grew - 1 900 m in the 475 years between 715 Ifr4.5years ago and 240 +25 years ago, a growth rate of -4 m yr-’ This growth rate and the assumed spit dimensions for the distal end equate to a distal end accumulation rate of - 56,000 m3 yr ‘. The accretion rate prior to 745 f 45 years ago may also have been - 56,000 m3 yr- ‘. However, there is no way to determine the accumulation rate prior to 745 +45 years ago using the collected data. The luminescence dates for the dunes on the proximal end of the spit cannot be used to determine spit age because, unlike the stable dunes at the distal end, the proximal end dunes have been reworked over the past 1000 years (see Fig. 4 and Table 2). If the entire spit accumulated at a rate of - 56,000 m3 yr-‘, it would be - 1950 years old. Since the peat layer at the bottom of core 4 was probably formed about 2200 years ago, it seems safe to assume that the proximal end of the spit existed at that time. These data suggest that Buctouche Spit is probably 32200 years old and that the long-term accumulation rate may be as high as 56,000 m3 yr- ‘. Since 24O_t25 years ago, the rate of sediment accumulation at Buctouche Spit has apparently been falling. This is likely because local sediment supply has decreased. Recall that the distal end of Buctouche Spit grew - 400 m between 1839 and 1945 ( 106 years). However, it was primarily the subaerial portion of the spit that grew. Although the data related to the size and shape of the spit platform in 1839 are limited, they do indicate that it was about the same size and shape in 1839 as it was in 1990. The data do not allow an accurate comparison of spit-platform elevations for this period. If it is assumed that the new distal end lobe ( Fig. 5) was deposited in 1 m of water on the existing spit platform surface, without any accompanying platform extension, the accumulation rate between 1839 and 1945 would have been -23,000 m3 yr-‘, less than half of the historic rate. Since 1945, there has been no measurable extension of Buctouche Spit. Present sediment supply to the spit, established using measurements of subaerial shoreline retreat, is - 8000 m3 yr-’ (New Brunswick Department of Natural Resources, 1975). It is possible that this rate may be somewhat

J. OIlerhead, R G.D. Davidson-Arnott/Marine

higher if additional sediment is being supplied from the Kouchibouguac/Richibucto barrier system or from offshore. Although there is no geomorphic evidence that sediment is being supplied from north of Richibucto Cape (Fig. 1), it is likely that at least small amounts of sediment are being transported south around Richibucto Cape into the Buctouche area from the Kouchibouguac/Richibucto barrier system. Conversely, it seems unlikely that any significant amount of sediment is being supplied to the spit from offshore at present. Given this evidence, it is doubtful that the current rate of sediment supply to Buctouche Spit is more than twice the New Brunswick Department of Natural Resources’ (1975) estimate of -8000 m3 yr-‘. Approximately 15,000 m3 of sediment would have to be evenly distributed over the subaerial surface of Buctouche Spit each year just to keep pace with a rate of relative sea-level rise of -0.9 m kyr- ‘. Because Buctouche Spit’s present rate of sediment supply is probably < 16,000 m3 yr-‘, it seems reasonable to conclude that Buctouche Spit’s present vertical growth rate is, at most, equal to the rate of relative sea-level rise at the spit and may be only half of the rate of relative sea-level rise. This would explain why there has been no significant net increase in spit area (including the platform) since at least 1945. The fact that the spit platform has not prograded/extended southward since at least 1839, even though it is still being supplied with sediment, suggests that it has reached a state of dynamic equilibrium. Spit growth has narrowed the gap between the platform and the mainland on the south side of Buctouche Bay to the point where ebb-tidal currents are strong enough to prohibit further extension. In other words, Buctouche Spit is now a constrained spit. Qualitative field observations confirm that sediment arriving at the distal end of the spit is being transported out into the Strait by ebb-tidal currents. 4.5. Dating results Radiocarbon dating The dated peat sample was collected from a peat layer centred at - -0.8 m elevation. It was

Geology 124 (1995) 215-236

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extracted from a pit dug into the beach on the Strait-side of the spit at the very proximal end (sample site 11, Fig. 4). It was found to have an age of 935 k 70 yrs cal B.P. (Table 3). This age equates to a calibrated radiocarbon age of 975 + 70 years before 1990 (1950 is 0 yrs cal B.P.). Given a rate of relative sea-level rise at Buctouche Spit of -0.9 m kyr- ‘, one would expect this peat sample to be -900 years old (relative to 1990) if it formed near the mean high tide line. Peats at Buctouche Spit are currently forming in locations up to 0.1-0.2 m above mean high tide, so it was recognized that this peat deposit could be up to 1100 years old. Thus, the age estimates for the peat sample from radiocarbon dating and from rate of relative sea-level rise are consistent, providing confidence in the estimated spit migration rate of 20.3 m yr- I. This result also provides support for the preceding conclusions based on the peat layers found in cores 3 and 4. The radiocarbon dates obtained for the shell samples collected at sites 2, 9, and 10 (Fig. 4) are inconsistent with morphologic/stratigraphic evidence observed and with the luminescence dates. For example, the shells collected at site 2 date as modern (i.e., post 1950) but the luminescence date is 660 k45 years. It is possible that these shells were deposited during an overwash event. Although there is currently an active overwash channel on the opposite side of the dune from where the sample was collected, there was no stratigraphic evidence in the sediments surrounding the collected shells to suggest that overwash had occurred (e.g., no evidence of current ripples). Another possibility is that the shells collected were contaminated with modern 14C but this seems unlikely. The shells collected at sites 9 and 10 were found to have ages of 1210f 100 and 1235f 100 yrs cal B.P. These ages equate to calibrated radiocarbon ages of 1250 k 100 and 1275 rf 100 years before 1990, respectively. The associated luminescence ages are - 200 years (Table 2). As well, the dune at site 10 is very close to the distal end of the spit and is likely ~250 years old. These shells must have been buried in the dunes in which they were found - 1050 years after their creators died. Thus, the shell dates are of little value because they

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J. Ollerhead,

R. G. D. L)rr~idson-rlrnottiMarine

cannot be related to the formation of the dunes in which they were found. Luminescence dating

Estimates of spit age from spit morphology and rate of relative sea-level rise are consistent with the luminescence ages. If a growth rate of 56,000 m3 yr-’ and the assumed spit dimensions are accepted, it would have taken 1950 years to fill the volume presently occupied by the spit. At this growth rate, the spit would have reached the point where sample 1-l was collected by - 850 years ago. This is in agreement with the luminescence age of 765 f 45 years for sample l-l (Table 2). The luminescence ages of samples 8-1, 9-1, 9-2, and 10-l are all consistent with -200 years (Table 2). Since 9-2 was collected 0.75 m above 9-1, it was expected that 9-2 would be slightly younger than 9-1. This is the case although the error bars overlap. Apparently, the dunes from which these samples were taken were disturbed approximately 200 years ago. Sample 8-1 came from a hummocky dune at the proximal end of the spit which appeared to have been reworked, whereas samples 9-l and 9-2 came from the foredune near the middle of the spit, where overwash is a fairly frequent occurrence. The dune from which sample 10-l was taken was also disturbed, or just forming, about 200 years ago (Table 2). Since the dunes at sites 8, 9. and 10, representing the proximal, middle, and distal portions of the spit respectively, were all forming or reforming about 200 years ago, it is possible that a major storm about 200 years ago caused a great deal of overwash and erosion at Buctouche Spit. Given the natural variability inherent in dune formation and growth. one would not expect the ages obtained from the dunes at sites 8, 9, and 10 to be identical. As noted, the luminescence ages of samples 9-1 and 9-2 (Table 2) are not significantly different. This indicates that the relevant dune developed fairly rapidly (tens of years or less). Other evidence presented also suggests that dune growth occurs quite rapidly at Buctouche Spit. Recall that a new distal lobe and its associated recurves developed between 1839 and 1945 (< 106 years). As well, the historic aerial photographs show that breaches in

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the active foredune caused by washover tend to heal in 3- 10 years. Hence, the geomorphic evidence presented is consistent with the luminescence ages obtained, giving confidence that the infrared optical dating technique used provides reasonable results, at least in this case. However, given the novelty of the method, several demonstrations of correct ages obtained on known-age samples, with no discordant results, are required before complete reliance can be placed on dates obtained by this method ( Ollerhead et al., 1994). 4.6. Spit evolution Short-term spit evolution (10s to 100s ofyears)

The luminescence dates for sites 8 and 9, along with field observations, the cores, and aerial photographs, confirm that overwash occurs frequently in both the proximal and transition zone sections of Buctouche Spit. The sequences observed in cores 3 and 4 illustrate a cyclic relationship between overwash/dune growth and the reestablishment of high tide marsh or deep swale vegetation as relative sea-level rises. The prevalence of overwash in these sections has also been observed at other flying spits such as Flat Island, Newfoundland (Shaw and Forbes, 1987) and Long Point, Lake Erie, Ontario (Davidson-Arnott and Fisher. 1992). Many workers have observed that overwash in the centre sections of flying spits is usually coincident with the focusing of wave energy on those sections by wave refraction (e.g., Shaw and Forbes, 1987). Although the centre section of Buctouche Spit is also very narrow and prone to overwash, there is no evidence that this is due to the focusing of wave energy by refraction (Ollerhead, 1993). The centre section of Buctouche Spit is most prone to overwash because it is the most unprotected part of the spit. The proximal section of the spit is protected by a shallow bedrock shelf which causes large waves to shoal and break before reaching it. The distal end of the spit is protected by its wide, accretional spit platform. It is the centre section of the spit, which is not presently protected by a wide, accretional spit platform, that is most prone to erosion and overwash.

J. Ollerhead, R G. D. Davidson-Arnottjkiarine Geology 124 (199.5) 215-236

Many workers have suggested that overwash is a primary mechanism for the landward migration of barriers (e.g., Yasso, 1964; Leatherman, 1979) and this is true for Buctouche Spit. However, barrier migration may also occur as a result of inlet formation (e.g., Armon, 1979; Leatherman, 1979; Hill and FitzGerald, 1992) and aeolian processes (e.g., Rosen, 1979). There is no evidence of inlet formation in the aerial photographs of Buctouche Spit (48 years of record). Despite a reference by Ganong (1908) to the fact that work was undertaken at Buctouche Spit at the turn of the century to prevent the sea from cutting through the spit, there is no evidence that inlets play a significant role in the present migration of the spit. Aeolian processes also appear to play a relatively minor role in the landward migration of the spit, probably because the prevailing winds are offshore and because there is a dense vegetation cover which limits sediment transport. The frequency and magnitude of overwash and inlet formation is tied to the frequency and magnitude of storms and tidal conditions. The major effect of extreme storm events at the proximal end of the spit is to erode the foredune and beach and move sediment to the lagoon by overwash. The net effect of these processes is proximal-end spit transgression. The effect of extreme storm events at the distal end of Buctouche Spit is to move sediment offshore from the beach to the nearshore zone and to redistribute sediments alongshore and across the progradational spit platform. It is the reservoir of sediment that accumulates on the spit platform that ultimately allows for the formation of new recurves. At Buctouche Spit, new recurves appear to form relatively quickly (i.e., tens of years). Major lobes appear to form over a longer period (i.e., tens to hundreds of years). New recurves and lobes are the cumulative result of both high-frequency/low-magnitude and low-frequency/high-magnitude environmental conditions. Many studies attribute the formation of recurves to wave refraction around a spit’s distal end (e.g., Evans, 1942; Holmes, 1944; Scheidegger, 1961; Yasso and Fairbridge, 1968; etc.). The evidence from Buctouche Spit suggests that wave refraction and diffraction are not the sole reasons for the

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formation of distal end recurves (Ollerhead, 1993). Recurves at Buctouche Spit appear to form in the same manner as those at Hurst Castle spit, Hampshire, England (King and McCullagh, 1971). The overall movement of littoral material down the coast and the main curvature of Buctouche Spit are a function of the dominant north to northeast waves that approach the spit and refract. These waves cannot refract completely around the distal end of the spit, however, because they shoal on the progradational spit platform. Individual recurves are formed by the movement of littoral material to the west, perpendicular to the main axis of the spit, by less-frequent waves from the east and east-southeast. Waves from the southeast and south build up the recurves until they are always above spring high tide. The lagoons behind new recurves are very shallow because of infilling by overwash. Large portions of the floors of the tidal ponds that exist between present recurves at Buctouche Spit are often emergent at low tide. Thus, the main axis of Buctouche Spit is drift-aligned while the recurves are swash-aligned (terms used by Carter and Orford, 1991). Once a new recurve is fully emergent, the prevailing westerly to southwesterly winds that occur at Buctouche Spit can move sediment from the beach into the embryo dunes. This builds the dunes and renders them less susceptible to future destruction by erosion. The resulting pattern of recurves is quite complex, with unequal spacing and varying lengths (Fig. 3). These patterns are the result of variability in the frequency and magnitude of waves approaching the spit from all relevant directions. The ends of the recurves are also modified by waves approaching the spit from the west (i.e., waves formed on Buctouche Bay). Old recurves at the proximal end of the spit have been modified to varying degrees by overwash. Another reason for the complex dune patterns at the distal end of Buctouche Spit is rapid dune accretion associated with the presence of sandwaves. At several distal end locations, it was observed that the 2-3 relict foredunes immediately behind the active foredune are very close together and the swales between them very shallow. A longshore sandwave may have been adjacent to these dunes when they formed, providing both

sufficient sediment and enough protection from wave energy for a series of parallel dunes to form rapidly. Since sandwaves migrate down the coast from north to south, the possibility for new foredunes to form rapidly in various locations along the spit always exists. Long-term spit evolution f 100s to 1000s of’~ww.~!

The primary obstacle to understanding the longterm evolution of Buctouche Spit is ascertaining where the sediment that is presently contained in the spit complex originated. The data indicate that until 150-250 years ago, Buctouche Spit accreted of sediment. If at a rate of _ 56,000 m3 yrl updrift sources can only provide 8000-16,000 m3 yr ’ of sediment, the remaining 40,000-48,000 rn’ yr-r of sediment must have been supplied from some other location. It is hypothesized here that until 150-250 years ago, 40,000-48,000 m3 yr ’ of sediment was supplied to the spit from offshore deposits which remained after the inundation,’ destruction of a former sandy barrier, which existed seaward of the present spit more than 2000 years ago. This hypothesis is not new. Ganong ( 1908) speculated that Buctouche Spit used to be anchored to North Patch, a shoal approximately 4000 m east of the present distal end of Buctouche Spit (Fig. 1). Ganong ( 1908) further asserted that when the spit was “deprived of support by its subsidence”, it began “swinging landwards at its freed end”. He recognized that all of the barrier systems along the west shore of the Northumberland Strait are migrating landward and noted that this was primarily due to overwash. He also described another shoal that stretches from North Patch south to Cocagne Head (- 15 km south of the distal end of Buctouche Spit). This shoal suggests that a barrier once stretched all the way from somewhere north of the present Buctouche Spit down to Cocagne Head (Fig. 1). Given the present understanding of the rate of relative sea-level change at the spit, Ganong (1908) was incorrect in assuming that subsidence led to the landward migration of Buctouche Spit. However, he was probably correct to suggest that the present spit is not the same spit or barrier that existed in this area >2000 years ago.

Johnson ( 1925) argued that Ganong’s ( 1908) interpretation was flawed because of the pronounced channel running N-S between the subaqueous bar at North Patch and Buctouche Spit. Johnson ( 1925) felt that this channel should be irregularly obstructed or even completely filled with sand if a large sand barrier had migrated over it. He also argued that the recurved ends in the bay behind the spit indicate that the present Buctouche Spit must have “been built progressively southeastward but very little in front of its present position”. Johnson (1925) further argued that “had the bar swung back to its present location from one far to the northeastward, in line with the North Patch, it could hardly possess such a series of recurved points as appear to be indicated both by the landward margin of the distal half of the bar and by the direction of the ridges obscurely indicated on some of the charts”. Johnson (1925) suggested that Buctouche Spit started as a flying spit growing towards North Patch but that as North Patch subsided and eroded (or more likely, as sea level rose), waves attacked the distal end of Buctouche Spit and deflected it landward to its present course. Both Ganong ( 1908) and Johnson ( 1925) were partially correct. It is likely that a barrier used to exist near, and may have been anchored to, North Patch when relative sea level was lower. The thick deposits of very fine and fine sand that underlie the current spit probably accumulated in the bay behind this barrier. Prior to 2000 years ago, the previous barrier was likely breached because the rate of sediment supply to the barrier was no longer sufficient to allow the barrier to grow vertically at a rate commensurate with the local rate of relative sea-level rise. After the previous barrier was destroyed. a new barrier, the present Buctouche Spit complex, began to form landward of it. This type of barrier migration has been termed overstepping. Similar cases of barrier migration by overstepping have been studied by others. For example. Forbes et al. ( 1991) documented the migration of the Story Head gravel barrier (Nova Scotia, Canada) and showed that it is likely migrating by overstepping. Hill and FitzGerald ( 1992) proposed an overstepping mechanism to describe the trans-

J. Ollerhead, R G.D. Davidson-ArnottlMarine Geology 124 ( 199.5) 215-236

gression of Duxbury Beach spit (Massachusetts, U.S.A.). They suggested that this spit used to be anchored to bedrock, drumlins, and various glacial deposits 2000-3000 m seaward of the present spit. The rise in relative sea level at this spit and a fluctuating sediment supply have produced cycles of barrier progradation followed by destruction and subsequent landward translation of the shoreline. Hill and FitzGerald (1992) found that the rate of relative sea-level rise at Duxbury Beach spit has been -1.1 m kyr-’ over the past 3 700 years and that this barrier has been migrating landward at an average rate of 0.27 ) 0.05 m yr- ‘. These values suggest that the estimated landward migration rate for Buctouche Spit of 0.3 m yr-’ is reasonable given the similar rates of relative sealevel rise at the two sites. The presented data indicate that Buctouche Spit is now in a limited sediment supply situation and that it has become a constrained spit. Sediment is being eroded from the centre section of the spit, transported to the distal end, and then carried away by ebb-tidal currents. The centre section of the spit will likely breach within the next hundred years. If a breach (or breaches) becomes permanently established in the centre section of the spit, the distal end will suffer significant erosion and may be destroyed. This might lead to significant rearrangement of the spit and the bay that it protects. After this, the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise. It is therefore concluded that the key to understanding Buctouche Spit’s long-term evolution is recognizing that it is migrating landward both by continuous processes like overwash, and by the more discrete and catastrophic process of overstepping.

5. Conclusions The general morphology of Buctouche Spit is similar to that of other flying spits. Five morphologically and functionally distinct subenviromnents are recognized at Buctouche Spit: ( 1) the dunes and swales, (2) the beach and nearshore, (3) the progradational spit platform, (4) the lagoon, and

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(5) the offshore. These subenvironments are also found at other flying spits and in all known cases they have similar characteristics. The subsurface stratigraphy of Buctouche Spit is also consistent with that found at other flying spits. The recurves that comprise Buctouche Spit are likely formed in the manner identified by King and McCullagh ( 1971). The overall movement of littoral material down the coast and the main curvature of the spit are controlled by the dominant north to northeast waves and resulting refraction patterns. Individual recurves are formed by the movement of littoral material to the west, perpendicular to the main axis of the spit, by less frequent waves from the east and east-southeast. The complexity of the observed pattern of dunes is due to variability in the frequency and magnitude of waves approaching the spit from the relevant directions. The luminescence dates obtained as part of this project proved very valuable. Although further testing is needed, the optical dating method in which infrared stimulation is used on potassium feldspars appears to be a very promising method for dating Holocene sediment deposits, yielding good resolution and having simplicity of measurement. The luminescence ages indicate that Buctouche Spit extended at a rate of -4 m yr-’ between 7 15 + 45 years ago and 240 f 25 years ago. This extension rate and the assumed spit dimensions for the distal end equate to a distal-end accumulation rate of - 56,000 m3 yr- ’ for this period. Because there is no way to determine the extension rate prior to 745 +45 years ago using the collected data, the accumulation rate is assumed to be -56,000 m3 yr-’ for that period as well. At this rate of accretion, the present Buctouche Spit would have formed during the past 2000 years. The core data indicate that the spit is 22200 years old, which is consistent with the preceding estimate. Since 240+25 years ago, the rate of sediment accumulation at Buctouche Spit has apparently been falling. This is likely because local sediment supply has decreased. It is concluded that the accumulation rate between 1839 and 1945 was -23,000 m3 yr-‘, less than half of the historic rate of -56,000 m3 yr-‘.

Since 1945, the distal end of Buctouche Spit has not prograded/extended measurably. It is esti15,000 m3 of sediment would have mated that to be evenly distributed over the surface of Buctouche Spit each year just to keep pace with its rate of relative sea-level rise of - 0.9 m kyr ’ Because Buctouche Spit’s present rate of sediment supply is probably < 16,000 m3 yr- ’ . it is concluded that Buctouche Spit’s present vertical growth rate is, at most, equal to the rate of relative sea-level rise at the spit and may be as little as half of the rate of relative sea-level rise. This explains why there has been no significant net increase in spit area since at least 1945. It is likely that the much larger sediment supply required to sustain an historic accretion rate of -56,000 m3 yr~i was derived from the reworking of a former sandy barrier located seaward, and possibly updrift of, the present spit. Buctouche Spit is now in a limited sedimentsupply situation and it has become a constrained spit. Sediment is being eroded from the centre section of the spit, transported to the distal end, and then carried away by ebb-tidal currents. The centre section of the spit will likely breach within the next hundred years. If a breach (or breaches) becomes permanently established in the centre section of the spit, the distal end will suffer significant erosion and may be destroyed. After this, the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise. It is therefore concluded that the key to understanding Buctouche Spit’s long-term evolution is recognizing that it is migrating landward both by continuous processes like overwash, and by the more discrete and catastrophic process of overstepping.

Acknowledgements

Jocelyn Ollerhead, Gillian Novoselac, and Anthony Gabriel are all thanked for their help with the field work. We are extremely grateful to Dr. David Huntley, Dr. Glenn Berger, and George Morariu for their help with the luminescence dating. We thank Marie Puddister and Brian

Morber for their help in preparing the diagrams. We thank Jacques Thibault for providing valuable advice and data, and the Irving family for allowing access to Buctouche Spit. Finally, we thank Dr. N.P. Psuty, S. van Heteren, and one anonymous reviewer for the helpful suggestions and comments they made while reviewing an earlier draft of this paper. This research was supported by grants to Dr. D.J. Huntley and Dr. R.G.D. Davidson-Arnott by the Natural Sciences and Engineering Research Council of Canada, and by a grant from Energy Mines and Resources Canada to Dr. R.G.D. Davidson-Arnott. This paper is a contribution to the International Geological Correlation 274, Coastal Programme (IGCP) Project Evolution in the Quaternary.

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