A model of coastal evolution in a transgressed thermokarst topography, Canadian Beaufort Sea

Marine GeoJagy, 106 ( i 992) 251-278 Elsevier Science Publishers B.V., Amsterdam

251

A model of coastal evolution in a transgressed thermokarst topography, Canadian Beaufort Sea Marie-H616ne Ruz a, A r n a u d H6quette a and Philip R. Hill b aCentre d'F.tudes Nordiques, Universitb Laval, Ste-Foy, Qubbec, Qu6. GI K 7P4, Canada 2Dbpartement d'Ocbanographie, Universit6 du Qubbec ~t Rimouski 310, all6e des Ursulines, Rimouski, Qub. G5L 3A l, Canada (Received June 7, 1991, revision accepted November 20, 1991)

ABSTRACT Ruz, M.-H., H6quette, A. and Hill, P.R., 1992. A model of coastal evolution in a transgressed thermokarst topography, Canadian Beaufort Sea. Mar. Geol., 106: 251-278. The coast of the Southern Canadian Beaufort Sea consists of bluffs of ice-bearing Quaternary sediments, and of spits and barrier islands enclosing (more or less completely) lagoons and complex embavments formed by the breaching of thaw lakes. The study of the evolution of this coast developed in a thermokarst topef, raphy and of the shallow stratigraphy of the inner continental shelf has resulted in the development of a conceptual model of coastal evolution. Five stages have been defined in this model which form an idealized continuum of coastal evolution. According to the model, the formation of embayments, spits and barrier islands is primarily controlled by the pre-existing topography, relative sealevei rise and local sediment supply. Sealevel rise over a topography of thermokarst basins formed in ice-bearing sediments leads to the formation of embayments and rapidly eroding headlands from which spits develop. Geomorphic evidence suggests that barrier islands mainly form by spit breaching or by erosion of tundra islands. Breached-lake basins act as sediment sinks and may restrict the development of coastal accumulation features. Spits and barrier islands are rapidly retreating landward, the mean retreat rates being 2.2 m a-~ and 3.9 m a-~ respectively. On an annual time-scale, spits and barrier islands retreat in response to episodic stormgenerated overwash events, but on a geologic time-scale, relative sealevel rise is the major forcing mechanism of landward migration. While retreating landward, these coastal accumulation features may be drowned and possibly partially preserved in breached lake basins. During the Hoiocene transgression, the shoreface has undergone erosional retreat, partly eroding the pre-existing topography but preserving the bottom of some breached thaw lakes and lagoons which are now overlain by a transgressive sand sheet on the inner shelf. Our observations provide the basis of a semi-quantitative model of shoreline changes for the barrier coast of the Canadian Beaufort Sea which relates coastal stability to bluff erosion, ground-ice content, sediment texture, offshore sediment supply, wave energy, volume of breached lake basins, relative sealevel rise, and regional slope.

Introduction

Coastlines resulting from transgression over thermokarst topography occur in high latitude regions, such as the northern coastal plain of Alaska, the Canadian Beaufort Sea coast and the northern coast of the U.S.S.R. The Canadian Beaufort Sea coastal plain is an area where postglacial thermokarst processes have produced a topography of thaw lakes surrounded by low hills of Correspondence to: M.-H. Ruz, Universit6 Laval, Centre d'l~tudes Nordiques, Ste-Foy, Qu6bec, Qu6. G IK 7P4, Canada. 0025-3227/92/$05.00

perennially frozen sediments. The coastline consists mainly of unconsolidated Quaternary sediment bluffs, spits, barrier beaches and barrier islands partially enclosing lagoons, and complex embayments formed by the breaching of thermokarst lakes (H6quette and Ruz, 1991). Spits, barrier islands and barrier beaches account for more than 20% of the total length of the coastline. The Holocene rise of relative sealevei has induced coastal retreat (Hill et al., 1986), and is still greatly responsible for the present coastal geomorphology and processes. Several studies showed that the Canadian Beaufort Sea coast is

© 1992 - - Elsevier Science Publishers B.V. All rights reserved

-5,..9 '~

undergoing rapid regional retreat (Mackay, 1963a,b, 1986; McDonald and Lewis, 1973; Lewis and Forbes, 1975; Harry et al., 1983; Forbes and Frobel, 1985). Bluffs are characterized by mean retreat rates of ! to 2 m a-~, but erosion rates in excess of 10 m a-~ have been documented at several locations (Harper, 1990). In addition to rapidly retreating bluffs, most spits and barrier islands are characterized by landward migration (Forbes and Frobei, 1985; H6quette and Ruz, 1991). In the Canadian Beaufort Sea, site-specific studies of coastal evolution have been carried out (Forbes, 1981; Mackay, 1986; Dallimore, et al., 1988; Hill, 1990), and conceptual models ofcoastai processes (Harper, 1990) and of sediment dispersal in coastal areas and on the continental shelf (Harper and Penland, 1982; Fissel and Birch, 1984) have been developed. Models of coastal evolution (Wiseman et ai., 1973; Reimnitz et ai., 1988) and of barrier island development (Short, 1979) have also been proposed for the Alaskan Beaufort Sea coast. However, the Canadian Beaufort Sea coast differs from the Alaskan Beaufort Sea coast due to the longer wave fetch and to the high sediment supply provided by the Mackenzie River. By analyzing shoreline changes and stratigraphic data, we propose a conceptual model of coastal evolution in the thermokarst-affected coastlands of the southern Canadian Beaufort Sea (Fig.l), including the evolution of thermokarst lakes during transgression, the development of spits, barrier beaches and barrier islands, and the resulting shallow stratigraphy on the continental shelf.

Methods The data used in this study include aerial photographs, stratigraphic data from borehole and vibracore samples, beach profiles and highresolution seismic profiles, sediment samples from beach, bluffs and the seafloor, and side scan sonar records. Measurements of shoreline changes (advance or retreat) were made by comparing 1950, 1972, and 1985 vertical aerial photographs, at scales varying from approximai,-ly I : 40 000 to I : 60 000. On two comparative photc.graphs, a known inland point was selected, and distances were measured to the

M.-H. R U Z ET AL.

waterline. The measurements were used to calculate annual retreat or advance rates of spits and barrier islands. Given the fact that tides are about 30 cm and that many shoreline retreat measurements were in the order of 100 to 200 m between 1950 and 1985, the possible error induced by tidal level variations is believed to be negligible in most areas. The error due to the measurement technique and to photographic distortion was estimated at + 5 % by comparing equivalent geographic distances on photograph sets of different years. High resolution seismic reflection profiles were collected on the inner shelf, seaward of the Tuktoyaktuk Peninsula, using an ORE Boomer system (300 to 3000 Hz band pass). A 7.0 kHz sub-bottom profiler system was also used in the same area and along the Yukon coast. Core samples were obtained using rotary drilling and vibracore methods.

Setting The Canadian Beaufort Sea coast consists of low-relief (< 60 m) unconsolidated Quaternary sediments. The Yukon coastal plain, west of ~he Mackenzie Delta (Fig.I), is covered by Wisconsinan and older glacial and marine deposits overlain in places by lacustrine and colluviai materials of Holocene age (Rampton, i aQ'~.,,,.p.Rio'bards, ....... Island (Fig.I) forms an upland area between two of the major distr'ibutary channels of the Mackenzie River. The surficiai geology of the island consists mainly of Wisconsinan glaciogenic deposits covered in places by Holocene lacustrine sediments (Rampton, 1988a). The Tuktoyaktuk Peninsula (Fig. 1) is p,i~cipally composed of Pleistocene sands overlain by a variable thickness of glaciogenic sediments of Wisconsinan age. The southwestern part of the peninsula is formed of coarse-grained ice-contact and morainal deposits overlain by lacustrine sediments of Holocene age, while the northeastern part consists of glacial outwash sands which are partially covered by eolian and lacustrine sediments. The Mackenzie River Delta is a dominant feature of the area, forming a low-lying plain between Mackenzie Bay and Kugmallit Bay. The coastline of the Mackenzie Delta is extremely low (1-1.5

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY.CANADA 140 °

7-53

136"

132"

i

i

!

/ -r'~(3~k'~\(~

_

71 o

•

- .."

~.~

,

CAPE B A I L L IE ~ . BAI"HURST

-~ ..

50Ore-

.

.P

RUSSEL

- . -

"

."---

"'

, .....

" '

-.

""-'"

i

"

,

.,

,' •~("~" " '

'.

,

,; "'-

"'"

,"

,',-

•

,'PELLY.

....

.~

B Y ~..... . " ' . ? ; . . ' . .

i. ~ "

..'.

. ..:

t'

,

"

-'.'.:

."'BAY

~

.....

~'"

".

:"...:'"

.'""

.~-.

"', .

".'.." '-"

....

.

.

~" " :.."

..::

• .':

:

136 °

".

"

"

"

.'"

•l

.'. : • "-" X3 ..~x~.~

/

. " .

.

.'" "

•

"" ." "

• _

',

":"~"~".X

McKINLEY-" " t" " CAPE " , ' ~" ':'.":"' : k B/~Y "" I ~D~.LHOUSI~':'.'...~

BAY..-.,,

' '

"'.

' --'",

,

-..%,

140 °

IS L A ND~.-'~

INLET .... " ' ' ,

lOOm

I

/" /-'~

- - -, "-.

,urn

~ - - - - - ~,

71"

..

m

"

i

.

,o

IOOC

- -.

128"

132 °

.

ii'"

:::

160"

140*

120"

Fig.l. Location m a p of the C a n a d i a n Beaufort Sea coast.

m) with delta channels, tidal flats and coastal wetlands. West of the Delta, the shoreline consists mainly of bluffs composed of ice-rich sediments, up to 50 m high. The Tuktoyaktuk Peninsula coast, east of the Delta, is formed of low sandy bluffs (generally < l0 m high) and of spits and barrier islands less than 2 m high (Table 1). Although some beaches, spits and barrier islands along the Yukon coast, on Richards Island and in Kugmailit Bay, are composed of coarse sand and gravel, most coastal landforms consist of wellsorted fine to medium sand (Lewis and Forbes, 1975; Forbes and Frobel, 1985). The frost table lies at shallow depth beneath the surface of these coastal accumulation landforms. Vibracoring showed that the depth of ice-bonded sediments ranged from 1.0 to 1.6 m below beach surface in the summer. The virtual absence of prograded beach ridges is noteworthy in the study area. Well developed spits and barrier islands usually face the northwest

or the north and are more exposed to high energy waves than smaller accumulation features located in more sheltered areas (Table !). Coastal morphology, and especially the configuration of spits, shows that net longshore sediment transport is generally oriented eastward along the coast, although lo~.al divergence in the direction of the littoral drif~ exist (Pelletier, 1975; Forbes and Frobel, 1985). Numerical modelling of longshore sediment transport obtained for six coastal sites (Pinchin eta;., 1985) confirm the geomorphological evidence, showing that potential sediment transport is directed eastward along the southern Canadian Beaufort Sea. The Canadian Beaufort Sea is covered by sea ice during eight or nine months of the year (Harper and Penland, 1982). During the open water season, from June to early October, the most powerful waves originate from the west and northwest in response to storm winds. Even during the ice-free period, wave energy is restricted by the presence

254

M.-H. RUZ ET AL.

TABLE I Geomorphic characteristics, exposure to incoming deep-water wave power, and mean retreat rate, between 1950 and 1985 for spits and barrier islands of the southern Canadian Beaufort Sea (see Figs.I and 4 for location of spits and barrier islands). The wave power data are derived from a wave hindcast model (Pinchin et al., 1985) based on 14 years of wind records at Tuktoyaktuk. The dimensions of the coastal accur, miation features are based on measurements on 1985 aerial photographs except where otherwise specified aMean retreat rate (m a - ~)

Incident deep-water wave power (kW m - ~)

- 10.60

!.70

! 20

- 7.90

2.22

100-220

- 2.85

1.80

- 3.80 -5.0 - 2.40 - 2.20 - !.60

2.06 3.15 2.0 2.50 2.24

-

3.17 3.34 1.58 4.32 3.34 2.73 3.12 4.92 4.92 2.46 5.05 4.64 4.47 5.25 4.60 5.25 5.16 5.25 3.20

Name of coastal accumulation feature Length (m)

Width (m)

Pelly Island Barrier Island

2500

80-300

2000 2000 (1972) 2600 (1985) 4550 1300 850 ! 650 470

Crest elevation (m)

(1950-1972) Pelly Island West Spit (1972-1985) Pelly Island East Spit 0972-1975) Pelly Island East Barrier Island Garry Island Barrier Island Tuktoyaktuk Barrier Island Tuktoyaktuk Spit West Tuktoyaktuk Spit Topkak Spit Tibjak Spit Mingnuk Spit Kukjuktuk Bay Barrier Island Tuft Pt. Spit Warren Pt. West Spit Warren Pt. East Spit Bols Pt. Spit West Atkinson Barrier Island Drift Pt. Spit Atkinson Barrier Island Atkinson Pt. South Spit Atkison Pt. North Spit East McKinley Bay North Spit East McKinley Bay South Spit East McKinley Bay Barrier Island South Phillip~ Island North Spit South Phillips Island South Spit Cape Dalhousie Barrier Island

2250 ! 560 ! 650 1850 ! ! 50 4800 5800 2270 10500 950 4420 2280 4900 920 3570 4000 3900 3700 7620

100-250 160 75 60 75

i.4

100 100 100 150 ! 80 300 500 160 250 185 160 100-150 350-500 150 130 160 330 180 250

!.5-3.0 !.0-2.0 0.5-1.0 !.5 I. l !.5 1.0 !.0 0.9

1.0 !.0

<

1.0

!.0 0.7 0.8 !.0 < 1.0 < !.0 1.0

i.0 0.4

i.10 1.0 2.20 3.50 1.90 1.75 1.0 !.20 3.30 - i.90 - 1.75 - 1.65 - 1.95 - 2.0 -2.15 - !.85 - 2.80 - 1.90 - 3.15

"Mean retreat rate of spits and barrier islands = 2.80 maMean retreat rate of spits = 2.20 maMean retreat rate of barrier islands = 3.90 m a -

of the pack ice offshore. Nearly 80% of deepwater waves are less than I m in height (Harper and Penland, 1982) and consequently, the Beaufort Sea can be considered as a moderate to low waveenergy environment. The Canadian Beaufort Sea is a microtidal environment, the mean tide ranging from 0.3 m to 0.5 m. In addition to normal tidal level fluctuations, the coastline is subject to positive storm surges up to 2.4 m above mean sealevel (Harper et al., i988). According to Hayes' (1979)

classification of shorelines as a function of tidal and wave regime, the Canadian Beaufort Sea coast is wave-dominated during the ice-free season. Seaice may play a significant role on coastal and nearshore sedimentary processes during freeze-up and break-up through the action of various mechanisms such as ice-scouring, ice-push or ice-rafting (Kovacs and Sodhi, 1980; Barnes, 1982; Reimnitz and Barnes, 1987; Reimnitz et al., 1990; Kempema et al., 1989; H6quette and Barnes, 1990) but the

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY, CANAP.A

255

relative importance of such processes on coa;tal sediment dynamics in the study area has net bcen precisely defined yet. In the Beaufort Sea, sealevel has risen approximately 70 m from a late Wisconsinan Iowstand prior to 12 ka B.P. (Hill et al., 1985). During the early and middle Holocene, the rate of relative sealevel rise was rapid (up to 14 m ka-~), but this rate decreased to less than I m ka-~ during the last 2000 years (Fig.2) as radiocarbon dates from the Yukon coast, the Tuktoyaktuk Peninsula and from archeological sites on Hershel Island (Fig.l) suggest (Hill et al., 1990). Tide-gauge data from Tuktoyaktuk saggest that relative mean sealevel is still rising. Although rates as high as 10 mm ahave been reported on the basis of a subset of annual mean water level data (1962-1978) from Tuktoyaktuk habour (Forbes and Frobel, 1985; Harper et al., 1985), least-squares regression analysis of monthly mean water levels from 1952 to 1980 gives a rate of relative sealevel rise of 2 mm a -t (D.L. Forbes, pers. commun., 1991). The ubiquitous shoreline recession and the fact that the Mackenzie Delta shows an aggrading rather than a prograding morphology (Lewis, 1985) are also evidence of contemporary sealevei rise. As a result of the postglacial transgression, the

coastal plain has been submerged, forming a broad shelf of gentle inclination (1:2000 in the eastern Beaufort Sea, and > 1 : 1200 seaward of the Yukon coast). Mud dominates the central and outer shelf while inshore of the 10 m isobath sand is generally abundant (Pelletier, 1975; Vilks et al., 1979). Subseabed permafrost underlies most of the Beaufort Shelf. It is relict from periods of lower sealevel, during the late Pleistocene, when the seabed was exposed to subaerial arctic conditions (Mackay, 1972).

Thermokarst topography On the Canadian Beaufort Sea coastal plain, the thermokarst topography consists of thaw lake basins and of low reliefs (10-20 rn) in which massive ice and icy sediments may be found. Cold climatic conditions during the Pleistocene led to the formation of continuous permafrost throughout the region to depths of several hundred metres (Judge, 1986), although taliks of unfrozen material occur under deep lakes and major river channels (Mackay, 1963a; Smith, 1976). Much of the topography in the area can be attributed to the presence or absence of subsurface ground ice (Rampton, 1988a). Ground ice is found throughout the region I

-10-

--20 °

E -3~

v

-1I-n LLI-40"

13

--50"

-60-

-70 10

9

8

.7

6

5

4

3

2

AGE (10 3 years B.P.) Fig.2. Holocene relative sealevel curve for the Canadian Beaufort Sea (after Hill et ai., 1990)

1

C)

256

either in the form of pore ice within the sediment or as ice bodies of various dimensions (Rampton and Mackay, 1971). Much of the ground ice is of segregation origin which formed during permafrost aggradation (Mackay, 1971). According to Rampton (1974), permafrost aggradation and ground ice formation proceeded rapidly once deglaciation started. Subglacial meltwater, incorporated into aggrading permafrost, resulted in an ice-cored topography. This topography is typical of Richards Island and of the southwestern part of Tuktoyaktuk Peninsula. Thermokarst, which is the process of ground ice melting and the accompanying collapse of the ground surface to form depressions, has been an active process during the Holocene and the latter part of the Late Wisconsinan. Radiocarbon dates in the area indicate that thermokarst activity started as early as 12.9 ka B.P. (Rampton, 1988a). Thermokarst activity was extremely intense between 10 and 9 ka B.P., probabiy in response to a climatic warming (Rampton, 1988b). Thermokarst appears to have remained active between 8.5 and 4 ka B.P., but with limited development of new thermokarst depressions. After 4 ka B.P., pollen records indicate a cooling of the climate to near present-day conditions (Ritchie and Hare, 1971); as a result, the landscape was partly stabilized (Rampton, 1988a). The initiation of the thermokarst lakes probably resulted from the development of small ponds, but the expansion of the lake basins occurred mainly through ground ice slumping. As the lakes expanded, the permafrost table degraded below the lake floor and further thermokarst subsidence may have occurred due to the melting of ice at depth. Today, thermokarst occurs where slopes are eroded by the sea, lakes or streams. Many lakes are drained and in many old basins polycyclic thermokarst has occurred. Although, thaw lakes occur throughout the area, they are particularly common on Richards Island and Tuktoyaktuk Peninsula. About 35% of the landscape is covered by lakes of various dimensions (Fig.3) and near!v 70% of the northeastern part of the Tuktoyakl:uk Peninsula consists of thaw lakes. Along this part of the Peninsula this process has been strongly affected by the wind regime

M.-H. RUZ ET AL

during summer in a manner that most of them are oriented perpendicular to the easterly winds (Mackay, 1956). Water depth can vary significantly from one lake to another, and is not usually related to the size of the lake. According to the topographic maps at ! :50 000 produced by the Canadian Army Service Establishment (1960), several lakes are very shallow (water depth < 3-4 m), but a fair number of lakes are l0 to 20 m deep while some others have water depths in excess of 30 m. In some areas, the mainland topography consists of low tundra surfaces characterized by a high density of thermokarst ponds, termed 'pitted thermokarst topography' in this paper. Model of coastal evolution

1. Formation of embayments and headlands In the southern Canadian Beaufort Sea, the Hoiocene rise in sealevel has favored rapid coastal retreat (Hill et al., 1986). Transgression over a thermokarst topography induces the drainage or the breaching of thaw lakes. The subsequent morphological changes of the coast will greatly depend on the initial depth of the lake. Some examples along the Canadian Beaufort coast show that a shallow lake (water depth < 3 m), perched above sealevel, will commonly be drained and the exposed bottom will be successively eroded by the sea. In the case of a deeper lacustrine basin, the breaching of the lake will result in the formation of a coastal embayment. Wide embayments evolve through the coalescence of breached thermokarst lakes in areas well exposed to deep-water waves. Examples of such topography include Hutchinson Bay and Russel Inlet (Fig.4). In some sheltered areas, such as east of Richards island (Fig.I), peat outcrops are found at a few metres water depth, outlining former lake contours (Hill and Frobel, 1991). A few embayments do not originate from thermokarst processes. McKinley Bay for example (Fig. 1) is believed to be an ancient meltwater channel of Wisconsinan age (Mackay, 1963a). The complexity of the shoreline depends on the local size and abundance of thaw lakes on the adjacent coastal plain. Where the landscape is covered by numerous thaw lakes, the shoreline is

257

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY, CANADA

Fig.3. Aerial photograph (A22884-197, copyright Department of Energy, Mines and Resources Canada) of the Toker Point area, Tuktoyaktuk Peninsula (see Fig.4 for location). Note the high c~,ncentrationof thermokarst lakes (35%) and the various types of coastal depositional landforms. highly indented, consisting of complex shallow embayments formed by the coalescence of breached lakes. In the Cape Dalhousie area for example (Fig.4), the coastal plain is characterized by large and numerous thaw lakes (75% of the land surface). The topography of the land between the lakes is very irregular and consists of higher relief areas and low tundra. In this area, the rising sealevel has caused the formation of many embayments separated by narrow headlands, and of small islands by flooding low tundra surfaces. The formation of embayments leads to the development of narrow peninsulas and eroding headlands of unconsolidated sediments. In addition to wave-induced erosion, thermal erosion promotes

rapid bluff retreat along the headlands (Mackay, 1986; Dallimore, 1988). When ground ice occurs in coastal bluffs, erosional processes such as block slumping, block failure, mud slumps, and retrogressive thaw failure are important, depending on the type and proportion of ground ice and on the textural composition of the bluff sediments (H6quette and Barnes, 1990).

2. Development of spits and barrier beaches The ubiquitous coastal erosion in the area favors the development of accretionary landforms by supplying sediment to the coastal zone. In newly formed embayments, a spit will usually develop

258

M.-H.

133'00

'70'00

132=00 i

1) Topkak Point 2) l"ibjak Point 3) Mingnuk Spit 4) Kukjukluk Barrier S) Tuft Point 6) Bols Point 7) West Atkinson Barrier 8) Atkinson Point Barrier 9) East McKinley Bay Barrier 10) East McKinley Bay South Spit 11) East McKinley Bay North Spit 12) South Phillips Island Spits

SEA

~ Phillips Is ~<.s,

"~1

RUSSELL q INLET d

fi

McKINLEY ~:"','. " : " BAY.11~..",.

"

" . ". "" ". ,.. • ...:..- • . . e

•

Otift P t ~ ' ! ~ i ' "

. .. . . .

BAY .1• °

Warren Pt

°

e

•

•

u

• " •

:... :" i" " " •

°

°

•"

•

°

o

•

KUGMALt ~T BAY

e

....-.L.~

-..~...

Toker Pt Fig.,8 3

ET AL.

13o-'ooCape Dalhousie

131°00

BEAUFORT

RUZ

2

::."

.

.

•

°

Q

o

°

•

Tukto r

Fig.

e

Q

5 • •

20 I

'o•i -I1~

km

Fig.4. Ea~*tern Beaufort Sea coast with location of spits and barrier islands•

from an updrift headland (Fig.3). The actual volume of clastic material available for the beach will depend on the percentage of sediments coarse enough to remain in the littoral system as well as the proportion of ground ice present within the bluff. The dimensions, shape and sediment texture of the accreting spits will be controlled by the availability of material in the adjacent bluffs, the grain-size of such sediments, and the local inshore wave climate. As a result, several typical spit and barrier beach morphologies are found in the area. A few examples will be described below. Mackay (1986) measured coastal erosion at a site on the south shore of Kugmallit Bay (Fig.4) where bluffs are mainly composed of diamicton containing interstitial and massive ground ice (F':g.5). On a 1935 oblique air photograph, two lakes of 500 to 1000 m in diameter were located at distances of about l l0 and 170 m from the shoreline respectively. From 1935 to 1950, the coastline retreated between 80 and i60 m. By 1950, the retreat had induced the breaching and flooding of the eastern lake, while the western lake had been drained• Dur-

ing the following years, the shoreline continued to retreat rapidly, at rates as high as i0 m a-~. By 1973, a sand and gravel spit, 300 m long, had formed across the small breached lake to the east. The relatively coarse character of the spit sediment is primarily controlled by the adjacent diamicton source. Because the head of Kugmallit Bay is a low wave-energy environment (Table l), waves at the coast induce only local longshore transport from the eroding headland to the spit. Spits located at the entrance of small embayments tend to close off former lakes. An example is provided by the development of the King Point barrier beach on the Yukon coast, a 50 to 250 m wide barrier with crestal elevation reaching 1.5 m above mean sealevel (Fig.6). This 1.6 km long sand and gravel barrier beach encloses a semicircular shallow lagoon formed by the breaching of a former thaw lake (Hill, 1990). Aerial photographs show that a spit extended approximately two-thirds of the way across the embayment in 1954, leaving a 400 m wide inlet to the south• Supplied with sediments eroded from the cliffs to the north, the

259

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY, CANADA

KUGMALLIT BA Y

j'~

o

~

I

500 i

i

i

o

T N

I

LOW BLUFF (--'5m)

ORIGINAL LAKE CONTOUR DRAINED BREACHED THERMOKARST LAKE (SUBAERIALLY EXPOSED LAKE BOTTOM)

WITH LITTLE MASSIVE ICECTON ~

ICY DIAMICTON OVERLAIN BY PEATWITH HIGH ICE CONTENT (PRESENCE OF NUMEROUS ICE WEDGES) BEACH SAND AND GRAVEL

Fig.5. Evolution of the coastline between 1935and 1973at one site on the southern shore of KugmallitBay, west of Tuktoyaktuk (see Fig.4 for location) [after Mackay, 1986, aerial photographs A 23477-12 and A 23477-13 (copyright Department of Energy, Mines and Resources Canada), and field investigations].Only the largest lakes are shown. spit had grown to completely enclose the lagoon some time before 1970 (Fig.6). At King Point, the closure of the lagoon may have been favoured by the seepage of inland water through the beach. Seepage processes occur in coarse-grained barriers as coarse sediments are highly permeable (Carter et al., 1984), although along the Beaufort coast icebonding at shallow depth may limit seepage. This form of lagoon discharge through the barrier limits the need for inlet formation and therefore contributes to a complete closure of the embayment by the barrier. Although, some embayments in the study area are closed by sandy barriers, in most cases the hydraulic pressure of tides and runoff from the lagoon combined with storm-induced overwash generally keep inlets open. In wide embayments, long spits extend for several kilometres from eroding headlands but do not completely close the embayments. At Atkinson

Point, two spits have developed from either side of a prominent headland cut into a 500 m long sandy bluff up to 2 m high. To the northeast, a broad, 5 km long sand spit extends into McKinley Bay (Fig.7). This wide (350-500 m) spit is characterized by washover flats. West of Atkinson Point a narrow (100 m wide) extends to the southwest for 2 km. Both spits are very low coastal features (Fig.7), consisting of well-sorted medium to fine sand and are extensively overwashed during storm events. The morphological and sedimentological characters of the spits at Atkinson Point are typical of most of the spits in the area.

3. Barrier island formation Barrier islands in the study area are low ( < !.5 m) and elongated in shape which is typical in a micro-tidal environment (Hayes, 1979). Several

260

M.-H. RUZ ET AL.

(m)

:! ~CINGPOINT

O m

BEAUFORS TEA

137*58'W

Fig.6. Evolution of the King Point barrier between 1954 and 1970 (based on aerial photographs), and beach profiles surveyed in 1985 (after Hill et al., 1986). MSL--mean sea level.

potential mechanisms can explain their formation. Geomorphic evidence indicates that the principal mode of formation of barrier islands in the study area results from the breaching of spits connected to the mainland or to small islands. Mingnuk Spit, on the coast of the Tuktoyaktuk Peninsula, was a low sandy spit attached to an ice-rich bluff which retreated at a rate of 2.1 m a -1 between 1950 and 1985 (Fig.8). During that period, the spit extended more than 300 m southeastward across a small breached lake. Field observations in 1988 and 1990 revealed that the spit was breached in its proximal part and the distal section transformed in a small barrier island (Fig.9) as a response to storm events and/or because sediment supply was not sufficient to maintain the dynamic equilibrium of the spit. Although there are no direct observations to show that larger-scale barrier islands form in a similar way, the distribution and location of barrier islands strongly suggest that most of them have developed from spits. A 4.4 km long barrier island

occurs southwest of Atkinson Point (Fig.7). There is evidence suggesting that the formation of the barrier island could have resulted from the breaching of the western spit. The barrier island and the spit have similar morphological characteristics: crest elevation (0.7 to 1.0 m), width (100 to 160 m) and grain-size (fine sand). Therefore, it seems reasonable to suggest that this barrier island formed when an inlet breached the original spit. In the Canadian Beaufort Sea barrier islands also form from erosion of tundra islands. At the tip of the Tuktoyaktuk Peninsula, the Cape Dalhousie barrier island system is a 7.5 km long barrier system facing the NNW (Fig. 10). Air photographs reveal that in 1950 the barrier system was formed of spits attached to small remnants of the eroded mainland transformed into islands. Between 1950 and 1985, the system migrated landward at a rate up to 4.5 m a - l ; the small islands were partly eroded and the spits merged into a continuous barrier island complex. Kukjuktuk barrier island (Fig.3) is another example of this type of barrier island formed by the longshore development of spits linked to a tundra island. Along the Tuktoyaktuk Peninsula, the West Atkinson barrier island is a 10.5 km long sandy barrier (Fig.l !) isolating a shallow lagoon less than 2 m deep. This barrier island seems anomalous compared to the other barrier islands in the area as it represents the only example in the study area showing no evidence of a former headland or tundra island from which it could have been developed. Because the mainland behind the barrier is an area of pitted thermokarst topography lying at less than 0.5 m above sealevel along the lagoon shoreline, mainland beach detachment cannot be categorically ruled out as an alternate mode of formation. The mainland beach detachment mechanism implies a rising sealevel partially submerging a beach ridge and inducing the formation of a lagoon by flooding a flat area behind the ridge (Hoyt, 1967). It is also possible that the formation of this barrier island resulted from a combiaation of mainland beach detachment and coastwise spit progradation, the littoral drift along this section of the coast leading to the formation of a long continuous barrier. In the study area, the barrier islands lie on a shallow platform substructure. East of Pelly Island,

261

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY. CANADA

~ 4

N

B

B'

,01

~

00

___

" ./7---

-1.0 /

'

05 0 '

.....

'

......

100 (m)

,o,,

..,. , ,

1990

~//~-,

,,

Atkinson Point. Core A2-87~.':"

A

McKINLEY BAY

A'

~.°I

"-I.o i o

so

/

lOOtin)

A'

o

A

I

Fig.7. The morphology of spits and barrier island at Atkinson Point, Tuktoyaktuk Peninsula (see Fig.4 for location). (l) Low tundra: (2) Pitted thermokarst topography: (3) Drained thaw lake: (4) Coastal marsh developed in breached thaw lake: (5) Spit, barrier island: (6) Foreshore flat. Datum of beach profiles is mean sea level.

for example, a 4 km long, narrow, sandy barrier island anchored to two mainland remnants rests on a very shallow platform of l to 2 m water depth which extends landward behind the barrier (Fig.12). The importance of a platform substructure for the development of barrier islands has been emphasized by several authors (Oertel, 1985; Otvos, 1985). The correspondance of barrier islands with shallow platforms in the southern Canadian Beaufort Sea suggests that the occurence of a platform substructure is one of the conditions required for barrier island formation.

4. Barrier and spit evolution Spits and barrier islands are experiencing landward migration at various rates (Table l).

Between 1950 and 1985, the mean recession rates were 2.2 m a-1 and 3.9 m a-1 respectively which are high rates compared to mid-latitude coastlines, given the short (3 to 4 months) ice-free season. At a short time-scale (event and annual timescales), overwashing represents the m~.ior mechanism of landward migration by transferring nearshore and beach sediments to the backbarrier region durin:~ storm events. This process is exemplified at Atki,,:son Point (Fig.7), where beach surveying of the northeastern spit in 1984 and 1990 showed that the beachface of the spit retreated over more than 20 m between these two dates while a progradation of 16 m occurred in the lagoon due to overwash sedimentation. Tidalinlet sedimentation plays a minor role in the landward retreat of the coastal accumulation

"s "~ ~6~-

M.-H. RUZ ET AL.

132° 5G'

~ '~

~

SPIT IN 1985 SPIT IN 1950 CLIFF RETREAT BETWEEN 1950 AND 1985

~.

|

~..'-""~ -

'

".'~i~ -

Locationof

690 39'

: ':

i :iii:.:-. Fig.8. Evolution of Mingnuk spit from 1950 to 1990. based on aerial photographs and field investigations (see Fig.4 for location).

features of the Canadian Beaufort Sea as floodtidal deltas are poorly developed. As shown by Bruun (1962), sealevel rise is a major cause of shore erosion. Along barrier coasts where overwashing is an important process, like in the Canadian Beaufort Sea, it has been shown that the rate of landward migration (6x/ft) is related to the rate of sealevel rise (6s/6t) divided by the transgression slope (tan fl,) defined as a slope extending from the middle shoreface, under the barrier, and into the lagoon (Inman and Dolan, 1989), i.e.

~x ~.

o~

6s/6t ,.

tan Pt

(I)

which is a modification of the Bruun's rule equation. In the study area, calculations of eqn. (i) have been carried out based on measurements of transgression profiles from hydrographic charts, and assuming a rate of relative sealevel rise of 2 mm a-~ (based on monthly mean water level data of Tuktoyaktuk tide-gauge from 1952 to

1980). Theoretical retreat rates are in agreement with measured barrier retreat rates at some sites, but in many cases they do not correspond to measured retreat rates. At Atkinson Point for example, calculations for profiles extended to depths of i0 m give shoreline recession ranging from 0.75 to 0.90 m a-~ although the retreat rates of the spits range from 1.65 to !.95 m a -~. Therefore, barrier migration in the southern Canadian Beaufort Sea cannot be explained in its entirety by applying the twa-dimensonal model for seaievel rise presented in eqn. (l) because most parts of the coast involve considerable longshore sediment transport in addition to shore-normal movement. In the southeastern Canadian Beaufort Sea, the rate of onshore migration of the spits and barrier islands is partly controlled by potential longshore sediment supply (Fig.13) (H6quette and Ruz, 1991). It can be demonstrated that a lack of sediment supply results in the rapid migration of barrier islands. At the northern extremity of Pelly Island, for example, a 2.5 km long barrier island was just detached from a small tundra island in 1950 (Fig.14). After 1950, the small island was completely eroded and between 1950 and 1972, the barrier island migrated landward at a mean rate of 10.6 m a -~ As the bluffs on the main island were retreating at a slower rate, the barrier island became attached to a headland, forming a double-spit some time before 1972. Subsequently, the spits increased in length but retreated at a much slower rate. This example illustrates dramatically the importance of longshore sediment supply on the rate of landward migration of spits and barrier islands. Generally, however, barrier islands in the Canadian Beaufort Sea do not weld to the mainland shore because the lagoon coastline retreats at a similar or higher rate, resulting in the landward migration of the complete barrier island-lagoon system. The mainland shore of the backbarrier lagoon of the west Atkinson barrier island (Fig. 11) has retreated rapidly as a result of the submergence of a low and flat coastal tundra plain. Because the mainland shoreline retreated at a higher rate than the barrier, the lagoon width increased by more than 50 m from 1950 to 1985. This mechanism

COASTALEVOLUTIONIN A THERMOKASTTOPOGRAPHY.CANADA

263

Fig.9. Oblique aerial photograph of Mingnuk barrier island, Tuktoyaktuk Peninsula (August 1988). The arrow shows the inlet formed in 1988.

contributing to strand the barrier island further offshore is similar to the process of "'backing tundra erosion" (Short, 1979, p. 100) described for some of the barrier islands of the Alaskan Beaufort coast. Despite rapid landward migration, the morphology of the barrier islands appears to be stable over time. On a map drawn in 1826 by Captain John Franklin (Franklin, 1828), the West Atkinson barrier island is present (although the scale of the map is not suitable for accurate measurement of the barrier dimensions and position). The width of this barrier island is similar on 1950 and 1985 air photographs, suggesting a fairly constant sediment volume despite a retreat rate in excess of 3 m a -t. In general, landward-retreating barrier islands lose sediment to the offshore through foreshore and shoreface erosion during storms (Maurmeyer, 1978; Niedoroda et al., 1985), and should therefore decrease in volume over time unless additional sediment is supplied alongshore. Because there is no headland acting as a direct longshore sediment source nor river sediment supply, some onshore-directed sediment transport

seems to be necessary for explaining the relative maintenance of this barrier island. Previous studies have shown that sand is abundant in the nearshore zone in this area (Vilks et al., 1979; H6quette and Hill, 1989). In addition to wave-induced onshore sediment transport, sea-ice processes may also contribute offshore sediment to the littoral system as it has been shown further west along the Alaskan Beaufort coast that onshore directed ice-push processes can supply sediment to beaches and barrier islands (Hume and Schalk, 1964; Reimnitz et al., 1990). At a longer time-scale, however (i.e. order of centuries or longer), relative sealevel rise is the major process forcing the landward retreat of spits and barrier islands. R!sing sealevel conditions result in more frequent storm overwashing which induces an almost continuous onshore barrier migration. Freshwater peat outcrops on the foreshore of numerous beaches and barriers in the region are evidence of long-term shoreward migration of coastal landfotms. At Atkinson Point (Fig.7), near the proximal part of the eastern spit, a freshwater peat exposed on the foreshore near

264

M.-H.

RUZ

ET

AL.

LOW TUNDRA

T

CAPE

DALHOUSIE 7"J

DRAINEDTHAW LAKE

N -~"

.

..

-.

:~.,z~.'~.-..->.-

,. J !

"

COASTAL MARSH

v <-~e ~ l . ~ . FORESHORE LIMIT

o~°

BARRIER ISLAND IN 1985

BARRIER ISLAND IN 1950

:

,'

." t : /

/,d~'

," /

,"/:,-;:-.~1 :

"'"

,' i~--'~.] !

."--.

t

"'4 .,:

I t

Utgcff_

,~,1

/

•

¢

~ ..,, s ~ql~

,; F , ' :

! ,,

,-

~,

""

/'

,~,

• .

:.

-,'' -3'

-','-~; ""

LIVERPOOL BAY

"'~

' ~.V'.,,~.'.';',~I

.,' i

,"~

,,-,~

(

: e'

'

i

C

.

.

"."

"':' v: - , ,

," .... :.,

',ao' .

.

I.. - \ ;

s

--

,

~.,o

,','

1

,..J

';;

-'~7.: , ~ ' 1 , ,.r~.1 #, "" ~''. . . . . i ', ,-. ~ - _ t - . ,~ , ~ . .'777,',..-'£2~_ ~ X t

, ), ,,-,~.~ ,

\\

.

I -"" i - 1 - #'-;

.,'; , ,.~2;- - , ~.~- ~I

", x ~ , . , -

6.L~LO2".~T

",,

[ t

,,,,

,;

~ " - ~ ~'4.,"~~

~

'

'

'

Fig.10. Landward migration of Cape Dalhousie barrier island system between 1950 and 1985, from aerial photographs (see Fig.4 for location).

low-water level has been dated at 2950_+70 yrs B.P. (Beta-28281). A core, collected from the edge of the lagoon about 500 m further landward (Core A2-87, Fig. 15) consists of freshwater peat, overlain by organic-rich silty sands containing algae and dinoflagellates. This flora corresponds to a brackish environment with a slight freshwater influence, interpreted as a shallow backbarrier lagoon (H6quette and Ruz, 1991). A radiocarbon date in this unit occurring about 50 cm below present

mean sealevel (Fig.15) yielded an age of 1280_+70 yrs B.P. (Beta-28282). These brackish sediments are overlain by modern backbarrier sand. The basal section of the overlying sand deposit is characterized by high-angle landwarddipping prograding beds which are interpreted as foresets deposited at the edge of a washover fan. Our interpretation of the depositional facies suggests that a slow sealevel rise induced barrierspit migration by overwash processes, which

265

COASTAL E V O L U T I O N IN A T H E R M O K A S T TOPOGRAPHY. C A N A D A

~

SANDYACCUMULATIONFEATURES (BARRIERISLANDS,BEACHES)

~.,// /~../

//

sw.o..s,.o,

FORESHOREFLATSWITHTRANSVERSEBARS

J..'.~

H COASTALMARSHDEVELOPEDIN BREACHED

LAKE

r - ~ ..n-,~o,.~-o.,,s.o~oo,,,~.,. ~ (TUNDRAWITH HIGHDENSITYOF

~_1 PIN~O

~/

~/

DRA,,,,EOTHER.O,=RST LAKE

....

///

S~i~]

/

~

i

t~

,~y

R ISLAND ,,,?~ / ~ R I E..............

.

.,~NN ~

.,.;~-

oo

"

•

•

O0o,

.

F.:I

u

1

¢

i~5~o ; Do ; o%; ~

. -~-- ~.~.

•~

•

~ ,i~ ~

•

~

~

.-

~

~ ....

Fig.l 1. Vertical air photograph and interpretation of sedimentary environments in the area of the West Atkinson barrier island, Tuktoyaktul< Peninsula (scc l~ig,4 for location).

266

M.-H. RUZ ET AL.

BARRIER 3.5.

BARRIER PLATFORM

1,5.

0J SHOREFACE

•

d,./c:r

Fig.12. Computer-generated three-dimensional plot and aerial photograph (A12857-409, copyright Department of Energy, Mines and Resources Canada) of the barrier island and platform east of Pelly Island.

5 p

a

~4

I

i IU

a

nu

i

i

B B

I

Barrier Islands

Spits

m

nil

Uui

&t

dMi & 0

I

I

I

!

I

!

I

I

0

1

2

3

4

5

6

7

POTENTIAL LONGSHORE SEDIMENT SUPPLY (103 m 3 a -1)

Fig, 13. Relationship between the landward migration rates of the spits and barrier islands of the southeastern Canadian Beaufort Sea and the rates of potential longshore sediment supply (based on data from H~quette and Ruz, 1991).

resulted in the burial of brackish backbarrier sediments. There are some examples, however, where it seems that the barrier migration could not keep pace with the rising sealevel. One kilometre seaward of Atkinson Point (Fig.4), high resolution seismic records have revealed that Recent sediments form a long linear sediment body at the seabed. In about 5 m water depth, this feature shows positive and negative relief, lies parallel to the major spit extending northeastward into McKinley Bay, and is up to 5 m thick while the average thickness of Holocene sediments at similar water depths in the area is about l m (H6quette and Hill, 1989). For this reason, we suggest that this linear feature may represent a partially preserved barrier-lagoon complex that was overstepped when the surf zone skipped from the front

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY, CANADA

267

3~°

34°

30°

BEAU'FORT SA E ~ 7 1 O

COASTALACCUMULATIONFEATURES 1950 1972 s . ~ . :.~.-.-~ •~.~

i~oo

BLUFFRETREAT

..- ~ ; . ~ . ~ "

~_~

[<~

~.=-"

1950-1972

.X<~i

•

•

•

.

~

,

~

,~, ~

..~,~.'~. =--

,

]

~

~ %

~

-'~'-'.,~ ",,'~'~i~,~"~,-~

f:::..

o,'~o 'p~"

,,, oo ~

Fig. 14• Evolution of the northern shore of Pelly island between 1950 and 1985 based on aerial photographs• MUD Ssaward

Facies I ~ r l p l l o n

Inlerpretatlon

Massive well-sorted (eolian?)sand

Backshore (eolian) flat

Low-angleplanar cross-bedded sand

Backshoro

Sub-horizontal plane beds

Washoverf~t

High-anglelandward dipping cross-beds

(foressts)

of the foundering barrier to the mainland spit behind. The in-place drowning of the barrier could have been caused by a decreasing sediment supply and/or by an acceleration in the rate of sealevel rise as proposed by Rampino and Sanders (1980) in their model of barrier overstepping. Another example of a possible overstepped barrier occurs seaward of Richards Island (Fig. 1), where a mediumto coarse-grained sand facies containing shell fragments of Holocene age overlies Pleistocene sands and is overlain by recent marine silt and clay. This shell-bearing sand facies has been interpreted as a barrier deposit buried beneath marine sediments during transgression (Hill and Nadeau, 1989).

Organic-rich silty sand

lagoon

Backbanier

5. Resulting inner shelf transgressive stratigraphy

Freshwaterpeat

Freshwaterbog

SAND

' C S 'VF F M' . . . . . .

MeBn Soa.Leve

Weshover fan

Fig.15. Lithological log, facies description and environmental interpretation of core A2-87, collected near the edge of the lagoon at Atkinson Point (see Fig.7 for location).

Over most of the Canadian Beaufort shelf, a sequence of several hundred metres of Pleistocene sediments is overlain by a variable thickness of Holocene marine deposits (Blasco et al., 1990). A well defined unconformity separates the recent

268

sediments from the underlying deposits. This surface is identified in high-resolution seismic profiles by a regional reflector and is clearly erosional because it truncates the underlying strata in some places. This contact represents the erosion surface produced by shoreface retreat as a result of Holocene seaievel rise. Over the eastern Beaufort Shelf, seismic profiles show that this unconformity is overlain by a seaward-thickening wedge of Recent sediment. This sediment reaches a thickness of about 4 m in water depths of 20 m. Vibracores collected seaward of the Tuktoyaktuk Peninsula coast reveal that this depositional sequence consists of fine to coarse sand interbedded with silty sand or silt laminae (H6quette and Hill, 1989). As water depth increases, the sands are overlain by a veneer of soft marine silty clays recently deposited under low-energy conditions on the shelf. The sands represent a transgressive sediment sheet generated by coastal and shoreface erosion during coastline retreat. As the shoreface migrates landwards, the adjacent inner shelf floor is a site of potential accumulation. Along the Tuktoyaktuk Peninsula, a detailed study has shown that strong offshore-flowing currents, responsible for seaward directed sediment transport on the shoreface, are generated during northwesterly storm events associated with positive surges (H6quette et al., !990). By this mechanism, foreshore and upper shoreface sediments are eroded and transported seaward on the lower shoreface and inner shelf by storm-induced offshore directed currents (Niedoroda et al., 1984; Swift et al., 1985). As over 50% of the Beaufort Sea coast is composed of unconsolidated bluffs, they are believed to represent the primary source of the advnncing sand sheet on the shelf. During winter, sea-ice scouring is a significant process contributing to sediment remobilization and erosion at the seafloor (H6quette and Barnes, 1990), but high resolution seismic profiles collected in the eastern Beaufort Sea (H6quette and Hill, 1989) show that the inner shelf undergoes net deposition and that ice-scouring reworks only the uppermost metre of the transgressive Holocene unit. Other studies have shown that the average scour depths on the Canadian Beaufort Shelf are in the order of 0.5 to l m (Lewis, 1978).

M.-H. RUZ ET AL.

Seaward of the Tuktoyaktuk Peninsula, numerous small-scale depressions underlay the transgressive sand sheet (Fig.16). The majority of these depressions are small (100-300 m wide) and show little negative relief. Based on their horizontal and vertical dimensions, which are similar to those of the thermokarst lakes presently seen on the adjacent Tuktoyaktuk Peninsula, and according to the seismic facies of the basin-fill sequences, most of these features have been interpreted as remnants of thermokarst basins which survived the erosion induced by the Holocene marine transgression (H6quette and Hill, 1989). Seismic records reveal that landward dipping progradational reflectors may occur in some basins (Fig.16), probably representing backbarrier sediments deposited in lagoons during landward migration of barriers. In these cases, the barrier superstructure was truncated when the active shoreface erosie.,', surface passed over it, but the basal portion of the transgressive barrier sequence was preserved in the depression. Thermokarst lake basin deposits have been penetrated by boreholes off Richards Island (Hill et al., 1990) and at King Point (Hill, 1990). Northwestward of Richards Island, two basins were present in water depths of 5 to 6 m, beneath up to 10 m of late Holocene sediment (Fig.17). The lacustrine sediment consists of organic rich silt and clay with fibrous peat intervals containing a distinctive nonmarine algal assemblage. Radiocarbon dates from the base of the lacustrine sequence gave ages ranging from 5580+80 yrs B.P. (B-9504) to 9470+ 100 yrs B.P. (B-9508) (ltill et al., 1990). The overlying transgressive deposits consist of graded fine sand beds and sand lenses, fining upwards into laminated silt and clay. At King Point, the shoreface erosion surface is steeper and the littoral sand prism extends only 50 m in front of the landward-migrating barrier beach (Fig. 18). Beyond this, lacustrine deposits are exposed on the shoreface to a water depth of at least 8 m. The lacustrine deposits consist of laminated to bioturbated organic silt and clay containing up to 30% organic material, as brown (when oxidised) amorphogen and plant fragments. In boreholes in or close to the barrier, sediments of this facies were frozen and contained veins of ice. The same deposits are found in the lagoon behind

269

COASTAL EVOLUTI.ON IN A THERMOKAST TOPOGRAPHY. CANADA

A

t

~

-

-

~,..~ ~_...~..

~I

" "~..~

50

~ = . ~ : ~.". . . . . .

~.--

~, .

~

.

::--

"~"

" ~-'~"

--

~'~=.,,~.-~'" = . ~ ' . .

"

~-"~=~."

•

.

....

....... •" .'t

y

.~,-.~.-=..-

~

I

0

-."-" . . ' ~ . . . .

-:

....

-..

L

:" ==.

" :

SHOREWARD----~ I0

SHOREFACE EROSION HOLOCENETRANSGRESSIVE

SEABED

Z

_

20

m

r

~

LACUSTRINE SEDIMENTS?

THER ~IOKARST LAKE BOTI'OM

30

'

,janlmw,a.

~

,-n="m'rru',,'=m ............ SEDIMENTS

EROSION SURFACE

,~mmmmamm~mwm|

THERMOKARST LAKE BO'R'OM

40 0

I

!

I

I

1O0

200

300

400

DISTANCE (m)

Fig.16. Shore normal high-resolution seismic profile and line drawing showing preserved thaw lake bottoms and unconformity representing shoreface erosional surface on the inner shelf seaward of the Tuktoyaktuk Peninsula (see Fig.4 for location). Note the onlap-fill rettection configurations in the larger basin at left compared with the prograded landward dipping reflectors in the smaller depression, suggesting that the submerged thaw lake at right has been partly infilled by a landward migrating barrier.

the barrie:r. An accelerator radiocarbon date of 1! 850_+ 1:;0 yrs B.P. (RIDDL-768) from the lacustrine mud near the bottom of the lacustrine sequence corresponds closely to the age of postglacial thermokarst activity in the region (Rampton, 1988a). On the eastern Beaufort Shelf, the preservation potential of lagoon and backbarrier facies is favoured by the pre-existing topography (i.e., thermokarst basins) and by the relatively shallow depth of shoreface erosion. In the eastern Canadian Beaufort Sea, shoreface erosion of the Pleistocene substrate by waves and currents is restricted to the first few metres of water in the nearshore zone. The transgressive sheet of recent sediments can be found in areas as shallow as 4-5 m water

depth (H6quette and Hill, 1989). Seaward of the Tuktoyaktuk Peninsula coast, the shallow depth of shorefac¢ erosion by waves and currents can be explained by the moderate to low wave regime ~nd by the low regional gradient. Previous studies of wave climate showed that a substantial loss of wave energy occurs by shoaling over the very flat shelf seaward of the Tuktoyaktuk Pen;nsula (Pinchin et al., 1985). The rate of relative sealev:l rise is also a major preservation factor (Belknap and Kraft, 1981). The rapid rate of sealevel rise during the Holocene (Hill et al., 1985) has favoured the preservation of breached-lake bottoms and backbarrier lithosomes on the shelf as the shoreface erosion surface has rapidly undergone landward and upward translation.

M..H. RUZ ET AL.

270

I

134o30 '

BEAUFORT SEA

--69045

00

~

.

6gO45~

32+00

RICI.I/~qDSL~'~r~ ¢

SW

NE

V M E A N S E A LEVEL

0

7+60

32 +00

34+00

.5.~

_

_

_

=====================

--,,

E

38+00

36+00 _

_

_

_

_

_

_

_

--

,-I uJ ~> w -Io._1

MARINE ..c 9470 ± 100

I.U ¢/) .... ,

,-I I.IJ -J5'IIuJ ¢3

PLEISTOCENE -20-

' DIAMICTON 5580 + 80 yr -25'

O

I

2kin

!

|

I

Fig. 17. Boreholestratigraphynorthwestwardof Richards Island. Note the presence of lacustrine basins overlain by a thick sequence of Holocene marine mud. Discussion Based on the preceding observations, a geomorphic model of coastal evolution is proposed (Fig.19). In this model, the formation and development of embayments, spits and barrier islands is

controlled by transgressive sealevel conditions and general coastal retreat. According to our model, spits develop from headlands, retreat landward, and possibly evolve as barrier islands, principally by inlet formation or by erosion of tundra islands. As a spit or a barrier island is retreating in response

COASTALEVOLUTIONIN A THERMOKASTTOPOGRAPHY,CANADA

KM13 LAGOON O

271

BARRIER BEACH

INNER SHELF

•

vMEAN SEA LEVEL

E'-IO

CL ~t~'20

-30

-40

~

-

:

,

:•

'

.

i". •

. ".

/

• "

,

.

~

2

,

."" ,

]

."

"

"

U

" 2 • .- • . .

N

•

~

"

..... ~

~

~FS

'

""

LACUSTRINEILAGOONALMUD

MARINE MUD I"- ." .I PLEISTOCENE DEPOSITS

•

I.

•

•

•

.

-

•

•

.

-."

.

•

"

j

-J

J

II tl tt I

I 0 I

E IO -r 20 po. i,i 30 Q 40

I

P.O~,LE.

o

.ETRES

~,

Fig.18. The record of a 7.0kHz sub-bottom profiler and borehole stratigraphy seaward of King Point, Yukon coast. Note unconformity (shoreface erosion surface) outer•ping at seabed in a water depth of approximately 12 m.

to storm overwashing, beach and nearshore sediments are transferred landward over the crest and consequently contribute to the infilling of the lagoon (Fig.20). Longshore sediment transport and coastwise spit progradation also play a role in lagoon sedimentation through spit platform construction at the distal end. If the water depth in the backbarrier-lagoon is too significant, landward transport of barrier sediment will result in the complete drowning of the barrier in the basin (Fig.20, stage 3). Barrier islands may also migrate onshore living off their stored reserves, recycling the backbarrier sands on the foreshore and shoreface as they retreat or they may be overstepped in certain circumstances like off Atkinson Point. However, according to our observations on the stratigraphy of the inner shelf and because of erosional shoreface retreat, such overstepped barriers are ephemeral features and are restricted to shallow areas as only an erosional surface overlain by a transgressive sediment sheet should be expected to remain after the passage of the shore-

face. Higher preservation of transgressive barrier facies may be found in lagoons/breached lakes where landward-migrating barriers may have been partially or completely drowned depending on the depth of the basin. Coastal and inner shelf transgressive stratigraphy, and modern sedimentary processes in the southern Canadian Beaufort Sea are schematically represented in a threedimensional diagram (Fig.21). Our observations provide the basis for the formulation of a conceptual semi-quantitative model of shoreline changes for the barrier coast of the Canadian Beaufort Sea. Swift et al. (1972) proposed to group the principal variables responsible for coastline changes in the form of the following equation:

K= (S/E)G- R

(2)

indicating that f~: a given sediment character (grain size (G) for instance), the ratio between the rate of sediment input (S) and the energy available for its dispersal (E) must be balanced by the rate

272

M . - H . R U Z ET A L

THERMOKARST

. _

Coastal accumulation features

Barrier Island drowning

Pleistocene sediments

I(1)

HEADLAND AND EMBAYMENT FORMATION

(2)

SPIT DEVELOPMENT

, -: . . . . .':.-

..

.

.

.

:

"..." •

.-..._,.._.

i,,D,a.~

Fig. 19, Schematic geomorphic model illustrating coastal evolution in the thermokarst topography of the southern Canadian Beaufort Sea.

of relative sealevel rise (R) for the coastline to remain stationary (K); otherwise, the coastline will advance (positive K values) or retreat. In the Canadian Beaufort Sea, the coastline is characterized by high rates of retreat, and therefore K must correspond to large negative values. As previously discussed, relative sealevel rise alone can not explain the high retreat rate of spits

and barrier islands (averaging almost 3 m a-~) in the study area (Table 1). Therefore, the regional retreat of these coastal features must be partly related to a disequilibrium between sediment input (S) and the energy available for dispersal of coastal sediments (E). In the Canadian Beaufort Sea, most of the energy responsible for sediment dispersal in the coastal zone is wave-generated as sediment

COASTALEVOLUTIONIN A TH.ERMOKASTTOPOGRAPHY,CANADA

273

-.,=.,=,,= .

vRSL 1

••°•• • o•

.

•

~

o

•

••

•

o

• • •

vRSL 2 .

.

.

.

.

.

.

.

.

•

°

•

....o..

•

~ ' .

o

°

•

.......i

.

° •

I~

°

•

•

° .

o

o

•o

•

•

•

,

•

°

•

~ ~ % " .."~

.

•

:..

°

.

•

.

•

•

.

•

o

.

.

•

•

.

•

.

•

•

.

•

•

.

o

.

•

•

.

•

•

°

v RSL 3

•

PLEISTOCENESEDIMENTS

~

BARRIERSANDS

~

LACUSTRINESEDIMENTS

MASSIVEICE

~

TRANSGRESSIVE INNERSHELFSANDS

~

LAGOONALSEDIMENTS

°

°

°

•

Fig.20• Idealized cross-section showing barrier landward migration and resulting transgressive stratigraphy in the southern Canadian Beaufort Sea. The dashed line represents the relative sealevel corresponding to RSL I.

transport out to the 10 m isobath is dominated by waves and wave-induced currents (Harper and Penland, 1982; Fissel and Birch, 1984). However, the southern Canadian Beaufort Sea coastal zone is a low wave-energy environment due to the very gentle gradient of the continental shelf and the fetch-restricting pack-ice. Despite this low energy, retreat rates are high, so it seems likely that there is a sediment deficit in the littoral zone. Several

authors have suggested that a net sediment deficiency, resulting from the presence of terrestrial ice and thus low volumes of actual sediments in the eroding bluffs, is the major cause of the rapid coastal retreat in the region (Mackay, 1986; Harper, 1990)• In the present study, it has also been shown that sediment supply is a major factor controlling barrier island and spit evolution. Along the Canadian Beaufort Sea coast, sediment input

274

M.-H. RUZ ET AL. TUNDRA CLIFFS MAINLAND RETROGRESSIVE \ ~ C H . ~ H - - ' , ~ THAW F A I L U R E ~ ~ ~..~', ' ~ ~ J-_-,~7_" J , C , MASSIVE

~

ICE

_

-.

_

TRANSGRESSIVE -...--~f~------AREWORKED ~ ~ ~-~:.~~ l LITTORAL ~ ~ l SEDIMENTS /

_

-:---------5--:I-

SEDIM

•~ LAC~'S~"~INE • M O D E R N S . . , ~ , , . : : : . . : . , : . . ~ - - " : • BARRIER " . ": PLEISTOCENE "." "" ,,". ". " . : • . ' .'.'

. ".'..

~ • "

... l - - ~ . i ~

I/~.~ ~1 "t./.~

SURFAGEE.---;LAG60N~,L " ~ . " ~ _ ~ ' . ~

. . ', ' . '

'BR~'AOHED MOKARST

' "PERMAFROST TABLE ./'." '

MAirqLA~ID MODERN SAND PRISIVl BACKBARRIER SHOREFACE LAGOON

INNER SHELF WAVE-DRIVEN SHORE-NORMAL CURRENTS ~_~ STORM-INDUCED OFFSHORE CURREN rs ,e,,= LONGSHORE SEDIMENT TRANSPORT ONSHORE COMPONENT OF ICE-PUSH FROZEN SEDIMENTS OVERWASH

Fig.21. Block diagram showing the various geomorphic elements and sedimentary processes interacting in the Canadian Beaufort Sea coastal zone, and the resulting stratigraphy in the coastal/inner shelf area.

for coastal accumulation features (Q~) is a function of at least four factors:

Q,- f(Q=, I, D, Qo)

(3)

where Q= is the total volume of material eroded from coastal bluffs, I is the excess ground-ice content in the bluff sediments, D is the bluff sediment type and more specifically the minimum grain-size (diameter) of sediments that are coarse enough to remain on beaches, and Qo is the volume of sediment supplied from offshore (either due to asymetrical wave-orbital velocities of shoaling waves or to ice-push processes). Then the volume rate of sediment available for deposition in coastal accumulation features (AQ~) may be expressed as:

AQ, = [AQ,(I - / ) ] d + AQo

(4)

where AQ= is the volume rate of bluff erosion, I is the fractional proportion of ground-ice in excess of the natural unfrozen porosity of the sediment, d is the proportion of sediment coarser than grainsize D, and AQo is the volume rate of offshore sediment supply. Another important parameter controlling shoreline changes is the ubiquituous presence of

breached thermokarst basins. Breached lakes are sediment sinks, not only for fine sediments transiting in the coastal zone, but also for sands which may be trapped in these basins (Fig.20). Cores collected in backbarrier lagoons along Tuktoyaktuk Peninsula revealed that lagoonal sediments are essentially sandy (Cloutier, in prep.). Assuming two sections of coast of identical length with the same sediment supply (Qs) and the same amount of wave energy (E) available for sediment dispersal in the coastal zone, the ratio QJE will decrease with an increasing occurrence of breached lakes. The volume of breached lake basins will affect the sediment availability for the coastal accumulation features after the wave energy has been expended in the coastal zone partly dispersing the sediments. Therefore, for a given length of coastline, the ratio QJE must be corrected for potential sediment loss in coastal embayments with a dimensionless coefficient (8) inversely proportional to the volume of the embayments that may be potentially filled by coastal sandy sediments. Consequently, this coefficient will also depend on the sedimentation rate of fine-grained sediments (i.e., mainly silt and clay) that contribute to infill the embayments.

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY. CANADA

Although there is at present a lack of information concerning sedimentation rates of fine sediments in breached lake/lagoons, these rates are likely spatialy variable along the coast and decrease with distance from the Mackenzie Delta, principal source of fine sediments delivered to the Canadian Beaufort Sea (Harper and Penland, 1982). Sediment input from the Mackenzie River is estimated at 150x 10 6 t a - t , 95% consisting of silt and clay (Davies, 1975). These fine alluvial sediments are transported in the form of a surface plume of high suspended sediment concentration and most of the sedimentation occurs within the first kilometres from the Mackenzie Delta (Hill and Nadeau, 1989). The importance of coastal embayments for coastal stability is dramatically exemplified at Cape Dalhousie (Fig.10), at the tip of the Tuktoyaktuk Peninsula, where the high density of breached lakes and the low supply of fine-grained sediments lead to rapid retreat rates of the barrier island system although sand is delivered to the barriers through the erosion of remnants of mainland. The coastal morphology of the northeastern part of the Tuktoyaktuk Peninsula is strongly related to the presence of breached lake basins. Even though the coast consists of relatively high bluffs (up to 15 m), the abundance of breached lakes results in sediment loss in these basins and consequently coastal accumulation features are rare. Therefore, coastal embayments and lagoons are probably the principal sediment sinks in the coastal zone rather than spits and barrier islands as previously suggested (McDonald and Lewis, 1973; Harper and Penland, 1982). Although additional quantitative datd concerning sedimentation rates in breached lakes/lagoons, volumes of these basins, and offshore sediment supply are needed to develop numerical models of shoreline changes in the Beaufort Sea, we propose the following semi-quantitative equation which presents the essential parameters controlling the evolution of the barrier coast in the region: AC =

AQc(I-I)d+AQo ~

Eb

(5) tan fl,

which can reduce to: A C = (AQs/Eb) ~ - A R / t a n fl,

(6)

275

where AC is the rate of change (retreat or progradation) of a given section of barrier coast, AR is the rate of relative sealevel rise, Eb is the wave energy at breaking (since breaking of surface gravity waves represent most of the energy expended in the coastal zone for sediment dispersal),//, is the transgression slope, and the other variables have been previously defined. With e and tan//, included in eqns. (5) and (6), we take into account the pre-existing topography in addition to the process variables originally proposed by Swift et al. (1972). As shown in this study, the evolution of spits and barrier islands in the Canadian Beaufort Sea is partly controlled by the pre-existing topography which has been defined as a significant controlling factor in recent models of coastal sedimentation (Belknap and Kraft, 1981, 1985; Boyd and Penland, 1984; Carter et al., 1987).

Conclusion Pre-existing topography, sealevel rise and sediment supply are the major factors controlling coastal evolution according to the conceptual model developed in this paper. Relative sealevel rise over a topography of thermokarst basins and low reliefs of ice-bearing sediments leads to the formation of embayments and rapidly eroding headlands from which spits develop. Subsequently, spits may retreat with their eroding sediment source or evolve as barrier islands, depending on sediment supply and local topography. Several lines of evidence suggest that the formation of the barrier islands of the southern Canadian Beaufort Sea mainly results from the breaching of spits connected to the mainland or to small islands. At a short time-scale (annual time-scale), spits and barrier islands retreat in response to episodic storm-generated overwash events, but at a longer time-scale (geologic time-scale), relative sealevel rise is the major forcing mechanism of landward migration. The occurrence of breached lake basins in the coastal zone plays a significant role in the continuum of coastal evolution in the area. These basins act as sediment sinks in which landward migrating barriers may be drowned or they may restrict the development of coastal accumulation features if their number and size is important.

276

During the Holocene transgression, the shoreface has undergone erosional retreat, truncating the pre-existing topography but preserving the bottom of some breached thaw lakes and lagoons in which backbarrier sediments may be found. While the shoreface was retreating, sedimentation took place on the adjacent inner shelf floor forming an advancing transgressive sediment sheet. The relative importance of shoreface erosion and shelf aggradation has varied locally, depending on a spectrum of local conditions and process-variable combinations including wave energy regime, volume of sediment supplied to the coastal system, gradient of the coastal plain and continental shelf, and rate of relative sealevel rise.

Acknowledgements This study was funded by the Northern Oil and Gas Actio~l Program and the Panel on Energy Research and Development (Department of Indian and Northern Affairs Canada), by the Geological Survey of C a n a d a , and through N S E R C operating grants to A. H6quette and P.R. Hill. Logistic support was provided by the Polar Continental Shelf Project. T h a n k s to Roy Sparkes who generated the three-dimensional plot o f the Pe!ly Island barrier island. D o n Forbes kindly provided the 1984 beach profile data of the Atkinson Point spit and made helpful suggestions on an earlier version of the manuscript. P.W. Barnes and an a n o n y m o u s reviewer critically read the manuscript and their comments are gratefully acknowledged.

References Barnes, P.W,, 1982. Marine ice-pushed boulder ridge. Arctic, 35: 312-316. Belknap, D.F. and Kraft, J.C., 1981. Preservation potential of transgressive coastal lithosomes on the U.S. Atlantic Shell Mar. Geol., 42: 429-442. lklknap, D.F. and Kraft, J.C., 1985. Influence of antecedent geology on stratigraphic preservation potential and evolution of Delaware's barrier system. Mar. Geol., 63: 235-262. Blasco, S.M., Fortin, G., Hill, P.R., O'Connor, M.J. and Brigham-Grette, J., 1990. The late Neogene and Quaternary stratigraphy of the Canadian Beaufort continental shelf. In: A. Grantz, L. Johnson and J.F. Sweeney (Editors), The Geology of North America, Vol. L, The Arctic Ocean region. Geol. Soc. Am., Boulder, Colo., pp. 491-502. Boyd, R. and Penland, S., 1984. Shoreface translation and the

M..H. RUZ ET AL.

Holocene stratigraphic record: examples from Nova Scotia, the Mississippi Delta and eastern Australia. Mar. Geol., 60: 391-4!2. Bruun, P., 1962. Sealevel rise as a cause of shore erosion. .I. Waterw. Harbors Div., Am. Soc. Cir. Eng., 88:117-130. Carter, R.W.G., Johnston, T.W. and Orford, J.D., 1984. Stream ,~utlets through mixed sand and gravel coastal barriers: examples from southeast Ireland. Z. Geomorphol., 28: 428-442. Carter, R.W.G., Orford, J.D., Forbes, D.U and Taylor, R.B., 1987. Gravel barriers, headlands and lagoons: an evolutionary model. Proc. Coastal Sediments 87. Am. Soc. Cir. Eng. Waterw. Div., New Orleans, pp. 1776-1792. Cloutier, M., in prep. Morpho-s6dimentologie et dynamique des littoraux de la Pointe Atkinson, P6ninsule de Tuktoyaktuk, T.N.O.M.A. thesis, Laval Univ., Qu6bec. Dallimore, S.R., Kurfurst, P.J. and Hunter, J.A.M., 1988. Geotechnical and geothermal conditions of near-shore sediments, southern Beaufort Sea, Northwest Territories, Canado Int. Conf. Permafrost, 5th (Trondheim, Norway), pp. 127-131. Dark,, K.F., 1975. Mackenzie River input to the Beaufort Se~. ~nst. Ocean Sci., Sidney, Beaufort Sea Proj., Tech. Rep. 15, 72pp. Fissel. D.B. and Birch, J.R., 1984. Sediment transport in the Canadian Beaufort Sea. Geol. Surv. Can. Arctic Sci., Sidney, 165 pp. (Unpublished). Forbcg~ D.L., 1981. Babbage River delta and lagoon: hydrology and sedimentology of an Arctic estuarine system. Ph.D. Thesis, Univ. British Columbia, Vancouver, 554 pp. Forbes, D.L. and Frobel, D., 1985. Coastal erosion and sedimemation in the Canadian Beaufort Sea. Geol. Surv. Can., Curr. Res., Pap. 85-1B, pp. 69-80. Franklin, Capt. J. 1828. Narrative of a second expedition to the shores of the polar seas in the year 1825, 1826, 1827. (Republished 1971, M.G. Hurtig, Edmonton, Alta., 320 pp.). Harper, J.R., 1990. Morphology of the Canadian Beaufort Sea coast. Mar. Geol. 9 !: 75-9 I. Harper, J.R. and Penland, S., 1982. Beaufort Sea sediment dynamics. Geol. Surv. Can., Woodward-Clyde Consult., Victoria, 125 pp. (Unpublished). Harper, J.R., Reimer, P.D. and Collins, A.D., 1985. Canadian Beaufort Sea: Physical Shore-Zone Analysis. Geol. Surv. Can., Open File 1689, 105 pp. Harper, J.R., Henry, R.F. and Stewart, G.G., 1988. Maximum storm surges elevations in the Tuktoyaktuk region of the Canadian Beaufort Sea. Arctic, 41: 48-52. Harry, D.G., French, H.M. and Clark, M.J., 1983. Coastal conditions and processes, Sachs Harbour, Bank Island, Western Canadian Arctic. Z. Geomorphol. N.F., Suppl. 47: 1-26. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: S.P. Leatherman (Editor), Barrier Islands: from the Gulf of St. Lawrence to the Gulf of Mexico. Academic Press, New York, pp. i-27. H6quette, A. and Barnes, P.W., 1990. Coastal retreat and shoreface profile variations in the Canadian Beaufort Sea. Mar. Geol., 91: 113-132. H6quette, A. and Hill, P.R., 1989. Late Quaternary seismic stratigraphy of the inner shelf seaward of the Tuktoyaktuk

COASTAL EVOLUTION IN A THERMOKAST TOPOGRAPHY, CANADA

Peninsula, Canadian Beaufort Sea. Can. J. Earth Sci., 26: 1990-2002. H6quette, A. and Ruz, M.-H., 1991. Spit and barrier island migration in the southeastern Canadian Beaufort Sea. ,i. Coastal Res., 7: 677-698. H6quette, A., Jenner, K.A. and Hill, P.R., 1990. Beach morphodynamics and sediment transport at Tibjak Beach, Canadian Beaufort Sea Coast. Geol. Surv. Can., Open File 2285, 43 pp. Hill, P.R., 1990. Coastal geology of the King Point area, Yukon Territory, Canada. Mar. Geol., 91: 93-111. Hill, P.R. and Frobel, D., 1991. Documentation of summer NOGAP activities, July 22-August 25 1990. Geol. Surv. Can., 84 pp. (Unpublished). Hill, P.R. and Nadeau, O.C., 1989. Storm-dominated sedimentation on the inner shelf of the Canadian Beaufort Sea. J. Sediment. Petrol., 59: 455-468. Hill, P.R., Mudie, P.J., Moran, K., and Biasco, S.M., 1985. A sealevel curve for the Canadian Beaufort Shelf. Can. ,i. Earth Sci., 22: 1383-1393. Hill, P.R., Forbes, D.L., Dallimore, S.C. and Morgan, P., 1986. Shoreface development in the Canadian Beaufort Sea. Proc. Symp. Cohesiv. Shores, Natl. Resour. Counc. Can., Ottawa, pp. 428-448. Hill, P.R., H6quette, A., Ruz, MH. and Jenner, K.A., 1990. Coastal investigations of the Canadian Beaufort Sea coast. Geol. Surv. Can., Open File 2387, 348 pp. Hoyt, ,I.H., 1967. Barrier island formation. Bull. Geol. Soc. Am., 78:1125-1136. Hume, .I.D. and Schalk, M., 1964. The effects of ice push on arctic beaches. Am. ,i. Sci., 262: 267-273. Inman, D.L. and Dolan, R., 1989. The Outer Banks of North Carolina: budget of sediment and inlet dynamics along a migrating barrier sytem. J. Coastal Res., 5: 193-237. Judge, A.S., 1986. Permafrost distribution and the Quaternary history of the Mackenzie-Beaufort region: a geothermal perspective. In: ,I.A. Heginbottom and ,I.S. Vincent (Editors), Correlation of Quaternary Deposits and Events around the Margin of the Beaufort Sea. Geol. Surv. Can., Open File 1237, pp. 41-45. Kempema, E.W., Reimnitz, E. and Barnes, P.W., 1989. Sea ice sediment entrainment and rafting in the Arctic. ,i. Sediment. Petrol., 59: 308-317. Kovacs, A. and Sodhi, D.S., 1980. Shore ice pile-up and rideup: field observations, models, theoretical analysis. Cold Reg. Sci. Tech., 2: 209-288. Lewis, C.F.M., 1978. The frequency and magnitude of driftice groundings from ice-scour tracks in the Canadian Beaufort Sea. Proc. 4th Port and Ocean Eng. under Arctic Cond., Mem. Univ. Newfoundland St-John's, 1977, pp. 568-579. Lewis, C.P., 1985. Delta front processes and morphology, Mackenzie Delta, N.W.T. 14th Arctic Workshop, Arctic Land-Sea Interaction, Bedford Inst. Oceanogr., Dartmouth, pp. 236-237. (Abstr.) Lewis, C.P. and Forbes, D.L., 1975. Coastal sedimentary processes and sediments, southern Beaufort Sea. Can. Dep. Environ., Beaufort Sea Proj. Tech. Rep., 24, 68 pp. Mackay, J.R., 1956. Notes on oriented lakes of the Liverpool Bay area, Northwest Territories. Rev. Can. G6ogr., 10: 175-178. Mackay, ,I.R., 1963a. The Mackenzie Delta area, N.W.T.,

277

Canada. Dept. Mines Tech. Surv., Geogr. Branch, Mere. 8, 202 pp. Mackay, ,I.R., 1963b. Notes on shoreline recession along the coast of the Yukon Territory. Arctic, 16: 195-197. Mackay, ,I.R., 1971. The origin of massive icy beds in permafrost, western Arctic coast, Canada. Can. ,i. Earth Sci., 8: 397-422. Mackay, J.R., 1972. Offshore permafrost and ground ice, southern Beaufort Sea, Canada. Can. ,i. Earth Sci., 9: 15501561. Mackay, ,I.R., 1986. Fifty years (1935 to 1985) of coastal retreat west of Tuktoyaktuk, District of Mackenzie. Geol. Surv. Can., Curr. Res., Pap. 86-1A, pp. 727-735. Maurmeyer, E., 1978. Geomorphology and evolution of transgressive estuarine washover barriers along the western shore of Delaware Bay, Newark, Delaware. Ph.D. Thesis, Univ. Delaware, 274 pp. McDonald, B.C. and Lewis, C.P., 1973. Geomorphic and sedimentologic processes of rivers and coast, Yukon coastal plain. Can. Environ. Soc. Comm., Northern Pipelines~ Task Force Northern Oil Dev., Rep. 73-39, 245 pp." Niedoroda, A.W., Swift, D.J.P., Hopkins, T.S. and Ma C.M., 1984. Shoreface morphodynamics on wave-dominated coasts. Mar. Geol., 60:33 !-354. Niedoroda, A.W., Swift, D.,I.P., Figueiredo, A.G. Jr., Freeland, G.L., 1985. Barrier island evolution, Middle Atlantic Shelf, U.S.A. Part 2: Evidence from the shelf floor. Mar. Geol., 63: 363-396. Oertel, G.F., 1985. The barrier island system. Mar. Geol., 63: 1-18. Otvos, E.G., 1985. Barrier platforms: Northern Gulf of Mexico. Mar. Geol., 63: 285-305. Pelletier, B.R., 1975. Sediment dispersal in the southern Beaufort Sea. Dep. Environ., Victoria, Beaufort Sea Proj. Tech. Rep. 25, 80 pp. Pinchin, B.M., Nairn, R.B., a,,~ : ~~"*t, K.L., 1985. Beaufort Sea Coastal Sediment Study: Numerical estimation of sediment ransport and nearshore profile adjustment at coastal sites in the Canadian Beaufort Sea. Geol. Surv. Can., Open File 1259, 712 pp. Rampino, M.R. and Sanders, J.E., 1980. Holocene transgression in south-central Long Island, New York. J. Sediment. Petrol., 50: 1063-1080. Rampton, V.N., 1974. The influence of ground ice and thermokarst upon the geomorphology of the Mackenzie-Beaufort region. In: B.D. Fahey and R.D. Thompson (Editors), Research in Polar and Alpine Geomorphoiogy. Proc. 3rd Guelph Symp. Geomorph., pp. 43-59. Rampton, V.N., 1982. Quaternary geology of the Yukon Coastal Plain. Geol. Surv. Can., Bull. 317, 49 pp. Rampton, V.N., 1988a. Quaternary Geology of the Tuktoyaktuk Coastlands, Northwest Territories. Geol. Surv. Can. Mere. 423, 98 pp. Rampton, V.N., 1988b. Origin of massive ground ice on Tuktuyaktuk Peninsula, N.W.T. Canada: A review of stratigraphic and geomorphic evidence. Int. Conf. Permafrost, 5th (Trondheim, Norway), pp. 850-855. Rampton, V.N. and Mackay, J.R., 1971. Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards

278

Island and nearby areas, District of Mackenzie. Geol. Surv. Can., Pap. 71-21, 16 pp. Reimnitz, E. and Barnes, P.W., 1987. Sea-ice influence on arctic coastal retreat. Proc. Coastal Sediments 87. Am. Soc. Civ. Eng. Waterw. Div., New Orleans, pp. 1578-159 !. Reimnitz, E., Graves, S.M. and Barnes, P.W., 1988. Beaufort Sea coastal erosion, sediment flux, shoreline evolution, and the eosional shelf profile. U.S. Geol. Sur., Map I-I182-G, 22 pp. Reimnitz, E., Barnes, P.W. and Harper, J.R., 1990. A review of beach nourishment from ice transport of shoreface materials, Beaufort Sea, Alaska. J. Coastal Res., 6: 439-470. Ritchie, J.C. and Hare, F.K., 1971. Late-Quaternary vegetation and climate near the Arctic tree line of northwestern North America. Quat. Res., I: 331-342. Short, A.D., 1979. Barrier island development along the Alaskan-Yukon coastal plains. Geol. Soc. Am. Bull., 90: 77-103. Smith, M.W., 1976. Permafrost in the Mackenzie Delta, Northwest Territories. Geol. Surv. Can., 75-28.

M.-H. RUZ ET AL.

Swift, D.J.P., Kofoed, J.W., Saulsbury, F.P. and Sears, P., 1972. Holocene evolution of the shelf surface, Central and Southern Atlantic Shelf of North America. In: D.J.P. Swift, D.B. Duane and O.H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchison and Ross, Stroudburg, pp. 499-574. Swift, D.J.P., Niedoroda, A.W., Vincent, C.E. and Hopkins, T.S., 1985. Barrier island evolution, Middle Atlantic Shelf, U.S.A. Part !: Shoreface dynamics. Mar. Geol., 63: 331-361. Vilks, G., Wagner, F.J.E. and Pelletier, B.R., 1979. The Holocene marine environment of the Beaufort Shelf. Geol. Surv. Can. Bull. 303, 43 pp. Wiseman, W.M., Jr., Coleman, J.M., Gregory, A., Hsu, S.A., Short, A.D., Suheyda, J.N., Waiters, C.D. and Wright, L.D., 1973. Alaskan arctic coastal processes and morphology. Coastal Stud. inst., Louisiana State Univ., Baton Rouge, Tech. Rep., 149, 17! pp.