Glacial and early postglacial lacustrine environment ...

2 downloads 0 Views 2MB Size Report
Cependant, durant la majeure partic de la pCriode qui precida I'inondation engendrCe par la montee des eaux du lac Ontario, le chenal Ctait occupi par uneĀ ...
Glacial and early postglacial lacustrine environment of a portion of northeastern Lake Ontario ROBERTGILBERT Department of Geography, Queen's University, Kingston, Ont., Canada K7L 3N6 AND

JOHN SHAW Department of Geography, University of Alberta, Edmonton, Alta., Canada T6G 2H4

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

Received February 25, 1991 Revision accepted August 5, 1991 A deep channel in bedrock extending to more than 25 m below sea level occurs along the north shore of the otherwise uniformly shallow Kingston basin of Lake Ontario. Bathyrnetr~cand subbottom acoustic data are used to map the topography of the channel and to reconstruct its late glacial and postglacial sedimentary hlstory. The results are interpreted as showing that the large channel and smaller channels nearby were created by hlgh-velocity subglacial meltwater flow. Acoustic facies assemblages of sediments deposited in the channels record patchy deposition, or deposition followed by partial erosion, of' glacial sediments on the bedrock of the channel floor. followed by deposition and episodic erasion of placiolacustrine sediment in a high-energy, ice-proximal lake. Palaeoslope analysis confirms that the early Holocene low-water phase of Lake Ontario resulted in the development of a iluvial system in part of the channel. Water level was controlled by a sill at Kingston. Kingston basin. the Ray of Quinte, and possibly. for a short time. a much larger area of ~ h upper e Great Lakes drained through the channel. However, for most of the perlod. until it was flooded by the rising waters of Lake Ontario. the channel was occupied by a small river on a wide lloodplain or it was tlanked by broad marshes.

On observe dans le lac Ontario, un chenal encaisse dans le socle rocheux, d'une profondeur de plus de 25 m sous le niveau de la mer, et qui longe la rive nord du bassin de Kingston normalement uniformkment peu profond. Le relev6 bathymktrique et les donnCes acoustiques pour les materiaux sous le fond du lac ont servi cartographier le model6 du chenal e t a reconstruire I'histoire ~Cdimentaireglaciaire tardive et postglaciaire. L'interprCtation des donnCes indique que le chenal principal et les chenaux secondaires rCsultent d'une Crosion due a un Ccoulement sous-glaciaire et rapide des eaux de fonte. Les assemblages de faciks stdirnentaires accurnul~sdans les chenaux. rnls en Cvidence par les donnees acoustiques, sont formCs de dCpBts isolks, ou de dip8ts parriellement i r d 6 , de sediments glaciaires accumulCs sur le socle rocheux au fond du chenal, ensuite recouverts de ddpijts glaciolacustres, kpisodiquernent trades. caractkristiques d'un contexte de grande Cnergie proximitk d'un lac glaciaire. L'ttude des paleopentcs confirme que la pCriode des eaux basses du lac Ontario, au debut de I'Holocene, a causP un dCveloppement du systi.rne fluvial dans une panie de ce chenal. Un seuil IocalisC a Kingston contrBlait le niveau de I'eau. Les eaux du hassin de Kingston, de la baie de Quinte et, possiblement, mais durant une @riode courte, celles d'une aire encore plus grande des Grands Lacs occupant un niveau plus ClevC Ctaient drainCes par ce chenal. Cependant, durant la majeure partic de la pCriode qui precida I'inondation engendrCe par la montee des eaux du lac Ontario, le chenal Ctait occupi par une petite rivikre coulant dans une grdnde plaine d'inondation ou il Ctait bordC par de vastes marCcages. [Traduit par la rCdaction] Can. 1. Earth

Sci. 29, 63-75

(1992)

Introduction The northeastern arm of Lake Ontario is a shallow basin (mean depth 23 rn) separated from the much deeper main body of the lake by a sill from Long Point through Main Duck and Galloo islands (Fig. 1). Shaw and Gilbert (1990) have proposed that. during glaciation. this Kingston basin served as part of a major conduit for the flow of subglacial water from the northeast, which sculpted much of the landscape of the upper St. Lawrence River valley and eastern Lake Ontario. The overdeepened interisland channels of the Thousand Islands and Kingston area ate an important part of the evidence for this phenomenon (Gilbert 1990). Following glaciation. the Ontario basin was occupied by ice-dammed Lake Iroquois, which had a surface in the study area much above the present level of Lake Ontario (Anderson and Lewis 1985). After the draining of this lake, the water level fell rapidly to below the present level, creating a series of lakes and rivers and separating the main body of Lake Ontario from a lake in the Kingston basin, referrcd to in this paper as "Lake Kingston" (Fig. I). The water surface rose through the Holocene in response to isostatic rebound, flooding Lake Ontario to its present level Printed In Canada 1 lrnpr~meau Canada

and leaving distinct suites of sediment as evidence of former environments (Dalrymple and Carey 1990: Gilbert 19%)). In this paper we examine the region north of Amherst and Woltii islands from Adolphus Reach through North Channel to Kingston Harbour (Fig. 1) to determinc the origin of the channels and their postglacial sedimentary environment. Understanding the palaeohydrology of this area is key to interpretation of the drainage and palaeogeography of Kingston basin to the south and of the Bay of Quinte, its drainage basin. and possibly, for a short period. the upper Great Lakes to the northwest. The manner in which these areas drained through the study site changed significantly during the Holocene. Evidence used in this study is based on detailed hathymetric mapping by the Canadian Hydrographic Service (Fig. 2) and subbottom acoustic survey (Fig. 3).

Acoustic record A survey was carried out across lower Adolphus Reach, North Channel, Upper and Lower gaps, and Kingston Harbour using a Datasonics SBP 5000 3.5 kHz acoustic system and a conventional 200 kHz echo sounder. Positions were

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

64

CAN. J. EARTH SCI. VOL. 29, 1992

FIG. 1 . Location of the study site in northeastern Lake Ontario showing the inferred location of Lake Ontario and the St. Lawrence River 11.4 ka BP, and of Lake Kingston 11.4 ka BP (broken line) and 8 ka BP (solid line), determined by projecting isostatic surfaces determined by Anderson and Lewis (1985) onto Canadian Hydrographic Service field sheets 8019, 8024, and 8294.

determined by sighting features recognizable on charts and air photographs and from a hull-mounted current meter linked to the graphic recorder of the acoustic system. Where subbottom sound penetration occurred, four distinct acoustic facies are reco9nized: an acoustically transparent sediment, which forms a layer on top of other sediments on the lake floor; a partially opaque. massive sediment; an acnustically well-stratified sediment; and a nearly completely opaque sediment underlying facies 3. From the patterns of these four. and the presence of completely opaque surfaces below which there is no acoustic definition. eight distinct acoustic assemblages are mapped in Fig. 3 and are described below. Exposed bedrock (Br) and bedrock with a thin veneer of sediment ( B r V ) On exposed shores in water depths less than 5- 10 m (all depths refer to hydrographic chart datum, 74 rn as]) the lake floor is swept clear of fine sediments and bedrock CBr) or coarse. hard sediment is exposed. The acoustic record shows an irregular opaque surface and many multiple echoes even at lowest settings of gain. In deeper water and in areas protected from wave action, a patchy or thin veneer of acoustically transparent sediment (facies 1) is found over the opaque surface (BrV). Recordings of wave height made 5.2 km southeast of Simcoe Island in Lower Gap (Marine Environmental Data Service, unpublished) indicate significant wave heights of 1 m are exceeded about 10% of the time and of 2 m about 1% of

the time. These correspond to a wave base of 8- 15 m (Sly 1978) on exposed coasts and in Upper and Lower gaps. Along the more protected coasts of North Chamel, wave base is estimated to be 3 -6 m except in sheltered areas such as the heads of Collins. Kerr, and Cataraqui bays, where waves rarely exceed 0.3 m and a veneer of sediment up to several metres thick occurs throughout. Sediment-mantled slopes (MS) A narrow zone, found only in the western portion of the study area (Fig. 3), marks the slope from the exposed or thinly covered bedrock shelves near shore to the lower shelf or the channel bottom sediments. The slope is covered by acoustically massive and partially opaque sediments (facies 2), which may be related to subaerial exposure of this area or may be derived from erosion on the bedrock shelf above. Upper shelves (US) and lower shelves (LS) flanking former channels and lakes West of Everett Point and throughout North Channel, nearly flat-lying shelves are found at 18-36 m depth (depths increasing westward). Those east of Kerr Point occur as a single platform along the sides of a well-defined channel (Fig. 4), while those to the west have a different acoustic character and rise in several steps comprising upper and lower shelves flanking the sides of a deeper basin occupying North Channel at Upper Gap. The largest (2.5 x 8.2 km) of the shelves occupies much of the eastern portion of North Channel (Fig. 3).

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

GILBERT AND SHAW

65

FIG. 2. Map showing names used in text and bathymetry of North Channel and Kingston Harbour drawn from Canadian Hydrographic Service field sheets 3079, 3080, 3233, 8078, 8079, 8293, and 8294. Isobath interval is 5 m below chart datum (74 m asl). Location is outlined in Fig. 1 .

The acoustic record of the upper shelf is characterized by a surface layer of acoustically transparent sediment (facies I), which is defined even in the 200 kHz record (Fig. 4a). It is about 7 m thick near the channel, decreasing to about 3 m thick at the backshore. Because on the large shelf the underlying surface is nearly horizontal, this pattern has created the shallowest depths near the channel (see the 16 and 18 m isobath patterns in Fig. 2). The few reflecting horizons that can be distinguished in facies 1 are nearly parallel to the present surface,-indicating that deposition at the outer edge has been consistently greater than that at the backshore throughout the period of submergence. In several places along the backshore (especially at Kerr Point (Fig. 5)) a small channel appears to be eroded in the underlying sediment. It is mantled by sediments of facies 1, indicating that it originated before they were deposited. Beneath the flat, strongly reflecting surface that marks the lower boundary of facies 1 is a zone of more opaque sediment (facies 2) that is largely massive, but with a few irregular reflectors. An irregular but strongly reflecting surface marks the lower boundary (Figs. 4b, 4c). Where sound penetrates facies 1 and 2 an underlying facies 3 is recorded (Figs. 4b, 4c). Sound return is weak because significant attenuation occurs in facies 2 above, so the sediment appears largely transparent. However, in some locations (Fig. 4b), relatively closely spaced parallel reflectors are recorded in facies 3. These are similar to those of well-defined glaciolacustrine sediment exposed or lying under a thin sediment cover in the main channels described below.

The lowest recognized surface marks the bottom of facies 3. It is irregular and strongly reflecting (Figs. 4b, 4c), suggesting that it is bedrock, glacial till, or a related nonlacustrine sediment. West of Kerr Point the flanking shelves have a different acoustic character (compare Fig. 4 with Figs. 6-8). The upper shelves (US) are similar to those east of Kerr Point and occur at the same level corrected for differential isostatic rebound (see below). However, everywhere in the surface layer (facies 1) of this portion of the shelves, subbottom echoes are completely masked by an opaque, fuzzy acoustic surface several metres into the sediment (Fig. 6). This is probably caused by the occurrence of gas associated with the decay of organic material (see also Schubel 1974). In those places where the shelf is narrow (Fig. 7) or at the outer and inner edges of the wider shelves (Fig. 6), a horizontal, strong reflector marks the bottom of facies 1 at a sharp, erosional unconformity with the underlying, layered sediments (facies 3). However, facies 2 is also found in some places under the larger shelves, where it forms an opaque, irregular zone, as under the upper shelf east of Kerr Point. In some places west of Kerr Point, there is a shelf (LS) downslope from the upper shelf (Figs. 6-8). The lower shelves consist of acoustically stratified sediment (facies 3), although the reflectors are relatively weaker and less uniform (Figs. 6, 8, 9). Internal reflectors are truncated at the surface especially toward the outer edges (Figs. 6-9). In some sections, gas partly masks reflectors over about one half of the lower shelf area. West of Upper Gap in Adolphus Reach, both

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

CAN. J.

EARTH SCI. VOL. 29, 1992

FIG. 3. Acoustic facies assemblages mapped from the tracks of subbottom profiles shown and interpolated from bathymetry (Fig. 2): bedrock at the surface (Br), bedrock with a thin or discontinuous veneer of sediment (BrV), slopes mantled with undifferentiated sediment (MS), upper (US) and lower (LS) shelves, glaciolacustrine sediments exposed at or near surface (GI), channel deposits (C), and floodplain deposits (F) with river courses (R). Locations of Figs. 4- 1 1 are marked by number and heavier track lines. Outline of this figure is the same as that of Fig. 2.

upper and lower shelves are absent, being replaced by the slope mantling sediment (MS) described above. Channel sediments (C) including glaciolacustrine (GI) and floodplain (F) assemblages A deep, submerged channel occupies a portion of the present waterway from Adolphus Reach through North Channel to Kingston (Fig. 2). Three facies assemblages recognized from their acoustic character occupy part of the channel: acoustically stratified sediments mantling the deep channel floor west of Kerr Point (Gl); a mixture of massive and stratified materials partly filling the channel between Kerr Point and Cataraqui Bay (C); and a zone of flat-lying, acoustically opaque sediments in Kingston harbour (F). The assemblage G1 is dominated by the acoustically stratified sediment (facies 3) consisting of many closely spaced parallel reflectors. West of a constriction created by the extension of lower shelves southeast of Bath (Figs. 2, 3), these sediments are about 10 m thick and conform to the underlying topography (Fig. 6). The upper surface is not eroded. To the east, the stratified sediments are considerably thicker and consist of two zones separated by a sharp erosional contact (at arrow in Fig. 9). In addition, the present channel is eroded into both stratified zones. Even though individual reflectors are caused by changes in acoustic impedance in the sediment (Shilts and Kettles 1987) and may not relate to particular sedimentary layers (Mayer

1979), the acoustic character of these sediments suggests deposition in a moderately high energy environment of rapid sedimentation where seasonal changes and episodic sedimentary processes, including turbidity currents, created distinct reflecting horizons (compare similar records, made with the same instrument, from glacial Harrison Lake, British Columbia: Desloges and Gilbert 1991). These conditions prevailed while glacial Lake Iroquois occupied the region (Naldrett 1991). The internal erosional contact may be the result of the creation of a sublacustrine channel similar to those reported in modern glacial lakes (Brodie and Irwin 1970) and fiords (Gilbert 1983) where channelized turbidity currents erode the floor. In Lake Iroquois erosion of this channel may have been associated with catastrophic draining from the nearby, damming glacier. The glaciolacustrine sediment is interpreted as lying directly on bedrock or glacial (nonlacustrine) sediment. Normally, the latter is acoustically opaque in records from the same equipment as that used in this study and cannot be distinguished from bedrock. However, in a few places (for example, Fig. 6) it can be recognized as a separate facies (4) on the basis of acoustic reflectors within and beneath the sediments. Where these sediments occur, they fill only a part of the channel, indicating that some have been subsequently eroded, as we suggest occurred to the glaciolacustrine sediments, or that they were deposited in contact with glacial ice that melted before

i

I

1

67

GILBERT AND SHAW

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

(a) South

North m

FIG.4. ( a ) 200 kHz echo profile showing the upper shelf and the channel. Bedrock surface is exposed at the ends of the run. (b, c) Detail of the upper shelf from the subbottom record. Numbers 1-3 refer to acoustic facies described in the text.

North

m

South 1

'loom 1 V.E. x 12.5

FIG. 5 . Subbottom record showing secondary channel on the shoreward side of the upper shelf at Kerr Point.

or as Lake Iroquois formed (cf. Shaw and Archer 1979; Kaszycki 1987). In the channel east of Kerr Point the assemblage of sedimentary deposits (C) is confined by shelves, especially on the south side. Channel fill (Fig. 10) consists of a lower zone of acoustically stratified sediment (facies 3). Although internal reflectors are not as clearly defined as in the glaciolacustrine sediment in the channel to the west, it was probably also deposited in glacial Lake Iroquois. There are massive sediments (facies 2) overlying facies 3, which probably account

for the weakness of the sound return from it. In most places the boundary between facies 2 and 3 is not clearly defined. A thin discontinuous layer of acoustically transparent sediment (facies 1) of modern origin caps the sequence. East of Cataraqui Bay the channel sediments have a different character (Fig. 11). There the lake floor is a wide, flat plane at about 16 m depth. Beneath a thin veneer of acoustically transparent sediment 0-3 m thick (facies 1) is an acoustically opaque surface with very little sound return from beneath except in a few locations where multiple reflectors characteris-

CAN. 1. EARTH SCI. VOL. 29, 1992

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

tic of the stratified glaciolacustrine sediments (facies 3) are observed. The only relief on this surface is a large channel toward the north side and several smaller channels (R in Fig. 11). These are interpreted as river channels (complete with levees) on the surface of a floodplain. A bedrock surface was not detected beneath these sediments, although at Penitentiary and Myles shoals it emerges above the sediment surfaces as bedrock knolls (Br in Fig. 11).

Interpretation Subglacial jluvial environments The bedrock channel in the study area is interpreted as part of an anastomosing series of channels in southeastern Ontario deeply incised in bedrock by subglacial fluvial erosion (Gilbert 1990; Shaw and Gilbert 1990). In our subbottom records, sound did not penetrate to bedrock on all transects because of the characteristics of the overlying sediment, especially the presence of gas and marl (Johnston 1978). In addition, it is not always possible to recognize the difference in the acoustic record between bedrock and glacial and nonlacustrine sediment. However, there are sufficient observations to map the bedrock surface approximately (Fig. 12). The bedrock surface is also mapped above the present water level from topographic maps on the assumption that the soil and sediment cover is thin. The anastomosing system of valleys, some of which are drowned (Shaw and Gilbert 1990), in the Kingston area clearly does not relate to the modern hydrologic regime. The subaerial parts of the valleys are occupied by underfit streams and show glacial features, drumlins, eskers, and transverse ridges on their floors. The glacial features show that there has been very little postglacial modification of the valleys. The anastomosing pattern of the present-day valleys and the presence of large-scale meltwater fluting on the interfluves among them indicate that at some time they functioned as large channels with occasional "overbank" flooding (Shaw 1988; Shaw and Gilbert 1990). Small-scale erosional marks formed by meltwater show flow toward the southwest in the large channels, opposite to the direction of immediately postglacial drainage inferred here from slopes of shelves discussed below and small channels inset in the large valleys. These small-scale marks carry striations, as does the large-scale fluting (Murray 1988; Shaw 1988). The flow direction and striations point to a subglacial origin for the anastomosing channel system. In addition, the deep scours and undulating long profiles of the channels suggest erosion under high subglacial hydrostatic pressure (Figs. 12, 13; Gilbert 1990). These valley or channel systems are probably of great antiquity, having operated through several glacial cycles. The inferred glacial deposits within a channel (Fig. 6) indicate the likelihood that at least the channel predates the last glacial advance. The occurrence of some of the valleys in the region may have been determined by faulting (for example, to the west of the study site in the Bay of Quinte) but some parts of the faults have no corresponding valley (McFall 1990, Fig. 2). Clearly, the faults themselves do not create valleys, but simply provide zones of weakness to be excavated by other agents including subglacial meltwater (Shaw and Gilbert 1990). Postglacial jluvial and lacustrine setting Anderson and Lewis (1985) presented a general Holocene palaeogeography of Lake Ontario determined from the pattern of isostatic rebound. The earliest phase of Lake Ontario

GILBERT AND SHAW

North

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

South

FIG. 7. Subbottom record showing small upper shelf on truncated glaciolacustrine sediments over bedrock. Abbreviations as in Fig. 3.

South GI

m 20 1

loom

,

--$--

LS

+

North

V.E. x 7 . 6

30 40

50 60

70 FIG. 8. Subbottom record showing adjacent upper and lower shelves, and glaciolacustrine sediments, all over bedrock. Abbreviations as in Fig. 3.

drained over Duck-Galloo sill south of Main Duck Island and thence through the present main channel of the St. Lawrence River south of Wolfe Island. North of that channel, Kingston basin was occupied by a lake (here designated Lake Kingston, Fig. 1) that drained to the north through North Channel and Kingston Harbour and then to the north of Wolfe Island. Also included in this drainage was water from the Bay of Quinte and its drainage basins to the west of the study area. The palaeogeography of the northern part of Kingston basin

from about 11.4 ka BP when the fast effect of ice damming ended until about 8 ka BP may be assessed using the detailed bathymetry of North Channel - Kingston Harbour (Fig. 2) and Kingston basin (from Canadian Hydrographic Service field sheets), and the subbottom acoustic patterns described above. Afier a b u t 8 ka, as isostatic rebound continued. water levels in the study area were no longer controlled by local sills but by backflooding related to sills downstream in the St. Lawrence River to the northeast.

70

CAN. J. EARTH SCI. VOL. 29, 1992

North

South

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

I

loom

I

V.E. x 70.6

FIG. 9. Subbottom record showing glaciolacustrine sedimentation with an internal erosion surface at the arrow, and subsequent erosion of a channel in those sediments. Gas masks the subbottom returns at the ends of the profile. Abbreviations as in Fig. 3.

According to Anderson and Lewis (1985), the isostatic surfaces slope with respect to the present horizontal surface 0.56 mlkm 11.4 ka BP, 0.38 mlkm 9 ka BP, and 0.24 mlkm 8 ka BP, each at an azimuth of 010". From this information, the profiles of base level (minimum water surfaces) along North Channel from Picton to Cold Bath Shoal (Fig. 1) were calculated for these dates (Fig. 14). The procedure involves lowering the isostatic surface until a point on the long profile of the channel floor emerges as a sill. The surface is then projected westward at that level (assuming water flow was from west to east). To the east of the sill, the surface is lowered again until another sill emerges. The surfaces, which were originally horizontal, appear uneven in Fig. 14 because the direction of the channel varies along its length (Fig. 1) with respect to the strike of the isostatic surface. Figure 14 shows that water level in North Channel - Kingston Harbour was controlled by a sill at Kingston (referred to here as the "Kingston Sill," Fig. 2), not by Cold Bath Sill (Fig. I), as suggested by Johnston (1978) and Anderson and Lewis (1985). Water passing over the sill (presently at 18 m depth) would have fallen about 1.5 m to a small lake where it was joined by water from the Cataraqui drainage to the north,

and thence over another rapids about 5 km east and into the channel controlled by Cold Bath Sill. It was probably not until about 8 ka BP that this second rapids was backflooded (Fig. 14) and somewhat later that Kingston Sill was backflooded. This presumes that the presence of the Champlain Sea did not influence the water levels in the study region or that any influence was short lived (Pair et al . 1988). West of Kingston Sill, the floor of the channel slopes downward to a maximum depth of 77 m at the west end of Amherst Island, then upward to emerge from the isostatic surface in Adolphus Reach where the mouth of a river occupying the bottom of the present Bay of Quinte would have migrated westward as the water level rose (see also Sly 1986, Fig. 3). East of Cataraqui Bay (Fig. 2) the present lake floor was a river floodplain with a main channel along the north side containing pools to more than 10 m depth. At least two secondary channels (Fig. 3), probably anabranches from the main channel, are found on the floodplain surface to the south. Sediment fill in the channel floors (Fig. 11) indicates that they silted up, either as discharge decreased or as backflooding occurred during isostatic rebound. Between Cataraqui Bay and Kerr Point the valley was

71

GILBERT AND SHAW

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

South

North

0

111 1

FIG. 10. Subbottom record typical of channel fill between Kerr Point and Cataraqu~Bay. Abbreviations as in Fig. 3.

m North

I

F

l00m

South

I

F

FIG. 11. Subbottom record showing a bedrock knoll (Myles Shoal) in the floodplain sediments containing a small river course with levees, and overlain by a thin, discontinuous veneer of modern sediment. Abbreviations as in Fig. 3.

FIG. 12. Surface of bedrock in and near North Channel interpreted from NTS topographic map 31 Cl2 and subbottom records shown in Fig. 3. Elevations refer to sea level; the contour interval is 12.5 m. The 75 m contour approximately outlines the present shoreline.

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

72

CAN. J. EARTH

FIG. 13. Map of the small channel in Lower Gap between Salmon Island and Melville Shoal (Fig. 2) drawn from Canadian Hydrographic Service field sheets 8078 and 8079. Isobaths in metres below chart datum approximately show the bedrock surface, as the sediment cover here is absent or thin (Fig. 3).

occupied by a larger, deeper channel with shallow shelves along the sides. The depth of this upper shelf is plotted in relation to the calculated water surfaces in Fig. 14b. The lowest surface (the strong reflector at the base of facies 1 in the most prominent of these shelves, for example in Fig. 4 ) occurs several metres below the lowest water surface. The difference in slope between a trend line through the shelf elevations and the 11.4 ka water surface is 0.10 mlkm. Since the stream flow-

SCI. VOL. 29,

1992

ing through North Channel would have had a slope from west to east, it may be assumed that this difference approximates that slope. We propose that this strong reflecting surface is an erosional base associated with this lowest water level immediately after the lake drained from the last ice dam. Sediments of facies 2 were deposited during the falling levels of Lake Iroquois or the earliest stage of Lake Ontario from glaciolacustrine materials eroded from higher land nearby now exposed. The presence of gas in the shelf sediments suggests the decay of abundant aquatic vegetation that may have established there and trapped glaciolacustrine sediment from the now exposed land to the north and south, and from the slow-moving river nearby. As the water levels rose, sediment accumulated on these shelves and they probably appeared much like the marshes flanking the Cataraqui River at Kingston today (Dalrymple and Carey 1990). The water level rose faster than the sediment accumulated and eventually aquatic vegetation could no longer survive. Sedimentation continued in the modern lake (facies I ) . In the absence of data on currents, we speculate that higher velocities along shore in a coastal jet (Csanady 1978) may account for the lower rates of sedimentation along the backshore of the shelves. The upper shelf is the only prominent surface, although several small shelves at higher elevations could be plotted from the records (Fig. 14b). These would have formed in remaining pockets of soft sediment as the lake level rose. The acoustic records show that there are almost no submerged cliffs in bedrock, although at the present water level in North Channel, cliffs 5- 10 m high are common in the Ordovician limestone. These modern shore cliffs appear to be created by wave action and especially freeze -thaw processes (cf. Trenhaile and Rudakas 1981) and ice push (Gilbert 1991). We suggest that the long period (since about 4 ka BP) during which the water has been nearly constant at the present level (Anderson and Lewis 1985) has allowed these to develop, whereas they could not in the more rapidly rising early Holocene water levels. West of Kerr Point the lake floor was up to 50 m below the lowest calculated water surface (Fig. 14). In this region North Channel would always have been occupied by a lake with inflow from the west and later the south (see below), and outflow into the channel east of Kerr Point. The lower shelves reaching a maximum of about 8 m below the lowest calculated water plain are found only in this region. We propose that these less prominent shelves represent wave base in this small lake. As the lake levels rose, modern sediments (facies 1 ) were deposited on top, but here may be indistinguishable from the earlier deposits in this slowly deepening lake. Why the shelves in the lake west of Kerr Point trapped gas and those to the west did not is unknown, although it may relate to coarser, more permeable sediments being deposited along the river than in the lake. The size of Lake Kingston (Fig. 1) and its relation to North Channel may also be calculated by applying the isostatically tilted surfaces (Anderson and Lewis 1985) to the bathymetric data in the same manner as was done to calculate the profile of North Channel. Figure 15a shows the profile of the small channel in Lower Gap ascribed to subglacial erosion (Fig. 13) in relation to the calculated water levels. A sill at 19.2 m below the present water surface would have controlled the water level in Lake Kingston until after 8 ka BP. In its earliest phase, Lake Kingston occupied the deeper, eastern portion of the basin (Fig. 1 ) and several small lakes and streams drained

73

GILBERT AND SHAW

pmmimnt shdf

rn

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

4

-

.

--Amherst Is.

*

m a l l &alf

4

FIG.14. (a) Long profile of Adolphus Reach to Kingston through North Channel showing calculated water surfaces during the early stages of Lake Ontario. (6) Detail of a portion of the channel showing the relation of the shelves to the calculated water planes.

from the northwest. The bathymetric data for the area to the southwest are insufficient to show whether a river connected Lake Ontario with Lake Kingston north of Main Duck Island at that time. A broad, gently sloping channel leads out of Lake Kingston to the sill in Lower Gap. Downstream to North Channel the slope was much steeper (0.96 m/km with respect to the 11.4 ka BP surface) and the old subglacial channel would have been scoured of sediment by the higher stream velocities here. By 8 ka BP Lake Kingston had doubled in area, flooding especially to the northwest. The calculated water levels (Fig. 156) indicate that between 9 and 10 ka BP Upper Gap also became an outlet for Lake Kingston to North channel, but that the slope and therefore the discharge were much lower than in Lower Gap. By 8 ka BP the isostatic surface of Lake Kingston at Upper Gap was actually 0.1 m lower than in North Channel, so discharge probably reversed and a small amount of North Channel flow went south into Lake Kingston at Upper Gap. Also by 8 ka BP it is probable that Lake Kingston and Lake Ontario were joined through a gap in the DuckGalloo sill to the west of Main Duck Island (Fig. 1). However. it is still probable that thc Lower Gap outlet was a relatively small stream and that it did not carry much of the outflow from Lake Ontario. Core 30 described by Anderson and Lewis (1985) from the channel leading out of Lake Kingston to Lower Gap indicates deposition-in quiet or slowly-flowing water around 8 ka BP. An important aspect of the development of North Channel is the role it played in drainage of southeastern Ontario and

possibly the upper Great Lakes. According to Lewis and Anderson (1989), Lake Algonquin and Lake Agassiz drained to Lake Ontario through the Kirkfield outlet for a period of at least several hundred years about 11 ka BP. That drainage would have been directed through the Bay of Quinte and North Channel to the St. Lawrence River. The dimensions of the cross section of the channel from Kerr Point east (Fig. 4) and the palaeoslope indicated by the difference in the slope between the prominent upper shelf and the calculated water plane (0.1 mtkm: Fig. 14) may be used in the Manning equation to estimate a bank-full (to the level of the strong reflecting surface under facies 1) discharge of the order of 10000 20000 m3/s. The present mean annual discharge of the St. Lawrence River is 9800 m3/s, with flood peaks to 150% of the mean. Thus, although the channel in North Channel could probably carry at lowest water level the present discharge of the St. Lawrence, there is some question that it could have carried the greater discharge that might be expected from glacial meltwater inflow to the upper Great Lakes plus the very large additional component from Lake Agassiz. Lewis and Anderson (1989) indicate that an outlet of the upper Great Lakes opened at Port Huron (the present drainage pattern) about 11 ka BP. This would have drawn off an increasing portion of the discharge until both outlets were closed when the outlet to the Ottawa valley opened about 10.5 ka BP. Thus, except in the earliest phase, North Channel may have carried less discharge than the total drainage of the upper Great Lakes. The absence of a large area of deltaic deposits in the western section of the study area also indicates that large inflows

74

CAN. J. EARTH SCI. VOL. 29, 1992

and postglacial environments were controlled by these channels. Acoustic facies include glacial or ice-proximal sediments overlying bedrock in part of the channel floor, glaciolacustrine sediments deposited and selectively eroded in a high-energy proglacial lake, sediments deposited by rivers responding to a rising local base level, and finally the lacustrine sediments of the modern Lake Ontario.

Acknowledgments

-

4

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

-

-

D

6 ka

-

-

35

0)

1 I

0

V.E. x 250

South 2

4 krn

I

North

6

8

FIG. 15. Long profile of ( a ) Lower and (b) Upper gaps showing calculated water surfaces during the early stages of Lake Ontario when Lake Kingston drained to North Channel. a accounts for the higher level of Lake Kingston due to the sill in Lower Gap.

to North Channel either did not occur or lasted for only a short time. After the closing of the Kirkfield outlet of the upper Great Lakes the drainage basin of North Channel would have been similar in area to the present basin, which is (including the area of Lake Kingston) 2.0 x lo4 km2. Present measured runoff in streams tributary North Channel averages 0.30 mla (data from the Water Survey of Canada gauging records). Thus, North Channel may have carried a mean discharge of about 200 m3/s, or about two orders of magnitude less than the maximum during the Kirkfield outlet stage. The geometry of the partially filled large channel on the floodplain at Kingston indicates that it had the capacity to carry only up to about three times that flow. This suggests that the channel became adjusted to this lower flow before the floodplain was backflooded by the rising water of Lake Ontario.

Conclusions The results of this study add to the growing evidence of the role of subglacial fluvial processes in shaping both large and small elements of landscape, and provide detail that supports the general model of the evolution of early Holocene geography and hydrology of northeastern Lake Ontario. We conclude that large volumes of meltwater under high pressure created or enlarged subglacial channels throughout the region at least once, and probably often, during the Pleistocene glaciations. North Channel, which extends from a general land surface elevation of about 100 m as1 to more than 25 m below sea level, is the largest of these, and is also the largest element of landscape in the region. Based on their shape and relation to North Channel, smaller forms, including the channel in Lower Gap, are also interpreted as having been created by this process. Subbottom acoustic data provide evidence that late glacial

The study was supported by grants from the Natural Sciences and Engineering Research Council of Canada to both authors. Comments by W. W. Shilts and an anonymous reviewer helped us significantly. Anderson, T. W., and Lewis, C. F. M. 1985. Postglacial water-level history of the Lake Ontario basin. In Quaternary evolution of the Great Lakes. Edited by P. F . Karrow and P. E. Calkin. Geological Association of Canada, Special Paper 30, pp. 231 -253. Brodie, J. W., and Irwin, J. 1970. Morphology and sedimentation in Lake Wakatipu, New Zealand. New Zealand Journal of Marine and Freshwater Research, 4: 479-496. Csanady, G. T. 1978. Water circulation and dispersal mechanisms. In Lakes chemistry geology physics. Edited by A. Lerman. Springer-Verlag, New York, pp. 21 -64. Dalrymple, R. W., and Carey, J. S. 1990. Water-level fluctuations in Lake Ontario over the last 4000 years as recorded in the Cataraqui River lagoon, Kingston, Ontario. Canadian Journal of Earth Sciences, 27: 1330- 1338. Desloges, J. R., and Gilbert, R. 1991. Sedimentary record of Harrison Lake: implications for deglaciation in southwestern British Columbia. Canadian Journal of Earth Sciences, 28: 800 - 8 15. Gilbert, R. 1983. Sedimentary processes of Canadian arctic fjords. Marine Geology, 36: 147 - 175. Gilbert, R. 1990. Evidence for the subglacial meltwater origin and late Quaternary lacustrine environment of Bateau Channel, eastern Lake Ontario. Canadian Journal of Earth Sciences, 27: 939-945. Gilbert, R. 1991. Ice pile-up on shores in northeastern Lake Ontario during winter 1990. GCographie physique et Quaternaire, 45: 241 -244. Johnston, L. M. 1978. Geolimnological studies in the Kingston Basin - Upper St. Lawrence River Region. Ph.D. thesis, Queen's University, Kingston, Ont. Kaszycki, C. A. 1987. A model for glacial and proglacial sedimentation in the shield terrane of southern Ontario. Canadian Journal of Earth Sciences, 24: 2373 -2391. Lewis, C. F. M., and Anderson, T. W. 1989. Oscillations of levels and cool phases of the Laurentian Great Lakes caused by inflows from glacial lakes Agassiz and Barlow-Ojibway. Journal of Paleolimnology , 2: 99 - 146. Mayer, L. 1979. The origin of fine scale acoustic stratigraphy in deep-sea carbonates. Journal of Geophysical Research, 84: 6177 6184. McFall, G. H. 1990. Faulting of a Middle Jurassic, ultramafic dyke in the Picton Quarry, Picton, southern Ontario. Canadian Journal of Earth Sciences, 27: 1536- 1540. Murray, E. A. 1988. Subglacial meltwater erosional marks in the Kingston, Ontario. Canada region: Their distribution. form and genesis. M.Sc. thesis, Queen's University, Kingston. Ont. Naldrett, D. L. 1991. Varves of eastern glacial Lake Iroquois. G e e logical Association of Canada, Program with Abstracts, 16: A89. Pair, D., ffirrow. P. F.. and Clark. P. U. 1988. History of the Champlain Sea in the central St. Lawrence iowland. New York, and its relationship to water levels in the Lake Ontario basin. In The late Quaternary development of the Champlain Sea basin. Edited by N. R. Gadd. Geological Association of Canada, Special Paper 35, pp. 107-124.

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of Alberta on 11/07/13 For personal use only.

GILBEIU A N D SHAW

Schubel, J. R. 1974. Gas bubbles and the acoustically impenetrable, or turbid, character of some estuarine sediments. In Natural gases in maritime sediments. Edited by I. R. Kaplan. Plenum Press, New York, pp. 275 -298. Shaw, J. 1988. Subglacial erosional marks, Wilton Creek, Ontario. Canadian Journal of Earth Sciences, 25: 1256- 1267. Shaw, J., and Archer, J. 1979. Deglacisrtion and glaciolacustrine sedimentation conditions, Okanagan Valley, British Columbia, Canada. In Moraines and varves. Edited by Ch. Schiichter. A. A. Balkema, Rotterdam, pp. 347-355. Shaw, J., and Gilbert, R. 1990. Evidence for large-scale subglacial meltwater flood events in southern Ontario and northern New York State. Geology, 18: 1169- 1172. Shilts, W. W., and Kettles, I. M. 1987. Tills of the Madawaska Highland and subbottom profiles of Lac Deschenes. In Quaternary

75

of the Ottawa region and guides for day excursions. Edited by R. J. Fulton. 12th International Union for Quarternary Research Congress, pp. 105-114. Sly, P. G. 1978. Sedimentary processes in lakes. In Lakes chemistry geology physics. Edited by A. Lerman. Springer-Verlag, New York, pp. 65 -89. Sly, P. G. 1986. Review of postglacial environmental changes and cultural impacts in the Bay of Quinte. In Project Quinte: Pointsource phosphorus control and ecosystem response in the Bay of Quinte, Lake Ontario. Edited by C. K . Minns, D. A. Hurley , and K. H. Nicholls. Fisheries and Oceans Canada, FS 41-31/86E, pp. 7-26. Trenhaile, A. S., and Rudakas, P. A. 1981. Freeze-thaw and shore platform development in GaspC, QuCbec. GCographie physique et Quaternaire, 35: 171 - 182.