Download The significance of sediment deposits in large lakes

The significance of sediment deposits in large lakes and their energy
relationships
P. G. Sly
Abstract.
Sediment deposits of large lakes, although strongly influenced by the characteristics
of their source material (basinal), reflect the changes of various energy controlled processes with
time and the comparable, and associated, horizontal and vertical components.
In large lakes the distributions, movements and behaviour of sediments reflect their low energy
environment which is marked by limited wave action, restricted current circulation, and generally
tideless conditions; only in the shallower and ncarshore areas are significant erosional-transportative
processes active. Transportational-depositional processes remain the dominant feature in most
lakes. The occurrence of turbidity, density and/or slump induced sediment distributions may,
however, add a further element which relates to the characteristics of basin bathymetry and
geometry.
Resume.
La nature du dépôt de sediments dans les grands lacs depend d'un côté des
caractéristiques de la source d'alimentation et de l'autre, de la répartition dans le temps et dans
l'espace des forces dynamiques des agents de transport.
Même dans les grands lacs, la distribution, le mouvement, et le comportement des sédiments
reflètent la faible énergie de leur environnement, qui se caractérise par une activité limitée des
vagues, un courant restreint, et généralement l'absence de marée. Les phénomènes d'érosion de
transports ne sont actifs que dans les zones peu profondes et littorales. Ce sont les phénomènes
de transport-dépôt qui dominent dans la plupart des lacs.
La turbidité, la densité, la distribution des sédiments selon ics zones d'effrondrement
peuvent, cependant, ajouter un élément supplémentaire, lié à la bathymétrie et à la géométrie
du bassin considéré.
INTRODUCTION
This paper draws upon much recent information which has been compiled from
published and continuing studies in the Great Lakes of North America, and combines
it with the results of studies in other large lakes and enclosed seas to discuss how the
characteristics of sediment distribution relate to energy controlled processes in large
lakes. Although mechanical energy is by far the most important single factor
controlling sediment distributions, the influence of thermal and potential energy
cannot be ignored. The author has not attempted to illustrate various correlations on
the basis of surficial sedimentary structures such as sand waves, sand ribbons, ripples,
fluting and grooving, etc. Such features are well documented in the Great Lakes but
the details of their formative processes have not been clearly established; discussion
would, therefore, pose probably more questions than answers at this time.
INFLUENCE OF SOURCE MATERIAL
The characteristics of shoreline and subaqueous materials have a considerable impact
upon the development of actively forming sediment deposits. Along sections of the
east shores of Georgian Bay and Lake Superior (Figs. 1 and 2), for example, virtually
no recent sediment accumulations are evident. This is because the combined effects
of surface weathering and wave attack upon the resistant Pre-Cambrian bedrock have
been insufficient to produce primary sedimentary deposits through active erosional
processes, since these lake levels effectively stabilized some five to six thousand years
384
P. G. Sly
fathoms
0-10
10-20
20-40
40-60
60-80
80-100
>100
Black
Lake Superior
Sarnia
FIGURE 1.
-À
m
•m
metres
0-18
18-37
37-73
73-110
110-146
146-183
>183
The significance of sediment deposits in large lakes
fathoms
0-10
10-20
20-40
40-60
60-80
80-100
>100
385
metres
0-18
18-37
37-73
73-110
lmm
i i 110-146
146-183
>183
St Lawrence River-
Lake Ontario
Black
River
Scarborough Bluffs
Toronto
Toronto
Islands
Oswego
Hamilton
Burlington
Bar
St Clair River
Detroit
River
\
0
kilomatres
FIGURE 2.
50
386
P. G. Sly
B.P. (Farrand, 1969; Lewis, 1969; Sly and Lewis, 1972). Along the western shore of
Georgian Bay, and around the opening with Lake Huron, another example of the
lack of active sediment formation can be observed. In this case, however, the shoreline
is composed of dolomitic lithologies which are subject to solution weathering.
In Black Bay, north Lake Superior, a further example of the lack of recently
forming sediment may be seen. Much of the floor of the Bay is covered by glacial clay
till materials which, once fragmented, may be easily reworked. Wave action, however,
is insufficient to initiate the reworking of such material, and significant recent
sediment deposits are again lacking (Mothersill, 1972).
Source materials may influence the sedimentary environment in other ways. Duane
(1967), for example, has shown how the St Clair River has acted as a pipeline for
sediment transported southward from Lake Huron. By analogy, the whole St Clair—
Detroit River System may be similarly described, hence the sedimentary materials
available for reworking as secondary deposits in western Lake Erie are greatly
influenced by the character of the particulates moving out of Lake Huron.
Perhaps one of the most interesting studies, in the Great Lakes, upon the control
of active sedimentary environment by source materials has been described by
Fricbergs (1970). The Pleistocene sands and silty sands of Scarborough Bluffs, near
Toronto on Lake Ontario, are presently being eroded at rates estimated to be
between 0.38 and 0.53 m per year. The Bluffs, which exceed 100 m in maximum
relief, yield approximately 300,000 m3 of material per year under the influence
of combined sub-aerial weathering and wave attack. Despite this considerable
quantity of material, only about 19,000 m3, or 6 per cent, enters the sediment
budget of the associated longshore drift. The remaining 94 per cent appears to be
transported offshore beyond the zone of active wave influence because the particle
size is too smali.
Many similar situations also exist in Lake Erie where shoreline erosion of 'soft'
glacial and post-glacial materials is even more rapid; recession rates of 1.0—1.5 m per
year are not uncommon [Great Lakes Levels Board, Shoreline Physical Characteristics
(Canadian Federal Department Public Works), and Surveys and unpublished data,
Haras, 1972], but quantitative studies have not yet been completed in this region.
CHANGES IN ENERGY CONTROLLED PROCESSES WITH TIME
The geological record, as evidenced by sedimentary deposits, is virtually the only
means by which changes in the energy controlled processes in lakes can be evaluated
over extended time spans and prior to the availability of historical data. There are
four main aspects to such changes; (1) basin-wide changes in energy levels which are
dependent upon climatic change; (2) basin-wide changes which are related to
differences in water level; (3) basin-wide changes which are related to expansion,
and/or shallowing, due to natural erosion—accumulation trends; (4) local changes
which affect only part of a basin such as the development of spits and bars, or
increased/decreased inflows due to river capture or similar drainage modifications.
In regard to the first aspect, outlined above, ample evidence exists in the Great
Lakes of very different energy regimes which persisted in immediate post-glacial
times. The glacio-lacustrine clays, for example (and varved deposits formed near
ice-fronts), remain in sharp contrast to the recent mud deposits even in the deepest
part of Lake Ontario where the depth of water has remained considerable since
immediate post-glacial times and sedimentation has been continuous (Kemp,
unpublished data). It is rarely possible, however, to imply climatic change purely
upon the basis of the sediment /energy relationships and confirmatory evidence
must be drawn from other sources.
Sediment changes which are related to both major and minor changes in water
levels, within the Great Lakes are of major significance. For example, the occurrence
The significance of sediment deposits in large lakes
387
of the high-level Lake Iroquois beach bar at Hamilton, western Lake Ontario, at an
elevation of approximately +30 m above present lake level (Karrow, 1963), and dated
approximately 12,000 years B.P.; the submerged Admiralty beach bar at in ele/ation
of approximately—70 to— 79 m below present lake level in western Lake Ontario,
and dated at about 11,000 years B.P. (Lewis, 1969); and the present Burlington beach
bar (existing lake level is about + 75 m above sea level), which has developed in
response to generally stabilized lake level conditions since the close of the Admiralty
phase of Lake Ontario.
Again, in Lake Ontario, the presence of extensive thin (1—5 cm) sand and silty
sand deposits along the northern and eastern parts of the lake immediately above the
glacio-lacustrine clays, and often beneath the recent deep water mud accumulations
(Lewis and McNeely, 1967; Sly and Lewis, 1972) provides strong supportive evidence
of rapid shallowing and then deepening of water depth consequent with the Admiralty
and post-Admiralty phases of Lake Ontario.
It is difficult to provide unquestionable data in support of basin-wide changes
which are related in infilling and shallowing as a result of separate input and/or
shoreline recession; the reason being that features reflecting such changes are often
localized or else developed at a comparable rate to those which more properly relate
to changing water level.
Nevertheless it may be seen that, with an estimated sediment accumulation rate
of about 0.2—0.3 cm/year in the western basin of Lake Erie (Kemp, personal
communication) and shoreline erosion rates of up to 1.5 m/year (previously cited),
significant energy changes could be reflected by sedimentary response over a water
level stability period of 2000 years or more. An uncorrected first-order estimate
based upon such values might imply a shallowing of the basin by 30—35 per cent and
an increase in its surface area by as much as 15—20 per cent. Details of such an
hypothesis, however, await proof from further sediment core studies in Lake Erie.
Within the Great Lakes there are also examples of local changes of energy,
particularly that related to wave action, as deduced from sedimentary features. The
development of the Toronto Island (Lewis and Sly, 1971) for instance, has provided
an extensive I ago on al area of low wave energy, the growth of which has really only
stabilized during the past 80 years or so under the influence of man-made structures.
In eastern Lake Erie, however, Long Point continues to extend southeastward s at a
rate of about 7 m per year and thereby adds about another 6-8 percent to the area
of the existing bay on the north side, in the form of a protected zone, each 100 years.
On the south shore of Lake Erie, at Presque'isle peninsula, a corresponding eastward
migration of about 5.5 m per year was recorded by Berg and Duane (1968) prior to
the implementation of techniques to slow the movement of the feature.
SIGNIFICANCE OF WAVE ENERGY
Surface waves interact with bottom sediments in two ways: (1) as breaking waves in
the beach zone, and (2) in that zone only affected by the orbital velocities of water
motion (beyond the breaker zone). The beach zone comprises a narrow vertical range,
either side of the water plane and is, in part, related to wave height. Beyond the beach
zone there remains an area which becomes progressively less affected by wave energy
(for every one-ninth of the wave length the orbital motion is approximately halved)
and at a depth of one-quarter wave length, motion is about 21 per cent; at a depth of
one-half wave length, motion is about 4 per cent; and at a depth of three-quarters of
the wave length, motion is less than 1 per cent of that of the surface.
When considering the influence of wave action, therefore, the two zones may be
viewed together and, as a generality, wave base (WB) is here defined as that depth
corresponding to not less than 25 per cent of the available wave motion. Using this
26
388
P. G. Sly
value, Table 1 indicates that percentage of each of the Great Lakes which lies within
effective wave base (as defined above) based upon the estimated development of
significant waves (H3) under conditions of cross lake fetch (maximum fetch values are
provided for comparative purposes; they are rarely developed).
TABLE 1.
Wave influence on Great Lake sediments, generalised
Lake
Max.
fetch
(km)
Superior
Michigan
Huron
Georgian Bay
Erie
Ontario
580
420
350
200
340
290
/Mm)
WB25%
<m)
Gross
fetch
(km)
/Mm)
WB 25% %
(m)
effected
7-8
6
5
3.5-4
5
4-4-5
58
46
42
28
42
37
250
95
160
70
90
75
4
1.5-2
2.5-3
1-L5
1.5-2
1.5
13
17
23
12
17
12
5%
5
±=S
=5
^20
5
Two interesting points emerge from this crude comparison: (1) the area affected
by wave activity does not markedly increase even if wave base is extended to a depth
of half the estimated significant wave length in any of the Great Lakes, except Lake
Erie; (2) that, despite the morphological difference between the other Great Lakes,
their effective zone of wave base remains at about 5 per cent or less of the lake area.
The distribution of surficial sediments in both Lake Ontario and Lake Huron have
been described by Thomas et a!. (1972). These authors have clearly demonstrated that
the sand-sized materials occur almost exclusively in the shallower shoreline and
nearshore areas, where they form 32 and 38 per cent, respectively, of the total
sediment composition of each of these two lakes. Such values can only imply that
the occurrence of sand-sized particulates, in these two lakes, is related to more factors
than surface wave energy along. In Lake Ontario it has been suggested (Jonys,
unpublished data) that about 55 per cent of the sand-sized material is contributed from
river inflows; the remaining 45 per cent, however, must be accounted for by processes
active in the nearshore zone, and of that about 28 per cent must be accounted for by
processes other than surface wave action. It seems probable that this discrepancy can
be largely accounted for by various current activities.
In the nearshore area wave action provides some of the transportée energy and
most of the erosional energy; such activity has two major components: (1) longshore
drift, and (2) onshore/offshore migration. The lack of sand particulates in most Great
Lakes deep-water sediments (Thomas et ai, 1972) indicates that the onshore/offshore
process is limited in extent and only rarely provides a 'sediment escape' from what is
otherwise an essentially 'closed-system' in the nearshore.
The response of Great Lakes beaches to longshore drift has been documented by
many authors, Berg and Duane (1968), Rukavina (1969, 1970), and Hands (1970),
and there seems to be little doubt that certain well-defined net sediment transport
paths have been active since lake levels and climatic conditions generally stabilized in
post-P!eistocene times. Bajorunas (1970) studied littoral transport phenomena in
southern Lake Huron where wave duration was found to be the significant variable
if less than about 12h, whilst effective shoreline length (20 km) became a critical
control for more extended time periods. The dimensional analysis indicated that
transportational rates increased with the square root of the sediment size (fine to
medium sands). It was found that the theoretical equations [e.g. material transported
— (KH2L cosa)/8 (Bascom, 1964)] which utilized wave energy, angle of approach and
shoreline length, could give a reasonable estimate of sediment movement if wind,
waves, and currents held the same general direction. However, in the complex
wave/current environment of the nearshore, such estimates could fail, and in such cases
The significance of sediment deposits in large lakes
389
current speed and direction became the determining factor. In this regard Saylor
(1966) showed that wind-generated currents could reach equilibrium with the wind
at about 0-033 of the wind speed. This occurred rather quickly (1—3 h), and such
currents could persist for days after wind cessation. Currents in the open lakes flowed
10—15° to the right of the wind direction with current speed and direction almost
independent of surface wave action. Local variations in the shoreline frequently
caused currents and waves to oppose, and under such conditions the data showed a
strong correlation between sediment transport and current speed. There appeared
to be no upper limit to this relationship.
As an example of this latter point Freeman et al. ( 1972) discussed the effects of a
severe storm surge at Sarnia, also in southern Lake Huron, during August 1971. During
this storm approximately 60,000 m"1 of sandy material were eroded, but only about
25,000m 3 were later redeposited on shore; the remainder was almost certainly
transported southwards, likely by a persistent longshore current having a continuing
velocity in excess of 1000 m/h (28 cm/s).
Studies on onshore/offshore sediment migrations in the Great Lakes have been
much less well developed than those related to longshore drift. Cook (1970) made an
important comment on this type of energy response by lacustrine beaches: 'Certain
differences seem to exist in the hydrodynamic regimes of high and low energy
shorelines. While directional inequalities in wave-induced oscillations are not
pronounced in the nearshore zone of an ocean beach, shoreward surges are notably
dominant in lakes or wave tanks. Also, rip currents are not well developed along
long energy beaches, and water may return offshore by other means. These disparities
cast doubt on the applicability of sand transport models for oceanic coasts to
protected lacustrine shores.' Lewis and Siy (1971), in their study off Toronto, in Lake
Ontario, suggested that there might be grounds for supporting the null-point theory
(Miller and Zeigler, 1964); this could be expected where wave affected environments
remained uninfluenced by tidal processes.
At the present time, the understanding of on shore /offshore processes has not been
well developed as it relates to the Great Lakes environment.
An additional comment might also be made at this point, in relation to internal
waves. Although internal waves are a common feature in the Great Lakes (Boyce,
personal communication), their significance in turbulent mixing zones is not known
as it may relate to sediment erosion—transportational mechanisms.
CIRCULATIONS AND FLOW
Current and circulatory phenomena in the Great Lakes represent an extremely
complex interaction of geostrophic circulation, wind stress response, thermal density
gradients, and kinetic flows related to inflowing waters. The observed long-term water
movements appear to be largely geostrophic in nature but with modification by
persistent (and occasional) baroclinic phenomena (e.g. Lake Michigan and southern
Lake Ontario) and the continuing driving force of large-scale inflows such as the
Detroit River into Lake Erie, and the Niagara River into Lake Ontario.
The geostrophic circulations have generally low velocities of the order of a few
centimetres per second; they also tend to show less velocity difference with depth.
The superimposed baroclinic circulations show considerably higher velocities (Csanady
and Pade, 1968, 1969; Scott ctal., 1971) of up to 50 cm/s or more at or near surface;
in their development along the south shore of Lake Ontario near Oswego, such currents
have been observed in a band 4—8 km offshore and to a depth of about 35 m. Scott
et al. (1971) also commented upon the mass transport which related to such currents
and estimated that mean daily transport was about 2.4 km 3 /day, and maximum
transport was possibly in excess of 5 km 3 /day. The magnitude of such figures may be
390
P. G. Sly
gauged by comparison with the average inflow of the Niagara River at a comparable
time; the baroclinic flow being four to six times greater than that of the Niagara River.
Studies on barotrophic circulations suggest that similar high velocities may aiso
occur.
Seiches are important in the Great Lakes and maximum ranges of from 2.56 m in
Lake Erie to 0.76 m in Lake Huron have been recorded (McDonald, 1954).
Oscillation periods in Lake Erie are from 14 to 16 h, and for Lake Ontario rather less.
Most seiches in the Great Lakes have responded to wind stress rather than barometric
gradients.
The development of upwelling circulatory phenomena in the Great Lakes is common
and has been observed throughout the year. Lee (1972) has discussed aspects of the
available data with regard to Lake Ontario, and Burns and Ross (1972) have discussed
similar phenomena in Lake Erie. It is worth commenting that measured epilimnion
currents in Lake Erie exceeded 75 cm/s for short periods and that a hypolimnion
current of more than 95 cm/s was also measured; mostly, however, epilimnion flow
exceeded hypolimnion flow by an order of magnitude.
The magnitude of inflow, by rivers, to various of the Great Lakes has already been
mentioned; however, it is interesting to note that the total discharge of the Niagara
River averages about 5500 m3/s and ranges between about 3400 and 6950 m3/s. The
current ai the point of inflow to Lake Ontario ranges between 30 cm/s and about
300 cm/s, with generalized flow of about 100 cm/s. This inflow to Lake Ontario is
significant in maintaining an easterly flow along the south shore of the lake and as a
driving force in maintaining circulatory systems throughout the lake. The extent of
the influence of the river flow in the lake has been clearly demonstrated by Simons
and Jordan (1972).
SEDIMENT RESPONSE TO CIRCULATORY PROCESSES
It is impractical at this juncture to try to correlate sediment distributions with
specific geostrophic, baroclinic or barotrophic regimes. Examples are, however,
provided which indicate 'preferred' relationships with geostrophic and other circulatory'
processes.
McAndrews(1972), discussed the presence of pollen in Great Lakes sediments and
in particular the correlation between unique point sources (the Black River, Lake
Ontario) and general lake circulation. It was demonstrated that fine particulates from
the Black River entered the main basin of the lake and distributed in a manner
probably reflecting a geostrophic gyre.
The computed circulations of Simons and Jordan (1972) have also been compared
with the distribution of silt and clay size fractions in the deeper waters of Lake
Ontario, as described by Thomas (1969), and Thomas et ai (1972). The presence of
three major circulatory 'cells' in the main basin of Lake Ontario bears a striking
resemblance to the three sedimentological sub-basins. In addition (Thomas,
unpublished data), there is strong evidence from the distribution and composition of
bottom sediments to suggest the incorporation of Niagara River derived materials in
the anti-clockwise movement of Simons' and Jordan's most westerly 'circulatory cell'.
In Lake Erie, Hartley (1968), and Burns and Ross (1972) demonstrated the
persistence of cross lake bottom currents having a strong northerly component to their
flow, particularly in the area immediately east of Point Pelée. A corresponding
association has been established with the fine bottom sediments of this area and
Lewis (persona! communication, and unpublished data) has evidence for the
continuing dispersal of fine materials, northwards in the area and in the form of
extending bottom deposits.
St John et ah (1972) discussed the implications of the distribution of mercury in
the sediments of Lake Ontario (based upon published and unpublished data). The
The significance of sediment deposits in large lakes
391
occurrence of mercury in the bottom sediments (as a geochemical tracer) implies
strong correiation with both the eastward flow along the south shore of the lake,
induced by the Niagara River, and has a further relationship to the (more easterly)
baroclinic circulation (Scott et ai, 1971) as exemplified by the dispersal pattern of
additional mercury inputs from the south shore.
Studies by Ayers and Hough (1964), Gross et at. (1970), Kennedy et al (1971),
Linebach et ai. (1970), Shimpeïa/. (1971), and Grosser ai (1972), have
demonstrated considerable correlations between sediment accumulations and
their sedimentological and geochemical characteristics, and the circulatory phenomena
of southern Lake Michigan; the correlation appears to be related both to geostrophic
conditions and the kinetics of inflow.
Seiches, as such, have little effect upon bottom deposits; rather the storm activity
with which they are associated induces severe turbulence and wave action. Their
significance lies in the fact that because of the considerable fluctuations in water
level, wave action is able to influence shoreline and nearshore. areas normally above or
below such activity. Hence, seiches are frequently associated with severe erosionai
conditions; the mass water movements which occur with seiches may also further
extend the transportive capability of local current and circulatory regimes.
Seiche effects, therefore, provide a means for extending the transportive capacity
of water movements under specific conditions and may account for the presence of
some coarser sediment deposits beyond the range of normal depositional conditions
(Author, personal observations).
In Lake Ontario a unique set of circumstances permit a discussion on the association
of bottom sediments to upwelling. The existence of deep-water return flows, and
upweiling has been implied by a number of workers, and Scott et ai (1971), and Lee
(1972) have indicated that the north shore of Lake Ontario (eastwards from Toronto)
may be considered as a weil-defined zone of fairly continual upwelling. The following
synopsis has been made from St John et ai (1972):
The occurrence of ferromanganese concretions has been reported from many
Canadian lakes (Kindle, 1932, 1936; Harriss and Troup, 1969; Cronan and Thomas,
1972). Many modes of formation have been suggested though the variations relate to
the source of the primary chemical constituents, Mn and Fe. In all cases (both marine
and freshwater) certain environmental conditions are a prerequisite:
(1) Nodules occur only in regions of a slow or nil depositional rate. Higher rates'
decrease exposure time at the sediment—water interface and reduce the possibility of
formation. Additionally, continual sedimentation and burial, with decreasing redox
potential with depth result in the solubilization of existing concretions.
A lack of sedimentation in a lake implies a medium energy regime sufficient to
prevent settlement, yet insufficient to cause breakage due to attrition. Mass water
flow over the region will also bring about continuous replenishment of the Fe and
Mn (in solution) necessary for the continued growth of such nodules. (This supposes
that precipitation from overlying waters is a major source of the Fe and Mn).
In Lake Ontario ferromanganese concretions occur in water depths of from 30 to
60 m, following the general outline of the north shore, and occur predominantly in
the form of a coated sand. The sand is of lag origin, well sorted at the surface, and
overlies glacial clays from which it has been derived by the winnowing of finer
sediments. This deposit exists under moderately eroding, or certainly non-depositing
sedimentary conditions and thus defines the existence of a significant medium energy
zone in an offshore lake bottom environment.
(2) The precipitation of Fe and Mn is dependent on Eh and pH conditions.
Under known pH conditions, it is thus possible to define the redox potential occurring
at the sediment—water interface. Further, variations in the Fe/Mn composition of the
concretions can be attributed to selective fractionation of the Fe and Mn during
392
P. G. Sly
precipitation under varying Eh or pH. The variation of the Fe/Mn ratio in concretions
relative to the spatial variation of sediment Eh has been demonstrated by Cronan and
Thomas (1972). These data have been confirmed by additional solubility experiments
on Lake Ontario concretions by Nriagu (1972).
The authors therefore imply that Fe/Mn coated sands receive a precipitate from
upweliing waters, and the selective fractionation of Fe and Mn which appears to exist
in this sedimentary deposit reflects the influence of changing sedimentary properties,
although perhaps also changes in the upweliing water itself.
TOTAL ENERGY RELATIONSHIPS
By using the data of Thomas etal. (1972) concerning the distribution of sediments in
Lakes Ontario and Huron, the data of Seibold (1967), and Seibold et al (197I)
relating to the Baltic Sea; the data of Pelletier (1968) on Hudson Bay; and that of Sly
(1966) on the sediment distributions in Liverpool Bay, a rough 'order of magnitude'
comparison may be made on sediment energy level of such differing environments.
It may be seen that the total sediment energy level of the nearshore environment of
the Great Lakes is considerably lower than that observed in open marine conditions;
it is also lower than that observed in restricted marine conditions (such as Hudson
Bay) but varies rather less in comparison to the Baltic Sea. The extremes are illustrated
by the sand: silt and clay ratios; Liverpool Bay 14.1:1, Lake Huron 1.69 : 1 , and
Lake Ontario 1.39:1.
Pelletier (1972) has considered such relationships further and considers that the
silt : clay ratio may be additionally diagnostic of low energy environments. Such a
relationship may not, however, be suitable for cross-correlation beyond the freshwater
environment because of the influence of flocculation.
At the present time it is not possible to indicate how the deep lacustrine sediments
may compare (In terms of relative energy levels) to those of the Baltic, Hudson Bay,
or the open marine environment; the variations are likely similar, though less extreme
(the silt : clay ratios of both Lake Huron and Lake Ontario are about 1.6 :1).
OTHER SEDIMENT-ENERGY RELATIONSHIPS
Under this heading consideration is given to density and turbidity currents, slump
flows, shear effects and convection.
The occurrence of density flows (for example river inflows having a different
temperature to that of the surrounding lake water) have been well documented from
many points of the Great Lakes and, during the spring runoff and early summer, the
discreet transport of sediment burdened water, as a density flow, is a common feature.
Many examples of density flows have been quoted in the literature, perhaps the most
applicable being that of Anderson and Pritchard (1951), which relates to density flows
in Lake Mead, and that of Forel (1895), and Dussart (1948), on Lake Geneva.
In the Great Lakes, however, little work has yet been undertaken to relate the
evidence of sediment distribution to such phenomena, although such relationships
are known to exist.
Turbidity currents, geologically speaking, have sometimes been regarded as distinct
from density currents, in that their very description implies some form of internal
turbulent mixing during the process of transportation. The deposits associated with
such flows are 'classically' typified by the presence of poorly sorted materials which
include a generally broad selection of particle types and sizes. The distinction between
density and turbidity flows in the Great Lakes may be considered 'fine', and the
interpretation of such phenomena by Gould (1960), as it relates to the Lake Mead
situation, is probably quite satisfactory for general application. The existence of a
The significance of sediment deposits in large lakes
393
turbidite type sediment has been recorded by Hough (1963), in Lake Michigan, and
no doubt similar deposits exist in other areas of the Great Lakes, but have so far
escaped detection.
Lewis and Sly (1971) have discussed the possible origin of sediment deposits,
offshore from Toronto; and seismic data suggests that slope instability may have given
rise to slumping. This is further supported by similar sedimentary structures noted
off the mouth of the Niagara River (Author). Evidence for sediment slumping, in the
deeper parts of Georgian Bay and Lake Huron, may also be found on many echo
traces, though the question arises as to whether or not such movement and
redistribution of material is in response to an unstable morphology (Bostrom and
Sherif, 1971), or in response to some form of current induced shear.
Lewis and Sly ( 1971 ) have also commented upon the possible origin of some recent
thin sediment accumulations off Toronto. It has been proposed that these anomalous
deposits may reflect the existence of a natural 'dumping-zone' at the 'vertical'
interface of the more rapidly moving nearshore circulation and the slower deep-water
basin regime. It is impossible at the present to further substantiate such a hypothesis,
but no doubt other situations such as the existence of thermal bars (Rodgers, 1968)
could provide a similar circumstance.
The influence of convection currents upon sediment distributions in large lakes has
not, to the author's knowledge, been considered in the Great Lakes. However, the
work of Gould and Budinger ( 1958) suggests that this may be a further aspect of
sediment-energy relationships which should be considered.
CONCLUSIONS
The relationships which appear to exist between large lake sediments, and the
associated energy levels of their environment, indicate a close correlation which is
specific to the lower energy fields of the sediment erosion—transportation—deposition
diagram of Hjulstrom (1939).
Because the influence of source materials upon derived sediments may control the
response of such sediments to various 'energy fields' particular care must be taken in
the interpretation of particle size variations; it may be necessary to consider more
sensitive indicators of response.
Models of nearshore processes designed for use in the marine environment do not
properly equate to the lacustrine environment, where both the factors and their
relative significance require reconsideration.
Lake systems are generally 'closed' and the associated sediment distributions
reflect decreasing energy from nearshore to offshore; such distributions, however, are
also sensitive to subtle changes in effective energy and selected variations can be
shown to reflect kinetic, baroclinic, barotrophic, upwelling and thermal situations.
At the present time sediment data relating to density, turbidity, slump, and convection
effects, in large lakes, is limited. The significance of such phenomena appears to be
restricted (there are exceptions, Lake Geneva for example) and its importance in
terms of the overall sediment—energy relationship is generally limited (though this
is not necessarily true of smaller lakes or reservoirs, which may be considered as
special cases).
Acknowledgements.
The author would like to express considerable thanks to many colleagues
ai the Canada Centre for inland Waters (CC1W) for their invaluable assistance, provision of data,
and valuable discussions. In particular, the help of R. L. Thomas, A. L. W. Kemp, C. F. M. Lewis,
and F. M. Boyce is gratefully acknowledged.
The author would like to indicate his appreciation for encouragement by R. A. Vollenweider,
and to R. L. Thomas for critical review of the manuscript.
The Director (CCIW), J. P. Bruce, is acknowledged for permission to publish this material.
394
P. G. Sly
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