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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 REFERENCES Anderson, E. R. and Pritchard D. W. (1951) Physical Limnology of Lake Mead: Navy Electronics Lab. Report no. 258. Ayres, J. C. and Hough, J, L. (1964) Studies ofWater Movements and Sediments in Southern Lake Michigan: Part II — The Surficial Bottom Sediments in 1962-1963: Univ. Mich., Great Lakes Res. Div. Spec. Publ. no. 19. Bajorunas, L. (1970) Littoral transport and energy relationships. Proc. Twelfth Coastal Engineering Conf., pp. 7 8 7 - 7 9 8 : Washington. Bascom, W. (1964) Waves and Beaches - the Dynamics of the Ocean Surface: Doublcday and Co. Ltd, New York. Berg, D. W. and Duane, D, B, (1968) Effect of particle size and distribution on stability of artificially filled beach, Presque'isle Peninsula, Peun. Proc. Eleventh Conf Great Lakes Res., pp. 161-178: Internat, Assoc, Great Lakes Res,, Milwaukee. Bostrom, R. C. and Sherif, M. A. (1971) The soil cover of the ocean floor. Proc. First UNESCONSF-Univ. Wash. Internat. Symposium on the Engineering Properties of Sea-floor Soils and their Geophysical Identification, pp. 140-155: Washington. Burns, N. M, and Ross, C. (1972) Project Hypo, an intensive study of the Lake Erie central basin hypolimnion and related surface water phenomena. Canada Centre for Inland Waters. Paper no. 6: United States Environmental Protection Agency, Technical Report no. TS-05-71-208-24. Cook, D. O. (1970) Models for nearshore sand transport. Proc. Thirteenth Conference on Great Lakes Research, pp. 210-216: Internat Assoc. Great Lakes Research, Buffalo. Cronan, D. S. and Thomas, R. L. (1972) Geochemistry of ferromanganese oxide concretions and associated deposits in Lake Ontario. Geol. Soc. Amer. Bull, 83, 1493-1502. Csanady, G, T. and Pade, B. (1968) Coastal jet project. Environmental Fluid Mechanics Laboratory Report no. I, Great Lakes Inst. Report no. PR 36: Univ. Waterloo. Csanady, G. T. and Pade, B. (1969) Coastal jet project. Environmental Fluid Mechanics Laboratory Report no. 2: Univ. Waterloo. Duane, D. B. (1967) Characteristics of the sediment load in the St Clair River. Proc. Tenth Conference on Great Lakes Research, pp. 115-132: Internat. Assoc. Great Lakes Research, Toronto. Dussari, B. (1948) Recherches hydrographiques sur le lac Léman. Ann. Station Centr. Hydro-biol. appl.,F2, 2, 187-206. Farrand, W. R. (1969) The Quaternary History of Lake Superior. Proc. Twelfth Conference on Great Lakes Research, pp. 181-197: Internat. Assoc. Great Lakes Research, Ann Arbor. Forel, F. A. (1895) Le Léman: Monographic Limnologique, vol. 2 -Mécanique, Chimie, Thermique. Optique, Acoustique: F. Rouge, Lausanne. Freeman, N. G., Murty, T. S, and Haras, W. S. (1972) A study of a storm surge on Lake Huron. Coil, Abstr, Third Canadian Océanographie Symposium, p. 21 : Burlington. Fricbergs, K. S. (1970) Erosion control in the Toronto area. Proc. Thirteenth Conference on Great Lakes Research, pp. 7 5 1 - 7 5 5 : Internat. Assoc. Great Lakes Research, Buffalo. Gould. H. R. (1960) Turbidity currents. Comprehensive Survey of Sedimentation in Lake Mead 1948-4949: US Geol. Survey. Professional Paper no. 295, US Dept. Interior. Gould, H. R. and Budinger, T. F. (1958) Control of sedimentation and bottom configuration by convection currents, Lake Washington, Washington./. Marine Res. 17, 183-198, Great Lake's Levels Board (1968). Publication on shoreline physical characteristics (by county). Canadian Federal Dept. Pub. Works, Ottawa. Gross, D. L , Linebach, J. A., White, W. A., Ayer, N. J., Collinson, C. and Leland, H. V. (1970) Preliminary Stratigraphy of Unconsolidated Sediments from the Southwestern part of Lake Michigan: Illinois Geol, Survey Environ. Geol. Note, no. 30. Gross, D. L-, Linebach, J. A., Shimp, N. F. and White, W. A. (1972) Composition of Pleistocene sediments in southern Lake Michigan, USA. Section 8 Rept, Twenty-fourth International Geology Congress, pp. 215-222: Montreal. Hands, E. B. ( 1970) A geomorphic map of Lake Michigan shoreline. Proc. Thirteenth Conference on Great Lakes Research, pp. 2 5 0 - 2 6 5 : Internat. Assoc. Great Lakes Research, Buffalo. Harriss, R. C. and Troup, A. G. ( 1969) Freshwater ferromanganese concretions, chemistry and interna] structure. Science 1 6 6 , 6 0 4 - 6 0 6 . Hartley, R. P. (1968) Bottom currents in Lake Erie. Proc. Eleventh Conference on Great Lakes Research, pp. 3 9 8 - 4 0 5 : Internat. Assoc. Great Lakes Research, Milwaukee. Hjulstrom, F. (1939) Transportation of detritus by moving water. Recent Marine Sediments, Symposium (P. D. Trask, editor): Amer. Assoc. Petrol. Geol., Tulsa, Oklahoma. Hough, J. L. (1963) Geological and Sedimentary Characteristics of the Freshwater Environment, pp. 134-139: Publ. no. 10, Great Lakes Research Div., Unit of Ann Arbor, Mich. The significance of sediment deposits in large lakes 395 Harrow, P, F. 11963) Pleistocene Geology ofthe Hamilton-Gait Area: Ontario Dept. Mines Geoi. Report no. 16, Toronto. Kennedy, E. J., Ruch, R. R. and Shimp, N. F. (1971) Distribution of mercury in inconsolidated sediments from southern Lake Michigan. Illinois Geo/. Survey Environ. Geot. Note, no. 44. Kindle, E. M. (1932) Lacustrine concretions of manganese. Amer. J. Sei. 24, 496-504. Kindle, E. M. (1936) The occurrence of lake bottom manganiferous deposits in Canadian lakes. Econ. Geo!. 3 1 , 7 5 5 - 7 6 0 . Lee, A. H. (1972) Some Thermal and Chemical Characteristics of Lake Ontario in Relation to Space and Time. Inst. Environ. Sci. and Engin., Great Lakes Inst. Div., Univ. Toronto. Report no. EG-6. Lewis, C. F. M, (1969) Lake Quaternary history of iake levels in the Huron and Eric basins. Proc. Twelfth Conference on Great Lakes Research, pp. 250-270: Internat. Assoc. Great Lakes Research, Ann Arbor. Lewis, C. F. M. (1969) Quaternary geology of the Great Lakes. Report on Activities, Part A: April to October 1968 (R. G Blackadar, editor), pp. 6 3 - 6 4 : Geot. Survey of Canada, Paper no. 69-1A. Lewis, C. F. M. and McNcely, R. M. (1967) Survey of Lake Ontario bottom deposits. Proc. Tenth Conference on Great Lakes Research, pp. 133-142: Internat. Assoc. Great Lakes Research. Toronto. Lewis, C. F. M. and Sly, P. G. 0 9 7 1 ) Seismic profiling and geoiogy of the Toronto waterfront area of Lake Ontario. Proc. Fourteenth Conference on Great Lakes Research, pp. 303-354: Internat. Assoc. Great Lakes Research, Toronto. Linebach, J. L., Ayer, N. I. and Gross, D. L. (1970) Stratigraphy of unconsolidated sediments in the southern part of Lake Michigan. Illinois Geol. Survey Environ. Geo!. Note, no. 35. McAndrews, J. H. (1972) Pollen analyses of the sediments of Lake Ontario. Sect. 8, Twenty-fourth Internat. Geol. Congress, pp. 223-227: Montreal. McDonald, W. E. (1954) Variation in Great Lakes levels in relation to engineering problems: Proc. Fourth Conference on Coastal Engineering, pp. 249-257: Council on Wave Research, Univ. California, Berkley. Miller, R. L. and Zeigler, J. M. (1964) A study of sediment distribution in the zone of shoaiing waves over complicated bottom topography. Papers in Marine Geology - Shcpard Commemorative Volume (R. L. Miller, editor), pp. 133 — 153. Mothersill, J. S. (1972) Report on Lake Superior Nearslwre Studies: Unpublished report, Canada Centre Inland Waters, Burlington. Nriagu, J. O. (1972) Dissolution characteristics of freshwater ferromanganese nodules under controlled Eh and pH conditions. Submitted to Nature. Pelletier, B. R. (1968) Submarine physiography, bottom sediments and models of sediment transport. Earth Science Symposium on Hudson Bay (P. J. Hood, editor), pp. 100-135: Geol. Survey Canada Paper no. 6 8 - 5 3 , Pelletier, B. R. (1972) Clastic sediment ratios and environmental models. Collected Abstracts Third Canadian Océanographie Symposium, p. 37: Burlington. Rodgers, G. K. (1968) Heat advection within Lake Ontario in spring and surface water transparency associated with the thermal bar. Proc. Eleventh Conference on Great Lakes Research, pp. 480—486: Internat. Assoc. Great Lakes Research, Milwaukee. • Rukavtna, N. A. (1969) Nearshore sediment survey of western Lake Ontario, methods and preliminary results. Proc. Twelfth Conference on Great Lakes Research, pp. 317-324. internat. Assoc. Great Lakes Research, Ann Arbor. Rukavina, N, A. (1970) Lake Ontario nearshore sediments, Whitby to Wellington, Ontario. Proc. Thirteen Conference on Great Lakes Research, pp. 266-273: Internal, Assoc. Great Lakes Research, Buffalo. Saylor, J. H. (1966) Modification of Nearsliore Currents by Coastal Structures: misc. paper. Lake Survey District, Corps of Engineers, Detroit, Michigan. Scott, J. T., Jekel, P. and Fenion, M. W. (1971) Transport in the barocfinic coastal current near the south shore of Lake Ontario in early summer. Proc. Fourteenth Conference on Great Lakes Research, pp. 6 4 0 - 6 5 3 : Internat. Assoc. Great Lakes Research, Toronto. Seibold, E. (1967) La Mer Baltique prise comme modèle de géologie marine. Revue de Géographie Physique et de géologie dynamique (2), IX, Fasc. 5, 371-384. Seibold, E., Exon, N., Hartmann, M., Kogler, F.-C, Krumm, H., Lutze, G. F-, Newton, R. S. and Werner, F. (1971) Marine geology of Kiel Bay. Sedimentology of Parts of Central Europe. Guidebook VIII. pp. 209-235: Internat. Sediment Congress, 1971. Shimp, N. F., Schleicher, J. A., Ruch, R. R., Heck, D. B. and Leland, H. V. (1971) Trace element and organic carbon accumulation in the most recent sediments of southern Lake Michigan. Illinois Geol. Survey Environ. Geol Note, no. 41. Simons, T. i. and Jordan, D. E. (1972) Computed Water Circulation of Lake Ontario for Observed Winds 20April-14 May 1971: Canada Centre Inland Waters Publ., Burlington. 396 P. G. Sly Sly, P. G. (1966) Marine geological studies in the eastern Irish Sea and adjacent estuaries, with special reference to sedimentation in Liverpool Bay and the River Mersey. Unpubl, PhD Thesis, University of Liverpool. Sly, P. G. and Lewis, C. F. M. (1972) The Great Lakes of Canada - Quaternary Geology and Limnology. Guide Book Trip A43: Twenty-fourth Internal. Geol. Congress, Montreal. St John, B. E.,Sly, P. G. and Thomas, R. L. (1972) The importance of sediment studies in western lakes as a key to basin management. Proc. Symposium on Lakes of Western Canada: Univ. Alberta, Edmonton. Thomas, R. L. (1969) The qualitative distribution of feldspars in surficial bottom sediments from Lake Ontario, Proc. Twelfth Conference on Great Lakes Research, pp. 364-379: Internat. Assoc. Great Lakes Research, Ann Arbor. Thomas, R. L., Kemp, A. L. W. and Lewis, C, F. M. (1972) Distribution composition and characteristics of the surficial sediments of Lake Ontario. J. Sed. Petrol. 42 (1), 6 6 - 8 4 .