Low-altitude organic deposits in Michigan: Evidence for pre-woodfordian Great Lakes and paleosurfaces

Transcription

1 Low-altitude organic deposits in Michigan: Evidence for pre-woodfordian Great Lakes and paleosurfaces RICHARD L. RIECK Department of Geology and Department of Geography, Western Illinois University, Macomb, Illinois HAROLD A. WINTERS Department of Geography, Michigan State University, East Lansing, Michigan ABSTRACT Reports of 136 glacially buried organic deposits below 200 m in altitude and other related data provide a basis for identifying relicts of several low-level Pleistocene paleosurfaces in southern Michigan. The carbonaceous material is concentrated in five areas and probably represents low places on pre-woodfordian landscapes with local base levels far lower than the present topography. These findings have implications regarding adjacent ancestral Great Lakes that existed sometime prior to final glaciation of the peninsula. PURPOSES The purposes of this paper are twofold. First we present evidence for low-level Pleistocene paleosurfaces in southern Michigan based on reports of organic deposits overlain by glacial drift. Second is an assessment of the implications of the data with regard to pre-late Woodfordian drainage base levels and their possible relationships to adjacent ancestral Great Lakes. PREVIOUS STUDIES Until recently, glacially buried organic material of Pleistocene age was thought to exist at relatively few places in Michigan's Southern Peninsula; only about 30 occurrences had been identified in the literature. Most of these deposits, generally described in a sentence or two, are situated in the subsurface (Leverett, 1899, 1904, 1917; Lane, 1900; Leverett and Taylor, 1915; Sherzer, 1917; Stanley, 1936; Mozola, 1954; Levin and others, 1965; Crane and Griffin, 1961; Sullivan and others, 1970; Buckley and Willis, 1972; Holman, 1976; Rieckand Winters, 1976; Eschman, 1978). Several other reports of buried carbonaceous material are more detailed (see Zumberge and Benninghoff, Figure 1. Distribution of low-level buried organic matter in the Southern Peninsula of Michigan. Cross-hachured areas indicate clusters of sites with subsurface elevations below 200 m. Open circles represent unclustered or miscellaneous sites with elevations of 175 m to 200 m; closed circles, sites below 175 m; squares, sites above 200 m with C u dates. Geological Society of America Bulletin, v. 93, p , 5 figs., August

2 ORGANIC DEPOSITS IN MICHIGAN ; Miller, 1973; Kapp, 1978; Eschman, 1980). There are also a few descriptions of exposed glacially buried organic material; but most of these deposits are now removed, covered, or inaccessible (see Cooper, 1906; Farrand and others, 1969; Zumberge and Benninghoff, 1970). To our knowledge, glacially buried organic matter may be observed at only one location in southeast Michigan and a few closely spaced sites in the northwestern part of the peninsula (Eschman, 1978, 1980; Stuiver and others, 1978). Recently, however, largely on the basis of unpublished well log data available at the Geological Survey Division of the Michigan Department of Natural Resources, we have identified more than 225 additional sites where organic matter in the subsurface is believed to be glacially buried (Rieck and Winters, 1980, and additional new data). On this basis more than 250 occurrences of glacially buried carbonaceous deposits are recognized in the Southern Peninsula. These reports of buried organic matter show that most of the deposits are concentrated in a number of relatively small areas and that many of the carbonaceous horizons within some groups have similar altitudes. Well logs show that most subsurface carbonaceous horizons are reported as > 1 m thick, many are described in increments of several metres, and some exceed 10 m. Furthermore, most exposures of Pleistocene buried organic matter described in the literature are reported as in place. All of these characteristics support the interpretation that the carbonaceous deposits are probably in situ. We expect, however, that some of these organic deposits may have been transported by glacial action, but because of the scarcity of references describing thick carbonaceous Pleistocene float within glacial deposits, they are probably few in number. Even so, some transported organic matter may remain significant because, as suggested by Dreimanis and Goldthwait (1973, p. 30), wood incorporated in drift has not been transported far, indicating that its source is a relatively short distance in an up-ice direction. A few other carbonaceous horizons may have accumulated in or along the shore of water bodies, but these are also generally reflective of local base level. Where the type of organic matter is identified, it is most often wood or other material generally associated with moist or poorly drained conditions. On the basis of site, situation, types of material reported, mmsl »ill i ELEVATION «Present L. Michigan- _J Huron level 177 m. i i i i i i i n NUMBER OF SITES DEPTH i i i i i i i i i r NUMBER OF SITES Figure 2. Elevation and depth of buried organic matter below 200 m asl elevation. and differing radiocarbon dates, we have concluded that the buried organic horizons may represent forest sites or low areas on several Pleistocene paleosurfaces with differing ages (Rieck and Winters, 1980). This indicates that most, if not all, of the carbonaceous material must have formed either near or somewhere above its respective local drainage base level(s). DISTRIBUTION AND ALTITUDE OF LOW-LEVEL ORGANIC DEPOSITS Data show that about one-half (136/257) of the known glacially buried carbonaceous horizons are below 200 m in altitude and more than one-quarter (72/257) are lower than 175 m. Organic deposits below 200 m are notable because practically all of the peninsula's existing topography is at a higher elevation (Fig. 1). Carbonaceous zones below 175 m and many are much lower than this figure are even more important because they are beneath the present local base level controlled by nearby Lakes Huron and Michigan (176.4 m). The interpretation that most, if not all, of the carbonaceous deposits are overlain by glacially deposited drift (Rieck and Winters, 1980) is further supported by additional data from the most recent soil surveys implying that the soil's parent material is till at 67 of the 136 sites considered in this study. In addition, comparisons of soil studies with well logs reporting the upper 1.5 m of sediment as either clay or clay with clasts larger than sand (drillers' terms include clay and stone, gravel-packed clay, and stony clay) show that such descriptions refer to till in 79% of the wells. Interviews with well drillers and our observations of drilling and logging concurred with these findings. We also found that well drillers who provided more detailed descriptions in logs are the ones who tended to report the most organic matter. On this basis, review of well log terminology for the 69 remaining sites (where the soil parent material is either not till or unknown) confirms the interpretation that most, if not all, of these carbonaceous horizons are also buried by till, but in the subsurface. It also supports the conclusion by Thwaites (1959, p. 110) that "the geologist can learn much from experienced drillers." Figure 2 shows the altitudinal distribution and overburden thickness for the buried organic matter pertinent to this study. Although one of these carbonaceous horizons is covered by more than 100 m of drift, the average overburden is between 35 m and 40 m. These variations in location and elevation provide a basis for considering separately each of the five groups and the "miscellaneous" sites shown in Figure 1. HUBBARDSTON SITES Thirty-five glacially buried organic deposits lower than 200 m in altitude are identified in the Hubbardston area (Fig. 3). These

3 728 RIECK AND WINTERS carbonaceous horizons range in elevation from to m with 30 forming a rather even altitudinal distribution between 175 and 196 m. All but one are buried by more than 20 m of drift; the average overburden thickness is about 45 m. Types of organic matter identified on well logs include, among others, muck and peat; but wood is by far the most common here. It is known that the most recent glaciation of the Hubbardston area took place during the Woodfordian, with the last possible readvance immediately following the Erie Interstade (Morner and Dreimanis, 1973); but such an event has not been BAY CITY CLUSTER i CROSWELL CLUSTER BP YY' 4 ' «CROSWELL o175.3 j. j ^ 144.1, Jl 71 oflb A PÜ CITY \ » 162.1*v \49._4«. I Bay Co. "141. (Saginaw Co. i >164.6, SanilacCo^ hit. Ciair Co ORT HURON CHARLEVOIX CLUSTER "I_ Charlevoix CCK ' Antrim Co. 0 mi. 5 1 ' ' Leelanau Co. Figure 3. Distribution and elevation of buried organic matter below 200 m in clusters. Open circles represent sites from m and closed circles, sites below 175 m.

4 ORGANIC DEPOSITS IN MICHIGAN 729 GRAND HAVEN I HOLLAND CLUSTER lko ' I 156.7* *1S9.4, » 162.2, 15 V» f ï 1 * 1^ l J»162.8 HUBBARDSTON proven here. Buried wood from an elevation of m northeast of the area has been dated as older than 30,000 yr, but W. R. Farrand (1979, personal commun.) stated that the sample (M-879) may be contaminated (Fig. 1). Another date for material from a horizon at m within the area indicates that burial took place more than 32,000 yr B.P. (W-2182; Sullivan and others, 1970; Miller, 1973) (Fig. 3). A third CLUSTER sample of wood (M-653) recovered at an elevation of m within a well located ~ 20 km west of the area was dated as >30,000 yr B.P. (Crane and Griffin, 1961) (Fig. 1). These dates indicate that pre- Woodfordian organic deposits exist in central Michigan. However, to our knowledge no finite radiocarbon date is available for glacially buried material from the Hubbardston area, and there is no sound basis for correlation with other deposits of known age. The present local base level, controlled by the nearby incised Grand River, is ~200 m, and the average topographic altitude of the area is more than 225 m. The glacially buried organic deposits believed to represent one or more paleosurfaces are many metres below both the present surface and its major drainage lines. More specifically, on the basis of data shown in Figure 3, it appears reasonable to conclude that major streams on the former surface(s) must have, at one time or another, been adjusted to local base levels near 1 or below 180 m and possibly < 150 m. Furthermore; if present stream gradients are representative of the past, rivers extending from this relatively remote location (situated at the present drainage divide of the Southern Peninsula and about 100 km from adjacent Great Lakes) to an ancestor of Lake Michigan or Lake Huron must have drained into a water body many metres lower than the present level of m. CROSWELL SITES Figure 3. (Continued). In the Croswell area (Fig. 3), 17 glacially buried organic deposits are known to exist below 200 m, of which 6 are below 175 m with altitudes that range from to m. Three of the organic deposits below 175 m are marl, indicating lacustrine conditions; two others at elevations of m and m are wood; and the sixth, at m, is muck, suggesting a mesic or paludal environment. All 11 of the organic deposits above 175 m but below 200 m are reported as wood and provide evidence for at least one subaerial paleosurface from 20 to 40 m lower than the present topography. Data in Figure 3 show that altitudes of the buried carbonaceous deposits tend to increase from ~ 150 m near Port Huron to > 190 m toward the north and west. The present-day topography, with most elevations between 175 m and 225 m, has similar trends, suggesting that it directly reflects variations on a paleolandscape(s) marked by the buried organic deposits. Most of the carbonaceous matter, lies proximal to the

5 730 RIECK AND WINTERS outer edge of the Port Huron moraine (Fig. 1). On this basis one might suspect that the material is associated with a south-southeasterly sloping paleosurface that was buried by the formation of the Port Huron moraine, dated about 12,500 B.P. (Farrand and Eschman, 1974, p ). But radiocarbon dates for several samples from the area one of 25,480 ± 700 B.P. (W-3667; K. Van Dellen, 1978, personal commun.), another of 48,300 ± 800 B.P. [QL-1215; Eschman, 1979; covered by at least two (and more probably three) pre-port Huron tills, D. F. Eschman, 1981, personal commun.], and a third, more than 37,000 B.P. recovered at 201 m (GX-5104; Eschman, 1980) all predate the ice advance that formed the Port Huron moraine by many thousands of years (Fig. 3). In addition, at least two of the buried-organic-matter sites lie distal to the moraine, making burial by that ice advance unlikely. These differing radiocarbon dates and positions indicate that remnants of two or more relatively low-level subaerial paleosurfaces exist in the Croswell area. Furthermore, they must have formed sometime prior to the Woodfordian, and the lowest paleosurface probably had a local base level at least 25 m below the present level of Lake Huron. CHARLEVOIX SITES We have reports for nine subsurface organic deposits with elevations lower than 200 m within Pleistocene drift in the Charlevoix area (Fig. 3). Five of the deposits are between 175 m and 200 m and are near or above present local base level controlled by Lake Michigan (176.4 m). These carbonaceous horizons are buried by 14 m to 40 m of drift and provide evidence for a former surface or some former surfaces lower than today's topography but higher than the modern lake level. The four other carbonaceous horizons are at much lower elevations and tend to be buried more deeply. Three, including the two lowest, consist of woody material, indicating formation in a subaerial landscape. These two drift-covered deposits, at altitudes of m and m, represent some of the lowest Pleistocene organic matter known to exist in the Southern Peninsula. The presence of these unusually low-level organic deposits indicates. the likelihood that, at one time or another, local base level was at least 50 m lower than the present level of Lake Michigan. These organic deposits cannot be associated with lower base levels related to Lake Chippewa (70 m) (Hough, 1958) because that body of water is essentially postglacial, and this carbonaceous material underlies glacial sediments. The last ice to override the area is generally believed to have advanced about 11,500 B.P. (Farrand. and Eschman, 1974; Evenson and others, 1976). No radiocarbon dates are available for relevant carbonaceous material from the area; the closest are several samples from a site ~ 50 km to the northeast, dating an ice advance that reached the northern part of the peninsula about 12,800 B.P. (Farrand and others, 1969) (Fig. 1). Another date of 45,800 ± 700 B.P. has been obtained for organic material exhumed in the bed of the Pine River about 70 km south-southwest of the area (QL-963; Stuiver and others, 1978; Eschman, 1980) (Fig. 1). These dates indicate that at two times, separated by about 34,000 radiocarbon years, carbonaceous material was buried by glacial advances that affected the peninsula. At present, however, there is no basis for associating the buried organic material near Charlevoix with specific phases of ice withdrawal or advance. Even so, these carbonaceous deposits below 175 m may represent relicts of one or more subaerial paleosurfaces with elevations that, at one time or another, were >50 m lower than present-day Lake Michigan (176.4 m). BAY CITY SITES Sixteen glacially buried deposits suggestive of one or more paleosurfaces below 200 m are known to exist in the Bay City area (Fig. 3). Nine of these are reports of marl above 175 m, indicating formation in a lacustrine environment. Six other reports, however, are for "organic matter," wood, peat, and leaves that must have formed subaerially; all are below 175 m in elevation (from m to m). Five of these lowest-level organic horizons are situated either beneath or proximal to the Port Huron moraine (Martin, 1955), four of which may have experienced burial by the ice advance that produced the feature about 12,500 B.P. (Fig. 1). The certain exception is a buried horizon at m containing ash, oak, and maple wood and leaves indicative of temperate climatic conditions (Cooper, 1906, p. 25). Such vegetation types cannot be associated with the short Cary-Port Huron Interstade described by Farrand and others (1969) or the Erie Interstade proposed by Morner and Dreimanis (1973). Instead, Cooper's 1906 description suggests that these organic deposits were first buried by some ice advance that marks the end of an interglacial or lengthy interstadial; apparently, the burial must have taken place no later than the early Woodfordian and may predate that time by many thousands of years. The sixth organic deposit, reported as peat at m elevation, is located ~5 km west of the Port Huron moraine's distal margin. A well log shows this deposit to be overlain by till. These spatial and stratigraphic relationships indicate that burial of this material must' also have taken place sometime prior to the ice advance that formed the Port Huron moraine.. In summary, low-level organic deposits in the Bay City area provide evidence for one or more paleosurfaces with local base levels that were 30 m or more lower than those of the present a level well below the modern landscape and even approaching the bottom plane of nearby Saginaw Bay. GRAND HAVEN-HOLLAND SITES Evidence of buried organic matter related to unusually low local base levels that existed sometime prior to final glaciation is especially abundant in the Grand Haven- Holland area (Fig. 3). Here 36 glacially buried carbonaceous deposits are known to exist below 200 m, and most of these haveelevations well under 175 m; more than four-fifths are lower than 175 m, and more than one-third below 160 m. Twenty-nine of the deposits are reported as wood, including the lowest at an elevation of only m. Thirty of the organic horizons have altitudes (153 m to 173 m) that are within 20 m of one another, much less variation than elsewhere in the peninsula for so large a number of sites. Overlying drift thickness tends to be similar at most of these sites. The present topography is a low-relief glaciolacustrine plain. The sum of these relationships suggests that the present landscape directly reflects one or more buried paleosurfaces of low relief represented by organic deposits. The Lake Michigan lobe last glaciated this area during the Woodfordian, possibly as late as an advance following the proposed Erie Interstade (but such an event is not documented in the area). No radiocarbon dates are available for glacially buried

6 ORGANIC DEPOSITS IN MICHIGAN 731 organic matter within this cluster, but several are known for nearby sites (Fig. 1). At Grand Rapids, a short distance to the east, samples (among others) of woody material collected from two locations at elevations of 201 m and 206 m have been dated, respectively, as >39,900 B.P. (1-4900; Buckley and Willis, 1972) and >51,000 B.P. (GrN-4614; Vogel and Waterbolk, 1972). Buckley and Willis (1972) and Eschman (1980) report on buried wood from a gravel pit just north of Grand Rapids. The material was recovered from an elevation of 198 m and dated at 33,300 ± 1,800 (1-5078). Farther north of the same city, wood from an altitude of 230 m has a date of about 25,000 B.P. (W-2897; Holman, 1976; Kapp, 1978). The dating of samples from these nearby sites indicates that pre-woodfordian carbonaceous material of different ages exists in this part of the state. Although correlations with this dated material cannot be made, the Grand Haven- Holland glacially buried organic deposits do provide evidence for one or more paleosurfaces with altitudes far below the level of the present landscape and as much or even more than 50 m lower than the level of Lake Michigan (176.4 m). MISCELLANEOUS SITES Twenty-three nonclustered glacially buried organic deposits also provide useful data (Fig. 1). Fourteen of the carbonaceous zones are below 175 m, and eight of these have altitudes less than 150 m. Seven of the eight lowest are near Lake Michigan's coast and further substantiate the existence of relicts from one or more relatively low subaerial paleosurfaces at the southwest margin of the peninsula (Figs. 1 and 3). Eight of the low-level deposits located within 50 km of the coast in the northern two-thirds of the peninsula may have been first covered by a late Woodfordian or younger ice advance; but, because a basis for correlation is lacking, the burial may be much older. All other nongrouped sites below 200 m must have been buried during or sometime before an ice advance immediately following the Erie Interstade of Morner and Dreimanis (1973), although, as stated above, such an event is not documented for most ofthe peninsula. The radiocarbon date of 33,300 B.P. from one of the sites near Grand Rapids indicates that at least some of the low-level organic material was buried prior to the Woodfordian Stadial of the Wisconsinan Stage (Fig. 1, see inset map). The subsurface altitude of carbonaceous horizons is of special interest at 11 of the miscellaneous sites nearest Lakes Michigan, Huron, and St. Clair; two are near 160 m, four are close to 150 m, five are below 145 m, and all have profound implications if they formed contemporaneously with an ancestor to the nearby lakes. Their proximity to the lake basins indicates that any contemporary body of water would probably have levels below 140 m, more than 35 m lower than present lake surface altitudes. Figure 4. Preglacial drainage of region. Bedrock contours from Horberg and Anderson (1956) (converted to metres). Drainage pattern after Spencer from White and Karrow (1971).

7 732 RIECK AND WINTERS IMPLICATIONS FOR PALEODRAINAGE AND ANCESTRAL GREAT LAKES It is generally agreed that preglacial strike valleys were modified by. multiple glaciation to form the Great Lakes basins. In addition, there is consensus that predecessors to the present Great Lakes existed in some form during Pleistocene interglacials and interstadials. All comprehensive geologic studies of the Great Lakes, however, commence with final déglaciation (see Leverett and Taylor, 1915, p. 319; Hough, 1958, p. 139). Specialized studies of particular areas or relationships of individual water bodies predating final glaciation have been made but are few in number. Examples include Lake Coleman; Lake Scarborough; unnamed lakes of the St. Pierre, Port Talbot I, Port Talbot II, and Plum Point Interstades (Dreimanis, 1969); Lake Ypsilanti (Kunkle, 1963); possible Sangamonian lakes (White and Totten, 1979); and an Erie Interstade water body in the Lake Erie basin (Môrner and Dreimanis, 1973). These lakes are associated with predecessors to Lakes Ontario or Erie, however, and their levels are not much different from existing lakes. Similar studies for ancestors of Lakes Huron and Michigan do not exist, and little information is available regarding pre-woodfordian lakes in these basins. Even so, the conclusion that bodies of water existed in ancestral lowlands during Pleistocene ice-free episodes is quite reasonable because of, among other things, the amount of glacial erosion required to form the lake basins, combined with the down-ice succession of complex deposits representing many glacials and stadials. Evidence of paleosurfaces and related local base levels recognized from lowaltitude glacially buried Pleistocene carbonaceous deposits in Michigan's Southern Peninsula do, however, provide a basis for certain observations regarding ancestral drainage levels associated with either nearby strike valleys or lakes that predate final glaciation of the Lake Michigan and Huron basins. Furthermore, variations in the types of carbonaceous deposits along with cited finite and infinite radiocarbon dates indicate that organic material of at least several different ages exists in the subsurface. Whether controlled by a major stream valley or one or more unrecognized ancestral Great Lakes, the lowest pre-existing local base levels identified on the basis of buried, organic matter must have been tens of metres lower than they are today. Bedrock data for nearby provinces and states indicate that the preglacial surface in the area was probably adjusted to Spencer's Laurentian River system (Fig. 4). This major drainage line, proposed in 1890 and supported by more recent data (see Horberg and Anderson, 1956; White and Karrow, 1971), drained eastward from a lowland now represented by Lake Michigan through the Lake Huron-Georgian Bay area toward Toronto. White and Karrow (1971) reported a threshold of 120 m on the buried bedrock surface associated with the valley in the Simcoe area, indicating that the upstream altitude of the preglacial valley floor (to the west) must have been somewhat higher (Fig 4). Interestingly, two buried organic deposits in Michigan's Southern Peninsula have about the same altitude (119.4 m, m) as the Simcoe threshold, and 134 are above that level, but not a single one is lower (Figs. 1, 2, and 5). Obvious complications exist, some discussed by White and Karrow (1971) and others that involve, among other things, glacial erosion or deposition, differential vertical movement from glacial isostasy, and drift loading. The complementary nature of the altitudes may be meaningful because the relative location and over-all geomorphic conditions do not vary greatly between the two lowest organic sites in southern Michigan and the Simcoe threshold almost directly to the east. The sum of these relationships suggests that the lowest organic sites in Michigan formed on a paleosurface adjusted to a lowland or a very low-level ancestral Great Lake that may well have been related to base levels controlled in some way by Spencer's Laurentian River. Most of the remaining buried organic deposits considered in this study vary in altitude from 150 m to 200 m (about onehalf above and one-half below 175 m); all of these are well above the lowest level of the Laurentian River; most tend to be situated near the middle or in the upper part of the drift column and are less deeply buried than the lowest-level carbonaceous deposits; some are known to be of different ages; and nearly all, by their nature, are believed to have formed within low sites at or above their respective local base level. All of these conditions, plus the constructional characteristics of the present topography support the conclusion that, although there are probably many exceptions and complications, the organic deposits tended to form on successively higher surfaces because of deposition by subsequent glacial episodes. Concomitantly, the bottoms of Lakes EROSION PREDOMINANT DEPOSITION PREDOMINANT Time 1 Time 2 Time 3 Today ORGANIC MATTER Today Time 3 Time 2 Time 1 PREGLACIAL BEDROCK SURFACE Figure 5. Generalized and hypothetical diagram showing relationship between buried organic matter, erosion of Great Lakes bottoms, construction of topography, and changing lake levels through time.

8 ORGANIC DEPOSITS IN MICHIGAN 733 Huron and Michigan, far below the 120-m threshold of Spencer's Laurentian River, indicate that a large amount of glacial erosion must have taken place to form the lake basins. Furthermore, numerous superposed drift sheets down-ice from the lake basins indicate that they formed as a result of many, rather than one or a few, glacial events. This periodic erosion of the lake basins must have resulted in deposition elsewhere, each phase presenting the opportunity for burial of a pre-existing landscape and the creation of a new terrain at a somewhat higher level. Although evidence exists for some highlevel glacially buried proglacial lake deposits associated with ancestors to Lakes Huron and Michigan (Leverett and Taylor, 1915, p ), the absence of known interglacial or interstadial lacustrine deposits at levels higher than these lakes is worth noting. In fact, known occurrences of such deposits are uncommon for all of the Great Lakes, and where they do exist, they are rarely much higher than present lake levels (see Dreimanis, 1969; Môrner and Dreimanis, 1973). High-level interstadial or interglacial deposits are either (1) unrecognized, (2) removed by erosion, or (3) non-existent because they were never deposited. We believe the presence of multiple low-level organic deposits of several ages in the subsurface favors the interpretation that highlevel ancestral nonglacial lakes in the Huron and Michigan basins either were rare or did not exist. We also recognize the possibility that some of the low-level organic deposits may have formed contemporaneously with unusual low-water bodies representing ancestral equivalents to Lakes Chippewa and Stanley. Such features, although not documented in the Huron and Michigan basins, may have formed following one or more of the many ice marginal recessions that predate final déglaciation. If such lakes did exist, their duration must have been short, existing only after the ice front receded from the northern Great Lakes area and before the occurrence of significant postglacial rebound. Reports of material types by Cooper (1906), the length of time necessary for vegetation to become established, and numerous reports of thick organic zones, however, do not favor the interpretation that the preserved carbonaceous matter is associated only with these short-lived lakes marking very special conditions that existed only during and immediately after deglaciation of certain areas. It is much more likely that at least some, and probably most if not all, of the organic matter formed during the middle or late (rather than earliest) phase of interglacials and interstadials, or immediately prior to overriding by a glacial advance. Our data and these conclusions indicate that lake surfaces tended to increase in altitude with successive erosion of the lake basins and corresponding deposition on adjacent areas. Figure 5 is an idealized and highly generalized illustration of the relationships between successive erosional events of the lake basins, resultant deposition in the Southern Peninsula, and progressively higher lake levels. It is important to note that this illustration does not take into account complications such as high proglacial lake levels, changing outlets, long-term lowering of lake levels by erosion of thresholds, or removal by a glacial event of pre-existing depositional topography and its associated paleosurface. The diagram does, however, show long-term effects indicated by findings in this study. In summary, most, if not all, of the numerous low-level glacially buried organic materials within drift are interpreted to represent relicts of paleosurfaces that formed near or above their respective local base levels. Types of materials, radiocarbon dates, position within the drift, and relative location indicate that the carbonaceous matter is of at least several different ages. The altitudes of these deposits are lower than the present landscape, indicating that multiple glaciation has tended to produce successively higher topographic levels within the peninsula. Furthermore, evidence presented in this study suggests that ancestral Great Lakes adjacent to the peninsula have generally tended to progress in size from smaller, lower, and shallower, with less volume of water, toward larger, deeper, and higher lakes containing greater amounts of water. ACKNOWLEDGMENTS We express appreciation to D. F. Eschman (University of Michigan), P. F. Karrow (University of Waterloo), and S. M. Totten (Hanover College) for offering comments on our manuscript. We also thank D. R. Currey (University of Utah) and L. H. Lattman (University of Cincinnati) for their suggestions as reviewers for the Geological Society of America. REFERENCES CITED Buckley, J., and Willis, E. H., 1972, Isotopes' radiocarbon measurements IX: Radiocarbon, v. 14, p Cooper, W. F., 1906, Geological report on Bay County: Geological Survey of Michigan Annual Report for 1905, p Crane, H. R., and Griffin, J. B 1961, University of Michigan radiocarbon dates VI: Radiocarbon, v. 3, p Dreimanis, A., 1969, Late-Pleistocene lakes in the Ontario and Erie basins: Great Lakes Research, 12th Conference Proceedings, p Dreimanis, A., and Goldthwait, R. P., 1973, Wisconsin glaciation in the Huron, Erie, and Ontario lobes: Geological Society of America Memoir 136, p Eschman, D. F., 1978, Pleistocene geology of the thumb area of Michigan, in Kesling, R. 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10 Geological Society of America Bulletin Low-altitude organic deposits in Michigan: Evidence for pre-woodfordian Great Lakes and paleosurfaces RICHARD L. RIECK and HAROLD A. WINTERS Geological Society of America Bulletin 1982;93, no. 8; doi: / (1982)93<726:LODIME>2.0.CO;2 alerting services Subscribe click to receive free alerts when new articles cite this article click to subscribe to Geological Society of America Bulletin Permission request click to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Geological Society of America