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Geological and Palynological Studies of Early Lake Erie Deposits C

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Geological and Palynological Studies of Early Lake Erie Deposits C Pub. No 15, Great Lakes es4cJi Pivisiou, The tiniversity of Michigan, 1966 • • _ • - y , r R , , 4.4 TO ULL A . g t, -_____•_:_:::._ .... .... .... / GEOLOGICAL AND PALYNOLOGICAL STUDIES OF EARLY LAKE ERIE DEPOSITS C. F. M. Lewis, T. W. Anderson, and A. A. Berti Geological Survey of Canada, Ottawa Abstract. Coring and echo sounding of Lake Erie bottom sediments have indi- cated a thin lag concentrate of sand, in places with plant detritus, pelecypods, gastro- pods and other fossils, underlying Recent silty clay muds and overlying clay till or late-glacial lacustrine clays. Buried shallow pond organic sediments in the western basin and relict beach deposits, wave-cut terraces and intrabasinal discharge channels in the central basin, some of which are buried, all indicate former low water levels in central and western Lake Erie much below those at present. This evidence, combined with radiocarbon dates of 10,200 and 11,300 years B.P. on the organic material and in- formation from nearby regions, suggests that Early Lake Erie came into existence about 12,400 years ago, with water levels, 100 ft (30 m) lower than at present, at approxi- mately 470 ft above sea level. From this stage lake levels rose rapidly as the outlet area at Buffalo, N.Y., was uplifted isostatically following deglaciation, and probably reached their present elevation 9,000 to 10,000 years ago. Examination of the cores indicated that pollen is sufficiently abundant and well preserved in the sediments for palynological studies. Pollen diagrams can be corre-. lated with one another, and with those outside of the Lake Erie basin. The presence of a legible pollen record indicates that sedimentation has been probably continuous and undisturbed at the sites investigated since low-level Early Lake Erie. Palynological studies support the geological evidence of a low lake stage and provide a means for dating and correlating sediment sequences which do not contain enough organic matter for radiocarbon analysis. INTRODUCTION Results of coring, echo sounding and SCUBA diving carried out from the research vessel C.C.G.S. PORTE DAUPHINE as part of a general study of the unconsolidated deposits in Lake Erie basin under the auspices of the Great Lakes Institute, University of Toronto, have suggested the existence of post- glacial low level stages of Lake Erie. Sediment samples at depths up to 11 and 12 m below the lake bed were collected by piston cores with core tubes having an inside diameter of 4.9 cm fabricated from AX drill casing. Three piston cores were studied for pollen content by personnel of the Pleistocene Palynology Laboratory, Geological Survey of Canada, to provide supporting en- vironmental and chronological evidence for conclusions regarding low lake stages based on geological evidence. The objective of this paper is to present geological and palynological evidence of postglacial low-level lake stages in Lake Erie and to integrate the sequence of these stages with the known history of the lower Great Lakes region. DESCRIPTION OF BOTTOM DEPOSITS Figure 1 is a generalized map of Lake Erie bottom deposits prepared from results of sampling and echogram interpretation. Two major bottom de- posit types were recognized: nearshore and ridge top areas where silty clay mud does not occur and offshore areas where mud does occur. 176 EARLY LAKE ERIE SEDIMENTS 177 82° 81° 80° 79 7 43 Portl Maitlan 4 42 oraln CLEVELAND BATHYMETRY AND BOTTOM MATERIALS LEGEND CONTOUR INTERVAL 10 METRES RECENT SOFT SILTY CLAY SEDIMENT............ !In DATUM: MEAN WATER LEVEL SAND, SAND VENEERED TILL OR GLACIO- 570 FEET ABOVE SEA LEVEL LACUSTRINE CLAY OR BEDROCK ................... (I GLD 1955) UNKNOWN ........................................................................ MILES 1244 • 0 25 50 75 100 I I I I I PISTON CORES 1 240 .................................... • 0 40 80 120 160 2226 ................................... 0 KILOMETRES FIG. 1. Lake Erie, showing bottom materials, bathyrnetry and location of piston cores used for palynological studies and radiocarbon analysis. In the nearshore areas and atop the Pelee-Lorain and Long Point-Erie ridges, clay till or dense glacio-lacustrine clay is usually overlain by a thin (few cm), patchy, rippled sand lag concentrate often with pebbles and detrital shell fragments. In places sand has accumulated to greater thicknesses and buried the clay surface. Bedrock is not exposed along the north shore of the central basin but outcrops along the shore of the eastern basin and is reported to be immediately offshore from much of the south shore of the central basin (Hartley 1961a). Bedrock commonly outcrops in the islands region of western Lake Erie. A soft, grey, silty clay mud occurs in all offshore areas. The surface of this mud is nearly level in the western (depth about 10 m) and cen- tral (depth between 20 and 24 m) areas of the lake. The eastern area alone is basin-shaped, reaching a maximum depth of 66 m off Long Point. Echo sounding and seismic reflection profiling have indicated mud thick- nesses approaching 5 to 10 m, 20 to 25 m and 40 m in the medial regions of western, central, and eastern basins respectively. The mud deposits thin to- wards shore and reach zero thickness along the boundaries shown in Fig. 1. An examination of piston cores which penetrated the mud deposit where its thickness did not exceed 12 m revealed a nearly uniform mud section. The mud is a faintly laminated, soft, grey silty clay which in places contains pele- cypod, gastropod and ostracod shells, both entire and broken. The shells com- monly occur with wood chips and plant debris as thin concentrations within or at the base of the mud. The material beneath the mud generally varies from basin to basin. Be- neath the mud in the northern half of the eastern basin is a zone (5 to 20 cm thick) of silt, fine sand and delicate shell fragments underlain by moderately dense, red laminated glacio-lacustrine clay. In the central basin, dense, red- dish grey clay till or glacio-lacustrine clay underlies the mud. Above depths of 178 LEWIS, ANDERSON, and BERTI 25 to 28 m, a thin zone of sorted and subrounded sand with pebbles and shell fragments overlies the dense clay. Below 25 to 28 m, the mud may directly overlie the clay or a thin horizon of shells may mark the contact; or the basal 10 to 50 cm of mud may be interstratified with sand. In the western basin the mud is siltier and is usually underlain by 5 to 20 cm of dark brown, spongy plant detritus. This detritus, occurring at depths ranging from 11 to 14 m below lake level and from 1 to 4 m below the sediment-water interface, is underlain by well sorted sand, dense clay till, or glacio-lacustrine clay or silty clay impregnated with plant detritus. The piston cores permitted the identification of a prominent subbottom reflection recorded on echograms of a Kelvin and Hughes MS26B Echosounder and a "Sparker" (seismic reflection profiler with electric discharge sound source) as the contact between the silty clay mud and the sand-veneered, dense clay till or glacio-lacustrine clay. A map showing the topography of the till and dense clay surface was prepared from the echograms (Fig. 2). Depth con- tours of this surface are considered reliable down to approximately 35 m in offshore areas. Below 35 m, reflections were sporadic and had to be interpo- lated. Near the base and edges where the overlying mud is admixed with sand, subbottom reflections are diffuse and in places absent. The salient features on this map are: (a) shallow basins in western Lake Erie; (b) a small basin northeast of Sandusky, Ohio, separated from (c) a large central basin by (d) a ridge between Pelee Point and Lorain, Ohio; (e) an incipient ridge between Erieau, Ont., and Cleveland, Ohio, constricting the central basin; (f) a broad ridge between the base of Long Point and Erie, Pa., partitioning the central and (g) the deep eastern basins. Note also the east-west trending channels at the southern termini of the Pelee-Lorain and Long Point-Erie ridges. Hartley (1960) has shown the Pelee Point-Lorain ridge to be composed of till and capped with sand and gravel. He states that the till surface in the associated ° 81 80° 79 4 2 Port 78 430 MaltIan BUFFALO Port S anl 4230 3 42° 41*30' CLEVELAND LAKE ERIE MAP SHOWING INFERRED TOPOGRAPHY OF PLEISTOCENE DEPOSITS DERIVED DEPTH CONTOURS IN------2 METRES APPROXIMATE FROM PISTON CORE AND ECHOGRAM - ASSUMED - ---2 DATA. MILES 0 25 50 75 100 DATUM: MEAN WATER LEVEL 570 FEET ABOVE SEA LEVEL (IGLD 1955). 0 40 80 120 160 KILOMETRES FIG. 2. Lake Erie, showing topography of Early Lake Erie basin. Contours are drawn on the surface of glacial or glacio-lacustrine deposits. EARLY LAKE ERIE SEDIMENTS 179 channel descends to a depth of 106 ft (32 m). The northern and southern sec- tions of the Erieau-Cleveland ridge are composed of till and are capped with sorted sand where the ridge descends beneath the mud surface. In the central and deepest portion, this ridge is composed of dense, glacio-lacustrine clay. The Long Point-Erie ridge is a broad rise 17-40 km wide, the western surface of which is mainly dense till veneered with a thin, rippled, sand lag concen- trate at depths below 18 to 20 m, and the eastern surface of which is sand- covered at depths less than 18 m. A piston core located approximately 18 km northwest of Erie has shown the channel floor at the southern end of this ridge to be clay till at a depth of 29 m.
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