Glaciomarine Deltas of Maine and Their Relation to Late Pleistocene-Holocene Crustal Movements

Maine Geological Survey Neotcctonics of Maine 1989 Glaciomarine Deltas of Maine and Their Relation to Late Pleistocene-Holocene Crustal Movements Woodrow B. Thompson Maine Geological Survey Augusta, Maine 04333 Kristine J . Crossen Geology Department University ofAlaska Anchorage, Alaska 99507 Harold W. Borns, Jr. Institute for Quaternary Studies University of Maine Orono, Maine 04469 Bjorn G. Andersen University of Oslo Oslo, Norway ABSTRACT The deglaciation of southern Maine in late Wisconsinan time was accompanied by marine submergence of the isostatically-depressed coastal lowland. Glacial meltwater streams deposited over 100 Gilbert-type deltas into the sea during the transgressive phase of submergence, between about 14,000 and 13,000 yr B.P. These deltas were deposited either in contact with the ice margin or very close to it. They are classified into four categories based on depositional environment: ice-contact deltas (39% ), esker-fed ice-contact deltas (30% ), leeside deltas (26% ), and distal outwash deltas (5% ). Most of the deltas for which subsurface data are available accumulated in water less than 80 m deep, and the ice-contact deltas are believed to have formed along a grounded tidewater-glacier margin. Bedrock strike ridges and other hills slowed the retreat of the ice margin, causing brief stillstands during which deltas were deposited adjacent to these topographic highs. The sequence of glaciomarine deltas in Maine probably formed when the transgressing sea had reached its maximum depth and relative sea level was beginning to fall in response to isostatic crustal uplift. The elevations of the contacts between the topset and foreset beds of 65 deltas in Maine and New Brunswick were measured in order to locate the positions of sea level to which the deltas were graded. These elevations were plotted and contoured to determine whether the late-glacial crustal uplift pattern has been modified appreciably by Holocene crustal movements. The elevation data for southwestern Maine indicate a minimum postglacial tilt of 2.82 ft/mi (0.53 m/km) in the central Kennebec Valley region, with higher elevations to the northwest. Deltas in eastern coastal Maine have anomalously low elevations relative to those in adjacent parts of Maine and New Brunswick. The delta elevations in the eastern region may have resulted from variations in the glacio-isostatic tilt direction across Maine and/or lowering of relative sea level by crustal uplift as the deltas were deposited. Alternatively, these 43 W. B. Thompson and others elevations may define a zone of Holocene crustal subsidence with maximum relative downwarp (at least 40 ft) in the Machias-Eastport area. Contouring of delta elevations also revealed variation in the spacing of the contours between central Maine and the southern extremity of the state. This variation probably resulted from regional differences in the history of late-glacial sea-level change and/or crustal uplift in response to deglaciation. INTRODUCTION A variety of recent studies suggest that down warping of the of Holocene time. Thus, this area is a potential source of earth's crust has occurred in parts of southern Maine during base-line glacio-isostatic uplift data against which the Holocene time (Anderson and others, 1984 ). The effects of the Washington County region can be compared. downwarp are clearest along the coast, where the sea has trans In 1982 the study was extended to the remainder of southern gressed over man-made structures (Smith and others, this Maine, including Washington and eastern Hancock Counties, as volume). Leveling data indicate that crustal subsidence during well as adjacent New Brunswick and New Hampshire. A the twentieth century has been most rapid -- perhaps as much as preliminary report by Thompson and others (1983) included a 0.9 m/century -- in eastern Washington County (Tyler, this map showing the locations of 69 deltas judged to be suitable for volume). However, until the present investigation, no data were determining the configuration of the uplifted plane of maximum available regarding the possible amount of relative downwarp marine submergence. Their map showed the elevation of the ing throughout Holocene time. contact between the topset and foreset beds in each delta, which Much of southern Maine experienced marine submergence indicates the position of relative sea level when the delta was during the retreat of the last (late Wisconsinan) continental deposited. A number of deltas in remote areas of Washington glacier that covered New England. This submergence occurred County did not show exposures of the topset/foreset contact, bur because the earth's crust was still depressed by the weight of the in some of these cases it was possible to measure the elevations ice sheet as deglaciation began, enabling the sea to flood Maine's of the distal ends of meltwater channels on the delta tops. A few coastal lowland in spite of the lower eustatic sea level of late prominent wave-cut terraces were also incorporated in the early glacial time. However, isostatic crustal upli ft occurred rapidly phase of the study. as the state was deglaciated (Belknap and others, 1987), with the The elevation data from which the marine-limit surface was result that glaciomarine deltas formed during the submergence contoured have been refined since the preparation of our 1983 now stand at elevations of up to 422 ft* ( 128.6 m) above present report. We have reevaluated some problematic localities, lo sea level. We examined these deltas in order to determine cated and surveyed fresh pit exposures in certain deltas, and whether their present elevations are solely the consequence of utilized new 1:24,000 -scale topographic maps. Figure 2 shows uplift resulting from deglaciation, or whether their positions the revised contour map of delta elevations, which chiefly com have been modified by Holocene crustal movements. The prises deltas whose elevations have been precisely determined hypothesis to be tested was that prolonged vertical crustal move by leveling. This map is the focus for much of the discussion ment at the rates that appear to be happening today in Washington that follows. Data from coastal New Hampshire are omitted County should have noticeably changed the elevations of the from Figure 2 because new and more accurate data on the glaciomarine deltas. A map on which these elevations are con elevations of glaciomarine deltas in this state are presently being toured should reveal the effects of postglacial tilt, and might also obtained by the U.S. Geological Survey (C. Koteff, pers. comm., show displacement of the isopleths (equal-elevation contours) 1988). The elevations of emerged shorelines that were shown on resulting from neotectonic crustal disturbance. The preparation our earlier map have also been omitted here. Many shorelines of such a map was one of the chief objectives of this study. We fit the elevation pattern defined by the deltas, and thus probably also collected information on the origin and stratigraphy of formed at the upper marine limit, but it is difficult to decide Maine's glaciomarine deltas, and those findings are presented exactly which points on shoreline features (wave-cut cliffs and here. terraces) should be surveyed to define the same water level as The first part of the investigation (during 1980-1982) was the deltas. carried out on deltas located between the Maine-New Hampshire border and Penobscot Bay (Figure 1). The stratigraphy of many of the deltas in this region can be examined in gravel pits. PREVIOUS WORK ON GLACIAL ISOSTASY Moreover, the central part of the coastal zone appears to have AND GLACIOMARINE DELTAS IN MAINE been an area of relative crustal stability during the period covered by Tyler and Ladd's leveling data and perhaps also during most The presence of marine clay deposits in southern Maine has been known since the earliest geological survey of the state by C. T. Jackson (I 837). Jackson concluded that these deposits • Elevations and related measurements are expressed in feet to facilitate comparison with elevations on topographic maps. reached their present position through uplift of the land, but he 44 G/aciomarine deltas ofMain e ST. GEORGE I MEODYBEMPSe \ WASHINGTON i -· co. \ Pineo MONROEe 0, • 0 10 20 30 40 50 Kilomet ers \ Figure 1. Map showing locations of geographic features mentioned in text. 45 W. 8 . Thompson and others i l, J \. ( :, '·-....:.·\.., , ......... _,.. \. .... ~ \ , '\ __ _ MAINE .._42 2 ~ N a· , I ' • 290 Surveyed eleva1 1on ot 1opse1·lorese1 contact in glac1omarme delta 6288 Eleva11on ot topset-loreset conlact, delermined by a 111me1er. •253 Surveyed elevation ot meltwate r channel on glac1omarme delta. 0244 EleYahon of me11wa 1er channel. determined by a111me1er - 300- Elevation contour ol marine-limit surtace. ~ ConHot point used 1n plotting elevation contours El•v• tions ar• 1n l•et 1bove meen sea level. 25 50 75 100 KILOMETERS Figure 2. Map showing elevations of surveyed glaciomarine deltas in Maine and adjacent New Brunswick. was unaware of their glacial origin. The former occurrence of ly received in the United States by the late 1800's (Flint, 1965). continental glaciation in New England was recognized by the The earlier acceptance of the latter theory resulted in part from mid l 800's, and shortly thereafter geologists began to srudy the Gilbert's work on the tilted shorelines of pluvial Lake Bonneville complex interactions of glacial ice with the changing levels of (Gilbert, 1890). Prior to Gilbert's report, Shaler ( J 874) proposed the land and sea. that the geologically recent depression of the land in Maine, As noted by Meyer (1986), the concept of lowering of together with the resulting marine submergence, was due to eustatic sea level by incorporation of sea water into glaciers was loading by glacial ice. Shaler also estimated the altitude of the not widely accepted until the early 1900's.

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