Re-Evaluation of Antevs' New England Varve Chronology and New Radiocarbon Dates of Sediments from Glacial Lake Hitchcock

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Re-evaluation of Antevs' New England varve chronology and new radiocarbon dates of sediments from glacial Lake Hitchcock

JOHN C. RIDGE Department of Geology, Tufts University, Medford, Massachusetts 02155 FREDERICK D. LARSEN Department of Earth Science, Norwich University, Northfield, Vermont 05663

ABSTRACT

A new varve record from sediments of glacial Lake Hitchcock in the Connecticut River Valley along Canoe Brook in Vermont matches and provides a test of Antevs' New England varve chronology for a span of more than 530 yr. Antevs' methods of correlation and for constructing the varve chronology appear to be valid. The varve record at Canoe Brook records weather-controlled variations in meteoric (nonglacial) discharge as well as glacial runoff. Organic sediment from the Canoe Brook site, which includes twigs, leaf debris, conifer needles, and fine disseminated organic detritus, lies 460-470 couplets above the base of the section and was deposited about 500 yr after déglaciation. The organic sediment was radiocarbon dated at 12,355 ± 75 yr B.P. (GX-14231), 12,455 ± 360 yr B.P. (GX-14780), and 12,915 ± 175 yr B.P. (GX-14781). Our radiocarbon dates are the first from sediments of Lake Hitchcock, and they provide the first real calibration of the New England varve chronology. The dates place the inception of Lake Hitchcock in central Connecticut at before 15,600 yr B.P. and déglaciation of the Canoe Brook site at about 12,900 yr B.P. An abrupt change in sediment types and thickness of varves 50 yr above the radiocarbon dates (about 12,400 yr B.P.) corresponds to a basin-wide change in the New England varve chronology and records the initial breaching of the dam for Lake Hitchcock at Rocky Hill, Connecticut. Nonglacial lakes, lower than Lake Hitchcock, persisted in the northern Connecticut Valley until at least 400 yr after the incursion of marine waters into the Champlain Valley.

INTRODUCTION

In the early 1920s, Ernst Antevs (1922) formulated the New England (NE) varve chronology from glacial lake sediments deposited during late Wisconsinan ice recession in New England and eastern New York. The NE varve chronology was part of an attempt to create a master chronology for North America, similar to a detailed chronology for Scandinavia (De Geer, 1912), and to determine the time and rate of recession of the Lauren- tide ice sheet (Antevs, 1922, 1925, 1928). Correlation with deposits in Europe was also attempted (De Geer, 1921; Antevs, 1928). Antevs (1922) measured the thicknesses of individual varve couplets (from bottom of silty summer bed to top of winter clay bed) in 73 exposures in the Connecticut Valley from Hartford, Connecticut, to St. Johnsbury, Vermont (Fig. 1). Additional varve records, one from sediments of Lake Ashuelot at Keene, New Hampshire, 17 from Lake Merri- mack in south-central New Hampshire, and 10 from Lake Albany in the Hudson Valley of New York, were also constructed. The Connecticut Valley measurements were mostly from sediments of Lake Hitchcock Figure 1. Location of the Champlain Sea and glacial lakes used by (Lougee, 1939), the highest lake level in the Connecticut Valley. Lake Antevs (1922) to formulate the New England varve chronology (modi- Hitchcock was dammed behind ice-contact stratified deposits at Rocky fied from Larsen, 1987).

Geological Society of America Bulletin, v. 102, p. 889-899, 4 figs., July 1990.

889

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Hill, Connecticut (Fig. 1), and its level was controlled by a stable bedrock Hartshorn, 1965). Summaries of the Quaternary geology of New England spillway at New Britain (Schafer and Hartshorn, 1965; Stone and others, (Schafer and Hartshorn, 1965; Hartshorn, 1976) and sedimentologic stud- 1982). The lake expanded northward in front of the receding ice sheet to at ies of Lake Hitchcock (Ashley, 1972, 1975) acknowledged the least Lyme, New Hampshire (Lougee, 1939; Goldthwait and others, disagreement between radiocarbon interpretations and the NE varve chro- 1951). Deltas of Lake Hitchcock have topset-foreset contacts that define a nology, but they could offer no solution to the problem. Disfavor with the planar, tilted surface and are now known to extend up the Connecticut NE varve chronology was marked by exclusion of any reference to it in the Valley to Littleton, New Hampshire (Koteff and others, 1987; Koteff and latest textbooks by Hint (1957, 1971). Antevs' (1962) final appraisal of Larsen, 1989). Initial failure of the Rocky Hill dam caused lowering, but the radiocarbon chronology of deglaciation in North America appeared to not complete drainage, of water in the Connecticut Valley (Schafer and carefully avoid any mention of discrepancies with the NE varve Hartshorn, 1965). chronology. Antevs constructed "normal curves" of varve thickness for separate Interpretations of the age of Lake Hitchcock based on radiocarbon regions by matching thickness records from individual sections and calcu- dates in the 1950s should be scrutinized, especially when one considers lating the average thickness of varve couplets for overlapping intervals. A that not one of the radiocarbon dates was from Lake Hitchcock sediment. large number of sections were matched to account for irregularities (miss- There also have been significant improvements in the laboratory treatment ing or extra couplets) in individual sections, and Antevs (1922) reported of samples for radiocarbon dating. More recent geologic evidence and close agreement between distant varve localities. Antevs was confident radiocarbon dating place doubt on the time constraints of Flint (1956). A that contemporaneous normal curves, from separate regions and lakes, radiocarbon date of 12,200 ± 350 yr B.P. (W-828, Colton, 1960) was matched because thickness variations on all normal curves were a function obtained from a log in a buried peat bog in one of many depressions on the of regional weather patterns that controlled glacial melting and runoff. surface of Lake Hitchcock sediments in Connecticut. The depressions are The NE varve chronology is composed of two continuous sequences probably permafrost features that formed shortly after the drainage of with a total of 4,152 varves. The varve chronology arbitrarily begins with Lake Hitchcock (Stone and Ashley, 1989). Lake Hitchcock, therefore, had NE varve 3001 and ends at NE varve 7400. Antevs was able to match to drain before 12,000 yr B.P. Ashley (1972) recognized that the persis- Connecticut Valley normal curves with curves constructed from sediments tence of glacial ice in the Connecticut Valley, and the persistence of Lake of Lakes Albany, Merrimack, and Ashuelot (Fig. 1). These correlations Hitchcock as a glacial lake as late as 10,700 yr B.P. was incompatible with allowed Antevs to cover gaps between normal curves constructed from the invasion of marine waters into the Champlain Valley about 1,000 yr varves in the Connecticut Valley. Only one gap, estimated by Antevs to be earlier (Parrott and Stone, 1972; Cronin, 1979; Lowdon and Blake, 1979). 200-300 yr (NE varves 6353-6600), was not covered by normal curves The contemporaneity of the Middletown Readvance and non- from outside the Connecticut Valley. arboreal pollen zones south of Middletown (Hint, 1956) cannot be dem- Since the 1950s, when the NE varve chronology was first challenged onstrated. This interpretation ignores the possibility of considerable lags in (Flint, 1956), many Quaternary geologists have viewed it with skepticism. organic sedimentation and very slow sedimentation rates for non-arboreal Our objective is to test the validity of Antevs' NE varve chronology by pollen-zone sediments (Cotter and others, 1983). Furthermore, recent comparing it with a new varve record from Canoe Brook, Vermont. We detailed surficial mapping has led to a different interpretation of sediments also present the first radiocarbon dates of Lake Hitchcock sediments and previously identified as those of the Middletown Readvance (Stone and the first real calibration of the NE varve chronology. The NE varve chro- others, 1982; London, 1985), and the existence of thg*readvance has been nology, the Canoe Brook varve record, and the radiocarbon dates together questioned. provide an important framework for studying the chronology of déglacia- tion in New England. THE NEW ENGLAND VARVE CHRONOLOGY AND PALEOMAGNETIC DATA THE VARVE CHRONOLOGY VERSUS RADIOCARBON INTERPRETATIONS Validity of the varve chronology is supported by successful attempts to use it as a chronologic framework for constructing high-resolution The NE varve chronology was challenged in the 1950s on the basis of records of late Pleistocene paleomagnetic declination and inclination. two sets of radiocarbon dates from southern New England. One set of McNish and Johnson (1938), and Johnson and others (1948) located and dates, from deposits at the southern end of the New Britain spillway remeasured varve sections studied by Antevs' (1922) and matched them channel and from fluvial deposits overlying the lacustrine sediments of with the NE varve chronology. Paleomagnetic declination records were Lake Hitchcock, was interpreted to constrain the drainage of Lake Hitch- constructed that had smooth changes in remanent declination over time, as cock in the Hartford area to about 10,700 yr B.P. (Flint, 1956). The would be expected for a continuous sequence of varves deposited under second set of dates was obtained from spruce pollen zones in cores of bogs time-varying geomagnetic field conditions. Verosub (1979a, 1979b) stud- south of Middletown (Fig. 1) and was interpreted to constrain the begin- ied the first 2,500 yr of the Antevs' chronology and was able to reproduce ning of Lake Hitchcock to 13,000 yr B.P. or later (Suess, 1954; Flint, the declination record of Johnson and others (1948) and to construct an 1956). The dates south of Middletown were from materials overlying inclination record. At 11 sites not seen by Antevs (1922), and as much as sediments with non-arboreal pollen assemblages that were interpreted to 15 km from sites used by Antevs to construct his normal curves, Verosub represent cold climatic conditions during the Middletown Readvance. The was able to match "year for year" his results with those of Antevs. As two sets of radiocarbon dates were interpreted to confine the age of Lake stated by Verosub (1979a, p. 245): "While there are still problems which Hitchcock in the southern Connecticut Valley to between 10,700 and must be resolved with regard to the deglaciation of New England, the fact 13,000 yr B.P. (Flint, 1956), a duration which is incompatible with the remains that in no case has the varve chronology itself ever been shown to 4,152 varves counted by Antevs (1922). Antevs' (1928) estimates of the be incorrect." A tentative correlation of paleomagnetic declination records duration of déglaciation in eastern North America, which were mostly from late Wisconsinan glaciolacustrine deposits in central New York, with based on analysis outside New England, were later recognized as clearly those from radiocarbon-dated sediments of Lake Hitchcock (Ridge and incompatible with later-derived time constraints (Flint, 1947; Schafer and others, 1990), represents an exciting possibility for interregional correla-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/889/3380905/i0016-7606-102-7-889.pdf by guest on 01 October 2021 Figure 2. A. Varve section located 150 m east of Route 5 along the north bank of Canoe Brook in Dummerston, Vermont. Locations of Figures B through F are shown. In Figures 2B through 2F, knife is 20 cm long, and black arrows indicate winter clay beds in varve couplets. B. Ice-proximal varves (top of couplet 7 through couplet 11) in the base of the Canoe Brook section. Occasional rippledfin e sand beds occur in laminated fine sandy to clayey silt summer beds.

Figure 2. (Continued). C. Thick, faintly graded silty clay bed in couplet 174. Top of thick graded bed displays load deformation by overlying fine sand. Several couplets above and below couplet 174 have rippled sand beds. Couplet thicknesses were measured in the troughs of clay beds draped over ripples.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/889/3380905/i0016-7606-102-7-889.pdf by guest on 01 October 2021 Figure 2. (Continued). D. Medium-sized, ice-distal varves (in the interval of couplets 175-225) in the Canoe Brook section. E. Thin ice-distal varves (in the interval of couplets 350-425) in the top of the Canoe Brook section. F. Load deformation in ice-distal couplets (in the interval of couplets 575-625) in the top of the Canoe Brook section.

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tion and the absolute dating of glacial events in the northeastern United and Johnson, 1938; Johnson and others, 1948; Verosub, 1979a) is difficult States. to explain if the couplets are not annual. Couplets 1-20 of the Canoe Brook section (Fig. 3) have very erratic VARVE STRATIGRAPHY AND RADIOCARBON DATES, thicknesses which average 16.1 cm, and 70% of the couplets are thicker CANOE BROOK, VERMONT than 10 cm (Fig. 2B). Less than 20% of couplets 21-200 exceed a thickness of 10 cm, and they have a mean thickness of about 4-8 cm (Figs. 2C A new 21-m exposure of varves along Canoe Brook in Dummerston, and 2D). Below couplet 21, current rippled sand beds and abundant, Vermont (Figs. 1 and 2), provides a rare site for testing the validity of the poorly sorted, very faintly bedded to massive clayey and sandy silt indicate NE varve chronology. The base of the exposure has an elevation of about deposition by pelagic rain-out from ice-proximal meltwater plumes in 80 m, and the projected water plane for Lake Hitchcock has an elevation addition to lake-bottom underflow currents (Gustavson, 1975; Gustavson of about 138 m at Canoe Brook (Koteff and Larsen, 1989). The Canoe and Boothroyd, 1982). Below couplet 21, and generally in the basal varves Brook site occurs about 0.4 km from the west shore of Lake Hitchcock in a of Lake Hitchcock, variations in glacial meltwater discharge (Ashley, place where the lake was about 1.75 km wide. Couplets at Canoe Brook 1975) and proximity to meltwater plumes at the front of the receding were numbered beginning with couplet 1 at the base of the exposure, and glacier account for most of the variations in varve-couplet thicknesses the thickness of each couplet was measured (Fig. 3). The Canoe Brook (Fig. 4). Paleocurrent directions inferred from current ripples at eight section records at least 645 yr of varve deposition in this part of the different levels between couplets 21 and 250 range from northeast to Connecticut Valley. The number of couplets which occur below the expo- southeast. These directions provide evidence of lake-bottom underflow sure is not known precisely because the basal 6 m of the section, down to currents emanating from the Canoe Brook Valley and the existence of a the level of Canoe Brook, is concealed by slump debris. The gradient of meteoric (nonglacial) sediment source. Variations in meteoric discharge Canoe Brook is steep, and the stream has probably cut entirely through the into Lake Hitchcock account to some extent for variations in couplet lake sediment so that alluvium rests on either till or bedrock. The basal 10 thicknesses above couplet 21. There does not appear to be any way to couplets of the measured section have a mean thickness of 19.1 cm, and determine the relative importance of meteoric- versus glacial-sediment the basal 30 couplets of the section increase in thickness downward. These sources for Lake Hitchcock in Vermont because, unlike in the southern part basal couplets are inferred to represent an ice-proximal environment of of the Connecticut Valley, these two sources do not have a different Lake Hitchcock. If the alluvium of Canoe Brook rests on till or bedrock, provenance. Some stream terraces in tributary valleys in Vermont and and varves continue to be as thick as in the base of the exposed section, the New Hampshire are graded to meteoric deltas deposited in Lake Hitch- slump debris cannot conceal more than 30 couplets, and the base of the cock and are also inset in deeply eroded till (Ridge, 1988). These deposits Canoe Brook section was deposited no more than 30 yr after déglaciation. suggest that large volumes of sediment were carried to Lake Hitchcock by The top of the Canoe Brook varve sequence is crosscut by an erosion meteoric discharges. Ashley (1975) has also recognized the thickening of surface that is overlain by fluvial, cross-bedded sand and gravel of a stream varves near the mouths of stream valleys and the importance of meteoric terrace deposit; over most of the exposure, however, artificial fill directly sediment sources in other parts of Lake Hitchcock. The gradual decrease in overlies the varves. couplet thickness above couplet 21 may be partly the result of decreasing erosion rates in the Canoe Brook Valley. Varves in the Canoe Brook section are slightly calcareous couplets that range in thickness from 0.7 to 33.2 cm (Figs. 2A-2F). Summer beds Two faintly graded, faintly laminated, silty clay beds that are 6.5 cm in the varves are dark gray (N4) or greenish-gray (5GY 5/1) to olive gray (couplet 48) and 7.0 cm (couplet 174) thick occur in two thick couplets (5Y 5/2), laminated, clayey to fine sandy silt beds with many light-colored (Fig. 2C). These beds are interpreted to be turbidites that represent either partings and laminated beds of micaceous fine sand and occasional, large storm events, mass movement on the floor of Lake Hitchcock, or rippled micaceous, fine to medium sand beds. Winter beds are dark gray to catastrophic floods ("drainage varves" of Antevs, 1922) produced by the greenish-gray (5Y-5G 4/1), apparently massive clay beds with occasional release of water from lakes impounded in tributary valleys at the margin of mid-bed silt laminae. the receding ice sheet. Evidence that the clay and silt couplets at Canoe Brook, and at other Mean varve thickness decreases upward to 2-4 cm in couplets rhythmite sections of glacial Lake Hitchcock, are annual have been out- 201-300 and to less than 2 cm in couplets 301-520 (Fig. 2E). Throughout lined by Ashley (1972,1975) and Ashley and others (1985). Key observa- the section from couplet 300 upward, isolated plant fragments and a small tions that indicate that the couplets are varves are as follows. leaf have been found on the bedding planes of summer layers. Winter 1. The over-all regularly repeated pattern and discrete lithologies of layers are sometimes capped by dark brown, paper-thin laminae of fibrous clay and silt beds indicate separate modes of deposition for silt and clay material that may be organic or an oxide precipitate. beds that are consistent with annual variations in sedimentation. Discontinuous, up to 1.0-cm-thick, organic-bearing beds that contain 2. There is a less variable thickness of clay beds in adjacent couplets leaf fragments, small twigs, conifer needles, and fine disseminated organic as compared to silt beds of more variable thickness. sediment have been found in the summer beds of couplets 449, 463, 469, 3. Multiple, micrograded layers in silt beds make a turbidite or and 470 (Fig. 3). Many clay laminae in the vicinity of the organic-rich storm-event origin for single couplets unlikely. beds exhibit a grayish to blackish-blue color resulting from organic reduc- 4. Sinusoidal crawling traces of nematode worms, and at least one tion. The organic-rich beds probably represent flood events that carried other organism, are on several different horizons within the silty lamina- organic debris from the Canoe Brook Valley into Lake Hitchcock. A 13C tions of one couplet. This observation is consistent with a long annual corrected radiocarbon date of 12,355 + 75 yr B.P. (SBC = -27.2 ppt; period of deposition and not deposition by single turbidity or storm events. GX-14232) was obtained from bulk material of a sandy organic bed in In addition, annual weather variation across the region covered by couplet 463. Tiny modern rootlets were suspected of contaminating the the NE varve chronology is the most likely explanation for the correspon- bulk sample. A smaller sample of hand-picked macroscopic organic debris dence of normal curves from within the Connecticut Valley and from from couplet 463, mostly small twigs and conifer needles, was submitted separate lake basins. Correlation of distant sections (Antevs, 1922; McNish for radiocarbon dating and yielded a 13C-corrected date of 12,455 ± 360

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/889/3380905/i0016-7606-102-7-889.pdf by guest on 01 October 2021 Figure 3. Varve record from Canoe Brook (couplet thickness versus couplet number) and normal curves (mean varve thickness versus varve year) of the NE varve chronology (Antevs, 1922). The normal curves are from Lake Albany (dashed line 14-N.Y.), Lake Hitchcock (solid line 14-VT., 15- VT„ 16-VT., N.H., 17-VT., N.H.), and Lake Ashuelot (dashed line 15-N.H.). All thicknesses are recorded in centime- ters, and several scale changes occur on the plots, for example at couplet 37 of the Canoe Brook record and at varve yr 5812 of the NE varve chronology. (Note overlap in center.)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/889/3380905/i0016-7606-102-7-889.pdf by guest on 01 October 2021 SYMBOLS Couplet thick- —ness (cm) off scale (47+48) Combined couplets kssss Gap at Canoe Bk. A Organic bed HP, Thick (>4cmj 15:2 graded silty ANOE clay bed BROOKJ 370- - VT 16z VT, N.H." 2.0 3.0 cm Thickness

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NORTH

Figure 4. Diagrammatic cross section of receding glacier, proglacial lacustrine environments, and varved sediment facies in the Connecticut Valley. No attempt is made to show diamicton, sand and gravel, or nonvarved facies associated with ice recession. The relationship of time lines to both varved facies and proglacial environments is shown diagrammatically. The ice-proximal environment is no more than 10 km long. Locally, ice-proximal sedimentation rates during the melting season are predominantly controlled by changes in meltwater discharge and proximity to vertically and horizontally shifting positions of meltwater plumes emanating from subglacial and ice-marginal streams. Time lines in ice-proximal sediments are separated by thicknesses of silt and clay that are not predominantly a function of regional weather variations. Ice-proximal and ice-distal environments are gradational. The ice-distal environment extends hundreds of kilometers south of the glacier and is dominated by slower sedimentation rates than in ice-proximal environments, except near the foreset slopes of meteoric (nonglacial) deltas. Sedimentation during the melting season in ice-distal environments is dominated by weather-controlled glacial and meteoric discharges. Time lines in ice-distal sediment are separated by thicknesses of silt and clay that are predominantly a function of variations in weather. (Note overlap in center.)

yr B.P. (Ô13C = -27.6 ppt; GX-14780). Clayey silt beds from couplets washed into Lake Hitchcock. It is not likely that conifer needles in the 469-470, which contain disseminated fine organic sediment, were also samples would remain intact for a significant interval on land before submitted for radiocarbon dating and yielded a 13C-corrected date of deposition in Lake Hitchcock. The disseminated organic material, dated at 12,915 ± 175 yr B.P. (Ô13C = -27.1 ppt; GX-14781). All radiocarbon 12,915 ± 175 yr B.P., is inferred not to be the same age as couplets samples were treated for both carbonate and dissolved organic contamina- 469-470. The disseminated organic sediment, which was partly decom- tion, and all samples have ô13C values which are consistent with terrestrial posed prior to deposition, may have resided on land for several hundred plant detritus. years before entering Lake Hitchcock. The sediment, furthermore, may The organic sediments may be slightly older than the couplets in contain a few percent of preglacial (>23,000 yr B.P.) organic sediment which they were deposited, especially the disseminated organics from that was incorporated into till during advance of the last glacier and was couplets 469-470 dated at 12,915 ± 175 yr B.P. All of the organic postglacially eroded and washed into Lake Hitchcock. Proglacial lacus- sediment may have resided on land before being washed into Lake Hitch- trine and fluvial deposits have been found beneath late Wisconsinan till in cock. Déglaciation of the Canoe Brook site predates deposition of the the Connecticut River drainage basin in areas north of Canoe Brook organic sediment in Lake Hitchcock by at least 470 yr. If the radiocarbon (Larsen, 1987; Ridge, 1988). It does not seem likely, given the S13C value dates accurately record the age of postglacial organic sediment, they pro- (-27.1 ppt) of sediment from couplets 469-470, that the sediment is partly vide minimum estimates of déglaciation. derived from aquatic plants that assimilated carbonate and bicarbonate All of the radiocarbon dates are from the same 10-varve interval, but ions from dissolved bedrock ("hard-water effect"; Donner and others, one date is about 500 yr older than the other two. We infer that the 1971; Shotton, 1972; Olsson, 1986). organic material dated at 12,355 + 75 and 12,455 ± 360 yr B.P. is about Beginning at couplets 520-525, varve thickness and sediment types the same age as couplet 463 because it is coarse terrestrial-plant debris, change abruptly (Fig. 2F). Below couplets 520-525, couplets have a mean mostly conifer needles, that was not significantly decomposed before it was thickness of less than 2 cm, and above those couplets, they have a mean

Features used to visually match the varve record and normal curves were SOUTH the over-all positions of peaks, abnormally high peaks, the relative heights of peaks, and the number of varves in peaks. Couplets 27-546 of the METEORIC Canoe Brook section show a year to year correspondence to Antevs' DISCHARGE (1922) normal curves from Lake Hitchcock (see 14-VT, 15-VT, 16-VT, (INWASH) NH and 17-VT, NH in Fig. 3) which were compiled from 9 individual .(. t -.o sections that are located 5-35 km north of the Canoe Brook site. Couplets v ^ „ 119-191 of the Canoe Brook section show a correspondence to Antevs' (1922) normal curve from Lake Ashuelot (see 15-NH on Fig. 3). Couplets 16-115 of the Canoe Brook section match a normal curve from Lake Albany which was used by Antevs to fill a gap in normal curves • (Offshore from: V from Lake Hitchcock (Antevs, 1922; see 14-NY in Fig. 3). Couplets 1- . «'¿'meteoric discharge) 15 at Canoe Brook cannot be matched to the Lake Albany curve due to erratic thicknesses of the basal 15 varves at Canoe Brook which were deposited in an ice-proximal environment. Normal curves from Lake Mer- rimack (NE varves 5709-5749 and 5771-6352 of Antevs, 1922) are not shown on Figure 3 because they would create too much congestion on the normal curve plots. The Lake Merrimack normal curves also show a close correspondence to the other curves and the Canoe Brook record. Differences in the number of years represented by normal curves and the Canoe Brook record appeared at nine places (Fig. 3). The positions of missing and extra couplets, similar to those recognized by Antevs (1922) between individual sections, were identified as points where the Canoe Time lines (tens of years) Brook record began to exhibit a 1- or 2-yr offset with a normal curve. For one continuous, 300-yr span of the Canoe Brook section (couplets 148- 447; NE varves 5836 through 6135), no offsets were observed. At thickness of more than 4 cm. This change is maintained to the top of the three places, single extra couplets, and at one place, two extra couplets, varve section (couplet 645). Couplets 525-645 are sandier in their appeared in the Canoe Brook record. Some of the extra couplets were summer layers than are units below; above couplet 546, they exhibit wavy recognized in the field as having faintly graded, silty clay beds, that were bedding as a result of ripples and syndepositional loading of sand. A single unlike the winter clay beds of other couplets, and they are probably couplet may vary in thickness from 0.5-10.0 cm, and above couplet 546, turbidites. At four places, the locations of single missing couplets, and at meaningful measurement of couplet thicknesses for the purpose of correla- one other place, the location of two missing couplets were identified. The tion was not possible. The sustained abrupt change in sediment types, thick graded silty clay beds in couplets 48 and 174 of the Canoe Brook bedding thickness, and structures is a facies change related to either rapid section appear to represent basin-wide events because they correspond to progradation of a delta at the mouth of Canoe Brook or a basin-wide extremely thick varves of the New England varve chronology (NE varves event. An abrupt sustained change in varve thickness at a correlative 5732-5734 and 5862). Above couplet 546 (not shown on Fig. 3) the position on normal curves of the NE varve chronology (Fig. 3, NE varves Canoe Brook section could not be matched with normal curves because of 6210-6215) is also apparent. The normal curves were constructed by wavy bedding in the Canoe Brook section (Fig. 2F). Antevs (1922) from sections in the Connecticut Valley that are 20-35 km north of the Canoe Brook site. The abrupt change in varve thickness at the IMPLICATIONS FOR THE GLACIAL CHRONOLOGY Canoe Brook section, therefore, represents a basin-wide event. A basin- OF NEW ENGLAND wide drop in lake level that caused rejuvenated erosion in the Canoe Brook Valley and elsewhere, as well as possible bathymetric and circula- The radiocarbon dates from the Canoe Brook section are important tion changes in the lake, is inferred to have produced the abrupt, sustained to the glacial chronology of New England because they are the first dates changes that occurred at least 520 yr after ice receded from the Canoe from sediments of Lake Hitchcock and they also provide the first real Brook site. This event is interpreted to be the drop in lake level caused by calibration of the NE varve chronology. Radiocarbon dates of 11,390 ± initial breaching of the Rocky Hill dam (Fig. 1) which occurred after ice 115 (WIS-919) and 11,540 ± 110 yr B.P. (WIS-961) obtained on wood receded north of Littleton, New Hampshire (Koteff and others, 1987; fragments in lacustrine sediment near Colebrook, New Hampshire (Fig. 1), Larsen and Koteff, 1988). Deltas have been found throughout the Connec- in the northern Connecticut Valley (Miller and Thompson, 1979) are not ticut Valley which represent deposition in lakes lower than levels con- from Lake Hitchcock sediment and have not been tied to the NE varve trolled by the New Britain channel (Koteff and Larsen, 1989). A lake chronology. The lacustrine sediment near Colebrook was deposited in remained impounded in the Connecticut Valley at Canoe Brook for at Lake Coos (Lougee, 1939) and is 65 km northeast of varve sections used least 125 yr after breaching of the Rocky Hill dam. to formulate the NE varve chronology (Antevs, 1922). If the Canoe Brook dates of about 12,400 yr B.P. (NE varve 6150) CORRELATION OF CANOE BROOK WITH THE and the NE varve chronology are correct, they place the beginning of the NE VARVE CHRONOLOGY varve chronology (NE varve 3001) at about 15,600 yr B.P. Given that NE varve 3001 at Hartford, Connecticut, occurs on top of at least 30 m of Comparison of a plot of varve thickness at the Canoe Brook site with clay (Antevs, 1922), both the beginning of Lake Hitchcock and recession normal curves of the NE varve chronology (Fig. 3) shows a close match of ice from Middletown would have to predate 15,600 yr B.P. by signifi- between the two records for 533 varve yr (NE varves 5702-6234). cant intervals of time. Correlations of the Lake Hitchcock normal curves

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and the Canoe Brook varve record with normal curves from Lake Merri- deltas of Lake Hitchcock indicating that Lake Hitchcock drained to a mack in south-central New Hampshire and Lake Ashuelot in southwestern lower level prior to ice recession and marine incursion in the Champlain New Hampshire (Fig. 1) indicate that these lakes were in existence by at Valley. Varves in the Connecticut Valley between Claremont and St. least 12,900 yr B.P. Johnsville (Fig. 1), that were used to construct the last 400 yr of the NE Earlier we inferred that an abrupt and sustained change in sediment varve chronology (11,500-11,100 yr B.P., NE varve years 7000-7400), types and couplet thickness in the Canoe Brook section at couplets therefore, were deposited in nonglacial lakes lower than Lake Hitchcock. 520-525 is a basin-wide change that represents the initial breaching of the Rocky Hill dam of Lake Hitchcock. If the NE varve chronology and the GENERAL IMPLICATIONS OF THE Canoe Brook radiocarbon dates are valid, they indicate initial breaching of CANOE BROOK SECTION the dam and drainage of Lake Hitchcock at about 12,300-12,400 yr B.P. If the Canoe Brook radiocarbon dates of about 12,400 yr B.P. accu- The Canoe Brook varve record is significant because of both its rately date couplet 463, the -500 varves below the dated sediment indi- sedimentologic and stratigraphic implications. First, in an ice-proximal cate déglaciation by at least 12,900 yr B.P. Initiation of Lake Hitchcock in environment (Fig. 4), such as is represented by couplets 1-15 of the Canoe Connecticut at or after 13,000 yr B.P. (Flint, 1956) is impossible because it Brook section, ice-marginal meltwater activity dominates over other sedi- would require more than about 150 km of ice recession and one glacial mentation processes (Ashley, 1975). Meltwater plumes that emanate from readvance in the Connecticut Valley (Antevs, 1922; Larsen, 1982) in no subglacial tunnels or ice-marginal drainage channels (Gustavson, 1975; more than 100 yr. Because of the ice-proximal character and thicknesses of Gustavson and Boothroyd, 1982) change their positions frequently, inde- varves in the base of the Canoe Brook section, it is unlikely that lacustrine pendent of weather variations. Both rapid and erratic sedimentation occurs deposits below the section represent more than 30 yr. If this is true, from place to place near glacier margins, and during the melting season, it déglaciation at Canoe Brook occurred at about 12,900 yr B.P. Ice reces- can mask variations in sedimentation in response to regional weather sion in the Connecticut Valley from Canoe Brook at 12,900 yr B.P. to patterns. For this reason, correlation of contemporaneous ice-proximal north of Littleton, New Hampshire (165 km), prior to failure of the Rocky varves based on couplet thicknesses is difficult. Ice-proximal sedimentation Hill dam (interpreted from our dates to be no later than 12,300 yr B.P.) processes account for the poor match of couplets 1-15 of the Canoe Brook would require an average ice-recession rate of at least 0.275 km/yr. section with the NE varve chronology and hinder the correlation of prox- Déglaciation at 12,900 yr B.P. in southern Vermont appears to be imal varve records with records at other localities. incompatible with some radiocarbon dates that provide minimum esti- Control of melting-season sedimentation rates by regional weather mates of déglaciation in northern New England (Stone and Borns, 1986), patterns seems to be the only explanation for varve records that can be especially a radiocarbon date of lake sediment (13,870 ± 560 yr B.P.) from correlated over 100 km. Only ice-distal environments (Fig. 4) that are not a core of Mirror Lake in northern New Hampshire (Fig. 1; Davis and dominated by unpredictable glacial meltwater activity, and that do not others, 1980; Davis and Ford, 1982). There appears to be no unequivocal occur near delta fronts where turbidity currents and slope failures are reason to reject either the Canoe Brook or Mirror Lake dates. It is possible, common, can have varve thickness variations that are mostly a function of however, that whole-sediment dates like the one from Mirror Lake are weather-controlled sedimentation rates. Ice-distal environments have dep- older than the radiocarbon dates of handpicked twigs and conifer needles osition with more regional uniformity and preserve relative varve at Canoe Brook because of the hard-water effect (Donner and others, thicknesses from place to place better than ice-proximal environments. 1971; Shotton, 1972; Olsson, 1986), and contamination by old organic Lake Hitchcock represents an ideal situation for constructing a varve sediment and calcium carbonate. No I3C analyses or pretreatment of chronology because ice-distal, weather-controlled varve deposition per- samples from Mirror Lake for carbonate has been reported. If any poten- sisted over most of the basin for thousands of years after deglaciation. tial error mentioned above is significant, the Mirror Lake date cannot be Because ice-distal varve records from Lake Hitchcock reflect weather used as a minimum estimate of déglaciation. There does not seem to be patterns, they record annual variations in temperature and/or precipita- any reason for the Canoe Brook sediment, that was treated for both tion. During the melting season, ice-distal environments (more than 10 km carbonate and dissolved organic contamination, to yield an anomalously from the ice front) may have had significant meteoric sediment sources 13 young date. Based on the C analysis of the Canoe Brook sediment and (Ashley, 1975), and varve thickness may not have been entirely a function the dating of hand-picked material, significant contamination by young of glacial melting rates or glacial meltwater discharges. If distal environ- organic material appears to be a remote possibility. The Canoe Brook ments were significantly influenced by meteoric discharges, sedimentation dates are compatible with radiocarbon dates of wood fragments in lake rates may have been controlled by precipitation as well as temperature sediment in the northern Connecticut Valley (Miller and Thompson, variations. Heavy winter snowfall resulting in large spring melts, and/or 1979). The disagreement between the Canoe Brook dates and some other heavy rainfall during the melting season may have caused increased radiocarbon dates in New England points out the need for pretreatment of meteoric discharges that influenced sedimentation rates in distal samples for contamination by dissolved organic material and carbonate, environments. 13 and for radiocarbon dates accompanied by C analyses. The Canoe Brook section has stratigraphic significance because it has Radiocarbon dates from Lake Hitchcock sediment place NE varve been radiocarbon dated and can be correlated to the NE varve chronology. year 6500 at about 12,000 yr B.P. If 11,500-12,000 yr B.P. is accepted as In addition, it spans a time for which Antevs (1922) was able to construct a possible range for the incursion of marine waters into the Champlain and match normal curves from four separate glacial lakes (Lakes Albany, Valley (Fig. 1), the NE varve chronology overlaps with the Champlain Sea Ashuelot, Hitchcock, and Merrimack) and it provides a test of Antevs' for a span of at least 400 yr. Lakes in western Vermont were impounded methodology. One cannot have confidence in the correlation of normal by ice in the Champlain Valley and drained eastward into the Connecticut curves of different lake basins if the method of normal curve construction Valley (Larsen, 1984, 1987). This eastward drainage fluvially downcut is in doubt. Normal curves from Lake Hitchcock span more than 500 yr of

Goldthwait, J. W., Goldthwait, L„ and Goldthwait, J. P., 1951, The geology of New Hampshire, Part 1, Surficiai geology: the Canoe Brook section and were pieced together from nine shorter Concord, New Hampshire, New Hampshire State Planning and Development Commission, 81 p. records (Antevs, 1922). The period of time represented by the Canoe Gustavson, T. C., 1975, Sedimentation and physical limnology in proglacial Malaspina Lake, southeastern Alaska, in Jopling, A. V., and McDonald, B. C., eds., Glaciofluvial and glaciolacustrine sedimentation: Society of Economic Brook section is one where the NE varve chronology would be suspect if Paleontologists and Mineralogists Special Publication 23, p. 249-263. Gustavson, T. C., and Boothroyd, J. C., 1982, Subglacial fluvial erosion: A major source of stratified drift, Malaspina one did not trust Antevs* procedure of normal curve construction. The Glacier, Alaska, in Davidson-Amott, R., Nickling, W., and Fahey, B. D„ eds., Research in glacial, glaciofluvial and correspondence of the Canoe Brook record with the Lake Hitchcock glaciolacustrine systems (Proceedings of the 6th Guelph Symposium on Geomorphology): Norwich, England, Geo Books, p. 93-116. normal curves provides a demonstration that Antevs' method of section Hartshorn, J. H., 1976, Quaternary problems in southern New England [abs.j, in Mahaney, W. C., ed., Quaternary stratigraphy of North America: Stroudsburg, Pennsylvania, Dowden, Hutchinson and Ross, p. 91-92. correlation within one lake basin is valid. Proof of the validity of Antevs' Johnson, E. A,, Murphy, T., and Torreson, O. W., 1948, Pre-history of the earth's magnetic field: Terrestrial Magnetism methods are especially important given that nearly all of the exposures and Atmospheric Electricity (now Journal of Geophysical Research), v. 53, p. 349- 372. Koteff, C., and Larsen, F. D., 1989, Postglacial uplift in western New England: Geologic evidence for delayed rebound, in used by Antevs to construct the NE varve chronology are no longer Gregersen, S., and Basham, P. W., eds., Earthquakes at North Atlantic passive margins: Neotectonics and available. Correlations of the Lake Hitchcock normal curves with those postglacial rebound: Norwell, Massachusetts, Kluwer Academic Publishers, p. 105-123. Koteff, C., Stone, J. R., Larsen, F. D., and Hartshorn, J. H., 1987, Glacial Lake Hitchcock and postglacial uplift (50th from other lakes, although not as clear as those between separate normal reunion of Friends of the Pleistocene guidebook): U.S. Geological Survey Open-File Report 87-329, 24 p. Larsen, F. D., 1982, Anatomy of the Chicopee readvance, Massachusetts, in Joesten, R., and Quarrier, S. S„ eds.. curves from Lake Hitchcock, are still striking and represent an exciting Guidebook for field trips in Connecticut and south-central Massachusetts (74th Annual Meeting of New England Intercollegiate Geological Conference): Connecticut Geological and Natural History Survey Guidebook 5, possibility for correlation of glacial events across New England with a p. 31-48. resolution superior to radiocarbon dating. 1984, On the relative age of glacial Lake Hitchcock, glacial Lake Wtnooski, and the Champlain Sea: Geological Society of America Abstracts with Programs, v. 16, no. I, p. 45. 1987, Glacial Lake Hitchcock in the valleys of the White and Ottauqueche Rivers, east-central Vermont, in Westerman, D. S., ed.. Guidebook for field trips in Vermont, Volume 2 (79th Annual Meeting of New England ACKNOWLEDGMENTS Intercollegiate Geology Conference): Montpelier, Vermont, p. 29-52. Larsen, F. D., and Koteff, C., 1988, Deglaciation of the Connecticut River valley: Vernon, Vermont, to Westmoreland, New Hampshire, in Bothner, W. A., ed., Guidebook for field trips in southwestern New Hampshire, southeastern We thank the New Hampshire Geological Survey under the direction Vermont, and north-central Massachusetts (80th Annual Meeting of New England Intercollegiate Geological Conference): Keene, New Hampshire, p. 103-125. of Gene Boudette, and the U.S. Geological Survey for funding of field London, E. H., 1985, Deglaciation of the Middletown basin and the question of the Middletown readvance, in Tracey, R. J., ed., Guidebook for fieldtrips in Connecticut and adjacent areas of New York and Rhode Island (77th Annual work and radiocarbon analyses. We especially thank Gail Ashley and Meeting of New England Intercollegiate Geology Conference): Connecticut Geological and Natural History G.S.A. reviewers Julie Brigham-Grette and Eric Leonard for careful re- Survey Guidebook 6, p. 323-352. Lougee, R. J., 1939, Geology of the Connecticut watershed: New Hampshire Fish and Game Department, Biological view of the manuscript which led to significant improvements. Carl Koteff Survey of the Connecticut Watershed, Survey Report 4, p. 131-149. Lowdon, J. A., and Blake, W„ Jr.. 1979, Geological Survey of Canada radiocarbon dates 19: Canada Geological Survey and Byron and Janet Stone provided stimulating discussions of the manu- Paper 79-7, 58 p. script. We also thank Harold Krueger of Geochron Laboratories, McNish, A. G., and Johnson, E. A., 1938, Magnetization of unmetamorphosed varves and marine sediments: Terrestrial Magnetism and Atmospheric Electricity (now Journal of Geophysical Research), v. 43, p. 401 -407. Cambridge, Massachusetts, for his advice concerning our discussion of Miller, N. G., and Thompson, G. G., 1979, Boreal and western North American plants in the late Pleistocene of Vermont: Journal of the Arnold Arboretum, v. 60, p. 167-218. radiocarbon dates. Olsson, I. U., 1986, Radiometric dating (Chap. 14), I. Radiocarbon dating, in Berglund, B. E., ed.. Handbook of Holocene palaeoecology and palaeohydrology: New York, Wiley & Sons, p. 275-298. Parrott. W. R.. and Stone. B. D., 1972, Strandline features and late Pleistocene chronology of northwest Vermont, in REFERENCES CITED Doolan, B. L.. and Stanley. R. S„ eds., Guidebook for field trips in Vermont (64th Annual Meeting of New England Intercollegiate Geological Conference): Burlington, Vermont, p. 359 376. Antevs, E., 1922, The recession of the last ice sheet in New England: American Geographical Society Research Series no. Ridge, J. C., 1988, The Quaternary geology of the upper Ashuelot River, lower Cold River, and Warren Brook Valleys of 11, 120 p. southwestern New Hampshire, in Bothner, W. A., ed.. Guidebook for field trips in southwestern New Hampshire, 1925, Retreat of the last ice sheet in eastern Canada: Canadian Geological Survey Memoir 146, 142 p. southeastern Vermont, and north-central Massachusetts (80th Annual Meeting of the New England Intercollegiate 1928, The last glaciation, with special reference to the ice sheet in northeastern North America: American Geology Conference): Keene, New Hampshire, p. 176 208. Geographical Society Research Series no. 17, 292 p. Ridge, J. C., Brennan, W. J., and Muller, E. H., 1990, The use of paleomagnetic declination to test correlations of late 1962. Transatlantic climatic agreement versus C-14 dates: Journal of Geology, v. 70, p. 194-205. Wisconsinan glaciolacustrine sediments in central New York: Geological Society of America Bulletin, v. 102, Ashley, G. M„ 1972, Rhythmic sedimentation in glacial Lake Hitchcock. Massachusetts-Connecticut: Amherst, Massa- p. 26-44. chusetts, University of Massachusetts, Geology Department, Contribution 10, 148 p. Schafer, J. P., and Hartshorn, J. H., 1965, The Quaternary of New England, in Wright, H. E., Jr., and Frey, D. G., eds.. - 1975, Rhythmic sedimentation in glacial Lake Hitchcock, Massachusetts-Connecticut, in Jopling, A. V., and The Quaternary of the United States: Princeton, New Jersey, Princeton University Press, p. 113-128. McDonald, B. C., eds., Glaciofluvial and glaciolacustrine sedimentation: Society of Economic Paleontologists and Shotton, F. W., 1972, An example of hard water error in radiocarbon dating of vegetable matter: Nature, v. 240. Mineralogists Special Publication 23, p. 304-320. p. 460-461. Ashley, G- M.,Shaw, J., and Smith, N. D., 1985. Glacial sedimentary environments: Society of Economic Paleontologists Stone, B. D„ and Borns, H., Jr., 1986, Pleistocene glacial and interglacial stratigraphy of New England, Long Island, and and Mineralogists Short Course 16,246 p. adjacent Georges Bank and Gulf of Maine, in Sibrava, V., Bowen, D. Q., and Richmond, G. M., eds., Quaternary Colton, R. B., 1960, Surflcial geology of the Windsor Locks quadrangle, Connecticut: U.S. Geological Survey Map glaciations in the northern hemisphere: New York, Pergamon Press, p. 39-52. GQ-137. Stone, J. R., and Ashley, G. M., 1989, Fossil pingos?: New evidence for active permafrost in southern New England Cotter, J.F.P., Evenson, E. B., Sirkin, L., and Stuckenrath, R., 1983, The interpretation of 'bog-bottom' radiocarbon dates during late Wisconsinan deglaciation: Geological Society of America Abstracts with Programs, v. 21, no. 2, p. 69. in glacial chronologies, in Mahaney, W. C., ed., Correlation of Quaternary chronologies: Norwich, England, Geo Stone, J. R., Schafer, J. P., and London, E. 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