Pleistocene stratigraphy of the Boston Harbor drumlins, Massachusetts

QUATERNARY

RESEARCH

34, 148-159 (1990)

Pleistocene WILLIAM *Department

Stratigraphy of the Boston Drumlins, Massachusetts

A. NEWMAN,*

of Geology,

Northeastern Geological

RICHARD C. BERG,? HERBERT D. GLASS? University, Boston, Survey, Champaign,

Harbor

PETER S. ROSEN,*

Massachusetts Illinois 61820

02115,

AND

and flllinois

State

Received February 7, 1989 Evidence from the Boston Harbor drumlins indicates that two superposed tills were deposited during glacier advances which were separated by a long nonglacial interval. At Long and Peddocks Islands, argillans and truncated clay-tilled fractures, along with discontinuities in clay-mineral composition, define the till contacts. Physical indicators separating the tills are not apparent at other exposures, where till boundaries were defined solely by discontinuities in clay-mineral composition. The weathering profile in the upper part of the lower till indicates extensive weathering under a chmate similar to that of today, and probably similar to that of the Sangamon Interglaciation. The depth of the weathering profile, the sequence of clay-mineral alteration products. and the presence of pedogenic features in the upper part of the lower till are comparable to Sangamonian weathering profiles in the midwestern United States, implying that the lower till is Illinoian or older. o 1%%1 University of Washington.

INTRODUCTION

Two superposed tills, known as the lower and upper tills, have been recognized in many parts of southern New England for more than a century. The upper till is regionally continuous and is interpreted to be late Wisconsinan in age. The lower till crops out discontinuously beneath the upper till and has not been correlated regionally. Because the lower till may represent one or more depositional events, determination of its age has been controversial. Some researchers have interpreted the lower and upper tills as late Wisconsinan lodgment and ablation deposits from one glacial episode. Others interpret the two tills as deposits from separate ice sheets, one dating to the late Wisconsinan (Woodfordian), and the other to the early Wisconsinan (Altonian) or earlier. Recent studies of the tills (Stone, 1975; Newton, 1978; Koteff and Pessl, 1985) have supported the interpretation that deep weathering of the lower till is the result of long subaerial exposure, thus implying that the two tills are

products of two separate glaciations of southern New England. Radiocarbon dates and pollen data from an organic horizon between tills along the Sandy River at New Sharon, Maine, led to the interpretation that the lower till was early Wisconsinan (Caldwell, 1959; Stuiver et al., 1960; Borns and Calkin, 1977). Subsequently, tills elsewhere in New England were correlated to this site. A reinvestigation of the New Sharon site (Weddle, 1986, 1988) yielded no evidence of a pre-late Wisconsinan till. Some researchers, however, interpret the lower till of New England as Illinoian or older on the basis of depth of its weathering profile (Oldale, 1962; Stone, 1975), amino acid racemization age estimates (Oldale et al., 1982), and the marine oxygen isotope record (Oldale, 1962; Oldale and Ekenasy, 1983). The range of hypotheses concerning the age of these tills is known as the “twotill problem” (Schafer and Hartshorn, 1965). The purpose of this paper is to define the till stratigraphy of Boston Harbor’s ex148

0033-5894190 $3.00 Copyright All rights

0 1990 by the University of Washington. of reproduction in any form reserved.

BOSTON

HARBOR

posed drumlin sections based on a comparison of clay-mineral discontinuities and weathering profiles with the observed stratigraphy. Results of the analyses suggest a resolution of the long-disputed two-till problem in Boston Harbor and offer a technique that may be used to establish continuity in regional Pleistocene stratigraphic investigations in New England. PREVIOUS WORK ON THE BOSTON HARBOR DRUMLINS

Several hundred drumlins occur in the Boston area. Those drumlins located in Boston Harbor and in Massachusetts Bay to the east are partly or entirely submerged and many of them are buried under glaciomarine sediments. Upham (1879, 1889a, b) reported that many of the drumlin exposures in the Boston Harbor islands have a core of lodgment till beneath a thin layer of ablation till of the same age. Upham characterized the lodgment (lower) till, and distinguished it from the ablation (upper) till, by its compactness, generally striated rock fragments, numerous smaller stones, and foliated parting structures. He also ascribed the depth of weathering on the drumlins (5-7 m) to progressive weathering since the recession of the last ice sheet when both units were deposited. LaForge (1932) and Judson (1949) did not recognize the two tills of Upham in the Boston Harbor area. Both believed that the till of the drumlins was deposited during a single glaciation. LaForge (1932) interpreted the till to be late Wisconsinan while Judson (1949) interpreted the till to be early or middle Wisconsinan. Kaye (1961) considered the lower till to be early Wisconsinan on the basis of its depth of oxidation. He believed that late Wisconsinan glaciers extended across Boston Harbor, but he did not identify a late Wisconsinan till in the harbor drumlins. Later, Kaye (1982) interpreted the Boston area drumlins as being composed of two superposed early Wisconsinan tills and in-

149

TILLS

ferred that the streamlined drumlin morphology was developed by glacial erosion subsequent to deposition of the drumlin till. Mickelson and Newman (1987) examined the tills of the Long Island drumlin complex (Fig. 1) and concluded that the bulk of the drumlin till was pre-late Wisconsinan. They inferred that the drumlins were shaped during late Wisconsinan glaciation by a sliding ice sheet eroding deeply oxidized preexisting till and that the late Wisconsinan till forms a thin layer over the sculptured drumlin form. Age estimates for the tills in the Boston Harbor drumlins have been based primarily on the visible depth of weathering (oxidation) and upon the presence of mollusk shells found in the lower till. Mercenaria mercenaria shells from a Boston Harbor drumlin yielded 14C ages older than 37,000 yr B.P. (H-1125; Kaye, 1976). Later, Belknap (1979) reported amino acid racemization age estimates of 200,000 and 214,000 yr B.P. (probable Illinoian) for Mercenaria shells found in the Peddocks Island lower till. However, the results have been reevaluated by Belknap (1980), who infers a probable Sangamonian age for the shells. METHODS

To characterize the tills of the Boston Harbor region, 15 profiles in drumlin exposures on eight islands and two peninsulas were sampled (Fig. 1). Samples were collected at OS- to 1.0-m vertical intervals measured from mean sea level. Approximately 500 g of till was collected for each of the 237 samples, and detailed field descriptions were made at each exposure. Texture, structure, color, sedimentary features, degree of compaction, presence of shell fragments and concretions, and degree of leaching and oxidation were noted at each sampling station. Color was determined in the field using a Munsell Soil Color Chart. The relative degree of carbonate content was tested with dilute hydrochloric acid. The clay-mineral composition was determined for each sample at the Illinois State

150

NEWMAN

I’

-

1‘1

ET AL.

&,‘+Wnthrop

Head

d

Boston Harbor

‘@‘a

Peddocks

FIG.

Island (

+&

/

1. Map of study area showing sampling locations.

Geological Survey laboratories using the Xray diffraction procedure of Glass (Hallberg et al., 1978; Killey, 1982). All samples were prepared using an oriented aggregate settling technique relying on the principle of Stokes Law. The <2-km fraction was dried on a glass slide and then run on a General Electric XRD-5 diffractometer with a log scale and Cu radiation at a recording rate of 2” per minute. Chlorite was defined by its 14-A periodicity, which is unaffected by ethylene glyco1 solvation or heating to 375°C for 1 hr. High-charge vermiculite was defined by its 14-A periodicity, and does not expand with ethylene glycol, but collapses to 10 A when heated. Low-charge vermiculite was defined by its expansion to 17 A with ethylene glycol and its collapse to 10 A when heated.

Low-charge vermiculite has the same diagnostic properties as smectite. Quantification of all clay-mineral data is based on peak heights. Replication of analyses indicated a precision of l-2%. Claymineral percentages from samples along representative profiles are shown in Figures 3-9. Particle-size analyses were performed on 134 samples using the standard hydrometer method (ASTM-422) (Figs. 3-9). STRATIGRAPHY OF THE BOSTON HARBOR DRUMLINS

In Boston Harbor, the lower till is extremely compact, faintly stratified, and commonly exhibits horizontal fissility. Cobble- to boulder-sized clasts are generally smaller and more striated than those in

BOSTON HARBOR TILLS

the upper till. The unweathered lower till is olive gray (5Y 4/2) and contains marine shell fragments. In the weathered lower till, the depth of oxidation may extend below the depth of carbonate leaching (i.e., Long Island-l, -2, -3, Rainsford Island, Peddocks Island-l, -2, and Great Brewster Island) or may approximate the depth of leaching (i.e., Winthrop Head, Allerton Hill, and Strawberry Hill; Fig. 2). The upper till (Fig. 2) is olive (5Y 4/3-5/4), oxidized, compact, faintly stratified, tends to contain more boulders than the lower till, and contains no marine shell fragments. The upper and lower tills have about the same lithologic composition. The clay-mineral composition is a more distinctive index of weathering in tills than are color changes, presence of shells, or leaching depths. A progression of claymineral alteration from chlorite to highcharge vermiculite to low-charge vermiculite indicates soil development. A buried soil profile is present if this trend in claymineral alteration occurs within the drum-

FIG.

151

lin section. Representative sections illustrating relationships of clay-mineral weathering to observable field evidence separating tills are presented below. Long Is’and Sections Long Island (Fig. 1) is composed of a complex of eight east- to southeast-trending drumlins, three of which were sampled. At Long Island profile 1, two tills are readily discernable on the basis of texture. The upper till is bouldery and separated from the lower till by a 2-m-thick sand layer interbedded with till (Fig. 3). The sandy loam upper till averages 56% sand, while the clay loam lower till averages 35% sand. The basal 3 m of the lower till contains numerous shell fragments in which chert has replaced most of the carbonate. The upper part of the lower till at Long Island profile 1 is oxidized and leached, has stains on ped faces and in joints, and contains argillans (clay skins) and truncated, clay-filled fractures. These features are characteristic of a C horizon, above which

2. Correlation of stratigraphic sections of Boston Harbor drumlins.

152

NEWMAN S 13- 53 1% 11-67 IO9-48 -1u _ a .-a- 26 (0E e 7- 30 6-39 54-37 321-41

m

sand

. . . . . . .

dwontlnuous boulder zone

3x53

weathered

FIG.

ET AL. SI 33

Cl 14

22

'1

37 41 41 47

15 33 29 14

36

27

34

25

LCV 6 7 7 9 a 13 15 18 19 17 17 17 15 S SI Cl LCV HCV IZ

zone

HCV

I

9 9 9 10 10 14 13 12 11 12 11 12 11

56 56 56 57 56

c 25 23 25 22 24

H 4 3 3 2 2

57 58 54 52 52 53 52 54

11 13 14 15 15 15 16

4 3 4 4 4 4 4

- sand I IllW - Sllt C chlorite - clay K - kaolmlte low charge vermwlite . high charge vermvzulite - Intermediate zone of clay mmeral composition

3. Textural and clay mineralogical data for Long Island-l,

the solum (the A and B horizons of the soil profile) is missing. Clay-mineral data (Fig. 3) show that chlorite percentages decrease upward in the lower till from 16% at the base of the exposure to 11% just beneath the sand. Maximum weathering of chlorite occurs just below the contact. Above the sand, chlorite increases abruptly with a concurrent decrease in both high- and low-charge vermiculite. Long Island profile 2 is located about 250 m northeast of profile 1 in an adjacent drumlin, The sandy loam upper till of profile 1 grades into a clay loam upper till at profile 2. The textural difference between the two tills at profile 1 is therefore lacking

14-134

at profile 2. However, profile 2 shows a decline of chlorite from 22% at the base of the exposure to 11% at an altitude of 12 m (Fig. 4). At 12.5 m, a discontinuity in clay mineral composition exists. Chlorite increases to 16%, with a corresponding decrease in vermiculite. This upward decrease of chlorite percentages in the lower till, followed by an abrupt increase of chlorite in the upper till, is similar to that observed at profile I. The upper till at profile 2 has lower percentages of chlorite than at profile 1 because it is more weathered. In the southwestern-most Long Island drumlin (profile 3), a sand zone separates the two tills, as in profile 1. The sand is a 40-cm-thick layer, the base of which lies 9.7

39 35

27 32

5 a

13 11

60 61

18 16

4 4

34 40

32 31

t4 15

11 IO 10 10

54 53 51 50

17 ,A 20 22

41 4, 5 4

14

~~~ m];;;-;;i; ; i ;1 14

FIG.

11

50

21

4. Textural and clay mineralogical data for Long Island-2.

4

153

BOSTON HARBOR TILLS

m above sea level (Fig. 5). Maximum weathering of chlorite occurs just below the sand. Chlorite values decrease from 14% at the base of the exposure to 9% at the till/ sand contact. Unaltered lower till is not present at profile 3. The texture of the upper till at profile 3 is loam to clay loam, similar to the upper till at profile 2. Clay-mineral data show higher chlorite and lower low-charge vermiculite percentages above the contact than below the contact. An intermediate zone at the base of the upper till (sample 13 in Fig. 5) contains 16% chlorite; above it the upper till contains 22% chlorite. This intermediate zone is believed to reflect the incorporation of eroded lower till into the upper till. The Long Island sections display a consistent stratigraphic succession applicable to the entire study area. Long Island profile 1 shows two texturally distinct tills separated by a sand layer. Long Island profile 3 shows two tills separated by a sand layer; however, the tills are texturally similar. Long Island profile 2 shows neither a textural distinction between the tills nor a sand layer separating the two tills. Common to all profiles, however, are truncated soil profiles in the upper part of the lower till. The same diagnostic clay-mineral discontinuity defining the upper and lower till contact at profiles 1 and 3 exists at profile 2. Distinct differences in clay-mineral assemblages and weathering profiles observed at Long Island exist throughout the study area. S

SI

Great Brewster Island

The Great Brewster Island bluff (Fig. 6) is the largest exposure in Boston Harbor. The section, 30.5 m high, is composed of uniform compact clay loam till, five boulder zones, and a 0.3-m-thick sand layer that extends across the bluff face. Chlorite percentages in the lower till average 25% through the lower 23 m of the exposure. Upward in the section, chlorite decreases to ll%, while there is a corresponding increase in high-charge vermiculite just below the contact with the upper till at a boulder zone at 26 m altitude. Above the lower till is a l-m-thick zone having a clay-mineral composition intermediate between that of the lower and upper tills (similar to Long Island profile 3). Here, chlorite percentages are slightly higher and vermiculite percentages lower than below the 26.5-m level. Above this zone, chlorite percentages decrease upward from 20 to IO%, with a corresponding increase in highand low-charge vermiculite. These changes represent the weathering of the upper till by modern soil-forming processes. At Great Brewster Island the contact between the two tills, defined by clay-mineral weathering, occurs beneath one of the live boulder zones present in the section. This shows that a boulder zone is not indicative of a till contact. Peddocks Island Sections

Peddocks Cl

--14-40_33_2~_~__?_6__54__22__4_ 30 30 13-40 12-136 33 31 11-142 30 26

FIG. 5. Textural and clay mineralogical

Island

is composed

LCV

HCV

I

C

K

11 16 20 21 23 22 22 20 16 23 16 22 20

15 15 13 12 12 11 10 10 9 12 11 10 11

53 55 54 54 53 55 55 54 60 52 55 51 52

16 9 9 9 8 8 IO 12 12 11 14 13 14

5 54 4 4 4 3 4 3 3 4 4 3

data for Long Island-3.

of five

154

NEWMAN

ET AL.

i

19J 2k?o g c

31

30

32

30

33

29

37

30

30

31

171s 1615- 38 16 i3m 121 lIO- 36 9a765- 33 431 $9

LCV

HCV

I

C

K

10 10

19 18

57 58

10 10

4 4

16 14 13 13 14 13 13 13 14 14 13 13 15 14 14 15 14 12 12 13 12 12 13

12 11 11 11 13 11 11 IO 11 12 II 12 12 11 12 11 11 11 11 11 10 11 10

47 47 47 47 46 47 47 48 45 45 45 45 45 46 44 44 47 48 48 47 48 47 49

21 24 24 25 23 24 24 24 26 25 26 26 24 25 25 24 23 25 25 25 26 26 24

4 4 5 4 4 5 5 5 4 4 5 4 4 4 4 6 5 4 4 4 4 4 4

FIG. 6. Textural and clay mineralogical data for Great Brewster Island.

drumlins. The northern-most drumlin was sampled along two profiles (Fig. 1). Till textures of the upper and lower tills are a relatively uniform loam to clay loam along both profiles at the exposure, similar to textures at Long Island profiles 2 and 3, Great Brewster, and other sampled exposures within the study area. Along Peddocks Island profile 1 (Fig. 7), shell fragments occur up to an altitude of 17 m. Four boulder zones occur in the lower 15.5 m of the exposure. A zone of large boulders (averaging 0.4 m in diameter) at 18.7 m separates the lower till from the upper till. The only unaltered till occurs at the base of the exposure. Chlorite percentages decrease steadily from 26% at the base of the exposure to 10% at 18.7 m altitude. Both high- and low-charge vermiculite percentages increase upward, corresponding with the loss of chlorite. Above the contact, at 18.7 m, is a 1.5-m-thick zone of till with an intermediate clay-mineral composition. This zone is characterized by 18% chlorite and a sharp decrease in low-charge vermic-

ulite. The upper till above the intermediate clay-mineral zone averages 24% chlorite. Peddocks Island profile 2 (Fig. 8) is located about 150 m northwest of profile 1. Three boulder zones are present but do not appear to correlate with those at profile 1. Clay-mineral data indicate that chlorite percentages steadily decrease from 22% at the base of the exposure to 10% at 18.8 m altitude. Both high- and low-charge vermiculite increase upward in the profile. Above the zone of minimum chlorite percentage is the intermediate clay-mineral zone of the upper till, similar to that found at Peddocks Island profile 1, Long Island profile 3, and Great Brewster Island. Here, chlorite ranges between 16 and 18% and low-charge vermiculite is less than in the lower till. A thin upper till with 21% chlorite lies at the top of the exposure. Close inspection of the tills at Peddocks Island profile 2 shows a subtle color change at the contact, with slightly redder hues characterizing the top of the lower till. In addition, argillans and silans (silt coatings)

155

BOSTON HARBOR TILLS

8

161514. 1312lllo9076543-

19 18 16 15 15 19 17 17 18 17 17 18 16

14 17 16 15 15 12 13 12 12 12 13 13 13 12

57 51 52 51 52 55 49 53 52 52 51 51 49 52

18 10 II 14 14 15 16 15 16 15 15 16 17 17

3 3 3 4 4 3 3 3 3 3 4 3 3 3

13 12 12 11 11 12

13 13 13 12 12 12 12 12 13

53 52 52 54 53 55 55 55 55 52

18 20 20 20 21 18 17 19 19 20

3 3 3 3 3 3 3 3 3 3

11 9

51 52

22 26

4 3

31

47

22

35 38

34 37

31 25

36

40

24

29

47

24

32

39

29

36

35

29

11 11 12

26

47

27

12 IO

FIG. 7. Textural and clay mineralogical data for Peddocks Island-l.

Other Profiles

permeate fractures in the upper part of the lower till; the lower till tends to be slightly more compact than the upper till. These visible characteristics are similar to those at Long Island profile 1, suggesting a C horizon and the remains of a buried soil. S --'7-3i2-Xi-3~-S_-l&-_52g-33 -14-55

30

13-35 12. 11439 ;?936 cn 8 31 2 ‘07 i

FIG. 8. Textural and clay mineralogical at 17.5 m).

At Prince Head, Spectacle Island, Strawberry Hill, Allerton Hill, Winthrop Head, and Thompson Island, the till textures and stone lithologies are nearly uniform throughSI

Cl

38

34 29 3 2:

34

27

34 38

31 31

33 32

29 30

:7 34

LCV a 12

HCV

1:

I

C

K

21

4

55 57

1”6

33 : : 3 3 3 3 3 4 4 4 5

II 14 15 11 13 14 13 13 14 12

1: 14 13 13 13 12 12 12 12 11

2 55 55 57 52 54 53 49 49 50

:4 15 14 14 16 19 17 19 22 21 23

12

10

51

22

data for Peddocks Island-2 (thick boulder zone indicated

NEWMAN

156

ET AL. TABLE

out both the lower and upper tills. At each of these exposures, the contact between tills of differing age can be defined on the basis of upward-decreasing chlorite percentages in the lower till followed by an abrupt increase in chlorite percentages in the upper till (e.g., Winthrop Head, Fig. 9). High-charge and low-charge vermiculite values inversely follow the same trend. All sections are similar to Long Island profile 2 where field evidence permitting the tills to be differentiated is lacking, yet a distinct weathering profile is present in the lower till. The profiles of Moon Island, Love11 Island, and Rainsford Island (Fig. 2) exhibit only one weathering profile; the upper till is absent. At the Rainsford Island profile, illite significantly decreases from 48% in the lower portion of the exposure to 38% in the upper portion.

1. MEANS

Upper Intermediate Lower

With the establishment of clay-mineral criteria for differentiating the upper and lower tills, the texture of each till can be examined separately. Table 1 shows that the mean texture of the two tills is a clay loam or loam. Although the upper till is slightly sandier and less clayey than the lower till, the sand and clay percentages exhibit a wider range of textures in the upper till. The silt component of each till exhibits a relatively large standard deviation. Statistical comparisons of standard devi-

FIG.

Clay

20 14 97

37.8 k 9.2 36.2 + 5.3 34.8 f 4.9

36.2 -c 5.4 36.7 k 5.0 37.1 t 6.0

26.0 + 6.1 27.1 + 5.9 28.1 2 3.8

= (n2

-

m1mY

-

l)(n2w2)2

where ~zr = sample number from the lower till, n2 = sample number from the upper till, S, = standard deviation of the lower till, and S, = standard deviation of the upper till. The analyses indicate that F = 0.272 for sand, 1.19 for silt, and 0.37 for clay when 0.59 < F < 1.69. The range of standard deviations shows the existence of two distinct populations and therefore confirms the existence of two separate tills in the study area. The average clay-mineral compositions SI

Cl

1 LCV

35 33

30 30

I (

14 13 19 24

'6.0~;,);,);,

Silt

HO

4dL3q-L-~*4

2 t" 35 11-1 IO-137

a

136

OF

Sand

h

S

DEVIATIONS

N

F

-13,

STANDARD

ations suggest that the data from the two tills represent distinct populations. An Fstatistic test was used to determine if the difference in textural variability is due to having fewer samples from the upper till or to compositional differences. Samples from the intermediate zone of clay-mineral composition were not considered part of the upper till for this analysis because they are believed to contain reworked sediment from the lower till and therefore have a blended lithology. At the 10% significance level,

REGIONAL CHARACTERISTICS OF TILLS

;\;<,,\:,\_I

AND

TILL TEXTURES

31

331

22 21 16 17

14 13 13 12 12 12 11 9 9 9 9

56 55 49 48 46 45 44 46 45 46 46

13 9 12 13 13 15 16 19 20 22 21

9. Textural and clay mineralogical data for Winthrop Head.

4 4 3 3 3 4 3 4 5 5 5

157

BOSTON HARBOR TILLS

and standard deviations for the unaltered upper and lower tills are shown in Table 2. Unaltered clay minerals are those that maintain relatively uniform percentages throughout the profile, do not display significant chlorite weathering, and occur in the lower portions of till units (e.g., Fig. 6, Great Brewster Island, samples 2 through 23). The definition of unaltered clay minerals in the upper till is based on chlorite percentage generally being 20% or more of the clay minerals (Figs. 3, 5, 6, 7, and 8). However, the till is oxidized and may be leached of carbonates. Standard deviations for all clay-mineral components of each unaltered till are small because of the uniformity of clay-mineral data for the two tills. The upper till averages 7% more illite than the lower till. The upper till is best characterized by illite percentages of 55 to 58%. Illite values of 45 to 51% best characterize the lower till. A triangular diagram (Fig. 10) shows that the two diagnostically distinct populations of unaltered clay minerals are associated with the two tills. The most striking difference between the two tills occurs with the quantity of vermiculite. The unweathered lower till always contains more low-charge vermiculite than the upper till. In summary, the contact between the lower and upper tills has been defined by analyzing clay-mineral alteration products; this approach is particularly useful in regions where clast and matrix characteristics of the tills are similar. Furthermore, we have separated tills on the basis of differences in their unaltered clay-mineral composition. Analyses of clay minerals thus provide a technique that is useful in estab-

TABLE

2.

MEANS

N

Upper Lower Note.

15 54

AND

STANDARD

LCV

6 k 1.3 13 k 2.9

DEVIATIONS AND

100%

A

K kaollnlte C - chlorlte l datapo~nt

LCV HCV

I +K+C

lowcharoevermul~te high chargeverm!culite

N = numberof

samples

FIG. 10. Triangular diagram of clay mineral composition of the upper and lower tills. More than one sample may be represented by a single data point.

lishing a regional stratigraphic framework for till investigations in New England. DISCUSSION

Our study of the lower and upper tills of Boston Harbor shows that they can be characterized by a significant difference in depth of weathering, as demonstrated by differences in clay-mineral assemblages. These differences exist because a major interval of subaerial weathering, followed by glacial erosion (Mickelson and Newman, 1987), occurred before deposition of the upper till. The contact between the upper and lower tills can be defined by using clay mineralogy, together with available physical evidence, to delineate the buried, truncated weathering profile in the lower till. Chlorite weathering under well-drained, acid, oxidizing conditions, such as exist in

OF CLAY MINERAL LOWER TILLS

COMPONENTS

FOR UNALTERED

UPPER

HCV

I

C

K

13 -c 3.2 11 5 1.2

5s 2 2.2 48 r+ 2.5

23 k 1.8 23 k 2.0

3 t 0.7 5 2 0.7

LCV, low-charge vermiculite; HCV, high-charge vermiculite; I, illite; C, chlorite; K, kaolinite.

158

NEWMAN

the harbor region, results in a predictable suite of alteration products. Deep weathering, represented by chlorite loss, occurs in the upper-most portion of the lower till in Boston Harbor. Drumlin exposures display only C horizons; A and B horizons have been truncated in all examined profiles. A few argillans were observed in the upper C horizon. The lower part of the upper till at many exposures is composed of a l- to 2-m-thick zone of intermediate clay mineral composition. The last glacier to flow across this region scoured the weathered surface of the lower till and redeposited the reworked sediments onto the eroded surface of the lower till. This zone of intermediate composition, containing slightly more chlorite than the weathered lower till, is often overlain by upper till with chlorite percentages similar to those in the unweathered lower till. Chlorite percentages decrease in the upper part of the upper till due to postglacial weathering at the surface. Although chlorite loss occurs in both tills, the lower till shows a much greater development of both low-charge and highcharge vermiculite (e.g., Fig. 7). If factors related to weathering intensity are similar for both tills, the loss of chlorite and the development of large amounts of both highcharge and low-charge vermiculite must reflect a longer weathering history for the lower till. The late Wisconsinan glacier that deposited the upper till and shaped the drumlins, receded from the Boston area approximately 14,000 years ago (Kaye and Barghoom, 1964). Traditionally, the lower till was considered to be early Wisconsinan (Kaye, 1961). Belknap’s (1980) Sangamonian age estimates for shells in the lower till support this view. However, an early Wisconsinan age for the lower till is inconsistent with the results of this study. Our data show deep weathering profiles (loss of chlorite, oxidation, leaching) associated with the lower till. The length of time and the climatic environment necessary to form a

ET AL.

strong weathering profile in the lower till are inconsistent with the length and character of the mid-Wisconsinan interval. Although we know very little about midWisconsinan (isotope stage 3) climate in the Boston Harbor region, we can assume that temperatures were cooler than present (Heusser and King, 1988) and that the interval was of shorter duration than the Sangamonian interglaciation (isotope stage 5). The weathering profile on the lower till indicates an extensive period of weathering under climatic conditions similar to those of today and to those presumed to have existed during the Sangamonian. The depth of the weathering profile, the sequence of clay-mineral alteration products, and the presence of pedogenic features in the upper part of the lower till are comparable to such characteristics of Sangamonian weathering profiles in the midwestern United States (Follmer, 1983). This suggests that the lower till may be Illinoian or older. Our age interpretations for the two tills are therefore in agreement with those inferred for exposures in Reading, Massachusetts (Oldale, 1962); in eastern Connecticut (Stone, 1975); and on Nantucket Island, Massachusetts (Oldale and Eskensy, 1983). ACKNOWLEDGMENTS This research was supported in part by a grant from Northeastern University. We thank Ms. Meribah Stanton, Director, Long Island Hospital, and Ms. Sue Heilman, Executive Director, Thompson Island Education Center, for permission to examine the exposures on those islands. We also thank the Massachusetts Department of Environmental Management and the Metropolitan District Commission for permission to visit, measure, and sample drumlin sections on their administered islands. We also thank Ms. Laura Toalson and Ms. Barbara Stiff of the Illinois State Geological Survey for drafting figures and tables.

REFERENCES Belknap, D. K. (1979). “Application of Amino Acid Geochronology to Stratigraphy of Late Cenozoic Marine Units of the Atlantic Coastal Plain.” Unpublished Ph.D. dissertation, University of Delaware. Belknap, D. F. (1980). Amino acid geochronology and the Quatemary of New England and Long Island.

BOSTON American Program

Quaternary

Association

Abstracts

HARBOR and

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