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Article 145

SEISMIC INVESTIGATIONS ON ,

By R. N. OLDALE and C. R. TUTTLE, , Mass.

Work done in cooperation with the Massachusetts Department Of Public Works and US. Bureau of Public Roads

Abstract.-Seismic studies on Cape Cod show that sedimentary made at Pilgrim Spring State Park in 1960 was 1,555 deposits of post- age, ranging from 250 to possibly more than 960 feet in thickness, overlie crystalline basement feet long. Thickness of layers and the depth to bed- rocks. A trough in the basement surface extending to about rock were calculated for each traverse, using recorded 900 feet below sea level was found on outer Cape Cod near velocities and critical distances. Truro. OUTER CAPE COD

Thirteen seismic-refraction traverses inade on Cape The upper two seismic layers (Li and L2) recorded in Cod between 1958 and 1963 form the basis for inter- this area probably represent unconsolidated deposits of pretations presented here. Additional traverses in or, in places, possibly of Tertiary age. A the Harwicli and Dennis areas have been reported third seismic layer (L)) recorded at traverse 4 may previously ( Oldale and others, 1962). Three of the represent semiconsolidated or consolidated deposits of 13 traverses reported were made in the Falmouth area Cretaceous(?) age. in October 1958 to determine the nature of the surficial The uppermost layer (Ll), having velocities of 1,300 deposits and the depth to bedrock along the proposed to 3,800 feet per second, was recorded at traverses 4,7, relocation of Massachusetts Route 28. The 10 other and 8 Gee accompanying table). This layer, which is traverses, made for a similar purpose, were along the thought to represent the unsaturated Pleistocene glacio- outer arm of Cape Cod between Orleans Beach and Thickness of the seismic layers, and depth, altitude, and average Race Point (fig. 145.1 ). Nine of these traverses were seismic velocity of the basement rocks at lS traverses on Cape Cod made in 1962 (Oldale and Tuttle, 1962); the other, an [Minimum thickness, depth, and aititude indicated where traverse was not long enough to detect underlying material] unreversed traverse (traverse 3, fig. 145.1 ), was made in 1960 at Pilgrim Spring State Park in Truro d a Altitude of Average Thickness of layers (feet) Computed basement, seismic deep borehole drilled by the Woods Hole Oceanographic Traverse depth to mean velocity of (fig.'145.1) basement ma level basement Institution (Zeigler and others, 1960). (feet) datum (feet/ All seismic traverses were of the inline refraction Li 4 4 (feet) second) type. Traverses before 1962 were made with a 12- Outer Cape Cod channel portable refraction amplifier and oscillo- graph. The 1962 traverses were made using the same A----2----/Im/ -__-----.-.-- 510 _._..._____-----._-_ 510 -500 21,25021.250 equipment coupled in "series" with a 12-channel re- •--22-*--22------88- 3•0 ---3486- >960>620 >-910>-580 .._..__....._-----_ fraction amplifier and oscillograph designed to record •------•} ------•} _•5 19,50014,750 30- 470 .._._._-- 500 -450 19,200 very low frequencies. The original equipment was •163-•KIIX-I- 50 480 ._._..____ 530 -480 23,500 used to record the first set of 12 geophones, and the 98-••ir• -*-Ii-•I.- •* •--I...--- 470400 _-__-__--_- 400470 -390-430 20,50014,100 very low frequency equipment recorded a second set of 12 geopliones. Traverses in the Falmouth area Falmouth Area were 1,100 feet long and those made in 1962 on outer 11...... | 80 • 200 •...... • 280 | -150 | 15.450 12_ ...... 1 70 180 2501 -150 1 17,750 Cape Cod between Orleans Beach and Race Point 13..--...... 1 80 | 170 •-<..3.--11 250 • -160 | 12.150 were 1,800 to 2,100 feet long. The unreversed traverse '

ART. 145 /N U.& GEOL. SURVEY PROF. PAPER 475-D, PAGES D-118-0122. 1964. 8118 OLDALE AND TUTTLE D119 70°30' 1 70°15' 70°00' Race Point »J1111Iwic,K••-3 \ -' TfT•,ro 0 ,PlyinjJ- Y Welitlett 06 B 7 3 7 t. CAPE COD \ Eastham BAY 0 Iii·li.,an,s•<••OrleansFi 1 0 01 Beach 41 °45'- 8)enni, t«3 9« Li'.-.,P%$sel Harwieh C]i:ithatii41< % t--0 0 02 »-»-Slij) 0, 2T ll ( 13 ) 1 Falmouth 0 5 10 MILES ·' ( 5 ,r-''0-3 111111 1 1,- *A.L...=.1

FIGURE 145.1.-Map of Cape Cod showing notthe length,locationandof. seismicnumbertraversesof seismicdescribed.traverse. Symbol shows approximate azimuth, but fluvial deposits above the water table, ranges froin 30 to the Woods Hole Oceanographic Institution at Pilgrim 60 feet in thickness. A second layer (L2) was recorded Spring State Park that found Eocene deposits at a at all traverses and has average velocities in the range depth of only 86 feet, overlain by materials of P]eisto- from 4,700 to 5,700 fps. The 1.,2 layer presumably rep- cene age. However, the contact between the acoustic resents glaciofluvial sand and gravel of Pleistocene age layers was at 175 feet, indicating no correlation between underlain by sand, silt, and clay of Pleistocene and, in the acoustic boundaries and geologic-time boundaries at sorne places, of Tertiary age (Woodworth and Wiggles- this site. worth, 1934,.and Zeigler and others, 1960). Two seismic traverses (1 and 2, fig. 145.1) near At traverse 3 the Le layer is made up of 2 distinct Provincetown in the vicinity of a second borehole made acoustic layers with velocities of 4,600 and 5,900 fps. by the Woods Hole Oceanographic Institution indicated This traverse Was made at a deep borehole drilled by average velocities of 5,250 and 5,500 fps, respectively, D120 GEOPHYSICS for the Ly layer. The borehole penetrated 193 feet of the order of 900 feet below sea level. The L, layer glaciofluvial material above 10 feet of sand and silt of occurs in what appears to be an eastward extension of Eocene age (Zeigler and others, 1960). The seismic a trough found in by Hoskins and Knott data gave no suggestion of this contact, indicating that (1961), the bottom of which is more than 700 feet below materials of Pleistocene and Eocene age have similar sea level. This trough is in part filled with material acoustic characteristics. Therefore, it is impossible to having velocities of 10,000 to 12,500 fps, interpreted by tell from seismic data alone whether unconsolidated Hoskins and Knott to be a semiconsolidated deposit of deposits of Tertiary age occur beneath the Pleistocene Cretaceous age. glaciofluvial deposits in this part of Cape Cod. The The crystalline basement in this area presumably L2 layer ranges in thickness from 390 feet at traverse 5 consists of Paleozoic or rocks similar to to more than 620 feet at traverse 3, where the traverse those exposed along the shore of Massachusetts Bay was not long enough to detect the velocity and depth of from Plymouth northward, because the average seismic the underlying material. velocities of 14,100 to 23,500 fps are comparable to the The absence of velocities in the 6,000 to 9,000 fps seismic velocities mea'sured in the outcrop areas of the range indicates that there is little or no co mpact till crystalline rocks. Comparison of variation in velocities beneath the glaciofluvial deposits in this part of Cape with azimuth suggests that in the Wellfleet area the Cod; compact till commonly has velocities between basement is composed of a bedded or foliated rock 6,000 and 9,000 fps in other parts of Massachusetts striking eastward, possibly similar to phyllite identified (Tuttle, 1961 ). Near Harwich, Mass., material having in a borehole near Harwich, M ass. (Koteff and Cotton, an average seismic velocity of 10,500 fps and under- 1962). At traverse 8 the average of the rock ve'locities lying 313 feet of stratified drift was identified in a bore- measured in an easterly direction was 23,500 fps, and at hole as compact till (Koteff and Cotton, 1962). Iii traverse 9 the average of the rock velocities measured in seismic studies in tlie Harwich and Dennis areas, ma- a northerly direction was 14,100 fps. Similar results terial having velocities of 6,000 to 11,000 fps and over- were noted near Harwich, where seismic studies in the lain by 82 to 357 feet of stratified drift has been inter- immediate vicinity of the borehole indicated an average preted to be compact till (Oldale and others, 1962). velocity of 23,250 fps in an easterly direction, and a A third seismic layer (L,) recognized at traverse 4 is traverse 1.1 miles southeast of the borehole indicated tliought to represen t semiconsolidated or consolidated an average velocity of 15,650.fps in a northerly direction deposits overlying crystalline basement rocks. The (Oldale and others, 1962). average velocity computed for the material below the As portrayed on figure 145.2 the basement surface is L2 layer at this traverse is 12,100 fps. This velocity deepest at traverses 3 and 4, where the seismic lines is about 7,000 fps slower than the average of the veloc- were not long enough to detect basement rocks. In- ities of crystalline basement rocks in the area and more stead, minimum depths were computed on the assump- than 2,000 fps slower than the lowest basement velocity tion that the critical distance for the basement surface recorded in tliis area by the authors. Similar velocities was equal to the length of the traverse. On this as- have been recorded on , R.I. (Tuttle and sumption the basement surface at traverse 3 was com- others, 1961 ), and frocll offshore seismic-refraction puted to be more than 580 feet below sea level. studies in. Cape Cod Bay (Hoskins and Knott, 1961), At traverse 4, two interpretations for the depth to in Sound south of Woods Hole, Mass. the basement surface are possible, depending upon (Ewing and others, 1950), in the Gulf of (Drake whether the 12,100 fps velocity recorded for the LB lind others, 1954), and south of Nova Scotia (Ofrieer layer is interpreted as representing crystalline basement and Ewing, 1954). 1n these investigations, velocities rocks or a semiconsolidated to consolidated rock over- of 9,750 to 13,000 fps were interpreted to represent lying the basement. By the latter interpretation and semiconsolidated or consolidated rock of Cretaceous on the assuniption that the critical distance for the age or rocks of age similar to tlie sedi ments basement is equal to the total lengtli of traverse 4, the found in the Valley. These in terpreta- altitude of the basement surface is computed to be 910 tions were based on other seismic measurements from feet below sea level. However,.if the '12,100-fps velocity areas wliere these lithologic units crop out or wliere actually represents the b asement rocks, the surface of seismic velocities and lithology can be correlated on the the basement in this area would be 440 feet below sea basis of borehole data. If the velocity recorded at traverse 4 for the L, level and the Inaximum depth would then be at trav- seisinic layer actually represents a semiconsolidated or erse 3. The authors' preferred interpretation is that consolidated layer, the layer is more than 450 feet thick the basement surface dips steeply north of traverse 5 and overlies crystalline basement rocks at an altitude on and south of traverse 2 to form a valley whose floor is OLDALE AND TUTTLE D121 N (3,. S The basement in this area is thought to be Paleozoic C sedimentary and crystalline rock, possibly the Dedham - ) m h to W r. 00 • Granodiorite that crops out along the west shore of Miaa ia 5f 5f 555fiE • Buzzards Bay. Average velocities measured on the + 1-- » 4 44 1- 1- 2 basement rocks range from 12,150 to 17,750 fps, and S• 1 1 1 1 1 1 1 seismic measurements on the bedrock in areas mapped LEVEL Unconsolidatedand Tertiary depositsPleistocene as Dedham Granodiorite by Emerson in New Bedford, -200'- Serniconsolidated or - Dartmouth, and Westport, Mass. (Emerson, 1917), -400'_ consolidated(Cretaceous?)sediments -- ranged from 11,900 to 16,600 fps. At traverse 13 the 600'--•i-»---li Basement (Paleozoicand 12,150-fps velocity could represent a semiconsolidated \j ' / Precambrian) rocks - or consolidated deposit of Mesozoic age similar to -800'- \\ / that recorded at traverse 4 in Truro, but is thought to ./ 0i iSMILES more likely represent a low-velocity facies of the Ded- -1000' ham Granodiorite similar to those recorded in New FIGURE 145.2-Generalized cross section of outer Cape Cod Bedford, Dartmouth, and Westport. The high per- altitudefrom Orleansof theBeachbasementto Race Point,surfaceshowingat 8 seismiccomputedtraverses.average ceniage of Dedham Granodiorite stones in the drift Dashed lines indicate alternate interpretations in area of of the Buzzards Bay (Mather and others, deepest basement rock. 1942) also suggests that the bedrock in this area is Dedham Granodiorite. more than 900 feet below sea level in the vicinity of traverse 4 (fig. 145.2). REFERENCES Drake, C. L., Worzel, J. L., and Beckmann, W. C., 1954, Geo- FALMOUTH AREA physical investigations in the emerged and submerged At 3 traverses on the Buzzards Bay moraine in Atlantic Coastal Plain, Part IX, Gulf of Maine: Geol. Falmouth (fig. 145.1), 2 seismic layers (Li and L•) Emerson,Soc. AmericaB. K., Bull.,1917, v.Geology65, p 957-970.of Massachusetts and Rhode representing unconsolidated deposits of Pleistocene Island: U.S. Geol. Survey Billl. 597. and possibly Tertiary and Cretaceous age were re- Ewing, W. M., Worzel, J. L., Steenland, N. C., and Press, Frank, corded over crystalline basement rocks. Velocities 1950, Geophysical investigations in the' emerged and sub- in the Ll layer ranged from 1,400 to 2,300 fps and are merged Atlantic Coastal Plain, Part V, Woods Hole, New York, and Cape May sections: Geol. Soc. America Bull., thought to represent an unconsolidated layer composed v. 61, p. 877-892. in part of unsaturated Pleistocene glaciofluvial sand Hoskins, Hartley, and Knott, S. T., 1961, Geophysical investi- and gravel and in part of unsaturated loose till similar gation of Cape Cod Bay, Massachusetts, using the contin- to sandy till found in other areas of Massachusetts. uous seismic profiler: Jour. Geology, v. 69, p. 330-340. The Li layer ranged in thickness from 70 to 80 feet. Koteff, Carl, and Cotton, J. E., 1962, Preliminary results of Velocities in the L• layer ranged from 2,900 to 6,600 recent deep drilling on Cape Cod, Massachusetts: Science, v. 137, p. 34. fps. The lower velocities in this layer probably repre- Mather, K. F., Goldthwait, R. P., and Thiesmeyer, L. R., 1942, sent loose sandy till. The intermediate and higher Pleistocene geology of western Cape Cod, Massachusetts: velocities are thought to represent a layer composed for Geol. Soc. America Bull., v. 53, p. 1127-1174. the most part of compact till similar to high-velocity Officer, C. B., and Ewing, M., 1954, Geophysical investigations till in other parts of Massachusetts (Tuttle, 1961). in the emerged and submerged Atlantic Coastal Plain, On the other hand, it may be a complex layer composed Part VII, Continental shelf, continental slope, and conti- of glaciofluvial deposits and till of Pleistocene age, and v.nental65, p.rise-653-670.south of Nova Scotia: Geol. Soc. America Bull., unconsolidated marine and lacustrine deposits of Oldale, R. N., and Tuttle, C. R., 1962, Preliminary report on the Pleistocene, Tertiary, and Cretaceous age. Uncon- seismic investigations in the Orleans, Wellfleet, North Truro, and Provincetown quadrang·les, Massachusetts: solidated marine and nonmarine deposits of sand, U.S. Geol. Survey open-file report. silt, and clay of Pleistocene, Tertiary, and Cretaceous Oldale, R. N., Tuttle, C. R., and Currier, L. W.. 1962, Prelimi- age crop out 20 niiles to the southwest on Marthas nary report on the seismic investigations in the Harwich Vineyard and 7 miles to the south on Nonamesett and Dennis.quadrangles, Massachusetts: U.S. Geol, Survey Island. Similar deposits of Tertiary and possibly open-file report. Cretaceous age are exposed 31 miles to the north at Tuttle, C. R., 1961, Seismic high-speed till in Massachusetts [abs]: Geol. Soc. America Spec. Paper 68, p. 288-289. Duxbury and Marshfield (Woodworth and Wiggles- Tuttle, C. R., AI]en, W. B., and Hahn, G. W., 1961, A seismic worth, 1934; Emerson, 1917). The L• layer ranges in record of Mesozoic rocks on Block Island, Rhode Island: thickness from 170 feet at traverse 13 to 200 feet at Art. 240 in U.S. Geol. Survey Prof. Paper 424-C, p. 0254- traverse 11. 0256. D122 GEOPHYSICS

Woodworth, J. B., and Wigglesworth, Edward, 1934, Geography Zeigler, J. AI., Hoffmeister, W. S., Geise, Graham, and Tasha, and geology of the region including Cape Cod, the Elizabeth Herman, 1960, A discovery of Eocene sediments in the sub- Islands, Nantucket, Martlias Vineyard, No Mans Land surface of Cape Cod, Massachusetts: Science, v. 132, p. and Block Island: Harvard Coll. Mus. Comp. Zoology 1397-1398. Mem., v. 52,322 p.

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