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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Article 145 SEISMIC INVESTIGATIONS ON CAPE COD, MASSACHUSETTS By R. N. OLDALE and C. R. TUTTLE, Boston, 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-Paleozoic 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 Pleistocene 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 Cape Cod Bay 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 Precambrian 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.