GEOLOGICAL SURVEY CIRCULAR 773 Stratigraphic Test Well, Nantucket Island, Massachusetts Stratigraphic Test Well, Nantucket Island, Massachusetts By D. W. Folger, J. C. Hathaway, R. A. Christopher, P. C. Valentine, and C. W. Poag GEOLOGICAL SURVEY CIRCULAR 773 Description of Triassic(?) basalt and Upper Cretaceous to Pleistocene sediments recovered from 514 meters of test coring 1978 United States Department off the Interior CECIL D. ANDRUS, Secretary Geological Survey W. A. Radlinski, Acting Director Library of Congress catalog-card No. 78-600008 Free on application to Branch of Distribution, U.S. Geological Survey 1200 South Eads Street, Arlington, Ma. 22202 CONTENTS Page Page Abstract ___________. 1 Results Continued. Introduction _____________. 1 Stratigraphy Continued Purpose ________-__. 1 Paleontology Continued Setting ____________. 2 Pliocene or Miocene 15 Previous work ___________ 3 Eocene ______ . 16 Acknowledgments ___________. 4 Paleocene _______ . 16 Methods ______________. 5 Borehole 6001 ______. 16 Results ___________________ 6 Pleistocene ________ 16 Stratigraphy ____________ 6 Eocene __ ______ 17 General lithology ______ 6 Upper Cretaceous _. 17 Upper zone ________ 6 Porosity and permeability _. 18 Middle zone ______ 7 Discussion _________ ___ _ ___. 18 Lower zone ____ 8 Regional stratigraphy ______ . 18 Saprolite and basalt 8 Regional structure _____________ 22 Paleontology _________ 15 Hydrology . 23 Coskata well _____ 15 Summary ______ ___ . 26 Pleistocene ___. 15 References cited ____ _ ________ 26 ILLUSTRATIONS Page FIGURE 1. Index map of Nantucket Island and the surrounding area _ 2 2. Cross section showing structure of part of Continental Shelf south and west of Nantucket ____ _- 4 3. Columnar section of borehole 6001 ___ ___ - 6 4-6. Diagrams showing variations for the following chemical compo­ nents in the basalt and saprolite zones of borehole 6001: 4. Chemical elements and minerals __________ __ _____ 14 5. TiC-2 and P205 _________________ 15 6. Mn02 and CaO ____________________________ 15 7. Stratigraphic sections showing regional correlation of Nantucket boreholes with other holes drilled in the Atlantic Coastal Plain to the west and south _______________ ____ 19 8. Section from New Jersey to Georges Bank showing environment of deposition of sediments _____________________________ 21 9. Map showing gravity anomalies in Nantucket region ___ _ 23 10. Seismic reflection profile through Great Round Shoal Channel and Nantucket Sound ________________________________ 24 11. Graph comparing salinity of interstitial water with electrical conductivity and lithology in borehole 6001 ___________ 25 TABLES Page TABLE 1. Partial chemical and X-ray modal analyses of volcanic rock and saprolite ________________________ _ _ 9 2. Porosity and permeability analyses ___________ _____________ 18 III Stratigraphic Test Well, Nantucket Island, Massachusetts By D. W. Folger, J. C. Hathaway, R. A. Christopher, P. C. Valentine, and C. W. Poag ABSTRACT local generation of petroleum hydrocarbons in this shoreward area of the continental margin is unlikely. The U.S. Geological Survey, in cooperation with the Freshwater or low-salinity brackish water was Massachusetts Water Resources Commission and the found in sediments far below the depth predicted by Nantucket Conservation Foundation, continuously cored the Ghyben-Herzberg principle. These interstitial 514 m of sediment and volcanic rock in a Stratigraphic waters are probably relict ground water emplaced and water-quality test. during times of low sea level during the Pleistocene. Stratified sediments were divided texturally into three zones: the upper zone (0-128 m) contains mostly coarse sand and gravel; the middle zone (128-349 m) INTRODUCTION contains mostly silty clay and a few beds of sand and silt; and the lower zone (349-457 m) contains soft, PURPOSE unconsolidated, clayey sand. Below the lower zone, a In 1975, the U.S. Geological Survey (USGS), saprolite, composed mostly of clay, grades abruptly downward at 470 m into partially altered basalt that in cooperation with the Massachusetts Water extends to the bottom of the hole at 514 m. Resources Commission, planned to drill a 122-m Fossils are sparse throughout the cores, but Pleisto­ hole near the geographic center of Nantucket cene, Eocene, and Upper Cretaceous assemblages were Island to assess the total thickness of fresh­ identified. On the basis of limited foraminifer and water in the sediments. Calculations based on ostracode assemblages, Pleistocene deposits are in­ the Ghyben-Herzberg principle predicted a zone ferred to extend to a depth of about 85 m. A Lower Eocene section 25 m thick is defined on the basis of of freshwater 120-150 m thick (Kohout and palynomorph and dinoflagellate assemblages. (How­ others, 1977). This principle is the theory of ever, samples from a water well drilled in 1933, 10 hydrostatic equilibrium between freshwater km to the north, contain Pliocene and Miocene, early and more dense seawater in a coastal aquifer; Eocene, and Pal eocene foraminifers.) Upper Cretaceous it states that for each meter of ground-water beds, inferred to extend from 128 to 349 m, are iden­ tified mostly on the basis of pollen and spores. The elevation above sea level, the freshwater lens oldest datable fossiliferous sample at 345 m is early- will depress the saltwater interface about 40 m to-middle Cenomanian in age. below sea level. The USGS later proposed to Samples found below 345 m are barren of indigenous deepen the test to about 500 m, the approximate fossils and are assigned an Albian Age on the basis depth of a prominent acoustic reflector (Oldale, of electric log correlations. Paleontologic evidence sug­ gests that the Pleistocene section was deposited under 1969), which was assumed to be basement. The nearshore marine conditions, the Tertiary section un­ primary objective of the additional drilling was der open-shelf marine conditions, and most of the to obtain new Stratigraphic information near Upper Cretaceous section under nonmarine conditions. the shoreward margin of the Georges Bank The unstratified clay that makes up the saprolite basin. Seismic reflection and refraction surveys was probably formed by subaerial weathering of basalt similar to that which underlies the saprolite. Chemical by oil companies and by the USGS have been evidence suggests that the basalt was probably de­ carried out for several years in the region of posited as a series of flows. The oldest K-Ar date ob­ Georges Bank, where leasing of tracts and ex­ tained in the basalt was 183±8 m.y. (Early Jurassic); ploratory drilling are imminent. however, because of alteration, this probably repre­ The nearest pre-Pleistocene sedimentary sents a minimum age. strata exposed on land are Tertiary and Cre­ Lignites and coal in the Upper Cretaceous section are of low rank and, on the basis of their thermal im­ taceous clay and sand that crop out at Gay maturity, have probably never been buried deeply; thus, Head, Martha's Vineyard (fig. 1). However, MASSACHUSETTS BAY 4I°25'- NANTUCKET SOUND Great Point COSKATA BOREHOLE ( T.D. 102m) Sankaty Head 41° 15'- USGS BOREHOLE NO. 6001 (T.D. 514m) 70°00' USGS BOREHOLE NO. 6016 USGS BOREHOLE NO. 6018 © and COST G1 STRATIGRAPHIC TEST 0 10 70° 00 FIGURE 1. Index map of Nantucket Island and surrounding area showing location of Nantucket stratigraphic test well (USGS borehole No. 6001) and other boreholes in the region. T.D. shows total depth of some of the boreholes. they are dislocated and highly folded and the coast of Cape Cod, Mass., on the eastern faulted, apparently from loading or "bulldoz­ Continental Shelf of the United States. It has ing" by glacial ice (Kaye, 1964), and they do a maximum elevation of only 33 m (Folger not provide a clear picture of the stratigraphy. Hill), and encompasses a total area of 132 km2. The Nantucket test-well site is located farther Exposed surficial sediments are mostly gla­ seaward from Martha's Vineyard, and it pro­ cial sand, gravel, and some clay (Richards, vided an opportunity to gain data where sedi­ 1962; Woodworth, 1934). These deposits are ments are thicker, closer to the basin center, part of the Ronkonkoma-Vineyard-Nantucket and presumably less affected by ice movements. moraine and its outwash plain that extends Furthermore, penetrating the sedimentary sec­ eastward to form part of Georges Bank (Pratt tion and identifying basement would be useful and Schlee, 1969). Controversy has surrounded in the interpretation of geophysical data col­ the age of shelly deposits underlying the sand lected to assess the resource potential of the and gravel at Sankaty Head (fig. 1), but most area. evidence available suggests that they are also of Pleistocene age (Richards, 1962; Gustavson, SETTING 1976). Nantucket Island is about 25 km long, as The well site (borehole 6001) is at lat much as 10 km wide, and lies 40 km south of 41°15/54.9// N. and long 70°02'16.72" W. near the geographic center of the island on land School student, and examined for microfossil owned by the Nantucket Conservation Founda­ content with the help of J. A. Cushman. On the tion, which, because of its interest in island basis of Foraminifera that he discovered in the water resource evaluation, kindly granted the fine-textured sediments, he assigned them a USGS permission to drill. The site was con­ Tertiary age. This work, remarkable for a high veniently located 60 m from Milestone Road, the school student, remained unpublished until now. main highway between Nantucket and Sias- The Coskata samples, which had been saved by conset (fig. 1), in a long linear depression Mr. Barrett, provided for us a Tertiary section known as Madequecham Valley that was prob­ that is mostly absent in the USGS borehole. ably eroded by a proglacial melt-water stream. Geophysical investgiations of the island to Ground surface elevation at the site is 10.9 m our knowledge have been sparse. Weston Geo­ above mean sea level. physical Co.