U.S. Geological Survey Open-File Report 02-002
Geological Framework Data from Long Island Sound, 1981-1990:
A Digital Data Release
Block Island Sound: Summary Report
The report presented below (Needell and Lewis, 1984) was previously published as a Miscellaneous Field Studies Map by the U.S. Geological Survey and is included here solely to broaden the perspective and understanding of those individuals interested in the geologic framework and the Late Quaternary depositional history of Block Island Sound and easternmost Long Island Sound. The results presented in Needell and Lewis (1984) are preliminary. Final interpretations and a regional perspective are presented in Lewis and Stone (1991) and Lewis and DiGiacomo-Cohen (2000).
GEOLOGY OF BLOCK ISLAND SOUND, RHODE ISLAND AND NEW YORK
Needell, S.W. and Lewis, R.S.
U.S. Geological Survey Miscellaneous Field Studies Map MF-1621
The stratigraphic framework and Quaternary geologic history of Block Island Sound have been interpreted from high-resolution, seismic-reflection profiles supplemented by information from previous geologic studies conducted in the sound and on land. Two structure-contour maps, four isopach maps, and a surficial geology map provide information on the framework of the sound. Six sedimentary units are inferred from the subbottom profiling: 1) coastal-plain strata of Late Cretaceous-early Tertiary(?) age, 2) glacial drift deposits of Pleistocene age, 3) glacial moraine deposits of Pleistocene age, 4) glaciolacustrine deposits of late Pleistocene age, 5) fluvial and estuarine deposits of Holocene age, and 6) marine deposits of Holocene age. The coastal plain and bedrock surface indicates that the preglacial landscape consisted of a lowland that was floored by bedrock and bounded to the south by the deeply incised escarpment of a cuesta of coastal-plain strata. Late Tertiary and early Pleistocene streams flowed southward across the bedrock and northward down the cuesta escarpment, tributary to mainstream flow westward along the foot of the escarpment. Glacial drift, including moraine deposits and glaciolacustrine sediments, was deposited over the preglacial surface during Pleistocene glaciations. Prior to sea-level rise, glacial-drift and older strata were eroded by streams that flowed southwestward and southeastward and merged at the head of Block Channel. During Holocene submergence the postglacial valleys were partly filled by fluvial, freshwater peat, estuarine, and salt-marsh peat deposits. Transgressing seas eroded and smoothed the sea floor. Marine sediments accumulated over the wave-cut surface.
An extensive high-resolution, seismic-reflection survey was conducted in Block Island Sound (Figure 1) by the U.S. Geological Survey, in cooperation with the State of Connecticut Geological and Natural History Survey, Department of Environmental Protection (D.E.P.), to determine the stratigraphic framework and Quaternary history of the sound. The survey included a total of 700 line km of data at a trackline spacing of approximately 2 km along dip lines and approximately 7 km along strike lines. The maps in this report adjoin those for western Rhode Island Sound, R.I. (Needell and others, 1983).
Previous studies of the subbottom in Block Island Sound have outlined the general structure of bedrock, coastal-plain, and glacial-drift surfaces. Oliver and Drake (1951) presented the first depth-to-bedrock map of Long Island and Block Island Sounds. Later, Tagg and Uchupi (1967) further described the bedrock surface as low-relief topography overlain by unconsolidated sediments. From 1965 to 1973 numerous subbottom studies were conducted (McMaster and others, 1968; McMaster and Ashraf, 1973a, b, c). These studies (1) identified an irregular basement surface and the submerged well-developed escarpment and cuesta of coastal-plain strata, (2) traced buried drainage systems, (3) discussed the extent and role of glaciation in the development of the sound, and (4) outlined the geologic history. Schafer (1961) located glacial end-moraine deposits across the inner shelf on the basis of sea-floor topography and bottom-sediment characteristics described by McMaster (1960). Bertoni and others (1977) identified freshwater-lake deposits in central Block Island Sound on the basis of bottom sediment samples, cores, and seismic profiles.
The authors wish to thank John J. Dowling, Calvin E. Crouch, and Lawrence Birch of the University of Connecticut Marine Sciences Institute (MSI) for their assistance and for allowing us to use the MSI docking facility. We are indebted to Arthur D. Colburn, captain of the RV ASTERIAS of the Woods Hole Oceanographic Institution for his assistance during the cruise. For his suggestions during the preparation of the report we express our appreciation to Robert N. Oldale.
Igneous and metamorphic rocks of pre-Mesozoic age probably dominate the bedrock lithology of Block Island Sound. The portions of southeastern Rhode Island that abut the sound are underlain by Precambrian metasedimentary and metavolcanic country rock that have been intruded by a pluton of Permian age (Hermes and others, 1981). Off the coast, granite underlies Fishers Island (Fuller, 1905), granite gneiss and schist underlie central and eastern Long Island (Suter and others, 1949; de Laguna, 1963), and crystalline rock similar to the bedrock of the mainland is thought to underlie Block Island and its vicinity (Oliver and Drake, 1951; Tuttle and others, 1961). Basement structural features in and around Block Island Sound trend predominantly northwest and northeast (McMaster and others, 1980). A transverse fault, the New Shoreham Fault, has been identified between Block Island and Montauk Point on Long Island and can be traced approximately 65 km seaward across the shelf. Vertical displacements on the fault vary from 20 to 40 m. The northernmost portion of the fault strikes northwest and offsets bedrock and Upper Cretaceous deposits (McMaster, 1971; Weston Geophysical Engineers, Inc., 1977).
A seaward-dipping, erosional remnant of the Cretaceous coastal-plain strata unconformably overlies the bedrock in the southern portion of Block Island Sound (McMaster and others, 1968). Coastal-plain sediments of unconsolidated and semi-consolidated gravels, sands, silts, and clays have been reported on Block Island (Tuttle and others, 1961; Sirkin, 1976), and along the north shore of Long Island (Fuller. 1914; de Laguna, 1963; Suter and others, 1949).
The bedrock and coastal-plain strata are unconformably overlain by glacial drift. Block Island, Fishers Island, and Long Island are capped by two glacial drift sheets representing two ice advances, one of late Wisconsinan age and one that predates the late Wisconsinan (Donner, 1964; Sirkin 1971, 1976, 1982; Mills and Wells, 1974).
Bottom sediments of the sound are predominantly composed of sand derived primarily from reworked subaqueous glacial deposits and glacial deposits derived from the mainland (Savard, 1966). In northern Block Island Sound, however, clay concretions occur locally and indicate the presence of Pleistocene freshwater lakes (Frankel and Thomas, 1966; Bertoni and others, 1977).
High-resolution seismic-reflection profiles (Figure 2) were obtained using an EG&G Uniboom seismic system. Seismic signals were bandpass-filtered between 400 and 4,000 Hz and were recorded on an EPC facsimile recorder using a 0.25-second sweep rate. The resolving power of the equipment generally ranged from 1 to 2 m. Tracklines were run at an average ship speed of five knots. Ship position was determined using Loran C navigation and fixes were recorded every five minutes and at course changes.
The base map for this work was generated from U.S. Coast and Geodetic Survey Bathymetric charts 0808N-51 and 0808N-53 that were published at a scale of 1:125,000 (U.S. Department of Commerce, 1967a, b).
The geologic significance ascribed to the major acoustic reflectors recognized in these data was derived from their inferred correlation to the stratigraphic control available in and adjacent to the survey area. The depth below present sea level to the acoustic reflectors was determined using assumed compressional-wave velocities of 1,500 m/s for sediments inferred to be of Pleistocene age, and 2,500 m/s for sediments inferred to be coastal-plain strata of Cretaceous age.
Block island Sound includes a variety of topographic features. Submerged highlands (as shallow as 10 m below sea level) form the margins of the sound. A ridge that extends 7.5 km northward from Block Island separates Rhode Island and Block Island Sounds. The highland between Montauk Point and Block Island is breeched near its midpoint by the head of Block Channel. The sea floor beneath the eastern portion of the sound is generally smooth and dips southward to a depth of 45 m. In the western portion of the sound the topography is very irregular, consisting of six major depressions and numerous shallow ridges and knolls. The depressions are generally oriented toward the southeast and they are deepest (as deep as 65-100 m below sea level) in the vicinity of Fishers Island.
Depth below present sea level to bedrock and submerged coastal-plain strata
A prominent acoustic reflector (Figure 10, unconformity fug in profiles A, B, C, D, E, F, G, H) marks the top of the bedrock and coastal-plain strata. In the northern portion of Block Island Sound the reflector is continuous and highly irregular. This unconformity lies at or near the sea floor and can be traced seaward to a depth of 120 m. In the southern portion of the sound, this reflector marks the top of the coastal-plain strata and is fairly smooth and continuous. In some places the coastal-plain strata crop out at the sea floor (Figure 9).
Well-defined internal reflectors were observed within the coastal-plain strata at several localities (Figure 10, profiles A, B, C and F). These reflectors, inferred to represent contacts between sedimentary units, gentle-dip southward and are truncated along the deeply incised, landward limit of the coastal-plain strata.
Figure 3 shows the morphology of the top of the bedrock and coastal-plain strata, which represents a fluvial and glacial unconformity. The bedrock surface dips southward and is incised by eight valleys having 40-60 m of relief that trend southwest. At a depth of 120 m below present sea level these valleys merge with the valleys of the escarpment of the coastal-plain cuesta. Along the western edge of the study area, the deepest valley incises coastal-plain strata and bedrock to a depth of 190 m below sea level. Coastal-plain strata (Figure 10, unit Ku in profiles A, B, C and E, F, G, H) form a cuesta with a north-facing escarpment and 5 outliers that lie to the north of the cuesta. The landward edge of the cuesta is very irregular and trends westward across Block Island Sound. Five large northwest-trending valleys, separated by distinct interfluves, cut into the cuesta and shoal to the south. In the western portion of the area, the coastal-plain strata appear to extend northward under Fishers Island.
Late tertiary and quaternary geologic history
The late Tertiary and Quaternary geologic history of Block Island Sound can be described on the basis of the interpretation of the seismic-reflection profiles and the previous studies conducted in the sound and on the southern New England inner shelf.
During the late Tertiary and early Pleistocene, a lowering of sea level (Vail and others, 1977) permitted streams to erode coastal-plain strata and remove an undetermined amount of material, re-exposing bedrock near the shore. This extensive erosion formed an inner lowland that was floored by bedrock nearshore and bounded to the south by the deeply indented, north-facing escarpment of the coastal-plain strata (Figure 3). A large interfluve north of Block Island separated this lowland from lowlands to the east (Needell and others, 1983; O'Hara and Oldale, 1980).
The late Tertiary and early Pleistocene drainage pattern was modified by glacial erosion during the late Pleistocene and by marine erosion during the Holocene. However, it appears that the preglacial streams flowed south-southwestward along bedrock and northward down the cuesta front and were tributary to a main stream that began at the divide north of Block Island and flowed westward along the base of the cuesta escarpment. South of Fishers Island the main stream may have turned southwestward and joined a larger drainage system that is only partly represented in this study. Drainage from the Thames River (Figure 2) may also have entered this drainage system.
During the late Pleistocene, glacial ice advanced southward across the inner lowland into the valleys of the coastal-plain escarpment, overtopped the cuesta, and terminated along a line extending from the south fork of Long Island to Block Island. As the ice advanced, it scoured the valley floors and walls, widening and deepening the valleys, especially in the semi-consolidated and unconsolidated sediments of the coastal-plain strata. End-moraine deposits accumulated at the seaward limit of the ice and, as the ice retreated, glacial drift was deposited over coastal-plain strata and bedrock. A second advance of late Wisconsinan age (Sirkin, 1976) pushed ice across the sound. The advancing ice forced much of the older drift out of the inner lowland and valleys of the escarpment and redeposited it on the interfluves and valley heads, forming end moraines (Figure 5). Eventually the ice overtopped these deposits and deposited a veneer of till.
As the ice made its final retreat, freshwater lakes formed between the outer end moraines and the retreating ice. Thick deposits of silt and clay (Figure 5) accumulated over the inner lowland and valleys. Later in the final ice retreat, readvancing ice formed an inner line of end moraines along the northern shore of Long Island across Plum Island, the entrance to Long Island Sound (Figure 5), Fishers Island and the coast of Rhode Island from Watch Hill to Point Judith (Schafer and Hartshorn, 1965). The ice then continued its northward retreat.
Prior to sea-level rise, the freshwater lakes drained through water gaps in the end moraines that had impounded the glacial-lake waters. A major water gap formed between Long Island and Block Island, and the lake waters of Rhode Island and Block Island Sounds flowed across Block Island Sound and out onto the shelf via Block Channel. The draining waters cut major valleys into the unconsolidated glaciolacustrine deposits and skirted the more erosion-resistant moraine deposits (Figure 7).
While the inner shelf was subaerially exposed, streams carried terrigenous material seaward to the shelf. As sea level rose, valleys and lowlands were partly filled by fluvial sediments and freshwater peats and were subsequently overlain by estuarine deposits and salt-marsh peat (Figure 8). Waves and bottom currents of the transgressing seas reworked the sediment, smoothed the sea floor, and in some places redeposited the sediment as beach and bar deposits over the wave-cut surface.
At the entrance to Long Island Sound and in the vicinity of Fishers Island, modern currents have cut through Holocene sediment into glacial drift, coastal-plain strata, and bedrock and have formed deep depressions in the sea floor. At the head of Block Channel, sand waves on the sea floor indicate active bottom-sediment transport by strong tidal currents. Where bottom currents are weak, modern marine sediments may accumulate at the sea floor.
Depth below present sea level to glacial-drift surface
The surface of the glacial drift (Figure 7) is a complex unconformity formed by Holocene fluvial and marine erosion (Figure 10, unconformities fu1) and mu in profiles A-H). A strong and continuous acoustic reflector in the subbottom profiles represents the smooth to very irregular glacial-drift surface.
The glacial-drift surface is channeled by valleys that lie as deep as 90 m below sea level and that have as much as 50 m of relief. Three major valleys, numerous tributaries, and one major depression cross Block Island Sound. One major valley lies to the north and west of Block Island, the second trends southeast across north-central Block Island Sound, and the third trends southeast from the entrance to Long Island Sound. The valleys merge between Montauk Point and Block Island to form one seaward-deepening valley. The large depression cuts into the glacial drift to a depth of 100 m below present sea level between Fishers Island and Gardiners Island.
Thickness and distribution of postglacial (holocene) deposits
Sediments of Holocene age as thick as 45 m bury the glacial-drift surface in most places (Figure 9, unit H). The Holocene deposits vary in their characteristics; they exhibit continuous, flat-lying and gently-dipping internal reflectors, cut-and-fill structures, and loss of distinct internal reflectors, all of which give the records a mottled appearance.
The Holocene sediments comprise both valley fill (Figure 10, unit Ofe in profiles A, B, C, D, E, F, G, H) deposited prior to submergence and marine sediments (Figure 10, unit Qb in profiles A, B, C and E, F, G, H) that accumulated during and after sea-level rise. A marine unconformity, marked by a generally flat-lying reflector (Figure 10, unconformity mu, profiles A, B, C, D, E, F, G, H), separates the valley fill and the post-transgressive marine sediments. The valley fill probably consists of fluvial sediments, freshwater peat, estuarine muds, and salt-marsh peat, similar to the Holocene deposits of coastal Connecticut (Bloom and Ellis, 1965) and the valley fill of eastern Rhode Island Sound (O'Hara and Oldale, 1980).
The marine sediments are derived primarily from glacial drift onshore and reworked glacial drift and younger material offshore (Savard, 1966). In some places, these sediments form beach and bar deposits and sand waves. The beach and bar deposits are as thick as 27 m and overlie the marine unconformity north of Block Island, offshore of the central Rhode Island coastline, and 5 km west of Block Island. Numerous sand waves with as much as 5 m of relief occur between Block Island and Montauk Point.
Thickness and distribution of glacial drift
Glacial drift (Figure 4) as thick as 160 m unconformably overlies bedrock and coastal-plain strata. The drift varies in acoustic character and is thought to represent glaciofluvial outwash, till, ice-contact stratified drift, drift deformed by overriding ice, and glaciolacustrine deposits. Glaciofluvial outwash is characterized by flat-lying and gently-dipping reflectors. Drift, represented by highly irregular and discontinuous internal reflectors, is thought to be till, ice-contact drift and ice-deformed drift. Drift that is characterized by acoustically laminated reflectors which mimic the underlying unconformity is inferred to be sediment deposited in proglacial lakes.
The moraine deposits (Figure 10, unit Odm in profiles A, B, C and F, G, H) are thought to consist of till, stratified drift, and older interglacial and preglacial strata deformed by advancing and overriding ice. These deposits are identified in the subbottom records by a well-defined surface reflector that obscures underlying reflectors. In most places where seismic profiles show moraine deposits to be exposed at the sea floor, bottom sediments are composed of boulders and gravel (U.S. Department of Commerce, 1981); these coarse materials are inferred to be lag deposits of the till that cap and (or) comprise the moraine deposits.
End-moraine deposits occur between Montauk Point and Block Island; between Montauk Point and Fishers Island; north, east, and south of Block Island; east of Gardiners Island; and west of Fishers Island (Figure 5). In most of the area the moraine deposits are less than 60 m thick, except west of Fishers Island where they are as much as 80 m thick. The end-moraine deposits overlie topographic highs of the bedrock and coastal-plain surface, except immediately west of Fishers Island where moraine deposits lie partly on the walls of a valley incised in the coastal-plain strata, and south of Fishers Island where they overlie irregular topography.
The glaciolacustrine deposits (Figure 10, unit Qdl in profiles A, B, C, D, E, F, G, H) are the most extensive of the deposits within the glacial drift (Figure 6). They fill the inner lowland of the bedrock surface and fill the valleys and depressions within the coastal-plain strata. The glaciolacustrine deposits are thickest in the deep valleys; they thin over the interfluves and pinch-out along the shallow coastal-plain strata and end-moraine deposits in the southern half of Block Island Sound.
Deposits of fluvial outwash and ice-contact drift (Figure 10, unit Odo in profiles A, B, C, D and F) occur in some places; they lie on valley floors and abut or overlie coastal-plain strata in central Block Island Sound. In most places, the fluvial outwash and ice-contact drift are overlain by glaciolacustrine or end-moraine deposits; however, locally they are unconformably overlain by sediments of Holocene age or are exposed at the sea floor (Figure 9, unit Odo).
Bertoni, Remo, Dowling, J.J., and Frankel, Larry, 1977, Freshwater-lake sediments beneath Block Island Sound: Geology, v. 5 p. 631-635.
Bloom, A.L. and Ellis, C.W., Jr., 1965, Postglacial stratigraphy and morphology of coastal Connecticut: State Geological and Natural History Survey of Connecticut Guidebook No. 1, 10 p.
de Laguna, Wallace, 1963, Geology of Brookhaven National Laboratory and vicinity, Suffolk County, New York: U.S. Geological Survey Bulletin 1156-A, 35 p.
Donner, J.J., 1964, Pleistocene geology of eastern Long Island, New York: American Journal of Science, v. 262, p. 355-376.
Frankel, Larry, and Thomas, H.F., 1966, Evidence of freshwater lake deposits in Block Island Sound: Journal of Geology, v. 74, p. 240-242.
Fuller, M.L., 1905, Geology of Fishers Island, New York: Geological Society of America Bulletin, v. 16, p. 367-390.
____1914, The geology of Long Island, New York: U.S. Geological Survey Professional Paper 82,231 p.
Hermes, O.D., Barosh, P.J., and Smith, P.V., 1981, Contact relationships of the late Paleozoic Narragansett Pier Granite and country rock, in Boothroyd, J.C., and Hermes, O.D., eds., Guidebook to geologic field studies in Rhode Island and adjacent areas, 73rd annual meeting of the New England Intercollegiate Geologic Conference, Oct. 16-18, 1981, p. 125-152.
McMaster, R.L., 1960, Sediments of the Narragansett Bay System and Rhode Island Sound, Rhode Island: Journal of Sedimentary Petrology v. 30 p. 249-274.
____1971, A transverse fault on the continental shelf off Rhode Island: Geological Society of America Bulletin, v. 82 p. 2001-2004.
McMaster, R.L., and Ashraf, Asaf, 1973a, Subbottom basement drainage system of Inner Continental Shelf off southern New England; Geological Society of America Bulletin, v. 84, p. 187-190.
____1973b, Drowned and buried valleys on the Southern New England Continental Shelf: Marine Geology, v. 15, p. 249-268.
____1973c, Extent and formation of deeply buried channels on the continental shelf off southern New England: Journal of Geology, v. 81, p. 374-379.
McMaster, R.L., de Boer, Jelle, and CoUins, B.P., 1980, Tectonic development of southern Narragansett Bay and offshore Rhode Island: Geology, v. 8, p. 496-500.
McMaster, R.L., LaChance, T.P., and Garrison, L.E., 1968, Seismic-reflection studies in Block Island and Rhode Island Sounds: American Association of Petroleum Geologists Bulletin, v 52, no. 3, p. 465-474.
Mills, H.C., and Wells, P.D., 1974, Ice-shove deformation and glacial stratigraphy of Port Washington, Long Island, New York: Geological Society of America Bulletin, v. 85, p. 357-364.
Needell, S.W., O'Hara, C.J., and Knebel, H.J., 1983, Maps showing geology and shallow structure of western Rhode Island Sound: U.S. Geological Survey Miscellaneous Field Studies Map MF-1537, 4 sheets.
O'Hara, C.J., and Oldale, R.N., 1980, Maps showing geology and shallow structure of eastern Rhode Island Sound and Vineyard Sound, Massachusetts: U.S. Geological Survey Miscellaneous Field Studies Map MF-1186, 5 sheets.
Oliver, J.E., and Drake, C.L., 1951, Geophysical investigation in the emerged and submerged Atlantic Coastal Plain, part 6 of the Long Island area: Geological Society of America Bulletin: v. 62, no. 11, p. 1287-1296.
Savard, W.L., 1966, The sediments of Block Island Sound: unpublished thesis, University of Rhode Island, Kingston, Rhode Island, 66 p.
Schafer, J.P., 1961, Correlation of end moraines in southern Rhode Island: U.S. Geological Survey Professional Paper 424-D, p. D68-D70.
Schafer, J.P., and Hartshorn, J.H., 1965, The Quaternary of New England in Wright, H.E., Jr., and Frey, D.G., eds., The Quaternary of the United States: Princeton, N.J., Princeton University Press, p. 113-127.
Sirkin, L.A., 1971, Surficial glacial deposits and postglacial pollen stratigraphy in central Long Island, New York: Pollen et Spores, v. 13, p. 93-100.
____1976, Block Island, Rhode Island: evidence of fluctuation of the late Pleistocene ice margin: Geological Society of America Bulletin, v. 87, p. 574-580.
____1982, Wisconsinan glaciation of Long Island, New York to Block Island, Rhode Island, in Larson, G.J., and Stone, B.D., eds., Late Wisconsinan Glaciation of New England: Dubuque, Iowa, Kendall/Hunt, p. 35-60.
Suter, Russell, de Laguna, Wallace, Perimutter, N.M., and Brashears, M.L., Jr., 1949, Mapping of geologic formations and aquifers of Long Island, New York: New York Water Power and Control Commission Bulletin, GW-18, 212 p.
Tagg, A.R. and Uchupi, Elazar, 1967, Subsurface morphology of Long Island Sound, Block Island Sound, Rhode Island Sound, and Buzzards Bay, in Geological Survey Research 1967: U.S. Geological Survey Professional Paper, 575-C, p. C92-C96.
Tuttle, C.R., Alien, W.B., and Hahn, G.W., 1961, A seismic record of Mesozoic rocks on Block Island, Rhode Island: U.S. Geological Survey Professional Paper 424-C, p. C254-C256.
U.S. Department of Commerce, 1967a,Bathymetric map of Block Island and Rhode Island Sounds: U.S. Coast and Geodetic Survey chart 0808N-51, Mercator projection, scale 1:125,000.
____1967b, Bathymetric map of Long Island Sound; U.S. Coast and Geodetic Survey chart 0808N-53, Mercator projection, scale 1:125,000.
____1981, Block Island Sound and approaches; National Oceanic and Atmospheric Administration chart 13205, Mercator projection, scale 1:80,000.
Vail, P. R., Mitchum, R. M., Jr., Thompson, S.., Ill, Todd, R. G. Sangree, J. B., Widmier, J. M., Bubb, J. N., and Hatlelid, W. G. 1977, Seismic stratigraphy and global changes in sea level, in Payton, C. E., ed., Stratigraphic interpretation of seimsic data: American Association of Petroleum Geologists Memoir 26.
Weston Geophysical Engineers, Inc., 1977, Marine Geophysical Survey New Shoreham Fault Investigation: Weston Geophysical Engineers, Inc., Weston, Massachusetts, 34 p, 13 plates.