Journal of Coastal Research 522-532 West Palm Beach, Florida Summer 2000

A Review of the Geologic Framework of the Long Island Sound Basin, With Some Observations Relating To Postglacial Sedimentation Ralph S. Lewis and Mary DiGiacomo-Cohen

State Geological and Natural History Survey of Connecticut Environmental and Geographic Information Center Department of Environmental Protection 79 Elm St., Hartford, Connecticut 06106, U.S.A.

. ~ .. LEWIS, R.S. and DIGIACOMO-COHEN, M., 2000. A Review of the Geologic Framework of the Long Island Sound .tltllllll:. Basin, With Some Observations Relating To Postglacial Sedimentation. Journal ofCoastal Research, 16(3), 522-532. ~ West Palm Beach (Florida), ISSN 0749-0208. Most of the papers in this thematic section present regional perspectives that build on more than 100 years of geologic euus~~ =-+; b--- investigation in Long Island Sound. When viewed collectively, a common theme emerges in these works. The major geologic components of the Long Island Sound basin (bedrock, buried coastal-plain strata, recessional moraines, gla­ cial-lake deposits, and the remains of a large marine delta) interact with the water body to affect the way the modern sedimentary system functions. Previous work, along with our present knowledge of the geologic framework of the Long Island Sound basin, is comprehensively reviewed with this theme in mind. Aspects of the crystalline bedrock, and the deltaic deposits as­ sociated with glacial Lake Connecticut, are examined with respect to their influence on sedimentation along the Connecticut coast and in the northern and western Sound. We also discuss the influence of the glacial drift that mantles the coastal-plain remnant along the north shore of Long Island and in the southern Sound. A total of approximately 22.7 billion m" of marine sediment has accumulated in the Long Island Sound basin. A significant portion (44%) of the fine-grained marine section in the central and western basins was redistributed there from the eastern Sound, as tidal scour removed slightly over 5 billion m" (5.3 X 10 12 kg) of fine material from glacial­ lake and early-marine deposits east of the Connecticut River. The remainder of the estimated 1.2 X 10I;~ kg of fine­ grained marine sediment that now resides in the central and western Sound can be accounted for by riverine input over the past 13.5 ka.

ADDITIONAL INDEX WORDS: Glacial Lake Connecticut, marine delta, glacial history, sedimentation, sediment bud­ get, marine geologic investigations, Long Island Sound.

INTRODUCTION to provide a geologic setting for several of the papers that follow. Selected past contributions to our geologic legacy are Most of the papers in this volume share a common thread. highlighted. The relevance of the bedrock, coastal-plain, gla­ The geologic components of the Long Island Sound (LIS) ba­ cial and postglacial components of the LIS basin to the de­ sin interact with the water body to influence the distribution velopment of the modern sedimentary system is examined and character of bottom sediments, sedimentary environ­ within the context of our inferred chronology of Quaternary ments, infaunal assemblages, and the patterns of sedimen­ events. A discussion of the volume and sources of postglacial tation, sediment transport and contaminant accumulation. sediments is also presented. Conceptually, this interaction appears to make simple com­ mon sense. However, the iterative process of understanding PREVIOUS AND RElATED WORK the components that collectively control the sedimentary sys­ tem of the Sound has taken more than 100 y to evolve. De­ Pioneering workers in the latter part of the 19th century velopment of this understanding has important implications (DANA, 1875, 1890; HOLLICK, 1893) and early 20th century for wise use and management of the Sound. (VEATCH, 1906; FULLER, 1914; ANTEVS, 1922; REEDS, 1927) Building on previous work, an 18-year cooperative between set the stage for future endeavors with their interest in the the U.S. Geological Survey and the State of Connecticut has origins of the LIS basin and its possible occupation by a gla­ produced an overall characterization of the Quaternary stra­ cial lake. From the late 1940's through the 1970's, remote tigraphy and history of the Sound (LEWIS and STONE, 1991; sensing and physical sampling technologies (developed dur­ STONE et al., 1998). Most recently, studies presented in this ing and after World War II) provided new opportunities for volume have tied the cumulative evidence together and col­ subbottom and surficial-process studies. lectively support a holistic view of sediment-related processes Initial studies of the subbottom in the vicinity of LIS and in LIS. Block Island Sound (Figure 1) outlined the general relation­ This paper draws from these, and other previous findings, ship of bedrock, coastal-plain, and glacial-drift surfaces. OL- Geologic Framework of Long Island Sound Basin 523

42° r------..,.::.-...... ,..------.~---__------__r_--__--__.,._--_r__-...... __;;...;.. , I I I ) \ I , / ) I , I I I RHODE I J IS LAND / I CO, NN ECTICUT I WESTERN UPLANDS I HARTFORD i NEW / BASIN " YORK

, ..~. \ ....~ \ NANTUCKET .> ISLAND

41° ~

OCEAN \

40ft) o 25 50 STATUTE MILES ! 25 75 100 KILOMETERS I I J

Figure 1. Index map showing the regional setting of the Long Island Sound (From LEWIS and STONE, 1991, with permission).

IVER and DRAKE (1951) presented the first depth-to-bedrock recent maps of the extent of glacial-lake deposits in Block map of Long Island and Block Island Sounds. Later, TAGG Island Sound and LIS were prepared by NEEDELL and LEWIS and UCHUPI (1967) further described the bedrock surface as (1984), LEWIS and STONE (1991), and STONE et al. (1998). low-relief topography overlain by unconsolidated sediments. STONE et al. (1985) clarified previous ambiguities by defining Several of the bedrock valleys in and around LIS were dis­ glacial Lake Connecticut as the lake that once occupied the cussed by UPSONand SPENCER (1964), McMASTERand ASH­ LIS basin. RAF (1973 a, b, c) and HAENI and SANDERS (1974). A total Several investigators have reported on glacial deposits as­ magnetic intensity map was prepared by ZURFLUEH (1962). sociated with the study area. Detailed studies of deposits on The aeromagnetic map of HARWOOD and ZIETZ (1977) and Fishers Island (Figure 2) and Long Island were completed by the free-air gravity map of DEHLINGER (1978) followed. FULLER (1905, 1914), and CRANDELL (1962) worked on Plum RODGERS (1985) and HERMES et al. (1994) compiled the Island (Figure 2). GOLDSMITH (1960, 1982), SCHAFER (1961), bedrock geological maps of Connecticut and Rhode Island. A and SIRKIN (1982) studied the moraines of eastern Connect­ summary of the geology of New York is contained in ISACH­ icut, Rhode Island, and Long Island (NY). STONE and BORNS SEN et al. (1991). WINTSCH et al. (1992, 1995) outlined the (1986) presented a Pleistocene glacial and interglacial stra­ bedrock terranes of eastern Connecticut and Rhode Island. tigraphy for the area. LEWIS and STONE (1991) also exam­ McMASTER et al. (1968), McMASTERand ASHRAF (1973 a, ined the nature and history of ice retreat across the LIS basin b, c) and NEEDELL and LEWIS (1984) described the regional and presented findings that were incorporated in the Qua­ relationship of a south-dipping bedrock surface, unconform­ ternary Geologic Map of Connecticut and Long Island Sound ably overlain by a remnant of Cretaceous coastal-plain strata, Basin (STONE et al., 1998). Goss (1993) used seismic-reflec­ in Block Island Sound. A similar relationship was mapped by tion data from Block Island Sound (NEEDELL and LEWIS, LEWIS and NEEDELL (1987) and NEEDELL et al. (1987) in 1984) to compare and integrate his interpretation of the southeastern and east-central LIS. Block Island Sound geology with ice-marginal sedimentation The concepts of HOLLICK (1893), ANTEVS (1922), REEDS and ice-retreat models proposed for LIS (LEWIS and STONE, (1927) and LOUGEE (1953), regarding a glacial lake that once 1991) and Narragansett Bay (PECK and McMASTER, 1991). occupied the LIS basin, were revived by NEWMAN and FAIR­ The composition and distribution of bottom sediments in BRIDGE (1960) in their naming of glacial Lake Antevs. Early and around LIS have been reported on by numerous inves­ physical evidence of the occurrence of glacial lake clay in the tigators te.g., McMASTER, 1960; BUZAS, 1965; MCCRONE, vicinity of LIS was supplied by FRANKEL and THOMAS (1966) 1966; SAVARD, 1966; DIGNES, 1976; WILLIAMS, 1981; FRIED­ and COCH et al. (1974). BERTONI et al. (1977) identified fresh­ RICH et al., 1986; KELLY and ALBANESE, 1989). BLOOM (1967) water-lake deposits in central Block Island Sound on the ba­ outlined the character of the Connecticut coast. BOKUNIEW­ sis of bottom samples, cores, and seismic profiles. The most ICZ et al. (1976) presented a sediment budget for LIS, and

Journal of Coastal Research, Vol. 16, No.3, 2000 524 Lewis and DiGiacomo-Cohen

WESTERN UPLANDS EASTERN UPLANDS \ I RI '.. t / 1: N CONNECTICUT \

00~'Q ~'Q S ,S",t" NY OC'f.. '0'"

ONSHORE DEPOSITS

""'" End moraine Ji, Deltaic deposits

OFFSHORE BEDROCK GEOLOGY

"~~-l "J"- Crystalline bedrock ...... Coastal plain strata o~ Miles tb-north-faCing cuesta scarp o 5 Km arrows indicate thalwegs of valleys <, ~ Anomalously deep bedrock

Figure 2. Map showing the distribution of crystalline bedrock and coastal-plain strata under Long Island Sound. The inferred position of the coastal­ plain cuesta scarp and major coastal-plain outliers is shown by hachures. The thalwegs of major bedrock and coastal-plain valleys are shown by arrows. Areas labeled "gas" indicate the presence of biogenic gas which precluded mapping. NH, New Haven; NL, New London; FI, Fishers Island; PI, Plum Island; OP, Orient Point (From LEWISand STONE, 1991, with permission).

FENSTER (1995) studied sand-sheet facies in eastern LIS. et al. (1970), GORDON (1980), WILLIAMS (1981), and LEWIS TRUMBULL (1972) and SCHLEE (1973) provided some infor­ and STONE (1991). These studies described the coastal-plain mation on the bottom sediments of Fishers Island Sound, but remnant as a highly dissected cuesta, cut in strata of Cre­ AKPATI (1974) conducted the first study specific to the min­ taceous age, that lies on an irregular, southeast-dipping bed­ eralogy and sedimentology of the area. POPPE et al. (1994) rock (Precambrian? and Paleozoic?) surface. Extensive gla­ outlined the distribution of surficial sediments in Fishers Is­ cial-lake deposits, and a marine delta, associated with the land Sound. draining of glacial Lake Hitchcock, have been mapped by In the first chapter of a three-part study, ZAJAC (1995) pre­ STONE et al. (1998). Special Issue No. 11 of the Journal of sented a comprehensive review of research on benthic com­ Coastal Research (GAYES et al., 1991) was dedicated to geo­ munities and sediments in LIS. Previously, an entire volume logic-framework, paleo-environmental, and sea-level studies of Advances in Geophysics (SALTZMAN, 1980) was devoted to in and around LIS. sediment, estuarine-physics, and chemistry studies in LIS, and FELDHAUSEN and ALI (1976) published on sedimentary GEOLOGIC SETfING environmental analysis. In 1998, the U.S. Geological Survey published a CD-ROM containing sidescan-sonar, seismic-re­ Long Island Sound is an elongate east-west estuary, which flection, bathymetric, sedimentary and bibliographic data is rather unique in that its long axis runs parallel to the coast pertaining to LIS (POPPE and POLLONI, 1998). (Figure 1). It has communication with the Atlantic Ocean at The inferred geology of LIS has been summarized by GRIM its eastern and western ends, and it receives drainage from

Journal of Coastal Research, Vol. 16, No.3, 2000 Geologic Framework of Long Island Sound Basin 525 the Iapetos, Newark and Avalonian terrane (RODGERS, 1985) The Iapetos and Avalonian terranes of central New Eng­ rocks of central New England, southeastern New York, and land, southeastern New York and Connecticut are character­ Connecticut. ized by north-south trending bedrock ridges and south-drain­ Crystalline bedrock has a strong influence on the depth ing bedrock valleys. The Hartford Basin is also oriented and configuration of westernmost LIS, and it controls the north-south (Figure 2). This north-south "grain" developed as shape of most of the Connecticut shoreline. Owing to its the area was compressed during the closing of the Iapetos southeast dip (GRIM et al., 1970), crystalline bedrock is gen­ Ocean, and rifted during the opening of the Atlantic Ocean. erally too deep to directly influence the shape of the north Subsequent fluvial and glacial erosion exploited north-south shore of Long Island, which is composed primarily of glacial weakness zones, and several south-draining fluvial systems, material and yields little surface runoff to the Sound. which feed into LIS, developed on the bedrock. The northeast trend of the Roanoke Point-Orient Point­ South-draining bedrock valleys, and bracketing south­ Fishers Island-Charlestown moraine (Figure 2) presently trending crystalline rock promontories, can be traced under controls the eastward constriction of the LIS basin. West of northern LIS (Figure 2). The large embayment in the New New Haven (Figure 2), the basin is confined by near-surface Haven area is bounded to the north by the rift basin rocks of bedrock to the north and west, and, along the south shore, the Newark terrane (Figure 2, Hartford Basin). East and by a series of north-northwest trending necks that reflect the west of New Haven, crystalline bedrock promontories often shape of the underlying coastal-plain deposits. The shape and extend, as south-trending bathymetric highs, quite far off­ composition of the LIS basin influence how physical processes shore. This is particularly true for the westernmost Sound, affect the sedimentary system of the Sound (see KNEBEL and and for the area east of New London (Figure 2). POPPE, this volume; SIGNELL et al., this volume). Bedrock valleys form numerous small embayments and coves along the north shore. These coves and embayments Bedrock usually contain fluviodeltaic deposits (STONE et al., 1998) as­ sociated with glacial Lake Connecticut. Some of the sedi­ The bedrock that lies under LIS is believed to consist of ment-related patterns noted by other workers in this volume metamorphic and igneous (?) rocks of pre-Mesozoic age. Pre­ (e.g. KNEBEL and POPPE, this volume; POPPE et al., this vol­ cambrian (?) and Paleozoic gneisses and granites of the Ava­ ume) are influenced by the north-south "grain" of the shallow, lonian Terrane (RODGERS, 1985) crop out along the Connect­ near-shore crystalline bedrock that underlies the northern icut coast east of New Haven (Figure 2, Eastern Uplands). Sound. North of Fishers Island, the Avalonian Anticlinorium is lo­ cally intruded by the Westerly Granite (Permian"), now noted Coastal Plain by HERMES et al. (1994) as the fine-grained component of the Narragansett Pier Plutonic Suite (RODGERS, 1985). Drill FULLER (1905) reported 21 ft (6 m) of Cretaceous blue clay, holes have encountered light-gray granite (presumably the resting on granite, on Fishers Island, but no Cretaceous de­ Westerly Granite) at about -86 m under Fishers Island posits are known to exist in northernmost LIS (Figure 2) or (FULLER, 1905). Gneiss/granitic gneiss and schist have been along the Connecticut coast. Coastal-plain sediments of Cre­ reported under eastern Long Island at depths ranging from taceous age (consisting of unconsolidated to semi-consolidat­ -153 m at Orient Point (Figure 2) to -488 m at Brookhaven ed gravels, sands, silts, and clays) have been reported along (SUTER et al., 1949; DE LAGUNA, 1963; PIERCE and TAYLOR, the north shore of Long Island (FULLER, 1914; SUTERet al., 1975). 1949; DE LAGUNA, 1963). The coastal-plain material that In the New Haven area, sedimentary and igneous rift basin crops out along the immediate north shore of Long Island is rocks of the Early Jurassic-to-Late Triassic Newark terrane not in place; it has been incorporated in glacial deposits, of underlie the Hartford Basin (Figure 2). Offshore, HAENIand varying thickness, that mantle the coast (LES SIRKIN, per­ SANDERS (1974) reported a deep bedrock valley south of New sonal communication, 1999). Haven, and LEWIS and STONE, (1991) defined an area of The overall shape of the coastal-plain remnant depicted in anomalously deep bedrock, trending southwestward from Figure 2 is consistent with earlier interpretations for Block New Haven (Figure 2). The deep valley mapped by HAENI Island Sound (McMASTERet al., 1968; NEEDELL and LEWIS, and SANDERS (1974) is partially filled with deposits that are 1984), where north-draining valleys incise and segment the older than glacial Lake Connecticut, but as yet unidentified. cuesta front. Inferred north-draining, coastal-plain valleys As of this writing, there is no direct evidence for the existence are associated with the north-shore bays of western Long Is­ of Newark terrane rocks under LIS. land, and with the submerged coastal-plain promontories of Westward of New Haven, schists, gneisses and phyllites of central and eastern Long Island (Figure 2). the Connecticut Valley Synclinorium (Middle-to-Early Paleo­ Seismic data from Block Island Sound show gently south­ zoic Iapetos terrane) crop out along the north shore of LIS dipping reflectors that have been interpreted as internal con­ (RODGERS, 1985; ISACHSEN et at., 1991), (Figure 2, Western tacts between coastal-plain strata (NEEDELL and LEWIS, Uplands). VEATCH (1906) reported outcrops of gneiss and do­ 1984). These reflectors are a useful indicator of the presence lomite in northwesternmost Long Island. The bedrock gets of coastal-plain strata because they are distinctive, and they deeper toward the east, and well data indicate that the crys­ are commonly truncated along the inferred, north-dipping talline bedrock surface under western Long Island dips scarp of the coastal-plain cuesta. Internal coastal-plain re­ southeast at about 25-50 m/km (NEWMAN, 1977). flectors have not been identified on any of the LIS seismic

Journal of Coastal Research, Vol. 16, No.3, 2000 526 Lewis and DiGiacomo-Cohen data reported on by LEWIS and STONE (1991). This compli­ STONE et al., 1998). An extensive ice-marginal lacustrine fan cates the identification of the north-facing cuesta front and seaward of Lordship (Figure 3) is thought to be associated opens the possibility that the glacial deposits of southernmost with an ice position involving the Norwalk Island moraine, LIS are thicker than LEWIS and STONE (1991) have indicat­ to the west, and the Old Saybrook moraine, to the east. The ed. tills that are exposed along the margins of the LIS basin con­ The configuration of the LIS basin is most obviously influ­ tinuously yield sediment to the estuary. This is evident in the enced by the shape and position of the coastal-plain remnant mapping of POPPE et ale (this volume). south and west of the Housatonic River (Figure 2). Shallow Deltaic and lake-bottom deposits of glacial Lake Connect­ bedrock limits the depth of this portion of the basin so that icut are found along the Connecticut coast and throughout the overlying coastal-plain deposits are more prominent. The the LIS basin (Figure 4). At its height, Lake Connecticut was net result is a confined basin where north-trending promon­ part of a series of glacial lakes that stretched eastward, oc­ tories (underlain by the coastal-plain remnant) nearly meet cupying Block Island and Rhode Island Sounds, and Narra­ the south-trending promontories of crystalline bedrock (Fig­ gansett and Buzzards Bays (Figure 1). Varved lake-bottom ure 2). The "Norwalk shoal complex" of KNEBEL and POPPE deposits of glacial Lake Connecticut (Figure 4) are commonly (this volume) is an example of this. The southern half of the 80 m thick and locally more than 150 m thick (LEWIS and "Stratford shoal complex" (KNEBEL and POPPE, this volume) STONE, 1991), west of the Connecticut River. These deposits is also associated with a coastal-plain promontory or outlier. were once similarly abundant to the east. Lake Connecticut The presence of these shoal complexes has an influence on was dammed by the Roanoke Point-Orient Point-Fishers Is­ the modern physical processes and habitats of the western land-Charlestown moraine (Figure 3), and its spillway was at Sound (e.g. KNEBEL and POPPE, this volume; POPPE et al., the Race (LEWIS and STONE, 1991; STONE et al., 1998). this volume; SIGNELL et al., this volume; ZAJAC et al., this Deltas, fed by meltwater streams and rivers that re-occu­ volume) pied south draining bedrock valleys, prograded into the East of New Haven, the bedrock is deeper, and the central northern margin of glacial Lake Connecticut as the Wiscon­ Sound has more fetch. The present northern Long Island sinan glacier retreated (STONE et al., 1998). Coalescing Lake shoreline curves gently northeastward. It is formed by the Connecticut deltas extended well into what is now northern Roanoke Point-Orient Point moraine, which rests on coastal­ LIS between the Norwalk Islands and New Haven, and be­ plain strata, and lies well south of the inferred position of the tween Guilford and the mouth of the Connecticut River (Fig­ buried cuesta front (Figure 2). Other than its role in forming ure 4). A large delta also was built southward from the the buried southern flank of the LIS basin, the coastal-plain Thames River valley. remnant is inferred to be too deep to have a great influence The extensive, coalesced, glacial deltas south and east of on modern processes in this area. Lordship, form the northern part of the "Stratford shoal com­ plex" of KNEBEL and POPPE (this volume), and glacial deltas Glacial Deposits comprise the northernmost part of the "Norwalk shoal com­ plex" (KNEBEL and POPPE, this volume) in the vicinity of the In Connecticut, and on the north shore of Long Island, the Norwalk Islands (Figure 4). The presence of these, and other bedrock and/or coastal-plain strata are unconformably over­ lain by two drift sheets. The older till is commonly attributed glacial deltas (e.g. Connecticut River mouth) (Figure 4) of the to pre-late-Wisconsinan events, whereas the younger deposits northern Sound, influence the sediment and habitat distri­ are a product of the late Wisconsinan ice advance (DONNER, butions reported on by several of the other authors in this 1964; SIRKIN 1971, 1976, 1982; MILLS and WELLS, 1974; volume (e.g. KNEBEL and POPPE, this volume; POPPE et al., RAMPINO and SANDERS, 1981). Two late Wisconsinan end­ this volume; ZAJAC et al., this volume). moraine lines cross Long Island (SCHAFER and HARTSHORN, Fluvial and fluviodeltaic deposits that fed the Lake Con­ 1965; SIRKIN, 1982; STONE and BORNES, 1986). The Ronkon­ necticut deltas (STONE et al., 1998) occupy south draining koma-Amagansett-Shinnecock moraine line, which marks the bedrock valleys all along the Connecticut coast (Figure 4). maximum extent of the late Wisconsinan glaciation, lies The overall distribution and magnitude of Connecticut's across central and southeastern Long Island (Figure 3) and beaches and marshes is tied to these deposits. In smaller val­ extends eastward across the shelf in the direction of Block leys, deltaic and/or feeder deposits are typically associated Island. To the north, a second moraine (Harbor Hill-Roanoke with pocket beaches and small marshes that are bracketed Point-Orient Point-Fishers Island-Charlestown, Figure 3) by rocky headlands. Postglacial erosion of the Lake Connect­ caps northern Long Island, Plum Island, and Fishers Island icut deltas has provided a source of sediment to the northern and extends eastward across southern Rhode Island (CRAN­ Sound throughout Holocene time. DELL, 1962; SCHAFER and HARTSHORN, 1965). Less promi­ Just north of the Race, tidal scour has completely removed nent recessional moraines (SIRKIN, 1982) lie between the two the lake deposits and re-exposed bedrock and till. The glacial major moraine belts on eastern Long Island. Several moraine lake-bottom deposits of adjacent Block Island Sound also segments have been mapped in southeastern Connecticut, have been extensively scoured in the vicinity of the Race and offshore (GOLDSMITH, 1982; STONE et al., 1998). Portions (NEEDELL and LEWIS, 1984). Some of this fine material, and of the Charlestown Moraine and most of the southeastern most of the fines stripped from the lake deposits of eastern Connecticut recessional moraines (Figure 3) have been LIS, are inferred to have been incorporated into the marine mapped and described as double linear belts (SCHAFER 1965; deposits of the central and western Sound.

Journal of Coastal Research, Vol. 16, No.3, 2000 Geologic Framework of Long Island Sound Basin 527

WESTERN UPLANDS EASTERN UPLANDS \ t I RI N ! CONNECTICUT \

NY

ONSHORE DEPOSITS .; End moraine Jt Deltaic deposits

OFFSHORE DEPOSITS ~ End moraine Ice-marginal lacustine fan deposits o 5 Miles ~ ~proximal facies o 5 Km L-J--distal facies

A small fans

Figure 3. Map showing submerged end-moraine deposits and ice-marginal lacustrine-fan deposits of glacial Lake Connecticut. Areas labeled "gas" indicate the presence of biogenic gas which precluded mapping. NI, Norwalk Islands; L, Lordship; NH, New Haven; C, Clinton; OS, Old Saybrook; NL, New London; FI, Fishers Island; PI, Plum Island; OP, Orient Point (From LEWISand STONE, 1991, with permission).

Postglacial Deposits The present irregular sea-floor topography in eastern Long Island Sound (KNEBEL and POPPE, this volume, Figure 2) is Glacial Lake Connecticut drained to the lowered sea, sub­ primarily the result of extensive tidal scour that removed the aerially exposing its lakebed, by about 15.5 ka (STONE et al., fine components of the eastern flank of the marine delta, and 1998). The paleo-channel system that carried drainage across the lakebed and out of the LIS basin (through the notch in most of the lake-bottom deposits of the eastern Sound. FENS­ the moraine at the Race) is depicted in Figure 5. Fluvial sed­ TER(1995) estimated that 2.1 billion m" of fine material has iments in the paleo-channels are overlain by estuarine sedi­ been removed from the lake-bottom deposits, and 2.0 billion ments that were deposited after eustatic sea-level began to m" of coarser material is missing from the eastern side of the rise between 16 ka and 15 ka (STONE et al., 1998). marine delta. KNEBEL and POPPE(this volume) estimate that As the stable phase of glacial Lake Hitchcock ended around an area of 195 km 2 was stripped of about 15 m of deltaic 13.5 ka, its lakebed (which extended over much of the Con­ material (2.925 billion m") since tidal scour began. necticut River valley) was also subaerially exposed. It is es­ Lag deposits, up to 17m thick, are now exposed at the sea timated that about 12 billion m" of lake-bottom sediments floor throughout most of eastern LIS (LEWIS and STONE, were removed and carried down the Connecticut River as the 1991, FENSTER, 1995, KNEBEL and POPPE,this volume). The lakebed was incised (STONE et al., 1998). Most of this sedi­ distribution of these lag deposits corresponds with the coarse ment (11.5 billion m") is inferred to have built the marine deposits shown on the map of POPPEet al. (this volume). The delta depicted in Figure 5 (LEWIS and STONE, 1991; STONE transition of sedimentary environments westward, from the et al., 1998). deeper area of erosion or nondeposition, through environ-

Journal of Coastal Research, Vol. 16, No.3, 2000 528 Lewis and DiGiacomo-Cohen

WESTERN UPLANDS EASTERN UPLANDS \ t I RI N I 1; CONNECTICUT \

00~Q ~Q S ,S\'-!>'- NY OC{.. 'O\.-

ONSHORE DEPOSITS

""" End moraine It Deltaic deposits

OFFSHORE DEPOSITS

" Deltaic deposits @ Varved lake-bottom deposits o~ Miles . o 5 Km

Figure 4. Map showing deltaic and varved lake-bottom deposits of glacial Lake Connecticut. Dashed lines indicate reconstruction of deposits where lack of data or removal of deposits by tidal scour precluded mapping. NI, Norwalk Islands; L, Lordship; NH, New Haven; G, Guilford; OS, Old Saybrook; NL, New London; FI, Fishers Island; PI, Plum Island; OP, Orient Point (From LEWISand STONE, 1991, with permission). ments of coarse-grained bedload transport, and up across the Given that LIS is an effective sediment trap (BOKUNIEW­ zone of sediment sorting and reworking, to the shallower en­ ICZ et al., 1976), Figure 6 is inferred to be a good approxi­ vironments of fine-grained deposition (KNEBEL and POPPE, mation of the total amount of marine sedimentation that has this volume, Figure 13) takes place across the "Mattituck occurred in the Sound. After digitizing hand-drawn isopachs, Sill" of BOKUNIEWICZ et al. (1976). This "sill" is not structur­ we used GIS technology to estimate that the total volume of ally controlled but rather it developed as a result of postgla­ sediment represented on Figure 6 is approximately 22.7 bil­ cial tidal scour, and did not exist prior to the marine incur­ lion m", sion. It is formed by remnants of the marine delta resting unconformably on relatively intact lake-bottom deposits to Chronology the west, and an eastward-thinning remnant of marine-del­ taic and lake-bottom deposits to the east. The Long Island Sound basin was an existing interior low­ The distribution and thickness of postglacial sediments in land, that had been fluvially eroded and modified by at least LIS is depicted in Figure 6. West of the Connecticut River, one glacier (Illinoian"), before the last ice advance. By about the postglacial section (including the marine delta) primarily 26 ka, the late Wisconsinan glacier was advancing into Con­ rests unconformably on the ravinement that developed across necticut (STONE et al., 1998). As it advanced, it scoured the the surface of the lakebed, as the sea invaded LIS. To the south-dipping bedrock surface, further modified the interior east, tidal scour has cut below the ravinement, and the post­ lowland, and reached its terminal position, on the coastal­ glacial section rests unconformably on a scoured, composite plain remnant, by about 21 ka. surface that locally includes sub-crops of bedrock, till, and By 19 ka (STONE and BORNS, 1986), the Wisconsinan gla­ glacial-lake deposits. cier had already formed the Ronkonkoma-Amagansett-Shin-

Journal of Coastal Research, Vol. 16, No.3, 2000 , I I I I [ ,I I I I I I I ~ I e::= I o I ~ \ \ \ ~ \ ~ \ \ Z \ \ \

./ ,""

/ ",,""

\ \ \ \ \

Long Island

~,. '~ ., (,. ~. Figure 6. Map showing the thickness (in meters) of the marine deposits in Long Island Sound (including the marine delta in the eastern Sound). This map was constructed from 3,500-line km of high-resolution, seismic-reflection data reported on by LEWIS and STONE(1991). Geologic Framework of Long Island Sound Basin 529

WESTERN UPLANDS EASTERN UPLANDS \ I RI Nf ., I " CONNECTICUT \

NY

ONSHORE DEPOSITS

" End moraine It Deltaic deposits

OFFSHORE DEPOSITS

Marine deltaic deposits

Channel deposits o 5 Miles Q ~ o 5 Km

Figure 5. Map showing early postglacial deposits; the paleo-channel system occupied by fluvial and estuarine channel-fill deposits (arrows indicate direction of channel-bottom slope); and the marine deltaic sequence. NI, Norwalk Islands; L, Lordship; NH, New Haven; NL, New London; FI, Fishers Island; PI, Plum Island; OP, Orient Point (From LEWIS and STONE, 1991, with permission). necock moraine (Figure 3) along its terminal position, and it Delta building commenced along the Connecticut coast as was about to retreat from its more northerly recessional po­ soon as the receding ice sheet lost contact with the lake, and sition along the Harbor Hill-Roanoke Point-Orient Point­ meltwater streams re-occupied emerging bedrock valleys. Fishers Island-Charlestown moraine (Figure 3). As the ice Southeastern Connecticut was deglaciated first, and a pro­ sheet retreated northward, meltwater was dammed behind gression of younger deltas built westward toward New Haven the Roanoke Point-Orient Point-Fishers Island-Charlestown (STONE et al., 1998). A lobe of ice, associated with the Hart­ moraine segment (Figure 3), and glacial Lake Connecticut ford Basin, lingered in the vicinity of New Haven until just was formed. The spillway for Lake Connecticut was estab­ before 16.5 ka, when coastal delta building ended (STONE et lished at the low point in the morainal dam at the Race (Fig­ al., 1998). The thick lake-bottom deposits of LIS (Figure 4) ure 4). At this time, sea level was about 100 m lower than are inferred to be contemporaneous with the coastal deltas. today. Water from Lake Connecticut joined with its neigh­ The spillway at the Race was being eroded, and lake levels boring lake in Block Island Sound, to spill across the exposed steadily lowered during (and after) coastal delta building continental shelf to the Atlantic Ocean. (STONE et al., 1998). It took the ice sheet about 1.5 ka to retreat from the posi­ The bed of glacial Lake Connecticut was completely ex­ tion of the Roanoke Point-Orient Point-Fishers Island­ posed by 15.5 ka. About 500 y later, the fluvial system drain­ Charlestown moraine to the position of the Norwalk Island­ ing the lakebed was well established (Figure 5), and the ris­ Lordship-Old Saybrook moraine (Figure 3). Lake Connecticut ing sea had started to invade the LIS basin through the erod­ continued to grow larger as ice retreat proceeded. It ulti­ ed spillway at the Race (STONE et al., 1998). The marine in­ mately covered an area roughly equivalent to the area of to­ cursion proceeded westward up the fluvial channels first, and day's LIS. eventually spread north and south over the exposed lakebed.

Journal of Coastal Research, Vol. 16, No.3, 2000 530 Lewis and DiGiacomo-Cohen

Wave action planed the lakebed surface, and the sea over­ the marine delta, and the lag deposits of the eastern Sound. topped outcrops of more resistant bedrock, coastal plain and Of the estimated 11.5 billion m" of sediment originally in the drift, forming a ravinement surface. marine delta (STONE et al., 1998), KNEBEL and POPPE (this The sea was well into the LIS basin by 13.5 ka. The estuary volume) calculate that approximately 2.9 billion m" of the is inferred to have developed sufficiently for continuous sed­ eastern flank of the delta has been redistributed, and the imentation to have commenced, and glacial Lake Hitchcock fine-grained component has been transported westward. The was beginning to drain down the Connecticut River valley. coarse fraction from the eastern flank of the delta was incor­ The Connecticut River is inferred to have delivered about porated into the lag deposits of the eastern Sound, which 11.5 billion m" of Hitchcock lake-bottom sediment to the FENSTER (1995) estimated to represent about 2.5 billion m", Sound between about 13 ka and about 9.5 ka (STONE et al., Subtracting the missing eastern segment of the marine delta 1998). This sediment buried the ravinement and built the (2.9 billion m") from its original volume (11.5 billion m") and marine delta (Figure 5). Eventually, the sea transgressed the adding the volume of the eastern lag deposits (2.5 billion m") nearshore deltas of glacial Lake Connecticut, and beaches will approximate the volume of the marine sediments that and marshes formed along the irregular Connecticut coast. were not redistributed westward (about 11.1 billion m"), As­ The marine transgression continues today. suming that 11.1 billion m", of the 22.7 billion m" of postgla­ As tidal currents developed, scouring and redistribution of cial sediment represented on Figure 6, is the eastern lag de­ glacial and younger sediments became common in constricted posit and the remaining marine delta, we estimate that 11.6 portions of LIS, particularly in the eastern part of the basin billion m" of fine-grained sediments lie west of the Connect­ (see KNEBEL and POPPE, this volume; POPPE et al., this vol­ icut River. Using the foregoing parameters for particle den­ ume; and SIGNELL et al., this volume). Slightly over 5 billion sity and porosity, this would equate to 1.2 X 1013 kg of rel­ m" of the lake-bottom (FENSTER, 1995) and fine-grained, ma­ atively fine-grained marine sediment. rine-delta deposits (KNEBEL and POPPE, this volume) of east­ Potential sources of this sediment are as follows: (1) KNE­ ern LIS are inferred to have been eroded and redistributed BEL and POPPE (this volume) report 2.5 X 1012 kg of fine westward over the past 13.5 ka. The 11.5 billion m" of Hitch­ material winnowed from the eastern flank of the marine del­ cock lake-bottom sediment that built the marine delta, ac­ ta and transported westward; (2) FENSTER (1995) reported count for slightly over half of the 22.7 billion m" of marine the equivalent of 2.8 X 1012 kg of lake-bottom deposits sediment that has been deposited in LIS. stripped from the eastern Sound; and (3) the total 13.5 ka Many buried structural elements of the western basin still contribution of sediment to the Sound from major rivers is have enough bathymetric expression to influence the physical 6.34 X 1012 kg (see previous calculation). As presented, these processes and sediment system of LIS (KNEBEL and POPPE, sources account for a total of about 1.2 X 1013 kg, an amount this volume). In the eastern Sound, the configuration of the equal to the 1.2 X 1013 kg of marine sediment derived using basin, and the erodability of the lake-bottom deposits, has Figure 6. promoted so much marine erosion that the irregular, armored Having arrived at the same number based on both volume seafloor is, itself, a major factor in how the modern sediment and input estimates, we feel that 1.2 X 1013 kg is a realistic system functions. approximation of the total mass of fine-grained marine sed­ iment in the central and western Sound. It is significant to POSTGLACIAL SEDIMENTATION note that this accumulation of marine sediment can be ac­ counted for without invoking offshore (open ocean) sources. The fine-grained sediments of Long Island Sound influence Redistributed glacial and early postglacial sediments from the distribution of habitats and contaminants throughout the within the LIS basin, and riverine contributions over the past basin (see related papers, this volume). This makes them of 13.5 ka, are sufficient to balance the estimated mass of fine­ interest to many workers, particularly resource managers. grained, marine sediment that has accumulated in Long Is­ BOKUNIEWICZ et al. (1976) estimated the volume of fine­ land Sound. grained sediment in LIS to be 5.3 billion m", Using a particle density of 2.65 X 106 g/m" and a porosity of 60% for silty ACKNOWLEDGEMENTS sediments (HAMILTON, 1974) this would equate to 5.6 X 1012 kg of marine mud. Assuming continuous deposition since 13.5 Since 1980, the State of Connecticut has been the benefi­ ka, and nearly 100% trapping efficiency (BOKUNIEWICZ et al., ciary of a successful, marine-oriented, cooperative relation­ 1976; GORDON, 1980), this amount is about 88% of the ac­ ship with the U.S. Geological Survey. We thank Sally Nee­ cumulation (6.34 X 1012 kg) that would be predicted from the dell, Robert N. Oldale, Janet Stone, Larry Poppe and Harley long-term contribution by major rivers emptying into the Knebel for their contributions to this effort. This manuscript Sound (4.7 X 108 kg/y) (BOKUNIEWICZ and GORDON, 1980; has benefited from critical reviews by Muriel Grim and Har­ GORDON, 1980). ley J. Knebel. The 3,500 line km of high-resolution, seismic-reflection data available to us (LEWIS and STONE, 1991), indicate that LITERATURE CITED BOKUNIEWICZ et al. (1976) underestimated the amount of AKPATI, B.N., 1974. Mineral composition and sediments in eastern fine-grained marine sediment in LIS by about one half. The Long Island Sound, New York. Maritime Sediments, 10,19-30. total amount of postglacial sediment represented on Figure ANTEVS, E., 1922. The recession of the last ice sheet in New Eng­ 6 is 22.7 billion m", This includes the remaining material in land. American Geographical Society Research Series, 11, 120p.

Journal of Coastal Research, Vol. 16, No.3, 2000 Geologic Framework of Long Island Sound Basin 531

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