Plum Island Surficial Geologic Map Sheet 1 by VIMS Coastal Geology

Massachusetts Geological Survey University of Massachusetts, Amherst

INTRODUCTION

Address: 269 Morrill Science Center, 611 North Pleasant Street, Amherst, MA 01003 Phone: 413-545-4814 E-mail: sbmabee@geo.umass.edu WWW: http://www.geo.umass.edu/stategeologist

This geologic map shows the distribution of surficial subaerial and subaqueous materials in the Newburyport East and northern half of the Ipswich 7.5' quadrangles (northeast Massachusetts) and the area of the Gulf of Maine immediately offshore, to an approximate depth of 80 m below modern mean sea level (MSL). This map was compiled from the onshore surficial geologic map of Stone et al. (2006) and the offshore surficial mapping of Barnhardt et al. (2009), and includes newly mapped shallow offshore geologic features. Onshore and offshore units are continuous across the shallowwater zone (0-20 m below MSL). The definition of map units is based on lithologic characteristics (grain size, mineralogy and structure), stratigraphic relationships and relative ages, and sedimentologic processes. The map describes the evolution of the surficial geology in terms of the sediment sources, transportation mechanisms, and depositional, post-depositional and modern processes that have acted on the late Quaternary sediments that compose these units. Cross sections are derived from subsurface data compiled from the literature and collected as part of this study.

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Topographic base modified from U.S. Geological Survey quadrangles, mapped in 1942, revised in 1966 and digitally re-projected to 1983 North American Datum. Shaded relief (5-m resolution) is a mosaic of digital terrain models (DTMs) derived from digital orthophoto images and bathymetric surveys (1947-1993) by the National Ocean Service (NOS) of the National Oceanic and Atmospheric Administration (NOAA), available from the Massachusetts Office of Geographic Information (MassGIS; www.mass.gov/mgis); Light Detection and Ranging (LiDAR) data collected by the Plum Island Ecosystems Long Term Ecosystem Research Station (2005; Valentine and Hopkinson, 2005) and the US Army Corps of Engineers (2007; USACE, 2009); bathymetric mapping by the U.S. Geological Survey (2004, 2005; Barnhardt et al., 2009); and unpublished real-time kinematic GPS surveys completed by the Plum Island Ecosystems Long Term Ecosystem Research Station (funded by National Science Foundation grant #0423565). Shoreline position was determined by Applied Coastal Research and Engineering (2009) using 2005 LiDAR data, 766 Falmouth Road, Suite A-1, Mashpee, MA 02649, 508-539-3737.

Freshwater Marsh Deposits (Qm Holocene) - fine-grained organic matter, and fibric and hemic peat that contains minor amounts of stratified and poorly sorted sand, silt, and clay in poorly drained areas. Most deposits are < 3 m thick and generally overlie glacial sediment or bedrock. In west-central Plum Island, this habitat has been artificially created by the emplacement of dikes across the estuarine sediments in the late 1940s. Freshwater marsh deposits also are commonly found at depth, underlying younger saltmarsh deposits; the contact between freshwater marsh and saltmarsh deposits marks the leading edge of the late Holocene transgression.

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APPROXIMATE MEAN DECLINATION 2010

CONTOUR INTERVAL 10 FEET DATUM IS MEAN SEA LEVEL BATHYMETRIC CONTOUR INTERVAL 10 METERS

Surficial geologic mapping by C. Hein and D. FitzGerald (2009-2011); B. D. Stone (1982) and E. Sammel (1958-1959) (see Stone et al., 2006); W. Barnhardt, B. Andrews, S. Ackerman, W. Baldwin and C. Hein (2004-2007) (see Barnhardt et al., 2009). Digital cartography and layout by M. Fernandez and C. Hein (2012-2013).

QUADRANGLE LOCATION

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ONSHORE-OFFSHORE SURFICIAL GEOLOGIC MAP OF THE NEWBURYPORT EAST AND NORTHERN HALF OF THE IPSWICH QUADRANGLES, MASSACHUSETTS SHEET 1: SURFICIAL GEOLOGY By Christopher J. Hein, Duncan M. FitzGerald, Walter A. Barnhardt and Byron D. Stone 2013

Saltmarsh Deposits (Qs Holocene) - fine-grained organic matter, and fibric and hemic peat interbedded with fine sand, silt and clay. Sediments are typically greater than 30% organic and 1-6 m thick. In the major estuaries (Plum Island Sound and the Merrimack River Estuary), saltmarsh deposits overlie estuarine and/or freshwater marsh deposits. These deposits are generally found in environments of low wave energy in backbarrier areas.

Thin Till Deposits (Qtt Late Pleistocene) - nonsorted, nonstratified matrix of sand, some silt, and little clay containing scattered gravel clasts and few large boulders; loose to moderately compact, generally sandy, commonly stony; predominantly till of the last (late Wisconsinan) glaciation. In offshore seismic-reflection profiles the top of this unit locally appears as a strong reflector that extends above thin till to the underlying bedrock reflector. In some places this unit is characterized by parabolic reflections in radargrams and seismic profiles, probably produced by scattered boulders. Till is mapped where it is generally less than 3-5 m thick including areas of shallow bedrock. Two till facies are present in some places: a looser, coarser-grained ablation facies, melted out from supraglacial position; and an underlying, more compact, finer-grained lodgement facies deposited subglacially. Both ablation and lodgement facies are sandy and stony, and are derived from coarse-grained crystalline rocks. Subsurface till overlies fresh, nonweathered bedrock and locally weathered rock.

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Dune Deposits (Qd Holocene) - mobile, well-sorted, fine and very-fine sand. Internal GPR reflections are commonly chaotic in nature. This unit is generally 1-10 m in thickness. The seaward edge of dunes is usually unvegetated and is mapped at the change in slope of the upper beach face above the storm high-tide line, which marks the toe of the dune. On the landward side of the unit, dune deposits are underlain by inactive estuarine and marsh deposits; on the seaward side they are underlain by beach deposits. Dune deposits are formed from eolian processes and may be vegetated with beach grass, beach pea, beach plum shrubs, coastal pine-oak-maple-cherry forest, or may be unvegetated. Beachface Deposits (Qb Holocene) - moderately sorted, fine to very-coarse sand, which is commonly flat-laminated. Coarser layers locally contain some granules and fine pebbles; finer layers contain very-fine sand and traces of silt. Surface beachface samples collected along the length of Plum Island indicate a general trend of increasing textural and mineralogical maturity away from the Merrimack River, reflecting winnowing and differential transportation of finer sand grains by wave action (FitzGerald et al., 1994). Beachface deposits are generally < 5 m thick and overlie inactive estuarine and saltmarsh deposits. This unit is dominated by sediment deposited along the shoreline by modern coastal processes (waves and currents) throughout the tidal cycle and is defined as the area between mean low water (MLW) and the dune toe. Textural variability is generally controlled by the texture and proximity of sediment sources (i.e., estuary mouth, eroding glacial deposits).

Drumlin Till Deposits (Qtd Pleistocene) - nonsorted, nonstratified matrix of sand, some silt, and little clay containing scattered pebbles, cobbles, and boulders in the shallow subsurface; at greater depths consists of compact, nonsorted matrix of silt, very fine sand, and some clay containing scattered small gravel clasts. In seismic-reflection profiles, the top of this unit appears as a strong, continuous reflector that extends smoothly down to the bedrock reflector. The upper part of this unit is characterized by parabolic reflections in radargrams and seismic profiles, probably from scattered boulders in the upper till. Profiles from deeper sections of this unit are devoid of internal reflectors. Qtd is mapped in areas where total till deposit is greater than 3-5 m thick, chiefly in smooth, elongate, northwest-southeast trending drumlin landforms in which till thickness commonly exceeds 30 m (maximum recorded thickness is 70 m; Stone et al., 2006). Late Wisconsinan till forms the shallow surface deposit known as the stratigraphic upper till; the underlying lower till of probable Illinoian age constitutes the bulk of the material in these areas. An oxidized, jointed, and iron-stained zone, the lower part of an interglacial soil profile, is generally present in the upper part of the lower till. Qtd commonly occurs in subaerially or subaqueously exposed clusters of drumlins (e.g., southern end of Plum Island). Qtd directly overlies bedrock.

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Ebb Tidal Delta Deposits (Qed Holocene) - medium to coarse sand with minor gravel, often with active surface bedforms and exhibiting herringbone ripple stratification and relict bedforms in cross section. This unit ranges from < 1 m to about 4 m thick and overlies Estuarine (Qe) and Sand Sheet (Qss) deposits proximal to modern inlets. Located on seaward side of tidal inlets, these deposits are actively reworked by tidal and wave-generated currents and smoothed by wave action along the seaward edge. This unit grades laterally into the Beachface (Qb) and Sand Sheet (Qss) units. Tidal Inlet Channel Deposits (Qtc Holocene) - thinly laminated medium to coarse sand interbedded with coarse sand, medium sand, fine sand, organic laminae and thin lenses of heavy minerals; the base of this unit is composed of coarse sand and granules. These deposits are thinner than Fluvial Channel (Qfc) deposits and were deposited by the daily and storm tidal currents that exchange water and sediment between estuarine water bodies and the coastal ocean. Inlet channel deposits receive only minor fluvial sediments from small local rivers draining into estuarine environments and then through inlets during ebbing tides (e.g., the Parker, Ipswich, and Rowley Rivers that drain through the modern Parker Inlet channel at the southern end of Plum Island). Under central Plum Island, GPR profiles reveal a relict tidal inlet at a depth of 2-7 m below MSL (the paleoParker Inlet) in the form of a U-shaped channel that can be divided into two complexes: a 3.5-m thick northern section dominated by southerly dipping reflections; and a southern complex of variable thickness (Hein et al., 2012).

Bedrock outcrops and areas of abundant outcrop or shallow bedrock (Precambrian to Paleozoic) - Precambrian granites and granitic gneisses, Precambrian to Ordovician metasedimentary rocks intruded by Ordovician to Devonian granites, and Silurodevonian metasedimentary igneous rocks. Solid color shows extent of individual bedrock outcrops on land. Line pattern indicates areas of shallow bedrock, areas where small outcrops are too numerous to map individually on land, or interpreted from seafloor mapping and seismic-reflection profiles offshore. In areas shown as shallow bedrock, surficial materials are < 3 m thick. EXPLANATION OF MAP SYMBOLS -20

Bathymetric contour (depth below msl in meters)

Estuarine Deposits (Qe Holocene) - largely massive, moderately well-sorted fine sand and silt, dominated by quartz but locally containing > 10% muscovite mica and traces of organic materials. This unit grades to coarse sand and fine gravel in relict and modern active tidal channels and along the landward side of the barrier complex. It dominantly contains weak, horizontal to sub-horizontal (< 2o dip), discontinuous reflections in GPR profiles, with little discernable pattern. Chaotic reflections are also common, as are smallscale (< 1 m, horizontally) truncations of individual clinoforms. Locally, reflections pinch out laterally and are generally indistinct, indicating a fairly homogeneous composition of fine sand-sized sediment with minimal internal structure. However, sets of sigmoidshaped radar-reflection sets are often visible in upper sections of this unit and correspond to medium to coarse sand. These are interpreted as former migrating estuarine tidal channels. This unit generally overlies Glaciomarine Silt-Clay (Qgsc) or Thin Till (Qtt) deposits and is as much as 20 m thick. Comprising backbarrier lagoonal sediments, tidalchannel sequences, and flood-tidal deltas, this unit is located underlying, and landward of, the modern barriers. This unit was generally sourced from offshore sediments during the Holocene transgression, with minor contributions from local rivers and upland deposits (i.e., bluff erosion, small streams with local drainages, etc.). Coarser units were generally deposited by overwash and inlet breaching during the landward migration of the barrier system; finer sediments were transported by tidal forces to the backbarrier through inlets and were reworked by tidal and locally wind-generated waves and currents. Fluvial Channel Deposits (Qfc Late Pleistocene-Holocene) - sorted and stratified medium to coarse sands with local gravel-rich and mud-rich layers. Channel deposits, < 1 to about 3 m thick, underlie the active channel of the Merrimack River and overlie bedrock, till, or local estuarine deposits or glaciomarine silt-clay. Channel deposits extend beyond the Merrimack Inlet to a tidal channel carved into the Ebb Tidal Delta (Qed) deposits. Buried Fluvial Channel Deposits extend under northern Plum Island and southern Salisbury Beach; they are also found under central Plum Island, extending in a dendritic channel pattern offshore. Under central Plum Island, GPR profiles reveal that this unit is eroded into underlying Glaciomarine silt-clay (Qgsc) deposits at depths of 1113 m below MSL. A sediment core revealed that this unit is composed of a 5-cm thick layer of mixed coarse sand to large pebbles with interstitial fine sand overlain by a 25-cm thick sequence of small, sub-angular pebbles in a matrix of very coarse sand and granules that fines upward to medium sand. This unit grades upward into nearly 7 m of Estuarine (Qe) deposits. Buried sections of this unit were deposited dominantly by fluvial systems at lower stands of sea level, though locally with some tidal influence.

Sediment cores

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Sediment samples Foresets bearing and dip Contact, location known Contact, location approximate Contact, location approximate and covered Contact, gradational

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Crossection Offshore buried fluvial channel deposits

Authors Affiliations Christopher J. Hein, Department of Physical Sciences, Virginia Institute of Marine Science, College of William and Mary 1357 Greate Road, Gloucester Point, VA 23062 Ph: 804-684-7533, hein@vims.edu Duncan M. FitzGerald, Department of Earth and Environment, Boston University 675 Commonwealth Avenue, Boston, MA 02215 Ph: 617-353-2530, dunc@bu.edu Walter A. Barnhardt, U.S. Geological Survey 384 Woods Hole Road, Woods Hole, MA 02543 Ph: 508-457-2355, wbarnhardt@usgs.gov Byron D. Stone, U.S. Geological Survey 101 Pitkin Street., East Hartford, CT, 06108 Ph: 860-291-6755, bdstone@usgs.gov

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Artificial Fill (Qf Anthropocene) - earth and anthropogenic materials that have been artificially emplaced, primarily in coastal structures (jetties), highway and railroad embankments, and infilled wetlands.

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Glaciomarine Silt-Clay Deposits (Qgsc Late Pleistocene) - silty clay, fine sand, and some fine gravel, containing dropstone gravel clasts, present as a widespread deposit draped over till and bedrock onshore and offshore throughout region. Where exposed or cores, it is nonfossiliferous and is highly compacted and dewatered. Onshore, these deposits consist of sandy sediments in the upper few meters, overlying thicker silty clay sediments (Sammel, 1963). The uppermost section (0.5-2 m) of Qgsc in cores through Plum Island is nonuniform and typically composed of indistinctly laminated and oxidized clay with some lenses of laminated organic-rich clay. Fine to very-fine sand, massive and laminated, is commonly present at the surface and grades downward into interbedded very-fine sand, silt, and silty clay. Lowest sections of silty clay and clay are massive and thinly laminated. The silty clay sediments consist of stratified to nearly homogeneous dark gray silty clay and clayey silt. Thin to thick (< 10 cm to > 1 m) lenses and beds of fine sand occur as interbedded units within the silty clay. Thin beds in these sandy units are planar or ripplelaminated, with silty draped laminae over ripples. The lower gray clay is weathered to reddish brown to yellow brown to depths of 1.5-4 m from the surface. Landward-most sections contain an upper part of Qgsc that is predominantly sand, locally containing minor fine gravel (Stone et al., 2004). Lower (glacier-grounding-line proximal) and upper (grounding-line distal) parts of unit Qgsc units (Belknap and Shipp, 1991) can be differentiated in some offshore seismic profiles. Lower Qgsc (seismic facies unit GM-M of Belknap and Shipp [1991]) sections appear faintly acoustically layered to massive (acoustically transparent) in seismic-reflection profiles; Upper Qgsc (seismic facies units GM-D and GM-P of Belknap and Shipp [1991]) sections reveal extensive acoustic stratification and lamination with a sharp upper bounding surface. In GPR profiles, Qgsc contains very few horizontal, discontinuous internal reflections and is dominated by data noise. Total unit thickness ranges from a thin drape (< 1 m) to > 25 m. Qgsc intertongues with Glaciomarine Deltaic and Fan (Qgdf) deposits beneath distal foreslopes onshore and is observed in seismic-reflection profiles as draped over Thin Till (Qtt) deposits and bedrock offshore. It was deposited during the regional sea-level highstand in environments of low wave energy along the coast and in river estuaries. Qgsc is correlated with the Presumpscot Formation of coastal Maine and New Hampshire (Bloom, 1963; Thompson and Borns, 1985).

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Glaciomarine Deltaic and Fan Deposits (Qgdf Late Pleistocene) - sorted and stratified gravel, sand, silt, and minor clay deposited by flowing meltwater in glacial deltas and submarine fans during retreat of the last ice sheet. Each delta or fan, evident on the map as a constructional topographic unit (a "morphosequence" of Koteff and Pessl [1981]) consists of a proximal part (head) deposited along the ice margin and a distal part deposited farther away from the ice. Both grain size and ice-melt collapse deformation of beds decrease from the proximal to the distal part of each morphosequence. The 24-28 m above MSL surface of the delta fluvial plain at Oak Hill Cemetery appears to be collapsed; the delta extends westward to delta plains at 28-31 m above MSL, which were graded to paleo-sea level, 28-29 m above MSL. Deltaic deposits have glacial-stream topset beds, 0.6-2 m thick, composed of sand and gravel and pebbly sand facies (Stone et al., 2004; Stone et al., 2005). Delta foresets here are probably sandy foreset facies that dip less than 25o. The sandy delta bottomset facies likely lies at the base. Total thickness of deltaic sediments is ca. 24 m. Deltaic deposits southwest of Rowley at 15 m above MSL extend westward to delta plains at 18 m above MSL; the plains at Rowley are probably underlain by thin coastal deposits that truncate the underlying deltaic sediments. Glaciomarine fan deposits ("kame" and "marine/estuarine" deposits of Sammel [1963]) preserve ice-contact slopes from 0 m to 18 m above MSL, and have hummocky top surfaces. Fans contain sand and gravel and sandy foreset and bottomset facies, and minor till, flowtill, and finegrained marine-bottom sediments. Deltas and fans overlie Till (Qtt, Qtd), bedrock, and Glaciomarine Silt-Clay (Qgsc) deposits.

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EXPLANATION OF COMBINED ONSHORE-OFFSHORE MAP UNITS

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Regressive Shoreline Deposits (Qrs Late Pleistocene) - sand and gravel in coastal deposits (barrier beaches, spits, regressive fluvial deltas) located above modern MSL. Pebbly sand forms a narrow ridge that slopes from 6 m to 19 m beneath High Road south of Rolfe Lane, and which are inferred to be a river-mouth bar or eroded delta. Other sandy deposits at 15-19 m above MSL at Newbury Old Town and south of the Parker River are local beach and spit deposits related to falling, post-glacial higher-than-present sea level. Coarse sand deposits also compose the barrier ridge at 12-21 m above MSL beneath Lafayette Road. Formed by coastal processes (waves, tides, wind) during falling sea level following the highstand. These deposits are distinguished from RegressiveTransgressive Shoreline (Qsrt) deposits in that they have not been modified by the subsequent sea-level transgression.

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Surficial materials in this region are dominantly unconsolidated deposits of glacial, alluvial, colluvial, eolian, marsh and swamp, estuarine, and shallow continental-shelf origin. Deposits of glacial origin are subdivided on the basis of grain size and sedimentary characteristics, origin, and age. Till deposits consist of unsorted sediments deposited from ice. The lower part of the Merrimack River valley contains extensive coarse sand to gravelly glacial drift and several glaciomarine deltas that formed following the Wisconsin sea-level highstand of about 17 ka (Stone and Peper, 1982; Edwards, 1988; Oldale et al., 1993). Rapid isostatic rebound resulted in sea-level fall and the deposition of regressive shoreline deposits and, in more distal reaches, a fluvial-marine delta. Sea-level lowstand was reached at 13-14 ka, at an elevation of ca. 45 m below MSL, as estimated by the change in bathymetric slope. The subsequent Holocene transgression progressed relatively rapidly and episodically for the first several thousand years (Oldale et al., 1993), during which time an unknown portion of the regressive and lowstand Pleistocene deposits were truncated and reworked by shoreline processes, forming a transgressive unconformity that extends from the depth of the lowstand to the modern shoreline. Sediment winnowed from the upper Pleistocene units during this transgression combined with material delivered directly by the Merrimack River along with minor contributions from local small rivers (Parker, Ipswich and Rowley Rivers) and the in situ erosion of glacial deposits (e.g., drumlins) to form broad sand shoals and a mobile nearshore sand sheet that discontinuously overlies the transgressive unconformity (Barnhardt et al., 2009). By ca. 4-5 ka, relative sea-level rise had slowed to ca. 0.6 ± 0.1 m yr-1 (Donnelly, 2006) and the shoreline reached its modern position by approximately 3-4 ka, at which time local barriers (Salisbury, Plum Island and Castle Neck) stabilized in their modern positions (Hein et al., 2012). These barriers have since aggraded, elongated and prograded as additional sediment has been delivered dominantly from the Merrimack River and shallow shelf.

Qmsc

Fluvial Terrace Deposits (Qft Late Pleistocene) - sand and gravel in inset fluvial terrace deposits. Fluvial sand and gravel facies, and pebbly sand facies (Stone et al., 2004; Stone et al., 2005), 1-4 m thick, underlie river-terrace surfaces at 3-16 m above MSL on the south side of the Merrimack River. Fluvial cross-bedded pebbly sand, 3-6 m thick, overlies Glaciomarine Silt-Clay (Qgsc) deposits beneath the terrace in the southwest corner of the map that slopes from 12-15 m above MSL (Stone et al., 2004).

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Geochronometry was provided by published dates for this region supplemented with additional radiocarbon analysis of 12 mollusk, wood, and peat samples collected from the Plum Island barrier lithosome. These samples were processed by Beta Analytic Inc. (Miami, FL, USA) and the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at Woods Hole Oceanographic Institution (Woods Hole, MA, USA).

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Lowstand Delta Deposits (Qdl Late Pleistocene) - fine to coarse, stratified sand and silt deposits. In seismic-reflection profiles, deposits are highly stratified with distinct, seawarddipping internal delta-foreset reflections. This unit can be divided into three sub-sections in seismic profiles (section A-A'): 1) the proximal section contains medium to coarse sand in high-angle delta foreset strata that appear as intermediate-angle (2-10o), concave-up, seaward-dipping foresets; 2) the intermediate unit contains fine, well-sorted sand and silt in gradually seaward-shallowing delta-foreset strata; 3) the distal subunit is dominated by gently-sloping delta bottomset beds that directly overlie Glaciomarine Silt-Clay (Qgsc) deposits and are covered by Offshore Marine (Qmsc) deposits. The upper contact of the proximal and intermediate sub-units is defined by the transgressive unconformity and it is discontinuously overlain by Sand Sheet (Qss) deposits. The approximate, partiallycovered contact between Qdl and the Regressive-Transgressive Shoreline (Qsrt) deposits shown on the map is determined from the proximal extent of delta foresets observed in seismic records. Qdl forms a broad delta that parallels the present coast and is ca. 20 km long, 4-7 km wide and up to 20 m in thickness (Oldale et al., 1993). It was deposited during the late stages of sea-level fall and during the 13-14 ka lowstand.

Qdl

Onshore map units (Sammel, 1963; Stone et al., 2006) were mapped from natural and artificial exposures and excavations, subsurface drilling data, and morphologic expression, with additional detail provided for Plum Island and Salisbury barrier systems through the collection of approximately 18 km of ground penetrating radar (GPR) data along both north-south (shore-parallel) and east-west (shore-normal) transects. All GPR data were post-processed (site-specific data filtering, variable-velocity migration, gain control) and time-depth converted using a combination of Radan (GSSI), RadExplorer (DECO-Geophysical Co. Ltd.), and GPR-Slice (Geophysical Archaeometry Laboratory) software packages. These geophysical data were supplemented with a series of vibracores, direct-push cores (Geoprobe Model 54DT), and auger drill cores (truckmounted Mobile B-40 drill rig). Additional subsurface data are from McIntire and Morgan (1964), McCormick (1969), Hartwell (1970), Rhodes (1973), Costas and FitzGerald (2011) and Hein et al. (2012).

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deposits that accumulated as sea level fell during the late-Pleistocene regression. Sediments in the uppermost part of this unit were derived from local shallow shoreface erosion and winnowing during the subsequent early-Holocene transgression. The contact (unknown depth) between these lower and uppermost sections is the transgressive unconformity. Continued erosion by storms and shelf currents has resulted in further winnowing of the uppermost part of this unit, contributing sediment to the overlying Sand Sheet (Qss) deposits.

Offshore data sources include high-resolution marine geophysical data, including swath bathymetry, acoustic-backscatter-intensity data and high-resolution seismicreflection data (Barnhardt et al., 2009). Additional shallow offshore surveys were completed using a surface-towed boomer seismic-reflection system and shallow-towed sidescan sonar. Ninety surficial-sediment grab samples were collected in September 2005 (79 samples; Barnhardt et al., 2009) and September 2006 (11 samples) using the U.S. Geological Survey's (USGS) SEABed Observation and Sampling System (SEABOSS; Valentine et al., 2000). Twenty samples were collected by L. Ward (University of New Hampshire) in August 2005 using a box-core grab sampler. Three additional samples were collected with an independent van Veen sampler in February and May of 2009. All sediment samples collected with the SEABOSS were analyzed using standard dry and wet sieving techniques (Folk and Ward, 1957) and a Beckman Coulter Multisizer 3 for fine fractions (Poppe et al., 2000). All other samples were analyzed using standard dry and wet sieving techniques; when applicable, fines (< 4 phi) were further analyzed on a Laser Diffraction Particle Size Analyzer at Wellesley College.

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Massachusetts Geological Survey, Geologic Map, GM 13-01 Onshore-Offshore Surficial Geologic Map of the Newburyport East and Northern Half of the Ipswich Quadrangles, Massachusetts, Sheet 1 of 3 2013

Qsrt

Transgressive Sand Sheet Deposits (Qss Late Pleistocene-Holocene) - relatively wellsorted fine to medium sand with minor quantities of silt and gravel. Ranges in thickness from < 1 m to ca. 9-10 m. The thickest deposits are located along the shoreface and offshore of southern Plum Island and Castle Neck (Barnhardt et al., 2009). Extends from approximately the postglacial lowstand shoreline (ca. -45 m contour) to MLW where it grades laterally into Beachface (Qb) deposits. Proximal to modern inlets, this unit grades into adjacent and overlying Ebb Tidal Delta (Qed) deposits. This is a time-transgressive unit, formed both from direct contributions from local rivers (predominantly the Merrimack River) and as a product of the reworking and winnowing of older transgressive, regressive, and lowstand deposits during the Holocene marine transgression. It likely also contains smaller contributions from remnants of either past barriers or intertidal to supratidal sand shoals deposited along the shoreline during the transgression. In its modern form, this unit undergoes varying degrees of reworking in relation to proximity to the modern shoreline. Along the shoreface, this unit demonstrates significant textural diversity over short spatial (< 100 m) and temporal (< 1 year) scales and is actively reworked by both tidal currents and storm waves. Further offshore, Qss is likely only impacted by rare, high energy events (dominantly, northeast storms). Here, this unit is dominated by low-amplitude (0.3-3.5 m), long-wavelength (150-900 m) bedforms that demonstrate a variety of orientations. These indicate reworking of the upper portion of the unit during high-energy events (Hein et al., 2007). To the northeast, in water depths of 35-50 m, these bedforms are oriented toward the northwest and have an average wavelength of 500 m and height of 1.9 m. Further south, in similar depths, bedforms are oriented toward to the north to north-northwest and have lower amplitudes (ca. 1.0 m) but similar wavelengths. Inshore of these features, the bedforms become more closely spaced (250 m), exhibit a west-southwest to west to west-northwest orientation, and have a mean amplitude of ca. 1.0 m. On a finer scale, the entire Qss unit is covered by three-dimensional cuspate (wave-formed) ripples with wavelengths of ca. 20 cm.

Comments to Map Users A geologic map displays information on the distribution, nature, orientation and age relationships of rock and surficial deposits and the occurrence of structural features. Geologic and fault contacts are irregular surfaces that form boundaries between different types or ages of units. Data depicted on this geologic map are based on field geologic mapping, compilation of published and unpublished work, geophysical surveys, borehole logs and descriptions and physical characterization of borehole and grab samples. Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given contact on to a topographic base map; therefore, the accuracy of contact locations depends on the scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause misunderstanding in the detail of mapping and may result in erroneous interpretations. Site specific conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent development may not be shown. We recommend reading Reading Maps with a Critical Eye: Becoming an Informed Map Reader by the Maine Geological Survey to make the best use of a geologic map. (http://www.maine.gov/doc/nrimc/mgs/mapuse/informed/informed.htm) Completion of this map was supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award Nos. G09AC00216 and G10AC00179. The crew of the F/V Venture (Gloucester, MA), Dann Blackwood, Emily Carruthers, Gene Cobbs Jr., Nicholas Cohn, Mary Ellison and Allen Gontz are acknowledged for their contributions in the field and lab. Finally, Wayne Newell, Joe Kelley, Ralph Lewis and Janet Stone are acknowledged for their careful reviews that have greatly enhanced the quality of this map.

Offshore Coarse Deposits (Qoc Late Pleistocene-Holocene) - dominantly shell-rich sand, but locally contains significant (> 25%) lithic gravel. This unit is present in sparse localities in the offshore area as a talus apron surrounding Bedrock and Till (Qtd) outcrops. This minor unit is interpreted as an erosional lag deposit from bedrock and boulders exposed on the sea floor.

The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government, the Commonwealth of Massachusetts, the Commonwealth of Virginia, the Massachusetts Geological Survey, Boston University, the Virginia Institute of Marine Science, or the University of Massachusetts.

Offshore Marine Deposits (Qm Late Pleistocene-Holocene) - fine sand to clay with variable textures. Shallower sections (ca. 40-50-m depth) grade into the seaward edge of the foreset beds of the Lowstand Delta (unit Qdl) and are sand-dominated with minor silt and clay (< 10%); intermediate sections (ca. 50-70-m depth) are mud-dominated with significant (> 10%) sand fractions; the finest fractions contain > 95% silt and clay and are generally found offshore of the 70-m depth contour and grade downward into the upper, distal sections of the Glaciomarine Silt-Clay (Qgsc) deposits. This unit is finely laminated in high-resolution seismic data. Its thickness increases in a seaward direction, exceeding 20 m in deep basins beyond the seaward edge of the Lowstand Delta (Qdl) deposits.

Citation: Hein, Christopher J., FitzGerald, Duncan M., Barnhardt, Walter A., and Stone, Byron D. 2013. Onshore-offshore surficial geologic map of the Newburyport East and northern half of the Ipswich Quadrangles, Massachusetts. Massachusetts Geological Survey, Geologic Map GM 13-01.

Regressive-Transgressive Shoreline Deposits (Qsrt Late Pleistocene-Holocene) continuous, 4-15-m thick deposit of unknown sediment type, capped by surficial lag of medium-coarse sand and granules, which generate high back scatter in sidescan-sonar data. Deposits are discontinuously overlain by finer Sand Sheet (Qss) deposits. The surface of the unit is fully exposed in an 8-10-km wide (perpendicular to the shore) and 16-km long tract east of the modern Merrimack River. Here, shore-perpendicular seismic-reflection profiles show a mostly transparent unit exhibiting little structure lying above Glaciomarine Silt-Clay (Qgsc) deposits. Along its seaward extent, it extends along and partially overlies proximal Lowstand Delta (Qdl) deposits. Sediments in the lower part of unit Qsrt are interpreted as wave-smoothed, offlapping, regressive shoreline

Scale 1:24,000. 3 sheets and digital product: Adobe PDF and ESRI ArcGIS database. This map was produced on request directly from digital files (PDF format) on an electronic plotter. This product supercedes Massachusetts Geological Survey OFR 10-01 and OFR 11-01. A digital copy of this map (PDF format), including GIS datalayers, is available at www.geo.umass.edu/stategeologist