NEIGC 2014 Hein & Stone Field Trip Guide
Post on 05-Apr-2016
DESCRIPTIONField trip guide to the lower Merrimack River Valley and Plum Island by Christopher Hein and Byron Stone. This field trip was part of the 106th meeting of the New England Intercollegiate Geological Conference (NEIGC), held at Wellesley College on October 10-13, 2014.
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ICE, WATER, AND WIND: A SOURCE-TO-SINK VIEW OF THE GLACIAL, PARAGLACIAL AND COASTAL SEDIMENTS AND
PROCESSES THAT HAVE SHAPED NORTHEASTERN MASSACHUSETTS
Christopher J. Hein1, Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA 23062
Byron D. Stone2, Eastern Geology and Paleoclimate Science Center, U.S. Geological Survey, 101 Pitkin St., East Hartford, CT 06108
Email addresses: email@example.com, firstname.lastname@example.org
Modern geologic maps have revealed the detailed extent of glacial, proglacial-meltwater, paraglacial, and postglacial deposits across the Merrimack Embayment area of northeastern Massachusetts (Sammel, 1963; Oldale, 1964; Cuppels, 1969; Stone et al., 2006; Hein et al., 2013). Recent offshore mapping studies have located critical evidence of submerged coastal features, which track the level and age of the late Wisconsinan postglacial marine regression and latest transgression (Oldale and Wommack, 1987; Oldale et al., 1993; Barnhardt et al., 2009; Hein et al., 2012, 2013, 2014). This trip will focus on the glacial, proglacial, paraglacial, and postglacial coastal deposits associated with the regressive and transgressive marine cycles following the retreat of the Laurentide Ice Sheet from northeastern Massachusetts. Relative sea level during deglaciation of this region reached a maximum of about 31 m above the present level. Glacial, fluvial and coastal processes resulted in the shaping of drumlins, deposition of glacial deltas and draped glaciomarine clay, and development of regressive shoreline features and fluvial terraces, all of which dominate the modern onshore landscape. Many of these deposits have been mapped continuously across the modern shoreline (Hein et. al., 2013) and into the submerged environment offshore of the barrier islands. Detailed mapping and stratigraphic analysis extend beyond the 13,000-year-old lowstand shoreline located 45 m below modern mean sea level (MSL). Reworking of these glacial and post-glacial sedimentary deposits by coastal processes during the most recent and ongoing marine transgression has culminated in construction of the massive barriers, beaches, marshes and dunes of the modern coastal zone. This trip will trace the shoreline from its highstand position at Haverhill, Massachusetts, down the Merrimack River valley, and out to Plum Island, tracking the various processes and deposits responsible for building the modern landscape. We will discuss how Quaternary glacial and marine geologists can come together to map continuous surficial geology across the modern shoreline.
Physiography of the lower Merrimack Valley / Merrimack Embayment This field trip covers a region in the lower Merrimack River valley downstream from Haverhill, Massachusetts,
to its mouth in the area known as the Merrimack Embayment, which extends from Cape Ann to north of the New Hampshire state line (Figs. 1, 2, 3). The Merrimack River basin, 13,000 km2 in area, heads in the White Mountains of New Hampshire and extends 180 km to the ocean (Fig. 2a). It drains areas dominated by granitic plutons and other crystalline rocks that have been eroded to produce quartzose, sandy glacial deposits (Fig. 2b). The lower part of the river flows through coarse, sandy glacial deltas that formed along the retreating ice front in the glaciomarine highstand sea and in subsequent glacial lakes up valley (Stone et al., 2006). In its coastal reach, the Merrimack largely flows over till and bedrock before draining into a mixed-energy, tide-dominated, inlet-influenced (FitzGerald and van Heteren, 1999) embayment in the western Gulf of Maine. The Merrimack Embayment coastal barrier system consists of a 34-km long series of barriers, tidal inlets, estuaries and backbarrier sand flats, channels and marshes (Fig. 3). This is the longest such barrier system in the Gulf of Maine. Individual barriers (Seabrook Beach, Salisbury Beach, Plum Island, Castle Neck and Coffins Beach) are 213 km long, generally less than 1 km wide and are backed primarily by marsh and tidal creeks that typically expand to small bays near inlets (Smith and FitzGerald, 1994). Barriers contain abundant, vegetated parabolic dunes that reach as much as 20 m in elevation. These are best developed along central and southern Plum Island and Castle Neck, reflecting abundant quartz sand associated with high rates of longshore transport.
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Figure 1. Simplified surficial geologic map and field trip stop locations 1-9 (circled numbers); AP-assembly point. White labels are geologic map units: d-drumlins and thick till, ld-glaciolacustine deltaic deposit, md-glaciomarine deltaic deposits, mc-glaciomarine clay deposits, r-regressive deposits, b-post-glacial beach and dune deposits (data from Stone et al., 2006). Detailed Holocene coastal deposits are shown in Fig. 3.
Bedrock geology and physiography The broad physiographic features of this area are related to the resistance of the complexly deformed,
metamorphosed bedrock units (Zen, 1983), which have reacted variably to chemical and physical erosive processes that shaped the bedrock surface during the late Tertiary and Quaternary periods. The local bedrock falls into several lithotectonic terranes (Hibbard et al., 2006): (1) Avalonia, extending north from Cape Cod, is dominated by Ediacaran granitoid rocks but intruded by Silurian Cape Ann Granite (Thompson and Ramezani, 2008) near the mouth of the Merrimack River; (2) the Ganderian Nashoba belt, comprising metamorphosed island arc assemblages and Ordovician to Devonian granites, that was welded onto North America during the Paleozoic (Hepburn et al., this volume); and (3) the Merrimack trough composed of Upper Ordovician to Silurian marine clastic rocks and younger Paleozoic plutons. North of the Clinton-Newbury fault (located 1.3 km south of the mouth of the Merrimack River), the New HampshireMaine Sequence is composed largely of Silurian metasedimentary turbiditic rocks (Berwick and Eliot Formations) in the western upland area, and Silurian intrusive igneous rocks of the Newburyport Complex
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along the river and adjacent uplands (Stop 3). Offshore, high-resolution bathymetry shows the submerged outcrop pattern of these granitic rocks to the east (Hein et al., 2013). South of the fault, the regional Avalon Belt includes Silurian and Devonian mafic metasedimentary rocks of the Newbury Basin, which underlies the central eastern lowland of the embayment. These contain red siltstone and sandstone that compose till-stone marker lithologies in local drumlins and till of Plum Island (STOP 5; Fig. 3). Red sedimentary rocks are bordered by hydrothermally altered Precambrian granodiorite to the south (Dennen, 1991). Silurian granite and local Devonian-Silurian felsic volcanics also are included in the Avalon Belt in the map area. A sloping seafloor plain extends eastward from the central lowland to depths below 85 m west and northwest of Jeffreys Ledge, which is a glacially modified submerged platform composed of coastal plain strata topped by thick glacial deposits.
Across the eastern area, bedrock hills lying below the glaciomarine highstand level (2832 m) are covered by only patchy thin till or thin glaciomarine silt-clay, the result of coastal erosion during the postglacial marine regression. Similar features extend offshore on the ocean bottom. The rock outcrops contain linear and orthogonal topographic patterns that reflect steeply dipping joint sets, faults, sheeting joints, foliation, and bedding, all of which facet and shape the extent of outcrops. Structural controls related to Palaeozoic tectonism also have shaped the coastal and nearshore regions of the western Gulf of Maine. Fault and fold patterns control the configuration of individual coastal compartments, and the gradients of the coastal lowland and proximal continental shelf control accommodation available for backbarrier and offshore deposits. Major rivers, such as the Merrimack, drain to the south-southeast, generally across the structural grain of bedrock, except locally where erosion of weaker rocks by both fluvial and glacial scouring controls local strike-aligned reaches.
Thick till deposits in drumlins stand in bold relief 2050 m above the surrounding bedrock surface (Fig. 1). The highest points of the landscape are underlain by thick till in drumlins: Powwow Hill, Amesbury (101 m); Prospect Hill, Georgetown (81 m); and Turkey Hill, Ipswich (77 m). In the valleys of the uplands, and across the central lowland of the area, glaciodeltaic deposits, glaciomarine silt-clay, and recent alluvial, swamp, marsh, and coastal sediments form a fairly continuous blanket deposit further obscuring relief on top of the bedrock surface. The lower Merrimack River flows in a narrow, rock-lined reach, below the base of drumlins, incised through glacial sand and clay deposits. The Parker, Egypt, Rowley, and Ipswich rivers, which drain into the Plum Island Estuary (backbarrier of Plum Island) similarly are incised into the surficial deposits lowland. The extensive coastal spit and tidal marsh deposits of Salisbury Beach, Plum Island, Castle Neck, and Coffins and Wingaersheek Beach (Fig. 3) are products of the late Holocene marine transgression to present sea level. These are pinned to bedrock promontories and high glacial till deposits in drumlins. Tidal inlets typically are situated in drowned bedrock-controlled river valleys.
Figure 2. Maps of the Merrimack River drainage basin (modified from Hein et al., 2014). (a) Extent of the Merrimack River drainage basin. (b) The distribution of plutons and sandy glacial deposits in the Merrimack River drainage basin (modified from FitzGerald et al., 2005).
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The earliest surficial geologic mapping studies in the Merrimack Embayment quickly followed publication of the first U.S. Geological Survey 15 topographic maps in the 1880s (Shaler, 1889). J.W. Goldthwait (in Antevs, 1922) summarized decades of previous work on the glaciomarine and glacial-lake features of the region in his map of the recession of the last ice sheet from New England and New York. Detailed geologic mapping in the region showed the extent of glacial, meltwater, and postglacial deposits (Sammel, 1963; Oldale, 1964; Cuppels, 1969). These maps differentiated glaciomarine deltas and fans from other glacial-lake deltaic deposits at higher altitudes in the Merrimack River valley (Koteff, 1976; Koteff and Stone, 2000; Stone et. al, 2006). Regressive marine features were recognized in later mapping studies of the Merrimack area (Stone and Peper, 1982). Although sparse, macrofossils were recovered from glaciomarine clay in the Danvers brick pits (Sears, 1906), in the Ipswich quadrangle (Sammel, 1963), and in sandy beds beneath diamict sediment in Gloucester (Tarr and Woodworth, 1903). The rich assemblage of shells at West Lynn, Massachusetts, provided two radiocarbon dates from the same shell (Kaye and Barghoorn, 1964) having calibrated ages of 17,410 62 and 16,410 489 (Calib 6.1; Stuiver and Reimer, 1993), based on standard marine correction of 400 yrs. They could be 600 years younger if further correction for old marine carbon is warranted. Offshore, dated macrofossils have placed ages on the marine clay (Birch, 1990), beaches and spits (Oldale and OHara, 1980; Oldale et al., 1983; Oldale, 1985; Oldale et al., 1993), and the post-glacial marine lowstand delta (Oldale et. al., 1993). Correlative deposits in sediment cores (Birch, 1990) have a maximum calibrated age of 16,330 400.
In the Merrimack Embayment, Goldthwaits 1922 crustal uplift isobases show tilted glaciomarine features rising linearly from altitudes of 15 m just south of Eastern Point, Gloucester (Cape Ann), to 30 m just north of Amesbury. The upward-tilt direction of these features is toward N45oW, having a slope of 0.37 m/km. Goldthwait extended the high glaciomarine inundation through the extensive delta deposits clogging the lower Merrimack valley to Manchester, New Hampshire. The slope of his marine limit increases to 0.92 m/km in the area of southeastern New Hampshire along the same direction of tilt. I.B. Crosby and R.J. Lougee (1934) revisited the field localities of Shaler, Tarr, Goldthwait, Woodworth, Horner, and others, and leveled the altitudes of 39 shoreline features. From these points they derived an upward tilt direction of N27.5oW, similar to the results of Horner (1929). Their plot of delta surfaces from Gloucester to Salisbury has a slope of 0.40 m/km, increasing sharply at the New Hampshire state line. Crosby and Lougee carefully discussed the effects of timing and rates of crustal tilting and
Figure 3. Merrimack Embayment barrier chain and marine lowstand delta. (Modified from Hein et al., 2012). Lowstand shoreline and delta locations are derived from Hein et al. (2013).
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relative sea-level (RSL) rise on plots of tilted marine deltas. They presented diagrams of tilted and curved lines of the marine limit based on different combinations of these variables. They envisioned the probable stability of the crust during deglaciation of the region, known from studies of tilted glacial-lake shorelines, and attributed the increase in tilt of the glaciomarine shoreline to RSL rise. Similar results were obtained by Stone and Peper (1982), Koteff et.al. (1993). Stone et. al. (2004) reanalyzed the older and some new data for the Merrimack Embayment and constructed a new RSL curve based on the concepts of Kaye and Barghorn (1964): 1) global eustatic sea-level changes (Bard et al., 1996), 2) local field evidence of delayed crustal rebound tilt (Stone and Peper, 1982, Koteff et al., 1993), 3) calibrated radiocarbon ages, and 4) constraining ages from the New England varve chronology (Ridge, 2004). This locally constrained model presents a time interval 1115 ka during which RSL varied only 10 m, from -30 to -40 m. This interval is a product of the RSL calculation using eustatic sea-level rise and a simple half-life rebound tilt function.
Following on the studies of McIntire and Morgan (1964), Oldale et al. (1983) presented an initial empirical post-glacial RSL curve based on their discovery of a lowstand paleo-delta offshore of the Merrimack River. Oldale et al. (1993) and Hein et al. (2012, 2013) later refined these curves with the incorporation of new data from the shallow shelf of the Merrimack Embayment and from the marshes and estuarine sediments of Plum Island. A recent comparison of RSL reconstructions for northern Massachusetts and coastal Maine (Fig. 4; Hein et al., 2014) demonstrates notable differences likely derived from the dearth of data in northern Massachusetts between 13.5 and 8 ka. In the absence of additional data, incorporation of the approach of Stone et al. (2004), supported by dated curves in Maine (Barnhardt et al., 1995, Kelley et al., 2010, 2013) (see Fig. 4), may offer some significant improvement on this section of the curve and demonstrate more consistent trends across the western Gulf of Maine.
Figure 4. Comparison of Late Pleistocene and Holocene sea-level changes in northern Massachusetts and Maine. Northern Massachusetts RSL data are from Hein et al. (2012). Coastal Maine RSL data are from Kelley et al. (2010). Rheologic-eustatic curve derived from Stone et al. (2004).
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Early studies of the Merrimack Embayment barrier-island chain developed hypotheses regarding its formation based on sediment-core data and morphological considerations (Chute and Nichols, 1941; Rhodes, 1973; McIntire and Morgan, 1964; Boothroyd and FitzGerald, 1989). Later, offshore mapping studies located submerged marine and coastal deposits, which locate the level and age of the postglacial marine regression and latest transgression (Oldale, 1985; Oldale et al., 1993). For example, geophysical studies of the nearshore zone of the Merrimack Embayment revealed the existence of a lowstand paleodelta located approximately 67 km offshore and trending parallel to the present coast (Oldale et al., 1983; Edwards and Oldale, 1986; Edwards, 1988; Barnhardt et al., 2009; Hein et al., 2013). Together, the results of these coastal and nearshore mapping studies further constrained theories for the formation of these barrier systems, which place various sedimentological units in their chronostratigraphic context and treat the evolution of the barrier system with respect to a varying RSL (McIntire and Morgan, 1964; FitzGerald et al., 1994, Hein et. al, 2013). In these models, shoreline and deltaic sediments were deposited on the shoreface, directly overlying eroded glacial and glaciomarine deposits, during post-glacial, rebound-induced RSL fall. Following maximum RSL lowering, coastal processes during the RSL transgression reworked these formerly stranded deposits into beaches. Stratigraphic units beneath Plum Island show that an initial barrier formed at 6.3 ka and migrated landward to its present location, which it reached about 3 ka.
QUATERNARY STRATIGRAPHIC FRAMEWORK
Sedimentary deposits in the lower Merrimack Valley and the Merrimack Embayment have mixed glacial, proglacial, paraglacial and nonglacial origins. Recent surficial mapping efforts by Stone et al. (2006), Barnhardt et al. (2009) and Hein et al. (2013) show the distribution of bedrock outcrops, Illinoisan and Late Wisconsin glacial deposits, deposits related to marine highstand, regression and transgression, and post-glacial (on land) and post transgression (coastal zone, offshore) deposits. Mappable units shown in the field trip map and stratigraphic successions in exposures and seismic profiles (Hein et al., 2013) provide relative stratigraphic order among deposits in relation to glacial to nonglacial origins from about 17 ka to present. Fig. 5 is presented here in order to facilitate discussion of the Middle-Late Quaternary history of the field trip area. Selective age and event annotations shown on the diagram are derived from the regional array of radiocarbon dates (Kaye and Barghoorn, 1964; McIntire and Morgan, 1964; Redfield, 1967; Keene, 1971; Oldale et al., 1983; Stone and Borns, 1986; Collins, 1989; Oldale et al., 1993; Stone et. al., 2004; Donnelly, 2006; Kirwan et al., 2011; Hein et. al., 2012, 2013), the regional varve chronology (Ridge et al., 2012), regional Be dates (Balco et al., 2009), and the presently known relative physical stratigraphy of glacial ice lobes (Stone et. al., 2004), glacial lakes and synglacial sea ice-margin positions.
Drumlins and till: Products of Illinoian and Wisconsinan glacial flow (Glacial Period) The dominant glacial deposit throughout the lower Merrimack Valley and Merrimack Embayment is sandy till
of late Wisconsinan age, which drapes the bedrock surface and the surface of numerous drumlin landforms, which coincide with our present map unit (Stone et al., 2006) of thick till (Fig. 1). Drumlins in northeastern Massachusetts, like nearly all drumlins in southern New England, contain a core of compact Illinoisan till, smoothed by glacial movements during the last two glaciations (Schafer and Hartshorn, 1965; Stone, 1989; Newman et al., 1993; Stone et al., 2004). Drumlins in the field trip area typically are elliptical in plan, varying in length from 0.35 km to 1.65 km, and 0.20.8 km in width. Vertical heights of the summit point above the closed contour at the drumlin base and vary from < 15 m to 54 m. These are likely constraining values for the possible maximum thickness of till within the drumlins. The position of the summit point of drumlin axes is variable; the group is about equally divided between hills with crests in the stoss side and those with crests in the lee side. Some clustered drumlins are welded in composite forms in which individual drumlin crests are aligned in downstream, transverse, and en echelon oblique directions. The orientation of long axes of drumlins in the Merrimack Embayment describes a fan distribution from south-southeast trends in the southern and western parts of the area, to mixed trends that include strong easterly directions in the eastern part. The easterly trend probably is present in drumlins that are submerged along the coast (Sammel, 1963). Several such clusters of drumlins are identified offshore to a depth of at least 70 m, extending up to the present-day shoreline in southern Plum Island (Hein et al., 2013; STOP 5) and western Castle Neck. Many drumlins in the coastal zone have undergone two post-glacial marine regression-transgression cycles, resulting in extensive erosion and their occurrence on the shelf as rubble lag, visible at low tide at STOP 5.
Till stratigraphy in the drumlins consists of two tills, known regionally as the lower (older) Illinoisan till in the core, and the upper (younger) late Wisconsinan till at the surface (Fig. 6). At STOP 5 the superposed stratigraphy consists of a nonweathered lower till that extends down to the bedrock surface. This unit is characteristically
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compact, clayey silty sand with only 510 % gravel-sized rock particles. The volumetric content of stones larger than 5 cm in drumlin till is probably about 5 %. Boulders larger than 1 m are rare in large excavations. The drumlin-till matrix (particles less than 2 mm in diameter) contains 2255 weight % silt and clay. The proportion of clay (3.9 microns particle size) is 624 %. Very coarse and coarse sand make up < 10% of the till matrix (Crosby, 1891; as derived from data from Boston Harbor).
Figure 5. Schematic diagram of the pattern of sedimentation in the lower Merrimack River Valley and the Merrimack Embayment coastal zone during and following glaciation. Modified from Hein et al. (2014).
Figure 6. Drumlin till stratigraphy and weathered zone in lower till.
The top of the lower till is characterized by an oxidized zone, typically 39 m thick. Weathering effects are progressive upward through this zone: amount of leaching increases, pH values decline, dissolution of garnets increases, color values of matrix stain increase, degree and darkness of iron-manganese stain on joint faces increase, blocky structure increases and is more densely developed, and illuviated clay on vertical joints increases. Laboratory data showing alteration of clay minerals and iron-bearing minerals further define the weathering gradient. The weathered zone is the upper part of the C horizon of a probable well-developed soil. The depth and degree of this weathering zone indicate a long or intense period of weathering, probably during the Sangamon and succeeding early Wisconsin time prior to advance of the last ice sheet. The base of the weathered zone is subparallel to the surface of the landform, indicating soil genesis after glacial smoothing by Illinoian ice. The late Wisconsinan till deposits on drumlins are thin and overlie the truncated weathering profile in the drumlin till, indicating that erosion
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and shaping of the drumlins by the last ice sheet did little to reshape many of these drumlin forms. The strong easterly ice-flow directions, confirmed by fresh bedrock striations, were similar during deglaciation of the last two ice sheets. Offshore drumlins (Barnhardt et al., 2009; Hein et al., 2013) are identified based on their shape, size, orientations, occurrence in groups, bathymetric and backscatter characteristics and seismic characteristics. One offshore reveals that the Holocene transgression nearly eroded the entire drumlin core till, leaving the lower extent outlined by an apparent ramp of eroded till, probably the upper till with boulders.
Overlying Illinoian drumlin tills is a thin upper till composed of nonsorted, nonstratified matrix of sand, some silt, and little clay, containing scattered gravel clasts and few large boulders. It is loose to moderately compact, generally sandy and commonly stony. This is predominantly till of the last (late Wisconsinan) glaciation. 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.
Late Wisconsinan glaciomarine deposits (Proglacial Period) Coarse- and fine-grained Late Wisconsinan meltwater deposits blanket the lowland in the eastern part of the
field trip area, disconformably onlapping older bedrock, drumlins and till deposits. They are nearly ubiquitous offshore and are prominent along the coast and extend up the Merrimack River Valley to Haverhill, MA (Fig. 1).
Glaciomarine delta deposits. Glaciomarine deltas in the Merrimack Embayment and river valley are typical glaciodeltaic morphosequences, characterized by ice-contact slopes on their northerly proximal sides, and lobate distal foreslopes on their distal sides which merge tangentially with marine-bottom plains (Fig. 7b; STOP 4). Fluvial beds at the surface and underlying foreset strata have been observed in almost all of these deposits during the past 100 years, so that their identification as ice-marginal glaciomarine deltas is consistent with our present understanding of the marine inundation. The surface of each delta is a gently sloping glaciofluvial plain, underlain by delta-topset fluvial sand and gravel, and punctuated by kettle depressions in the proximal part of most deposits. Deltaic deposits vary from 7001700 m in width, and 2002000 m in length. In some examples, ice-channel ridge deposits extend north in a series of irregular linear forms. Local relief along the ice-contact backslope of these deltas varies from 8 m to 20 m. Recessional ice-margin position line symbols show the trend of ice-contact slopes generally to the northeast, but some are deflected around buttressing bedrock and drumlin hills in pocket basins. Aligned deltas and recessional ice-margin lines in Newburyport-Amesbury and up the Merrimack valley indicate recession of the ice margin toward the northwest. The positions and sizes of individual ice-marginal deltas in the Merrimack Valley demonstrate the successive positions and sizes of local depositional basins uncovered by wasting ice masses during systematic retreat of the stagnant ice zone in this lowland. Ice-contact deltas have distal plain elevations that reach up to 33 m in Salisbury. The general rise of these surfaces permits their differentiation from higher glacial-lake deposits along the western part of the area and further southwest along the western margin of the glaciomarine incursion (Cuppels, 1969; Oldale, 1964; Stone and Peper, 1982).
A generalized diagram of the internal structure of these deltaic deposits is shown in Fig. 7a. The contact at the base of topset strata and underlying inclined foresets is inferred to be coincident with sea level at the time of delta deposition. The level probably represents low-tide level, based on observations of modern glaciofluvial distributary channel scour-and fill processes during low-tide cycles. Topset deposits consist of glaciofluvial sedimentary facies
Figure 7. Glaciomarine delta stratigraphy. (a) Generalized model of an ice-contact glaciomarine delta. (b) Glaciomarine delta at Ipswich, MA; topset-foreset contact and collapse faults behind Harold Nielson.
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in horizontally stratified, alternating beds of planar- or cross-bedded gravel and sand. Gravel clasts are well rounded to subrounded; gravel clasts commonly are imbricated. The sand matrix in gravel beds is poorly sorted, medium to coarse sand. Individual sets of gravel planar beds or sets of cross beds are less than 1.5 m thick. All beds are bounded by erosional contacts; individual beds or sets of beds extend laterally as much as 15 m. In the general model, foreset strata show a progradational sequence from the proximal surface of the delta, but an upward aggradational sequence from marine fan to upper delta foresets also is possible (J.C. Boothroyd, pers. comm., 1992). Delta foreset beds have variable dips (1540) and are grouped in disconformable sets, commonly 210 m thick. Sets of foreset beds are planar tabular in proximal parts of deltas and trough-shaped in distal parts. Foreset strata contain planar, cross bedded, and ripple-laminated beds. Total thickness of foreset beds varies from 2 to > 15 m. Foreset facies generally intertongue with bottomset beds beneath the distal parts of each delta. Bottomset strata are subhorizontally layered and contain alternating sets of ripple and planar laminations composed of medium to fine sand, silt, and clay. Bottomset beds are grouped in conformable flat-lying sets or in disconformable trough-fill sets that dip less than 10. Bottomset beds commonly extend gradationally from the lobate depositional front of the delta to the marine-bottom deposits.
Glaciomarine fan deposits. Glaciomarine fans are ice-marginal morphosequences, smaller than nearby glacial deltas, and characterized by ice-contact slopes on their northerly proximal sides, and lobate distal foreslopes on their distal sides which merge tangentially with marine-bottom plains. These deposits do not attain the local level of the projected general rise of the deltas. Surfaces of fans are variable. Some show an irregular hummocky surface underlain by sandy and silty sediments. Others have a gently sloping surface plain, underlain by fluvial or marine littoral sand and gravel that disconformably overlies sandy and silty beds. Fan surfaces do not contain kettles or sharp ice-contact slopes or ice-channel feeder deposits. Some fans have bedrock outcrops that protrude through the deposits, indicating anchoring of the deposits around a local bedrock core or glacial-margin grounding point. Fan deposits vary from 2001100 m in width and 100500 m in length. Local relief along the ice-contact backslopes of these fans varies from 610 m. Recessional ice-margin position line symbols show the trend of ice-contact slopes generally to the northeast. Aligned fans and recessional ice-margin lines in Ipswich indicate recession of the ice margin toward the northwest.
Glaciomarine-bottom deposits. Finer-grained marine-bottom sand, silty sand, and silt-clay deposits form an extensive blanket across the area, onlapping the slopes of the older ice-contact deposits. Glaciomarine-bottom deposits extend continuously offshore (Oldale and Wommack, 1977) where they overlie till and older deposits on the bottom of the Gulf of Maine. They extend under Plum Island and its associated backbarrier marshes and are found up to the highstand shoreline in the Merrimack River Valley (+ 33 m), but are discontinuous on land. For example, this unit does not extend beneath the regressive terrace deposits at the Plum Island Airport (STOP 4). Marine-bottom deposits are reportedly 1823 m thick over the deepest parts of the underlying bedrock surface. This deposit is known locally and throughout New Hampshire and Maine as the Presumpscot Formation (Bloom, 1963; Thompson and Borns, 1985) and in the Massachusetts Bay area as the Boston Blue Clay (Kaye, 1961) (see trip B5). This unit ranges in thickness from a thin (< 1 m) drape to > 30 m (McIntire and Morgan, 1964; Rhodes, 1973; Stone et al., 2006; Barnhardt et al., 2009). It is composed of Lower (glacier-grounding-line proximal) and Upper (grounding-line distal) glaciomarine units (Belknap and Shipp, 1991). Its strongly micaceous composition suggests derivation from glacial erosion of metamorphic rocks (Kelley, 1989). 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. Lower silty clay and clay is massive and thinly laminated (Sammel, 1963; Oldale, 1964; Stone et al., 2006). 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 ripple-laminated, with silty draped laminae over ripples. Solitary pebble and cobble dropstones reveal the presence of glacial, coastal, or river ice during deposition of most of the unit. In cores, the uppermost section of tends to be oxidized, as result of subaerial exposure following deposition (Hein et al., 2012).
Late Wisconsinan fluvial deposits (Early Paraglacial Period) River terrace deposits. Postglacial stream terrace deposits along the Merrimack River and smaller rivers are
present at altitudes below the glaciomarine deltas and marine coastal deposits. Terrace segments are discontinuous and merge downstream with extensive plains at 1015 m altitudes near the mouths of the rivers. These deposits disconformably overlie finer marine-bottom deposits. They are composed of fluvial sand and gravel facies, and pebbly sand facies (Stone et al., 2004, 2005), 15 m thick. Along the south side of the mouth of the Merrimack River, these deposits are subaerially exposed within 5 m of MSL and have provided a flat morphology onto which the Plum Island Airport was constructed (STOP 4).
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River channel cut-and-fill deposits. Fluvial cut-and-fill deposits have been mapped by Hein et al. (2012, 2013) as dendritic structures offshore of the Rowley and Parker rivers, several kilometers south of the Merrimack River. The absence of evidence of these features offshore of the mouth of the Merrimack River can likely be attributed to the coarse grain sizes, rather than non-deposition. This unit is composed of sorted and stratified medium to coarse sands with local gravel-rich and mud-rich layers. 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, ground-penetrating radar (GPR) profiles reveal that this unit is eroded into underlying Glaciomarine silt-and-clay deposits at depths of 1113 m below MSL (Fig. 8; STOP 7). 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. Buried sections of this unit were deposited dominantly by fluvial systems at lower stands of sea level, though locally with some tidal influence. More recent, late Holocene through 19th century paleo-channel deposits, < 1 to ~3 m thick, underlie the active channel of the Merrimack River, southern Salisbury Beach (Costas and FitzGerald, 2011) and northern Plum Island (Nichols, 1942; STOP 8).
Late Wisconsinan marine and coastal deposits (Early Paraglacial Period and Paraglacial Sand Maxima) Discontinuous, disparate regressive, sand and gravel coastal deposits (barrier beaches, spits, regressive fluvial
deltas) have been identified between the highstand shoreline (+ 33 m) and modern MSL throughout the field trip area. These continue discontinuously offshore.
Marine beach and spit deposits. These regressive deposits are composed of sand or sand and gravel, and underlie morphologic features that are similar to the size and shape of modern coastal beaches, spits, and tombolos. These deposits are smaller and thinner than nearby glaciodeltaic or fan deposits. They characteristically extend as overlapping sedimentary bodies onto the sides of glaciomarine deposits or drumlin hills that were islands in the postglacial Merrimack Embayment. In some locations, these deposits extend along the base of steep wave-cut slopes. In most cases, the surfaces of these coastal deposits are 12 m higher than the distal surfaces of local glaciomarine deltas. Pebbly sand forms a narrow ridge that slopes from 619 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 1519 m above MSL at Newbury Old Town and south of the Parker River are local beach and spit deposits. Coarse sand deposits also compose the barrier ridge at 1221 m above MSL beneath Lafayette Road. These deposits were formed by coastal processes (waves, tides, wind) during falling RSL following highstand. These deposits are distinguished from regressive-transgressive shoreline deposits in that they have not been modified by the subsequent transgression.
Figure 8. Raw and interpreted shore-parallel GPR profile collected across river channel cut-and-fill deposits associated with the lowstand Parker River channel. Data collected with a Mala Pro-Ex GPR with a 100 MHz antenna. TWTT: two-way travel time. Solid vertical white lines in upper image indicate locations of cores. Dashed lines indicate unit boundary designations shown in lower image. Modified from Hein et al. (2012).
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Regressive-transgressive shoreline deposits. This subaqueous unit in the Merrimack Embayment is composed of a continuous, 415-m thick deposit of unknown sediment type, capped by surficial lag of medium-coarse sand and granules, which generate high backscatter in sidescan-sonar data. Deposits are directly overlie glaciomarine silt-and-clay deposits and are discontinuously capped by finer Holocene deposits. Along its seaward extent, it extends along and partially overlies proximal sections of the lowstand paleodelta. The surface of the unit is fully exposed in an 810-km wide (perpendicular to the shore) and 16-km long tract east of the modern Merrimack River (Barnhardt et al., 2009). This mostly transparent unit exhibits little structure in seismic sections.
Lowstand paleodelta. Initial geophysical investigations of the shallow shelf of the Merrimack Embayment (Oldale et al., 1983, Edwards, 1988) revealed the extent of the submerged Merrimack River lowstand delta located 67 km offshore and trending parallel to the present coast. The seaward-prograding lowstand delta foresets are well imaged in seismic reflection transects (Oldale et al., 1983; Barnhardt et al., 2009; Hein et al., 2013; Fig. 9). This unit can be divided into three sub-sections in seismic profiles (Fig. 9): 1) the proximal section contains medium to coarse sand in high-angle delta foreset strata that appear as intermediate angle (510o), concave-up, seaward-dipping foresets in seismic profiles; 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 clay and silt (Barnhardt et al. 2009; Hein et al., 2013).
Figure 9. Seismic-reflection profile across the Merrimack Embayment (see Fig. 3 for location) collected using an EdgeTech Geo-Star FSSB system and a SB-0512i towfish (Barnhardt et al., 2009). A constant seismic velocity of 1500 m/s through both water and sediment was used to convert travel time to depth.
Holocene marine deposits (Middle to Late Paraglacial periods) Offshore marine deposits. In depths > 40 m, the seafloor is dominated by fine sand to clay with variable
textures. Shallower sections (ca. 4050-m depth) grade into the seaward edge of the foreset beds of the lowstand paleodelta and are sand-dominated with minor silt and clay (< 10%); intermediate sections (ca. 5070-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-and-clay 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 paleodelta. Locally, as a talus apron surrounding subaqueous exposures of bedrock and eroded till, coarse-grained marine deposits dominate. These are predominantly composed of shell-rich sand, but locally contain significant (> 25%) lithic gravel. This minor unit is interpreted as an erosional lag deposit from bedrock and boulders exposed on the sea floor.
Nearshore marine transgressive deposits. The shallow (< 45 m) sea floor of the Merrimack Embayment is discontinuously overlain by transgressive sand sheet deposits composed of relatively well-sorted fine to medium sand with minor quantities of silt and gravel (Barnhardt et al., 2009). This unit ranges in thickness from < 1 m to ca. 910 m. The thickest deposits are located along the shoreface and offshore of southern Plum Island and Castle Neck (Barnhardt et al., 2009). They extend from approximately the postglacial lowstand shoreline to mean low water, where they grade laterally into barrier and beach deposits.
Holocene barrier system deposits (Middle and Late Paraglacial and Post-paraglacial periods) The modern barrier systems of the Merrimack Embayment are very similar to those found along coastlines
throughout the world (Fig. 10). Each is bounded by tidal inlets with associated ebb and flood tidal deltas, backbarriers with marshes, tidal flats and tidal channels, active shorefaces, and dunes of varying heights and degrees of vegetation. The units comprising these systems remain active, undergoing reworking by modern coastal processes (wind, waves, tides, storms). Although traditionally lumped as coastal deposits in surficial geologic maps, they are split here based on detailed study of their sedimentological and stratigraphic characteristics and relationships to their bounding combined onshore and offshore geologic units.
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Figure 10. Idealized barrier-island cross section, extending
from the backbarrier, dominated by lagoonal, estuarine and/or
salt-marsh deposits, across the barrier island and into the
nearshore zone and shallow shelf.
Freshwater Marsh Deposits. At the contact between the modern barrier system and upland areas, as well as locally preserved at the base of the barrier-island sequence is freshwater marsh deposits. These are composed of 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 salt-marsh deposits; the contact between freshwater marsh and salt-marsh deposits marks the leading edge of the late Holocene transgression.
Estuarine deposits. This expansive deposit underlies the entire Great Marsh system behind the Merrimack Embayment barriers and extends under each of the barriers and likely into the shallow nearshore zone, where it is covered by active beachface and sand sheet deposits. This unit is composed of 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 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 small-scale (< 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 sigmoid-shaped 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-and-clay deposits and is as much as 20 m thick. It comprises backbarrier lagoonal sediments, tidal-channel sequences, and flood-tidal deltas.
Salt-marsh deposits. Estuarine sediments are discontinuously overlain by salt-marsh deposits composed of fine-grained organic matter, and fibric and hemic peat interbedded with fine sand, silt and clay. Sediments are typically greater than 30% organic and 16 m thick. In the major estuaries (Plum Island Sound and the Merrimack River Estuary), salt-marsh deposits overlie estuarine and/or freshwater marsh deposits. These deposits are generally found in environments of low wave energy in backbarrier areas.
Tidal inlet channel deposits. Located in the primary inlet channels and proximal backbarrier channels at each of the inlet systems in the Merrimack Embayment, this unit is composed of 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 generally composed of coarse sand and granules, reflecting the strong tidal flows transferred through these inlets. These deposits are deposited by the daily and storm tidal currents that exchange water and sediment between estuarine water bodies and the coastal ocean. Modern 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 27 m below MSL (the paleo-Parker Inlet; Hein et al., 2012) 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 (Fig. 11; STOP 6).
Ebb-tidal-delta deposits. Offshore of the primary inlet channels are ebb-tidal deltas, composed of medium to coarse sand with minor gravel, often with active surface bedforms and exhibiting herringbone ripple stratification
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and relict bedforms in cross section. This unit ranges from < 1 m to about 4 m thick and are actively reworked by tidal and wave-generated currents and smoothed by wave action along the seaward edge.
Beachface deposits. Modern beach deposits are deposited along the shoreline by coastal processes (waves and currents) throughout the tidal cycle. The beachface is defined as the area between mean low water and the dune toe. It is composed of 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. Beachface deposits are generally < 5 m thick and overlie inactive estuarine and saltmarsh deposits. Beachface deposits extend under each of the barrier islands. In the landward 10 % of most barriers they are dominated by storm overwash sediments, whereas the remaining 90% of the barrier beachfaces grew through seaward progradation and spit elongation (Dougherty et al. 2004; Hein et al., 2012). Alongshore textural variability is generally controlled by the texture and proximity of sediment sources (i.e., estuary mouth, eroding glacial deposits).
Dune deposits. Sand dunes in the Merrimack Embayment are composed of mobile, well-sorted, fine and very-fine sand. The seaward edge of dunes (the dune toe) is usually unvegetated and grades from beachface deposits, with the contact noted as 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. Internal GPR reflections are commonly chaotic in nature. This unit is 110 m thick. 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.
Figure 11. Buried inlet sequence at Plum Island, as imaged in GPR profiles. Modified from Hein et al. (2013, 2014). Profile was collected with a GSSI SIR-2000 with a 200 MHz antenna. Profile will be observed in detail at STOP 6.
QUATERNARY HISTORY OF THE LOWER MERRIMACK VALLEY AND THE MERRIMACK EMBAYMENT: INSIGHTS FROM SEAMLESS ONSHORE-OFFSHORE SURFICIAL MAPPING
The coastal zone represents the boundary between land, sea and air. It also marks the traditional contact between marine and terrestrial geological investigations. Through careful study of this transition zone, surficial geologic units can be seamlessly mapped across these environments (Fig. 12), thereby providing insights into the overall development of the marine and terrestrial landscapes that would be impossible through traditional, disciplinary study.
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Figure 12. Stratigraphic cross sections of the Merrimack Embayment (see Fig. 3 for locations). (a) Shore-normal cross section. Eastern half is based on seismic-reflection data (Fig. 9), ground-truthed with surficial sediment samples and one offshore vibracore. (b) Shore-parallel cross section across four barriers in Merrimack Embayment. Note: vertical exaggeration of (b) is twice the vertical exaggeration of (a). Modified from Hein et al. (2013, 2014).
A source-to-sink summary of the post-glacial history of the lower Merrimack River Valley and the Merrimack Embayment
Multiple Quaternary glaciations resulted in significant erosion (tens of meters) of coastal plain sediments and bedrock across the Gulf of Maine, and in extensive scouring of bedrock-controlled fluvial valleys on land. Physical and chemical weathering of intrusive granitic (with minor gabbro and granodiorities) plutons common to inland regions throughout the Gulf of Maine, as well as glacial excavation of saprolite, generated much of the sand-rich sediment that was later reworked into glacial and paraglacial deposits (Hanson and Caldwell 1990; Thompson et al., 1989; FitzGerald et al., 2005). Ice sheets of the most recent glaciations, the Illinoian and Wisconsinan, left behind non-stratified glacigenic sediments (drumlins and thin till) throughout the Gulf of Maine and in adjacent New England. Contemporaneously, meltwater streams drove the accumulation of extensive, quartz-feldspar-rich, stratified ice-contact sediments throughout the region, both under and in front of the ice sheet. The distribution of sandy eskers, outwash plains and fans, and coarse sandy ice-marginal deltas reflect the ice-sheet extent and recession (FitzGerald et al., 2005).
Upper Quaternary glaciation drove complex Late Pleistocene and Holocene RSL changes (Fig. 4) which resulted from the combined forcings of global eustatic sea-level rise and regional glacio- and hydro-isostatic adjustments. These set the stage for the redistribution of glacially generated sediment across the across the lower Merrimack River Valley and the Merrimack Embayment. The resulting stratigraphic units reflect the erosion, reworking, transport and deposition sediment during and after glaciation. This occurred over multiple periods, each marked by the deposition of distinct sedimentary units and the formation and modification of specific landforms (Fig. 5; Hein et al., 2014).
Deglaciation and sea-level highstand (Glacial and Proglacial periods). The Laurentide Ice Sheet of the Wisconsinan glaciation reached its maximum extent in southern New England between 27 and 23 ka (Balco et al., 2002; Rittenour et. al., 2012; Stone, 2012). While near its maximum extent, the mass of the Laurentide Ice Sheet depressed the lithosphere across southern New England. The ice sheet receded northward as climate warmed, exiting
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present-day Massachusetts by ca 1716 ka (Borns et al., 2004). Eustatic sea level rose rapidly during this period, submerging isostatically depressed areas immediately upon their deglaciation in a tidewater setting (Bloom, 1963). As such, for the period of retreat through the western Gulf of Maine, sediment carried by meltwater streams was immediately modified by waves and currents. Glaciomarine deposits consist of bedded gravel, sand, and mud deposited either under the ice sheet (e.g., eskers) or at the front of the retreating ice sheet by meltwater streams (glaciomarine deltas; grounding-line fans; glaciomarine silt-and-clay).
Isostatic rebound, sea-level fall and lowstand (Early Paraglacial Period). The maximum marine limit was reached at 3133 m above modern MSL in northern Massachusetts (Fig. 4) (Stone and Peper, 1982; Oldale et al., 1983; Ridge, 2004; Stone et al., 2004). It is identified by an ice-contact marine delta at + 30 m in the lower Merrimack River valley, estimated to be about 16,000 years old. Rapid crustal rebound coincident with deglaciation resulted in RSL fall and attendant forced shoreline regression. River mouths and channels followed the regressing shoreline and coastal and fluvial processes modified and redistributed sediments across the open Merrimack Embayment that developed on the emergent glaciomarine plain. This resulted in the deposition of coarse sand to gravel as fluvial channel-fill deposits which have been mapped under modern barriers and extending onto the shallow shelf (Hein et al., 2012, 2013). Upstream, falling base level forced fluvial downcutting and the stranding of river terrace deposits (alternating layers of gravel and sand, STOP 9, 9A) above modern RSL. Simultaneously, regressive shoreline sediments were deposited by waves, currents and wind action (Stone et al., 2006). Discontinuous, disparate remnants of regressive beaches and spits and fluvial terraces have been identified between the highstand and modern coastlines along much of Gulf of Maine and lower Merrimack Valley (Retelle and Weddle, 2001; Stone et al., 2006). Off the mouth of the Merrimack River, abundant coarse sediment produced a 10-km wide deposit that parallels the shore for 16-km and is 4 to 15-m thick (Barnhardt et al., 2009). The exact nature and origin of this regressive-transgressive shoreline deposit remains unknown. It is the subject of intense debate, even among the authors of this guide (CJH & BDS), who variously argue that it is either a fully regressive, late Pleistocene fluvial unit, minimally modified in the surface < 1 m by coastal processes during the Holocene transgressive (CJH), or a chiefly transgressive unit, overlying coarse fluvial beds in local tributary channels to the low delta, and composed of sandy sediments from the Merrimack River and the erosion of upper beds of the lowstand paleodelta (BDS).
Continued isostatic rebound resulted in rapid RSL fall as regional uplift outpaced eustatic sea-level rise. By 1314 ka, RSL stabilized as rebound decelerated and temporarily matched the rate of eustatic sea-level rise. This produced a relative marine lowstand at ca. -41 m below MSL (as estimated from the change in slope of the drowned paleodelta foresets) (Oldale et al., 1993; Hein et al., 2012) or in two calculated lows at ca. 12 and 14.5 ka (Stone et. al., 2004) (Fig. 4). Deceleration and subsequent cessation of RSL fall led to the deposition of a lowstand delta at the mouth of the Merrimack River (Oldale et al., 1983). This occurred during a period of large-scale delivery to the coast of sediments derived from glacial and primary paraglacial deposits within the Merrimack River valley (the Paraglacial Sand Maxima) (Hein et al., 2014). The resulting lowstand delta is the seaward extent, and significantly-finer component, of the regressive-transgressive shoreline deposits. It is 20 km long, 47 km wide, up to 20 m in thickness, and lies beneath 4150 m of water (Oldale et al., 1993). The delta contains ca. 1.3 billion m3 (Oldale et al., 1983) of fine to coarse, bedded sand and silt deposits.
Late Pleistocene and early Holocene rapid sea-level rise (Middle Paraglacial Period). Following the lowstand, RSL likely rose rapidly in the Merrimack Embayment, leading to erosion of regressive shoreline and lowstand deposits. Seismic profiles across Merrimack lowstand paleodelta delta demonstrates the presence of a smooth, gently-dipping erosional surface that truncates the upper parts of delta foresets, indicating deep scouring during the early transgression (Oldale et al., 1983). The resulting transgressive unconformity was discontinuously overlain by transgressive sand sheet deposits composed of relatively well-sorted fine to medium sand with minor quantities of silt and gravel (Barnhardt et al., 2009). This broad time-transgressive sand sheet 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, this unit is likely only impacted by rare, high energy events (dominantly, northeast storms). Here, this unit is dominated by low-amplitude (0.33.5 m), long-wavelength (150
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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).
Middle to late Holocene slow sea-level rise (Late Paraglacial Period). Sea-level rise in the Merrimack Embayment gradually slowed to 0.6 0.1 m/yr by 45 ka (Fig. 4). Continuing, but slowed, marine transgression drove sediments from coastal and river sources farther onshore to the locations of modern barrier systems. Thin and mobile protobarriers were pinned to contemporaneous emerged drumlins and bedrock outcrops proximal to modern barrier positions by 4 ka (Hein et al., 2012). There is little evidence of overwash following this original pinning phase. Abundant GPR profiles collected along Salisbury Beach and Plum Island reveal few overwash deposits along the proximal landward side of the barriers (Costas and FitzGerald, 2011; Hein et al., 2012). Although preserved washovers are somewhat more prevalent along the Crane Beach section of Castle Neck (Dougherty et al., 2004), evidence of progradation, aggradation and spit elongation generally dominate (greater than 90%) barrier widths (Hein et al., 2012).
Freshwater marsh deposits, composed of organic matter and peat that contains minor amounts of stratified and poorly sorted sand, silt, and clay, formed at the leading edge of the transgression (e.g., McIntire and Morgan, 1964). This was followed by the deposition of massive, moderately well-sorted fine sand and silt. These estuarine deposits comprise backbarrier lagoonal sediments, tidal channel sequences and flood tidal deltas. This unit was generally sourced from offshore deposits (regressive fluvial shorelines and deltas) and moved onshore by wave and tidal processes during the landward migration of proto-barrier systems. Finer sediment was transported by tidal currents to the backbarrier through inlets and additional sediment was likely derived from upland deposits (e.g., 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 (Hein et al., 2012). Within these estuarine deposits are tidal channel-fills, deposited by migrating backbarrier tidal channels and tidal inlets. One such inlet-fill sequence (the paleo-Parker Inlet) was identified under central Plum Island (STOP 6), highlighting the role played by backbarrier infilling and tidal-inlet closure in barrier formation: inlet closure was driven by the reduction of bay tidal prism over time by backbarrier sedimentation and was closely followed by the rapid expansion of backbarrier marshes and the aggradation, elongation and progradation of the barriers themselves (Hein et al., 2011, 2012).
Fresh and brackish marsh deposits initially formed at the leading edge of the transgression (McIntire and Morgan, 1964). The cause and timing of subsequent salt-marsh expansion in Merrimack Embayment estuaries has been attributed to a late-Holocene decrease in the rate of RSL rise. Most observations suggest a rapid transition from barren tidal flats to well-developed high marsh around or after 4 ka (Oldale, 1989; FitzGerald et al., 1994).
The Merrimack Embayment in the Anthropocene Unlike many barrier islands throughout the world, the Merrimack River barriers, including Plum Island, are
generally stable. They remain progradational and, except locally (i.e., the southern tip of Plum Island), show few signs of retreat and migration. However, these systems remain dynamic, responding to the coastal processes of wind, waves, storms and tides. This is especially the case proximal to tidal inlets, where complex river / inlet / tidal-delta interactions create highly dynamic depositional settings. Although most of the barriers of the Merrimack Embayment remain in semi-natural states, both Seabrook Beach and northern Plum Island are developed. The final stops of this field trip will be on the beaches of northern Plum Island, within the town of Newbury, and is thus the focus of the history of Anthropocene modifications.
Plum Island was first settled as pasture land in the early-1600s (Waters, 1918). The island was sub-divided between the towns of Newbury (2/5), Ipswich (2/5) and Rowley (1/5) in 1649 (Toppan, 1905) and by the late 19th century had become a popular tourist destination featuring several large hotels and private homes. The PRNWR was established across parts of the island and its backbarrier in 1942, followed more recently by the Sandy Point State Reservation (Mass. Department of Conservation and Recreation). Today, the northern 1/3 of PI is shared by the towns of Newbury and Newburyport. Located immediately seaward of the harbor of Newburyport and downstream of the important mill towns of Lawrence, Lowell and Haverhill, the entrance to the Merrimack River at the northern end of Plum Island quickly became a critical pathway for commercial and, later, recreational boating. However, driven by processes of southerly longshore transport, spit elongation and ebb-delta breaching, the Merrimack River Inlet at that time actively migrated across the area currently covered by southern Salisbury Beach and northern Plum
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Island (Figs. 13, 14). This caused not only periods of severe erosion and accretion along proximal sections of these barriers. Following a significant ebb-delta breaching event in the mid-1800s, a large sediment bar welded onto the northern end of Plum Island, forming the landward-most piece of the eastern fork of northern Plum Island, and abandoning the former river channel in what is today known as The Basin, an estuarine lowland between the east and west forks of the northern section of the island (Fig. 15; FitzGerald, 1993). The US Army Corps of Engineers (USACE) initiated a series of public works projects at the mouth of the river in the 1880s to stabilize the Merrimack River Inlet in its new configuration for navigation purposes (Fig. 15). These included the construction of North and South Federal Jetties, the latter of which was lengthened in the late 1960s.
The inlet stabilized by these jetties is periodically dredged by the USACE. The fate of the spoils has varied over time, including dumping both offshore or directly on the beach face as beach nourishment. Several groins were also installed along the northern sector of Plum Island around this same time. This section of the barrier is now densely populated (ca. 1200 homes and > 100 roads), but has experienced long-term erosion at rates of ca. 0.2 m/yr (averaged from 18502000) and nearly 0.5 m/yr in the past 30 years (EOEEA, 2010). Over longer timescales, shoreline retreat is not a steady state, or a storm- or season-linked process, but includes variability over small areas and varying timeframes; this variability is central to understanding the overall sediment dynamics of the barrier. For example, abundant evidence indicates the existence of a decadal-scale cycle of erosion and accretion along northern Plum Island. Refraction of northeast storm waves around the ebb-tidal delta seaward of the Merrimack River jetties causes a reversal of net longshore transport, driving sediment north across the developed portion of the shoreline and leaking sand back into the inlet through degraded sections of the jetty. While this reversal has created a broad accretional deposit adjacent to the inlet, it is at the expense of beach to the south. Superimposed on the local transport system are smaller-scale variations in refraction and wave energy along the shore which appear to correspond to shifting bars on the ebb-delta platform. Shifting of these shoals creates specific erosion hotspots that, when active, can focus severe erosion on a small portion of the developed barrier, while leaving other areas stable or accretionary (EOEEA, 2010). These hotspots can last months to years in an aperiodic pattern.
Beginning in the late 1950s and early 1960s, a range of beach structures, including groins and riprap seawalls were constructed as a result of this cyclic erosion; many have been lost to erosion since then. Recent erosion-mitigation techniques have included the installation of geotubes (2008), hay bales and coir rolls (2009, 2010), beach nourishment (2010), and, most recently, controversial citizen-led programs of beach-scraping (2011, 2012), the construction of rip-rap revetments (2013) and artificial nourishment (20132014). Nonetheless, severe erosion persists, exposing groins and revetments from earlier periods of erosion, and threatening residential and commercial properties along the shoreline with every high-energy event. The variability in the timescales and patterns of beach erosion at Plum Island has contributed to the lack of any unified understanding, policy, or response to erosion when it does threaten structures. This is compounded by generation-long (multi-decadal) cycles of accretion, which contribute to a belief in holding ground during erosion cycles until the accretion area moves down the beach again.
Figure 13. Northeastern section of the map of the Town of Newbury by Philander Anderson, 1831. Note the absence of the northern section of Plum Island and the southerly extension of Salisbury Beach.
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Figure 14. Ground-penetrating radar section collected with a Mala Pro-Ex GPR with a 100 MHz antenna across southern end of The Basin, the early 19th century channel of the Merrimack River. Time-depth conversion uses a depth-averaged radar velocity of 8.5 cm/ns and ground-truthed with sediment core (Geoprobe core PIG26). Note eastward-dipping reflections signifying eastward migration of the river / inlet. This former channel was abandoned as the river breached the ebb-tidal delta to the north, truncating southern Salisbury Beach.
Figure 15. Changes at the mouth of the Merrimack River prior to and following construction of the jetties. Modified from the The Daily News of Newburyport, as derived from Nichols (1942) and earlier USACE maps.
Glacial and paraglacial sediment sources and coastal sediment sinks of the Merrimack Embayment Previous studies have identified a number of likely sediment sources that contributed the Merrimack
Embayment barrier system. Based on their experience studying the development of the Boston Harbor Islands, Chute and Nichols (1941) proposed that the barriers in the Merrimack Embayment formed relatively recently, from the in situ erosion and reworking of drumlins during the latest stages of the modern transgression. Although the modern shoreline is dotted with subaerial drumlins and nearshore boulder lag deposits from eroded drumlins, the possible sediment contributions from these features is dwarfed by even conservative estimates of the sediment found comprising the barriers (~75 x 106 m3; Table 1).
Paraglacial sedimentary deposits on the shallow shelf (regressive-transgressive shoreline deposits, lowstand delta) provide a second possible source for the Merrimack Embayment barriers. Derived largely from primary glacial and proglacial deposits upstream in the Merrimack River Valley, these shelf deposits serve as both long-term and temporary sediment sinks in the system. For example, seismic records show that the upper portion of the paleodelta foresets are truncated (Fig. 9), indicating that the surface of the delta was reworked during the early Holocene transgression (Oldale et al., 1983; Barnhardt et al., 2009). FitzGerald et al. (1993) present a four-stage evolution for the system in which sediments of the upper paleodelta were eroded during the early transgression, reworked onshore as an expansive sand sheet, and pinned to drumlins and shallow till and bedrock between 5 and 4
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ka. While much of this general evolution is accepted, shallow vibracores through the paleodelta foresets (Edwards, 1988) show that delta is composed of fine sand to silt, as opposed to the barrier which is dominated by fine to coarse sand. Recent studies have identified the coarse grained (sand to gravel) regressive-transgressive shoreline deposits as a secondary source for the sediments comprising the barrier system. This unit represents, at least to some degree, a lag deposit of the most recent transgression. Sediments eroded from this unit were a likely source for sediments that formed the Holocene shelf sand sheet and barrier systems (Hein et al., 2007; Barnhardt et al., 2009; Hein et al., 2013, 2014).
Table 1: Sediment reservoirs associated with the Plum Island barrier system. System Type Name Est. Sediment Volume Data Sources / References Barrier Beaches Plum Island 2.95E+07 Hein et al., 2012
Castle Neck 2.10E+07 Dougherty et al., 2004 Coffins Beach 8.50E+06 McKinlay, 1996 Salisbury Beach 6.80E+06 Costas and FitzGerald, 2011 Seabrook Beach 3.64E+06 Costas and FitzGerald, 2011 Hampton Beach 3.42E+06 Estimated
Backbarriers Plum Island, Castle Neck & Salisbury Beach backbarriers 8.50E+08 Hein et al., 2011
Tidal Deltas Merrimack River ETD 1.20E+07 Hubbard, 1971 Parker / Ipswich ETD 2.43E+07 FitzGerald, et al., 2002 Essex River ETD 1.04E+07 FitzGerald et al, 2002 Annisquam River ETD 8.58E+06 FitzGerald, unpublished Essex River FTD & Sand Shoals 9.40E+06 Rhodes, 1973
Nearshore Holocene Reservoirs
Mobile Sand Sheet 1.21E+08 Barnhardt et al., 2009 Unmapped Nearshore Sand Sheet 1.36E+08 Estimated
Shallow Shelf Pleistocene Reservoirs
Lowstand Paleodelta 1.30E+09 Oldale et al., 1983 Regressive / Transgressive Braidplain Delta undetermined N/A
A final source of sediments for the Merrimack Embayment barriers is the rivers draining into the coastal zone. Three small rivers currently discharge into the backbarrier of Plum Island: the Ipswich River (drainage area [A]: 402 km2; annual freshwater discharge [Q]: 0.56 km2), the Parker River (A: 167 km2; Q: 0.033 km2) and the Rowley River (A: 36 km2; Q: negligible [tidal]) (Sammel, 1967; Simcox, 1992). Although these may have contributed some inorganic sediment to the development of the backbarrier marsh system in the past (e.g., Kirwan et al., 2011), modern flow is dominated by tidal fluxes and sediment exports are minimal. The only estuary in this system with significant freshwater discharge is the Merrimack River, which has an average discharge of 6.5 km3/yr and total suspended sediment load of 0.2 Mt/yr (Milliman and Farnsworth, 2012). Dredging operations from the Merrimack River Inlet over a 62 year period removed 2.6 x 106 m3 of sand and gravel, suggesting an average annual deposition rate of 4.16 x 104 m3/yr since the mid-1900s (Hein et al., 2012), a time of reduced sand and gravel supply as compared to earlier in the Holocene (Fig. 5). This sediment ranges in size from fine to coarse sand and granules. Southerly- and ebb-oriented bedforms within the ebb-tidal-delta complex seaward of the jetties indicate southerly migration, corroborating sedimentologic evidence of a southerly-fining trend across the ebb delta, and a general trend of increasing textural and mineralogical maturity to the south, away from the Merrimack River. This trend reflects winnowing and differential transportation of finer sand grains by wave action (FitzGerald, 1993; FitzGerald et al., 2002) and demonstrates the dominance of southerly longshore currents and the continued contribution of sediment to the Merrimack Embayment barrier system by the Merrimack River. These recent findings suggest that it is likely that the barriers of the Merrimack Embayment formed from a combination of the truncation of paleodelta forsets, winnowing of the regressive-transgressive shoreline deposits, and a continual fluvial source (Hein et al., 2012).
The magnitudes of changes in the relative contributions from each of the major sediment sources (fluvial erosion and reworking of upstream glacial and paraglacial deposits; erosion and reworking of shallow shelf and
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nearshore glacial and paraglacial deposits by coastal processes during RSL rise) over time are currently unknown (Fig. 16). Detailed mapping and sampling of the shallow shelf offshore of the Merrimack Embayment (Barnhardt et al., 2009; Hein et al., 2013) has revealed the existence of several large bedform fields formed by coastal set-up during intense extratropical cyclones that regularly affect the coast (Hein et al., 2007). The result is a medium-term (101 to 1010 yrs) sand-circulation gyre that may actively exchange sediment with the barrier system (Hein et al., 2007). However, the rates and net-mass fluxes between the barrier and nearshore, and how these fluxes will be affected by climate change (SLR, changes in storminess), remain unknown. Thus, there is an enhanced reliance on new sediment inputs from local rivers for continued barrier stability. The Merrimack River, the ultimate sediment source for all post-glacial subaerial and subaqueous deposits in the Merrimack Embayment, has undergone extensive anthropogenic modifications which have decreased the sediment supply from the river in recent centuries. Major dams were constructed along the river in the 19th century, most notably at Lowell (1820s) and Lawrence (1847). Additionally, large river-dredging projects below Lowell were carried out beginning in the 1870s as part of an effort to create a navigable waterway on the Merrimack River from Newburyport to Lowell (Bradlee, 1920). While major dredging of the river in recent decades has been centered at the mouth of the Merrimack under the aegis of the USACE, hydraulic changes to rivers channel and sediment loading are a product of the many historic alterations to the land and streams within the Merrimack Valley. This, combined with the natural stabilization of slope, terrace and floodplain deposits, and the river channel, by bedrock knickpoints and vegetation cover, has greatly reduced fluvial sediment supply to the Merrimack Embayment over time.
Figure 16. Conceptual model of the spatial and temporal
scales of the factors influencing sediment supply
to the Merrimack Embayment.
SOURCES AND ACKNOWLEGEMENTS
Portions of this text were derived and edited from Stone et al. (2004), Stone et al. (2006), Hein et al. (2012) Hein et al. (2013) and Hein et al. (2014). The authors wish to thank the following landowners and organizations for permission to observe geologic features on their properties: farm gravel pit Haverhill, Plum Island Airport, and Nancy Pau and Matt Poole of the Parker River National Wildlife Refuge. Editor Meg Thompson solicited this trip and remained supportive during the final editing stages.
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FIELD TRIP ROAD LOG
This trip will leave from the MARKET BASKET parking lot in the HAVERHILL PLAZA SHOPPING CENTER, 285 Lincoln Ave, Haverhill, MA at 8:15am. Directions are given from Wellesley College to the meeting location. Road Log is listed in cumulative miles, beginning at STOP 1. Times are cumulative within direction sets. Street addresses and GPS coordinates are included in each STOP and DRIVEBY locality. Future users of this road log are reminded that all STOPS are on private or publically administered property; it is the responsibility of future visitors to request permission from the land owners to visit these locales. Maps used in trip stop locations for STOPS 1-6 are the USGS 7.5-minute, 1:25,000-scale topographic quadrangles. For STOPS 7 and 8, map is from NOAA Chart 13282, Newburyport Harbor and Plum Island Sound. Black asterisks on maps show stop locations.
Roads on this trip are paved and in good condition, with the exception of the road to and from STOP 6, which is a dirt road through the Parker River National Wildlife Refuge (PRNWR). There is also limited parking at STOPS 6 and 7. Thus, STOP 5 will be a brief stop in a large parking lot just beyond the entrance to the PRNWR, on our way south to STOPS 6 and 7. At STOP 5, we will combine into as few vehicles as possible, leaving others behind. Restroom facilities are also located at STOP 5. We will return to STOP 5 for our final stop, STOP 8.
Mileage Time Directions (Wellesley College to Haverhill Plaza Shopping Center [Meeting Place])
00.0 0 min (UTM 19N: 310343 E, 4684951 N) Start at Wellesley Science Center Parking Lot. 00.6 5 min Make left on Washington St. from Wellesley College 01.2 8 min Turn right onto MA-16 E / Washington St. 04.4 15 min Make slight right to merge onto I-95 N / MA-128 N toward Portsmouth, NH 44.7 55 min Follow I-95 N (do NOT follow signs for 128N). Take exit 54B from I-95 to MA-133 W
toward Georgetown 51.1 1 hr, 12 min Follow MA-133W and MA-97N onto Groveland St. in Haverhill, MA; cross over
Merrimack River 51.2 1 hr, 13 min Make second left after the bridge (immediately after Gulf station) onto Lincoln Ave. 51.3 1 hr, 14 min Make left into Haverhill Plaza Shopping Center (UTM 19N: 333272 E, 4736621 N)
Mileage Time Directions (Haverhill Plaza Shopping Center [Meeting Place] to STOP 1)
00.0 0 min Start (8:15am) at Haverhill Plaza Shopping Center (UTM 19N: 333272 E, 4736621 N) 00.1 1 min Make right onto Lincoln Ave. 00.2 2 min Turn left (west) onto Groveland St. 00.7 4 min Turn right (NE) onto East Broadway 01.5 7 min Turn left onto Eves Way 01.5 8 min Turn right onto Seven Sister Rd. 01.6 9 min Arrive at STOP 1, at 78 Seven Sister Rd.
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STOP 1: SEVEN SISTER DRUMLIN (30 MINUTES) start at 8:30am (UTM 19N: 333718 E, 4737621 N)
Figure 17. Topographic map showing the location of STOP 1.
This stop is along the Seven Sister Rd. in a new(ish) housing development built atop the Seven Sister Drumlin. Note the similarities in the subaerial forms of the drumlins proximal to STOP 1 and those along the coast, proximal to STOP 6 (Fig. 18). The stratigraphically lower Illinoisan till is exposed in the cores of both hills. Typical of the area (Weddle et al., 1989) the lower till has a dark gray (5Y 3.5-5/1) clayey silt matrix of low plasticity and medium dry strength, small medium-coarse sand component, and scattered pebbles and few small cobbled-size gravel clasts. Principal axes of elongate gravel clasts show a northwesterly preferred direction of shallow dip. The oxidized zone of the lower till is exposed beneath sandy till of the mixed-till zone at the top of the small drumlin. Effects of weathering can be seen to increase upward in the oxidized zone above a transitional contact with underlying gray till. Color values of matrix stain increase from olive-olive gray (5Y4-5/2-3) at the base to olive brown 2.5Y 4-5/3-5). Oxidized halos in matrix surrounding individual garnet grains can be seen at the top. The degree and darkness of iron-manganese stain on joint faces increase. Blocky structure increases and is more densely developed along horizontal and vertical joints. The extent of the excavation in the small drumlin reveals that the base of the weathered zone is subparallel to the surface of the landform, indicating prolonged or intense soil genesis after till deposition and glacial smoothing in the Illinoisan. The mixed zone of weathered and fresh lower till fragments within a sandier Late Wisconsin till matrix is present in the upper 2 m of the excavation. Soils developed in the surface till and mixed-till zone since late Wisconsinan deglaciation are inceptisols, characterized by B-horizons that contain < 20 percent illuviated clay, and weakly modified clay mineralogy. Typically, Canton and Charlton soils series develop in the surface tills, and the Paxton soil develops in the thin mixed-till zone on drumlins (Fuller and Holtz, 1981).
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Figure 18. LiDAR maps of drumlin patches. (a) LiDAR imagery from vicinity of Seven Sister Hill drumlin (STOP 1), from 2011 MassGIS LiDAR imagery database. Elevations range from +80 m (light shades) to -1 m (Merrimack River; dark shades). (b) LiDAR imagery from vicinity of South Plum drumlin at the southern tip of Plum Island (STOP 6). Data from the Plum Island Ecosystems Long-Term Ecological Research project (Valentine and Hopkinson, 2005). Elevations range from +40 m (Plover Hill; light shades) to -2 m (water; dark shades).
Mileage Time Directions (STOP 1 to STOP 2)
01.6 0 min Start at STOP 1. Head back out Seven Sister Rd. 01.6 1 min Make a left onto Eves Way 01.6 2 min Make a left (northeast) onto East Broadway 04.5 8 min Make hard left onto Country Bridge Rd. 04.5 9 min Arrive at STOP 2; park alone side of road.
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STOP 2: KIMBALL HILL FARM PIT (60 MINUTES) start at 9:30am (UTM 19N: 334794 E, 4740280 N)
Figure 19. Topographic map of drumlins, glaciomarine deltas, terrace, and alluvial flood plain deposits at STOP 2.
The topographic map of the area (Fig. 19) shows flat delta topset plains at 30 m altitude of at least four ice-marginal glaciomarine deltas. These extend continuously from Rocks Village to Kimball Hill thick till deposits and two other thick-till hills to south of Millvale Reservoir. Either previous pits south of Greenwood cemetery or the active pit on Country Bridge Road will be STOP 1, depending on pit conditions and landowner permissions. We hope to see field examples of features shown in Fig. 7. Numerous other glaciomarine deltas are at this altitude down valley to Newburyport. Thus, all deltas here are aligned more or less with the crustal-uplift isobases of Goldthwait (in Antevs, 1922), Koteff et. al. (1995), who use some topset/foreset data from these pits, and Stone et. al. (2004). Note the wide stream terrace deposit at 12 m altitude, and the wide alluvial flood plain at 3 m altitude south of Kimball Hill. As you drive to the next stop, note how the river is deeply incised into the glacial deposits, and is not bordered by a wide flood plain. The river here has been transgressed by the sea. The river is tidal and part of the estuary.
DRIVE BY: The route to STOP 2 takes you along a textbook example (BDS) of the erosional contact between the glaciomarine deltaic deposits in Newburyport and the highest regressive post-glacial river-terrace deposit that resulted from crustal uplift and tilting. This scarp contact can easily be seen along Ferry Road at the 80-ft surface and High Rd, at the 50 ft surface.
Mileage Time Directions (STOP 2 to STOP 3)
04.5 0 min Start at STOP 2. Return along Country Bridge Rd. towards East Broadway 04.6 1 min Turn left onto East Broadway; continue onto East Main St. 05.1 3 min Turn left onto River Rd. and follow road along Merrimack River (to your right) 08.2 8 min Turn right onto Pleasant Valley Rd. ; turns into Merrimac St. (13.1 mi) (stay right) 13.5 14 min Take first right onto Main St.; road turns into Evans Pl. and then back to Main St. 14.1 19 min Cross Merrimack River on Main St.; turns into Spofford St. 14.3 20 min At traffic circle, go straight to stay on Spofford St. 14.4 21 min Make a slight left onto Ferry Rd. 14.6 22 min Merge onto High St. / MA Rt. 113 15.2 25 min Turn right onto Toppans Ln.; turns into Hale St. 15.9 30 min Turn left down road to Little River Hale St. Stop; park along road
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STOP 3: LITTLE RIVER HALE STREET GLACIOMARINE OUTCROP (30 MINUTES) start at 11:00am (UTM 19N: 344215 E, 4740517 N)
Figure 20. LiDAR map of bedrock outcrops and areas of shallow bedrock exposed above the top of eroded glaciomarine clay of the Presumpscott Formation. (a) Location of Stop 3 (symbol) on topographic map, 3 m contour interval. (b) LiDAR imagery from 2011 MassGIS LiDAR imagery database.
At this stop you can evaluate the control by rock fractures of outcrop size, shape, and relief of relatively homogeneous intrusive rocks of the Silurian Newburyport Complex (SOngd) granodiorites of the New Hampshire-Maine sequence. The outcrops, shown by lighter shades of gray in the LiDAR data (Fig. 20), are clustered, showing orthogonal topographic expression. In marine high-resolution bathymetric surveys to the east, similar contrasting shades, shapes, and bathymetric expression also reveal mappable outcrops and areas of shallow rock surrounded by eroded till or glaciomarine clay of the Presumpscot Formation.
Mileage Time Directions (STOP 3 to STOP 4)
15.9 0 min Start at STOP 3. Return to Hale St. and make right 16.3 2 min Continue on Hale St. back onto Toppans Ln. 16.6 4 min Make right onto High St. 18.4 11 min Make left onto Rolfes Ln.; turns into Ocean Ave. 18.8 13 min Make right onto Water St. at end of Ocean Ave.; Pass entrance to PRNWR Visitors
Center; Water St. becomes Plum Island Turnpike 19.3 15 min Arrive at Plum Island Airport (STOP 4) parking lot on right. Park in lot.
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STOP 4: PLUM ISLAND AIRPORT FLUVIAL TERRACE (60 MINUTES, including lunch) start at 11:45am (UTM 19N: 349444 E, 4739823 N)
Figure 21. Topographic map of the regressive deposits and recent back-bay marsh at Stop 4.
The Plum Island Airport and our USGS drilling site are located on a very low flat plain, which straddles the 3 m contour and dips very slightly to the east. This plain is considered to be the lowest regressive river terrace along the Merrimack River. Take time to view the extent of the plain and its relief above the river (Figs. 1, 21). The corehole revealed about 19 ft of medium to coarse sand with some pebbles in the upper part, and cobble gravel at the base. This fluvial sequence overlies fine, nonoxidized sand with some silt to a total depth of 52 ft. We did not encounter organic sediments or fine back-bay deposits that underlie the coastal marsh to the east. Thus, the low terrace sits disconformably on slightly older Pleistocene deposits related to the post-glacial marine regression. The topography and position of Stop 4 (Fig. 21) show why drilling operations were challenged in these low-lying deposits. Extreme heaving of sand into the core barrel stopped us at 52 ft. Proximity and permeability of regressive marine deposits to the west provide a direct path for ground water flow from the water-table mound to the drill site.
Mileage Time Directions (STOP 4 to STOP 5)
19.3 0 min Start at STOP 4. Make right (east) onto Plum Island Turnpike from airport parking lot. Cross over Plum Island River via Plum Island Turnpike
20.7 5 min Turn right onto Sunset Dr. 21.2 7 min Go through entrance to Parker River National Wildlife Refuge (PRNWR) (no fee
required if with field trip group); road becomes Refuge Rd. 21.3 8 min Make left into large parking lot (Parking Lot #1) of PRNWR (STOP #5/8)
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STOP 5: PRNWR PARKING LOT #1 (5 MINUTES) (UTM 19N: 352078 E, 4739076 N)
Figure 22. Topographic / bathymetric map of northern Plum Island showing the location of PRNWR Parking Lot #1 (STOP 5).
This is a brief stop for use of the restrooms and to consolidate into as few vehicles as possible (there is limited parking at STOPS 6 and 7). Extra vehicles can be left in parking lot, which is the location of the final stop (STOP 8) of trip. Trip will leave PRNWR Lot #1 to continue to STOP 6 at 1:00pm. The drive to STOP 6 will be partially along a flat, dirt road. Please plan accordingly.
Mileage Time Directions (STOP 5 to STOP 6)
21.3 0 min Start at STOP 5 in as few vehicles as possible. Make left onto Refuge Rd. 27.6 20 min Drive (slowly!) to Sandy Point State Reservation at southern tip of Plum Island. Note that
road becomes dirt at 3.5 miles along route. Arrive at STOP 6 (small dirt parking lot on left) and carefully find parking. Additional parking is available along road to right, another 0.2 miles down. If needed, walk back to STOP 6 from overflow parking lot.
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STOP 6: SANDY POINT DRUMLIN (30 MINUTES) start at 1:20pm (UTM 19N: 354797 E, 4729596 N)
Figure 23. Topographic / bathymetric map of southern Plum Island showing the location of STOP 5, at Sandy Point State Park. Parking is just north of South Plum drumlin.
The southern tip of Plum Island is occupied by a lone drumlin (South Plum) which rises above the relatively flat topography in front of you when driving south. South Plum is one of a patch of drumlins, including Cross Farm Hill ca. 1.5 km to the north and the drumlins that make up Plover Hill and Little Neck on the mainland (west) side (in Ipswich, MA) and Castle Hill across the Parker River Inlet on Castle Neck / Cranes Beach (Fig. 19b). Stage Island is composed of till, but its origin is unknown. These comparatively erosion-resistant drumlins serve as pinning points for the barrier systems, reducing wave energy through refraction. At lower stages of RSL, these drumlins also served to route local fluvial systems, such as the Ipswich River. The lowstand river channel was later re-occupied by the Parker River Inlet that separates Plum Island and Castle Neck.
Walking east from the parking lot to the beach, you will encounter two boulder fields which are fully exposed at low tide. These are lag deposits from drumlins that have been eroded in recent centuries and which likely served as earlier pinning points for southern Plum Island before it migrated landward to its present location. Additional lag deposits from former drumlins in this cluster are found offshore to depths of 3540 m. These were eroded by waves and tides as the shoreline transgressed across that area during the early Holocene. Finer sediments were eroded out, possibly contributing some small volume of sand to the developing barrier / backbarrier systems of the Merrimack Embayment (it is noted that these drumlins contain little sand), and gravel lags were left behind on the modern seafloor.
Evidence of the ongoing landward migration of southern Plum Island is also evident in peat deposits which occasionally outcrop along the low- and mid-tide beach in this location. These peats are several meters thick and are remnants of marsh systems that once lived behind Plum Island, when it was positioned further offshore. The ages of these peats are currently unknown but are under investigation (Hein, unpub.).
To the south of the drumlin is Sandy Point, the active recurve of the Plum Island spit. Up to 60 % of Plum Island formed through spit elongation (Hein et al., 2012), a process that continues at the southern end of Plum Island today. Much of Sandy Point is a spit platform building into the Parker River Inlet. However, the position of this inlet is stabilized by the South Plum and Castle Hill drumlins and cannot migrate further south to accommodate further
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elongation of Plum Island. If time and tide permit, the sands of Sandy Point are commonly formed into some of the most spectacular intertidal bedforms in the northeast. These were a major focus of a 1989 SEPM coastal geology field trip (Boothroyd and FitzGerald, 1989).
Mileage Time Directions (STOP 6 to STOP 7)
27.6 0 min Start at STOP 6. Head back north along dirt Refuge Rd. 31.6 15 min Drive (slowly!) along Refuge Rd. to PRNWR Sub-Station (STOP 7). Make left into lot.
Additional parking is available in small lot 0.15 mi south of STOP 7 lot (before arriving at STOP 7).
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STOP 7: PRNWR SUBSTATION (45 MINUTES) start at 2:05pm (UTM 19N: 352323 E, 4735390 N)
Figure 24. Topographic / bathymetric map of central Plum Island showing the location of STOP 6.
At this stop you are standing at the PRNWR sub-station. Underneath your feet lies barrier washover sands, nearly 20 m of backbarrier sediments, and at least 15 m of glaciomarine silt-and-clay. These units are captured in a sediment core which we will open at this location to see how middle Holocene transgressive barrier deposits lie unconformably on 17,000-year-old glaciomarine deposits, which are the same as those seen exposed at STOP 3.
Had you been here 700013,000 years ago, you would have been standing along the north bank of the Parker River (seen heading to the west from the Plum Island River to your west), which at that time extended its course due east out onto an exposed plain of glaciomarine clay and regressive shoreline deposits. Though small, this river helped to feed the growth of the lowstand paleodelta, located in 40 m of water, ca. 7 km offshore of Plum Island. Fluvial cut-and-fill structures can be imaged in seismic and GPR data both offshore of this location and under this section of Plum Island (Fig. 9). By 3600 years ago, the lower Parker River had become fully tidal and developed in this location into a nearly 700-m wide tidal inlet, the paleo-Parker Inlet (Fig. 11). We will look in detail at GPR and sediment-core data from this buried inlet sequence. Over the course of hundreds of years, this inlet migrated, underwent ebb-delta breaching, shoaled and, by ca. 2000 years ago, closure (Hein et al., 2012). This final step resulted in the continued elongation of Plum Island, driven by dominant southerly sediment transport.
As in much of Plum Island, remnants of overwash fans can be seen along the western edge of the island in this location (these are largely covered south of the substation by freshwater marsh formed by the construction of dikes by the PRNWR). Overwash makes up only the western ca. 10 % of this island. The other 90 % of its width was built through progradation (seaward-building) (Hein et al., 2012). Plum Island owes this unusual feature to the fact that it has an ample source of sediment from the Merrimack River through which to continue to widen and build vertically.
Mileage Time Directions (STOP 7 to STOP 8)
31.6 0 min Go back out to Refuge Rd. and turn left (north) 33.9 10 min Continue (slowly!) along Refuge Rd. to PRNWR Parking Lot #1 (STOP 5/8). Make right
into lot. Ample parking is available.
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STOP 8: PRNWR PARKING LOT #1 (60 MINUTES) start at 3:00pm (UTM 19N: 352078 E, 4739076 N)
Figure 25. Topographic / bathymetric map of northern Plum Island showing the location of PRNWR Parking Lot #1 (STOP 5). From here, we will walk to the beach and north to Center Island (east of end of Plum Island Turnpike).
A 200 m walk east of the parking lot across aeolian sand dunes brings you to the beach at the northern end of the PRNWR. From here you can see Cape Ann to the south and, on a clear day, Boars Head to the north. These bedrock promontories mark the boundaries of the Merrimack Embayment. Offshore to a depth of ca. 60 m, sitting atop glacial till and post-glacial glaciomarine silt-and-clay, is an enormous volume (likely more than 2 billion m3; Table 1) of gravel, sand and silt that compose the late Pleistocene and Holocene regressive-transgressive braidplain delta, lowstand paleodelta, and sand sheet. The latter of these remains mobile and likely includes remnants of former barrier islands that existed during lower stages of sea level as well as sediments that have continued to be fed from erosion of sandy glacial deposits and bedrock upstream in the Merrimack River basin. Through erosion and reworking during the most recent marine transgression (13 ka to present), each of these paraglacial units has fed the development of the wide, long Plum Island barrier system with its high, well-developed dunes over which you just walked to arrive at the beach. The Merrimack Embayment barrier, backbarrier and shallow shelf system is enormous, and owes its entire existence to the sandy deposits left behind by the Laurentide Ice Sheet and the Merrimack River, which transported those sediments to the coast for reworking by waves, tides and wind.
A shallow cross-shore trench through the beach and the vertical scraping of the dune toe illuminate the processes by which waves, winds and storms rework sediment along and across the Plum Island beach and dunes.
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Cross bedding in the dune face demonstrates the act of winds to move sand from the beach, over the foredunes, and into the large, well-vegetated dunes on Plum Island. Multiple heavy mineral layers are evident in the profile. Though of similar grain size to the surrounding sediments, these layers are composed dominantly of red garnet. Some say this is where Plum Island got its name; others attribute it to plum bushes in the vegetated dunes. This garnet has a higher density than quartz sands and is therefore requires higher wave energy to move. It becomes concentrated along the beach face during storms and then buried during quieter times. These heavy-mineral layers thus mark old storm events. They are also responsible for most of the radar reflections visible in GPR lines.
Looking north again, one can see the south jetty of the Merrimack River Inlet. Constructed in the 1880s, lengthened several times throughout the 20th century, and reinforced and heightened as recently as Spring 2014, these jetties serve to stabilize the mouth of the Merrimack River. Without these jetties, the entire north fork of Plum Island (the section of island east of The Basin, the former channel of the Merrimack River) would not exist. Nor would most of the hundreds of homes that populate the northern 1/3 of Plum Island. Prior to the installation of the jetties, the Merrimack River mouth would migrate alongshore by several kilometers, nearly as far south as where you now stand.
However, the fight to control the coastal system of Plum Island continues. Walking north from the path over the dunes, you will pass out of the PRNWR and into the town of Newbury. A series of shore-normal groins extend from the dunes out into the surf zone. Mostly built in the 1970s during a former period of severe erosion, these had been fully covered by sand as the northern section of Plum Island prograded and accreted. As recently as 2007, their existence had been forgotten. However, the return of the erosion hot spot, driven by wave refraction around the Merrimack River Inlet ebb-tidal delta and its associated bars, at Center Island (the eastern end of Plum Island Turnpike) in 20072008 exposed the northern-most groin. Several homes were lost to erosion during this time (Fig. 26a). Southerly migration of this hot-spot has now exposed two additional groins and greatly intensified erosion along Annapolis Way, causing additional homes to fall into the Gulf of Maine (Fallon et al., submitted). The response by residents has been swift . . . and not always to the letter of the law. A group of homeowners starting a program of beach-scraping in 2012 and 2013, followed by the emplacement of a rip-rap revetment in the form of large boulders in front of their homes in 2013, and most recently, nourishment of the beach. While this has undoubtedly saved many homes, it has also changed the nature of the beach forever. Or has it? The exposure of an earlier (1970s?) revetment wall under what appeared only years ago to have been natural dunes suggests that the erosion that appears so devastating today is in fact cyclical, returning once every 2530 years. Perhaps, residents just need to wait this cycle out and their homes can be save for another generation, or at least until the forces of accelerated RSL rise and enhanced storminess overcome the natural inlet-sediment cycles.
Figure 26. Photos of recent erosion on Plum Island. (a) Erosion north of Center Island in February, 2009 (image courtesy of the Boston Globe, 9 February 2009). (b) High tide along Annapolis Way, 17 December 2013 (image courtesy of G. Cliffords, Plum Island Resident). Note groin (dating to 1970s) in foreground and rip-rap revetment along eroded dune line (installed in spring 2013). Although this section of beach used to look like the beach in the PRNWR as recently as 2008, little beach remains today at high tide.
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END OF TRIP (4:00pm)
RETURN TRIP TO WELLESLEY COLLEGE
Return to Wellesley College, approximately 1 hour, 15 min (without traffic) for Welcome Reception and registration (at The College Club at Wellesley College), starting at 5:00pm.
Mileage Time Directions (STOP 8 to Wellesley College)
33.9 0 min Start at STOP 5/8, PRNWR Parking Lot #; make right onto Refuge Rd and leave through PRNWR entrance gate
34.5 2 min Make left onto Plum Island Turnpike; becomes Water St. 36.4 5 min Make left onto Ocean Ave.; becomes Rolfes Ln.; cross over High Rd. (Rt. 1A); road
becomes Hanover St. 38.1 12 min Go straight at light at Rt 1 (Newburyport Turnpike); road becomes Middle Rd. 38.5 13 min Turn right onto Highfield Rd. 39.2 14 min Turn left onto Scotland Rd. 41.3 16 min Turn left to merge onto I-95 S 85.5 58 min Follow I-95 S. Take exit 21B-22 from I-95 S for Grove St. toward MA-16 W / Wellesley;
Take exit 21B on the left toward MA-16 W / Wellesley 89.4 1 hr, 5 min Merge onto Quinobequin Rd. 89.6 1 hr, 6 min Turn right onto MA-16 W / Washington St. 92.7 1 hr, 10 min Turn left onto Washington St. 93.4 1 hr, 15 min Arrive at The College Club at Wellesley College, 727 Washington St., Wellesley, MA
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