High-Resolution Subsurface Imaging & Sedimnetoogy of Coastal Ponds, Maine, USA - JSR, 2003

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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 73, NO. 4, JULY, 2003, P . 559–571Copyright 2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-559/$03.00

HIGH-RESOLUTION SUBSURFACE (GPR) IMAGING AND SEDIMENTOLOGY OF COASTAL PONDS,MAINE, U.S.A.: IMPLICATIONS FOR HOLOCENE BACK-BARRIER EVOLUTION

ILYA V. BUYNEVICH* AND DUNCAN M. FITZGERALD Department of Earth Sciences, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, U.S.A.

e-mail: [email protected]

ABSTRACT: Ground-penetrating radar (GPR) transects and sedimentcores have been used to examine the basement morphology, stratig-raphy, and environmental history of maritime ponds along the pen-insular coast of Maine. Silver Lake, Lily Pond, and North Pond areshallow ( 3 m) water bodies bordered by steep bedrock ridges in thenorth, east, and west, and sandy barriers to the south. The bedrockbasins of the ponds are formed in metasedimentary rocks surroundedby resistant pegmatitic intrusions. A dense network of GPR traversesobtained over the ice-covered Silver Lake reveals a series of prominentwavy-parallel and basin-fill reflector geometries terminating againstthe bedrock or grading into the barrier sediments and interpreted asorganic lake-bottom facies. The transparent units represent sand-richhorizons, mostly eolian in origin. Convex-up structures found both onthe surface and within the basin-fill sequence are interpreted as pre-

served parts of coastal dunes. The present study indicates that fresh-water conditions prevailed since at least 4.6 ka, with an initial sedi-mentation rate of 1.7 mm/yr. The position of this unit below the con-temporary sea level suggests presence of a welded barrier by that time.Radar profiles taken along the shores of Lily Pond, a small water bodybehind the Sand Dune Barrier, indicate a significantly larger arealextent of the pond in the past. A succession of organic deposits over-lying a Pleistocene glaciomarine unit indicates progressive inundationof the paleo-lagoon by rising sea level. Saltwater peat seaward of LilyPond was buried by washover sands about 1.2 ka, and a narrow pondexisted here prior to dredging and artificial infilling of its eastern partin the 1950s–60s. The organic and eolian units are absent in the NorthPond, where sedimentary fill consists of glaciomarine clay overlain bymarine sands. A proposed three-stage model of pond evolution alongan embayed coastline consists of: (1) organic accumulation in an up-

land depression during lower sea level; (2) predominantly washover ortidal deposition in a lagoon (Stage 2a) or blocked coastal pond (Stage2b) during initial transgression, and (3) mainly eolian and organic de-position behind a prograded or aggraded barrier. Future acceleratedrise in relative sea level and inadequate sediment supply will causemany back-barrier ponds to reenter Stage 2 of the proposed model.

INTRODUCTION

Lakes and ponds of diverse origins (closed lagoons, dune swales, defla-tion basins, bedrock depressions, thermokarst, deltaic, etc.) are commonfeatures along many coastlines (Reeves 1968; Aronow 1982). Their sedi-mentary fills serve as archives of depositional events that result from cli-matic, oceanographic, and geomorphic changes in coastal regions (Liu and

Fearn 1993; Devoy et al. 1996). One of the first comprehensive investi-gations of a coastal pond is the geological and oceanographic study of Oyster Pond, Massachusetts by Emery (1969), in which he used a shallowseismic technique (‘‘acoustic bottom penetrator’’) to examine pond stratig-raphy. With the advent of high-resolution geophysical techniques, such asground-penetrating radar (GPR), much greater resolution has been achievedin delineating the internal architecture of fluvial, periglacial, eolian, deltaic,and coastal barrier settings (Jol and Smith 1991; FitzGerald et al. 1992;

* Present address: Geology & Geophysics Department, MS #22, Woods HoleOceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A.

Schenk et al. 1993; Jol et al. 1996a; Jol et al. 1996b; van Heteren 1996;van Heteren et al. 1996; Leclerc and Hickin 1997; Smith and Jol 1997; Jolet al. 1998; van Heteren et al. 1998; Busby and Merritt 1999; Smith et al.1999; Neal and Roberts 2000). However, the use of GPR in sedimentolog-ical and stratigraphic investigations of basin-fill sequences of lakes andponds has been limited.

Ground-penetrating radar has been successfully used in geological stud-ies of inland lakes where the system was moved over the ice surface (Lob-ster Lake, Maine; Caldwell and FitzGerald 1995) and towed through thewater column (Lake Michigan, Sauck and Seng 1994; Lake Erie, Grant etal. 1996). In a study of the Saco barrier complex, Maine, van Heteren etal. (1996) demonstrated the continuity of the back-barrier stratigraphy byextending the radar profile over an ice-covered pond. To date, however, nostratigraphic studies have been reported using GPR to investigate the basin-

fill stratigraphy of shallow coastal lakes and ponds.Because the GPR signal is attenuated by saltwater, the freshwater natureof many coastal ponds and sedimentological heterogeneity of their basinfills (organic, eolian, washover, and tidal-inlet deposits) make them idealsettings for subsurface geophysical investigation of back-barrier stratigra-phy (Fig. 1). In addition, the flat surface of ice-covered ponds and nearlyhorizontal areas adjacent to many ponds reduce the need for topographiccorrection.

The aim of this study is to use GPR records in conjunction with sedimentcores to examine in detail the contrasting Late Quaternary geological his-tories of three sites along the peninsular coast of Maine. Based on geo-morphological, geophysical, and chronostratigraphic data, we propose athree-stage evolutionary model of back-barrier pond sedimentation alongindented coastlines. The applicability of the model is demonstrated by areview of published Holocene morphostratigraphies at a number of North

American and European sites.

PHYSICAL SETTING

Silver Lake, Lily Pond, and North Pond are located along the centralpeninsular coast of Maine (Table 1; Fig. 2). Silver Lake (43 44.7 N; 69

47.3 W) occupies a depression behind the Hunnewell Beach barrier onthe western flank of the Kennebec River estuary (Fig. 2). It has an openwater area of 51,430 m2 and an average depth of 1.2–1.5 m, reaching over3.0 m in the center. Steep vegetated bedrock ridges of Sabino Head andRockledge border the lake to the west and east, respectively (Figs. 2, 3).To the north, a gently sloping till-covered bedrock surface descends intothe lake with a prominent high ledge in the center. The bedrock ridges arecomposed of relatively resistant Devonian pegmatitic granites, whereas thedepression of the lake has been formed in less resistant Precambrian–Or-dovician metasedimentary rocks of the Casco Bay Group (Osberg et al.1985; Kelley 1987; Hussey 1989). The contact between the two rock typesis revealed along the edge of the lake. The southern border of the lake isformed by the 250–300 m-wide Hunnewell Beach dunefield. Historically,several dunes have migrated into the lake, producing steep slopes alongthe southern shoreline. The extreme southwest and southeast parts of thelake represent narrow, elongated extensions between steep bedrock ridgesand dunefield (Fig. 3).

Lily Pond (43 43.5 N; 69 51.4 W) is a small water body located atthe southern end of Hermit Island (Fig. 2). It is less than 1 m deep andhas an area of 7,500 m2 (Table 1). The pond is bordered to the north and



560 I.V. BUYNEVICH AND D.M. FITZGERALD

FIG. 1.—Generalized diagram of sedimentfluxes into bedrock-framed back-barrier waterbodies. Arrows show the relative magnitude andcontinuity of the depositional processes (see keyat bottom right). Along previously glaciatedcoasts, fluvial and marine reworking of glacialdeposits may be an important sediment source.Note the importance of eolian sedimentation anddune migration along relatively wide barriersystems.

west by steep ridges of metasedimentary rocks intruded by multiple graniticand pegmatitic bodies. Historically, Lily Pond had a greater area until the

1950s–60s when some material was dredged from the western part of thepond and placed on its eastern side (Iris Downs; Fig. 2). In a seawarddirection, the pond is bordered by the narrow (80–100 m) welded barrierof Sand Dune Beach. A large maritime Typha sp. bog forms the boundarybetween Sand Dune Beach and Head Beach to the southeast.

North Pond (43 46.2 N; 69 45.5 W) is located at the head of Saga-dahoc Bay (Fig. 2). To the south, North Pond is separated from the tidalflats of the bay by artificial impoundment and is presently regulated by asmall drainage canal. A steep, north–south bedrock ridge forms the easternshore of the lake and abuts a narrow sandy barrier to the east. The present-day open water area is the result of flooding due to a beaver dam.

Water-level fluctuations in the ponds depend on seasonal and long-termprecipitation–evaporation balance. During high rainfall, large quantities of water are shed from the surrounding bedrock ridges into the ponds andlakes. Forest fires, such as the Hermit Island Fire of 1938–1939 (I. Bugler,

personal communication), may have resulted in temporary loss of vegeta-tion and higher rates of runoff and eolian input. Topographic surveyingindicates that the average water surface elevations in Silver Lake, LilyPond, and North Pond are 1.5 m, 0.7 m, and 0.5 m above the ocean meanhigh water (MHW) level, respectively. A mean tidal range of 2.6 m (springrange is 3.5 m) and mean shallow water wave height of 0.5 m (Jensen1983) place this part of the Maine coast in a mixed energy, tide-dominatedenvironment (Hayes 1979; FitzGerald et al. 1994). The mean high waterin this region is 1.36 m above the National Geodetic Vertical Datum of 1929 (NGVD-29; Gehrels et al. 1996).

METHODS

The internal stratification the sedimentary sequences of the ponds andassociated barriers was investigated using a Geophysical Survey Systems,Inc. SIR-3 ground-penetrating radar system. The radar control unit wasmounted on a cart for increased mobility. The 120 MHz transceiver fre-quency allowed for optimal penetration (6–10 m) and resolution of 0.2–0.7 m (see Topp et al. 1980; Jol 1995; Conyers and Goodman 1997; andvan Heteren et al. 1998, for description of the theoretical aspects of GPRtechnique). A dense network of GPR transects was obtained over SilverLake during the winter months when the lake surface was ice-covered (Fig.3). At Lily Pond and North Pond, geophysical transects were run along themargins of the ponds and across the barrier. For travel-time to depth con-version, mean signal velocities of 0.15 and 0.07 m/nanosecond were usedfor unsaturated and saturated sand, respectively. Where available, the

depths to key subsurface reflectors were also matched with major litholog-ical changes in sediment cores. GPR records were topographically corrected

where the relief was greater than 0.5 m. Because of the nearly flat surfaceof pond margins and ice-covered surfaces of the ponds, no topographiccorrection of these areas was needed. The attenuation of electromagneticGPR signal by saltwater was a limiting factor only at the seaward extremeof one shore-normal profile. In this paper, depths are presented relative tomean high-water level, unless noted otherwise.

Geophysical data were groundtruthed with five vibracores and eightpulse-auger cores that were taken along GPR survey lines from the pondbasins and adjacent coastal areas. Core penetration ranged from 3 to 8 m.Textural characteristics of the core samples were analyzed using the RapidSediment Analyzer at the U.S. Geological Survey/Woods Hole Sedimen-tology Laboratory. This information, together with compositional charac-teristics, provides a basis for interpreting the depositional environments of basin-fill lithofacies. Conventional radiocarbon dates on organic sedimentsare reported as uncalibrated, 13C-corrected ages.

RESULTS

Silver Lake

Shore-Parallel Transect.—The geophysical profiles taken over the ice-covered surface of Silver Lake show a number of reflectors of varyingintensity and geometry. The western segment of profile SL-2 reveals aconvex-up reflector on the otherwise flat lake bottom with several promi-nent subhorizontal to slightly convex-up signal returns (Fig. 4A). The mor-phology of this reflector and its location basinward of a large lakeshoredune suggest that it is the surface of the submerged extension of a dune.Several subhorizontal reflectors extend to a depth of at least 6 m. VibracorePB-1 taken through the lake fill sequence east of the submerged dune con-tained several layers of muddy freshwater peat interbedded with well-sortedmedium sand (Figs. 4A, 5). Similar stratigraphy was found in core PB-22at the westernmost extension of the lake (Fig. 3).

The eastern part of profile SL-2 shows a similar reflector with irregularconvex-up reflector configuration indicative of a submerged dune (Fig. 4B).The western part of this dune can be traced beneath recent lake sediments.To the east, the dune borders a depression 2.0–2.5 m deep in the lakebottom. Beneath this low area is a series of closely spaced, wavy-paralleland basin-fill reflectors characteristic of lacustrine sedimentary sequences(see van Heteren et al. 1998 for discussion of GPR facies). Core PB-28penetrated three layers of muddy peat at 0–0.23 m, 1.27–1.60 m, and 2.65–2.75 m below the lake bottom (Figs. 4B, 5). Bedrock was encountered at



561 HIGH-RESOLUTION IMAGING OF COASTAL PONDS

FIG. 2.—Location of the study area and the three sites discussed in the text. Note the embayed nature of the coastline with numerous bedrock ridges and islands providingpinning points for coastal accumulation forms. MLW, mean low water.

2.75 m and is a continuation of the Rockledge headland, which descendsabruptly into the lake (Figs. 3, 4B).

Shore-Normal Transect.—A bedrock promontory on the north shore of Silver Lake (Fig. 3) can be traced on GPR profiles to depths of over 7 mbelow the lake surface (profile SL-10; Fig. 4C). In core PB-30 just westof the profile, a thin layer of till was collected above the refusal. Alongthe southern part of the trace the margin of the dunefield is seen as a steeply

descending surface reflection with several basinward-dipping beds. Belowthe flat lake-bottom reflector, a sequence of convex-up and wavy-parallelreflectors can be traced along the profile. A prominent convex-up reflectorwith a transparent central part is analogous to the submerged dune signa-tures shown in Figs. 4A and 4B.

A 7-m-long pulse-auger core PO-9 penetrated several units of muddylake-bottom sediments (gyttja) intercalated with organic-rich sandy hori-




TABLE 1.—Comparison of pond physiography and stratigraphy at the threestudy sites.

Silver Lake Lily Pond North Pond

Maximum length (m)Maximum width (m)Area (m2)1

Average depth (m)Barrier width (m)

410200

51,4301.2–1.5

250–300

110100

7,5000.5

80–100

14090

12,3200.5

15–20Sedimentology of basin fill:2

eolian sandwashover sandfreshwater peatsaltwater peatlowermost unit

CRCAtill

CCCC

glaciomarine

ACAA

glaciomarine

1 Post-1970 values.2 Occurrence of sedimentary deposits: C—common; R—rare; A—absent.

FIG. 3.—Vertical aerial photograph of Silver Lake showing locations of GPR transects and sediment cores. Note vegetated dunes along the south shore of the lake withrecent evidence of migration into the lake basin.

zons. The mean grain size of sediment samples from the lower part of thecore was compared to that of nearby cores PO-4 and PO-5 collected fromthe Hunnewell dunefield (Fig. 3). In all cores, a prominent shift to coarsergrain sizes occurs between 6 and 8 m below MHW (Fig. 6). However, atthis depth in core PO-9 the moderately well to very well-sorted fine-grainedsands closely resemble the upper eolian units in cores PO-4 and PO-5.Therefore, the deep unit in core PO-9 was likely formed by wind deposition

in the proto-Silver Lake and was contemporaneous with coarse to mediumproto-barrier sands.

Sediment Accumulation Rates.—A sandy freshwater peat unit ( 13C 29.5‰) at 8.30–8.52 m below the water surface (5.95–6.17 m belowlake bottom) was collected at a similar depth in cores PO-4 and 5 (Figs.3, 6). The top of this was dated in core PO-4 at 4,600 65 14C yr BP.We assume that the continuation of the peat horizon cored in Silver Lakehas a similar age. Therefore, following the deposition of this lower peatunit, the time-averaged sediment accumulation rate is calculated at 1.8–1.9mm/yr. The only age estimate for sediments in Silver Lake is 2,395 4014C yr BP reported by Nelson (1979). This date was obtained on freshwaterpeat at 2.2 m below the lake bottom from a piston core located approxi-mately 40 m east of core PO-9. When considering this intermediate age,the time-averaged accumulation rate of lake sediments is somewhat lowerthan above, decreasing from 1.6–1.8 mm/yr between 4.6 and 2.4 ka to 0.9mm/yr after 2.4 ka.

Lily Pond

Shore-Parallel Transect.—Ground-penetrating radar profile LP-1 wascollected along the seaward side of Lily Pond and Iris Downs (Fig. 2).Although Iris Downs are capped by the dredged material, the GPR recordpenetrated to 2.5–3.0 m into the original pond sequence. The bottom por-tion of the record exhibits irregular to wavy-parallel reflectors which at-

tenuated the radar signal. In core SD-2, these coincide with the top of Pleistocene glaciomarine sandy clay (Presumpscot Formation, Bloom 1963;




FIG. 4.—A) Western segment of GPR profileSL-2 taken over the ice-covered Silver Lake.Several subparallel reflectors within the lake-basin sequence represent organic-rich horizonsinterbedded with siliciclastic units, mainly eolianin origin, penetrated by core SL-1. Thesubmerged feature on the lake bottom coincideswith the extension of the dune along the southshore of the lake. B) Eastern segment of GPRprofile SL-2, showing infilled bedrockdepression, which is also manifested in lake-bottom morphology. The steep western flank of

the depression is a slipface of a submerged dunesimilar to that in Fig. 4A. C) A shore-normalGPR profile SL-10 showing the steeply plungingbedrock surface along the north side of the lake.The slipface of a dune is revealed at the southend of the profile. The convex-up reflector at 3–4 m depth is indicative of a buried dune cappedby organic-rich lake sediments. See Fig. 3 forlocations of GPR profiles and sediment cores.For detailed core logs of vibracores PB-1 andPB-28, and pulse-auger PO-9 refer to Fig. 5.




FIG. 5.—Selected core logs used for interpretation of subsurface reflections and for paleoenvironmental reconstruction. (PB-1 and PB-28 are vibracores; the remainderare pulse-auger cores.)




FIG. 6.—Variation in mean grain size with depth in Silver Lake core PO-9 andcores PO-4 and 5 taken through the barrier sequence (see Fig. 3 for locations). Theprominent coarsening of barrier sediments between 6 and 8 m depth coincides with

similar trend in lake core 9. The textural boundary between beach/washover andeolian sands is based on sedimentological characteristics of recent estuarine andbarrier sediments (Buynevich 2001). The mean-grain-size values in the coarse-grained interval in the two dunefield cores is similar to modern fluvial–estuarinesediments.

Figs. 5, 7). Overlying this reflector are at least two prominent, continuous,undulating reflectors which correspond to peat horizons in core

SD-2. The two layers of dark-brown to black compact peat with woodfragments and detrital organics, diagnostic of terrestrial wetland conditions,are separated by a coarse sand unit at the 2.48–2.80 m interval in the core.A thin layer of light-brown, muddy peat with Spartina patens rhizomes(2.22–2.39 m) occurs in the middle of the upper freshwater peat unit (seeFig. 5). Both the glaciomarine and the overlying organic sediments laponto the bedrock that crops out to the southeast. The transparent unit abovethe 2 m depth is composed mainly of fine-grained, well-sorted sands witha bounding-surface configuration indicative of a coastal dune (van Heterenet al. 1998). The low-frequency chaotic reflections at the top of the traceare due to human disturbance and the placement of the fill (Fig. 7).

Shore-Normal Transect.—The NE–SW oriented GPR profile LP-2 wastaken from the edge of Lily Pond to the beach (Figs. 2, 8A) and ground-truthed with four pulse-auger cores (SD-1, 1A, 2, and 3; Fig. 8). Thistransect reveals a prominent, thick reflector extending beneath the pond andascending in a seaward direction. This reflector is likely the result of amal-gamation of freshwater and saltwater organic units discussed above (profileLP-1). At least two separate horizons of light-brown peat with remains of

saltmarsh vegetation (i.e., Spartina sp.) were identified in sediment cores(Figs. 5, 8B, 9). All cores contain a unit of coarse sand to sandy gravelthat pinches out in a landward direction beneath the pond. This unit isinterpreted as a washover deposit. A saltwater peat ( 13C 17.9‰)immediately below this unit was dated at 1,170 105 14C years BP pro-viding a maximum age for washover deposition. The bottom reflector cor-responds to bedrock overlain by glaciomarine clay in seaward cores SD-1and 1A (Fig. 8B).

North Pond

The GPR profile NP-1 taken along the seaward edge of North Pondexhibits a prominent concave-upward reflector that descends to over 6 min the middle part (Fig. 10). Based on pulse-auger core N-1, it representsthe top of Pleistocene glaciomarine clay. Multiple reduction-stained lami-nae and burrow fills were present throughout the 2.2-m-thick clay unit (Fig.5). The subsurface outline of the basin fill extends to the surrounding bed-rock ridges, indicating that the original depression was similar to or largerthan the present-day open-water area of the North Pond. The hyperbolicreflections beneath the clay are indicative of isoclinally folded bedrock,which is exposed adjacent to the pond. The image of several hyperbolicreflectors along the western segment of the basin fill suggests that the claylayer is relatively thin there (Fig. 10). Where the clay unit reaches over

0.2–0.5 m in thickness, the GPR signal is commonly attenuated (van Het-eren 1996), as seen along the eastern half of the basin. Along this section,the bedrock surface is placed at 5.2 m on the basis of core refusal. A seriesof concave-up, nested reflectors in the middle of the record coincides witha sequence of interbedded quartz-rich, coarse to medium sand beds. Lumpsof sand-coated clay were found at the base of the unit. In contrast to theother two sites, no peat or fine-grained eolian sand were present in the core(Figs. 5, 10; Table 1). Organic deposits are confined to the top 5–10 cmof the pond sediments.

DISCUSSION

Back-Barrier Sedimentation and Implications for Coastal Development

The results of GPR and coring studies illustrate contrasting histories of

Holocene sedimentation in Silver Lake and Lily Pond. A thick ( 7 m)sedimentary sequence in Silver Lake consists of muddy organic units,which produce prominent wavy-parallel and basin-fill reflectors on GPRrecords (Fig. 4). The predominantly freshwater nature of these units issupported by: (1) presence of terrestrial organic debris (bark, twigs, woodchips of cedar and pine); (2) abundance of cladoceran beetle carapaces andpennate diatoms (Nelson 1979); (3) black color, and (4) strongly negative 13C value of at least the lowermost horizon (20 to 30‰; Stuiver andPollach 1977). The lowermost sandy freshwater peat layer beneath the bar-rier dunes and the lake is 2.5–3.0 m below the contemporaneous sea level(Fig. 6; Gehrels et al. 1996). Such a large elevation difference cannot beexplained by compaction due to high sand content of the peat unit. Themost probable explanation of this stratigraphy is that organic sedimentswere deposited in a large proto-lake or seasonal bog behind a coastal bar-rier. In an analogous fashion, organic-rich sediments are presently formingin Silver Lake at a depth of over 2 m below present ocean mean high waterlevel. The existence of the lowermost nonmarine organic deposits indicatesthat a coastal barrier had welded to both Sabino and Rockledge bedrockridges by at least 4.6 ka. Moreover, this finding suggests that sedimentfrom the nearby Kennebec River must have been adequate to maintain thebarrier and even cause its progradation through mid- to late Holocene. Thelarge extent of the coastal wetland in this area and throughout the regionby 4.6 ka may be due to increasingly humid climatic conditions ineastern North America at that time (Delcourt and Delcourt 1984; Webb etal. 1993).

Stabilization of the barrier was facilitated by buried bedrock pinnacles,




FIG. 7.—Shore-parallel GPR profile LP-1 taken along the seaward margin of Lily Pond (see Fig. 2 for location). A relatively thin Holocene sequence of interbeddedfreshwater peat, saltmarsh peat, washover sand, and dune deposits overlies the bedrock, which is capped by Pleistocene glaciomarine clay in a landward section. The basal

freshwater peat represents sedimentation in an upland pond or wetland during lower sea level. The subsurface record reveals a much larger former extent of the pond. SeeFig. 5 for descriptions of cores SD-1 and SD-2.

which rise from 60 m below MHW to within 10 m under the uppershoreface (Buynevich et al. 1999). The low area between this antecedenthigh and the Sabino–Rockledge ridge formed a natural depression that con-tained the original water body or wetland. The proposed development of awelded barrier is similar to ‘‘barrier blocking’’ and subsequent formationof freshwater wetlands described for Atlantic coastal sites of Europe byDevoy et al. (1996). In these areas, however, the high-energy wave regimeresulted in deposition of coarse-grained washover units. In contrast, thefine-grained eolian sands have dominated the clastic component of SilverLake sedimentary fill, particularly throughout the late Holocene prograda-

tional phase of barrier development (Buynevich 2001; Buynevich and Fitz-Gerald 2001).

The convex-upward reflections with characteristically transparent centralparts found on the lake floor and within its basin fill represent parts of recently active and buried dunes, respectively. The occasional subparallelto convex-up reflectors within the dune facies may represent periods of organic accumulation when dunes became less mobile. The drape of or-ganic sediments over both recent and buried dunes appears as a strongGPR reflector and improves delineation of their morphology and dimen-sions. Possible mechanisms for renewed dune migration include devege-tation due to disease and deforestation. Drought and forest fires were alsoproposed as possible causes of parabolic dune migration (Nelson 1979).

In contrast to Silver Lake, Lily Pond has a relatively thin ( 4 m)sedimentary sequence of Late Quaternary age (Figs. 6, 7). The Holocenecoastal deposits overlie the Pleistocene glaciomarine sandy clay sequenceand are separated from it by the basal unconformity (Ub; Figs. 5, 8B). Thissurface, which can be traced offshore, represents a regressive erosionalunconformity produced by relative sea-level fall associated with the post-glacial isostatic rebound (Kelley et al. 1987; Belknap et al. 1989; Barnhardtet al. 1997). Formation of freshwater peat was enhanced by runoff fromsteep bedrock ridges and accumulation of detrital organic matter in topo-graphically low areas. At the Lily Pond site, a transverse bedrock ridge inthe middle of the section produced a topographically low area behind it,which became a locus of organic deposition. Additionally, it provided alocal pinning point for barrier stabilization during its onshore migration.The lowermost contact between freshwater organics and overlying salt-

marsh peat and washover sands represents the leading edge of transgres-sion. If a time gap is present between the deposition of the two organicunits, this contact will be equivalent to a transgressive unconformity (Ut),which separates the freshwater peat from the overlying washover unit (Figs.5, 8B). Saltmarsh peat horizons and coarse-grained washover units in themiddle of the sequence indicate that tidal inlet(s) penetrated through anarrow and low barrier around 1.2 ka (Buynevich 2001).

Several well-defined GPR reflectors were used to extrapolate the dataover a large area. These correspond to specific organic-rich sedimentaryunits in sediment cores. The nature of organic deposits (freshwater vs.

brackish/saltwater) was used to estimate the extent of freshwater (pond/ bog) and marine (lagoon) deposition, respectively (Fig. 9). The saltwaterpeat units sandwiched between freshwater peat horizons suggest prolongedperiods of back-barrier salt marsh deposition (tens to hundreds of years;see Pethick 1981). The deceleration in sea-level rise along this coast andsubsequent welding and aggradation of the Sand Dune Barrier closed thetidal inlet and transformed the saltmarsh into a freshwater pond that persiststo the present day. The entire sedimentary sequence within Lily Pond rep-resents an alteration between increasing and decreasing salinity similar tothe stratigraphic record of many back-barrier lagoons along the Maine coast(Duffy et al. 1989).

In contrast to Silver Lake and Lily Pond, the geophysical and strati-graphic results from North Pond suggest a prolonged period of nondepo-sition following the accumulation of glaciomarine clay. The top of theglaciomarine unit represents the basal unconformity (Ub), which in thiscase coincides with the transgressive unconformity (Ut). The erosion of theexposed Pleistocene unit is marked by sand-coated clay lumps immediatelyabove the unconformity. The lack of organic or eolian deposits in NorthPond contrasts with previous sites. This finding suggests that at least theseaward part of the pond was subaerially exposed between the depositionof glaciomarine and overlying transgressive units. The latter are most likelytidal-flat and washover deposits, as evidenced by their coarse-grained na-ture. High content of quartz and lack of till exposures indicate that thesewere derived from a seaward source rather than erosion and reworking of local glacial deposits. Because Sagadahoc Bay is presently floored by fine-grained, mica-rich sands, the transgressive deposits of North Pond suggest




FIG. 8.—A) Shore-normal GPR profile LP-2 extending from the seaward part of Lily Pond to the foredune ridge. B) Stratigraphic section based on GPR transect andfour sediment cores. Note the complex stratigraphy underlying the pond and extending beneath the barrier. The pinnacle in the middle of the transect is a cross-sectionalview of a transverse bedrock ridge with a topographically low area behind it. Ub, basal unconformity, Ut, transgressive unconformity. See Fig. 2 for location.

that a high-energy regime (i.e., strong tidal currents and/or storm waves)prevailed at the head of the bay during their deposition. The infilling of the pond in an eastward direction in a series of nested basins shows howthe accommodation space of the original depositional basin was reducedover time (Fig. 10). The entire basin-fill sequence has high preservationpotential with the continuing transgression.

Evolutionary Model of Pond Infilling

On the basis of geophysical and sedimentological data, we propose athree-stage evolutionary model of sedimentation in barrier-fronted pondsalong embayed coastlines (Fig. 11). In Stage 1, freshwater, organic-richsediments accumulate over bedrock and regolith of variable thickness. The




FIG. 9.—Reconstruction of the spatial extent of freshwater wetland and salt marsh based ongeophysical and sedimentological data. Thepresence of saltmarsh peat between freshwater

units is indicative of seawater incursion througha tidal inlet that subsequently closed.

FIG. 10.—Shore-parallel GPR profile NP-1 along the seaward margin of North Pond. The radar signal is attenuated along the eastern margin by a thick clay unit, whereas severalbedrock pinnacles can be seen through a thinner clay horizon along the western side of the basin fill. Several nested basins represent the pattern of pond infilling and furtherdemonstrate the ability of GPR to provide high-resolution subsurface information that cannot be obtained by other methods. See Fig. 5 for a log of pulse-auger core N-1.

organic sedimentation takes place in an upland depression during lowerstages of sea level (i.e., before local sea level reaches the site) and may bepreserved only in the deepest part of the basin. The organic, gyttja-like siltof Duffy et al. (1989) is an example of the resulting deposit found at thebases of several back-barrier sequences in Maine. In the Silver Lake basin,sediments representing this stage were not encountered in the cores and areprobably confined to the deep central part of the basin. The basal peat inLily Pond immediately overlying the Pleistocene clay was probably formedduring period of lower sea level during Stage 1. In the North Pond, this

stage is represented by the amalgamated basal and transgressive unconfor-mities. The reason for the absence of organic units here is unclear, andperhaps is related to the lack of sufficient drainage required for transportof organic detritus into the depression combined with its originally smallsize. The upland ponds that have not experienced inundation during theHolocene are still in Stage 1 of their development (e.g., Big Pond on CapeSmall, Fig. 2).

In areas where rising sea level and abundant sediment supply result inbarrier formation, Stage 2 of pond sedimentation begins (Fig. 11). At this




FIG. 11.—Diagram of a three-stage model of back-barrier evolution along an embayed paraglacial coastline. The presence of till and glaciomarine deposits exemplifies theparaglacial coast of Maine. Stage 1—organic-rich detritus accumulates in bedrock depressions during lower stages of relative sea level (RSL). Stage 2—with rising sea leveand abundant sediment supply from the inner shelf or longshore sources (rivers, coastal bluffs) a narrow barrier is formed. The presence of a tidal inlet determines whetherlagoon (2a) or pond (2b) occupies the back-barrier depression. Stage 3—with progradation and heightening of the barrier, eolian deposition and dune migration become theprimary modes of clastic sediment input into a coastal lake or pond. Note the reversal of conditions to Stage 2 that may result from subsequent accelerated sea-level rise.

time, the water body behind the barrier may be connected to the ocean viaa tidal inlet (Stage 2a—lagoon; e.g., Lily Pond, North Pond; see also Duffyet al. 1989). Alternatively, the back-barrier depression can be separated fromthe ocean by a welded barrier and no record of long-term seawater influencemay result (Stage 2b—blocked pond; e.g., Silver Lake). In this stage, theposition, dimensions, and dynamics of the barrier, rather than sea-level his-tory, may largely dictate the back-barrier sedimentation processes. An anal-ogous situation of coastal morphodynamics controlling back-barrier accretionhas been reported by Jennings et al. (1997) for gravel barrier-enclosed seep-age lagoons of Nova Scotia. Where inlets are small or absent and the frontingbarrier is narrow and low, washover sedimentation is the dominant mecha-nism of clastic sediment input into the pond/lagoon (Figs. 1, 10). Switchingbetween Stages 2a and 2b may result from inlet opening and closure, and ismanifested in intercalated horizons of brackish/saltwater and freshwater peat.According to this model, modern lagoons represent Stage 2a, and may ormay not enter the next stage of development.

In Stage 3, barrier progradation (due to deceleration in sea-level rise,coastal uplift, or increase in sediment supply) and dune heightening pre-

clude washover deposition or tidal inlet formation. Exceptions may include:(1) anomalously high storm water levels capable of overtopping or erodingrelatively wide barriers; (2) episodic coastal subsidence, and (3) artificialexcavation of outlets for drainage or aquaculture. Most of the clastic sed-iment input to the back-barrier lake or pond at Stage 3 is through eoliantransport (deflation and dune migration during de-stabilization). In areaswhere dune sands have been stabilized by vegetation, organic deposition(detrital peat or gyttja) becomes the dominant process of pond bottom sed-imentation (Figs. 1, 10). With continuing sea-level rise, coastal lakes andponds, especially those fronted by low barriers, experience increased fre-quency of saltwater inundation and are likely to reenter Stage 2 of theirgeological history. Regardless of environmental changes, the basin-fill se-quences discussed in this paper have an excellent preservation potentialduring sea-level rise.

Implications for Coastal Stratigraphic Research

The applicability of the proposed model is demonstrated by comparingthe characteristics of the three study sites to those from several existing




TABLE 2.—Physiography and evolutionary stages of selected coastal ponds and wetlands. See Figure 2 for locations of New England sites.

Site LocationPond Length/

Width (max, m)Long AxisOrientation

InletType1

BarrierWidth (m)

BarrierTexture

EvolutionaryStages2 Reference

Silver Lake, south-central MaineLily Pond, south-central MaineNorth Pond, south-central MaineShort Pond, south-western MaineJasper Beach lagoon, north-central Maine

410/200110/100140/90130/70300/150

shore-parallelshore-parallelshore-normalshore-parallelshore-parallel

nnan

t s

250–30080–10015–20

200–23060–80

sandsandsandsandgravel

(1)-2b-31-2a-32a-32a-32a

this studythis studythis study12

Provincelands intradunal ponds, Mass.

Oyster Pond, Nova Scotia, CanadaRugged Head Pond, Nova ScotiaBlack Island ponds, Nova Scotia

800/500

3,000/5002,300/1,700

30/20

shore-parallel

shore-normalshore-parallelshore-parallel

n

t sa s

n

800–1,500

100–15050–25060–80

sand

gravelgravelgravel

3

2a2a2a-2b

3

445

Lough Cahasy, Co. Mayo, W. IrelandBran Lough, Co. Clare, W. IrelandEtang de Nerizelec, B rittany, FranceMarais de la Joie, Brittany, France

320/12070/50

350/1001,000/700

shore-normalshore-normalshore-normalshore-parallel

nasa

150–180100–120320–350200–250

sand gravelsand gravelgravelsand

2a2b-32a-2b2a-3

6, 7777

1 Inlet type: t—natural tidal inlet; a—artificial channel/sluice; s—water exchange by seepage through the barrier; n—no inlet.2 Basis for assignment: Stage 1—presence of basal freshwater peat; Stage 2a—saltwater peat, lagoonal silt/clay, or tidal inlet deposits (e.g., flood-tidal delta); Stage 2b—no sedimentary record of long-term saltwater

conditions during Holocene transgression; Stage 3—absence of a natural inlet; dominant clastic input through aeolian sedimentation, where available.References: 1, van Heteren et al. (1996); 2, Duffy et al. (1989); 3, Winkler (1992); 4, Jennings et al. (1997); 5, Carter et al. (1989); 6, Delaney and Devoy (1995); 7, Devoy et al. (1996).

databases on back-barrier ponds and lagoons along temperate coastlines of various origins (Table 2). The presence of freshwater organic-rich units inthe lower part of the stratigraphic record at Silver Lake and Lily Pondillustrates the importance of antecedent bedrock depressions for initial or-ganic sedimentation (Stages 1 and 2; Fig. 11). Stage 1 is absent in coastalwater bodies formed within the barrier or along a low-gradient upland (seeother examples).

A sequence of interbedded saltmarsh peats and washovers in many back-barrier water bodies documents a history of active tidal exchange duringmost of the Holocene (Stage 2a. e.g., semi-enclosed lagoons of Nova Sco-tia, Ireland, and France; Table 2). On the other hand, the absence of salt-marsh peat in lake-basin stratigraphy suggests that the back-barrier has notexperienced long-term inundation by seawater and evolved as a blockedpond (i.e., freshwater basin separated from the ocean by a welded barrier,Stage 2b). Such features would be more common along indented, rockycoasts (e.g., Silver Lake, Bran Lough, Table 2) or those with glacial de-posits acting as anchor points for the barriers. Several mechanisms fortransforming the lagoon into a pond during Stage 2 include: (1) decrease

of back-barrier tidal prism due to rapid infilling; (2) elongation of the bar-rier due to increase in longshore sediment supply, and (3) artificial inletclosure. In all cases, the barrier may still be relatively narrow and low andsubject to washover sedimentation or breaching. Table 2 indicates thatseepage of water through gravel barriers plays a significant role in con-trolling back-barrier water levels and sedimentation (Carter and Orford1984; Carter et al. 1989; Jennings et al. 1997), a process which is notimportant in sandy barrier lithosomes.

The stability of back-barrier lakes and ponds in Stage 3 is reflected inbasin-fill stratigraphy by organic-rich peat and gyttja interbedded with lam-inae of windblown sand or thicker beds produced by dune migration (Figs.1, 10). In some areas, eolian deposits may undergo partial reworking andresedimentation by active streams (e.g., Silver Strand, Ireland; Delaney andDevoy 1995). Some coastal ponds may form and evolve entirely during

Stage 3, such as intradunal ponds formed during the period of Holocenetransgression but removed from any seawater influence (e.g., within widestrandplains or large dune systems, such as Provincelands Dunefield; Wink-ler 1992; Figure 2; Table 2).

The present study demonstrates the use of GPR profiling complementedby sediment cores in high-resolution lithostratigraphic analysis of back-barrier freshwater bodies. Our findings and the proposed model emphasizethat in addition to relative sea-level history and sediment supply alongsand-dominated barrier coasts, the local antecedent basement morphology,barrier morphodynamics, and dune stability have to be considered in orderto accurately reconstruct the depositional history of coastal lakes and ponds.

CONCLUSIONS

High-resolution ground-penetrating radar records of barrier and back-barrier sequences demonstrate the importance of antecedent topography in

Holocene coastal evolution. In some cases, topographically high areas sea-ward of the pond basins not only isolated natural depressions for wetlanddevelopment but may have also aided barrier stabilization and progradation.

Silver Lake experienced periods of organic accumulation punctuated byeolian sedimentation. GPR profiles reveal both buried and recently activedunes along the south shore of the lake. Absence of saltmarsh peat andmicrofauna indicates that nonmarine conditions prevailed in this area sinceat least 4.6 ka. Occurrence of freshwater organic units several meters belowcontemporary sea level suggests deposition behind a welded proto-barriersince mid-Holocene. In contrast, the stratigraphy of Lily Pond indicates atleast two prolonged periods of saltwater incursion within the past 2 ka.The clastic input into the pond was through washover, and later, eoliandeposition. At present, the open-water area of Lily Pond is less than 15%of the original freshwater body. The stratigraphy of North Pond suggests

nondeposition over the Pleistocene clay followed by washover and tidalflat deposition during the late Holocene when local sea level reached thehead of Sagadahoc Bay.

Holocene stratigraphy of back-barrier ponds along the indented coast of Maine is a function of in situ organic deposition interrupted by periods of clastic sedimentation of variable duration and magnitude and can be ex-plained by a 3-stage evolutionary model. The initial stages of pond devel-opment involve organic sedimentation or nondeposition in an upland de-pression during lower sea level (Stage 1). As sea level rises, tidal inlet andwashover deposition play an important role, particularly if the barrier islow and narrow (Stage 2a). Abundant sediment supply may preclude for-mation of a tidal inlet as the landward migrating barrier is welded to bed-rock ridges on both sides (Stage 2b). During the phases of decelerated sea-level rise, local uplift, or increase in sediment supply, eolian depositionbecomes dominant both through deflation of dunes and large-scale dune

migration (Stage 3). Further rise of sea level and reduction in the supplyof clastic sediments may cause many back-barrier ponds to revert to Stage2 of the evolutionary sequence. A review of the existing studies in physi-ographically similar settings demonstrates the applicability of the proposedmodel.

ACKNOWLEDGMENTS

This study was funded by American Association of Petroleum Geologists Grant#528–12–01, American Chemical Society Contract #32527-AC8, and GeochronLaboratories Radiocarbon-Dating Award. We thank Brent Taylor, U.S. GeologicalSurvey, Woods Hole for his help with RSA data processing and Ian Bugler for




historical information and access to Hermit Island. We extend our gratitude to Sytzevan Heteren, Amy Dougherty, Paul McKinlay, Sarah Mills, Donald Hunt, and An-drew Lorrey for their assistance in the field. Reviews and criticisms by JosephKelley, Walter Barnhardt, Jan Alexander, Daniel Belknap, and David Marchant sig-nificantly improved the manuscript.

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Received 17 July 2000; accepted 18 December 2002.

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High-Resolution Subsurface Imaging & Sedimnetoogy of Coastal Ponds, Maine, USA - JSR, 2003