high-resolution subsurface (gpr) - Geology Journal

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 sediment cores have been used to examine the basement morphology, stratigraphy, and environmental history of maritime ponds along the peninsular coast of Maine. Silver Lake, Lily Pond, and North Pond are shallow (, 3 m) water bodies bordered by steep bedrock ridges in the north, east, and west, and sandy barriers to the south. The bedrock basins of the ponds are formed in metasedimentary rocks surrounded by resistant pegmatitic intrusions. A dense network of GPR traverses obtained over the ice-covered Silver Lake reveals a series of prominent wavy-parallel and basin-fill reflector geometries terminating against the bedrock or grading into the barrier sediments and interpreted as organic lake-bottom facies. The transparent units represent sand-rich horizons, mostly eolian in origin. Convex-up structures found both on the surface and within the basin-fill sequence are interpreted as preserved parts of coastal dunes. The present study indicates that freshwater conditions prevailed since at least 4.6 ka, with an initial sedimentation rate of 1.7 mm/yr. The position of this unit below the contemporary sea level suggests presence of a welded barrier by that time. Radar profiles taken along the shores of Lily Pond, a small water body behind the Sand Dune Barrier, indicate a significantly larger areal extent of the pond in the past. A succession of organic deposits overlying a Pleistocene glaciomarine unit indicates progressive inundation of the paleo-lagoon by rising sea level. Saltwater peat seaward of Lily Pond was buried by washover sands about 1.2 ka, and a narrow pond existed here prior to dredging and artificial infilling of its eastern part in the 1950s–60s. The organic and eolian units are absent in the North Pond, where sedimentary fill consists of glaciomarine clay overlain by marine sands. A proposed three-stage model of pond evolution along an embayed coastline consists of: (1) organic accumulation in an upland depression during lower sea level; (2) predominantly washover or tidal deposition in a lagoon (Stage 2a) or blocked coastal pond (Stage 2b) during initial transgression, and (3) mainly eolian and organic deposition behind a prograded or aggraded barrier. Future accelerated rise in relative sea level and inadequate sediment supply will cause many back-barrier ponds to reenter Stage 2 of the proposed model. INTRODUCTION

Lakes and ponds of diverse origins (closed lagoons, dune swales, deflation basins, bedrock depressions, thermokarst, deltaic, etc.) are common features along many coastlines (Reeves 1968; Aronow 1982). Their sedimentary fills serve as archives of depositional events that result from climatic, oceanographic, and geomorphic changes in coastal regions (Liu and Fearn 1993; Devoy et al. 1996). One of the first comprehensive investigations of a coastal pond is the geological and oceanographic study of Oyster Pond, Massachusetts by Emery (1969), in which he used a shallow seismic technique (‘‘acoustic bottom penetrator’’) to examine pond stratigraphy. With the advent of high-resolution geophysical techniques, such as ground-penetrating radar (GPR), much greater resolution has been achieved in 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 Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A. JOURNAL OF SEDIMENTARY RESEARCH, VOL. 73, NO. 4, JULY, 2003, P. 559–571 Copyright q 2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-559/$03.00

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; Jol et 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 sedimentological and stratigraphic investigations of basin-fill sequences of lakes and ponds has been limited. Ground-penetrating radar has been successfully used in geological studies of inland lakes where the system was moved over the ice surface (Lobster Lake, Maine; Caldwell and FitzGerald 1995) and towed through the water column (Lake Michigan, Sauck and Seng 1994; Lake Erie, Grant et al. 1996). In a study of the Saco barrier complex, Maine, van Heteren et al. (1996) demonstrated the continuity of the back-barrier stratigraphy by extending the radar profile over an ice-covered pond. To date, however, no stratigraphic studies have been reported using GPR to investigate the basinfill stratigraphy of shallow coastal lakes and ponds. Because the GPR signal is attenuated by saltwater, the freshwater nature of many coastal ponds and sedimentological heterogeneity of their basin fills (organic, eolian, washover, and tidal-inlet deposits) make them ideal settings for subsurface geophysical investigation of back-barrier stratigraphy (Fig. 1). In addition, the flat surface of ice-covered ponds and nearly horizontal areas adjacent to many ponds reduce the need for topographic correction. The aim of this study is to use GPR records in conjunction with sediment cores to examine in detail the contrasting Late Quaternary geological histories of three sites along the peninsular coast of Maine. Based on geomorphological, geophysical, and chronostratigraphic data, we propose a three-stage evolutionary model of back-barrier pond sedimentation along indented coastlines. The applicability of the model is demonstrated by a review 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 central peninsular coast of Maine (Table 1; Fig. 2). Silver Lake (438 44.79 N; 698 47.39 W) occupies a depression behind the Hunnewell Beach barrier on the western flank of the Kennebec River estuary (Fig. 2). It has an open water area of 51,430 m 2 and an average depth of 1.2–1.5 m, reaching over 3.0 m in the center. Steep vegetated bedrock ridges of Sabino Head and Rockledge border the lake to the west and east, respectively (Figs. 2, 3). To the north, a gently sloping till-covered bedrock surface descends into the lake with a prominent high ledge in the center. The bedrock ridges are composed of relatively resistant Devonian pegmatitic granites, whereas the depression of the lake has been formed in less resistant Precambrian–Ordovician metasedimentary rocks of the Casco Bay Group (Osberg et al. 1985; Kelley 1987; Hussey 1989). The contact between the two rock types is revealed along the edge of the lake. The southern border of the lake is formed by the 250–300 m-wide Hunnewell Beach dunefield. Historically, several dunes have migrated into the lake, producing steep slopes along the southern shoreline. The extreme southwest and southeast parts of the lake represent narrow, elongated extensions between steep bedrock ridges and dunefield (Fig. 3). Lily Pond (438 43.59 N; 698 51.49 W) is a small water body located at the southern end of Hermit Island (Fig. 2). It is less than 1 m deep and has an area of 7,500 m 2 (Table 1). The pond is bordered to the north and

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FIG. 1.—Generalized diagram of sediment fluxes into bedrock-framed back-barrier water bodies. Arrows show the relative magnitude and continuity of the depositional processes (see key at bottom right). Along previously glaciated coasts, fluvial and marine reworking of glacial deposits may be an important sediment source. Note the importance of eolian sedimentation and dune migration along relatively wide barrier systems.

west by steep ridges of metasedimentary rocks intruded by multiple granitic and pegmatitic bodies. Historically, Lily Pond had a greater area until the 1950s–60s when some material was dredged from the western part of the pond and placed on its eastern side (Iris Downs; Fig. 2). In a seaward direction, the pond is bordered by the narrow (80–100 m) welded barrier of Sand Dune Beach. A large maritime Typha sp. bog forms the boundary between Sand Dune Beach and Head Beach to the southeast. North Pond (438 46.29 N; 698 45.59 W) is located at the head of Sagadahoc Bay (Fig. 2). To the south, North Pond is separated from the tidal flats of the bay by artificial impoundment and is presently regulated by a small drainage canal. A steep, north–south bedrock ridge forms the eastern shore of the lake and abuts a narrow sandy barrier to the east. The presentday open water area is the result of flooding due to a beaver dam. Water-level fluctuations in the ponds depend on seasonal and long-term precipitation–evaporation balance. During high rainfall, large quantities of water are shed from the surrounding bedrock ridges into the ponds and lakes. Forest fires, such as the Hermit Island Fire of 1938–1939 (I. Bugler, personal communication), may have resulted in temporary loss of vegetation and higher rates of runoff and eolian input. Topographic surveying indicates that the average water surface elevations in Silver Lake, Lily Pond, and North Pond are 1.5 m, 0.7 m, and 0.5 m above the ocean mean high water (MHW) level, respectively. A mean tidal range of 2.6 m (spring range is 3.5 m) and mean shallow water wave height of 0.5 m (Jensen 1983) place this part of the Maine coast in a mixed energy, tide-dominated environment (Hayes 1979; FitzGerald et al. 1994). The mean high water in 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 and associated barriers was investigated using a Geophysical Survey Systems, Inc. SIR-3 ground-penetrating radar system. The radar control unit was mounted on a cart for increased mobility. The 120 MHz transceiver frequency 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; and van Heteren et al. 1998, for description of the theoretical aspects of GPR technique). A dense network of GPR transects was obtained over Silver Lake during the winter months when the lake surface was ice-covered (Fig. 3). At Lily Pond and North Pond, geophysical transects were run along the margins of the ponds and across the barrier. For travel-time to depth conversion, mean signal velocities of 0.15 and 0.07 m/nanosecond were used for unsaturated and saturated sand, respectively. Where available, the

depths to key subsurface reflectors were also matched with major lithological changes in sediment cores. GPR records were topographically corrected where the relief was greater than 0.5 m. Because of the nearly flat surface of pond margins and ice-covered surfaces of the ponds, no topographic correction of these areas was needed. The attenuation of electromagnetic GPR signal by saltwater was a limiting factor only at the seaward extreme of one shore-normal profile. In this paper, depths are presented relative to mean high-water level, unless noted otherwise. Geophysical data were groundtruthed with five vibracores and eight pulse-auger cores that were taken along GPR survey lines from the pond basins and adjacent coastal areas. Core penetration ranged from 3 to 8 m. Textural characteristics of the core samples were analyzed using the Rapid Sediment Analyzer at the U.S. Geological Survey/Woods Hole Sedimentology Laboratory. This information, together with compositional characteristics, provides a basis for interpreting the depositional environments of basin-fill lithofacies. Conventional radiocarbon dates on organic sediments are reported as uncalibrated, d13C-corrected ages. RESULTS

Silver Lake Shore-Parallel Transect.—The geophysical profiles taken over the icecovered surface of Silver Lake show a number of reflectors of varying intensity and geometry. The western segment of profile SL-2 reveals a convex-up reflector on the otherwise flat lake bottom with several prominent subhorizontal to slightly convex-up signal returns (Fig. 4A). The morphology of this reflector and its location basinward of a large lakeshore dune 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. Vibracore PB-1 taken through the lake fill sequence east of the submerged dune contained several layers of muddy freshwater peat interbedded with well-sorted medium sand (Figs. 4A, 5). Similar stratigraphy was found in core PB-22 at the westernmost extension of the lake (Fig. 3). The eastern part of profile SL-2 shows a similar reflector with irregular convex-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 lake bottom. Beneath this low area is a series of closely spaced, wavy-parallel and basin-fill reflectors characteristic of lacustrine sedimentary sequences (see van Heteren et al. 1998 for discussion of GPR facies). Core PB-28 penetrated 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

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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 providing pinning points for coastal accumulation forms. MLW, mean low water.

2.75 m and is a continuation of the Rockledge headland, which descends abruptly 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 m below the lake surface (profile SL-10; Fig. 4C). In core PB-30 just west of the profile, a thin layer of till was collected above the refusal. Along the southern part of the trace the margin of the dunefield is seen as a steeply

descending surface reflection with several basinward-dipping beds. Below the flat lake-bottom reflector, a sequence of convex-up and wavy-parallel reflectors can be traced along the profile. A prominent convex-up reflector with a transparent central part is analogous to the submerged dune signatures shown in Figs. 4A and 4B. A 7-m-long pulse-auger core PO-9 penetrated several units of muddy lake-bottom sediments (gyttja) intercalated with organic-rich sandy hori-

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

Maximum length (m) Maximum width (m) Area (m 2)1 Average depth (m) Barrier width (m) Sedimentology of basin fill: 2 eolian sand washover sand freshwater peat saltwater peat lowermost unit 1 2

Silver Lake

Lily Pond

North Pond

410 200 51,430 1.2–1.5 250–300

110 100 7,500 0.5 80–100

140 90 12,320 0.5 15–20

C R C A till

C C C C glaciomarine

A C A A glaciomarine

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

zons. The mean grain size of sediment samples from the lower part of the core was compared to that of nearby cores PO-4 and PO-5 collected from the Hunnewell dunefield (Fig. 3). In all cores, a prominent shift to coarser grain sizes occurs between 6 and 8 m below MHW (Fig. 6). However, at this depth in core PO-9 the moderately well to very well-sorted fine-grained sands 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 medium proto-barrier sands.

Sediment Accumulation Rates.—A sandy freshwater peat unit (d13C 5 229.5‰) at 8.30–8.52 m below the water surface (5.95–6.17 m below lake 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 6 65 14C yr BP. We assume that the continuation of the peat horizon cored in Silver Lake has a similar age. Therefore, following the deposition of this lower peat unit, the time-averaged sediment accumulation rate is calculated at 1.8–1.9 mm/yr. The only age estimate for sediments in Silver Lake is 2,395 6 40 14C yr BP reported by Nelson (1979). This date was obtained on freshwater peat at 2.2 m below the lake bottom from a piston core located approximately 40 m east of core PO-9. When considering this intermediate age, the time-averaged accumulation rate of lake sediments is somewhat lower than above, decreasing from 1.6–1.8 mm/yr between 4.6 and 2.4 ka to 0.9 mm/yr after 2.4 ka. Lily Pond Shore-Parallel Transect.—Ground-penetrating radar profile LP-1 was collected along the seaward side of Lily Pond and Iris Downs (Fig. 2). Although Iris Downs are capped by the dredged material, the GPR record penetrated to 2.5–3.0 m into the original pond sequence. The bottom portion of the record exhibits irregular to wavy-parallel reflectors which attenuated the radar signal. In core SD-2, these coincide with the top of Pleistocene glaciomarine sandy clay (Presumpscot Formation, Bloom 1963;

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 with recent evidence of migration into the lake basin.


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FIG. 4.—A) Western segment of GPR profile SL-2 taken over the ice-covered Silver Lake. Several subparallel reflectors within the lakebasin sequence represent organic-rich horizons interbedded with siliciclastic units, mainly eolian in origin, penetrated by core SL-1. The submerged feature on the lake bottom coincides with the extension of the dune along the south shore of the lake. B) Eastern segment of GPR profile SL-2, showing infilled bedrock depression, which is also manifested in lakebottom morphology. The steep western flank of the depression is a slipface of a submerged dune similar to that in Fig. 4A. C) A shore-normal GPR profile SL-10 showing the steeply plunging bedrock surface along the north side of the lake. The slipface of a dune is revealed at the south end of the profile. The convex-up reflector at 3– 4 m depth is indicative of a buried dune capped by organic-rich lake sediments. See Fig. 3 for locations of GPR profiles and sediment cores. For detailed core logs of vibracores PB-1 and PB-28, and pulse-auger PO-9 refer to Fig. 5.

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


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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 gravel that pinches out in a landward direction beneath the pond. This unit is interpreted as a washover deposit. A saltwater peat (d13C 5 217.9‰) immediately below this unit was dated at 1,170 6 105 14C years BP providing a maximum age for washover deposition. The bottom reflector corresponds to bedrock overlain by glaciomarine clay in seaward cores SD-1 and 1A (Fig. 8B). North Pond The GPR profile NP-1 taken along the seaward edge of North Pond exhibits a prominent concave-upward reflector that descends to over 6 m in the middle part (Fig. 10). Based on pulse-auger core N-1, it represents the top of Pleistocene glaciomarine clay. Multiple reduction-stained laminae 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 bedrock ridges, indicating that the original depression was similar to or larger than the present-day open-water area of the North Pond. The hyperbolic reflections beneath the clay are indicative of isoclinally folded bedrock, which is exposed adjacent to the pond. The image of several hyperbolic reflectors along the western segment of the basin fill suggests that the clay layer 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 Heteren 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 series of concave-up, nested reflectors in the middle of the record coincides with a sequence of interbedded quartz-rich, coarse to medium sand beds. Lumps of sand-coated clay were found at the base of the unit. In contrast to the other 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 cm of the pond sediments. DISCUSSION

FIG. 6.—Variation in mean grain size with depth in Silver Lake core PO-9 and cores PO-4 and 5 taken through the barrier sequence (see Fig. 3 for locations). The prominent coarsening of barrier sediments between 6 and 8 m depth coincides with similar trend in lake core 9. The textural boundary between beach/washover and eolian sands is based on sedimentological characteristics of recent estuarine and barrier sediments (Buynevich 2001). The mean-grain-size values in the coarsegrained interval in the two dunefield cores is similar to modern fluvial–estuarine sediments.

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 wood fragments 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 (see Fig. 5). Both the glaciomarine and the overlying organic sediments lap onto the bedrock that crops out to the southeast. The transparent unit above the 2 m depth is composed mainly of fine-grained, well-sorted sands with a bounding-surface configuration indicative of a coastal dune (van Heteren et al. 1998). The low-frequency chaotic reflections at the top of the trace are due to human disturbance and the placement of the fill (Fig. 7). Shore-Normal Transect.—The NE–SW oriented GPR profile LP-2 was taken from the edge of Lily Pond to the beach (Figs. 2, 8A) and groundtruthed with four pulse-auger cores (SD-1, 1A, 2, and 3; Fig. 8). This transect reveals a prominent, thick reflector extending beneath the pond and ascending in a seaward direction. This reflector is likely the result of amalgamation of freshwater and saltwater organic units discussed above (profile LP-1). At least two separate horizons of light-brown peat with remains of

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 GPR records (Fig. 4). The predominantly freshwater nature of these units is supported by: (1) presence of terrestrial organic debris (bark, twigs, wood chips of cedar and pine); (2) abundance of cladoceran beetle carapaces and pennate diatoms (Nelson 1979); (3) black color, and (4) strongly negative d13C value of at least the lowermost horizon (220 to 230‰; Stuiver and Pollach 1977). The lowermost sandy freshwater peat layer beneath the barrier 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 be explained by compaction due to high sand content of the peat unit. The most probable explanation of this stratigraphy is that organic sediments were deposited in a large proto-lake or seasonal bog behind a coastal barrier. In an analogous fashion, organic-rich sediments are presently forming in Silver Lake at a depth of over 2 m below present ocean mean high water level. The existence of the lowermost nonmarine organic deposits indicates that a coastal barrier had welded to both Sabino and Rockledge bedrock ridges by at least 4.6 ka. Moreover, this finding suggests that sediment from the nearby Kennebec River must have been adequate to maintain the barrier and even cause its progradation through mid- to late Holocene. The large extent of the coastal wetland in this area and throughout the region by ; 4.6 ka may be due to increasingly humid climatic conditions in eastern North America at that time (Delcourt and Delcourt 1984; Webb et al. 1993). Stabilization of the barrier was facilitated by buried bedrock pinnacles,

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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 interbedded freshwater 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. See Fig. 5 for descriptions of cores SD-1 and SD-2.

which rise from 260 m below MHW to within 210 m under the upper shoreface (Buynevich et al. 1999). The low area between this antecedent high and the Sabino–Rockledge ridge formed a natural depression that contained the original water body or wetland. The proposed development of a welded barrier is similar to ‘‘barrier blocking’’ and subsequent formation of freshwater wetlands described for Atlantic coastal sites of Europe by Devoy et al. (1996). In these areas, however, the high-energy wave regime resulted in deposition of coarse-grained washover units. In contrast, the fine-grained eolian sands have dominated the clastic component of Silver Lake sedimentary fill, particularly throughout the late Holocene progradational phase of barrier development (Buynevich 2001; Buynevich and FitzGerald 2001). The convex-upward reflections with characteristically transparent central parts found on the lake floor and within its basin fill represent parts of recently active and buried dunes, respectively. The occasional subparallel to convex-up reflectors within the dune facies may represent periods of organic accumulation when dunes became less mobile. The drape of organic sediments over both recent and buried dunes appears as a strong GPR reflector and improves delineation of their morphology and dimensions. Possible mechanisms for renewed dune migration include devegetation due to disease and deforestation. Drought and forest fires were also proposed 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 Holocene coastal deposits overlie the Pleistocene glaciomarine sandy clay sequence and are separated from it by the basal unconformity (Ub; Figs. 5, 8B). This surface, which can be traced offshore, represents a regressive erosional unconformity produced by relative sea-level fall associated with the postglacial isostatic rebound (Kelley et al. 1987; Belknap et al. 1989; Barnhardt et al. 1997). Formation of freshwater peat was enhanced by runoff from steep bedrock ridges and accumulation of detrital organic matter in topographically low areas. At the Lily Pond site, a transverse bedrock ridge in the middle of the section produced a topographically low area behind it, which became a locus of organic deposition. Additionally, it provided a local 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 transgression. If a time gap is present between the deposition of the two organic units, 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 the middle of the sequence indicate that tidal inlet(s) penetrated through a narrow and low barrier around 1.2 ka (Buynevich 2001). Several well-defined GPR reflectors were used to extrapolate the data over a large area. These correspond to specific organic-rich sedimentary units 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 saltwater peat units sandwiched between freshwater peat horizons suggest prolonged periods of back-barrier salt marsh deposition (tens to hundreds of years; see Pethick 1981). The deceleration in sea-level rise along this coast and subsequent welding and aggradation of the Sand Dune Barrier closed the tidal inlet and transformed the saltmarsh into a freshwater pond that persists to the present day. The entire sedimentary sequence within Lily Pond represents an alteration between increasing and decreasing salinity similar to the 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 stratigraphic results from North Pond suggest a prolonged period of nondeposition following the accumulation of glaciomarine clay. The top of the glaciomarine unit represents the basal unconformity (Ub), which in this case coincides with the transgressive unconformity (Ut). The erosion of the exposed Pleistocene unit is marked by sand-coated clay lumps immediately above the unconformity. The lack of organic or eolian deposits in North Pond contrasts with previous sites. This finding suggests that at least the seaward part of the pond was subaerially exposed between the deposition of glaciomarine and overlying transgressive units. The latter are most likely tidal-flat and washover deposits, as evidenced by their coarse-grained nature. High content of quartz and lack of till exposures indicate that these were derived from a seaward source rather than erosion and reworking of local glacial deposits. Because Sagadahoc Bay is presently floored by finegrained, mica-rich sands, the transgressive deposits of North Pond suggest


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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 and four 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-sectional view 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 how the accommodation space of the original depositional basin was reduced over time (Fig. 10). The entire basin-fill sequence has high preservation potential with the continuing transgression.

Evolutionary Model of Pond Infilling On the basis of geophysical and sedimentological data, we propose a three-stage evolutionary model of sedimentation in barrier-fronted ponds along embayed coastlines (Fig. 11). In Stage 1, freshwater, organic-rich sediments accumulate over bedrock and regolith of variable thickness. The

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FIG. 9.—Reconstruction of the spatial extent of freshwater wetland and salt marsh based on geophysical and sedimentological data. The presence of saltmarsh peat between freshwater units is indicative of seawater incursion through a tidal inlet that subsequently closed.

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

stage is represented by the amalgamated basal and transgressive unconformities. The reason for the absence of organic units here is unclear, and perhaps is related to the lack of sufficient drainage required for transport of organic detritus into the depression combined with its originally small size. The upland ponds that have not experienced inundation during the Holocene are still in Stage 1 of their development (e.g., Big Pond on Cape Small, Fig. 2). In areas where rising sea level and abundant sediment supply result in barrier formation, Stage 2 of pond sedimentation begins (Fig. 11). At this

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 several bedrock 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 further demonstrate 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.


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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 the paraglacial 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 level and 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 whether lagoon (2a) or pond (2b) occupies the back-barrier depression. Stage 3—with progradation and heightening of the barrier, eolian deposition and dune migration become the primary 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 via a tidal inlet (Stage 2a—lagoon; e.g., Lily Pond, North Pond; see also Duffy et al. 1989). Alternatively, the back-barrier depression can be separated from the ocean by a welded barrier and no record of long-term seawater influence may result (Stage 2b—blocked pond; e.g., Silver Lake). In this stage, the position, dimensions, and dynamics of the barrier, rather than sea-level history, may largely dictate the back-barrier sedimentation processes. An analogous situation of coastal morphodynamics controlling back-barrier accretion has been reported by Jennings et al. (1997) for gravel barrier-enclosed seepage lagoons of Nova Scotia. Where inlets are small or absent and the fronting barrier is narrow and low, washover sedimentation is the dominant mechanism of clastic sediment input into the pond/lagoon (Figs. 1, 10). Switching between Stages 2a and 2b may result from inlet opening and closure, and is manifested in intercalated horizons of brackish/saltwater and freshwater peat. According to this model, modern lagoons represent Stage 2a, and may or may 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 eroding relatively wide barriers; (2) episodic coastal subsidence, and (3) artificial excavation of outlets for drainage or aquaculture. Most of the clastic sediment input to the back-barrier lake or pond at Stage 3 is through eolian transport (deflation and dune migration during de-stabilization). In areas where dune sands have been stabilized by vegetation, organic deposition (detrital peat or gyttja) becomes the dominant process of pond bottom sedimentation (Figs. 1, 10). With continuing sea-level rise, coastal lakes and ponds, especially those fronted by low barriers, experience increased frequency of saltwater inundation and are likely to reenter Stage 2 of their geological history. Regardless of environmental changes, the basin-fill sequences discussed in this paper have an excellent preservation potential during sea-level rise. Implications for Coastal Stratigraphic Research The applicability of the proposed model is demonstrated by comparing the characteristics of the three study sites to those from several existing

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I.V. BUYNEVICH AND D.M. FITZGERALD TABLE 2.—Physiography and evolutionary stages of selected coastal ponds and wetlands. See Figure 2 for locations of New England sites. Site Location

Silver Lake, south-central Maine Lily Pond, south-central Maine North Pond, south-central Maine Short Pond, south-western Maine Jasper Beach lagoon, north-central Maine Provincelands intradunal ponds, Mass. Oyster Pond, Nova Scotia, Canada Rugged Head Pond, Nova Scotia Black Island ponds, Nova Scotia Lough Cahasy, Co. Mayo, W. Ireland Bran Lough, Co. Clare, W. Ireland Etang de Ne´rize´lec, Brittany, France Marais de la Joie, Brittany, France

Pond Length/ Width (max, m) 410/200 110/100 140/90 130/70 300/150 800/500 3,000/500 2,300/1,700 30/20 320/120 70/50 350/100 1,000/700

Long Axis Orientation

Inlet Type1

Barrier Width (m)

shore-parallel shore-parallel shore-normal shore-parallel shore-parallel shore-parallel shore-normal shore-parallel shore-parallel shore-normal shore-normal shore-normal shore-parallel

n n a n t1s n t1s a1s n n a s a

250–300 80–100 15–20 200–230 60–80 800–1,500 100–150 50–250 60–80 150–180 100–120 320–350 200–250

Barrier Texture

Evolutionary Stages 2

Reference

sand sand sand sand gravel sand gravel gravel gravel sand 1 gravel sand 1 gravel gravel sand

(1)-2b-3 1-2a-3 2a-3 2a-3 2a 3 2a 2a 2a-2b 2a 2b-3 2a-2b 2a-3

this study this study this study 1 2 3 4 4 5 6, 7 7 7 7

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 in the lower part of the stratigraphic record at Silver Lake and Lily Pond illustrates the importance of antecedent bedrock depressions for initial organic sedimentation (Stages 1 and 2; Fig. 11). Stage 1 is absent in coastal water bodies formed within the barrier or along a low-gradient upland (see other examples). A sequence of interbedded saltmarsh peats and washovers in many backbarrier water bodies documents a history of active tidal exchange during most of the Holocene (Stage 2a. e.g., semi-enclosed lagoons of Nova Scotia, Ireland, and France; Table 2). On the other hand, the absence of saltmarsh peat in lake-basin stratigraphy suggests that the back-barrier has not experienced long-term inundation by seawater and evolved as a blocked pond (i.e., freshwater basin separated from the ocean by a welded barrier, Stage 2b). Such features would be more common along indented, rocky coasts (e.g., Silver Lake, Bran Lough, Table 2) or those with glacial deposits acting as anchor points for the barriers. Several mechanisms for transforming the lagoon into a pond during Stage 2 include: (1) decrease of back-barrier tidal prism due to rapid infilling; (2) elongation of the barrier due to increase in longshore sediment supply, and (3) artificial inlet closure. In all cases, the barrier may still be relatively narrow and low and subject to washover sedimentation or breaching. Table 2 indicates that seepage of water through gravel barriers plays a significant role in controlling back-barrier water levels and sedimentation (Carter and Orford 1984; Carter et al. 1989; Jennings et al. 1997), a process which is not important in sandy barrier lithosomes. The stability of back-barrier lakes and ponds in Stage 3 is reflected in basin-fill stratigraphy by organic-rich peat and gyttja interbedded with laminae of windblown sand or thicker beds produced by dune migration (Figs. 1, 10). In some areas, eolian deposits may undergo partial reworking and resedimentation by active streams (e.g., Silver Strand, Ireland; Delaney and Devoy 1995). Some coastal ponds may form and evolve entirely during Stage 3, such as intradunal ponds formed during the period of Holocene transgression but removed from any seawater influence (e.g., within wide strandplains or large dune systems, such as Provincelands Dunefield; Winkler 1992; Figure 2; Table 2). The present study demonstrates the use of GPR profiling complemented by sediment cores in high-resolution lithostratigraphic analysis of backbarrier freshwater bodies. Our findings and the proposed model emphasize that in addition to relative sea-level history and sediment supply along sand-dominated barrier coasts, the local antecedent basement morphology, barrier morphodynamics, and dune stability have to be considered in order to accurately reconstruct the depositional history of coastal lakes and ponds.

CONCLUSIONS

High-resolution ground-penetrating radar records of barrier and backbarrier sequences demonstrate the importance of antecedent topography in Holocene coastal evolution. In some cases, topographically high areas seaward of the pond basins not only isolated natural depressions for wetland development but may have also aided barrier stabilization and progradation. Silver Lake experienced periods of organic accumulation punctuated by eolian sedimentation. GPR profiles reveal both buried and recently active dunes along the south shore of the lake. Absence of saltmarsh peat and microfauna indicates that nonmarine conditions prevailed in this area since at least 4.6 ka. Occurrence of freshwater organic units several meters below contemporary sea level suggests deposition behind a welded proto-barrier since mid-Holocene. In contrast, the stratigraphy of Lily Pond indicates at least two prolonged periods of saltwater incursion within the past 2 ka. The clastic input into the pond was through washover, and later, eolian deposition. 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 tidal flat deposition during the late Holocene when local sea level reached the head 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 explained by a 3-stage evolutionary model. The initial stages of pond development involve organic sedimentation or nondeposition in an upland depression during lower sea level (Stage 1). As sea level rises, tidal inlet and washover deposition play an important role, particularly if the barrier is low and narrow (Stage 2a). Abundant sediment supply may preclude formation of a tidal inlet as the landward migrating barrier is welded to bedrock ridges on both sides (Stage 2b). During the phases of decelerated sealevel rise, local uplift, or increase in sediment supply, eolian deposition becomes dominant both through deflation of dunes and large-scale dune migration (Stage 3). Further rise of sea level and reduction in the supply of clastic sediments may cause many back-barrier ponds to revert to Stage 2 of the evolutionary sequence. A review of the existing studies in physiographically similar settings demonstrates the applicability of the proposed model. ACKNOWLEDGMENTS

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

HIGH-RESOLUTION IMAGING OF COASTAL PONDS historical information and access to Hermit Island. We extend our gratitude to Sytze van Heteren, Amy Dougherty, Paul McKinlay, Sarah Mills, Donald Hunt, and Andrew Lorrey for their assistance in the field. Reviews and criticisms by Joseph Kelley, Walter Barnhardt, Jan Alexander, Daniel Belknap, and David Marchant significantly improved the manuscript. REFERENCES

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