Ecosystem Modeling in Cobscook Bay, Maine: A Boreal, Macrotidal Estuary 2004 Northeastern Naturalist 11(Special Issue 2):51–74 Controls on Surficial Materials Distribution in a Rock-Framed, Glaciated, Tidally Dominated Estuary: Cobscook Bay, Maine JOSEPH T. KELLEY 1,* AND ALICE R. KELLEY 1 Abstract - Surficial materials were mapped on the bottom of Cobscook Bay, ME, through aerial photography of intertidal habitats, side-scan sonar, and seismic reflection profiling of subtidal regions. Like many other estuaries in northern New England, this rocky, macrotidal estuary has only slight riverine input and contains an abundance of till and fine-grained glacial-marine sediment. Contrary to conceptual models of estuarine sediment and habitat distribution, grain size does not become finer and habitats lower in energy in a landward direction within the estuary. The irregular shoreline shape, imparted by bedrock, forms a series of narrow constrictions separating broad bays. More than 70% of the bottom of the estuary is floored by gravel and rock; mud deposits are located in shallow-water coves throughout the Bay and in two large deposits in the Central Bay. Here, circulation models predict two large gyres form because water cannot pass through a bedrock constriction quickly enough. Natural gas is present in sufficient quantities in the sediment column to facilitate sediment mass movements near the mud deposits. Almost 60% of the intertidal zone is composed of mudflats that are uniformly distributed within and along the outside margin of the Bay, with increasing abundance of bedrock in a landward direction. Small beaches occur wherever coarse-grained glacial sediment erodes from bluffs. These observations depart from existing conceptual models of estuarine sediment distribution based on coastal plain estuaries and suggest that better understanding of biotic habitat or contaminant distribution in rocky glaciated estuaries will require more localized models. These estuaries appear more complex than coastal plain estuaries because of the unique outcrop pattern of bedrock and glacial deposits in each bay. Introduction In 1937, Krumbein and Aberdeen applied the newly described logarithmic measurement of sediment grain size to the spatial distribution of sediments in a Louisiana estuary. They mapped sandy material near tidal inlets and channels, with a fining of grain size in a landward direction. Many other studies have subsequently described similar results (Holliday et al. 1993) and generally attributed the grain size distribution to a decline in wave and current energies in a direction away from the sea (Nichols and Biggs 1985). 1Department of Earth Sciences, University of Maine, Orono, ME 04469-5790. *Corresponding author - jtkelley@maine.edu. 52 Northeastern Naturalist Vol. 11, Special Issue 2 Most early geological work on estuarine sediment distribution was based on observations from coastal plain estuaries, and a major focus was often on understanding the spatial distribution of sand bodies in relation to the development of models for petroleum exploration. Dalrymple et al. (1992) attempted to extend and generalize our understanding of estuarine facies globally, and elegantly summarized the salient features of macrotidal and microtidal estuarine sediments. In their models, the estuary is given a simple shape, a box or a triangle, within which wave and tidal energies dominate the seaward end, while riverine energy most influences the landward side. Sand and mud are assumed to be abundant, thus sandy barriers (microtidal) or flats (macrotidal) dominate the estuarine mouth, while muddy marshes fill the upper estuary. The overall geological framework of a coastline defines the type and shape of its estuaries (Inman and Nordstrom 1971) and strongly influences many estuarine processes. In northeastern North America, bedrock and glacial deposits are ubiquitous components of most estuaries (Belknap et al. 1994, Kelley 1987, Kelley et al. 1986, Knebel et al. 1999, McMaster 1960, Roman et al. 2000). In attempting to “distill away all local variability and retain only the common features” of the world’s estuaries, Dalrymple et al. (1992, p. 1133) neglect bedrock and glacial deposits, and weaken the applicability of their model to some important regional problems. In northeastern North America, for example, many estuarine scientists seek to better understand the spatial distribution of contaminants in estuaries (Larsen and Gaudette 1995, Mecray and Bucholtz-Ten-Brink 2000) and to map “essential fish habitat” (Zajac et al. 2000). These tasks clearly require incorporation of rock and glacial deposits into estuarine facies models because large areas of the seafloor in the northeast are covered with pre-Holocene material (Roman et al. 2000). In addition, natural gas is a widespread feature in Holocene estuarine sediments in the northeast (Fader 1991, Fleischer et al. 2001, Gontz et al. 2002, Kelley et al. 1994). Gas reduces sediment strength, leading to failure and submarine mass movements (Kelley et al. 1989). Gas is also associated with pockmark fields, which are major morphologic components of muddy northeast estuaries (Fader 1991, Kelley et al. 1994). There are many salmon aquaculture sites within Cobscook Bay, ME (Fig. 1), and potential exists for further growth (Brooks 2004, Department of Marine Resources 1996, Sowles and Churchill 2004). Whether these facilities are adding nutrients to the Bay, through fecal remains and unconsumed food, in sufficient quantities to lead to local eutrophication is an important policy question. Existing sediment transport models indicate that pen-associated mud is mobile (Dudley et al. 2000), and conceptual models (Dalrymple et al. 1992) suggest that the landward reaches of a bay are the most likely sites for fine-grained sediment accumulation and 2004 J.T. Kelley and A.R. Kelley 53 organic matter build up. Those models may not apply to a rock-framed, glaciated estuary, however. The need to understand estuarine sediment distribution provides the basis for this research. Here we describe the surficial materials of Cobscook Bay in relation to these earlier models. On the basis of differences between our observations and existing models, we suggest additional model components needed to understand sediment dispersal mechanisms in rock-framed, glaciated estuaries. Regional Setting and Previous Work Cobscook Bay is located in the northeastern corner of the United States, on the border between Maine and New Brunswick, Canada (Fig. 1). Cobscook Bay is connected to Oak and Passamaquoddy Bays, which primarily join with the Gulf of Maine through a large passage in northern Passamaquoddy Bay as well as through a smaller opening to the southeast. With a mean, semi-diurnal tidal range of 5.7 m, and spring tides in excess of 7 m (National Oceanic and Atmospheric Administration chart 13328), this region is characterized as macrotidal. Approximately half a cubic kilometer of water enters and leaves with each flood or ebb tide; a half-tide volume comparable to the discharge of the Mississippi River (Brooks et al. 1999). The largest stream entering the estuary, the Dennys River (Fig. Figure 1. Location and bedrock geology of the Cobscook Bay region within the Gulf of Maine. Bedrock geology simplified from Osberg et al. (1985). Many other bedrock faults shape the smaller coves within the estuary, but are not shown because of scale. Gulf of Maine 54 Northeastern Naturalist Vol. 11, Special Issue 2 2), has an estimated maximum discharge of only 8 m3/sec, and represents less than 1% of the intertidal volume (Brooks et al. 1999). The second largest stream, the Pennemaquan (Fig. 2), is about 45% the size of the Dennys (Brooks et al. 1999). The irregular outline of the Cobscook Bay region is shaped by Paleozoic bedrock lithology and structure (Osberg et al. 1985). The relatively straight outer Atlantic shoreline is formed by a fault zone separating much younger Mesozoic sedimentary rocks of the Bay of Fundy from the older rocks of Maine (Kelley 1987). The weakly metamorphosed volcanic, volcanic-sedimentary, and sedimentary rocks of Cobscook Bay form a plunging anticline (Bastin and Williams 1914). Anticlinal limbs composed of erosion-resistant volcanic rocks of the Easport, Edmunds, Leighton, and Hersey Formations form separate peninsulas; the easily eroded sedimentary rocks of the Perry Formation and fault zones within the volcanic rocks, occupy the bays (Fig. 1). Glacial-marine sediment is mapped along more than 50% of the Bay shoreline (Thompson and Borns 1985). This generally muddy sediment was deposited during deglaciation, approximately 14 ka (thousands of years ago), when the isostatically depressed land was inundated by the sea (Belknap et al. 1987). Bouldery-clayey till crops out only near the tips of Seward, Hersey, and Crow Necks (Fig. 2), and along a small part of the Inner Bay shore. The remainder of the shoreline, approximately 40%, has < 1 meter of glacial sediment (Thompson and Borns 1985). Figure 2. Bathymetry and location of place names cited in text. The inner, central and outer areas of the Bay are indicated by arrows. 2004 J.T. Kelley and A.R. Kelley 55 The sea-level history of the area includes the late-glacial drowning until about 11.5 ka, followed by emergence to a sea-level lowstand at 55 m depth around 10.5 ka (Barnhardt et al. 1995). At the time of the lowstand of sea level, the bathymetric depressions forming much of Cobscook Bay today were probably occupied by lakes, just as lakes are common features inland from the present coast (Thompson and Borns 1985). Sea level has risen at varying rates since the lowstand, and is rising from 2–3 mm/yr today (Aubrey and Emery 1991). The bathymetry of the Bay is locally very uneven, but becomes generally shallower from sea to land and towards the smaller bay margins (Fig. 2). The Outer Bay channel averages about 30 m deep at mean low water (MLW), with a maximum depth of 45 m in several places. The Central Bay reaches a maximum depth of 30 m (MLW) in the channel, but the remaining area averages about 10 m depth. The Inner Bay reaches a maximum depth of 10 m (MLW) at the entry place of the main channel, but is mostly less than 6 m deep. Uncharted bedrock shoals with more than 10 m of bathymetric relief abound throughout the Bay. For this paper, the area of the Bay that is under study includes the outer shoreline of the Bay from Eastport to the northwest where the peninsula joins with the mainland. This is done because a comparison is made below with models of estuarine sediment distribution that also include the land that frames the outer areas of estuaries. Thus, for this paper, about 41.5 km2 of intertidal flats are mapped, compared with 37.2 km2 measured solely within the Bay by Larsen et al. (2004). This is a large proportion of a total high water Bay area of 111 km2 (Larsen et al. 2004). Tidal circulation was evaluated with a three-dimensional numerical model supported by field observations (Brooks et al. 1999). At times of peak flow, both ebb and flood tidal currents approaching 2 m/s were modeled from the Bay entrance throughout the main channels, with declining velocities toward each of the Bay margins (Fig. 3). Notable features suggested by the model are gyres to either side of the main channel in the Central Bay (Fig. 3c,d) that seem to be confirmed by satellite imagery (Larsen et al. 2004). These form during the flood tide because the inflow cannot be accommodated by the narrow bedrock constriction leading to the Inner Bay (Fig. 3). Extreme turbulence, with many small eddies and standing waves, exist within and adjacent to the areas of bedrock constriction. Methods Intertidal environments (all units were greater than 150 m2), mapped from 1960s low-tide aerial photographs (Timson 1976), were digitized in a geographic information system (GIS) to a 1:24,000 scale (Ward 56 Northeastern Naturalist Vol. 11, Special Issue 2 Figure 3. a) Overall circulation model for Cobscook Bay near peak ebb flow. b) Note strongest currents are located along the main channel axis in the Outer and Central Bays, and decline greatly to the margins and in the Inner Bay; c) Overall circulation model for Cobscook Bay near peak flood flow. d) Note the gyres that are modeled to either side of the “reversing falls”. a b 1999) and evaluated using a GIS. Because this study is concerned with the zonation of environments from the outermost to inner parts of the estuary, areas along the eastern edge of the peninsula on which Eastport Currents after 5 Hours Currents after 5 Hours Kilometers Kilometers Kilometers Kilometers 2004 J.T. Kelley and A.R. Kelley 57 c d 58 Northeastern Naturalist Vol. 11, Special Issue 2 rests were included in area estimates. Thus, areas are greater in extent than listed by Larsen et al. (2004). Subtidal habitats were imaged with an EG&G SMS 260 slant-range corrected side-scan sonar in 1994 and 1998. The swath area was 200 m (100 m range) over most of the Bay; in areas deeper than 35 m, a 300 m swath (150 m range) was used. Data were interpreted on the basis of the strength of the acoustic return (relative darkness on the analogue record at a constant gain setting) and morphology of the seafloor (bedrock fractures, ripples, boulders, etc.). In this way, bedrock, gravel, sand, and mud were distinguished (Barnhardt et al. 1998, Kelley et al. 1998). In areas of complex seafloor in which two of the four seafloor types occurred over a small area (< 10,000 m2), the more abundant surficial material was mapped as dominant (“rock with gravel” means rock more abundant than gravel and “mud with rock” means mud more abundant than rock; Barnhardt et al. 1998). Ten bottom samples were collected with a Smith-McIntyre grab sampler to verify the side-scan sonar interpretations. About 70 km of 3.5-kHz seismic reflection profiles were gathered in 1984 (Kelley et al. 1989). These records imaged through all surficial materials to bedrock, except where thick till or natural gas occurred. The nature of surficial material was interpreted on the basis of the strength of the surface acoustic return in the seismic record and its morphology and context (Barnhardt et al. 1998, Kelley et al. 1998). Navigation was by LORAN-C during collection of the seismic records, and by DGPS for the side-scan sonar and bottom samples. The LORAN data was converted to Latitude/Longitude with LORCON (Barnhardt et al. 1998). All data were compiled in a GIS for computation of areas and map production. Results Outer Bay Covering almost 90% of the seafloor (Table 1), a gravel bottom dominates the subtidal area of the Outer Bay, with subordinate rock and sand cropping out locally (Fig. 4). Seismic records trace bedrock outcrops from the intertidal zone into the shallow subtidal region where rock is overlain by till and glacial-marine sediment. The truncated acoustic reflectors of the glacial-marine sediment and the outcropping patch of till suggest a hard and eroding bottom, as do accompanying side-scan sonar images (Fig. 5a, b). Lineations from upper right to lower left in Fig. 5a represent re-working of coarse-grained sediment by strong tidal currents. Sand and gravel were gathered in bottom samples just north of this area in a similar setting (Lehmann 1991). Exposed bedrock and boulders form the slope along the Eastport side of the channel (Fig. 5a). Farther within the Bay, the gravel bottom is interrupted only by occa2004 J.T. Kelley and A.R. Kelley 59 sional bedrock outcrops (Fig. 4b). Bedrock relief in the subsurface, in excess of 30 m over horizontal distances of several hundred meters, was commonly observed. Mud is mapped only in a cove sheltered from waves by islands and shoals in the southern part of the Outer Bay (Figs. 2, 4b). Fishing drag marks are well preserved in the mud, which abruptly changes to gravel on its seaward border. A large, shallow unmapped area in the northeast part of the Outer Bay is also very sheltered, and probably contains muddy sediment, too. Rock and gravel also appear around the scoured fringe of a small island in the Southern Bay (Fig. 4b). The intertidal zone of the Outer Bay is dominated by extensive mud flats (53.5%), punctuated with many small bedrock outcrops (16.7%; Fig. 4b, Table 1). Sand and gravel pocket beaches (19.8%) and gravel flats (6.2%) are common ( Table 1) between rock outcrops where bluffs of till are eroding along the Bay margin. Marshes exist along small stream mouths in protected coves. Central Bay The Central Bay is connected to the Outer and Inner Bays by narrow bedrock constrictions to the northeast and southwest, respectively (Figs. 4a, 6a). Subtidal environments in these constrictions, along with adjacent regions in which strong currents flow, are dominated by gravel (54%, Table 1; Figs. 7a, 8). Bedrock outcrops are relatively rare (< 2%) (Fig. 6a). Where glacial sediment is abundant, as on Birch Point and Seward Necks, bedforms of sand and gravel occur on the bottom (Figs. 4, 8). These appear to represent reworking of underlying till. The tip of Birch Point contains a double tombolo, derived from the same till Table 1. Areas of intertidal and subtidal regions. Note that these values differ from Larsen et al. (2004) because they include the eastern edge of the outermost peninsula as well as that peninsula’s western shore. Inner Central Outer (km2) (%) (km2) (%) (km2) (%) Intertidal habitats Beach 0.40 3.3 1.49 8.3 2.28 19.8 Mudflats 6.77 56.0 10.73 60.0 6.15 53.5 Gravel flat 0.14 1.1 0.76 4.2 0.72 6.2 Bedrock 3.74 31.0 3.82 21.4 1.92 16.7 Marsh 1.02 8.5 1.09 6.1 0.29 2.5 Artificial 0.01 0.1 0.005 0.0 0.15 1.3 Total 12.08 17.89 11.51 Subtidal habitats Bedrock 0.12 2.1 0.26 1.3 1.50 7.2 Gravel 4.85 82.8 11.00 54.0 18.88 89.7 Mud 0.89 12.9 9.09 44.7 0.66 3.1 Total 5.86 20.35 21.04 60 Northeastern Naturalist Vol. 11, Special Issue 2 a Cobscook Bay Intertidal and Subtidal Environments deposit, that is the largest beach in the estuary (Duffy et al. 1989). In the very narrow passages into the Inner Bay, currents in excess of 2 m/ sec (Brooks et al. 1999) have removed all sediment finer than cobbles (Fig. 9). Boulders up to 7 m in diameter are exposed on the seafloor and bedrock outcrops up to 15 m in relief impede the tidal flow. Although extreme turbulence (whirlpools, standing waves) made seismic reflection records difficult to interpret, not much sand-sized and finer Quaternary sediment remains in these areas. Toward the middle and marginal reaches of the Central Bay, large deposits of mud and mixed mud and gravel occupy 44.7% of the Bay bottom. As in the Outer Bay, the transition from gravel to mud is abrupt (Fig. 10). Unlike those in the Outer Bay, however, the Central Bay mud deposits are not sheltered by islands. Drag marks from fishing gear completely mark the muddy areas, but largely disappear in gravel sediment. The gravel bottom is several meters deeper than the adjacent mud in most places, and is up to 5 m deeper near bedrock outcrops. This observation implies non-deposition of mud in gravel areas, or scouring 2004 J.T. Kelley and A.R. Kelley 61 Figure 4 (opposite page and above). a) Surficial geology map for Cobscook Bay. Boxes are areas shown in detail in Figures 4b and 6a,b. Map units are shown in Figure 4b. b) Surficial geology map for outer Cobscook Bay. Labeled boxes and line are shown in other Figures. Location of map in relation to entire Bay is shown in Figure 4a. b Outer Bay Intertidal and Subtidal Environments 62 Northeastern Naturalist Vol. 11, Special Issue 2 Figure 5. a) Side-scan sonar line CC98-1. The strong (dark) acoustic reflectance on this image indicates a hard gravel-dominated bottom. Bedrock crops out in the upper part of the image, with boulders at the base of the slope. See Figure 5b for seismic reflection record CC84-10 crossing this image. b) Seismic reflection line CC84-10. This profile extends from Eastport, ME, to Campobello, NB (modified from Kelley et al.1989). On this and other seismic records, Br is interpreted as bedrock, T as till, and Gm as glacial-marine sediment. The dark surface return and truncated acoustic reflectors suggest a sand/gravel lag deposit developed on eroding beds of the till and glacial marine sediment. The area imaged by side-scan sonar in Figure 5a is located at the top of the record. b a 2004 J.T. Kelley and A.R. Kelley 63 of overlying mud deposits. A grab sample from a transition area between gravel and mud yielded a very poorly sorted mud-cobble mixture (Fig. 7b). This was from an area dragged for scallops. A seismic line across the Central Bay shows the rapid change from mud to gravel near the axial channel (Fig. 11). The mud is acoustically transparent (light) with a weak surface return. A reflector, interpreted as natural gas, occurs about 5 m below the seafloor and obscures all lower acoustic reflectors. The irregular margin of the axial channel is interpreted as slumping blocks of muddy sediment. Denser till, with a strong surface return, covers the seafloor near an island. The intertidal environments of the Central Bay are dominated by mud flats (60%). Bedrock is almost ubiquitous along the shoreline (Fig. 6a), but the steepness of the rock limits its areal exposure to 21.4% of the intertidal zone (Table 1). Coarse-grained beaches (8.3%) and gravel flats (4.2%) occur where erosion has reworked till deposits. Marshes fringe the shoreline in protected coves in the uppermost margins of the Bay, especially in the coves between Denbow and Crow Necks (Figs. 4, 6a). Inner Bay Gravel covers most of the channel area of the Inner Bay and comprises 82.8% of the mapped subtidal area (Fig. 6b, Table 1). Much more of the gravel is associated with small mud and rock outcrops (Fig. 12) than in other sections of the Bay (Table 1). Mud covers much of the upper reaches of the Bay. Large areas of relatively shallow water may also contain numerous small mud pockets between rock and gravel outcrops, but these areas were too hazardous to permit complete mapping. Rock or gravel projects through surficial mud in many places, and the contact between mud and gravel is very irregular. The intertidal region of the Inner Bay is occupied mostly by mudflats (56%), although bedrock comprises 31% of the area (Table 1). In addition to a fringe of bedrock along the shoreline, rock also occurs as many small, discrete environments throughout the intertidal zone. Gravel flats and generally gravel beaches are of minor spatial extent, but marshes comprise almost 9% of the Inner Bay intertidal zone (Table 1). Some of the marshes are probably freshwater wetlands contiguous with salt marsh (Ward 1999). Discussion Estuaries in rocky, glaciated landscapes may differ greatly from generalized estuarine models in their sediment facies patterns. Irregular bedrock outcrops are a major feature that distinguishes these estuaries from existing models. First, bedrock structure and compostion control the overall shape of the estuary (Fig. 1). Cobscook Bay clearly reveals a 64 Northeastern Naturalist Vol. 11, Special Issue 2 Figure 6 (above and upper opposite page). a)Surficial geology map for central Cobscook Bay. Labeled boxes and line are shown in other Figures. Location of map in relation to entire Bay and map units are shown in Figure 4a. b) Surficial geology map for inner Cobscook Bay. Labeled boxes and line are shown in other Figures. Location of map in relation to entire Bay and map units are shown in Figure 4a. a Central Bay Intertidal and Subtidal Environments 2004 J.T. Kelley and A.R. Kelley 65 b Inner Bay Intertidal and Subtidal Environments Figure 7 a) Photograph of gravel-size sediment collected from Cobscook Bay. The cobbles range from 3–10 cm in nominal diameter and are coated with pink encrusting algae. Sample is located in Figure 4b. b) Photograph of mixed mud and gravel sediment. Sample is located in Figure 6a. 66 Northeastern Naturalist Vol. 11, Special Issue 2 folded rock structure that descends into the earth (a plunging anticline) through its many peninsulas and bays (Fig. 1). Weaker rock formations were preferentially eroded away by glacial and other weathering processes to leave bays, with more erosion-resistant formations comprising Figure 8. Side-scan sonar line CC98- 10. Bedforms with a spacing of 5–10 m and up to 50 m in length are developed from till deposits near Birch and Seward Points. The very strong acoustic return (dark) indicates that the seabed is all gravel. Figure 9. Side-scan sonar line CC98-4. Bedrock and gravel form the seafloor in this image of a narrow channel. The bathymetric trace beneath the towfish track depicts more than 15 m of relief across a rock pinnacle. Light areas are in acoustic shadows behind rock pinnacles. 2004 J.T. Kelley and A.R. Kelley 67 peninsulas. Bedrock faults have also led to cove formation by imparting a weakness to the rocks through which they pass. Within the Bay, individual rock types exhibit differing outcrop patterns. For example, more bedrock crops out in the intertidal area of the Inner Bay than elsewhere Figure 10. Side-scan sonar line CC98-9. The transition from gravel (dark) to mud (light) occurs across a distance less than 100 m. Drag marks are more apparent in the mud than in the gravel. Figure 11. Seismic line CC84-35. Br is interpreted as bedrock. Slump blocks are interpreted where the mud deposit abuts the Central Bay channel. Figure 12. Side scan sonar line 98-8. This is a typical area along the Inner Bay where small bodies of rock surrounded by gravel occur just seaward of the intertidal zone. Mud has collected in areas where the rock shelters the bottom from strong currents. Rock or gravel appear to project through the mud. 68 Northeastern Naturalist Vol. 11, Special Issue 2 (almost twice the proportion of the Outer Bay) because the Edmunds Formation (Fig. 1) only occurs in the Inner Bay. This formation is apparently locally variable in its resistance to erosion. Bedrock structure also manifests its influence in controlling tidal current velocities. At rock constrictions like Birch Point and Denbow Neck, tidal currents are accelerated and made more turbulent by the narrow channel width and irregular bottom (Fig. 9; Brooks 2004). This creates areas of non-deposition or erosion of all but the largest boulders. Bedforms were only observed on the bottom near bedrock constrictions, where scour depressions were observed as well (Fig. 8). Finally, even in sheltered areas where mud does accumulate, rock outcrops exist and are always surrounded by “halos” of gravel at the base of outcrops. Where directly observed elsewhere, the gravel consists of angular rock fragments eroded from the outcrop and/or shells from formerly attached organisms (Kelley et al. 1998). In addition to bedrock, glacial processes and deposits may significantly affect sedimentary facies in estuaries. Glaciated regions have typically experienced drainage derangement owing to sediment filling of pre-glacial valleys (Kelley et al. 1986). Thus, large, rock-framed embayments that may have been cut by a river in pre-glacial time, often do not possess significant river input today. It is not clear what river was once associated with Cobscook Bay, perhaps the St Croix, which passes over waterfalls before abruptly entering Oak Bay 25 km to the north. Clearly the Dennys and Pennamaquan Rivers are “underfit” streams in the sense that they are disproportionately small for the size of their estuary. As a result, their discharge is too slight to significantly modify the circulation of Cobscook Bay (Brooks et al. 1999), and there is no difference between sediments near river mouths and in the many other small coves in the Bay located far from fluvial influence. The largest salt marshes, for example, are associated with even smaller streams to either side of Denbow Neck (Figs. 4a, 6a). The lack of significant river-sediment input accentuates another distinguishing feature of glaciated estuaries in general and Cobscook Bay in particular: the sediment contribution from reworked glacigenic material. In Cobscook Bay, till crops out between bedrock exposures along the shoreline and in many places on the seabed (Fig. 5a). Erosion of till results in localized sand and gravel beaches, gravel flats, and a gravel seabed. Though more beaches occur in the Outer Bay than in the Inner, the difference is less than suggested by models (Dalrymple et al. 1992). In subtidal areas, gravel is the most abundant seafloor material in each arm of the Bay (Table 1, Fig. 13). Where boulders are common on the seafloor (Figs. 5a, 9), and in gravel “halos” surrounding outcrops of bedrock, overlying sediment has been removed (Fig. 12), and till is exposed at the surface. Over much of 2004 J.T. Kelley and A.R. Kelley 69 the Bay, however, glacial-marine sediment crops out on or very near the seafloor (Figs. 5 and 7). This material is generally muddy, and the overlying gravel may be a lag deposit winnowed from coarser components of the glacial-marine sediment, such as dropstones and other ice-rafted debris (Belknap and Shipp 1991). Alternatively, gravel that overlies glacial-marine mud may represent a winnowed stream deposit that formed during the lowstand of the sea or components of till redistributed by tidal currents. The general lack of sandy sediment precludes the formation of largescale barrier beaches and associated environments. Beaches within this fetch-restricted bay occur only where coarse-grained glacial deposits crop out on the shoreline. Large sandy bedforms are also absent in subtidal regions. Instead, only small bedforms are observed in areas where Figure 13. Relative abundance of intertidal (top) and subtidal (bottom) environments in Cobscook Bay, ME. Data are from Table 1. Cobscook Bay Habitat Area 70 Northeastern Naturalist Vol. 11, Special Issue 2 coarse-grained glacial sediment occurs and currents are strengthened by bedrock constrictions (Fig. 9). Mud deposits are not restricted to the most landward reach of the estuary. Each sub-bay’s landward margin is generally muddy, but the most extensive mud deposit is associated with a current gyre in the middle of the Central Bay (Figs. 3, 6a). The mud presumably accumulates as a result of sedimentation during the prolonged residence time in the gyre. The excellent spatial correlation between the modeled gyre and the distribution of mud in the Bay indicates the great utility of the numerical model in understanding and predicting the movement of sediment and contaminants in formerly glaciated embayments. Organic matter is a component of the Central Bay mud deposit. Microbial breakdown of the organic matter results in the generation of methane, which was observed in the shallow subsurface. Although this may be “modern” gas derived from late Holocene mud, it is possibly gas derived from older organic matter. At the time of lowerthan- present sea level, the Central Bay possibly existed as a lake or wetland basin. In this scenario, the older terrestrial carbon, existing in deposits below and obscured by the gas bubbles, is the source of the methane. Whatever its source, the gas has reduced sediment strength and led to sediment failure along the channel margins (Fig. 11), as in other Maine estuaries (Barnhardt and Kelley 1995; Kelley et al. 1989, 1994, 1998). Because “spoils” from aquaculture facilities are relatively fine grained, this material probably accumulates with the mud and organic- rich sediments of the Bay. Thus, instead of becoming focused strictly at the landward reaches of the Inner Bay, as suggested by models (Dalrymple et al. 1992), material introduced by aquaculture facilities may largely become concentrated in the muddy basins of the Central Bay. Conclusions Existing conceptual models are of limited usefulness in understanding and predicting the spatial distribution of sediments, contaminants, and habitats in rocky, formerly glaciated estuaries. Bedrock plays a complex role that controls the overall shape of the estuary, thereby dictating the spatial distribution of current velocities. Sediment grain size is strongly influenced by the distribution of currents, but is further affected by the inherited, patchy distribution of older glacial deposits. In Cobscook Bay, gravel beaches occur wherever till erodes, but mudflats dominate the overall intertidal region because of the widespread occurrence of glacial-marine muddy sediment. Cobscook Bay, and many 2004 J.T. Kelley and A.R. Kelley 71 other similar estuaries, lacks significant freshwater input compared to its tidal prism, and receives minimal river-sediment input to its landward reaches. This results in a well-mixed estuary with no concentration of sediment near river mouths. Lack of “new” river-sediment enhances widespread cannabilization of glacial materials across the relict seabed. Finally, natural gas is associated with thick mud deposits in Cobscook Bay and in estuaries throughout northeastern North America (Fader 1991; Kelley et al. 1989, 1994; Barnhardt and Kelley 1995). Gas reduces sediment strength and leads to mass movements where slopes are relatively steep (Fig. 11). This gas may be derived from microbial breakdown of modern organic matter or from older, buried terrestrial organic matter dating from times of lower sea level. If particulate organic matter from aquaculture facilities in Cobscook Bay is retained within the Bay, it is not simply accumulating in the most landward reaches of the estuary. The organic matter is probably accumulating with mud in the shallow subtidal regions and on tidal flats in coves throughout the Bay, and in the mud deposits of the Central Bay. Acknowledgments We acknowledge support for this research from the Maine Chapter of The Nature Conservancy, and are particularly grateful for assistance from Mr. Jim Dow. The earlier seismic work was supported by a grant from the US Nuclear Commission to the Maine Geological Survey. Literature Cited Aubrey, D.G., and K.O. Emery. 1991. Sea Levels, Land Levels and Tide Gauges. 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