Skip Maine state header navigation
Skip First Level Navigation | Skip All Navigation
|Home | Contact Us | Publications|
The study area is in southern and outermost Saco Bay, centered about 5.6 km (3 nautical miles) from the nearest land at Biddeford Pool (Figure 1 and Figure 2). This area is due south of areas previously investigated in the outer bay. The depth ranges from about -25 m to -80 m, and focused on the presumed lowstand depth of -60 m (Kelley and others, 2003).
Area C possesses the least bathymetric relief, and Area B possesses the greatest change in depth (Figure 4). Area A extends in a landward (northwest) direction out of the region of multibeam coverage, while Area B trends northeast-southwest out of the area of multibeam observations along the border of Areas A and C. Area C extends seaward of the region of multibeam coverage. It is apparent from close examination of the multibeam data that the depth range of the presumed lowstand, -50m to -60 m, is relatively flat compared to other depths, and could be a drowned wave-cut platform veneered with littoral deposits.
The bathymetry imaged by multibeam is mirrored by a similar pattern of changing acoustic reflectivity on the side-scan sonar mosaic (Figure 5). The shallow, western Area A from the multibeam mapping is defined by high acoustic reflectivity uniformly over its surface. Some of the shallowest locations are the most reflective. The grid of linear depressions evident in the multibeam is not nearly so marked in the side-scan mosaic, and most of the shallow regions are "hard." A more detailed examination of the data (Figure 6 and Figure 7) reveals that the hard reflector in Area A is bedrock. The fractured nature of the rock resembles outcrops on land (Kelley and others, 1998) with steep edges to the rock bodies. In the fractures and between bedrock outcrops, strong reflections emanate from low-relief gravel occurrences. The intermediate Area B of the multibeam map is represented by a complex mix of acoustically reflective and non-reflective areas on the side-scan images. Areas of bedrock are apparent as in Area A, but they are relatively small "islands" of rock surrounded by sediment. The sediment directly observed in cores and bottom samples correlates well with acoustic reflectivity. The areas of acoustically stronger reflections, as seen in the mosaic in Figure 6 and Figure 7, contained fine or medium sand in the upper part of the core or bottom samples. Areas of even stronger reflectivity are probably marked by concentrations of shells or pebbles such as those found in many samples. Areas of low reflectivity regions contained muddy surficial material. Area C, the deepest, most uniform and gentle area on the multibeam imagery is the most uniform and least acoustically reflective area. Some parts of the "channels" of Area C separating the "islands" of Area B are more acoustically reflective than the eastern edge of the study area, and are floored by sand.
The surficial sediment map (Figure 8), which synthesizes the side-scan sonar, multibeam, and core and bottom samples, reveals the overall area as one dominated by rock and mud. The rock areas are shallow, extensive and mostly bordered by mud. The bathymetric depression to the southwest, labeled a Shelf Valley by Kelley and others (1998), transitions from sand to mud between 30 and 70 m depth, with abrupt bedrock borders on its margins. Areas dominated by gravel and sand are relatively smaller and flank the rock in the northeast part of the study area. Gravel and sand-floored depressions also occur in the southernmost part of the study area.
There are many smaller gravel and sand areas that cannot be depicted on the scale of the map (Figure 8). The fractures between rock outcrops and the margins of the rock are places where shell fragments and gravel exist as lag deposits or are recent deposits derived from contemporary erosion of the bedrock and associated attached fauna.
Seismic Reflection Profiles
Reflectors in the upper glacial-marine sediment are truncated in depths shallower than 65 m. Seismic observations are obscured in the uppermost few meters of sediment overlying truncated reflectors, but core observations described below reveal that sand often caps the sedimentary section. In many places sand deposits possess a sigmoidal shape where sand is banked up against a bedrock cliff (Figure 9), and in other locations sand deposits form moderate-relief swells.
Basin 1 is located in the Shelf Valley (Kelley and others, 1998) on the southwestern edge of the study area (Figure 3 and Figure 11). The Shelf Valley contains the largest volume of sediment in the study area, but most of the sediment is interpreted as glacial-marine, muddy material (Figure 9). The prominent reflectors in the glacial-marine sediment are probably sand or gravel layers, but most are buried beneath more than 10 m of muddy sediment and are not considered in this report.
Clean sand is restricted to the upper 20 cm of this basin, although sandy mud continues to greater than a meter's depth in core SCVC04-05 (Figure 9). This core was collected from the center of the basin. Although more sand may exist higher up on this bank, sand thickness is not likely to exceed 20 cm across the remainder of the Shelf Valley. Sand probably increases in thickness in a landward direction, and decreases to seaward (Kelley and others, 1998).
Basin 2 is on the northern edge of the study area, and borders the region of extensive investigation in the 1990's (Kelley and others, 2003). Seismic lines depict a transition from the rocky, shallow area (labeled A, Figure 4) across the lowstand shoreline complex (labeled B, Figure 4) and into deep water (labeled C, Figure 4). The deepest feature, approximately -65 m, interpreted as a shoreline borders the extensive muddy region (labeled C, Figure 4) (Figure 12). Rock and till separate this landform from a more extensive and thicker sand deposit that was cored in 1992 (Core SCVC92-01). Almost 3 m of coarse and medium-size sand and some gravel were recovered in the core from an apparently widespread deposit that is banked up against a shallow bedrock outcrop. Between 3.8 x 105 m3 and 5.7 x 105 m3 (Table 2) of clean sand are interpreted to exist within this lowstand shoreline complex inside the study area. The shoreline complex trends out of the study area to the northeast; so this estimate is a minimum for the bay as a whole.
On the eastern edge of the study area, Basin 3 borders the muddy plain to the north and fits between two rocky "islands" to the northeast and southwest (Figure 3 and Figure 11). The core sample, VC04-01, (Figure 13) penetrated about 60 cm of clean sand over sandy mud and then mud. Medium and fine sand layers were encountered below 3.5 m depth in the core. These layers appear to be strong acoustic reflectors recognized in the seismic line and are not lowstand sand deposits. They are not included in the sand volume estimation since the reflectors do not have spatial continuity as the shoreline deposits do. The volume estimate for this basin assumes 0.6 m of sand across the basin and totals 1.11 x 106 m3 (Table 2).
Located near the center of the study area, Basin 4 borders the extensive muddy area to the northeast (Figure 3 and Figure 11). It is a relatively flat feature with somewhat stronger surface acoustic reflectivity than the muddy area (Figure 5). Core SCVC04-09 was gathered from a ridge that trends across Basin 4 (Figure 14). The ridge appears to be a constructional landform, possibly a beach, but Core SCVC04-09 contained less than 0.5 m of clean sand overlying sandy mud and glacial-marine mud. Owing to its relatively small area and apparently thin deposit of sand, Basin 4 is estimated to contain only 3.28 x 105 m3 (Table 2).
Basin 5 abuts Basin 4, but is separated from that Basin by a shallow, rocky ridge (Figure 3 and Figure 11). Seismic and core data (Figure 15) suggest that this basin may contain more sand than any other part of the study area. Core SCVC04-10 penetrated more than 3 m of coarse-grained sediment, including some pebble layers. Finer sediment accumulated near the bottom of the core, but the glacial-marine mud, inferred on the basis of the seismic data, was apparently not reached by the core.
The substantial sand body here, 1.6 x 107 m3 equals maximum value, 3.25 x 105 m3 equals minimum value (Table 2), appears to be a shoreline complex partly eroded into glacial-marine mud and overlying valley fill material (Figure 15).
Basin 6 lies along a narrow Shelf Valley in the central part of the study area (Figure 3 and Figure 11). side-scan sonar reveals that the core location is in a narrow portion of this valley, and up against the western side (Figure 4). Seismic data show that the core, SCVC04-20, was located in the middle of a bathymetric high (Figure 16). The core penetrated about 2 m of medium sand and muddy sand overlying glacial-marine mud. This suggests that the bathymetric high represents a sandy littoral remnant drowned when sea level rose past 65 m depth. The maximum sand volume estimate is based on a sand thickness of 2.0 m across the area of Basin 6, yielding 3.4 x 106 m3; the minimum estimate, 6.8 x 104 m3, assumes that the sand thickness is only 0.4 m thick (Table 2). The latter, minimum value corresponds to the thickness of sand inferred in the deeper part of the valley away from the bathymetric high.
Basin 7 is on the southeast border of the study area and is divided into two parts (Figure 3 and Figure 11). In the eastern region a ridge containing a substantial sand deposit was cored (Figure 17). The core was all medium-coarse sand, and refusal was met before the core reached the glacial-marine mud interpreted below the sand. The deposit, at 54 m depth, is above the inferred lowstand depth and is thicker than the thin veneer of sand at greater depths nearby. Though small in area, Basin 7 appears to have between 3.8 x 106 m3 and 1.52 x 106 m3, one of the largest deposits in the study area (Table 2). More sand exists in the western part of Basin 7, but no cores were gathered from that area and so it is not considered in the volume characterization.
Last updated on November 1, 2006
|Copyright © 2006 All rights reserved.|