Assessment of Sand and Gravel Resources along the Inner Continental Shelf of Maine

Results

Multibeam Bathymetry

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).

Figure 4

Multibeam coverage is conveniently broken into three regions (Figure 4). Area A forms the western edge of the multibeam study area, and appears as an "island" to the south. Area A ranges from -25 m to -35 m depth. The large, almost rectangular and shallow regions in area A are separated by a crude grid of linear depressions averaging -40 m. Area A is bordered by Area B on most of its seaward side, where the seafloor abruptly descends to -65 m. Isolated regions within Area B project upward as relative shoals in a generally seaward sloping area. Two "islands," also mapped as Area B, exist to the southeast. These are each centered on a shallow area approximately -35 m deep, around which the seafloor deepens on all sides. Most of the eastern side of the study area, mapped as Area C, is deeper than 60 m and generally flat. Deep "channels" of Area C surround the three small "islands" of Area A and B.

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.

Side-Scan Sonar

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

Figure 9

The subbottom geology of the study area is similar to other places studied in the region (Kelley and others, 1998). Bedrock, with up to 25 m of local relief, forms acoustic basement (Figure 9). Glacigenic sediment fills in basins in the bedrock. Till is not widespread as moraines in the study area, but appears to occur as smaller deposits only a few meters thick. Glacial-marine sediment is the thickest deposit in the region, with up to 30 m of material in some places. Numerous coherent reflectors are observed in the glacial-marine sediment, and may be sand deposits, but all are located beneath a thick, muddy cover.

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.

Cores

Figure 10

Vibracores provided essential ground truth regarding the surficial sediment texture over the study area and the nature of the uppermost few meters of the sedimentary section. Most cores met refusal in glacial-marine muddy sediment (Figure 10). This distinctive stiff muddy sediment is gray to blue in color when first exposed to air and has been recognized in many cores collected from the region (Barnhardt and others, 1997; Kelley and others, 2003). Sand either gradually increases in abundance above the mud or as in core SCVC04-22, for example, the sand rests with a sharp contact over the glacial-marine mud. The sharp contact between sand and mud coincides with truncated acoustic reflectors interpreted in the seismic record. The overlying sand is typically fine to medium in size (Appendix A), with a mud content ranging from 0 to more than 25%.

Sand Volume

Figure 11

Bedrock punctuates the study area, breaking it up into eight, generally unconnected basins containing sand deposits (Figure 11). Between these basins, bedrock or mud are the major surficial materials, with the possible exception of some of the area of multibeam coverage where no seismic data were collected. To evaluate sand thickness, side-scan sonar data were used to delineate areas of surficial sand. Cores and seismic data were then used to evaluate the thickness of sand across an area. Because of the relatively large number of geologically unique areas, no extrapolation from basin to basin was possible. Within basins, however, extrapolation across a relatively large area was required by the relatively few cores. Because sand thickness varied across individual basins, maximum and minimum estimates were made for many basins. The true volume of sand is a value between these estimates.

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).

Table 2

The sand volume estimate of 7.96 x 10⁵ m³ assumes a uniformly thin sandy deposit across the basin (Table 2). Based on an interpretation of the seismic line (Figure 9), channels were possibly incised into the glacial-marine sediment at the time of sea-level lowstand. These may have been filled with valley fill material during transgression. The limited core data did not encounter sandy material at depth, however, so we are constrained to report a relatively small deposit here.

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.

Figure 12

Figure 13

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).

Figure 14

Figure 15

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.

Figure 16

Figure 17

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.

Figure 18

The sand deposit in Basin 8 resembles the deposit in Basin 1, the Shelf Valley (Figure 3 and Figure 11). Basin 8 is also a bedrock-framed valley-like feature that is largely filled with glacial-marine mud (Figure 18). A core from the center of the basin recovered about 1 m of slightly muddy sand overlying dense mud interpreted as glacial-marine material. Although the volume of the deposit, 4.8 x 10³ m³ of sand (Table 2), is the smallest evaluated for this study, the basin extends outside of the area of study.

Introduction Previous Studies Geologic Setting Methods Results Discussion References Appendix

Last updated on November 1, 2006