OPEN-FILE NO. 88-6
Geomorphology and Sedimentary Framework of the Inner Continental Shelf of Central Maine ,., , , , ' ...... ·"-r... , ...... ,, ' : by , ' { I o I Joseph T. Kelley ,.. ' I Daniel F. Belknap • •I •> /' .~ ,--1'.1 Maine .. -- -~• I I I I I I I ,JI I I I I I I I I I I I • • . -l -- --...; .... -
.. ~- .. ---.--, .I ' . ' .•
Walter A. Anderson, State Geologist Maine Geological Survey DEPARTMENT OF CONSERVATION GEOMORPHOLOGY AND SEDDOOITARY FRAMEWORK OF
THE IHBER COHTillEllTAL SHELF OF CENTRAL KAillE
Joseph T. Kelley Maine Geological Survey State House Station #22 Augusta, Maine 04333
and
Daniel F. Belknap Center for Marine Studies Oceanography Program Department of Geological Sciences University of Maine Orono, Maine 04469
Preparation of this report was supported by the United States Minerals Management Service Continental Margins Program through Cooperative Agreement 14-12-0001-30296.
Walter A. Anderson, State Geologist Maine Geological Survey DEPARTMENT OF CONSERVATION
1988
Open-File No. 88-6 GEOMORPHOLOGY AND SEDIMENTARY FRAMEWORK OF THE INNER CONTINENTAL SHELF OF CENTRAL MAINE
Joseph T. Kelley Maine Geological Survey State House Station #22 Augusta, Maine 04333
and
Daniel F. Belknap Center for Marine Studies Oceanography Program Department of Geological Sciences University of Maine Orono, Maine 04469
ABSTRACT
This report summarizes the Year Three research in Maine of the Minerals Management Service Continental Margins Program. Approximately 275 kilometers of high resolution seismic reflection profiles, 100 kilometers of side-scan sonar records, and 107 bottom samples were collected and investigated to elucidate the sedimentary framework of the central Maine inner shelf. On the basis of this, the shelf may be divided into four physiographic zones: Nearshore Basins, Shelf Valleys, Rocky Zones, and Outer Basins. These are distinguished on the basis of surficial sediment texture and composition, geometry of sedimentary deposits, and late Quaternary geological history. The driving force behind shelf sediment deposition, and the process which unifies the shelf stratigraphic framework is sea level change. Following deglaciation, the shelf experienced two marine transgressions and a regression which led to sediment deposition and erosion at various places across the shelf in the past 14,000 years.
INTRODUCTION
This report describes the submarine geomorphology, surficial sediments, and Quaternary stratigraphic framework of the western Gulf of Maine along the inner continental shelf of central Maine seaward of Muscongus Bay (Figures 1, 2). Although reference is made to pertinent terrestrial observations, the research focuses on the nearshore region to a depth of 100 m. Within this area, bedrock of complex origin ranges in age from Ordovician to Devonian; with a Devonian intrusive, the Waldoboro Pluton, the most common coastal outcrop (Osberg et al., 1985). Bedrock is widely exposed in the coastal zone and exercises a primary control on the morphology of the shoreline (Kelley, 1987c). The Muscongus Bay region from Pemaquid Point to Penobscot Bay is the northernmost embayment of the Indented Shoreline coastal compartment. This area is characterized by elongate peninsulas of metamorphic rocks separated by narrow muddy estuaries (Kelley, 1987c).
1 66'' 68° 74'' 72" 70° GULF OF MAINE BATHYMETRY 45'' 45° - ISOBATH5 IN METERS !:::'.':I CENTRAL MAINE i;;;;;:i INNER SHE lf .:-:·NOVA I• () 25 50 75 100 1 , , • 1 1 I I I Nautical Miles ;'." SCOTIA ·" 44° 0 25 50 75 100 1,,. •• 1 I I I Kilometers 44°
l'
43°
41"~ ~<:) Browns Bank ~G"""" N I «.; 42" ~~ I 42' ~.,.(\-.. e'> (,eo~"?; .pl'. -141"
bb" 68° 74" Figure 1. Location of the study area in the western Gulf of Maine. as• 10· a.- 30· u• 20·
BATHYMETRY OF MUSCONGUS BAY
.....__,..s-- ISOBATHS IN METERS
0 ' 2 u• oo 44' 00 --NAUTICAL MILES I
0 1 2 3 MM MI KILOMETERS
43" 50
6~ Oo
Figure 2. Bathymetric map o:f the s"tudy area, Muscongus- Bay. The irregular contour interval was employed because inadequate bathymetry exists :for this region. Modified :from NOS Chart 13301.
3 Like other shelf areas of New England and the Canadian Maritimes, central Maine has probably experienced numerous Quaternary glaciations, and relatively thin glaciogenic sediment only partly mantles submerged bedrock exposures (Needell et al., 1983; McMaster, 1984; Knebel, 1986; Piper et al., 1983). Unlike the outer regions of the Gulf of Maine and beyond, however, local, relative sea level has fluctuated profoundly in southwestern Maine due to isostatic crustal movements as well as eustatic sea level changes related to growth and disintegration of the Laurentide Ice Sheet (Stuiver and Borns, 1975; Schnitker, 1974; Belknap et al., 1987a,b). Within the past 14,000 years the study area has experienced a deglaciation, two marine transgressions, and a regression of the sea. It is these changes in sea level, which have permitted a variety of terrestrial and marine processes to repeatedly operate over the inner shelf, that have established the regional stratigraphic framework, and most significantly affected the nature of surficial sediments. The purpose of this paper is to describe the surficial sediments of the area in the context of a stratigraphic framework dictated by Holocene sea level fluctuations.
PREVIOUS WORK
Observations on the late-Pleistocene geology of the land surrounding Muscongus Bay have been made since the late 19th century, although there are no published reports dealing exclusively with Quaternary events in this area (Table 1). Stone (1899) first recognized the large moraine system near the head of Muscongus Bay, the Waldoboro Moraine, and this feature was subsequently mapped and discussed in greater detail by Leavitt and Perkins (1935). They also observed accumulations of stratified sand and gravel associated with the moraine, and mapped Quaternary marine sediments around the perimeter of Muscongus Bay. They called the marine sediments "wash plains" and described their intimate association with closely-spaced ridges of till which they called "moraine banks". Although he did not concentrate his work in central Maine, Bloom (1960, 1963) described the significance of the raised marine sediment in nearby southern Maine, and named the material the Presumpscot Formation.
More recently, Smith has remapped the Quaternary geology of the land surrounding Muscongus Bay and his observations have been compiled onto the state surficial map (Thompson and Borns, 1985). Smith (1985) has also provided radiocarbon dates on fossils within the Presumpscot Formation near the study area, which bracket its time of deposition between about 11,500 and 13,000 years before present (BP). He established that the older portions of the Presumpscot Formation were deposited in close association with a marine-based ice sheet (Smith, 1982). According to Smith (1982, 1985) a floating ice shelf existed in the area in the late Pleistocene, and where it occasionally became grounded on high areas, a moraine with associated subaqueous outwash and glaciomarine mud was deposited. Where this occurred frequently, such as in central Maine, fields of "moraine banks", or washboard moraines (Smith, 1982) are found. In discussing the land surrounding the study area Smith (1982) states: "Here, moraines are larger and more continuous than in any other part of the central and southern coastal zone (of Maine)" (p. 198).
4 Table 1
Quaternary Geology of Central Maine and Adjacent Inner Continental Shelf Region: Previous Work
Study Location Data
Stone, 1899 Central Maine Terrestrial mapping of moraines
Leavitt and Central Maine Terrestrial mapping Perkins, 1935 of moraines, marine sediment
Bloom, 1960 Southwestern Maine Regional terrestrial mapping
Bloom, 1963 Southwestern Maine Study of sea level changes
Ostericher, 1965 Penobscot Bsy Seismic stratigraphy, coring
Knebel, 1986 Penobscot Bsy Seismic stratigraphy
Kelley et al., 1987a Southwestern Maine Seismic stratigraphy, inner shelf bottom samples
Kelley et al., 1987b South central Maine Seismic stratigraphy, inner shelf bottom samples
Stuiver and Borns, Coastal Maine Study of sea level 1975 changes
Smith, 1982 Coastal Maine Terrestrial mapping of moraines
Thompson and Borns, Maine State surficial map 1985
Smith, 1985 Southwestern Maine Regional terrestrial mapping
5 69" 30" 69' 20' 59" 10
BOTTOM SAMPLE STATIONS IN MUSCONGUS BAY
•SAMPLE STATIOlllS
0 ' , d• oo· 44• 00 NAUTICAL MILES -0 1 - 2 3 ---KILOMETERS MUSCOlllGUS ~: ., 'e
·:·'· ROUlllO POlllD~;·
.,,, ., @ 'e .., ., ~ ,, g •,, I •,,, .;/ •,..,,.. ·-. lllEW HAR~~i;t··~' •,o: •,, •,o., .., ., ' •., :f.: •,, .., ~'f,,~t P"..~~~TUIO •e, ·.• •,,c ·.. .,, ·~ •,, ...., •., .., .,, .,, •,, ..• ··. .,,
·. ., ... ..,, ., ..
el" 20· n• 10·
Figure 3. Location of bottom sample stations.
6 In nearby Penobscot Bay, Ostericher (1965) first employed coring and seismic reflection techniques to examine submarine stratigraphy in coastal Maine. He described reflectors that correlated with bedrock, till, and the Presumpscot Formation, as well as surficial sediment textures from Penobscot Bay. He recognized the regressive unconformity on the surface of the Presumpscot Formation and dated wood fragments from cores of its surface at 7,390 years before present. On the basis of this he concluded that the "post-Presumpscot Formation" lowstand of sea level occurred at that time at a depth of 15-20 m. Knebel and Scanlon (1985), and Knebel (1986) have re-occupied Ostericher's (1965) lines with better seismic equipment and described details of submerged moraines and provided sediment thickness maps.
Schnitker (1972, 1974) also used seismic reflection methods to examine the central coastal region and interpreted subaerially dissected till from the records to a depth of 65 m, where he recognized a "berm". At depths greater than 65 m, he interpreted "undissected till" from the seismic records and constructed a widely cited sea level change curve which depicted the relationship of the land and sea between 14,000 years BP and the present. Belknap et al. (1986, 1987a; Kelley et al., 1987a,b) reinterpreted Schnitker's (1974) "undissected till" as natural gas, but otherwise acknowledged a 65 m lowstand shoreline and generally accepted the sea level curve (Belknap et al., 1987b).
On the northern border of Maine, Piper et al. (1983) described the evolution of parts of the Nova Scotian coast on the basis of seismic profiling. Fader et al. (1977) mapped the seafloor of the northern Gulf of Maine off the Bay of Fundy using seismic methods and bottom sampling, and King and Fader (1986) extended that work along the Scotian shelf. To the south, Birch (1984a,b) presented a structure contour map of the buried bedrock surface and isopach maps of seismic units representing early Cenozoic sediments, till, the Presumpscot Formation, as well as surficial deposits of sand and mud winnowed from the older units. He recognized a sea level lowstand at 35 m depth on the basis of truncated deltaic foreset beds at that depth (Birch, 1984b).
METHODS
Bottom Samples
During the summer of 1986, 139 bottom samples were collected from the central Maine inner shelf off Muscongus Bay by means of a Smith-Macintyre grab sampler (Figure 3). The device reliably collects a .25m3 sample of gravel, sand, or mud with minimal loss of material. Sample stations were located on LORAN-C time-delay intersections. The position of all samples was obtained by LORAN-C and depth measured by Raytheon Fathometer.
All samples were frozen immediately after collection and field description (Table 2). After standard sample splitting, gravel was screened out of the material for carbon analysis. While gravel was also screened out of carbonate analysis splits, the weight of gravel was noted for carbonate analyses.
7 Bottom Sample (2-5 kg) 1 Frozen
Splitl 1 1 Sal'!\ple Crushed,l Split Sieved (2mm) (50-lOOg) Sieved at 2 mm (lOg) (0.5-2.0 g)
Weight of Gravel
Carbonatel Removed Organics Removed Filtered atl 0.45 µm (HCl vapor) (2g) (NaOCl) (10% HCl)' °'
! ~ Weight of Caco3 I Carlo-Erbe Model Washed, Wet! Sieved 1106 .Elemental Analyzer at 63 µm
I • c, • N I I Weight of Sand J ~~~~II~~~~ Sand Dried, Split Dispersed~ Mud (101' g) (Na("f 4)6)
Heavy Minerals Pip.3tte Analysis Separated (C H Br ) 2 2 4 [ Weight of Silt, Clay I [ Weight of Heavy Minerals I
Table 2. Flow diagram for laboratory analyses. The carbonate analyses were performed by the acid filtration technique of Molnia (1974). Gravel was removed from the sample prior to crushing, although the weight of gravel was incorporated into the calculation of percent carbonate. This technique accurately evaluated known standards to within 0.5% (Kelley et al., 1987a).
The textural analyses followed the procedure outlined by Folk (1974). For all samples, a complete grain-size analysis was performed. The proportion of gravel is probably not well represented owing to the difficulty of sampling such large clasts (Folger et al., 1975).
Seismic Reflection Profiles
Approximately 275 km of seismic reflection profiles have been collected from the study area (Figure 4). Navigation was by LORAN-C and position fixes were made every 4 or 5 minutes, with ship speed varying from 3 to 5 knots. Two types of seismic systems were employed in our study: a Raytheon RTT 1000A unit and an O.R.E. Geopulse system. In general the two systems were operated simultaneously, although there were times when only one device was in operation. In addition, seismic data gathered by the U.S. Geological Survey in 1974 using an EG&G Uniboom system was examined (Figure 4).
The two systems we used operate in a complementary fashion. The Raytheon unit runs on a 3.5/7.0 kHz frequency and simultaneously at 200 kHz. The 200 kHz signal provides an accurate trace of the bathymetry while the 3.5 kHz signal generates a high resolution record of sub-bottom acoustic reflectors in generally shallow water ( The seismic records were used to deduce the nature of the subbottom geology as well as of the surficial material. In the latter capacity, side-scan sonar and bottom sampling provided ground truth "calibration" for interpreting surficial texture as revealed by the relative intensity of the surface acoustic return and overall geometry of the upper acoustic unit. The seismic lines were of great use, thus, to interpolate the surficial geology between the relatively widely spaced bottom samples (Folger et al., 1975). Interpretation of the subbottom geology was less direct, and inferences drawn from observations on land, in borings and core holes, and from nearby studies were employed to identify the acoustic reflectors. Bedrock was never penetrated by the seismic systems and its surface usually formed the lower-most reflector on a record. Relief on the surface of the bedrock was extreme, and ranged over tens of meters across short horizontal distances. In other nearby areas (Birch, 1984a,b; Fader et al., 1977) early Cenozoic sediment unconformably overlies bedrock. No such deposits were identified in the study area, however. Instead, a seismic unit with chaotic internal reflectors and an irregular surface commonly rested on 9 89" 20' SEISMIC PROFILE LINES OF MUSCONGUS BAY ++-+-SEISl.llC REFLECTION -~SIDE SCAN SONAR. F~. S£!Sl.llC R£FLECTION ~ J FluH Location 0 ' ' 44° 00 NAUTICAL MILES -0 -' 3 ---KILOMETERS -:· ">' .-,. < "·'\ 43" 50' ee•30· ee•20· 8g' 10' Figure 4. Location of seismic reflection profiles and side-scan sonar lines. 10 bedrock. This has been interpreted as till, and several prominent moraines were recognized as in nearby locales (Oldale, 1985; Knebel and Scanlon, 1985; Knebel, 1986). Frequently, a relatively transparent acoustic unit with closely spaced basal reflectors mantled the till or bedrock. The hard surface return of this unit was usually flat except in valleys where it was channel-shaped. Where it outcropped near the surface and was cored for other projects, this unit was identified as the glaciomarine Presumpscot Formation (Belknap et al., 1987b). Because of its readily identifiable characteristics, the glaciomarine sediment has been called Presumpscot Formation far offshore, and well outside the area within which it was originally described (Bloom, 1960). Its upper surface on land marks the regressive unconformity, terminating its deposition, while the offshore surface of the Presumpscot Formation is probably capped by the transgressive unconformity. Overlying the Presumpscot Formation, an acoustically transparent unit of modern mud was identified in some locations. Modern deposits are relatively smooth on their surface and generally lacking internal reflections. As in nearby areas (Kelley et al., 1986) natural gas occurrences were recognized in the study area. Seafloor Observations Observations on the seafloor itself were made by side-scan sonar profiling (Figure 4). The side-scan system used was the EG&G SMS 260 Seafloor Mapping System. This system automatically provided slant range corrections to the analogue output and was operated successfully at all depths in the study area. It was usually run at a 100 m or 200 m range (to either side of the vessel) and allowed bottom sample ground truth to be widely extrapolated. In deep water (>75 m) the 300 m and 400 m range were used. BATHYMETRY Unlike the inner shelf of New Hampshire and Massachusetts (Birch, 1984; Folger et al., 1975), the physiography of the study area is extremely irregular (Figure 2). As a submerged extension of Maine's coastal lowland (Denny, 1982) its bathymetry is dominated by bedrock exposures and glacial deposits. Glaciation has probably exaggerated the local relief by preferentially eroding foliated rocks and leaving resistant knobs of plutonic bodies (Kelley, 1987c). Most of the islands in the study area, for example, are composed of intrusive rocks. Unlike most portions of the United States continental shelf, no bathymetric charts of Muscongus Bay and the central Maine inner shelf are yet prepared. Thus, the bathymetry was taken from NOS Chart 13301, which employs seven irregularly-spaced contours between the shoreline and the 100 m isobath. As in previous work (Kelley et al., 1987a,b), the complex bathymetry is best understood when simplified to four distinct physiographic regions (Figure 5). The Nearshore Basins are shallow (30 m), generally low-relief regions adjacent to the mainland. These basins are separated from other areas by extensive shoals or chains of islands extending seaward from the mainland. The contact between the basins and the intertidal upland is gradational 11 59• 30' 66' 20 MUSCONGUS BAY INNER SHELF PHYSIOGRAPHIC ZONES NB· Nearshore Basin SV·Shell Valley RZ·Rocky Zone OB·Outer Basin H" oo· ••• 00 0 ' ' --NAUTICAL MILES=1 0 ' 3 -KILOMETERS- - . [). RZ RZ .i:t' 50' OB OB RZ OB OB R2 OB RZ RZ OB OB OB RZ OB 69' 30' 69' 10· Figure 5. Map depicting the physiographic provinces of the central Maine inner shelf. 12 along mudflats in very protected areas, but may be abrupt, with greater than 5 m of vertical relief in more exposed locations. While the seafloor of the basins is generally smooth, it is often cut by channels in narrow constrictions between islands, and is bordered by rock margins or punctuated by rock outcrops occasionally. Shelf Valleys are long, narrow depressions which extend from Nearshore Basins to gradational terminations in deep water. The valleys are very steep-walled along their margins and frequently encounter irregular areas along their otherwise smooth seaward slopes. They are most common in the depth range from 25-55 m. Rocky Zones are regions of extreme local relief ranging from 5 m vertical bedrock cliffs, to extensive areas of boulders. The highly foliated nature of bedrock in central Maine, in contrast to the inner shelf of the southwestern part of the State (Kelley et al., 1987a), is especially characterized by long narrow bedrock ridges and troughs. Although sediment is often found in the troughs, it is of minor abundance compared to the extensive ledges. In some areas, Rocky Zones extend along the trend of morainal axes in outer Muscongus Bay. The final physiographic zone is the Outer Basins which generally form the seaward border of the study area. This is the least known portion of the study area, largely due to a lack of good bathymetric control. On the basis of small scale charts (1:1,000,000) it appears to extend from about the 55 m isobath to Platts Basin in the deeper Gulf of Maine. BOTTOM SEDIMENT TEXTURE Bottom sediment texture on glaciated shelves is notoriously heterogeneous (Trumball, 1972). Virtually all components of the particle size spectrum were encountered in bottom samples from central Maine's inner shelf (Figure 6). Although only four samples (3%) were true gravels, some material coarser than 2 mm in diameter was collected from 27% of the stations occupied (Figure 6). As in other studies (Folger et al., 1975), the abundance of gravel was probably underrepresented by bottom sampling due to the difficulty of collecting large objects in the sampler, and of obtaining enough gravel to perform statistically meaningful grain size analyses. Despite these limitations, several large patches of gravel bottom are mapped in the study area (Figure 7). These are located in channels between islands where tidal currents are concentrated, adjacent to islands and along the margins of Nearshore Basins and Shelf Valleys, and in an area of glacial moraines (discussed below) north of Monhegan Island. As presented below, side-scan sonar observations reveal coarse-grained gravelly sediment surrounding nearly all rock outcrops in the region, as was reported from other portions of Maine's shelf (Kelley et al., 1987a,b). Thus it is not surprising that half of the gravel samples come from regions mapped as "rocky zone" (Figure 5). Sandy gravels and gravelly sands are more common substrates than pure gravels, and each category represents about 11% of the stations occupied. It is significant that all of these samples are from Rocky Zones or Nearshore Basins (Figure 6), and most are shallow water deposits. In the 13 100% Gravel 6 6 • 6 6 6 6 6 6 6 6 • • • 6 06 6 • • 6 6 • 6 6 • • 6 • 100%~-----~·------"~• 0~0,______.!.!.. ______,.100% Sand Mud ~00% Sand c. - Rocky Zone • - Nearshore Basin o - Shetf Valley x - Outer Basin • • 0 0 • 0 • 0 • •o• • • 0 0 • o• • • o• x 0 • D xx e D 0 • eoo Do xx x • • 100% x.O. XX XX f X X 100% Silt ~------~'--'----"--''-'---'------"c1ay Figure 6. Ternary diagrams depicting the texture of sediment samples from the central Maine inner shelf. 14 811' 30' n•" 59• 10 MUSCONGUS BAY INNER SHELF SURFICIAL GEOLOGY """'"'-C2nvn) '~f----1 o• oo' 44• 00 ROUND PONO 0 1 ' NAUTICAL MLES --0 t 2 3 --KILOMETERS M M BR BR 811" 30' 811' 10' Figure 7. Map of sediment texture on tne central Maine inner conoinental shelf. 15 Nearshore Basins this sediment is located in channels, or along the margin of the basin. Many of the gravel, or sandy gravel deposits in these locales were observed on side-scan sonar, but are too small and irregular in extent to map. The Rocky Zones often surround islands and shoals (Figure 5) and gravelly sediment frequently armors small sediment ponds between outcrops, or thinly veneers the rock. In the large Rocky Zone surrounding Monhegan Island, bedrock dominated the side-scan sonar imagery but gravelly sediment was collected in grabs. Probably more gravel exists here than is shown (Figures 5, 7). Only 2% of the stations contained muddy gravel (Figure 6) and these extremely poorly sorted samples very likely came from the boundary between muddy deposits and rocky areas. There were no clean sands found in the study area. Although this is unusual compared to observations made along other parts of the Maine shelf (Kelley et al., 1987a,b), it is not surprising if one considers that there are no sandy beaches or Nearshore Ramps in this area such as those found in southern Maine. It is more unusual to note that only 3% of the samples are muddy sands, because sediment of that texture was the most common found south of this study area (Kelley et al., 1987a,b). As a generalization, one must conclude that there is simply little sand in this region. Although there is little sand, there is a great deal of mud in Muscongus Bay (Figure 6). Almost 47% of the samples are classed as muds (more than 50% of their sediment is less than sand size). The muddy samples come from Shelf Valleys, Nearshore Basins, and Outer Basins, but most of the samples with more than 95% mud are from the Outer Basins. Even in areas that possessed an irregular bottom, suggestive of a rock outcrop, yielded muddy samples. GEOPHYSICAL OBSERVATIONS Bottom sediment properties correlate well with environmental settings defined by bathymetry (Figures 5, 6, 7). Considerable variation in geological properties exists, however, within the various regions. This variability is best explained by the more detailed examination permitted by side-scan sonar coupled with observation of the subbottom geology made with seismic reflection profiling. The seismic reflection profiling also provides insight into the geological history of the region, which further abets complete understanding of the overall sedimentary framework. Nearshore Basins Three large, unconnected Nearshore Basins are recognized in the study area (Figure 5). Each of these is bordered by the mainland and sheltered by large islands or extensive shoal areas on two sides. These basins begin in estuaries, although the discharge of the Medomak and St. George streams is insignificant (<100 m3/sec). The basins each terminate in a seaward direction in a Shelf Valley. The border with Shelf Valleys occurs where the sheltering influence of islands ends, and more exposed conditions prevail. The most prominent characteristic of Nearshore Basins is an extensive, smooth muddy seafloor (Table 3; Figure 8). All bottom samples from these 16 Table 3. Subdivision of the physiographic zones into facies of differing properties. Physiographic Zone/Facies Depth(m) Morphology Texture Structure Composition Origin NEARSHORE BASIN Smooth muddy 5-15 flat soft mud, bioturbated, high C, N Holocene mud dark patches? fishing gear low Caco marks 3 Channels 5-25 steep sided, mud - gravel slumped blocks ? modern slumping slump scars due to tidal currents? Rocky 0-25 high local muddy, sandy chaotic high CaC0 reworked glacio relief gravel low C, N 3 rnarine, modern carbonate -lo ------~ ROCKY ZONE Exposed Bedrock 0-100 high relief no sediment circular high CaC0 modern carbonate outcrop pattern 3 Gravel Plain 30-65 low relief muddy sandy chaotic high CaC0 reworked till gravel with low C, N 3 boulders Sediment Ponds 30-65 low relief, sandy gravel; megaripples high Caco reworked paleo- seaward dip muddy gravel low C, N 3 shoreline; reworked glaciomarine Table 3. Continued. SHELF VALLEY Thalweg 30-65 U-shaped muddy sand bioturbation high C, N paleochannel, now channel common low Caco rarely active 3 Rippled, Scour 30-65 elongate gravelly sand megaripples low C, N recently scoured Zones patches in high CaC0 area channel 3 Channel Margin 30-65 high relief rocky, sandy megaripples, low C, N modern carbonates gravel talus pile very high and reworked Caco glacial material 3 OUTER BASIN Muddy Basin 65-100 flat, smooth sandy mud, bioturbation high C, N modern hemipelagic --' CP muddy sand common high CaC0 sediment, 3 turbidi tes? Low stand 55-85 moderate relief gravelly sandy ? high C, N shoreline cut into Shoreline concave up mud high CaC0 glacial, glacio 3 marine sediment modern mud Rocky 55-100 high relief, muddy gravel ? high C, N reworked glacio chaotic gravelly mud high Caco marine, modern mud 3 areas are muds (Figure 6), and side-scan sonar records of most of the basins are uniformly white (low reflectivity), Seismic reflection profiles indicate more than 30 m of sediment in some basins with up to 10 m of acoustically transparent material (i.e., mud) at the surface, On the basis of unpublished core data through this acoustic unit for archeological research, we infer it is Holocene in age as has been observed elsewhere (Belknap et al., 1986; Kelley et al., 1986, 1987b). A pronounced acoustic reflector marks the bottom of this unit in most areas. This reflector has been recognized in many areas as the transgressive unconformity which caps the Presumpscot Formation (Kelley et al., 1987b). Where bedrock pinnacles bring this reflector near the surface, gravel occurs on the seafloor (Figures 8, 9, 10). Thus, we interpret gravel occurrences along the rocky margins of Nearshore Basins, and adjacent to outcrops within the basins as a result of the exposure of coarse-grained Pleistocene sediment. Relatively strong currents near bedrock outcrops may be presently eroding these areas, or simply preventing deposition of Holocene mud. Beneath its strong surface reflector the acoustic unit identified as the glaciomarine Presumpscot Formation possesses many continuous, parallel reflectors which mimic the underlying topography. Where these are truncated at the seafloor we infer on-going erosion of the bottom sediment. This is very common in narrow channels of the Nearshore Basins (Figure 10) where current action is usually strong. In the central axes of the Nearshore Basins as acoustically opaque reflector masks all lower material. This is interpreted as natural gas, and it is a common feature in Maine's fine-grained Quaternary sediment (Figure 8). In one area a circular pit 10 m in diameter was observed above a deposit of gas-charged sediment, and may have formed as a result of escaping gas. No slump deposits were observed in the Nearshore Basins of this study as were noted in Casco Bay (Kelley et al., 1987b). Shelf Valleys As in other locations along Maine's inner shelf, there are more Shelf Valleys than presently-existing rivers. In Muscongus Bay there are only two minor streams entering the bay, yet four significant drowned valleys are recognized, One of the smaller valleys (Figure 11a) exhibits most of the components of Shelf Valleys discussed elsewhere (Kelley et al., 1987a,b), In the central area a muddy seafloor is dominant. The mud appears to overlie a gravel margin adjacent to the bedrock walls of the valley, Sample 107 (Figure 11a) was gathered from this area and is a gravel. Although this small valley contains little sediment today (Figure 11b), larger valleys (Figures 12, 13) hold more than 30 m of sediment. As in the Nearshore Basins, the gravel deposits at the edge of the valley appear to represent the outcrop trace of prominent subbottom reflectors. In many locations the walls of the valleys are bordered by flat-topped surfaces partly incised from inferred Pleistocene sediment (Figures 12b, 13b). These resemble terrestrial river terraces or drowned shorelines, and may be submerged remnants of such features. Little evidence of current action (i.e., bedforms like megaripples, gravelly scour surfaces) was observed in Muscongus Bay Shelf Valleys. Currents may be present, but the general absence of sand may preclude 19 bedform formation and migration as inferred to occur elsewhere (Kelley et al., 1987s,b). Outer Basins The Outer Basins generally begin at the seaward termination of Shelf Valleys where the gradient of the valleys effectively ends, and the seafloor flattens (Figures 5, 14). This is usually around the 55-65 m isobath, which is inferred to be the depth of lowest Holocene sea level (Belknap et al., 1987). While bedrock is relatively common in the Outer Basins (Figures 14, 15), it is usually blanketed by Holocene mud. All samples returned from these areas were muddy (Figure 6), even when rock exposures were common. The average sand content of Outer Basin samples was less than 5% (Figure 6). Seismic records often reveal a thin cover of acoustically transparent sediment overlying an irregular, bedrock surface in the Outer Basins. We interpret this as mud, since even areas with considerable relief associated with rock exposures (Figure 15) yielded muddy bottom samples. In the deepest locations, none of the winnowing of seafloor sediment near rock outcrops, as reported from the shallower environments, was observed. Instead, as discussed below, Pleistocene sediment is generally covered by inferred, muddy Holocene sediment. Seafloor with the same character as the Outer Basins very likely extends into the deeper Gulf of Maine. Rocky Zone The Rocky Zone is the most areally extensive portion of the study area (Figure 5), and the central Maine inner shelf is the rockiest shelf area of Maine yet observed (Kelley et al., 1987a,b). As described from other shelf areas in Maine, many regions of exposed bedrock are located in water depths less than 50 m. Exposed rock is especially common surrounding shoals and islands in the central part of Muscongus Bay. Perhaps, because the bedrock in the study area is mostly intrusive bodies (Osberg et al., 1985), there are fewer sediment ponds between rock ridges as reported from nearby Casco Bay (Kelley et al., 1987b). Where sediment ponds were observed (Figure 16), the material between rock exposures was gravel. Seismic profiles reveal that the sediment ponds contain less than 10 m of sediment as a rule, and the coarse-grained surface sediment is typically underlain by an acoustically transparent unit inferred to be muddy. Gravel plains comprise a significant portion of the area mapped as Rocky Zone. To the north of Monhegan Island a large area of the seafloor is covered with sand, gravel, and boulders (Figures 17, 18). Side-scan sonar imagery shows that the surficial deposits of this very heterogeneous region are organized around a series of northwest-southeast oriented ridges having a relief up to 5 m. Seismic reflection profiles indicate that the surficial deposits are part of an acoustic unit with no continuous interns! reflectors, and which is difficult to distinguish from the underlying bedrock. We interpret this sediment as glacial till, and the field or ridges as a series of washboard moraines like those described from the nearby mainland (Smith, 1982). Where these ridges border a low area like a 20 Shelf Valley (Figure 18), the till ridges are not observed to extend into the valley. Thus, the washboard moraines seem confined to areas shallower than about 60 m. EVOLUTION AND SEDIMENTARY FRAMEWORK OF THE CENTRAL MAINE INNER CONTINENTAL SHELF AT MUSCONGUS BAY The sedimentary framework of central Maine's inner continental shelf has been controlled by the late Quaternary deglaciation of the region, and accompanying sea-level fluctuations (Stuiver and Borns, 1975, Belknap et al., 1987). Although the general time in which retreating ice reached the present coast of Maine is not in dispute, disagreement exists about the style of deglaciation (King and Fader, 1986; Oldale, 1988). Seismic evidence from the study area and nearby (Knebel, 1986) supports the idea of a marine-based ice-sheet in the nearshore regions of Maine about 13,000 years ago (Figure 19). Large moraines and fields of smaller washboard moraines are indicative of active ice during deglaciation. The washboard features result from till (labeled t in Figures 19-22) deposition at topographically high pinning points. In lower areas, where the ice was not grounded, glaciomarine mud, accompanied by dropstones, accumulated (Smith, 1982). Mass movements were probably a common occurrence under these unstable conditions, and debris flows, or subaqueous outwash deposits are tentatively recognized near moraine fields north of Monhegan Island (Figure 19). Accompanying retreat of the ice, glaciomarine sediment, the Presumpscot Formation, was deposited. At first, when the ice was nearby, rapid deposition of the glaciomarine mud draped that sediment over till or bedrock in the vicinity of the ice. This resulted in the unit labeled gm-d in the seismic cross sections (Figures 19-22). Seismic reflectors within this acoustic unit are continuous, and mimic the undulations of the underlying relief across hundreds of meters. After removal of ice from the area, marine sediment accumulated more slowly through physical sedimentation. This led to a parallel-bedded unit (gm-p, Figures 19-22) overlying the draped sediment. During deposition of this sediment, the land surface was undergoing isostatic uplift as a result of withdrawal of the ice and unloading of the crust. It is still unclear exactly when, but by around 9,500 BP, sea level had fallen to its lowest Holocene elevation of around 65 m depth (Belknap et al., 1987b). Evidence for the depth of the lowstand of sea level has been (Schnitker, 1974) and remains (Kelley et al., 1987a,b) the morphology of the seafloor in the 55-65 m depth range (Figure 20), and the stratigraphy seaward of that depth compared with more landward areas (Figure 21). Lowstand shoreline terraces are interpreted from several locations in the study area (Figures 19, 20) and typically exist on the flank of an Outer Basin or Shelf Valley. The terrace commonly dips in a seaward direction and may consist of (apparent) constructional features as well as eroded notches, The surficial sediment comprising these terraces is usually a sand and gravel mixture, and in locations outside the study area, may be overlain by a thin deposit of mud. 21 Seaward of the inferred lowstand shorelines there are no prominent reflectors to mark the Holocene-Pleistocene boundary, because sedimentation has been more or less continuous through that interval (Figure 20). In these areas the ponded acoustic facies (gm-p) extends with few acoustic reflectors to the present seafloor. In the shallower water above the inferred depth of sea-level lowstand, a strong acoustic reflector appears to represent a transgressive unconformity on the surface of the Presumpscot Formation (Figures 8, 9, 21). This has been cored in other locations and confirmed as a widespread feature in the nearshore waters of Maine (Ostericher, 1965; Belknap et al., 1986, 1987). The sedimentary framework of the central Maine inner shelf, thus, has been established by the late Quaternary deglaciation and sea-level changes. Most of the seismic units overlying acoustic basement are interpreted as glacial or early post-glacial in origin. Surficial sediment appears to be largely derived from reworking of older sediment. In the Outer Basins, the mud which blankets the seafloor over wide areas, has probably been derived from the glaciomarine Presumpscot Formation. We infer that sediment accumulation here is slow today, but it may have been faster in the past when sea level was lower. The extensive Rocky Zones were probably thinly covered with sediment prior to the sea-level regression. Passage of the surf zone across these regions, during times of falling as well as rising sea level, stripped them of most of their cover of unconsolidated sediment. While a lag deposit of boulders and gravel is all that remains today in these areas, sandy lowstand shoreline deposits, and mud in the Outer Basins and beyond may have originated from erosion of the Rocky Zone's former sedimentary cover. The Shelf Valleys are bedrock-framed features, and with the exception of the valley extending into Penobscot Bay (Figure 5), may never have hosted significant streams. No clear ravinement (regressive) unconformity is recognized in the valleys, although one may have existed in areas presently obscured by natural gas. If the Shelf Valleys were gullied during the regression, bluff erosion removed much of their sediment during the transgression. Erosion may still be occurring within the Shelf Valleys today, and exposed glaciomarine sediment on the flanks of the valleys may reflect this process. The Nearshore Basins are a landward extension of the Shelf Valleys and have much in common with them. Because of greater protection from waves, and possibly from tidal currents, the basins possess a relatively thick deposit of inferred Holocene sediment. Current action appears to effectively prevent sediment accumulation only near bedrock protuberances (Figures 8, 9) and in restricted channels (Figure 10). ACKNOWLEDGMENTS We wish to acknowledge funding for this project from the United States Minerals Management Service's Continental Margins Program. The field assistance of Captain Michael Dunn, and University of Maine students Louise McGorry, Christopher Mulry, Jean B. Pelletier, and Stephanie Staples is gratefully acknowledged, as well as that of Robert Johnston of the Maine Geological Survey. 22 REFERENCES CITED Belknap, D. F., Shipp, R. C., and Kelley, J. T., 1986, Depositional setting and Quaternary stratigraphy of the Sheepscot Estuary, Maine: a preliminary report: Geographie Physique et Quaternaire, v. 15, p. 55- 70. Belknap, D. F., Kelley, J. T., and Shipp, R. C., 1987a, Quaternary stratigraphy of representative Maine estuaries: initial examination by high resolution seismic reflection profiling, in FitzGerald, D. M., and Rosen, P. S., eds., Treatise on Glaciated Coasts, Academic Press, New York. Belknap, D. F., Andersen, B. G., Anderson, R. S., Anderson, W. A., Borns, H. W., Jr., Jacobson, G. L., Jr., Kelley, J. T., Shipp, R. C., Smith, D. C., Stuckenrath, R., Jr., Thompson, W. B., and Tyler, D. A., 1987b, Late Quaternary sea-level changes in Maine, in Nummedal, D., Pilkey, O. H., Jr., and Howard, J. D., eds., Sea Level Rise and Evolution of Coastal Systems Architecture, Society of Economic Paleontologists and Mineralogists Special Publication. Birch, F. S., 1984a, A geophysical survey of bedrock on the inner continental shelf of New Hampshire: Northeastern Geology, v. 6, p. 92-101. Birch, F. S., 1984b, A geophysical study of sedimentary deposits on the inner continental shelf of New Hampshire: Northeastern Geology, v. 6, p. 207-221. Bloom, A. L., 1960, Late Pleistocene changes of sea level in southwestern Maine: Maine Geological Survey, Augusta, Maine, 143 p. Bloom, A. L., 1963, Late-Pleistocene fluctuations of sea level and postglacial crustal rebound in coastal Maine: American Journal of Science, v. 261, p. 862-879. Denny, C. s., 1982, The geomorphology of New England: U.S. Geological Survey Professional Paper 1208, 18 p. Fader, G. B., King, L. H., and MacLean, B., 1977, Surficial geology of the eastern Gulf of Maine and Bay of Fundy: Geological Survey of Canada Paper 76-17, 23 p. Folger, D., O'Hara, C., and Robb, J., 1975, Maps showing bottom sediments on the continental shelf off the northeast United States: Cape Ann, Mass. to Casco Bay, Maine: U.S. Geological Survey Misc. Inv. Ser. Map I-839, Washington, D.C. Folk, R. L., 1974, Petrology of Sedimentary Rocks: Hemphill Publ. Co., Austin, Texas, 182 p. Kelley, J. T., Kelley, A. R., Belknap, D. B., and Shipp, R. C., 1986, Variability in the evolution of two adjacent bedrock-framed estuaries in Maine, in Wolfe, D., ed., Estuarine Variability, Academic Press, 23 Kelley, J. T., Shipp, R. C., and Belknap, D. F., 1987a, Sedimentary framework of the inner continental shelf of southwestern Maine: Maine Geological Survey, Open-File Report No. 87-5. Kelley, J. T., Shipp, R. C., and Belknap, D. F., 1987b, Sedimentary framework of the inner continental shelf of south central Maine: Maine Geological Survey, Open-File Report No. 87-19. Kelley, J. T., 1987c, Sedimentary environments along Maine's estuarine coastline, in FitzGerald, D. M., and Rosen, P. S., eds., A Treatise on Glaciated Coasts, Academic Press, New York, p. 151-176. King, L. H., and Fader, G. B., 1986, Wisconsinan glaciation of the Atlantic continental shelf of southeast Canada: Geological Survey of Canada Bull. 363, 72 P• Knebel, H. J., 1986, Holocene depositional history of a large glaciated estuary, Penobscot Bay, Maine: Marine Geology, 73:215-236. Knebel, H. J., and Scanlon, K. M., 1985, Sedimentary framework of Penobscot Bay, Maine: Marine Geology, v. 65, p. 305-324. Leavitt, H. W., and Perkins, E. H., 1935, Glacial geology of Maine, v. 2, Maine Tech. Expt. Sta. Bull. 30, 232 p. McMaster, R. L., 1984, Holocene stratigraphy and depositional history of the Narragansett Bay system, Rhode Island, USA: Sedimentology, v. 31, P• 777-792. Molnia, B. F., 1974, A rapid and accurate method for the analysis of calcium carbonate in small samples: Jour. Sed. Petrol., v. 44, p. 589-590. Needell, S. W., O'Hara, C., and Knebel, H. J., 1983, Quaternary geology of the Rhode Island inner shelf: Marine Geology, v. 53, p. 41-53. Oldale, R. N., 1985, Upper Wisconsinan submarine end moraines off Cape Ann, Massachusetts: Quaternary Res., v. 24, p. 187-196. Oldale, R. N., 1988, The glaciomarine mud of the Gulf of Maine and adjacent onshore regions: Geol. Soc. America, Abs. with Prog., p. 59. Osberg, P.H., Hussey, A. M., II, and Boone, G. M., 1985, Bedrock geologic map of Maine: Maine Geological Survey, Augusta, Maine, 1:500,000. Ostericher, C., 1965, Bottom and subbottom investigations of Penobscot Bay, Maine in 1959: U.S. Nav. Ocean. Off. Tech. Rept. TR-173, 177 p. Piper, D. J. W., Letson, J. R. J., Delure, A. M., and Barrie, C. Q., 1983, Sediment accumulation in low sedimentation, wave dominated, glaciated inlets: Sedimentary Geology, v. 36, p. 195-215. Schnitker, D., 1972, History of sedimentation in Montsweag Bay: Maine Geological Survey Bull. 25, Augusta, Maine, 20 p. Schnitker, D., 1974, Postglacial emergence of the Gulf of Maine: Geological Society of America Bull., v. 85, p. 491-494· Smith, G. W., 1982, End Moraines and the Pattern of Last Ice Retreat from Central and South Coastal Maine, in Larson, G., Stone, B., eds., Late Wisconsin Glaciation of New England, Kendall-Hunt Publ. Co., Dubuque, Iowa, p. 195-210. Smith, G. W., 1985, Chronology of late Wisconsinan deglaciation of coastal Maine: Geological Society of America Special Paper 197:29-44· Stuiver, M., and Borns, H. W., Jr., 1975, Late Quaternary marine invasion in Maine: its chronology and associated crustal movement: Geological Society of America Bull., v. 86, p. 99-104. Stone, G. H., 1899, Glacial gravels of Maine and their associated deposits: U. S. Geological Survey Monog. R-34, 489 p. Thompson, W. B., and Borns, H. W., Jr., 1985, Surficial Geologic Map of Maine: Maine Geological Survey, Augusta, Maine, 1:500,000. Trumbell, J. V., 1972, Atlantic shelf and slope of the United States: sand size fraction of bottom sediments, New Jersey to Nova Scotia: U.S. Geological Survey Professional Paper 529-K, 45 P• 25 ··~ .,. ~ · r image of smooth seafloor of Nearshore Basin with small bedrock outcrop. Sample 135 was a sand collectetl from dark mantle surrounding outcrop. The a rrow on the side-scan sonar image lines up and points i_n the same direction as the arrow on the seismic r eflection profile of Figure Figure is located in Figure 4. /' ,,.,,, ,.,, ., ''"""' '' ··--"-' ~~:w- ·:~s~x~~PVi~A1V-v~· ···"··"...... ,,.. ,,""' h :. "'0 ..~ E IO .,...C\I E IO ,,' ' '! . ~.· I ...... 6 0 • ., g ...... , .... 0 ,..0 ,..., ,,...... c""'0 ., .... E! t., 81 E ...... c L C\I .,,.. .., p C\I o~ ..lC\IJI . ... 0 i '.,...... E! "" ....."' ""' J' I .,... Jl ~ !' :;o ' ' =a. ..a. en a> co ~c ~ ....~ r.. 27 ,. r ... "' " .. mud " ... Side-scan sonar image of bedrock outcrop in central Muscongus Bay. The muddy Nearshore Basin (sample 130 is a mud) contrasts with the gravel surrounding a bedrock outcrop. on the side-scan sonar image lines up and points in same direction as arrow on the seismic profile of Figure 9b. Figure is located in Figure 4. ~} MS 87-20 ":'. Depth 10-36m 10m HOLOCENE t> 100m !·· ---·~-:-- HOLOCENE .·· ~ l ; .M MUD .· ... .. : ' 1)5 Figure 9b. Seismic reflection profile showing truncation of subbottom reflector near bedrock outcrop. The seafloor trace of the seismic reflector coincides with the appearance of gravel in the side-scan sonar image (Figure 9a). , ------· ------.. ------· - .. --· ~~~ . Side-scan sonar image of channel area within Nearshore Basin. Because of high current velocities here, the bottom is coarse-grained (sample 115 is a gravelly sand) . The arrow lines up and points the same _direction as arrow in Figure 10b. Figure is located in Figure 4. ------· ------...... ---~ ... --·-····- -··- .• MS 87-14 !::::-... •"t,-:•.. :::-:~- -=~.~~" '::~~ ~-~ ' 125m : ~ ~ ; ' Depth 13-20m .. • • • . • Side scan ;•· HOLOCENE MUD . image .. ' .. , - ~:.:.~.;;,,a;c;. GLACIOMARINE "'~ ~ ;-.~~;~:~~.. +::~~7~. ! ~·.:.:LL!::_, ____...-, I'.-:.. ~·; •. :-_---...-..--.----~·-·~~ ~..:2'"~~·~. Figure 10b. Seismic reflection profile adjacent to side-scan sonar record from channel in Nearshore Basin. The numerous truncated reflectors indicates this is an area of erosion, MS 87-3 DEPTH 28-53m 10m fl HOLOCENE 120m '"•;' MUD-- HOLOCENE \jJ MUD \jJ ~- . -.._~j} .-~ ::-...... ::.; I' .. :~;:_;;\~: :~:.~--::-, •...- Figure 11b. Seismic reflection profile shows the small valley is largely empty of sediment. Note that the notch etched in the right margin of the valley may be a terrace. MS 87-10-11 Depth 34-50m - --1------·-----r------. -r------··--··----smT1~---·. o-o-m ; • --··-··------·------.. -+---.. · --.. ------· ----- RIPPLED GRAVEL ~-- -· ------··------ \.>J \J1 ... '• . ·.· .. ,. ···~ .... - - '· ·.··; =-· -.. , ,.-. ,.,.·".\' ?.:; (::~~-~ ... ·~~-;,:· :~:-~\- ~\: ·,; ~ I·. . ·:.:~.--·~/~~;~----·-+--..,...... _,___ - ,. '• "• .', . ·--: 1.:: " '_: __,. - '·;'-._:.:,;~ }~!\ 1JL ., - ):?. 10m ' .. 125m w------..- ..... --·-·· ::------·-----·--···--·--~- .•.. -~. GLACIOMARINE ~ Figure 13b. Seismic reflection profile of ssme scene as Figure 13a. The flat-topped surface on the right margin of the valley appears to represent a terrace etched in coarse Pleistocene sediment. · ~'"f· ·-~ ..... :::~'\ ·~ t '°'" . 't!.. . . ~ ,:~"" - ~~ ;,;.; .,., .~.:,~~· '."'fl- · ... .. 'i ~'~~ ' -}~ . ~' ...... " .. ... ' ...... .it;t("'.~::~/ ·i· Side- scan sonar image from near the boundary between a Shelf Valley and the Outer Basin. Sample is a mud, and probably is typical of the extensive smooth area of the deep water portion of the study area. The arrow lines up and points in the same direction as the arrow in Figure 14b. The figure is located in Figure 4. ·-· ------~---··-····--·----~---~~'""------·· .. -.. ----··~···~. -"----·····------.. , MS 87-5 ~ Figure 14b. Seismic reflection profile across the same area as Figure 14a. The flat nature of the seafloor is very evident here, as is the thin cover of sediment. . . ,,•.t ' . ." '·· ,f' . - ~~: ~:: •~: . · ~r:'..~· ·· ~ ;.. ·~ •·· · ~- T .. ;•. ~ ·.~. · ·-.·_··.~; ;.,.i · · · . -r: • ,...... " . . . . ·;,. ~ . •• ,_ . ' II rt\'f>' ...... • '"'° ~ . .'• ' '· " • ' I l\.'l . ;4\lr-f' ' •• tl ~ . r " ...... - ...... ,; .i. ~.i..:...Llli & .l. ~~.,zJ!:a:~ -~! y ~ ..· ,. .. ..r ...... , , ,~1~~'''1~ ..,...... - . . ;: .. . l!', .,. . . JI' •• ... . . 4ii ...... ;c ~i· s: -...... r r t:_ .. , .. . .,t ...... , r r· j. ' t, ·., ...... , ~ ---~ MS 87-6 Depth 70-93m 10m ·I 1 100m I --·--· HOLOCENE MUD :;;-::~-- ,. ~ ':.f,r"('·, Figure 15b. Seismic reflection profile across the same area as Figure 15a. The relief associated with the bedrock outcrop is evident, yet all samples from this area are muds. Subbottom reflectors do not extend to the seafloor in the deep Outer Basin as is common in shallower environments. of sediment ponds and exposed bedrock near a granite island in central 4). Gravel floors the sediment ponds and is covered by mud in the area away up and points in the same direction as the arrow in Figure ------·------~------·------~-·~-r---·-·------MS 87-22 Depth 25-45m ... ·--·-·---·------·------~-.:,,=------·. ·,, ... 10m . ·,·. '. ·:: ·!; ':, 110m ~~ ·; cl. ... "" ·-----·----·------ · ..... -? \jj Figure 16b. Seismic reflection profile of the same area as Figure 14a. The thin nature of the sediment ponds is evident as is the abundance of exposed rock. north of Monhegan be till is around this area. MS 87-8 I Depth 53-65m Side scan image ----·----- l> i I 10m 100m ,_____ ...... _._ .... ------·---·-·--·-·}i--~------' ·--'-----~7•-______J .. ------11------., , .. {M~RAINE. " . ,. MORAIN~): ~- .·· ..l..' . - ~V£22&j_ ·, ~.-••-J~l'i '':..~··•<'•....-. Figure 17b. Seismic reflection profile of gravel immediately adjacent to side-scan sonar image of Figure 17a. Although it is difficult to pinpoint the contact between the acoustic unit identified as till and the underlying bedrock, the morphology of the seabed features resembles washboard moraines on land. MS 87-3 Depth 50-70m ------10mr ~som , HOLOCENE MUD ~ -':.::l-~ ::._:-· " -~ ··c ·' . :.- I•· .. "· ::.. :. ~~~'.~-{. --'---~ ~ <-_~-:--~·-·.: .. _~.. ~;.:~.:;_~·,_:.. - --·-_::.:::::::, ~~;~-=-:~: -'~-· Figure 18b. Seismic reflection profile of same area as Figure 18a. The nearly acoustically opaque unit interpreted as till is shaped into a series of ridges on the topographic high, but disappear beneath the muds of the deeper, nearby Shelf Valley. The small bulge at the base of the ridges of till is covered with mud, and does not appear like till. Its origin is uncertain. SE ,.00 NW 30 u ~ "E 82 w ::;; >= ...J w > ..a: I- 134 >- ~ N' 0 0.5 E=3 E=3 E3 MS-87-7 7/15/87 KILOMETER VE= 7.Gx ORE GEOPULSE 0 M 0.5- 2.0 kHz NAUTICAL MILE 30 END PALEO- ICE- FLOW MORAINE 30 MONHEGAN SHELF u BASIN 50 " ~ ~" " E .s ~ 82 w ;; 70 .:::::"' I- (/) ...J a: 90 w w I- > w ..a: ::;; I- 134 J: >- 110 I- ~ 0..w I 0 N 130 w• 12/2Blf17 4 47.70 N 4~ 88.33' N 69"1!5.98 w 69- 16.72' w Figure 19. Interpreted seismic profile from the border between a Rocky Zone and a Shelf Valley (Figure 4 for location). The labels for seismic units in this and following figures are as follows: br, bedrock; t, till; gm, glaciomarine; gm-d, glaciomarine draped; gm-p, glaciomarine ponded; m, modern mud; sg, sandy gravel; ng, natural gas; ird, ice-rafted debris. SE 1325 1330 '"5 NW ~·'1'! L...50 ...... " ~" E -"' LO " 706 E"' (J) a:: w f w 112 w ::;: 90::; i= I --'w f > a.. <( w a:: D f- 110 >- <( :;: I C\J 130 MS-87-7 .5 VE= 7.8 x KILOMETER 7/15/87 0 05 ORE GEOPULSE 0.5-2.0 kHz NAUTICAL MILE 60 5C MONHEGAN SHELF " BASIN L.OWSTAND '"' " r--sHORELINE 70 E " TERRACE -"' "'E ""! 112 w ::;: 90~ i= w f- --' w w ::;: ><( a:: 110 :I: f- f- a.. >- w <( D :;: 164 I C\J 130 DF8 /23187 43° 46.91'N 69° 14.96'w ~~0-~~~~ Figure 20. Interpreted seismic reflection profile with inferred constructional shoreline terrace. 49 50 '2 -· -~- ·' "/ ~ ----'~" .,<..> -..."' E - 112 .>< <..> ______,____ _ " .5"' w .-.--:-.-~ .. (f) ::2' a:: ~ w 110 1- w -'w ::2' > 150 - -~:, ' . 0 0.5 MS-87-5 7/15/87 KILOMETER \I.E.= 7.7x ORE GEOPULSE OE::::=::=::=::::i::;;:::;::=:::::;:::::::;::::E::=::=::=::=J:=:=::=::=::=:J::=::=::=~O:::'i.5 0.5- 2.0 kHz NAUTICAL MILE 60~-----L------L------~--, 50 MANANA SHELF ...A.A..!'-' SEA-LEVEL LOWSTAND BASIN 70 <..> <..> ~" E " .>< "'E112 CONFORMITY' INDISTINCT LO PLEISTOCENE- HOLOCENE 90 w TRAN ITION ::2' , f- (f) -'w a:: > 12/2'.iJ/81~· 150 4 44.3 N 43° 43.98 N 69°20.74' w 69° 20.26'W Figure 21. Interpreted seismic reflection profile showing indistinct boundary between inferred Holocene and Pleistocene sediments. The depth of the lowstand of sea level is sketched. 50 SE 1810 0 '" ' " 5 NW 0 ------~- ~ .. ·---~· ~- 0 ~. --·-··--~·-,---~----r,,------(J -- -- - 10 0 w Cf) I ------~"' .5 I 20 ~ w 30 ~ ~ f- 40 Cf) _J a: w 65 > 50~ - 70 f- a.. ii1 w I BOO N 90 130 0 0.5 KILOMETER VE =9.7x MS-87-3 7/14/87 0 0.5 I ORE GEOPULSE NAUTICAL MILE Q5- 2.0 kHz 0 0 (J -;:; w Cf) 10 ~ .5 WESTERN EGG SHELF VALLEY E 20 ~ w ::;; "' ;:: 30 Cf) _J a: w 40w > HOLOCENE/PLEISTOCENE f- - 60 :I: ii1 \ f- 70 N' -t11111~~1111ri ~H~~~ 8J 11 0 , I ~ og 1 80 A'\ I ti< 90 130 OCB 12/23/Bl 43•ro.28'N 43•50.92'N 69•24.20'W 69•25.oa'w Figure 22. Interpreted seismic reflection profile across a Shelf Valley near the depth of the sea-level lowstand. 51