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COI Funded Project: The nature and extent of the Upper Cape Cod groundwater aquifer beneath Falmouth and Nantucket Sound

Project Funded: 2003
Key Words: Groundwater Discharge, Geophysical Profiling, Cape Cod, Sedimentary Record, Aquifer

Proposed Research

Geologic materials hosting the groundwater system on Cape Cod consist of unconsolidated sediments deposited during the Wisconsinan glaciation. The primary aquifer occurs in sandy glacial outwash deposits, with less important water resources present within finer-grained glaciolacustrine sediments laying below the outwash. Drainage channels carved into the lacustrine deposits were backfilled by the outwash and form preferential flowpaths at depth. Correlating onshore stratigraphy described in borehole logs (Mulligan and Uchupi, submitted) with offshore stratigraphy revealed by seismic reflection (Oã?â°Hara and Oldale, 1987) suggests that these drainage channels may, in fact, extend into Nantucket Sound (Uchupi and Mulligan, in prep). However, the offshore channels have been interpreted to occur in Illinoian deposits. So, while the assumed ages of the sediments onshore and offshore do not correlate, the elevation and locations of the channels clearly do. Both the onshore and offshore data sets are sparse to nonexistant near shore, making correlation difficult at best.

We propose an innovative program of geophysical profiling that effectively crosses Falmouth's southern shoreline in order to resolve many questions associated with the history of Cape Cod and the effect of this history on groundwater flow (including discharge to the coast) today. The geophysical profiles will be tied to well borings and other geophysical data recently collected by collaborators. The results will allow us to place more accurate constraints on the relationships between sedimentary deposits onshore and offshore, which will ultimately improve our understanding of the early sedimentary history of Cape Cod and the nature of the groundwater aquifer in the coastal region. The approach that we initiate in Falmouth can be used to study similar environments with coastal groundwater problems in towns elsewhere on the Cape and along the southern glaciated coast of New England.

Final Report

Abstract           

Understanding the early sedimentary history of Cape Cod and the nature of the groundwater aquifer in the coastal region requires improving our knowledge of the geologic structure both onshore and offshore.   As a first step towards this goal, we collected seismic reflection profiles in Nantucket Sound between Falmouth and Martha's Vineyard.   Despite problems with data quality and depth of penetration, we were able to locate many sediment-filled channels within 3 km of the shoreline of Falmouth.   In water depths of 20-30 m, we obtained images of the shallow portion of two deep channels eroded during the Pleistocene.   We can tentatively trace one channel northward from Vineyard Haven across the Sound to Waquoit Bay.   These channels probably extend beneath Falmouth and Martha's Vineyard.   We have proposed continuation of the survey on land to determine the correlation with channels mapped beneath Falmouth.   Off Waquoit, we discovered two shallow accumulations of stratified sediments in which there are several diapirs, which are tectonic structures formed when deeply buried mud pushes upwards through shallower layers.   One appears to be actively deforming the seafloor.

Introduction

It is generally agreed that the Wisconsin ice sheet advanced southward across southeastern Massachusetts in three lobes.   In the center, the Cape Cod Bay lobe reached Martha's Vineyard and Nantucket at ~24,000 yrs ago while the Buzzards Bay lobe (west) and the Great South Channel lobe (east) reached farther south.   Subsequent northward retreat paused with ice lobes to the west, north and east of what is now Cape Cod.   A readvance of unknown distance at ~19,000 yrs ago formed the Buzzards Bay and Sandwich moraines.   Coastal plain sediments (>2 Ma) are found on the islands and presumably pinch-out on crystalline basement beneath Nantucket Sound because boreholes on Cape Cod indicate that late Pleistocene age sediments lie directly on bedrock.

The origin of the deeper stratigraphic layers beneath Cape Cod and Nantucket Sound is unclear.   Oldale (1992) noted that silts and clay units are more common lower in the section and suggested that these units were deposited in a pro-glacial lake bounded by ice fronts on three sides and by the terminal moraine connecting Martha's Vineyard and Nantucket.   Mulligan and Uchupi (in press) examined a large number of well records and mapped a north-south oriented channel in the fine-grained unit beneath Falmouth.   They considered the possibility that fine-grained sediments were deposited sub-glacially and that the channels were eroded by sub-glacial streams beneath the Cape Cod Bay ice sheet.   They argued, however, that the lake was most likely pro-glacial and dammed by thick ice-contact deposits along the south shore of Cape Cod, most of which have since been removed by coastal processes.   They suggested that the channels were eroded when breaches formed in the southern dam.   Thus, the models for the early sedimentary development on Cape Cod are: (1) a pro-glacial lake extending from an ice front located near the present northern shore of Cape Cod to a terminal moraine along Martha's Vineyard and Nantucket, (2) a pro-glacial lake that extended only to the south shore of Cape Cod and that emptied catastrophically through narrow breaches, and (3) sedimentation in subglacial lakes and erosion by subsequent sub-glacial channels.


Approach

Previous studies have not collected sub-surface data in coastal Falmouth with sufficient density and resolution to distinguish between the proposed models.   Well borings are expensive, so we did a marine seismic reflection survey to determine the location of any subsurface structures in the near-shore region.    The survey was a joint effort between WHOI and the US Geological Survey Woods Hole Field Center.   A proposal to the 2004-2006 WHOI Sea Grant program for funds to carry-out a land-seismic reflection survey in collaboration with John Ebel and students from Boston College was turned down.   Swift has applied to NSF for funds to determine the structure along the coast of Waquoit with on-land seismic and drilling methods.  

Methods

O'Hara and Oldale (1987) acquired the only regional survey of seismic reflection profiles in Nantucket Sound using a boomer sound source towed along lines spaced ~2 km apart that extend to about 1.5 km of the shoreline.   In 2003, we used the R/V Asterias to acquire digital seismic profiles by towing the USGS EdgeTech 512 chirp subbottom profiler at a line spacing of ~220 m (Figure 1).   We collected digital side-scan sonar images on about half of the lines.   Profiles recorded by the EdgeTech chirp will have substantially finer vertical (no ringing, higher frequencies) and horizontal resolution (much higher ping rate).    Penetration depths varied considerably.   Rough seafloor within ~1 km of the Falmouth shoreline west of ~70°35'W scattered   considerable energy yielding no subbottom penetration.   On the other extreme, interbedded sand and silt fill in Pleistocene channels exposed at the seafloor yielded penetration of at least 30 m before interference by the seafloor multiple.  

Results

Numerous small, sediment-filled channels were found within 1-1.5 km of the shoreline along much of the Falmouth coastline.   Many of these small channels appear to be fluvial in origin, so they probably formed prior to 6,000 yrs ago, when local sea level was well below its present position.   Some small channels occur near breaches in barrier beaches, both present and past as revealed by ground-penetrating radar (I. Buynevich, personal communication).   These channels may be part of ebb tide deltas formed in the last 6,000 yrs as a result of strong currents flowing through the tidal inlets.

We imaged the shallow portion of at least two subsurface channels extending to >30 m below seafloor.   The largest has a width of ~2.4 km and the smaller is ~0.6 km across (Figure 2).   The channels are identified by continuous subsurface reflections from horizons in the channel fill that are truncated against the walls of the channels.   Portions of the channel reflections are folded.   Because there is no published evidence that the Falmouth region has undergone significant compressive tectonic stress in the last 100,000 yrs and no salt deposits have been identified in boreholes from the Cape and islands, it is likely that the folds are post-depositional deformation formed by ice-loading or by mud diapirism (see below).   O'Hara and Oldale (1987) identified the larger channel but missed the smaller one.   The channels were likely eroded by high-pressure melt-water streams beneath one of the Pleistocene ice sheets (Uchupi et al., 1996).   The dip and truncations of seismic horizons within the channels suggest that density-driven, sub-aqueous currents deposited the channel fill.   It is likely that the channels were filled just after retreat of an ice sheet.

Using our data and profiles from the 1970s USGS survey (O'Hara and Oldale, 1987),   we traced one channel across the Sound from Vineyard Haven northward to Waquoit Bay.   Within about 0.5-1 km of the shoreline off Waquoit, the channel disappears in the profiles.   This appears to be due to thickening of the overburden rather than northward termination of the structure.   It is unclear whether the overburden is part of the glacial outwash that covered Falmouth when the ice lobes stalled near the present Buzzards Bay and Sandwich moraines or whether along-shore sediment transport covered the features during the late Holocene.

Off Waquoit Bay we found two shallow accumulations of stratified sediments reaching 12-16 m thick, in which several diapirs deform laterally coherent seismic reflectors (Figure 3).   Diapirs are tectonic structures formed when deeply buried sediments push upwards through shallower layers.   Within the cores of the diapirs, there must be soft sediment that can be easily deformed and flow at slow rates.    Diapirs, with cores comprised of mud from glacial lake deposits, are exposed in the sea cliffs of Truro (Oldale et al., 1993).   The Waquoit diapirs are similar in size (50-150 m width with apparent relief of 8-9 m) and are likely to have formed by upwelling mud as well, probably driven by buoyancy or over-pressured water.   One of the Waquoit diapirs appears to be actively deforming the seafloor (Figure 3b).    In our chirp profiles, diapiric deformation also appears in the stratified channel fill sequences (Figure 2b).   The key feature of the diapir profiles in Figure 3 is the lateral continuity of the reflectors whose truncations and deformation we use to image the core of the diapir.  There are no other shallow stratified sequences in the western Nantucket Sound covered by our survey. Most of the region is floored by laterally incoherent seismic structure identical to that on the western edges of the profile segments shown in Figure 2.   As a result, it is impossible to detect where diapiric deformation occurs and whether it is active.    After discovering these features, we went through records from a reconnaissance survey that the U.S. Geological Survey did in the mid-1970s and looked for sediment ponds and diapirs.   They used a lower frequency system and plotted the data with a longer sweep, so we can detect the diapirs in sediment ponds, but we can not resolve the tops of the features to tell if they are moving.   This review shows that diapirs occur in most regions of the Sound.   There is some suggestion in the records that the diapirs are associated with deeply eroded channels similar to those in Figure 2.

Discussion

Our marine survey produced a dense grid of high-resolution seismic profiles that greatly improves our understanding of the sub-seafloor structure between Falmouth and Vineyard Haven.   Despite the new structural control, the southward extent of channels mapped in Falmouth by Mulligan and Uchupi (2003) is unclear.   The flanks of the onshore channels appear to extend from <20 m below sea level to >40 m below sea level (Mulligan and Uchupi, 2003).   These channels are too large and deep to be directly related to the smaller-scale channels that we found within 1-1.5 km of the shoreline of Falmouth.   The deeper channels that we found farther offshore (Figures 1 and 2) have similar horizontal and vertical scales and are positioned at similar stratigraphic levels.   Based on this similarity, it is tempting to correlate the two sets of structures.   In particular, the larger offshore channel (Figure 2a) projects northward to the sand spit west of the Waquoit Bay inlet, and Mulligan and Uchupi (2003) map a channel that projects south to the east side of the inlet.   Control on both sides of the shoreline is uncertain enough that these structures could be connected.   However, the lithology of the fill in channels on either side of the shoreline is dramatically different.   The onshore channels are filled with coarse-grained, fluvial outwash, whereas a vibracore along the profile in Figure 2a indicates the fill is silty sand with thin beds of clay.   The difference may indicate a major difference in the nature of the onshore and offshore sequences as proposed by O'Hara and Oldale (1987) and by Mulligan and Uchupi (2003).   If the Falmouth channels end at the shoreline, discharge of groundwater is likely to be controlled by the north-south transition in structure.   Alternatively, channel structures may be continuous across the shoreline, and the lithology differences may just indicate a facies change from fluvial deposition to finer-grained deltaic deposition in a lake constrained by the walls of the channel to the south.   If the latter alternative is correct, then it is possible that groundwater flowing south beneath Falmouth may become trapped by low-permeable, fine-grained layers in the offshore fill allowing a portion of the groundwater - and any contaminants - to continue flowing south beneath Nantucket Sound.   It is clear that more field work must be done near the shoreline - particularly on land where well log control is sparse- to define the structural continuity.   Towards this end, Swift submitted a proposal to NSF/Earth Sciences to collect a seismic profile and well logs along the Waquoit Bay barrier beach.

The chirp profiles across the sediment pond southeast of Waquoit Bay indicate that mud diapirism is actively deforming the seafloor in Nantucket Sound.   Uplift of the seafloor such as that pictured in Figure 3b is an obvious hazard to construction of permanent seafloor structures, such as the windmills proposed for Horseshoe Shoals.   Unfortunately, there is too little information to understand what controls the occurrence of diapirs and their activity.   Diapiric structures are apparent in the 1970s USGS boomer profiles, but these data have insufficient resolution to image reflectors above the diapirs in enough detail to reveal whether or not movement is occurring at the present time.   With the available profiles, we are constrained to imaging diapirs only where they deform well-stratified sediment sequences.   Because the chirp profiles southeast of Waquoit Bay (Figure 3) indicate that the source material for the diapirs is coming from below the floor of the sediment pond, sediment ponds are probably not necessary for diapirs to occur.   Thus, it is likely that diapirs occur in many places in Nantucket Sound where seafloor sediment is not well-stratified and available profiles can not resolve their presence.   The data are equivocal, but diapirs appear to be more common along the edges of the deep, sub-glacial channels mapped by O'Hara and Oldale (1987).   We do not know enough about the deep structure of the diapirs to understand why channels walls correlate with uplift.   If the correlation proves tenable and the channels do, indeed, continue beneath Cape Cod and the islands as Uchupi et al (1996) suggest, then diapirism may be occurring beneath land as well as beneath the seafloor of Nantucket Sound.

Conclusions

1.   Offshore, sub-seafloor structures appear to fall into two categories: (a) small shallow features within 1.5 km of the shoreline that are related to Holocene fluvial activity or to late Holocene tidal currents, and (b) deeply eroded, relatively steep-sided channels.   The latter are similar to channels mapped by O'Hara and Oldale (1987) and interpreted by Uchupi et al (1996) as features eroded by sub-glacial streams.   The axis of one offshore channel can be tentatively extrapolated north across the shoreline to a channel mapped by Mulligan and Uchupi (2003) using well logs.

2.   Mud diapirs with widths of 50-150 m occur beneath Nantucket Sound.   At least one diapir appears to be active and deforms the seafloor upward in a region of active tidal sediment transport.   Diapirism appears to be wide-spread, but the nature of uplift is poorly constrained.

References

Mulligan, A.E., and E. Uchupi, in press, Wisconsin glacial lake sediments in the subsurface of Cape Cod, Massachusetts, Northeastern Geology and Environmental Science.

Mulligan, A.E., and E. Uchupi, 2003,   New interpretation of the glacial history of Cape Cod, Massachusetts, may have important implications for groundwater contaminant transport, EOS, 84, 177, 182-183.

Oldale, R.N., 1982, Cape Cod and the Islands: The Geologic History, East Orleans, MA, Parnassus Imprints, 208 pp.

Oldale, R.N., J.V. O'Connor, B.B. Tormey, and R.S. Williams, 1993, A geologic overview of Cape Cod and a geologic transect of Wellfleet from the Atlantic Ocean to Cape Cod Bay, U.S. Geological Survey Open-File Report 93-618, 26 pp.

O'Hara, C.J., and R.N. Oldale, 1987, Maps showing geology, shallow structure, and bedform morphology of Nantucket Sound, Massachusetts, U.S. Geological Survey Miscellaneous Field Studies Map MF-1911, 4 sheets.

Uchupi, E., G.S. Giese, D.G. Aubrey, and D.-J. Kim, 1996, The late Quaternary construction of Cape Cod, Massachusetts: a reconsideration of the W.M. Davis model, Geol. Soc. Amer. Spec. Paper 309, 69 pp.

Originally published: January 25, 2003