ONSHORE-OFFSHORE SURFICIAL GEOLOGIC MAP OF THE WELLFLEET QUADRANGLE AND PORTIONS OF EASTERN CAPE COD BAY, BARNSTABLE COUNTY, MASSACHUSETTS

Abstract

The surficial geologic map of the Wellfleet 7.5-minute Quadrangle incorporates
a previously available onshore surficial geologic map (Stone and
DiGiacomo-Cohen, 2009) and new data collected offshore out to approximately
20 m below mean sea level (MSL). The offshore surficial data
include: sidescan sonar imagery, swath bathymetry and surface grab samples.
In addition, subsurface data were collected offshore using both high
frequency ‘Chirp’ and lower resolution ‘Bubble Gun’ seismic reflection
profilers. In addition, 17 vibracores and peat cores were collected within
the Wellfleet Quadrangle to help map sediment distribution in conjunction
with seismic data, collect representative peat and marsh deposits, and track,
if possible, the rate of Holocene marine transgression. Some minor updates
of the extent of previously mapped terrestrial units were made to better
represent conditions at the time of this publication. For example, many
coastal areas have eroded, or accreted, since the initial field data were
collected in 1968 by R. Oldale and some post-glacial units were updated
and/or mapped using aerial photography from 2005 (Stone and DiGiacomo-
Cohen, 2009). Efforts were made here to capture the largest of these
changes though likely many still remain. The map units are lithologically
based and include characteristics such as grain size, composition and sedimentary
features and structures, including where appropriate, subaqueous
bedforms. These lithological characteristics are produced by ongoing and/or
dominant processes such as winds, waves, tides and currents on multiple
temporal and spatial scales.
The Center for Coastal Studies (CCS) used a suite of instruments in order to
conduct high-resolution, vessel-based acoustic surveys. The Edgetech 6205
is a dual-frequency, phase-measuring sidescan sonar. Its operating frequencies
are 550 and 1600 kHz for backscatter imagery and 550 kHz for
bathymetry. The sidescan sonar resolution is 1 cm, and the horizontal
beamwidth is 0.5 degrees at 550 kHz and 0.6 cm and 0.2 degrees at 1600
kHz. The bathymetric resolution is 1 cm. The respective bandwidths at 550
and 1600 kHz are 67 and 145 kHz (Edgetech, 2014). The effective bathymetric
swath width is 6-8 times the height of the sonar over the bottom. A
Teledyne TSS DMS-05 Motion Reference Unit mounted on the sonar
collects data on heave, pitch, and roll, measuring heave to 5 cm and roll and
pitch to 0.05° (Teledyne TSS, 2006). A HemisphereGPS™ V110 vector
sensor in differential GPS (DGPS) was used to measure heading. Two
differential GPS receivers spaced 2 m apart yield heading accuracies of
<0.10° RMS (HemisphereGPS, 2009). A Trimble® R8 GNSS receiver utilizing
Real-Time-Kinematic GPS (RTK-GPS) is used for positioning and tide
correction. A YSI Castaway™ CTD was used for casts to the seafloor. The
Edgetech 6205 has an internal sound veloci-meter. The data were collected
using Hypack acquisition software, the bathymetry was processed using
CARIS HIPS v7.x and the sidescan sonar imagery was processed using
SonarWiz v5.x. Fledermaus v7.x was used for data visualization and analysis
and ArcGIS v10.x was used for displaying data and map creation. Sidescan
sonar data were collected at greater than 200% coverage and swath
bathymetry at or near 100% coverage throughout the study area. Sediment
grab samples (n = 28) were collected in the study area in the summer of
2015 all samples were analyzed using standard granulometric techniques
(Folk, 1974).
The earth materials that comprise the surficial units in the Wellfleet Quadrangle
are glacial and post-glacial deposits or anthropogenic fill. Glacial and
post-glacial deposits are all unconsolidated deposits consisting of eolian,
marsh and swamp, estuarine, beach, and shoreface materials. There are no
bedrock outcrops in the quadrangle. The high energy open ocean coast, the
west-facing, moderate energy bay coast and the low energy embayment
(Wellfleet Harbor) are three distinct coastal zones within the quadrangle. The
open ocean coast within the quadrangle was not mapped for this effort and
is not discussed here in detail. For information about this area in the adjacent
quadrangle to the north see Borrelli et al. (2014).
The surficial geology offshore of the moderate energy bay coast has similar
lithologic characteristics as the open ocean with regards to shoreface and
inner shelf deposits. The relative water depths at which these deposits occur,
their extent and distance from the shoreline vary due to decrease in overall
wave energy. For example, on the open ocean coast the upper shoreface
deposits extend approximately to the 10 m isobath and are up to 1 km from
the shore and the lower shoreface deposits extend beyond the 20 m isobath
in places and are >2 km away from the shoreline (Borrelli et al., 2014). In
contrast, on the bay side the upper shoreface extends to approximately the
5 m isobath and ranges from 10s of meters offshore to up to 1 km. The lower
shoreface ends at approximately the 10 m isobath in places and ranges from 10s of meters to 1.5 km from the shoreline. The wide ranges within the bay tself are a result of significantly different energy regimes in the area.

Full text

The research conducted for this geologic map and the preparation of this geologic map were funded in part by the U.S. Geological
Survey, National Cooperative Geologic Mapping Program, under STATEMAP Award Number G16AC00125. The views and conclu-
sions contained in this document are those of the authors and should not be interpreted as necessarily representing the official
policies, either expressed or implied, of the U.S. Government, the Commonwealth of Massachusetts, the Massachusetts Geological
Survey or the University of Massachusetts. This map is submitted for publication with the understanding that the United States
Government is authorized to reproduce and distribute reprints for governmental use.
Funding for this project was also provided by the Town of Wellfleet. The authors wish to thank Mark Adams of the Cape Cod Nation-
al Seashore for logistical support, Christina Kennedy at the Center for Coastal Studies for collecting sediment samples, Dr. Jeff
Donnelly at the Woods Hole Oceanographic Institution for use of his lab for grain size analysis, and Captain Ted Lucas. The authors
also wish to thank Brian Yellen, Nick Venti, Doug Beach, and Paul Southard at the Massachusetts Geological Survey for helping
conduct the coring for this project and for many helpful conversations in preparing the geologic map.
Citation: Borrelli, M., Oakley, B.A., Smith, T.L., Mabee, S.B., Legare, B., McFarland, S., Woodruff, J.D., Giese, G.S., 2017.
Onshore-offshore surficial geologic map of the Wellfleet quandrangle and portions of eastern Cape Cod Bay, Barnstable County,
Massachusetts, 1:24,000 scale, 3 sheets. Massachusetts Geological Survey, Open File Report 17-01.
Scale 1:24,000. 3 sheets and digital product: Adobe PDF and ESRI ArcGIS database.
This map was produced on request directly from digital files (PDF format) on an electronic plotter.
A digital copy of this map (PDF format), including GIS datalayers, is available at http://mgs.geo.umass.edu
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Explanation
Roads
Route 6
artificial fill
beach and dune deposits
cranberry bog
swamp and marsh deposits
valley-floor fluvial deposits
Water
glacial stratified deposits, coarse
Vibracore or Peat Core
33
34
31
30
29
38
35
27A
27
37
25
26
36
3A
14 15
24
Figure 1. Location map of vibracores and peat
cores.
0.99 m
(NAVD88)
0.63 m
(NAVD8)
Silt
1.3 m
(NAVD88)
-1.15 m
(NAVD88) 1.47 m
(NAVD88)
1.03 m
(NAVD88)
0.44 m
(NAVD88)
0.7 m
(NAVD88)
-0.22 m
(NAVD88)
0.09 m
(NAVD88)
3320 14C yrs
2750 14C yrs
2440 14C yrs
2080 14C yrs
Industrializtion 1850-1900
137Cs Onset 1950
Carbon-14 Dates
Industrialization
Figure 6. Age model for Core 24. Black line is the median of simulations and is the most
likely age. The dark and light blue lines are the 68% and 95% uncertainty bounds,
respectively.
Figure 5. X-radiographs and relative
elemental abundances for lead, zinc,
potassium, titanium and bromium for
Core 24. Lead and zinc are often
used to record the onset of industrializa-
tion due to the appearance of heavy
metals in sediment and is taken to repre-
sent 1850 to 1900. In Core 24,
zinc provided the best indicator of indus-
trialization. Units are relative and repre-
sent integrated peak areas.
Introduction
During September 2016, a series of vibracores and peat cores
were collected at 17 locations within the Wellfleet Quadrangle.
Cores were collected along the Pamet River (Cores 33 and 34),
the Herring River (Cores 27, 27A, 29, 30, 35, and 38), Duck
Harbor (Core 31), Great Island (Core 36), Blackfish Creek
(Cores 14, 15, and 24) and in Cape Cod Bay and Wellfleet
Harbor (Cores 25, 26, 3A, and 37) (Figure 1 and Sheet 1). The
purpose of the work was to help map sediment distribution in
conjunction with seismic data, collect representative peat and
marsh deposits, and, if possible, track the rate of Holocene
marine transgression.
Field Methods
Three-inch diameter vibracores were collected from a raft with
two canoes serving as pontoons (Figure 2). Cores were vibrat-
ed into place and then removed using a farm jack. Excess pipe
was cut away in the field and the retained core labeled. Floral
foam was placed in the ends and the cores were sealed with
electrician’s tape. Two-inch diameter peat cores were collected
from marshes. The blade of the corer is inserted into the marsh
to the desired depth, twisted to cut a semicircular slice of the
marsh and then retrieved by pulling the corer out of the hole
(Figures 3 and 4). This step is repeated in successive drives
until refusal. Each drive is 50 cm in length. The samples were
preserved by placing them in PVC pipes that were split in half
lengthwise, labeled and sealed with plastic wrap. Peat cores
were stored in a cooler. Upon return to the University of Mas-
sachusetts, vibracores and peat cores were stored in a cold stor-
age room at 35°F until analysis.
Figure 2. Drilling platform used for vibracoring in water with
canoes serving as pontoons.
Figure 4. Sample from peat corer.
Figure 3. Peat corer in marsh before twisting to acquire sample.
Laboratory Methods
Vibracores were split longitudinally with electronic tin snips.
One half was wrapped and archived in the cold storage room
at the University of Massachusetts. The other half was photo-
graphed and described. Peat cores were unwrapped, photo-
graphed, and described. Based on the core descriptions, core
location and length, two cores (Cores 31 and 24) were selected
for further analysis. X-radiograph images were obtained on the
cores by scanning the sections at 500 µm resolution on an Itrax
Core Scanner. Black and white inverted x-radiographs reveal
density variations that may not be evident through visual
examination. Temporal constraints on sediment deposition
were determined by using radiocarbon, cesium-137 (137Cs),
and the onset of industrial heavy metals (as identified in rela-
tive abundance-depth profiles of Pb and Zn). The global onset
of 137Cs in the sediment record corresponds to 1954 CE, and
the peak in 137Cs dates to 1963 CE, or just prior to the signing
of the Nuclear Test Ban Treaty (Pennington et al., 1973). 137Cs
was measured using a Canberra GL2020R Low Energy Ger-
manium Detector. Sediment samples with a dry mass greater
than 2 grams were powdered, put in 6 cm diameter plastic jars,
and counted for 48-96 hours. 137Cs activities were computed
spectroscopically using the 661.7 keV photopeak.
In the Northeastern U.S., concentrations of heavy metals
increase significantly in sediment between 1850 and 1900 CE,
corresponding to the rise of factories during the Industrial Rev-
olution (Woodruff et al., 2013). Depth profiles of Pb and Zn
were employed to identify the depth of this industrial horizon
(Figure 5). Pb and Zn were obtained on the cores with the
ITRAX Core Scanner using a Molybdenum tube and operating
at 30 kV and 55 mA for 10 seconds per measurement. To
extend ages beyond heavy metal and 137Cs derived constraints,
radiocarbon dates were obtained at sediment depths of 575,
430, 375, and 315 cm in Core 24 and 80 cm in Core 31. The
radiocarbon age with 1 sigma uncertainties was converted to
calendar age probabilities using the IntCal13 radiocarbon
calibration curve (Reimer et al., 2013). Radiocarbon dates
were determined at the National Ocean Sciences Accelerator
Mass Spectrometry (NOSAMS) facility at the Woods Hole
Oceanographic Institution.
Monte Carlo simulations (Haslett and Parnell, 2008; Parnell et
al., 2008) were used to derive Bayesian age constraints
between chronological controls in Core 24 (Figure 6). For each
of the large number of simulations a discrete age is drawn
randomly yet weighted using the sample’s obtained probability
radiocarbon-derived distribution. A specific age is defined for
the 1963 CE and 1954 CE 137Cs constraints, and a randomly
drawn age between 1850 and 1900 CE for the heavy metal
onset, with probabilities evenly distributed over this
1850–1900 CE interval. A date of 2016 CE was also defined
at the top of the core. Random ages were generated at
random depths between the radiocarbon, 137Cs, and heavy
metal control points such that ages increase monotonically
with depth (i.e. no age reversals). The median of all simula-
tions for a particular depth is defined as the most likely age,
with bounds presented for 68% and 95% uncertainties (Figure
6).
K was also measured on the ITRAX Core Scanner and is often
used to determine the input of illite clay in sediment derived
from terrestrial sources. Ti is used also as a proxy for clastic
sediments derived from terrestrial sources. Higher relative
values of K and Ti suggest higher input from clastic sources. Br
is often used as an indicator of marine organic matter. Br often
correlates well with percent organic matter determined from
loss on ignition.
Results
Most of the cores provide a good record of the depth to the
sand-peat/estuarine deposit contact. A few cores (Cores 29,
33, and 35) exhibited severe compaction and/or the sand/peat
interface was not encountered so these cores were not ana-
lyzed. Attempts to core in Cape Cod Bay at sites 25 and 26
failed. Site 26 consisted of cobbles and gravel, possibly a till
lag, and was deemed uncoreable. Site 25, west of the Gut,
was cored 101 cm in sandy gravel but all the sample was lost
out the bottom of the core barrel when retrieved. The core
barrel at site 3A had to be vibrated out so the sample was
disturbed and unusable. The core barrel at site 27 was lost and
had to be abandoned in place. Core 27A experienced over a
meter of compaction. The remaining 9 cores are shown here
on Sheet 3.
Swamp and marsh deposits constitute the upper units of the
cores, are generally 1 to 4.5 m thick and consist primarily of
peat and organic muck with sand and silt. In major estuaries,
the swamp and marsh deposits are underlain locally by estua-
rine deposits. The estuarine deposits consist of fine sand, silt
and silty clay with substantial organic matter and are 1 to >3
m thick. The swamp, marsh and estuarine deposits overlie
coarse glacial stratified deposits that consist of fine to coarse
sand, silt and occasional gravel.
A radiocarbon date at the bottom of the peat layer in Core 31
at 80 cm returned a modern 14C date suggesting that it is 65
years old or younger. Core 31 is located in Duck Harbor and
most likely indicates the rapid infilling of the harbor once
Bound Brook Island and Griffin Island were connected by a
tombolo.
Core 24 was retrieved at the head of Blackfish Creek just east
of the Route 6 and former railroad crossings (now the bike
path). A total of 6 m of continuous core was obtained with
ample organic matter for radiocarbon dating. The sand inter-
face was not encountered. This core provided the most com-
plete and continuous record of all 17 cores. Four 14C dates
were obtained ranging from 3600 years to 2100 years ago.
Onset of industrialization was noted in the Zn profile (Figure 5)
at about 140 to 150 cm depth and onset of 137Cs at about 60
cm depth (Figure 6).
The age model captures the late Holocene transgression after
3600 years ago (Figure 6). The estimated linear rate of relative
sea level rise from 3600 to 2100 years ago is about 1.7 mm
per year slowing to 1.1 mm/year between 2100 and 116 years
ago. These values are comparable to rates summarized by
Engelhart and Horton (2012). However, there is a lack of
dating control between 2100 and 1900. The flattening of the
curve could be related to the slowing of relative sea level rise
or could represent an erosional unconformity. From 1900 to
the Present the rate of sedimentation increases considerably
with a rate of sedimentation exceeding 12 mm/year. This
unprecedented sedimentation rate is most likely related to con-
strictions due to the Route 6 and railroad culverts and acceler-
ated development.
The slowing of the rate of relative sea level rise at 2100 years
ago also coincides with a reduction in K and Ti suggesting that
the influx of clastics was reduced at this time (Figure 6). Fur-
thermore, the reduction in K and Ti correlates well with a
steady increase in Br signaling a greater influx of organic
matter. These higher levels of organic matter correlate also
with the rapid infilling that commenced between 1850 CE and
the Present.
References Cited
Engelhart, S. E. and Horton, B. P., 2011, Holocene sea level
database for the Atlantic coast of the United States, Quaterna-
ry Science Reviews, v. 54, pp. 12-25.
Haslett, J. and Parnell, A., 2008, A simple monotone process
with application to radiocarbon-dated depth chronologies,
Journal of the Royal Statistical Society, Series C, v. 57, pp.
399–418.
Parnell, A. C., Haslett, J., Allen, J. R. M., Buck, C. E. and Hunt-
ley, B., 2008, A flexible approach to assessing synchroneity of
past events using Bayesian reconstructions of sedimentation
history. Quaternary Science Reviews, v. 27, pp. 1872–1885.
Pennington, W., Tutin, T., Cambray, R. and Fisher, E., 1973,
Observations on lake sediments using fallout 137Cs as a tracer,
Nature, v. 242, pp. 324-326.
Reimer, P.J. et al., 2013, IntCal13 and Marine 13 radiocarbon
age calibration curves 0-50,000 years cal BP, Radiocarbon, v.
55, pp. 1869-1887.
Woodruff, J. D., Martini, A. M., Naughton, T. N., Elzidani, E. Z.,
Kekacs, D., MacDonald, D., 2013, Off-river waterbodies on
tidal rivers: human impact on rates of infilling and the accumu-
lation of pollutants, Geomorphology, v. 184, pp. 324-326.
University of Massachusetts, Amherst
Massachusetts Geological Survey
Address: 269 Morrill Science Center, 627 North Pleasant Street, Amherst, MA 01003
Phone: 413-545-4814 E-mail: sbmabee@geo.umass.edu
WWW: http://mgs.geo.umass.edu/
MGS Open File Report No. 17-01
Onshore-Offshore Surficial Geologic Map of the Wellfleet
Quadrangle, Barnstable County, Massachusetts, Sheet 3 of 3
2017
Mark Borrelli1, Bryan A. Oakley2, Theresa L. Smith1, Stephen B. Mabee3, Bryan Legare1, Samantha McFarland1, Jonathan D. Woodruff4, Graham S. Giese1
Author Affiliation: 1Center for Coastal Studies, 5 Holway Avenue, Provincetown, MA 02657, 2Eastern Connecticut State University, 83 Windham Street, Willimantic, CT 06226
3Massachusetts Geological Survey, University of Massachusetts, 627 North Pleasant Street, Amherst, MA 01003, 4Department of Geosciences, University of Massachusetts, 627 North Pleasant Street, Amherst, MA 01003
2017
ONSHORE-OFFSHORE SURFICIAL GEOLOGIC MAP OF THE WELLFLEET QUADRANGLE AND PORTIONS OF
EASTERN CAPE COD BAY, BARNSTABLE COUNTY, MASSACHUSETTS
SHEET 3: REPRESENTATIVE CORE DESCRIPTIONS, GEOCHEMISTRY AND AGE MODEL