BUXTON.PDF

SAND RESOURCES OF THE NORTH CAROLINA OUTER BANKS
2nd INTERIM REPORT: ASSESSMENT OF BUXTON STUDY AREA
Prepared for the
Outer Banks Task Force and the
North Carolina Department of Transportation
by
Dr. Stephen K. Boss
Department of Geosciences
113 Ozark Hall
University of Arkansas
Fayetteville, AR 72701
and
Charles W. Hoffman
North Carolina Geological Survey
Coastal Plain Office
Mail Service Center 1620
Raleigh, NC 27699-1620
Submitted: June 1999
Revised: August 1999

EXECUTIVE SUMMARY
A review of available geophysical (single -channel seismic reflection and side -scan sonar
records) and sedimentological (core descriptions, images, and textural analyses) data from an
area offshore Buxton, North Carolina was undertaken on behalf of the Outer Banks Task Force
and the North Carolina Department of Transportation to determine the potential of this area as a
source of sand for possible beach nourishment programs on the North Carolina Outer Banks.
Results of this review are presented as an outline of the stratigraphic architecture of the Buxton
Study Area (BSA) derived from interpretation of seismic reflection and side -scan sonar data,
description of the gross textural attributes of sediment in cores collected within the BSA, and
assessment of potential sand reserves within the BSA that might be utilized for future beach
nourishment programs.
Nine principal seismic reflectors (designated Ro through R8) were correlated throughout
the BSA and form the upper and lower boundaries of eight principal stratigraphic units
(designated S1 through S8) extending from the seafloor to approximately 60 in sub -sea.
Individual seismic units are relatively thin, averaging 5.4 in throughout the BSA. The seismic
signatures of units within the BSA are quite variable, ranging from acoustically "transparent"
units (i.e. lacking internal reflecting horizons) to units with multiple, closely spaced parallel
reflectors. These variations in seismic character are indicative of rather variable geologic or
sedimentologic units.
Side -scan sonar records throughout the BSA indicate that seafloor over three -fourths of
the study area is characterized by weak acoustic reflectivity. This phenomenon most commonly
indicates very fine sand or finer sediment at the seafloor. In addition to imaging of fine-grained
sediments, side -scan sonar data from the BSA scanned significant occurrences of low- to high -
relief hardbottoms exposed on the seafloor over a good portion of the northernmost quarter of the
study area (north of line 022).
Twenty- seven vibracores were collected within the BSA. These cores contain variable
sediment types ranging from very fine-grained sand and mud to shell gravel. The variable nature
of sediment contained within cores demonstrates the highly variable nature of stratigraphic units
throughout the BSA. A cluster of five cores (189, 190, 192, 193, 195) are composed of greater
than 97% sand and shelly gravel and appear to be associated with seismic unit S7 in the southern
portion of the study area.
Among the eight seismic units exposed within the BSA, only one, S7, seems to satisfy the
necessary conditions to be considered a candidate sand resource: 1) it crops out relatively close to
shore within the southern portion of the BSA near a site of critical shore erosion, 2) it crops out
in relatively shallow water and thus is accessible to presently available dredging technology, 3)
its seismic signature and reflector geometry are indicative of a sand -rich depositional
environment (fluvial channel or inlet fill), and 4) cores within this unit confirm the presence of
appreciable quantities of sand. Unit S7 is estimated to contain in excess of 375 million cubic
yards of sand over the surveyed area. A smaller sand resource target within S7 is identified with
an area of 11 million square yards and an estimated sand volume of 60 million to 180 million
cubic yards, depending on assumptions regarding thickness of unit ST
1

INTRODUCTION
Project Background
Following preliminary meetings and discussion of problems related to maintenance of
North Carolina Highway 12 in 1993 and 1994, the Outer Banks Task Force agreed to conduct a
large-scale geophysical survey of the northern Outer Banks from Oregon Inlet to Ocracoke Inlet.
The primary intent of this survey was to collect reconnaissance data (single -channel, high -
resolution seismic reflection and side -scan sonar profiles) over a broad area of the northern Outer
Banks (Oregon Inlet southward to Cape Hatteras, then westward to Ocracoke Inlet; Fig. 1).
These data were to be used to acquire baseline knowledge regarding the shallow (<100 in depth)
stratigraphy, sea -floor characteristics, and sand resource potential of the continental shelf within
waters under state jurisdiction (to 3 nautical miles offshore). The geophysical survey was
conducted during July and August 1994 by Dr. Stephen W. Snyder (North Carolina State
University) under contract to the North Carolina Department of Environment and Natural
Resources with the North Carolina Geological Survey (NCGS) acting as contracting agency.
The following summer, a sampling survey was authorized to provide "ground truth" for
geophysical data. Vibracores were collected during 8 weeks (July — August 1995) aboard the
United States Army Vessel Snell from Oregon Inlet southward to Cape Hatteras, across Diamond
Shoals, then westward to Ocracoke Inlet. Upon completion of the field -sampling program, all
cores were transferred to the Coastal Plain Office of the North Carolina Geological Survey for
processing. All cores were halved lengthwise, described, digitally imaged, and sampled to
determine textural attributes. The digital images of each core were archived on CD-ROM and
placed into the public domain at the Coastal Plain Office of the North Carolina Geological
Survey. Core sediment samples were processed using standard methods by the Soils Testing
Laboratory of the North Carolina Department of Transportation and textural attributes were
compiled and archived on CD-ROM at the Coastal Plain Office of the North Carolina Geological
Survey.
In December 1998, a contract was executed between the North Carolina Department of
Environment and Natural Resources and the University of Arkansas (Dr. Stephen K. Boss,
Principle Investigator). The purpose of this agreement was to complete analyses of existing
geophysical data (single -channel seismic reflection and side -scan sonar profiles) and assess the
sand resource potential of four study areas offshore of the northern Outer Banks (Fig. 1).
The following report is the second project deliverable, and is organized into several
sections to facilitate understanding of the rather complex data. Section I describes the available
geophysical data and presents results of interpretations of BSA stratigraphy. Section II
documents textural attributes of sediment in vibracores collected within the Buxton Study Area
(BSA) during 1995. Finally, Section III provides information pertinent to assessing the BSA as a
potential resource of sand for beach nourishment along the critically eroding beach north of
Buxton, NC.

IN Oregon Inlet
0 5 10
Pea
nautical miles
Island
erosional "hot spot"
(Stone et al., 1991)
individual study area
33-5°N
• vibracore location
trackline for seismic
and side -scan data
Buxton
Buxton
"hot spot" —
t
Diamond
flattelas. Inlet
Shoals
O
c:
owe
O
o
o
0
35ON
0
Fig. 1. Location map showing the Outer Banks Task Force sand resource project area. The four principal project
areas are labeled along with locations of erosional "hot spots" (red line segments) with potential to impact
NC Highway 12. Geophysical tracklines (seismic reflection and side -scan sonar profiles) are indicated by
thin solid lines. Vibracore locations are shown as solid circles. Green study area is the subject of this
report.
3

SECTION I: GEOPHYSICAL DATA
The BSA is approximately rectangular, measuring about 18.0 km x 7.3 km (9.9 nautical
miles x 3.9 nautical miles) and occupying an area of 132 km2 (39 nm2). Geophysical data consist
of single -channel, high -resolution seismic reflection profiles and side -scan sonar records from
the BSA (Fig. 2). These data were collected simultaneously during the 1994 research cruise and
are subdivided into 28 trackline segments constituting 204 km (110 nautical miles). Tracklines
are oriented with 5 lines spaced at approximately 1 km (0.5 nautical mile) intervals parallel to the
coast from 0.5 to 3.0 nautical miles (limit of state jurisdiction offshore). These shore -parallel
lines are crossed by a series of zig-zag tracklines oriented approximately perpendicular to the
coast and extending from near shore to approximately the 3-mile limit.
Seismic Reflection Profile Interpretation and Analysis
Seismic reflection data were archived as paper scrolls printed at the time of acquisition
and in digital format on CD-ROM. Paper copies of these data printed at the time of acquisition
were of limited utility because their quality is greatly influenced by physical sea -state at the time
of the research cruise and by the acquisition software processing parameters. However, digital
records of these data (archived on CD-ROM) were reprocessed using specialized software to
enhance signal-to-noise relations and thus provide more interpretable versions.
Seismic reflection data were collected to a maximum "depth" of either 100 or 120
milliseconds two-way travel time (the standard vertical axis on seismic reflection profiles) during
the initial survey. Seismic reflection profiles from the BSA were reprocessed and interpreted to a
maximum "depth" of 60 milliseconds two-way travel time. This depth was chosen as a
compromise providing sufficient depth to assess the geological architecture of the BSA while
also enabling relatively fine -scale resolution of individual sedimentary units. In addition, data
below 60 ins are of little value to the goal of assessing sand resources since sediments beneath
this level are too deep beneath the seafloor to be considered for conventional dredging.
Precise conversion of two-way travel time to true depth requires knowledge of the
velocity ofp-waves through both seawater and sedimentary deposits, parameters that typically
are not available during a survey. Thus, figures showing "depth" to a particular reflecting
horizon (e.g. Figs. 3, 4, 6) are presented in milliseconds two-way travel time, the parameter
recorded during data acquisition.
For this study, estimates of the thickness of stratigraphic units were obtained by assuming
uniform p-wave velocity through the sediment column. A reasonable estimate ofp-wave
velocity of 1800 m/sec was obtained from published values of typical unconsolidated, surficial
marine sand (Dresser Atlas, 1982), and this value was adopted for this study. This value was
chosen as a conservative estimate, since it is likely that p-wave velocities in the subsurface are
greater than 1800 m/sec. Thus, estimates of sediment thickness reported herein are considered to
be minimum estimates since velocities of seismic transmission greater than 1800 m/sec will
result in thicker deposits (Table 1).
4

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I�
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026
a '
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024 <175
174 176
°23 i
1,78
177 i
a 179 - 022 182 001
Vibracore
020 181 ; seismic and side scan
184 / sonar trackline (some
183 lines patterned for
oJ9 clarity)
185 � 048
188 1866 Contour Interval = 3m
Opp
i
016 i
187
189 i
erosional 191 r' 190
014 '
hot spot 1,920
°I3
.� 012
1-93 194
01"
197 19�6 � �
195 i
198 , 0 2.5 5
°09 ;
Cape 008 Nautical Miles
�J 199 200
Hatteras M
O �
i
i
1�5 004
001 f
Fig. 2. Detailed location map of Buxton Study Area (BSA) showing tracklines of seismic reflection and side -scan
sonar profiles (thin lines) as well as locations of vibracores (solid cirlces) used in this sand resource
assessment. Bathymetry from NOAA hydrographic data (NGDC, 1999).

TWO-WAY TRAVEL THICKNESS
(seconds)
p-WAVE VELOCITY (m/sec)
THICKNESS (m)
0.020
1500
15
0.020
1800
18
0.020
2100
21
Table 1. Example calculations showing the dependence of estimated deposit thickness onp-wave velocity.
Example assumes a stratigraphic unit with measured "thickness" 0.020 seconds two-way travel time on a
seismic reflection profile. The change in true thickness of the unit with increasing p-wave velocity is
evident. For this study, a conservative p-wave velocity of 1800 m/sec was assumed to arrive at estimates
of sediment thickness. The equation relating p-wave velocity, two-way travel time, and thickness is: (t212)
x vp = z where t2 = two-way travel time, vp =p-wave velocity, z = thickness.
Seismic reflection profiles were interpreted using an iterative correlation method whereby
prominent seismic reflectors are identified and correlated among closely spaced seismic profiles.
An attempt is then made to extend these initial correlations throughout the entire surveyed area,
cross-referencing and checking for appropriate "ties" frequently until the entire data set is
interpreted. This process constitutes the first iteration through the data.
Following completion of the initial interpretation cycle, all profiles were reviewed, and
refinements to the initial interpretations made. This process constitutes the second iteration
through the data.
Finally, the geographic locations and depths of principal seismic reflectors are tabulated
for each time -event mark (approximately every 500 seismic shot points) and line crossing. These
data are compiled in a spreadsheet and checked for consistency; the position and depth of a
reflector should be the same on crossing seismic profiles. Anomalous reflector depth pairs are
noted, and the associated interpreted seismic profiles checked again for accuracy. This process
constitutes the third iteration through the seismic data.
Once satisfied that correlations among major reflectors were reasonable, the digitized
locations of seismic reflectors were updated using spreadsheet software and the results exported
to Geographic Information System (GIS) software to generate maps of reflector surfaces and
seismic stratigraphic unit thickness throughout the BSA. Mapping of reflector surfaces in three
dimensions made it possible to estimate the volume of material contained within the major
depositional sequences throughout the BSA.
Geologic Framework of the Buxton Study Area
Interpretations of seismic profile data indicate that the detailed geologic history of the
BSA is more complex than that of Diamond Shoals. Seismic units can be grouped into eight
principal depositional units. The major reflectors separating these principal units are labeled
beginning with the seafloor reflector as Ro and others designated Rl, R2, R3, R4, R5, R6, R7, and
R8 (Fig. 3).
6

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10
20
E
LU
30
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J
w
40
Q
O
3:
50
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LINE 107 — BUXTON STUDY AREA
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10
20
30
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B
Fig. 3. Four kilometer segment (2.1 nautical miles) of seismic reflection profile 107 between shot points 52000 (left) and 56000(right). Segment trends parallel
to shore of BSA from north (left) to south (right). The principal reflectors (Ro through Rg) and stratigraphic units (Sl through Sg) correlated throughout
the BSA are indicated. Vertical scale in meters is approximate based on a sub-bottomp-wave velocity of 1,800 meters/second.

Within the seismic units defined by the nine major reflectors, the acoustic character of
contained stratigraphic units is somewhat distinctive, aiding in the correlation of these units
around the Buxton area. By convention, each unit (or sequence) is named according to the label
of its basal reflector. Therefore, the ocean water column above Ro could be labeled So, the
sedimentary package between Ro and Rl is termed S1, that between Rl and R2 is called S2, etc.
Brief descriptions of some of these seismic units are provided below.
Seismic Unit SI
Unit S1 is recognizable throughout the BSA. However, it is only continuous east
(seaward) of line 096 (Fig. 2). West (or shoreward) of line 096, unit S1 and its lower bounding
reflector, RI, are discontinuous from line 018 northward. Unit S1 is missing altogether south of
line 018, having been truncated by later erosion related to development of R7. Unit S1 has a
tabular geometry and averages 4.1 in thick (range 0 in to 10 m) throughout the BSA. This
reflector dips very gently seaward from the nearshore area where it occurs between 5 and 10
milliseconds two-way travel time and reaches maximum depth of about 25 milliseconds near the
eastern (seaward) boundary of the survey area.
The basal reflector of Unit S1 appears to crop out in the northernmost portion of the study
area (north of line 022), and may be the horizon occurring as hardbottom throughout the northern
portion of the BSA.
Unit S1 is the uppermost stratigraphic unit within the northern and eastern portions of the
study area, and it is a unit for which direct sedimentological data are available from cores. In
addition to core sediment data, the surface expression of S1 is represented on the side -scan sonar
records north of line 018 and seaward of line 096. These data indicate that S1 is of somewhat
variable composition throughout its area of occurrence. Cores 181, 183, 184 show that the upper
portion of unit S1 is dominantly silty fine sand to silty very fine sand. However, down core, as
one approaches the basal reflector of unit S1, sediments become coarser and lithified.
Occurrences of hardbottom composed of lithified coarse to medium sand (cores 174 to 177)
correspond to areas where reflector Rl crops out on the seafloor.
Seismic Unit S2
The next seismic unit identifiable among the BSA seismic reflection data is also
recognizable throughout the study area. Reflector R2 is also somewhat discontinuous. North of
line 017 and east of line 096, R2 can be identified in all seismic data. However, south of line 017
and west of line 096 (Fig. 2), R2 has been truncated by erosion during development of R7. The
R2 reflector also dips gently seaward from 15 — 20 milliseconds two-way travel time in the
nearshore area to about 30 ms near the eastern boundary of the BSA. The stratigraphic unit
bound by Rl and R2 is termed S2 in this report.
Unit S2 is recognized and correlated throughout the BSA. S2 also displays a tabular
geometry, averaging 3.4 in thick (range 0 to 18 m) throughout the study area. The unit is missing
in an area bound by lines 096, 107, 017, and 007. It is not clear whether any cores have
penetrated this unit.
8

Seismic Unit S3
Like reflectors Rl and R2, reflector R3 has been truncated by erosion and development of
R7 in the southern, nearshore portion of the BSA. Unit S3, therefore, is missing south of line 016
and west of line 096 (Fig. 2). Unit S3 averages 6.5 m thick (range 0 to 30 m) where it can be
found within the BSA. The unit is somewhat tabular, but shows evidence of some channel
development, particularly in the northernmost part of the study area where it also displays the
greatest thickness. In general, the unit dips seaward (east), with the basal reflector, R3, occurring
between 20 — 25 milliseconds two-way travel time nearshore and deepening to 35 milliseconds at
the seaward (eastern) limit of the BSA. No cores penetrate unit S3, so its sedimentary
constitution is not presently known. However, its depth beneath the surface (20 — 35 m) is
sufficiently great to preclude its consideration as a potential sand resource.
Seismic Units S4, S5i S6
Reflectors R4, R5, and R6 form the basal reflectors of units S4, S5, and S6. Each of these
units is too deep beneath the surface to be considered as potential sand resources, and are
mentioned here only for completeness. Both R4 and R5 dip gently seaward. R4 is recognized
throughout the entire study area, occurring at about 35 milliseconds two-way travel time in the
western part of the BSA and descending to 40 — 43 milliseconds along the eastern boundary of
BSA. Unit S4 has a uniform thickness averaging 8.8 m throughout the BSA. R5 is also
recognizable throughout the BSA, and dips gently eastward from about 40 milliseconds two-way
travel time in the western (nearshore) portion to about 53 milliseconds along the seaward
boundary of the survey area. S5 also has a relatively uniform thickness averaging about 8.6 m.
Reflector R6 is a very strongly reflecting horizon and easily correlated throughout the
entire BSA. It is the deepest reflector observed within this study area and may correlate with R5
observed within the Diamond Shoals Study Area. R6 is a very stable reflecting horizon, and
varies in depth between 50 and 60 milliseconds two-way travel time everywhere in the BSA.
Once again, this unit is too deep beneath the surface to be exploited as a sand resource, but it is a
good seismic/stratigraphic marker unit.
Seismic Units S7 and S8
Reflector R7 is the most significant feature within the BSA. R7 occurs only in the
southern portion of the BSA, in an area bound by lines 007, 018, 096, and the shoreface. R7
intercepts the seafloor in the vicinity of line 018 and rapidly descends to 24 - 27 milliseconds
two-way travel time such that it truncates reflectors Rl, R2, and R3 throughout the entire southern
area of the BSA shoreward of line 096 (Fig. 4).The average thickness of unit S7 overlying R7 is
12.4 m (Fig. 5).
Note that the northern edge of occurrence of R7 is approximately coincident with a
dramatic change in the width of Hatteras Island (Fig. 4). North of the occurrence of R7, the
island is sufficiently wide to accommodate the community of Avon. South of the occurrence of
R7, however, the island is much narrower and lower in relief. This area of the island is presently
the focus of dune restoration efforts.

i
020
_ °18—j—
�� 0
erosional
hot spot I �`•
1-2
I '
Cape
Hatteras
9 Surface of
i
Reflector R7
CI = 5ms
/Seismic and side scan
sonar trackline
Vibracore
8 ms
10 ms
-------------------15 ms
------20 ms
- - - 25 ms
-----------30 ms
0 2.5
Nautical Miles
Fig. 4. Structure contour map derived from seismic reflection data showing depth (in milliseconds two-way travel
time) from sea level to top of reflector R7 within the BSA. Contours on surface of R7 labeled with 5 ms
contour interval (approximately 4 meters). Note that R7 occurs only in the southern portion of the study
area.
10

,
024
2�
022
,
020
ov
- Olg
Thickness of
Unit S7
CI = 5m
erosional :� ;:: �
,•�_.; ., W4 Seismic and side scan
hot spot j sonar trackline
Of3
_012 Vibracore
1 410� S m
,. �.
'-------------------- 10 m
---------- —15m
—20 m
��`� 0 2.5
Cape
Hatteras Nautical Miles
Fig. 5. Isopach map of unit S7 in the southern portion of the BSA. The average thickness of S7 is 12.4 m. Contour
interval is 5 m.
11

On seismic profiles (e.g. Fig. 3), the northern boundary of R7 appears as a concave
reflector reminiscent of one sidewall of a channel. No similar sidewall has yet been located to
the south. When viewed on shore -parallel seismic profiles (i.e. lines 107 and 138), the seismic
appearance of sediments overlying R7 (that is, sediments of unit SO is that of a continuous series
of thin, tabular cross -strata, dipping to the south. When viewed on shore -perpendicular seismic
profiles (i.e. Lines 007 to 018), the seismic character of S7 is that of numerous small,
superimposed channels. This seismic signature is interpreted to represent a migrating channel
complex which initiated at the northern edge of R7 occurrence and progressively migrated toward
the south, perhaps to Cape Hatteras, before closing. As this channel system migrated, it eroded
all pre-existing stratigraphy and redeposited channel -fill sediments (mostly sand and gravel).
Riggs (personal communication, 1999) suggests that these channels evolve from fluvial systems
that incise into the continental shelf during lowstands of sea -level. When sea -level rises again,
the channels are inundated and back fill with estuarine sediments. During the next sea -level
lowstand, channels may incise into estuarine sediments once more and rework or redeposit
coarser channel -fill sediment.
An alternative hypothesis is that the migrating channel complex represents southward
advance of a tidal inlet system. Under this scenario, an inlet opened at the northern limit of R7
(indeed, R7 represents the northern "wall" of this inlet). Over time, southerly longshore drift
along the Outer Banks forced migration of the inlet toward Cape Hatteras and as it migrated, it
eroded through all pre-existing stratigraphy (truncating reflectors Rl, R2, and R3) and redeposited
inlet fill (mostly sand and shell gravel). A limitation of this model is that the breadth of R7
offshore may be too great to be explained as migration of an inlet.
Like reflector R7, R8 also has a restricted areal distribution within the BSA (Fig 6). R8
occurs as a horizontal reflector throughout most of the area of R7, and S8 occurs as a thin capping
stratum over S7. S8 averages 2.9 in thick (range 0 to 10 in; Fig. 7) and appears to be composed of
interbedded silty very fine sand, mud, and gravel (cores 186, 191, 196, 198). In areas where this
unit is thick, cores are of highly variable quality, though relatively thick mud layers impart
overall mud contents in excess of 10%. In areas where S8 is thin (e.g. seaward of line 138), cores
appear to penetrate through approximately 0.8 — 0.9 in of unit S8 into unit S7 and their overall
textural attributes are very good. If one overlays a map showing the seaward limit of R8 onto a
map showing the seaward limit of R7, and compares this map to the location of favorable cores, it
is evident that the best cores sampled unit S7 beyond the seaward limit of S8 (Fig. 8). Thus, it
appears that unit S8 is the likely source of poor sediment quality observed shoreward of line 107
in the southern part of the BSA.
Side -Scan Sonar
Side -scan sonar data were collected concurrently with the seismic data using an EG&G
(now Edgetech) DF-1000 system. The digital signal was processed through a deck control unit
and then written to a thermal plotter as well as digital tape. For this study, the hardcopy records
from the thermal plotter were reviewed. The thermal plotter records a gray -scale image of the
seafloor, known as a sonogram, which is sensitive to the textural characteristics of the surface
sediments. Higher reflectivity (darker record) is typically associated with coarser -grained
sediments; lower reflectivity (lighter record) with finer grained sediments. Topographic
12

erosional
J 1
hot spot
-- =
01-2
�1
1
—4-
010�,
Cape
Hatteras
Surface of
Reflector R8
CI = 5ms
/ Seismic and side scan
sonar trackline
Vibracore
---- 8ms
10 ms
-----15 ms
-------------- -20 ms
0 2.5
Nautical Miles
Fig. 6. Structure contour map derived from seismic reflection data showing depth (in milliseconds two-way travel
time) from sea level to top of reflector Rg within the BSA. Contours on surface of Rg labeled with 5 ms
contour interval (approximately 4 meters). Note the limited areal extent of Rg, occupying only the southern
portion of the study area. Also note that R8 occupies a more restricted area than R7..
13

,
,
�024
I � ,
a '
i�
j 022
020
,
01-8 019
,
Thickness of
i Unit S 8
CI = 2m
� r�
/
/ Seismic and side scan
erosional , ,
sonar trackline
hot spot Vibracore
,
012 if --- 0m
--4 m
•�---------
-6 m
--------
' —8 m
008
0 '
Cape ��`' 0 2.5
Hatteras Nautical Miles
Fig. 7. Isopach map of unit S8 in the southern portion of the BSA. The average thickness of Sg is 2.9 m. Contour
interval is 2 m.
14

024
j 022
� oil
020
0T8 9 i Areal Extent
I of
�I7 '
01-6 Unit S7 vs. S8
I '
I
erosional
014 '
hot spot S 7
11 of3 ;
� 0 to S8
1 11
I ;
01-0
1 � ;
008 09
' Seismic and side scan
00sonar trackline
) ,
Vibracore
Cape ��`' 0 2.5
Hatteras Nautical Miles
Fig. 8. Comparison of areal extent of units S7 and S8, illustrating that unit S7 extends beyond the limits of S8. Thus,
cores in this area will sample S7 with without penetrating poorer quality sediment of S8.
15

irregularities in the seafloor such as escarpments, bedforms, or even man-made debris can impart
character to the record as well. Time -event marks on the sonograms were cross-referenced to
known navigation fixes taken during the data collection and could be referenced to the seismic
data and GIS basemap.
All sonograms were recorded with a 400-meter swath width (200 in to each side of the
towfish). Ideally, the towflsh should "fly" at a relatively constant and recorded height above the
seafloor. In the BSA, especially the lines running perpendicular to the shoreline, the fish would
need fairly continuous monitoring and adjustment for optimal data capture. No such activity is
noted on the data records or logs. The consequences of operating at an improper height typically
include 1) the imaged seafloor swath typically is less than 400 in, 2) there are significant portions
of sonograms where acoustic returns from the sea surface obscured seafloor data (especially in
rough weather), and 3) it is difficult to maintain the bottom -track of the sonar fish in the
shallowest portions of the survey area -- resulting in poor sonogram quality across these areas.
Furthermore, specific operating parameters of the side -scan sonar instrument during
acquisition were not available. Thus, it was not possible to determine whether slight changes in
acoustic character were related to actual variability of seafloor physical properties or to
adjustment of operating parameters (such as gain) at the time of acquisition. Thus, careful
evaluation of the record is necessary to differentiate "real" featureless data from poor data.
Indeed, much of the BSA side scan record was fairly featureless and indistinct. On the other
hand, several areas of hardbottom were imaged quite well, so it is apparent that the equipment
was functional.
The southern three -fourths of the Buxton area side -scan record is dominated by a weak to
moderate acoustic return (Fig. 9). This acoustic character of seafloor sediment suggests a
predominance of very fine- or fine- to medium -grained sand as the surface sediment type. Along
the shore -parallel lines, much of this bottom type is rippled to suggest further that sandy surficial
material is mobile. Ripples are not imaged very well on the shore -perpendicular lines, even
where these lines cross distinctly rippled sediments along the shore -parallel tracklines. This may
be due to an unfavorable orientation in terms of imaging, but also may be due to the fact that the
lines were collected several days to over a week apart from each other. The shore -perpendicular
lines were collected under somewhat rough sea conditions and both the side scan and seismic
data quality suffered as a result.
Figure 9 illustrates the extent of different generalized seafloor types throughout the BSA.
Hardbottom areas are relatively common in the northern quarter of the surveyed area. The
hardbottoms are very distinctive on the side scan sonograms (Fig. 10) and show relief along
generally east -facing scarps. Some areas appear to be rock rubble. Whereas the presence of
hardbottom (and more specifically the flora and fauna that develop and flourish on this bottom
type) is a strongly negative factor in terms of offshore mining, the area north of line 022 should
not be considered for mining even if it does prove to contain otherwise viable sand resources.
Similarly, the inner mile extending south from line 022 to about line 017 should be disregarded.
Surprisingly, the hardbottom area has no characteristic signature on the single -channel
seismic profiles, so seismic data alone would be insufficient to locate these features. They are
16

I
W1
I
`I
`I-
V
I
1W I
I
1 I
a � v
V
1
W
/ I
v
1
— Hard Bottom
v ! v Low to moderate reflectance
W erosional (f ner grained material)
hot spot V Moderate to high reflectance
V (coarser grained material)
— Data missing or uninterpretable
L
Ripples observed on
' < < side scan record
I Bottom types that are dicontinuous are
indicated by dashed pattern
Y
0
Cape 2.5
Hatteras Nautical Miles
Fig. 9. Seafloor map derived from analysis of side scan sonar imagery.
17

scarps
Mini. `•sue •t..tq{ ! .. 1 Y '
fine 50 m
sand
0 I I jJ pI
Fig. 10. Side scan sonogram along line 196 between lines 023 and 024.
fine
=f sand

best located using side -scan sonar. Cores 174 and 175 are located along trackline segments
marked as hardbottom and contain clasts and zones of cemented shell debris (coquina) within
medium- to coarse -grained sand and gravelly sand. Core 176, located further offshore from the
other two and in an area mapped as a sandy bottom type, contains approximately 2.5 in of muddy
fine-grained sand at the top of the core. But this overlies the sandy, cemented lithology. Thus,
the hardbottom "unit" is buried beneath younger material further offshore.
In the southern portion of the BSA, side -scan sonar reveals areas of moderate acoustic
reflectivity indicative of coarser sediment (medium to coarse sand or gravel). The principal area
where this seafloor type is observed (Fig. 9) corresponds well to the mapped distribution of unit
ST
SECTION II: SEDIMENT TEXTURAL CHARACTERISTICS FROM CORES
Twenty-seven vibracores were collected within the BSA during the summer of 1995
aboard the U.S. Army Vessel Snell (Fig. 11; Table 2). Core lengths range from 1.96 in to 6.11 in,
with an average length of 3.97 in. The cores are distributed throughout the study area and range
from approximately 0.5 miles to 3 miles offshore. These locations generally coincide with
seismic line crossing or end points.
Using the p-wave velocity adopted for this study of 1800 m/sec, the average core length
would be represented on seismic profile data by 4.4 ms two-way travel time. Minimum and
maximum length cores would be represented on seismic profile data 2.2 ins to 6.8 ins two-way
travel time. Thus, it is clear that cores penetrate to very shallow depths within the BSA sediment
package.
Despite the fact that cores penetrate to relatively shallow depths, it was possible to sample
different stratigraphic units within the BSA because the areal extent of stratigraphic units was
limited. In particular, it appears that north of line 018 and east of line 096, unit S1 was the likely
stratigraphic unit sampled by cores. South of line 018 and west of line 096, cores appear to
sample both units S7 and S8. (Figs. 5, 7, 11).
Sediment textural data (Table 2) are summarized from original core descriptions
(composed at the time cores were opened in 1995). Images of cores archived on CD-ROMs (also
composed at the time cores were opened), and textural analyses (standard textural parameters
such as weight percent size fractions, mean grain size, sorting, etc.) were compiled by Hoffinan
and Boss on computer spreadsheets in 1996 (unpublished data). All of these data, including core
halves, are archived at the Coastal Plain Office of the North Carolina Geological Survey in
Raleigh, NC.
Within the BSA, sediment texture is varied within most individual cores as well as from core
to core. Several sediment types, varying from mud to gravel, are present within the cores. The
three northernmost cores (numbers 174, 175, and 176) all contain clasts and zones of cemented
shell debris (coquina) indicative of hardbottom areas. These zones occur within medium- to
coarse -grained sand and gravelly sand and are altered to a dull yellowish brown color.
Approximately 2.5 in of muddy fine-grained sand overlies the sandy, cemented lithology in the
18

026
o
� -024
177�4 175
'
023
1176
-,17K
—x�
°
022
'
179 1 �
182
021 I;
020— 1�84
184
183
0 '
01-8 19
• Hardbottom
185 j
18641 188
ol� ;
016 ,
High mud content
,
I 0 1 187
I:
I S
189
(> 10 percent)
erosional �014
Sandy cores w/
hot spot 191 192 */ I
< 10 percent mud
012 l
19 01 193 194
� 1I
195
/—� ;j
197
198,
�09 it
OOg
200
00)
�170
1�5 * 004 �
Cape
2.5
Hatteras 001
Nautical Miles
Fig. 11. Map showing distribution of cores coded to indicate quality of sediment with respect to suitability for beach
nourishment.
19

N
Table 2. Summary textural data from vibacores.
Water
Length
MUD
SAND
GRAVEL
Mean
Mean
St. Dev.
CORE No.
Depth (m)
(m)
(wt. %)
(wt. %)
(wt. %)
Grain
Grain
(0)
General Description
Size(0)
Size(nun)
uniform fine-grained sand; laminae of very coarse skeletal
SNL-001
7.3
2.82
2.23
97.14
0.39
2.49
0.18
0.65
sand 80-120 cm
finer -grained sand in upper meter and medium- to coarse -
SNL-004
17.4
2.18
6.86
84.75
8.08
2.10
0.23
1.63
grained sand in lower meter; shell gravel at 1.5-1.9 meters
ardbottom area; 0-209cm--medium to coarse shelly sand
with cemented zones (coquina); 209-256cm--mud; 256-
SNL-174
10.97
3.62
18.28
72.87
8.83
1.74
0.30
1.76
362cm--muddy, fine burrowed sand grading down to clean
medium to coarse sand
ardbottom area; uniform coarse sand and gravelly sand
SNL-175
12.80
4.24
8.52
60.18
31.19
0.52
0.52
1.75
with abundant cemented clasts and zones of cemented shell
debris (coquina)
0-250cm--dominantly muddy fine sand; 250-481cm--
SNL-176
16.15
4.81
6.85
77.70
15.30
1.83
0.28
1.71
gravelly coarse and medium to coarse sand with cemented
ones and clasts (hardbottom lithology)
sandy throughout core; dominantly fine to medium sand with
SNL-177
15.85
3.62
3.79
76.99
19.12
1.15
0.45
1.65
ones also containing coarser sand and shell fragments
dominantly medium sand; 0-146cm--fine to medium sand;
SNL-178
10.97
6.10
12.24
73.89
13.80
1.59
0.33
1.83
385-432cm--several mud layers; otherwise medium to coarse
shelly sand
numerous variable textured layers 10-50cm thick ranging
SNL-179
8.23
3.87
4.62
84.02
11.41
1.86
0.28
1.63
from fine to medium sand to medium to very coarse sand;
several gravelly and shelly zones
overall muddy core; 0-211cm--silty fine and very fine sand;
SNL-181
12.50
5.09
37.01
61.81
1.00
3.10
0.10
1.31
11-509cm--mostly muddy with thin sand lenses
mostly silty very fine to fine sand; 320-378cm--medium to
SNL-182
15.24
3.78
10.59
85.07
4.23
2.59
0.17
1.27
coarse sand and gravelly sand

N
N
Table 2. Summary textural data from vibacores (continued).
Water
Length
MUD
SAND
GRAVEL
Mean
Mean
St. Dev.
CORE No.
Depth (m)
(m)
(wt. %)
(wt. %)
(wt. %)
Grain
Grain
(0)
General Description
Size(0)
Size(nun)
SNL-183
13.72
3.24
4.98
94.22
0.68
2.81
0.14
0.80
Dverall fairly uniform silty fine to silty very fine sand
0-152cm--silty fine and silty very fine sand; 152-160cm--
SNL-184
11.28
4.80
3.52
84.30
11.57
1.79
0.29
1.61
mud lens; 160-480cm--medium to coarse sand with shell
gravel
mostly fine to medium sand with several 10 to 20cm-thick
SNL-185
7.62
3.74
3.37
89.48
6.88
2.07
0.24
1.39
layers of medium to coarse sand; mud -filled burrows
throughout the core
interlayered sand and mud; 0-49cm--fine to very fine sand;
SNL-186
11.58
5.97
41.49
57.29
1.08
3.04
0.12
1.43
9-150cm--mud; 150-425cm--silty fine to medium sand with
mud -filled burrows; 425-597cm--mud with thin sandy zones
SNL-187
14.33
1.96
4.70
89.15
5.97
2.35
0.20
1.39
mostly silty fine to very fine sand; several 5 to 10cm-thick
lenses of medium to coarse sand within 120-150cm interval
0-130cm--layers of silty fine sand, silty very fine sand, mud,
SNL-188
16.76
2.20
6.23
86.07
7.52
2.05
0.24
1.50
and medium to coarse sand; 130-220cm--clean fine sand
with a few mud filled burrows at top of interval
sandy throughout core; fine to medium sand with lenses of
SNL-189
15.54
2.35
2.68
88.10
9.05
1.93
0.26
1.43
medium sand with shell gravel 50-60cm and 90-100cm
0- 1 50cm--very uniform fine to very fine sand; 150-425cm--
SNL-190
12.50
4.24
2.38
77.30
20.27
1.24
0.65
1.78
coarse to very coarse sand with abundant shell clasts
0-95cm--very fine to fine sand; 95-180cm--mud; 180-
SNL-191
8.23
4.80
13.53
84.67
1.52
2.17
0.22
1.39
42cm--silty fine sand; 242-480cm--medium to coarse sand
0-115cm--very uniform fine to very fine sand; 115-180cm--
SNL-192
12.50
3.04
2.66
90.34
6.93
1.90
0.27
1.42
medium to coarse sand; 180-282cm--coarse to very coarse
sand with abundant shell clasts; 282-304cm--medium sand

N
W
Table 2. Summary textural data from vibacores (continued).
Water
Length
MUD
SAND
GRAVEL
Mean
Mean
St. Dev.
CORE No.
Depth
Grain
Grain
General Description
(m)
(m)
(wt. %)
(wt. %)
(wt. %)
(0)
Size(0)
Size(mm)
sandy throughout the core; dominantly medium to coarse
SNL-193
15.85
4.20
2.18
87.79
9.89
1.37
0.39
1.31
sand with several gravelly layers in upper 140cm
sandy throughout the core; silty fine sand upper 75cm;
SNL-194
18.59
2.23
10.34
68.51
21.14
1.53
0.35
1.94
mostly fine to medium sand below 75cm with thin lenses of
shell gravel
coarse to very coarse sand and shell gravel; significant shell
SNL-195
15.54
2.51
1.87
56.33
41.74
0.28
0.82
1.62
gravel content 0-50cm and 140-251cm
layered sand and mud; 0-82cm--silty very fine to fine sand;
82-90cm--gravelly zone; 90-243cm--mud grading down to
SNL-196
11.89
3.92
15.70
82.85
1.27
2.48
0.18
1.23
muddy sand by 150cm; 243-392cm--dominantly medium
sand with shell gravel toward base
relatively uniform fine sand; some mud as burrow fillings
SNL-197
5.79
3.91
2.65
96.26
0.95
2.50
0.18
0.67
and linings; very coarse sand at 113-120cm
layered sand and mud; 0-85cm--silty very fine sand; 85-
140cm--medium to coarse sand with shell gravel; 140-163--
SNL-198
11.58
6.05
22.20
76.58
1.08
2.75
0.15
1.33
fine sand to silty fine sand; 163-415cm--mud with shells in
lower m; 410-605cm--medium sand
sandy throughout the core; 0-425cm--uniform medium to
SNL-199
10.36
6.11
4.42
92.90
2.29
1.86
0.28
1.07
coarse sand; 425-611cm--uniform silty fine sand to fine
sand
0-130cm--mud with several layers of fine to very fine sand;
SNL-200
17.68
2.85
41.12
50.51
6.51
2.67
0.14
1.86
130-165cm--coarse sand and shell gravel; 165-190cm--mud;
190-285cm--coarse sand
Average
12.85
3.97
11.07
78.66
10.05
1.97
0.29
1.46
Maximum
18.59
6.11
41.49
97.14
41.74
3.10
0.82
1.94
Minimum
5.79
1.96
1.87
50.51
0.39
0.28
0.10
0.65

outermost of these three cores. These cores probably sample unit S1, and its basal seismic
reflector, R1, is likely the indurated sediment forming the hardbottom. Unit S1 in this location
does not represent an area of potential sand resources.
Elsewhere across the BSA, unit S1 is sampled by cores 178, 181, 182, 183, 184, 187, 188,
194, and 200. Textural attributes of these cores are highly variable, though they averagemore
than 10% mud and thus are unsuitable as potential sand resources.than 10% mud and thus are
unsuitable as potential sand
Another group of cores consists of interbedded sand and mud. This group includes cores
186, 196, and 198. The mud content of these cores ranges from 15 percent to over 40 percent,
thus rendering these deposits unsuitable as beach nourishment material as well. These cores
sample unit S8, which averages 2.9 in thick throughout the BSA. Review of the core logs for
these cores indicates that sediment with coarser textural attributes (medium to coarse sand)
occurs in all cores below approximately 2.5 meters. It is suggested that the lower portion of
these cores is sampling unit S7.
Cores 189, 192, 193, 195, and 199 all contain acceptable amounts of mud with all but one
of these cores containing less than 5 percent mud. When the locations of these more prospective
cores are considered, the cluster comprised of cores 189, 190, 192, 193, and 195 form an area
that is approximately 2.5 square nautical miles. This area coincides with the area where unit S7
is interpreted to crop out directly on the seafloor. Theses 5 cores average 2.35% mud, 79.97%
sand, and 17.58% gravel. The mean grain size of 1.34 (0.395 mm; _ -loge of grain diameter
in millimeters, Pettijohn, 1975) falls within the medium grained sand classification.
SECTION III: SAND RESOURCE ASSESSMENT
The primary goal of this survey was to determine the potential for the BSA to serve as a
source of sand for future beach nourishment of the critically eroding shoreline immediately
onshore of the BSA (Figs. 1, 2). The geophysical data have aided in determining the
stratigraphic architecture of the BSA (from seismic reflection data) and characteristics of the
surface sediment (from side -scan sonar). Interpretations of these data have been verified to some
extent through sedimentological analysis of available cores. The final step in the process of
assessing the sand resource potential is to merge the geophysical interpretations and core data to
determine which stratigraphic units (if any) might serve as suitable sand resources and to arrive
at an estimate of the total volume of sand within suitable units. In arriving at sand volume
estimates, a purposeful effort has been made to use conservative measures wherever possible.
Thus, values reported in this section should be considered to be minimum estimates of the total
sand volume contained within suitable units of the BSA.
Volume estimates for each stratigraphic unit can be made if the thickness and area of each
unit are known. Recall that the thickness of stratigraphic units (in meters) was estimated by
assuming that the speed of propagation of seismic impulses (p-wave velocity) through the
sediments was 1800 m/sec and that this was considered to be a minimum velocity; higher
velocities would yield greater thickness for each unit. To represent the final result in appropriate
24

volume units, the thickness of each unit (in yards) was determined by dividing the estimated
thickness in meters by a conversion factor (yards = meters/0.9144).
The area of each unit was determined utilizing an automatic feature of the GIS software
package which will calculate the area of any contoured region in units specified by the user. For
this study, it was appropriate to determine the area in square yards bound by the contours of Fig.
5.
For each contoured area, the value of thickness used is that of the lower contour. For
example, a contoured region bound by the 5-m and 10-m contour ranges in thickness from 5 m to
10 m. For the purpose of estimating the volume of material bound by these contours, it was
assumed that the area had the minimum thickness of 5 m throughout its areal extent. Once the
total area bound by different contours was determined, the volume of sand within these contours
was calculated by multiplying the area and minimum thickness. The resulting volume, expressed
in millions of cubic yards (yd3), is presented in Table 3 below.
For this study, the only stratigraphic unit considered to be a potential sand resource was
unit S7 (interpreted as a channel fill complex). While deeper stratigraphic units might also yield
quality sand, their depth beneath the surface is considered to make the cost of their exploitation
prohibitive versus dredging the more easily available surficial material.
As can be seen in Table 3, the estimated minimum volume of sand available within S7
across the BSA is appreciable. The total volume of sand within unit S7 across the BSA is at least
375 million cubic yards. However, as has been discussed previously, unit S8 overlies unit S7 over
much of the southern portion of the BSA. Given that the average thickness of unit S8 is 2.9
meters, it may be impractical to attempt to dredge through S8, especially since available cores
indicate that S8 sediments are rather undesirable as beach fill material.
THICKNESS (m)
UNIT S7 VOLUME (millions yd )
5
80.5
10
157.1
15
77.6
20
49.3
25
10.7
TOTAL
375.2
Table 2. Estimated volume of sand in stratigraphic unit S7. Volumes were calculated by measuring the area (in yd2)
for each contour in a 5-m contour interval using GIS software. These areas were then multiplied by
minimum thickness of the contoured unit (meters/0.9144 = yards) to obtain volume in cubic yards. Recall
that the p-wave velocity used to estimate unit thickness is also conservative. Thus, results above represent
minimum volume estimates of sand within the upper two stratigraphic units of the BSA.
If the area of S7 chosen as the potential sand resource is arbitrarily restricted to that area
Of S7 where the thickness of overlying S8 is less than 0.5 m thick, a refined estimate of the
quantity of available sand can be made (Fig. 12, Table 4). The area and thickness of S7 within
this box must be estimated. For the sake of this illustration, assume several scenarios for the
thickness of S7 in this box; 5 m, 10 m, and 15 m. The estimated volume of sediment in this box
using these values is given in Table 4 below.
25

erosional
hot spot
0 �
23
/ 020—
I ,
018�
1 �
—gy p J6 01
I 0fS I
I 014
I
i
Cape
Hatteras
,
Seismic and side scan
I sonar trackline
I
T
S7
S8
Core Texture Type
Hardbottom
High mud content
(> 10 percent)
Sandy cores w/
< 10 percent mud
0 2.5
Nautical Miles
Fig. 12. Map showing hypothetical sand resource area (box) located to access sediment from unit S7 with a
minimum from unit S8. Area of this box is 11 million ydz. Estimates of sand resource within this box are
presented in Table 4 assuming scenarios for thickness of S7 of 5 m, 10 m, and 15 m..
26

ESTIMATED VOLUME OF SAND IN BOXED AREA (Fig. 12)
Area of Box on Map = 11 million yd2
ASSUMED SEDIMENT THICKNESS (m)
ESTIMATED VOLUME OF S7R (million yd3)
5
60.1
10
120.3
15
180.4
Table 4. Estimated volume of S7 restricted (S70to area where overlying Sg is less than 0.5 m thick. Sand Volumc
calculated using same method as previous.
Obviously, not all of the sand contained within unit S7 is economically recoverable, but
this exercise illustrates that a significant amount of sand is available within the BSA that could
be exploited to nourish the critically eroding shoreline immediately onshore. Whether or not a
decision is made to utilize this sand will depend on factors such as technological capability (e.g.
dredging methods); logistics (e.g. mobilizing and operating a dredge in this somewhat remote
location); environmental considerations (e.g. potential impacts of dredging operations on
fisheries); social concerns (e.g. public perception of beach nourishment or dredging in waters
offshore national seashores); and economic factors (e.g. cost of transporting sand from BSA to
nourishment sites). These considerations, however, were beyond the scope of this
reconnaissance -level assessment.
The issue of compatibility of the offshore sand with the native beach sand will need to be
addressed. To date, no systematic sampling and testing of the native beach material within the
erosional hot spots has been conducted. More detailed, feasibility -oriented studies of potential
nourishment projects, will likely involve this work.
REFERENCES CITED
Dresser Atlas Industries, Inc, 1982, Well logging and interpretation techniques: The course for
home study: Dresser Atlas Industries, Inc., 228p.
Krumbein, W.C., 1934, Size frequency distribution of sediments: Journal of Sedimentary
Petrology, v.4, p.65-77.
National Geophysical Data Center (NGDC), 1999, Coastal Relief Model - volume 2 - U. S.
South East Atlantic coast, 1 CD-ROM.
Pettijohn, F.J., 1975, Sedimentary Rocks, 3rd Edition: New York, Harper -Row Publishers, 627p.
Stone, J., Overton, M., Fisher, J., 1991, options for North Carolina Coastal highways vulnerable
to long term erosion [unpublished]: Contract Report to North Carolina Department of
Transportation.
27