SHOREFACE MAPPING AND SAND RESOURCE INVENTORY: NORTH TOPSAIL BEACH AND SURF CITY, NORTH CAROLINA
Kenneth T. Willson
A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science
Department of Geography and Geology
University of North Carolina Wilmington
2009
Approved by
Advisory Committee
James A. Dockal Paul A. Thayer
William J. Cleary Chair
Accepted by
Dean, Graduate School
This thesis has been prepared in a style and format
consistent with
The Journal of Coastal Research
ii TABLE OF CONTENTS
ABSTRACT ...... v
ACKNOWLEDGEMENTS ...... vi
LIST OF TABLES ...... viii
LIST OF FIGURES ...... ix
INTRODUCTION ...... 1
Study Area ...... 3
North Topsail Beach ...... 9
Surf City ...... 10
Previous Work ...... 11
METHODS ...... 15
Subbottom Profile and Seismic Surveys ...... 15
Sidescan Sonar Surveys ...... 16
SCUBA Diver Ground Truthing Surveys ...... 20
Vibracore Survey ...... 20
Additional Methods ...... 22
RESULTS ...... 25
Seafloor Characteristics ...... 25
Subsurface Analysis ...... 33
iii
Vibracores ...... 33
Chirp Seismic Analysis ...... 58
DISCUSSION ...... 71
Area 1 ...... 78
Area 2 ...... 83
Area 3 ...... 87
CONCLUSION ...... 97
LITERATURE CITED ...... 99
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ABSTRACT
North Topsail Beach and Surf City, located along the north and central portions of Topsail
Island, North Carolina respectively, are two of the most vulnerable coastal communities in the
State in regard to beach erosion and storm impacts. This is primarily due to the relatively low topography of Topsail Island and the lack of sand available on the shoreface to nourish the beaches. Over the past decade a profusion of data has been collected offshore Topsail Island in an attempt to locate beach quality sand for beach nourishment projects along the Island. This study compiled and utilized these data to map the shoreface off North Topsail Beach and Surf
City with respect to surficial and subsurface geomorphology. The data utilized in this study includes vibracores, sidescan sonar data, subbottom profile data, and SCUBA diver ground truthing data and grab samples.
A number of Oligocene age units associated with the River Bend Formation were identified throughout the course of the study which crop out on the seafloor surface and compose the underlying strata of the shoreface. None of the units mapped as Oligocene age show potential for being a viable source of beach quality sand due to a combination of fine grain material, high silt content, and/or high carbonate percent. Despite the underlying units composing the shoreface exhibiting little potential for sand resources, three channel like features that were incised into the Oligocene units where identified due to the density of data throughout the survey area. These three areas suggest that isolated deposits of beach quality sand may exist on the
shoreface off Topsail Island.
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ACKNOWLEDGEMENTS
I would like to express my genuine thanks and appreciation to all those who have assisted
me in countless ways to complete this work. A heartfelt appreciation to my wonderfully encouraging and understanding wife Ali and my son Jack who have sacrificed much while I have juggled a career and the completion of this manuscript. I express my thanks to my parents
Walter and Sylvia Willson who have been a constant encouragement and motivation. I wish to recognize my brothers Walt and Stephen for their love and support throughout this endeavor. It has truly been a pleasure to work under the tutelage of Dr. William J. Cleary as he prepared me for a career in coastal geology. My sincere thanks go to Dr. Paul A. Thayer and Dr. James A.
Dockal for their encouragement, support, and guidance throughout this process.
Countless students, faculty, and staff of UNCW and the Center for Marine Science have played a vital part in this work. A debt of gratitude to my fellow SCUBA divers who assisted in the collection of data including, Adam Knierim, John Welsh, Jay Styron, Jason Souza, Andy
Mcleod, Mark Rauscher, Osku “Jonny” Backstrom, Glenn Taylor, Sharon Kessling, and Ken
Johns. Also I would like to thank the staff of the research vessels, who made this study possible including Capt. Dan, Capt. Gerry, Capt. Mike, and Capt. Ron. My sincere thanks and appreciation go out to my colleagues and friends in the Coastal Geology Lab that made my time in the lab so enjoyable including, Ben McGinnis, C.J. Jackson, Adam Knierim, John Welsh,
Dave Doughty, and Leighanne Budde. I am indebted to all of the professors who have played a role in both my graduate and undergraduate education at UNCW especially Dr. Michael Smith and Dr. William Harris.
I also wish to thank my colleagues and supervisors at Coastal Planning & Engineering for their support, assistance, and understanding in affording me the opportunity to complete this
vi thesis especially: Mr. Jeff Andrews, Mr. Tom Jarrett, Mr. Tom Campbell, Mr. Beau Suthard,
Mrs. Melany Larenas, Mrs. Dawn York, and Dr. Charles Finkl.
Finally thanks to the UNCW Geography and Geology Department, the Center for Marine
Science, the National Undersea Research Center, and HDR Engineering for providing assistance in the completion of this study.
vii
LIST OF TABLES
Table Page
1. Sieves used in grain size analysis conducted by CPE on vibracore samples and native beach samples...... 24
2. Results of insoluble residue analysis ...... 51
3. Percentage of major compositional elements of thin section modal analysis for selected samples inside the study area ...... 54
4. Percentage of major compositional elements of thin section modal analysis after JOHNSTON (1998) ...... 56
5. Cumulative results of insoluble residue analysis conducted on three partially unconsolidated, sandy units interpreted to be Oligocene in age that underlie and crop out on the seafloor in the study area ...... 75
viii
LIST OF FIGURES
Figure Page
1. The Onslow Bay region of southeastern North Carolina illustrating the continental shelf geology (After Snyder et al. 1994), and the tracks of landfalling hurricanes (1996 - 1999)...... 4
2. Location map of Topsail Island and the study area ...... 6
3. Oblique aerial photograph of North Topsail Beach after Hurricane Floyd (1999) ...... 8
4. Stratigraphy of upper Mesozoic and Cenozoic units in southeastern North Carolina (Modified from HARRIS et al., 2000) ...... 13
5. Location map showing U.S. Army Corps of Engineer data used in the study ...... 17
6. Location map showing Coastal Planning & Engineering data used in the study ...... 18
7. Location map depicting UNCW Coastal Geology Lab data used in the study ...... 19
8. Generalized seismic stratigraphic column showing seismic sequences and respective boundaries ...... 23
9. Map depicting sidescan sonargraph with digitized areas of interpreted and ground truthed hardbottoms ...... 27
10. Map depicting surficial sediment type based on UNCW sidescan sonar data, SCUBA diver grab samples, and vibracores ...... 28
11. Map depicting sidescan mosaic exhibiting sorted bedforms ...... 29
12. Map depicting surficial sediment type enhanced by the addition of USACE vibracore data ...... 31
13. Photograph of a submarine rock outcrop interpreted to be of the upper Oligocene River Bend Formation ...... 32
14. Photographs of hardbottom scarp outcrops interpreted to be of the upper Oligocene River Bend Formation ...... 34
15. Sidescan mosaic on which the location of Ground Truthing Sites “SDA” and “Top 6” are superimposed ...... 35
16. Location map of eight cross sections depicted in Figures 17 – 24 ...... 37
ix
17. Cross Section Diagram of Transect A-A’ ...... 38
18. Cross Section Diagram of Transect B-B’ ...... 39
19. Cross Section Diagram of Transect C-C’ ...... 40
20. Cross Section Diagram of Transect D-D’ ...... 41
21. Cross Section Diagram of Transect E-E’ ...... 42
22. Cross Section Diagram of Transect F-F’ ...... 43
23. Cross Section Diagram of Transect G-G’ ...... 44
24. Cross Section Diagram of Transect H-H’ ...... 45
25. Photo-mosaic of vibracore VC-31 depicting back barrier sequences ...... 46
26. Map depicting Pleistocene sediment filled depressions ...... 47
27. Photomicrograph of sample TI03-VC-274 from the dark olive gray sandstone ...... 50
28. Photomicrograph of sample SC-105 from the moldic limestone ...... 55
29. Seismograph of Line 3 Day 3 with superimposed vibracore logs ...... 59
30. Seismograph of Line 3 Day 3 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units ...... 60
31. Seismograph of Line 53 with superimposed vibracore logs ...... 61
32. Seismograph of Line 53 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units ...... 62
33. Seismograph of Line 2 Day 3 with superimposed vibracore logs ...... 63
34. Seismograph of Line 2 Day 3 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units ...... 64
35. Seismograph of Line 2 Day 2 with superimposed vibracore logs ...... 65
36. Seismograph of Line 2 Day 2 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units ...... 66
37. Map depicting three areas in which thick deposits of beach quality sand were identified ...... 79
x
38. Annotated and unannotated chirp seismograph of the portion of Line 2 Day 3 that bisect Area 1 ...... 80
39. Map depicting the location of the mud filled paleo-channel contained within a sand filled channel in the vicinity of Area 1 ...... 82
40. Annotated and unannotated chirp seismograph of the portion of Line 2 Day 2 that depicts the channel feature that is the landward extension of Area 2 ...... 84
41. Map depicting Area 2 and the outline of the channel feature identified using CPE Chirp Seismic Data ...... 86
42. Seismographs from four lines surveyed by CPE depicting the variability of the channel morphology observed throughout Area 2 ...... 88
43. Schematic drawing of the interpretation of the channel feature from Area 2 grading from an estuarine or tidal creek channel into an inlet complex ...... 89
44. Map depicting Area 3 in detail with channel like feature superimposed upon it ...... 91
45. Annotated and unannotated chirp seismograph of a portion of Line 117 collected by CPE that depicts the channel feature that identified within Area 3 ...... 93
46. Oxygen Isotopic-based sea-level estimates for the past 9 Ma after MILLER et al. (2005) ...... 94
47. Map depicting the primary and secondary channel feature in relation to the modern drainage pattern ...... 96
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INTRODUCTION
Owing to an influx in population to coastal communities over the past few decades, the beach
has become increasingly important to those invested in its many provisions. Worldwide, the
beach is a place to enjoy a picturesque view of a sunset and to spend relaxing time away from the
stresses of everyday life. Entire industries have developed around beaches and beach
communities including tourism, which provide significant revenue for local governments and
private entrepreneurs. Beaches, however, provide much more than revenue and fun in the sun.
Perhaps the most important role a beach and its associated dunes play are in mitigating the
effects of erosion and storm surge flooding during tropical and extra-tropical storms. Such
storms with increased wave energy and storm surge, can cause severe erosion, breach dune
systems and overwash topographically low portions of islands. It is in recognizing the storm protection attributes of a beach that one begins to realize why such growing emphasis is being put on the “health” of beaches in southeastern North Carolina. Local governments and citizens alike are concerned with frequent storms, steady shoreline retreat rates and the sand starved nature of the region.
Chronic beach erosion has become a serious issue for many who live in the coastal communities of southeastern North Carolina. A number of these coastal communities have recovered from the impacts of recent hurricane activity (1996-2005) including Wrightsville
Beach and Carolina Beach; however, many severely impacted areas such as Topsail Island are at higher risk as a result of a historic regional sand deficit and little or no replenishment of sand.
Many of these high-hazard communities are undertaking a reexamination of the shoreface and adjacent inlet systems for sand resources that can be used for storm mitigation beach nourishment projects. The three communities situated on Topsail Island (North Topsail Beach,
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Surf City, and Topsail Beach) are among the most vulnerable communities in southeast North
Carolina with regards to historically low topography of the island. The low topography coupled
with the effects of hurricane activity experienced during the 1990’s and into the twenty-first
century have given rise to great concern for the residents Topsail Island.
Currently, the State of North Carolina has one of the most restrictive shoreline stabilization
policies in the nation in the form of the Coastal Area Management Act (CAMA) of 1974
(CLEARY et al., 2004). One of the most limiting regulations, banning the construction of
shoreline hardening structures was enacted in 1984 which is stated in Article 7, Section 1, 113A-
115.1 Paragraph B:
No person shall construct a permanent erosion control structure (a breakwater, bulkhead, groin, jetty, revetment, seawall, or any similar structure) in an ocean shoreline (the Atlantic Ocean, the oceanfront beaches, and frontal dunes, including an ocean inlet that exhibits characteristics of estuarine shoreline.)
Other than the relocation of erosion-threatened homes, the only other viable option for
erosion mitigation is beach nourishment (CLEARY et al., 2004). Beach nourishment is an engineering practice that involves mechanically placing large quantities of sand onshore or in the nearshore zone to artificially compensate for a loss of sand in the beach system (FINKL et al.,
2005). Along ocean shorelines, beach nourishment is advantageous over structural hardening
methods of shore protection such as groins, revetments, and seawalls because the procedure
preserves the aesthetic and recreational value of beaches by replicating the protective
characteristics of natural beach and dune systems (e.g. FINKL, 1981; NRC, 1995; WALKER and
FINKL, 2002; CAMPBELL et al., 2003). Many coastal communities in southeast North Carolina
have a history of replenishment, whereas those without have feasibility studies in progress aimed
at stemming chronic erosion. Given the success of beach nourishment projects along the U.S.
2
East Coast, beach nourishment has become the accepted solution for mitigating erosion along
southeastern U.S. shorelines.
Aside from considerations of how to finance such projects, the availability of a viable source
of beach quality sand is the primary factor in determining the feasibility of nourishment. A
viable source of sediment is one that contains sufficient volumes of compatible sediment, which
can be economically transported to the recipient beach. Neither North Topsail Beach nor Surf
City has had any significant nourishment to mitigate for beach erosion experienced through
recent hurricanes or to protect property and infrastructure on the island from subsequent storm
events. Consequently, there have been extensive investigations into the offshore portions of
these communities in search of a viable sand source.
The purpose of this study is to provide a geologically-based examination and mapping of offshore sand resources, which could be used by North Topsail Beach and Surf City for beach nourishment. This study maps the distribution of several facies of what is interpreted as
Oligocene age rock units and concludes that significant amounts of sediment are being derived from these units. The study also identifies several channel-like deposits in the area and discusses the potential for these deposits to yield beach quality sand. The study employs seismic, sidescan, vibracore, and ground truthing survey methods to evaluate the sediment bodies found on the inner continental shelf off Topsail Island North Carolina.
Study Area
The shape of the North Carolina coastal system reflects major differences in the underlying
geological framework (RIGGS et al., 1995). The 326-mile coastline of North Carolina is
separated by Cape Lookout into two distinct provinces. The southern province extends from
Cape Lookout to Cape Fear (Figure 1). The embayment located between Cape Lookout and
3
Figure 1 The Onslow Bay region of southeastern North Carolina illustrating the continentat shelf geology (After Snyder et al. 1994), and the tracks of recent hurricanes (1996-2005).
4
Cape Fear is Onslow Bay. Onslow Bay is underlain by Cenozoic era rock units (SNYDER et al.,
1982; SNYDER et al., 1994; CLEARY et al., 1996; WILLSON and CLEARY, 2003). These units are
associated with the Carolina Platform, which underlies the region between Myrtle Beach, South
Carolina and Cape Fear, North Carolina. This structural platform has undergone relative uplift
over geologic time resulting in the truncation of the rock units by the migrating shoreface.
Consequently, an erosional topography exists throughout Onslow Bay with only a thin veneer of
sediment and widespread exposures of rock across the shoreface.
MILLIMAN et al. (1972) classified the Onslow Bay shelf sediment cover as residual (derived
from the erosion of underlying sediments and rocks). The sand-poor nature of the shoreface in
the area is associated with the thin veneer (several centimeters to < 2 meters in most areas) of
Holocene sediment covering shallow rock units that frequently crop out on the seafloor as scarps or flat hardbottoms. Relatively low sediment input into Onslow Bay due to a lack of fluvial input and minimal sediment exchange between adjacent shelf embayments further exacerbates the situation. (CLEARY and PILKEY, 1968; BLACKWELDER et al., 1982)
Topsail Island is a 35 km (22 mi) long barrier island located in the central portion of the
Onslow Bay shoreline (Figure 2). The Island is bordered by New River Inlet to the north and
New Topsail Inlet to the south. Three separate communities, North Topsail Beach, Surf City,
and Topsail Beach are situated on the Island. This study encompasses an area that extends from
1.8 km to 9.3 km (1 to 5 nautical miles) offshore of the towns of North Topsail Beach and Surf
City located in the northern and central portions of Topsail Island respectively (Figure 2).
Both North Topsail Beach and Surf City are situated in a zone of chronic overwash. Topsail
Island as a whole is topographically low and the only dune protection is a very narrow (1 - 3 m
wide) dune field. This low topography and narrow dune system has resulted in numerous
5
Historic Location of Stumpy Inlets
Figure 2. Location map of Topsail Island and the study area.
6
overwash events experienced during hurricanes, tropical storms, and nor’easters (CLEARY and
PILKEY, 1996). Storms between 1944 and 1962 as well as winter nor’easters during the late
1980’s were particularly destructive (CLEARY and HOSIER, 1979; CLEARY, et al., 2000). In July,
1996 Hurricane Bertha made landfall in Southeastern North Carolina eroding a significant
portion of the dune field with the exception of the area immediately downdrift of New River
Inlet. Washover occurred throughout North Topsail Beach and Surf City evidenced by large
washover fans on the back side of the island. The only beach recovery undertaken prior to
landfall of Hurricane Fran in early September, 1996 was profile manipulation. This artificial
profile manipulation involved pushing sand from the road and the beach onto the dune. These
efforts did little to protect homes and infrastructure behind the unvegetated artificial dunes.
In early September, 1996 Hurricane Fran made landfall in the immediate vicinity of Topsail
Island. Much of the barrier was inundated destroying nearly all of the fronting dunes and a large
number of homes and infrastructure. HDR ENGINEERING INC. OF THE CAROLINAS, (2002 b.) states that the high water line along North Topsail Beach receded ~11 to 20 meters. Once again washover features occurred throughout the island. Extensive exposures of peat and stump forest on the foreshore indicate that portions of the island were on the brink of an acceleration in rollover rates (HDR ENGINEERING INC. OF THE CAROLINAS, 2002 b.). At least six temporary
breaches were formed on the island during the storm. Several of these breaches remained open
for several months (CLEARY, et al., 2000).
Hurricane Bonnie made landfall in the vicinity of Topsail Island in August, 1998 followed by
the landfall of Hurricanes Dennis and Floyd in August and September of 1999, during which
time similar overwashing and breaching of the island occurred (Figure 3). Numerous nor’easters
7
Peat Outcrop
Washover Breach
Figure 3. Oblique aerial photograph of North Topsail Beach after Hurricane Floyd (1999) looking north toward New River Inlet. Note the washover fans, breach in the island, and peat outcrop. Photograph provided by Dr. William Cleary.
8
also impacted the Topsail Island shoreline from 1996 through 2005 exacerbating erosion of the
beach and dune system of the barrier island.
North Topsail Beach
North Topsail Beach comprises the northern 19.3 km of Topsail Island. New River Inlet
borders the town to the North. The natural realignment of the outer bar channel of New River
Inlet in a more northeasterly orientation has promoted significant erosion of the beach and dunes
along the northern end of the Island (CLEARY et al., 2003). Since 1997, shoreline erosion has
ranged from 13.7 m (45 ft) to 47.2 m (155 ft) along the northern 1.2 km (4,000 ft) of the Town.
In May 2002, approximately 229,400 m3 of material was dredged from the confluence of the
Atlantic Intra-coastal Water Way and the New River and placed along the eroding shoreline in
an attempt to mitigate the erosion. This was short lived with only a fraction of the material
remaining after several winter storms. Similar routine maintenance events, subsequent to the
2002 effort, have done little to stem erosion on the northern end of the Island.
The town of North Topsail Beach also faces a different problem from many other
communities in southeastern North Carolina due to the COASTAL BARRIER RESOURCE ACT of
1982 . Enacted by the United States Congress, this legislation establishes Coastal Barrier
Resource Act (CBRA) Zones for the purpose of “… minimizing the loss of human life, wasteful expenditure of federal revenues and the damage to fish, wildlife and other natural resources associated with the coastal barriers along the Atlantic and Gulf Coasts…”. This legislation was implemented to restrict future federal expenditures and financial assistance that would have the effect of encouraging development of coastal barriers. Eleven kilometers (7 mi) of the 18 km
(11 mi) shoreline of North Topsail Beach falls within a CBRA Zone (Figure 1). This is
9
significant because for the town to consider a beach nourishment project within the CBRA Zone,
it must secure funding without any federal contributions.
Surf City
Surf City occupies the central 9.7 km of Topsail Island. Most of the Town is situated atop the
portion of the island, which fronts the relict flood-tidal deltas of Stumpy Inlet. This inlet opened
and closed several times during the 1800’s (HDR ENGINEERING INC. OF THE CAROLINAS, 2002 b.)
(Figure 2).
According to the North Carolina Division of Coastal Management (DCM), the average long- term erosion rates for the southern portion of Surf City, prior to the hurricane activity beginning in 1996, ranged from 0.0 to 0.6 m/yr (0-2 ft/yr). The northern segment of the Town’s shoreline was actually characterized by accretion of up to > 0.9 m/yr (BENTON et al., 1993). These long
term shoreline rates were not indicative of storm event erosion experienced in given locations.
The dunes along much of the town were low (< 2 m), scattered and often scarped indicating that
the beach was experiencing active erosion. Some of the most devastating structural damage
from the storms were experienced in sections of the shoreline characterized by long-term
accretion. Washover terraces in these segments extended across much of the island and into
finger canals dredged in the area of the relict flood tidal delta of Stumpy Inlet. The southern
portion of Surf City is characterized by higher foredune and adjacent dune fields.
Located along the central 9.7 km of Topsail Island, Surf City has no adjacent inlets, which
could provide beach fill material. In contrast, several beach communities in southeastern North
Carolina, such as Ocean Isle Beach, Oak Island, Bald Head Island, Wrightsville Beach, and
North Topsail Beach, are able to dredge sediment from adjacent inlets during maintenance
10
dredging or channel reorientation projects and place the material on the beach. The central
location of Surf City on Topsail Island precludes the community from receiving such material.
Previous Work
Great interest has been taken in the morphology and lithology of the underlying and surficial
units of Onslow Bay. Many studies have been conducted in the region to determine the link
between barrier island morphology and storm response and the underlying geological
framework. Likewise, a great deal of investigation has been aimed at locating sufficient
quantities of beach quality sand for beach nourishment projects.
MEISBURGER (1977, 1979) collected low-resolution seismic data on the inner continental
shelf from New River Inlet to Cape Fear and west to the North Carolina/South Carolina border as
part of the Cape Fear Region Inner Continental Shelf Sediment and Structure Program (ICONS).
MEISBURGER (1979) delineated the general geology and stratigraphy of the inner shelf. SNYDER et al. (1994) established a three-dimensional stratigraphic framework for the inner continental shelf between Fort Fisher, North Carolina and Mason Inlet located at the northern end of
Wrightsville Beach, N.C. Using seismic data, two unconformities were traced and mapped, which delineated three sequences in ascending order as Kpd (a Late Cretaceous sequence
correlated to the Pee Dee Formation); Ech (a middle Eocene sequence correlated to the Castle
Hayne); and Orb (an early Oligocene sequence correlated to the River Bend Formation).
BAUM et al. (1978), HARTS (1998), and HARRIS et al. (2000) mapped Tertiary and Quaternary rock and sediment units on the coastal plain of southeastern North Carolina from the Neuse
Hinge to the Cape Fear Arch. BAUM et al. (1978) and HARRIS et al. (2000) developed a
stratigraphic nomenclature for the middle Miocene through lower Eocene strata. The
11
nomenclature developed by HARRIS et al. (2000) is utilized in this study (Figure 4). HARRIS et
al. (2000) described the upper Oligocene River Bend Formation as a sandy, molluscan-mold
grainstone and calcareous, quartz sand, whereas the lower Oligocene River Bend Formation is
described as very fine to fine calcareous, quartz sand. HARRIS et al. (2000) concluded that the
lower part of the onshore River Bend Formation occurs offshore in Onslow Bay off Kure Beach
based on Sr isotopic ratios of foraminifera collected from onshore and offshore core samples.
JOHNSTON (1998) and MCQUARRIE (1998) conducted similar studies using geophysics to
examine the underlying geology and surficial morphology of areas immediate northeast and
southwest respectively of the study area for this thesis. JOHNSTON (1998) mapped the surficial
characteristics and subbottom profile of the shoreface of the northern section of Topsail Island
and Onslow Beach. He defined the uppermost stratigraphic unit that crops out over most of the
New River Inlet shoreface as the upper Oligocene Belgrade Formation. A second unit crops out
south of New River Inlet near the northern boundary of this study area. This unit is correlative to
the lower Oligocene Trent Formation. The Trent and Belgrade Formations are compositionally
similar and are separated in the Belgrade Quarry by “a highly phosphatized and bored diastem”
(JOHNSTON, 1998), characterized by a surface pitted with gastropod and mollusk borings.
JOHNSTON (1998) also mapped two types of paleochannels incised into the Belgrade Formation.
One channel type was lined with the Aquitanian age oyster Crassostrea gigantissima. The other
channel type was Holocene in age. This channel was identified and mapped off of New River
Inlet as Quaternary paleo-fluvial channel fill. The Holocene channel features were backfilled
with unconsolidated sands and estuarine mud. MCQUARRIE (1998) used sidescan sonar, high-
resolution seismic reflection analysis, and diver-retrieved vibracores to examine the stratigraphic
framework, surficial morphology, and influence of the underlying geology on the surficial
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Stratigraphic Units Stratigraphic Units Harris et al. (2000) Era Period Epoch Stages Baum et al. (1978) Onshore Offshore MESSINIAN TORTONIAN SERRAVALLIAN PUNGO RIVER Not Addressed PUNGO RIVER LANGHIAN FORMATION FORMATION BURDIGALIAN CRASSOSTREA BELGRADE MIOCENE AQUITANIAN Silverdale Fm. ? Belgrade Fm. FORMATION RIVER BEND CHATTIAN FORMATION UNDIFFERENTIATED
TERTIARY OLIGOCENE CENOZOIC
RUPELIAN TRENT FORMATION RIVER BEND
OLIGOCENE FORMATION PRIABONIAN BARTONIAN CASTLE HAYNE CASTLE HAYNE CASTLE HAYNE LIMESTONE LIMESTONE LUTETIAN LIMESTONE EOCENE ROCKY PT. UPPER MEMBER Not Addressed Not Addressed CRETACEOUS PEEDEE
MESOZOIC FORMATION
Figure 4. Stratigraphy of upper Mesozoic and Cenozoic units in southeastern North Carolina (Modified from HARRIS et al., 2000).
13
morphology of the shoreface seaward of Topsail Beach, Lea and Coke Island. The study
focused on short-term morphological changes of the shoreface as a result of hurricanes Bertha
and Fran in 1996. The study defined the principal stratigraphic unit underlying the Topsail
Beach shoreface (south of this study area) as the middle Oligocene River Bend Formation. The unit, sampled in vibracores, consisted of olive green, silty, often dolomitic, fine quartz sand.
Along with the dominant olive green unit, MCQUARRIE (1998) also mapped a variety of
Quaterary age fluvial channel features on the shoreface. Vibracores taken in the channel
sequences were infilled with dark gray estuarine mud.
WILLSON and CLEARY (2002) re-examined offshore areas and the ebb-tidal deltas of adjacent
inlets for three sites in southeastern North Carolina, including Onslow Beach, Topsail Bach, and
Figure Eight Island. The re-examination compiled preliminary sand quality and thickness data for the sites to be used in erosion and storm damage reduction projects. The results of the re- examination indicated that the relatively thin sequences of sediment, the nature of individual sand deposits and the widespread rock outcrops exclude the use of major portions of the shoreface for long-term borrow needs. The study concluded that significant volumes of beach fill quality sand that could support long term beach nourishment projects are contained in ebb- tidal deltas. However, the restrictive nature of North Carolina’s regulatory policies present significant hurdles to permit a borrow area for beach fill projects in the vicinity of these ebb-tidal deltas.
HDR ENGINEERING INC. OF THE CAROLINAS (2002 a. and 2002 b.) conducted two preliminary
sand searches, along the entire offshore area of Topsail Island. These preliminary sand searches
were the basis for the Unites States Army Corps of Engineers (USACE) authorizing the
examination that involved a large-scale reconnaissance level seismic survey of Topsail Island
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(Ocean Surveys, Inc., 2004) coupled with the collection of approximately 369 vibracores, which focused on specific target areas where potential borrow sites for sand were thought to exist.
METHODS
This study employed a number of methods to investigate seafloor characteristics and subsurface geologic units of the shoreface including: subbottom profile and seismic surveys, sidescan sonar surveys, SCUBA diver ground truthing, vibracore surveys, jetprobe surveys, mechanical sieve grain size analysis, carbonate analysis, and thin-section analysis. Data aquisition was undertaken by three organizations: The University of North Carolina
Wilmington’s (UNCW) Coastal Geology Lab, the USACE, and Coastal Planning & Engineering,
Inc. The author of this thesis participated in and in most cases supervised the collection of data by the UNCW Coastal Geology Lab and Coastal Planning & Enginnering, Inc. Data collected by the USACE were independently analyzed by the author. All data were incorporated into a single data set to examine the quantitative and qualitative characteristics of the geological units and local geomorphology to determine whether substantive amounts of beach quality sand is present on the shoreface off North Topsail Beach and Surf City.
Subbottom Profile and Seismic Surveys
Data from two separate seismic surveys were utilized during the course of this investigation.
The first survey was completed in September, 2004, by the USACE. The survey was conducted for a study aimed at evaluating sand resource potential offshore Topsail Island for Federal shoreline protection projects. The USACE study employed two different systems for the collection of seismic data, the EdgeTech X-star “Chirp” Subbottom Profiling System and the
15
Applied Acoustics Engineering, Inc. “Boomer” Seismic Reflection System (OCEAN SURVEYS,
INC, 2004). This data set is comprised of 583 km of seismic trackline of which 222 km were
analyzed (Figure 5). The second seismic survey was conducted by Coastal Planning and
Engineering, Inc (CPE), during a sand search conducted for the town of North Topsail Beach
(FINKL et al., 2007). The CPE study employed the EdgeTech 512i X-STAR full spectrum sonar
(FINKL et al., 2007). The focus of the effort was to identify potential sand sources for a beach
restoration project. This CPE survey collected 200 km of seismic tracklines (Figure 6).
Sidescan Sonar Surveys
Data from two separate sidescan surveys were utilized in this study. The first survey was conducted by the UNCW Coastal Geology Laboratory under grants from the National Undersea
Research Center (NURC) and HDR Engineering of the Carolinas. This sidescan sonar survey employed the EdgeTech DF 1000 sidescan sonar system. A low frequency of 100 kHz was used to allow for an increased range (250 m) for survey lines to maximize coverage of the final mosaic. The data was processed with XSONAR (USACE software) and exported to a geo- referenced .tiff file that was later imported into GIS for analysis. This survey was conducted in several different phases from 1999 to 2001 (Figure 7).
The second sidescan sonar survey was conducted by CPE, during its sand search conducted for the town of North Topsail Beach along the same 209 km (130 mi) of trackline that seismic data were collected (Figure 6). The CPE sidescan sonar survey employed the EdgeTech 4200 FS system which uses full-spectrum chirp technology. The sonar package included the portable configuration with laptop computer running Discover® acquisition software and a 120/410 kHz
16
Figure 5. Location map showing U.S. Army Corps of Engineer vibracore sites and tracklines along which subbottom profile data were collected.
17
Figure 6. Location map depicting Coastal Planning & Engineering geophysical survey tracklines (seismic, bathymetric, and sidescan surveys), jetprobes and vibracore sites.
18
Figure 7. Location map showing the locations of vibracore samples, ground truthing surveys and samples, and sidescan sonar collected by the UNCW Coastal Geology Lab. Note that Sidescan data shown to the extreme southeast is from McQuarrie (1998).
19
dual frequency towfish running in high definition mode (FINKL et al., 2007). Dual frequency
provided a differential aid to interpretation. The range scale was set at 150 m (492 feet),
providing over 100% coverage (FINKL et. al., 2007).
SCUBA Diver Ground Truthing Surveys
One hundred six locations were occupied by SCUBA divers in order to conduct ground
truthing of seismic, sidescan, and bathymetric data (Figure 7). This ground truthing took place between June 1999 and August 2002 and was performed by divers from the UNCW Coastal
Geology Lab including the author. Ground truthing was done from the UNCW R/V Seahawk. A
Differential Global Positioning System (GPS) was used to navigate and record site locations.
Through differential correction, the accuracy of the GPS system used in this study, provides for a position accuracy of 0.3 m (1 ft) to 1.2 m(4 ft) . Divers collected hand samples of the surficial sediment as well as subbottom samples usually 1 m below the seafloor surface. Divers would, on most occasions, make an effort to determine the sediment thickness above rock with a 1.5 m
(5 ft) metal probe by driving it into the sediment until 1.5 m was reached or rock refusal occurred.
Vibracore Surveys
Data from three separate vibracore surveys were utilized in this study. The first survey was conducted by the UNCW’s Coastal Geology Laboratory and included 35 vibracores (Figure 7).
The second vibracore survey was conducted by the USACE during their evaluation of potential sand sources off Topsail Island. The USACE data set includes 369 cores of which 159 were used in this study (Figure 5). The third vibracore survey was conducted by CPE during its sand
20
search conducted for the town of North Topsail Beach. Twenty-Eight vibracores were taken
during the course of the CPE investigation (Figure 6). Vibracores and surface sediment samples
provided a means of ground truthing the sidescan sonar mosaics and seismic subbottom profiles
and assessing the distribution and nature of hardbottoms.
The vibracores taken by the UNCW Coastal Geology Lab were SCUBA diver retrieved
vibracores taken from the UNCW R/V CapeFear. The vibracore apparatus consists of a
pneumatically driven piston head, BH-4 Linear Vibrator being mounted on to the top of a three-
inch aluminum pipe with a standard length of 3 m. The vessel used Differential GPS to navigate
to the targeted location. Typical core lengths recovered using this method were between one and
two meters. The cores were subsequently logged, photographed, and sampled.
The vibracores taken by the USACE and CPE employed a 271B Alpine Pneumatic
Vibracore, configured to collect undisturbed cores 6 m (20 ft) in length. This self-contained, freestanding pneumatic vibracore unit consists of: an air-driven vibratory hammer assembly, an aluminum H-beam which acts as the vertical beam upright on the sea bottom, a steel coring pipe, a cutting edge, and a depth recorder for measuring penetration of the core pipe into the sea
bottom. An air hose array provides compressed air from the compressor on deck to drive the
vibracore.
Vibracore logging procedures varied between data sets. Terminology such as trace, little,
and some were used for the CPE vibracores and adopted into the description conducted for the
UNCW Coastal Geology vibracores and the USACE vibracores. These terms are specifically
defined in the Army Corps of Engineers Unified Soils Classification System to denote the
following (ACOE, 1985):
• Trace – 1% to 10% of a component such as silt, shell hash, shell fragments, etc.
21
• Little – 10% to 20% of a component such as silt, shell hash, shell fragments, etc.
• Some – 20% to 35% of a component such as silt, shell hash, shell fragments, etc.
Additional Methods
Due to lithologic variability of the units that occur throughout the study area it was necessary to develop a generalized stratigraphic framework to describe and define the units identified through interpretation of subbottom chirp data. These stratigraphic units and the unconformities observed as seismic reflectors are depicted in Figure 8.
The data collected throughout these surveys were used in the construction of a GIS
(Geographical Information System) using ArcGIS® 9.1 software. The GIS allows the data to be more easily accessed and displayed so that quantitative and qualitative analysis could be undertaken.
Several different sediment analyses were performed on chosen SCUBA diver retrieved samples and vibracore samples. Grain size analysis, through mechanical sieving in accordance with ASTM standards (1987), was conducted on vibracore samples collected by CPE and the
USACE. Table 1 lists the sieves used in the mechanical sieve analysis for CPE and Table 2 lists those used by the USACE. Wet sieving was performed on a representative portion of the samples in a manner consistent with Ingram (1971) to determine the percent fines in those respective samples. Insoluble residue analysis was performed on a select number of samples in a manner consistent with IRELAND (1971) to determine the carbonate fraction of the rock and sediment samples. These data assisted in the classification of sediment and rock units.
22
Figure 8. Generalized seismic stratigraphic column showing seismic sequences and boundaries.
23
Table 1.
Sieves used in the grain size analysis conducted by CPE on vibracore samples and native beach samples.
Sieve Seive Size Sieve Size Number (Phi) (Millimeters) 3/4 " ‐4.25 19.03 5/8 " ‐4.00 16.00
7/16 " ‐3.50 11.31 5/16 " ‐3.00 8.00 3.5 ‐2.50 5.66 4 ‐2.25 4.76 5 ‐2.00 4.00 7 ‐1.50 2.83 10 ‐1.00 2.00
14 ‐0.50 1.41 18 0.00 1.00 25 0.50 0.71 35 1.00 0.50 45 1.50 0.35
60 2.00 0.25 80 2.50 0.18 120 3.00 0.13 170 3.50 0.09 200 3.75 0.07
230 4.00 0.06
24
Rock samples from diver retrieved grab samples were sent off to Tulsa Sections of Coweta,
Oklahoma for thin-section production. These thin-sections were analyzed using a petrographic
microscope in order to identify lithostratigraphic and biostratigraphic characteristics. A number
of thin-sections where stained with Alizarin Red to differentiate between calcite and dolomite.
RESULTS
Analysis of seismic survey data, sidescan sonar data, vibracores, and diver ground truthing
data suggest the shoreface of North Topsail Beach and Surf City is a complex series of highly
weathered rock outcrops and unconsolidated Tertiary lithologic units, interspersed with incised
channels and covered by a variable veneer of Holocene sandy sediment. Data derived from
sidescan sonar surveys and ground truthing surveys allowed for the delineation of boundaries
between shoreface hardbottoms and adjacent sediment bodies. Vibracores provided samples that
allowed for the determination of unit lithology, color, percent silt, and percent calcium
carbonate. Seismic data coupled with vibracore data was used to map subsurface geologic units.
Compilation of these data allowed for the mapping, qualification, and quantification of
subsurface sediment units with a potential for yielding beach compatible sand.
Seafloor Characteristics
The first task in describing the offshore geology was to analyze the sidescan sonar data and
diver retrieved samples to identify the location and nature of rock outcrops. The broad range
scale (250 meter) used to collect the sidescan data provided a regional insight into the nature of the seafloor (Figure 7). Analyses of sidescan mosaics allowed for the digitization of boundaries
between hardbottom areas and areas of sediment accumulation.
25
A total of 106 diver-retrieved grab sample locations and 35 diver-retrieved vibracore locations
were occupied during this initial phase of the study to determine seafloor morphology (Figure 7).
Acquisition of morphologic and sediment thickness data provided a means to groundtruth
seismic profiles and assisted in the sidescan interpretation to depict hardbottom occurrence and
shell hash/sand interfaces throughout the study area. Figure 9 depicts the sidescan sonar mosaic
overlain with interpreted areas of hardbottom based on diver retrieved grab samples and
vibracores.
The sidescan and ground truthing surveys indicate the shoreface is characterized by thin (< 2
m), disjunctive, sheets of sand interspersed with undulating, relatively flat hardbottom platforms.
Scattered low relief scarps bound these flat hardbottoms in some locations. The thin sediment
cover is highly variable and includes sand with trace to little shell material, shelly sands, silty sand, mud, and muddy sand mixtures (Figure 10).
An examination of sidescan data also shows a number of shore-normal to shore-oblique bottom features on the shoreface. The features ranged in length from 1.5-5.0 km and extend
from just outside the surf zone onto the inner shelf (Figure 11). Following ground truthing, the
features were confirmed to be similar to the sorted bedforms identified by THEILER, et al. (1995 and 1998), off Wrightsville Beach, North Carolina, who originally referred to the features as rippled scour depressions. Recent research conducted by GUTIERREZ et al. (2005) refers to the
features as sorted bedforms. These sorted bedforms exhibit a concave cross section floored with
rippled coarse shell and lithoclastic gravel. The adjacent areas between the sorted bedforms exhibit a raised topography that appears as ridges in bathymetric cross sections of the area.
These ridges are composed of fine quartz sand and silt with varying amounts of shell hash.
Aside from the coarse shell and lithoclastic gravel found in the sorted bedforms, patches of
26
Figure 9. Map showing sidescan sonargraph with digitized areas of interpreted and ground truthed hardbottom outcrops. Note the locations of ground truthing surveys.
27
Figure 10. Map showing sidescan sonargraph overlaid with Surficial sediment types based on UNCW diver grab samples and UNCW diver retrieved vibracores. Note the locations of ground truthing surveys.
28
Figure 11. Map depicting sidescan sonar mosaic exhibiting sorted bedforms.
29 similar material surround shoreface limestone outcrops, which are the sources of this gravel.
Samples of the gravel contain lithoclasts with the same mineral composition as well as the same
molds and bioclasts as those that compose nearby hardbottoms. This suggests that physical and
chemical weathering is taking place through bio-erosion and wave quarrying, eroding the
submarine outcrops and adding sediment to the system.
A second phase of mapping surficial sediment was undertaken with the addition of the
USACE vibracores in 2004. Using the USACE vibracores as another tool for ground truthing,
the surficial sediment cover map shown in Figure 10 was enhanced. This map shown in Figure
12 depicts, in greater detail, the sorted bedforms discussed previously. The map also shows the
areas of shelly sand and gravel skirting the edges of many of the large-scale limestone outcrops.
Mobile pockets of fluidized, black to olive gray mud are scattered throughout the survey area. Because of poor visibility in the areas of the fluidized mud, it was difficult to characterize the underlying bottom types. Divers reported observing the mud over hardbottom and sandy areas alike. It is likely that the mud is restricted to topographic lows and may be derived from the reworking of exposed mud-filled channels and depressions, as well as new, fine material that
is being discharged from the New River through New River Inlet. On several occasion re-
occupation of a ground truthing site found fluidized mud covering the bottom where it had not
previously been observed and vise versa.
Most rock outcrops observed within the study area are composed of light gray to yellowish-
brown, sandy, moldic limestone (Figure 13). The majority of molds are after a variety of
pelecypods and gastropods. The low-relief hardbottom scarps observed by divers, appear to be
landward facing features ranging from 0.3 to 1.1 m (1 to 3.5 ft). Information from HDR
ENGINEERING INC. OF THE CAROLINAS (2002 b) support these findings, stating that “shore-normal
30
Figure 12. Map showing sidescan sonargraph overlaid with Surficial sediment types based on UNCW diver grab samples, UNCW diver retrieved vibracores, and USACE vibracores. Note the locations of ground truthing surveys and vibracores.
31
MOLDS
MOLDS
Figure 13. Sample of submarine rock outcrop interpreted to be of the upper Oligocene River Bend Formation. Note the abundant molds. Locations of these samples are shown in Figure 15.
32
fathometer profiles that were collected rarely exhibit more than 1 m (3.3 ft) relief at the locations
of the identified landward facing scarps.” The exposed seaward slope of the limestone outcrops
are hummocky with numerous irregular fractures and shallow depressions. These hardbottom
areas were observed supporting a diversity of life including encrusting corals, macro algae,
sponges and a variety of fish, crustaceans, bivalves, and gastropods all of which are actively
contributing to bio-erosion of the unit (Figure 14). This bio-erosion as well as the physical erosion of surge and wave energy on the bottom result in the adjacent surface gravel and shelly sand discussed previously.
In one particular location off Surf City (SDA), a very fine grained, dark olive gray, sandy, poorly consolidated, siltstone unit was observed at the base of a limestone scarp. This is interpreted to be the contact between the upper Oligocene River Bend Formation and the lower
Oligocene River Bend Formation (Figure 15). This unit is similar in composition and appearance to that mapped by MCQUARRIE (1998) as the Oligocene River Bend Formation off
Topsail Beach. Though only one outcrop of this lithology was observed a number of vibracores
indicate that the unit is widespread over the southwest portion of the study area.
Subsurface Analysis
Vibracores
Analysis of the suite of thirty-five (35) diver-retrieved vibracores (UNCW), one hundred fifty (150) alpine vibracores (USACE) as well as observations from diver surface sample surveys indicates that most of the Holocene shoreface sediment sequences are thin and consist of fine to very fine quartz sand interbedded with sandy gravel, muddy gravel, muddy sand, and gravelly sand. Thickness of the modern sediment across the entire study area, seaward of the active
33
Figure 14. Examples of hardbottom scarp outcrops interpreted to be of the upper Oligocene River Bend Formation. Note arrows indicate (A): sponge growing on top of rock ledge; (B) gravel talus at the base of the scarp; (C): macro algae species growing on top of Ledge; (D): gravel talus at the base of the scarp.
34
Figure 15: Sidescan mosaic on which the location of Ground Truthing Sites “SDA” and “Top 6” are superimposed.
35
active beach, ranges from less than one centimeter (0.5 in) in hardbottom areas to about one m
(3.3 ft) in intervening regions. Eight lithologic sequences, greater than 2 m (6.6 ft), were observed in the vibracores. Two of the units are interpreted to be Pleistocene age sedimentary units; whereas, six units are interpreted to be Oligocene. Figure 16 shows the location of a series of shore-parallel and shore-normal vibracore transects depicting the variability of sediment types and thickness of units comprising the shoreface sediment sequence (Figures 17-24). An inspection of the cross-sections show the variability in the different sediment sequences.
One of the two Pleistocene age sediment units consist of mud-rich and clay-rich back-barrier sequences. Often these sequences occur in the vibracores as gradational sequences of sand grading into muddy sand, grading into alternating layers of sand and clay or grading into consolidated/partially lithified silty clay with trace sand, and trace whole oyster shells. The presence of abundant oyster shells in situ in these units confirms a back-barrier depositional environment. Figure 25 shows a digital photo mosaic of vibracore VC-31 which depicts this back-barrier mud-rich and clay-rich sediment unit. Sequences of these back-barrier deposits in excess of 2 m are diagnostically depicted in Figures 18, 22, and 23. Figure 26 shows the locations of areas in which local deposits of the back-barrier sequences may occur based on the vibracore and cross section data.
The second unit interpreted to be Pleistocene age consists of fine sand with trace shell hash and trace silt. In shore parallel cross sections A – A’, C – C’, and D – D’ (Figures 17, 19, 20), local, discontinuous deposits of this sandy unit were found in excess of 2 m. The sand deposits occur in channel-like features or depressions incised into the underlying units. Though the sand units occur in several vibracores throughout the survey area, the nature of the deposits within isolated channel-like features and depressions, coupled with the relatively wide
36
. Figure 16: Location map of eight cross sections constructed for this study. Note the location of three seismic tracklines for which seismographs are shown in figures 16 through 23. Note the location of spotdive location SC-105.
37
Figure 17: Cross Section Diagram of Transect A-A’. Note the sediment depositional basin infilled with sandy material between cores TI03-VC-328 and TI03-VC-311. Note Limestone section between VC-42 and TI03-VC -330 is based on Sidescan Interpretations.
38
Figure 18: Cross Section Diagram of Transect B-B’. Note the sediment depositional basin infilled with sand over muddy sand material between cores 9(29) and TI03-VC-317.
39
Figure 19: Cross Section Diagram of Transect C-C’. Note the sediment depositional basin infilled with sandy material between core TI03-VC-260 and spot dive 140 as well as between cores TI03-VC-45 and TI03-VC-43.
40
Figure 20: Cross Section Diagram of Transect D-D’. Note the sediment depositional basin infilled with sandy material between cores TI03-VC-279 and TI03-VC-92A as well as the depositional basin infilled with sand over muddy sand material between cores TI03-VC-88 and TI03-VC-70.
41
Figure 21: Cross Section Diagram of Transect E-E’. Note the sediment depositional basin infilled with sand over muddy sand material between cores TI03-VC-262 and TI03-VC-260.
42
Figure 22: Cross Section Diagram of Transect F-F’. Note the sediment depositional basin infilled with sand over muddy sand material between cores TI03-VC-329 and TI03-VC-339.
43
Figure 23: Cross Section Diagram of Transect G-G’. Note the sediment depositional basin infilled with sand over muddy sand material between cores TI03-VC-327 and TI03-VC-325 as well as the depositional basin infilled with sandy material between cores TI03-VC-85 and TI03- VC-69.
44
Figure 24: Cross Section Diagram of Transect H-H’.
45
TOP OF CORE
BOTTOM OF CORE
Figure 25. Photo-mosaic of vibracore VC-31 showing a gradational transition from fine sand, into muddy sand , into alternating layers of sand and clay into consolidated/partially lithified, silty clay with trace sand, and trace whole oyster shells.
46
Figure 26: Map depicting sediment filled depressions based on analysis of cross sections and surrounding vibracore data.
47 spacing of vibracores throughout the survey area (usually in excess of 1.5 km); make it difficult
to accurately map the sand deposits. These sequences are stratigraphically lower than the
Pleistocene mud and clay back-barrier sequences where both units occur in the same vibracore,
indicating the back-barrier sequences are younger than the sand. Figure 26 shows the locations
of areas in which local sand deposits occur based on the interpretation of vibracore and cross
section data. These deposits are the types of features being targeted in the re-evaluation of
sediment for beach nourishment projects.
Six additional lithologic units identified through grab samples and vibracores and depicted in
the cross sections are interpreted to be Oligocene in age based on correlation of the units with
those described by Johnston (1998) and McQuarrie (1998) to the northeast and southwest
respectively. The consolidated limestone unit observed in the grab samples and vibracores was a
light gray to yellowish brown bio-moldic, sandy limestone. The other five units observed were
generally unconsolidated units; however, many vibracores contained portions of these units that
were partially lithified. The units were: a light olive gray to gray silty sand; an olive gray to
olive, very fine sand with little (10% - 20%) to some (20% - 35%) silt, and little (10% – 20%)
shell hash; a light olive gray to olive gray, very fine sand, with little (10% - 20%) to some (20%
- 35%) silt, trace (1% - 10%) shell hash, and often contains echinoid spines; a unit that was generically described as an olive green siltstone, and a highly weathered, silty, sandy, limestone.
Based on the work of JOHNSTON (1998) and MCQUARRIE (1998) which bound the study area
to the northeast and southwest respectively, strata in this area is interpreted to be early to late
Oligocene. According to HARRIS (2007, Personal Communication) there are locally multiple facies of both the upper and lower Oligocene River Bend Formations. Accurate identification of these stratigraphic units based on sediment and mineralogical data obtained in this study is not
48
possible. For this reason informal nomenclature has been assigned to the stratigraphic units identified in this study. Based on subbottom profile data discussed in the following section, it is possible to determine relative age of these units based on superposition, and dip angles of seismic reflectors.
The oldest unit in the vibracores is the unit generically described as an olive green siltstone.
Due to the variability of data sources, not all vibracores were available for examination by the author. This resulted in a number of units within vibracores being described as an olive green siltstone. In actuality, the color of the unit is dark olive gray (5Y 3/2) according to the Munsell
Soil Color Charts (1994). Based on grain size the unit is not a siltstone but rather a fine to very fine sandstone. Grain size analysis and measurements made during thin section analysis of the unit yielded a mean grain size of 0.13 mm. A mean grain size of 0.13 mm indicates that the unit is not a siltstone but a fine grain sandstone.
There is some debate as to whether the dark olive gray fine to very fine grain sandstone can be correlated to the Oligocene River Bend Formation that crops out on the shoreface from
FigureEight Island south to Fort Fisher. This unit is similar to the unit described by MCQUARRIE
(1998) off Topsail Beach as Middle Oligocene. Figure 27 shows an example of the dark olive
gray, very fine sandstone in thin section. The unit is composed of sub-angular quartz grains and is classified, according to FOLK et al. (1970) as a quartzarenite . Data from insoluble residue analysis for eight samples taken from vibracores in this area that contained the dark olive gray sandstone indicate that the calcium carbonate fraction averaged 13.5 percent by weight (Table 2).
Modal analyses of thin-sections from siltstone samples collected from the Kure Beach shoreface suggest a 17 percent carbonate constituent within the dolomite-rich sample (MARCY
AND CLEARY, 1997). Comparable calcium carbonate fractions were determined for the siltstone
49
Figure 27. Photomicrograph of sample TI03-VC-274 (210 cm below top of core) from the dark olive gray sandstone.
50
Table 2.
Results of insoluble residue analysis.
Percent Carbonate By Weight
NUMBER MINIMUM MAXIMUM STANDARD UNIT DESCRIPTION OF MEDIAN MEAN VALUE VALUE DEVIATION SAMPLES
Sandy Mud, Muddy Sand Mixture 4 3.1 27.8 15.4 11.4 11.2
Highly Weathered, Silty, Sandy, Limestone 7 7.8 34.8 21.3 22.9 10.8
Dark Olive Gray Sandstone TI-B 8 2.9 34.8 18.9 13.5 9.9 Olive Gray to Olive, Very Fine Sand, With Little (10% - 20%) to Some (20% - 6 10.8 56.8 33.8 30.2 16.6 35%) Silt And Little Shell Hash (10% - 20%) TI-D Light Olive Gray to Olive Gray, Very Fine Sand, With Little (10% - 20%) to Some 4 25.9 43.5 34.7 30.8 8.5 (20% - 35%) Silt And Trace Shell (1% - 10%) TI-E
51
observed on the Kure Beach shoreface and the dark olive gray sandstone sampled in this study.
However, only trace amounts of dolomite were identified in thin sections from the unit off of
Topsail Island.
Differences in mineralogy and general lithologic characteristics among samples from the
sandy unit through the course of this study off Topsail Island, are a testament to the complexity
of the units interpreted to be Oligocene in age within Onslow Bay. Although several generalized
descriptions of upper and lower Oligocene units have been developed over the last thirty years
there exists a complexity of different sub groups and facies changes throughout the unit that
make identification of these units based on lithologic characteristics alone nearly impossible
(HARRIS, 2007, Personal Correspondence). Regional facies differences may be attributed to
paleo-environmental and geomorphologic differences.
Regardless of the discrepancies between the facies, the poorly consolidated to unconsolidated
sandy unit appears to significantly contribute to the surficial sediment of the area. The quartz
rich nature of the unit adds significant amounts of fine sand to the surrounding area. The
preferential weathering of this unit due to its lack of cementation and fine to very fine grain size
size explains the scarcity of outcrops on the seafloor in comparison to the moldic limestone
facies interpreted as the upper Oligocene River Bend Formation, which comprises the majority
of rock outcrops on the shoreface.
The dark olive gray, very fine sandstone unit is depicted in cross section A-A’ (Figure 17) in
vibracore TI03-VC-330 (labeled as Olive Green Siltstone (Generically Described)) underlying a
moldic, sandy limestone unit. A significant portion of the very fine sand found in surface
sediment samples collected on the shoreface is similar in grain size and physical characteristics to the dark olive gray siltstone. This further supports the notion that the unit is contributing to
52 the sediment budget of the region through the periodic exposure and subsequent physical and biological erosion of the unit interpreted as lower Oligocene. Although other cores within the cross sections were field described as olive green siltstone (TI03-VC-313 in Figure 17, TI03-VC-
159 in Figure 18, and TI03-VC-83B in Figure 19), it is the position of this study that these units are part of younger lithologic units.
The second oldest unit, a sandy-moldic limestone, makes up the majority of the hardbottom in the study area. The unit occurs in cores as tan to olive gray fragments of sandy moldic limestone, with trace shell hash (1% - 10%), in a carbonate mud matrix. Diver bottom surveys and grab samples at both low relief (< 0.5 meters) flat hardbottom locations and higher relief ( >
0.5 meters) scarp locations yielded rock samples described as a yellowish brown, to olive-gray, to tan, sandy bio-moldic limestone. Most of the moldic pore-space in the rock resulted from the leaching of aragonitic shell material of pelecypods and gastropods. This unit is depicted in A-
A’, B-B’, C-C’, E-E’, and G-G’ (Figures: 17, 18, 19, 21, and 23).
Table 3 contains data from a suite of thin sections from representative samples of the moldic limestone unit. Figure 28 depicts a photomicrograph of a thin section cut from a sample collected at spotdive location SC-105, an exposure of the sandy-moldic limestone (See Figure 16 for location). Quartz grains are comparable in size and roundness to those found in the dark olive gray sandstone. Large bio-moldic pore spaces can be seen as well as micrite cement.
Varying percentages of microcrystalline calcite, equant calcite, quartz grains, moldic porosity, and allochems were determined from modal analyses of the samples. Samples SC-85, SC-89, and SC-105 have statistically similar percentages of compositional elements. Likewise, when compared to samples analyzed by JOHNSTON (1998) as Belgrade Formation (Table 4), these three
53
Table 3.
Percentage of major compositional elements of thin section modal analysis for selected samples inside the study area.
Percent of Major Compositional Elements Microcrystalline Equant Interparticle Moldic Sample Phosphorite Quartz Allochems Other Calcite Calcite Porosity Porosity
SC-85 2.6 2.3 28.5 7.2 15.1 0.3 42.3 1.7 SC-89 2 39.4 0.7 23.5 3.4 1.7 27.2 2.1 SC-105 3.7 25.1 10.8 23.8 6.7 0.3 25.4 4.2 SDA 16.7 11.9 15 29 2.3 1 22.4 1.7 TI-V-81 112 cm 25.9 31.1 0 21.7 2.8 1.9 12.6 4 AVERAGE 10.2 22.0 11.0 21.0 6.1 1.0 26.0 2.7 STANDARD 10.7 14.9 11.7 8.2 5.3 0.8 10.7 1.3 DEVIATION
54
Figure 28. Photomicrograph of a thin-section from sample SC-105 which was a bio-moldic limestone. Note A) moldic pore space; B) echinoid spine bioclast; C) micrite cement.
55
Table 4:
Percentage of major compositional elements of thin section modal analysis after JOHNSTON (1998).
Percent of Major Compositional Elements Microcrystalline Interparticle Moldic Sample Equant Calcite Phosphorite Quartz Allochems Other Calcite Porosity Porosity A01 0.0 36.0 2.3 26.0 5.0 1.3 24.3 5.1 A02 2.3 34.0 7.0 29.3 5.0 3.0 19.0 0.4 A03 2.0 24.0 1.0 29.6 11.0 5.0 24.3 3.1 A04 0.3 2.3 23.0 16.3 17.0 13.0 22.0 6.1 A05 11.0 22.3 10.6 19.0 9.0 1.3 21.6 5.2 A06 1.0 4.3 43.3 16.6 9.3 2.3 22.6 0.6 A07 8.3 13.0 10.3 19.3 31.6 0.3 15.1 2.1 A08 1.0 20.0 31.6 25.0 1.0 2.0 17.1 2.3 A09 1.0 26.0 7.6 36.6 10.3 4.6 10.0 3.9 A10 0.0 16.6 32.3 25.1 9.3 4.0 6.6 6.1 A11 0.0 31.3 3.0 37.3 5.6 1.0 17.0 4.8 A12 0.0 12.0 36.0 7.6 14.0 0.3 30.0 0.1 A13 0.0 10.0 0.6 35.6 5.6 2.6 33.6 12.0 A14 2.3 38.0 0.0 23.6 1.0 0.1 32.7 2.3 AVERAGE 2.1 20.7 14.9 24.8 9.6 2.9 21.1 3.9
56
samples are comparable in compositional elements. The other two samples in Table 3 (SDA and
TI-V-81 112 cm) are quite dissimilar in percent compositional elements once again owing to the
complexity of the shoreface units.
Three of the additional units are very similar and were only distinguished by slight color and shell content characteristics. These units were described as an olive gray to olive, very fine sand with little (10% - 20%) to some (20% - 35%) silt, and little (10% – 20%) shell hash; a light olive gray to gray silty sand; and a light olive gray to olive gray, very fine sand, with little (10% -
20%) to some (20% - 35%) silt, and trace shell (1% - 10%), often containing echinoid spines.
These units are most often depicted in the cross sectional diagrams as discontinuous (Figures 17
-24). As stated previously, it is documented that there exist a number of facies of both the upper and lower Oligocene units in the region (HARRIS, 2007, Personal Communication) and thus these
three lithologic units are interpreted as facies changes among the upper and lower Oligocene.
The eighth and youngest of the units interpreted to be Oligocene in age and described in the
vibracore samples is a highly weathered, silty, sandy, limestone. This unit was observed mostly
in vibracore samples taken in the northeastern portion of the study area and was more prevalent
in samples collected farther offshore. JOHNSTON (1998) and HDR (2003) described Oligocene
age sandy limestones in the vicinity of New River Inlet and assigned them to the Belgrade
Formation. The samples collected for this study described as a sandy, silty, limestone also are
consistent with the description of the Belgrade Formation found to the north (JOHNSTON, 1998)
as was the case for the moldic limestone previously described. The samples of this unit obtained
from vibracores are understandably siltier and sandier than samples of the same unit found at
submarine outcrops because of the vibracore process. Based on more recent updates to the
stratigraphic nomenclature of the region the Belgrade Formation is Miocene in age (HARRIS et
57
al., 2000). Although the highly weathered, silty, sandy limestone described in this study is
similar to the unit described by JOHNSTON (1998) and HDR (2003) as the Belgrade Formation
(Miocene), it is interpreted here to be part of the Upper River Bend Formation (Oligocene). In cross section diagrams C-C’ and D-D’ (Figures 19 and 20), a moldic limestone unit occurs to the northeast and stratigraphically above the sandy silty, limestone. This is interpreted as a facies change within the Upper River Bend.
Insoluble residue analysis was conducted on 29 samples collected from the vibracores. Table
2 shows the results of these analyses. Four of the 29 samples were collected from the
Pleistocene back-barrier mud-filled channel units. These samples had a mean carbonate weight percentage of 11.4. Seven samples were collected from the sandy, silty, limestone interpreted as upper Oligocene. These samples yielded a mean carbonate weight percentage of
22.9. As previously stated, the dark olive gray sandstone unit yielded a mean carbonate weight
percentage of 13.5. A value of 30.2 weight percent carbonate was determined for the olive gray
to olive, very fine sand with little (10% - 20%) to some (20% - 35%) silt, and little (10% – 20%)
shell hash. A value of 30.8 weight percent carbonate was determined for the light olive gray to
olive gray, very fine sand, with little (10% - 20%) to some (20% - 35%) silt, and trace shell (1%
- 10%), often containing echinoid spines.
Chirp Seismic Analysis
Three tracklines of Chirp seismic data collected by the USACE in 2004 (Line 2 Day 2 and
Day 3, Line 3 Day 3, and Line 53 All) cross the study area in the shore parallel direction (Figure
16). The data collected along these tracklines provide a seismic stratigraphic framework of the
study area depicted in Figure 8. Figures 29 through 36 are seismographs of the chirp data
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Figure 29. Seismograph of Line 3 Day 3 with superimposed vibracore logs. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar.
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Figure 30. Seismograph of Line 3 Day 3 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar.
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Figure 31. Sismograph of Line 53 with superimposed vibracore logs. Note the locations of hardbottom outcrops identified with the use of sidescan sonar.
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Figure 32. Seismograph of Line 53 All with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar.
62
Figure 33. Sismograph of Line 2 Day 3 with superimposed vibracore logs. Note the locations of hardbottom outcrops identified with the use of sidescan sonar. Note also the location of a solution collapse feature.
63
Figure 34. Seismograph of Line 2 Day 3 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar. Note also the location of a solution collapse feature.
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Figure 35. Sismograph of Line 2 Day 2 with superimposed vibracore logs. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar.
65
Figure 36. Seismograph of Line 2 Day 2 with superimposed vibracore logs, digitized reflectors, and shaded interpretations of lithologic units. Note the locations of hardbottom outcrops that were identified with the use of sidescan sonar.
66 collected along these three tracks. Figure 33 and 34 show the southwest portion of the seaward-
most line (Line 2 Day 3) while figures 35 and 36 show the northeast portion (Line 2 Day 2).
Three unconformities designated α, β, and γ can be identified on the three lines and are considered regional unconformities separating mapable seismic units (TI-A, TI-B, TI-C, TI-D,
TI-E, and TI-F (TI designation refers to Topsail Island)). Vibracores taken along the tracklines were superimposed onto the images to allow for correlation (Figures 29 – 36). Likewise, rock samples and diver observations from seafloor outcrops as well as sidescan sonar interpretations were used in correlating the units.
Line 3 Day 3. Seismic Line 3 Day 3, shown in figures 29 and 30, is the most landward of the USACE tracklines (Figure 16). None of the vibracores collected along seismic lines and examined in this study, penetrated γ nor recovered material from TI-A (Figures 30, 32, and 34); therefore, no assumptions are made as to the lithology of this unit. Vibracore TI03-VC-265 is the only vibracore on Line 3 Day 3 to penetrate and recover material from TI-B (Figure 30). The material recovered from TI-B is the unit described herein as a dark olive gray unconsolidated sand to partially consolidated sandstone. This material, as noted previously, is thought to be
Lower Oligocene making it part of the Rupelian stage Lower River Bend Formation of HARRIS
et al. 2000. The samples are comparable to the unit described by MCQUARRIE (1988), off
Topsail Beach and Lea / Coke Island as the middle Oligocene River Bend.
Moving from southwest to northeast and stratigraphically upward along the line (left to right
across the page), unit TI-D crops out across nearly half of the study area. Two diver grab sample
locations [9(29) and 119] occur along the section of the seismic track where the unit crops out
(Figure 30). Observations at both sites indicate the presence of rock outcrops on the seafloor. A
flat to low relief rocky bottom was observed at location 119 covered by as much as 70 cm of
67 sand. Samples collected from site 9(29) reveal a moldic limestone similar to the one shown in figure 13. Three vibracores along this seismic trackline penetrate into unit TI-D (TI03-VC-349,
TI03-VC-348, and TI03-VC-338). The material observed in vibracores that correspond to TI-D are described as light olive gray to olive, very fine sand with little (10% - 20%) to some (20% -
35%) silt, and little (10% - 20%) shell hash, becoming slightly lithified at the base.
TI-E, the unit above, is separated from TI-D by unconformity β. This unit appears to crop out on the seafloor along 40 percent of the trackline. Several mud and clay filled channels are incised into TI-E, suggesting TI-E is less resistant or more easily erodable than TI-D. This seismograph illustrates the discontinuous nature of the incised channels compared to the portrayal of the mud and clay filled channel extent in the cross section (Figure 18). Sediment observed within vibracores TI03-VC-326 and TI03-VC-318 that corresponds to TI-E is light olive gray to gray, silty sand with trace (1 – 10%) shell hash. Sediment observed in vibracore
TI03-VC-317 that corresponds to TI-E is light olive gray to olive gray fine sand with little (10%
- 20%) to some (20% - 35%) silt, trace (1 – 10%) shell hash, echinoid spines, and is partially lithified locally.
Line 53. Line 53 (Figure 31 - 32) is located approximately 4.8 m (3.0 mi) offshore of
Topsail Island and crosses only about half of the survey area (Figure 16). Two vibracores (TI03-
VC-262 and TI03-VC-271) along the southwest end of the seismic line recovered material from unit TI-B. Vibracore TI03-VC-262, shown at the southwest end of the seismograph (left side of page), recovered 2.97 m (9.7 ft) of material from TI-B. The material differed from material observed from unit TI-B in Line 3 (Day 3) because it contained a calcareous, partially lithified, limestone. Vibracore TI03-VC-271, located 0.63 km (0.39 mi) to the northeast, penetrates through a local unconformity representing the base of an incised, mud and clay filled channel.
68
Below this local unconformity, the vibracore contains material described as a dark olive gray
unconsolidated sand to partially consolidated sandstone. This material is consistent with that recovered from unit TI-B in vibracoere TI03-VC-265 along Line 3.
Moving stratigraphically upward along the line to the northeast, TI-D outcrops on the
seafloor. As was the case in Line 3 Day 3, there are two occurrences of hardbottoms observed
on the seafloor in the section where TI-D outcrops (Figures 31 and 32). Vibracores TI03-VC-
350 and VC-39 contain sediment similar to that collected from vibracores that recovered material
from TI-D along Line 3 Day 3. A local unconformity labeled τ can be seen in both Line 53
(Figure 33) and Line 2 (Figure 34 and 36), as the upper boundary of TI-C. None of the
vibracores examined in this study recovered material from unit TI-C; hence, the lithology is
unknown.
Unconformity β, which represents the base of TI-E, intersects the seafloor in the vicinity of
VC-39 (Figures 31 and 32). Vibracores TI03-VC-347 and TI03-VC-340 contain light olive gray
to olive gray very fine sand with little (10% - 20%) to some (20% - 35%) silt, trace (1 – 10%)
shell hash, echinoid spines, and is partially lithified locally. This lithology is consistent with the
description for TI-E from Line 3 Day 3 and thus supports the correlation of the units over the
study area.
Line 2. Due to the naming convention used by the USACE’s subcontractor, Line 2 is split
into two different sections, Day 2 and Day 3. Line 2 Day 3 (Figures 33 - 34) represents the
southwestern half of Line 2. Unlike the previously discussed seismic tracklines, Line 2 does not
show unconformity α intersecting the seafloor surface. Two vibracores were recovered along
Line 2 slightly southwest of where α appears to terminate. TI03-VC-260 was taken within an
incised channel denoted by the local unconformity. TI03-VC-273 contains 375 cm (12.3 ft) of
69
fine grained sand with trace (1 – 10%) shell hash. The occurrence of both an incised channel and
a thick deposit of high quality sand suggests the area is a depression that was incised into TI-B
and TI-D and filled with Pleistocene sand. This sequence of a channel being incised and subsequently infilled with Pleistocene sand explains the truncation of unconformity α in this
particular seismograph. The mud and clay filled channel may be a later depositional event.
A subsurface topographic low can be seen in the seismograph of Line 2 Day 3 between the
two areas interpreted as hardbottom (Figures 33 and 34). The parallel-wavy nature of the
reflectors in this location indicate that the origin of this feature is younger in age than TI-E. This
feature may represent a deep-seated karst event. It is noteworthy that the two areas mapped as
hardbottom outcrops are composed of material from TI-D, which is characteristic throughout the
central to southwest portion of the study area. Only one vibracore (TI03-VC-341) recovered
material from unit TI-D along Line 2 (Figure 34). The material recovered was comparable to the
sediment recovered from TI-D in Lines 3 and 53, which included light olive gray to olive, very
fine sand, with little (10% - 20%) to some (20% - 35%) silt, little (10 – 20 %) shell hash, and
partially lithified at base. This partial lithification refers to a gradational change in the
consolidation of the sediment with depth in specific vibracores.
In all three seismic lines, vibracore data suggests that there is at least one and possibly two
units that overly TI-E with in the survey area. Vibracores TI03-VC-158 (Figure 30), TI03-VC-
324 (Figure 32), and TI03-VC-85 (Figure 36) contain material descried as a highly weathered,
light gray, silty, sandy, limestone that is partially lithified. Figure 31 shows a distinct reflector
that separates this unit from the rest of TI-E. Data for the other two lines do not show a reflector
that would separate this unit from TI-E. Because the seismic data does not support the
identification of an additional unit, no such unit is shown on the seismographs; however, it is
70
noted that such a unit may exist. Similarly, vibracores TI03-VC-159 (Figure 30) and TI03-VC-
83B (Figure 36) contain material that appears to be part of a separate unit. This material was described as an olive green siltstone. This transition although depicted in vibracores along two different lines is not evident in the seismic record and therefore is not represented as such. Due to extensive hardbottoms along the northeast end of Line 2 Day 2 (Figure 36) there is little vibracore data to correlate any of the non-differentiated unconformities observed in the seismic data. Therefore, no attempt at differentiating the units observed in the seismographs in this portion of the survey area was made.
DISCUSSION
The profusion of data available within the study area is uncommon for a shoreface environment. This abundance of data is a testament to the importance of finding offshore sources of beach quality material for beach nourishment projects. The collection of reconnaissance geophysical and geotechnical data undertaken by UNCW and the USACE allows for the unraveling of the complex geologic nature of the North Topsail Beach and Surf City shoreface. Key to understanding the nature of the shoreface is a regional knowledge of the geologic nature of Onslow Bay. Like many high-energy shelves on passive continental margins,
Onlsow Bay is a sediment starved coastal system (MILLIMAN, 1972; MILLIMAN et al. 1972; and
TUCHOLKE, 1987). Very little fluvial input exists within the system as few tidal inlets are
directly hydrologically connected to rivers that drain the coastal plain. Flow velocities are not
sufficient to carry sand sized sediment through the extensive back-barrier tidal flats and
marshlands that trap the majority of what little fluvial sediment enters into the intertidal system.
71
The cape system of North Carolina further exacerbates the depletion of sediment within Onlsow
Bay as Cape Fear and Cape Lookout preclude sand bypassing between adjacent embayments.
These factors are superimposed onto the Coastal Plain Province of the South Atlantic Bight.
The Coastal Plain Province is characterized by a series of depositional basins separated by
topographic highs (HARRIS and LAWS, 1997 and HARRIS et al. 2000). In North Carolina the
Albemarle Embayment is the major depositional basin with the Norfolk Arch bordering it to the
North and the Cape Fear Arch bordering it to the South. With the influence of the Cape Fear
Arch to the Southwest and the Albemarle Embayment to the northeast the Mesozoic to Cenozoic sediments that make up the coastal plain units inherently dip to the northeast as demonstrated by seismographs herein. Units identified in this study are interpreted to be Early to Late Oligocene in age. This is consistent with previous work from the vicinity, both onshore and offshore
(CLEARY et al., 1996; HARRIS et al., 2000; JOHNSTON, 1998; MCQUARRIE, 1998; and WILLSON and CLEARY, 2003).
Given the underlying geological control of the region and the factors contributing to the
characterization of the region as sand starved, no significant deposits of Holocene material exist
within the study area. A thin veneer of Holocene sediment blankets much of the study area,
equating to a sizeable volume of material. However, no significant deposits of Holocene
material have been identified with a thickness such that it would provide an economically feasible deposit to mine for beach nourishment. The sand fraction of the Holocene surficial sand
is comparable in mean grain size, color, mineralogy, rounding and other physical characteristics
to the sand fraction in the consolidated limestone and siltstone outcrops on the shoreface as well
as the sand fraction in the unconsolidated sub-surface units. Shell hash and lithic gravel deposits
observed at the base of many of the limestone outcrops were shown to be derived from the
72
adjacent outcrops based on near identical bioclasts and lithoclasts. Similar bioclasts and
lithoclasts were observed in many of the surface samples collected over 1 km away from any
outcrops. This suggests that not only is new material being derived from the weathering of rock
outcrops and unconsolidated sub-surface units, but the material is being transported and mixed with other sediment throughout the study area.
Oligocene lithoclasts and fragments of the extinct fossil oyster Crassostrea Gigantismus
occur in great abundance on Onslow Beach and Topsail Island following storms (CROWSON
1980; CLEARY and HOSIER 1987; CLEARY et al. 1996). These lithoclasts and bioclasts are
derived from the bioerosion and physical weathering of rock outcrops on the inner shelf and are
subsequently transported onshore during high energy storms (CLEARY et al. 1996). Despite the
evidence that significant quantities of sediment are being derived from the offshore units and that
some of the material migrates landward during storm events, it is not feasible that a significant
portion of the newly derived sediment is deposited on the beach to stem the 0.6 to 1.1 m/yr (2.0 –
3.5 ft/yr) erosion rate for North Topsail Beach and Surf City (NC DCM, 2007).
The options for communities seeking a source of beach quality sand for beach nourishment
projects are limited to the underlying geologic units. Recently the state of North Carolina (NC
DCM) has adopted a set of Technical Standards for Beach Fill Projects that must be adhered to
for placement of fill material on beaches in North Carolina (15A NCAC 07H .031). These
standards require extensive testing of both fill material and native beach material so that
compatibility can be determined. Among the parameters that help determine compatibility by
the North Carolina Technical Standards are percent fines (< 0.0625 mm) and percent by weight
calcium carbonate. For fill to be deemed compatible by the Technical Standards the percent
fines (< 0.0625 mm) must not exceed the native value + 5 percent. Likewise, carbonate percent
73
by weight for fill may not exceed the native value + 15 percent. Results from a recent study
conducted by Coastal Planning & Engineering, Inc. for the Town of North Topsail Beach
indicated that the native beach at North Topsail contains 1.5 percent fines and 25.8 percent
carbonate by weight (FINKL et al. 2007).
A number of lithologic units, described in previous sections were assessed for their potential
as a source for beach nourishment. The upper most unit identified in this study, thought to be
Oligocene in age, is a highly weathered, light gray, calcareous, quartz sand that is locally
lithified. This unit shows little potential as a mineable source of beach quality sand. High silt
percentages, high percentages of carbonate and the fact that the unit is partially to well
consolidated in most of the samples recovered discount the unit as a targeted sand source.
Clearly the unit is not a mineable source for beach quality sand; however, when exposed on the
seafloor surface the unit allows for the process described by SWIFT (1976) as shoreface
bypassing to occur. This process of regenerating sediment through the mechanical, chemical, and biological erosion of the unit provides additional sediment to the system. This sediment,
when winnowed to its quartz sand composite, is compatible with modern day beach sand.
The results of the vibracore and chirp seismic analyses suggest there are three partially
unconsolidated, sandy, units interpreted to be Oligocene in age that underlie and crop out on the seafloor in the study area, TI-B, TI-D and TI-E (Figures 30, 32, 34, and 36). TI-E was described as both a light olive gray to gray silty sand and a light olive gray to olive gray, very fine sand, with little (10% - 20%) to some (20% - 35%) silt, trace shell (1% - 10%), often containing echinoid spines. Table 4 indicates the unit, on average contains 30.8 percent carbonate by weight. The average mean grain size for 10 samples taken from this section was 0.18 mm (Table
5). The percent fines for the same 10 samples was 10.3 percent. This unit may have some
74
Table 5.
Cumulative results of insoluble residue analysis conducted on three partially unconsolidated, sandy units interpreted to be Oligocene in age that underlie and crop out on the seafloor in the study area.
MEAN PERCENT NUMBER GRAIN FINES SORTING CLASSIFICATION OF SIZE (< 0.0625 (phi) SAMPLES (mm) mm) TI-B 7 0.13 18.7 0.6
TI-D 8 0.21 17.2 1.2
TI-E 10 0.18 10.3 0.7
75
potential as a site for future exploration; however, the relatively high percentage of fines may
preclude exploitation of the unit.
All shoreface rock outcrops in the central to southwest portion of the study area are
associated with unit TI-D (Figures 30, 32, and 34). As described previously, these outcrops are composed of bio-moldic, sandy limestone units. Analysis of chirp seismic data indicated that these moldic limestone outcrops are the surface portion of an unconsolidated unit averaging 4 m
(13.1 ft) thick. No seismic reflectors are evident that would suggest the moldic limestone units are anything other than thin surficial expressions of the unconsolidated material. Of the three lithologic units mapped in this study (TI-B, TI-D, and TI-E) that were interpreted to be
Oligocene in age, TI-D contained the highest amount of shell material and carbonate mud.
The unconsolidated portion of TI-D is an olive gray to olive, very fine sand, with little (10%
- 20%) to some (20% - 35%) silt and little shell hash (10% - 20%). This is thought to be the more representative lithology for TI-D as opposed to the occasional moldic limestone rind that occurs along local exposures on the seafloor. On average, TI-D contains 30.2 percent carbonate by weight (Table 3). The average mean grain size for eight samples taken from this section was
0.21 mm (Table 5). The average percent fines for the same eight samples is 17.2 percent (Table
5). This unit has little potential for future exploration because of the high silt content and high percent carbonate.
The oldest of the three units interpreted as Oligocene in age is TI-B. This unit is believed to be correlative to the unit described by MCQUARRIE (1998) as middle Oligocene in age. This unit has a significantly lower percent carbonate than TI-D and TI-E at 13.5 percent carbonate by weight (Table 3). Unit TI-B is the most fine grained unit with an average gain size of 0.13 mm and average percent silt (< 0.0625mm) of 18.7 percent (Table 5). TI-B also has little potential
76
for future exploration based on the very fine mean grain size and the high percentage of fine
grained material.
The underlying geological units of the shoreface off Surf City and North Topsail Beach hold
little if any potential as a mineable sediment resource for beach nourishment projects due to
sediment incompatibility between offshore units and the native beach. Nevertheless, all the units
that underlie the shoreface off Surf City and North Topsail Beach contain high percentages of
quartz sand. This suggests that when shoreface bypassing occurs all of these units have a great
potential for producing quartz-rich sandy sediment. If in fact these units were deposited in the
Oligocene epoch, one could surmise that over the last 23.7 million years the units have
experienced extensive erosion and reworking as sea level has fluctuated.
It has been well documented that many portions of the underlying units of the Onslow Bay shoreface have been incised by the valleys of major Piedmont drainage systems and infilled with complex sediment sequences (PEARSON, 1979; HINE and SNYDER, 1985; RIGGS et al., 1992;
SNYDER et al., 1994). SNYDER et al. (1994) identified a valley greater than 1.5 km wide and
greater than 15 meters deep off Wrightsville Beach, North Carolina. This valley feature, which was cut into the underlying Oligocene strata was infilled with Plio-Pleistocene age sediment and referred to as a Plio-Pleistocene valley-fill lithosome. Also identified in the study off
Wrightsville Beach, North Carolina were at least five features interpreted to be fluvial channel fill features. These features were between 7 and 11 m deep and 0.3 to 3.0 km wide. Although most of the valley fill features mapped by SNYDER et al. (1994) were infilled with carbonate units, they demonstrate that fluvial systems sufficient to carve large scale channel and valley features into the underlying strata have existed in the region during post-Oligocene time.
77
MCQUARRIE (1998) mapped a number of mud-filled fluvial channels off Topsail Beach that
were incised into the underlying unit thought to be correlative with TI-B of this study. A number
of small scale paleo-fluvial channels infilled with back-barrier sandy mud and muddy sand were
identified on the seismographs analyzed in this study. Some of these channels identified were up
to 4.6 m deep and 760 m wide, which would provide significant accommodation for a mineable
sediment supply. With the well documented regional existence of valley fill and paleo fluvial
channels and the sand rich underlying geological units, it seems feasible that there may occur
sand filled channel–like deposits on the shoreface.
Analysis of vibracore cross sections, constructed for this study, allowed for the identification
of three (Area 1, 2, and 3) relatively thick deposits of sand. The sand deposits are quartz sand
with grain sizes, silt percentages, and carbonate percentages compatible with the native material
found on North Topsail Beach (Figure 38) (See Figures 17, 19, and 23 for cross section diagrams). The occurrence of these relatively thick layers of beach quality sand on the shoreface where units interpreted to be Oligocene in age compose the underlying regional geology, suggest
that vibracores recovered material from sand filled channel-like features incised into Oligocene
units. By examining seismographs that bisect these areas it is possible to verify if the relatively
thick layers of beach quality sand within the vibracores represent sand filled channel-like
features.
Area 1
An analysis of vibracore TI03-VC-273 along profile C – C’ (Figure 19) suggests that there may
exist a mineable deposit of unconsolidated sand with less fine grained material and carbonate than the surrounding units. This area is designated Area 1 and is shown in Figure 37. Figure 38
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Figure 37. Map depicting three areas (shown in yellow) in which thick deposits of beach quality sand were identified. Note the relationship between the three areas and the chirp seismic lines shown in figures 30 – 37. Also note the location of chirp seismic lines collected by CPE in 2005 in relation to Area 3.
79
Figure 38. Annotated and unannotated chirp seismograph of the portion of Line 2 Day 3 that bisect Area 1. Note the truncation of the reflector previously labeled Unconformity α.
80
shows a portion of Line 2 Day 3 where the line crosses Area 1. Note the location of vibracore
TI03-VC-273 and the truncation of the seismic reflector that represents Unconformity α. This
truncation suggests a channel-like feature cut into the underlying strata. Vibracore TI03-VC-
260, located southwest of TI03-VC-273, shows a channel-like feature incised into the underlying sediment and infilled with sandy mud, typical of a back-barrier environment. The presence of the secondary mud filled channel incised into the larger-scale feature suggests that the larger-
scale feature may have a preferential weathering characteristic relative to the surrounding units.
This preferential weathering characteristic may be related to the unconsolidated material that has infilled the larger scale channel-like feature. Analysis of several other vibracores in the area allowed for the tracing of the mud filled channel feature over a distance of 4.5 km (Figure 39).
At its landward-most position, the channel material is first evident in core TI03-VC-269A, which falls on chirp seismic line 53. The channel feature shown in Figures 31 and 32 in the vicinity of vibracore TI03-VC-269A, appears to be approximately 4.5 meters deep in the thalweg.
Although Figure 32 shows unconformity α terminating at the edge of this mud filled channel a closer examination of the seismic data suggests the reflector representing the unconformity may actually be terminated 170 m to the northeast of the mud filled channel. Although no vibracore data exists to ground truth the physical characteristics of the material between the edge of the mud filled channel and the termination of unconformity α it is feasible that given the depositional sequence present in Line 2 Day 3 (Figure 38), the material may be clean sandy material.
The channel generally trends south. Analysis of vibracore data from TI03-VC-269A, TI03-
260, TI03-VC-259, TI03-VC-257, and TI03-VC-255 suggest the channel continues south to southwest offshore for approximately 2.5 km (Figure 39). Beyond 2.5 km a lack of data precludes the continued tracing of the channel. TI03-VC-260 contains 3.74 meters of sand;
81
Figure 39. Map depicting the location of the mud filled paleo-channel contained within a sand filled channel in the vicinity of Area 1.
82
however an examination of Figure 38 indicates the deposit may be as thick as 7 meters. In
addition to data from the two seismic lines that bisect the channel complex, analysis of data from vibracore TI03-VC-270A suggests that a channel fill sand deposit may exist. Subsequently a smaller scale channel was incised into this sand filled feature in which mud eventually infilled.
TI03-VC-270A contains 1.7 m of sandy material that is underlain by 1.65 m of alternating sequences of fine sand and sandy mud. Although the vibracore data supports the idea of a sand deposit it also confirms the highly variable nature of these channel fill deposits.
Area 2
Analyses of cross sections D – D’ and G – G’ (Figures 20 and 23 respectively) suggest relatively thick (1.5 m – 2.0 m) deposits of sand in the vicinity of where the two cross section intersect (Figure 16). Further examination of the vibracores collected near the confluence of the two cross sections suggest a channel feature illustrated in Figure 37 as Area 2. Although no vibracore data were available to provide evidence of the channel-like feature landward of Line 2
Day 2, seismic data suggests the feature extends beyond the boundary of Area 2. Figure 40 shows the portion of Line 2 Day 2 along which this channel-like feature occurs. The material contained within the channel does not exhibit the degree of backscatter that is evident in mud filled channels such as the one depicted in Figure 38 in the vicinity of vibracore TI03-VC-260.
This observation suggests the material contained in the channel fill is more homogenous and more well sorted than the back-barrier deposits observed in many of the other vibracore samples analyzed in this study. The channel feature shown in Figure 40, is approximately 5.5 meters deep with a cross-sectional width of 250 m.
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Figure 40. Annotated and unannotated chirp seismograph of the portion of Line 2 Day 2 that depicts the channel feature that is the landward extension of Area 2. Note the truncation of the reflector and the basal channel reflector.
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Chirp Seismic data collected by Coastal Planning & Engineering during a sand search
conducted for the Town of North Topsail Beach provided a means for the further tracing of this
channel feature. The feature is traceable on four lines within Area 2 (Line 159, 157, 155, and
153) and continues past Line 2 Day 2 across eight additional chirp seismic lines collected by
CPE (Figure 41). Upon crossing Line 2 Day 2 the alignment of the feature curves to the
northeast in a shore parallel orientation and then trends north in a shore normal orientation. The
extent of the channel as shown in Figure 41 is not nearly as wide as the original area designated
Area 2. This is due to the fact that the feature designated as Area 2 was originally based on
vibracore data alone. It is possible; however, that the sand deposit that exists within Area 2 is
more extensive than the channel identified in the chirp seismic data. Beach compatible material
was identified within vibracore TI03-VC-82, which was collected outside the channel feature to
the west along Line 157 (Figure 41).
As stated previously, material contained within Area 2 is compatible with beach quality sand
with respect to percent silt, grain size, and percent carbonate by weight. Further examination of
the sediment recovered from vibracores within the channel-like feature suggests that as the
channel crosses Line 2 Day 2 and continues landward of Area 2, there exists a transition from
sandy material to muddy material. This transition may reflect the transition evident in modern
coastal systems. In modern coastal systems tidal creeks feed into the estuarine system that
separates barrier islands from the mainland. Tidal channels that cut through the back-barrier
estuarine system feed into the inlet systems, which exchange water from estuary to ocean. The
tidal creeks that feed into the modern estuary in this region as well as many of the channels carved through the back-barrier estuary system are infilled with varying layers of muddy sand,
sandy mud, clay, shell hash, and fine sand (MCGINNIS, 2004). Vibracore data collected from
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Figure 41. Map depicting Area 2 and the outline of the channel feature identified using CPE Chirp Seismic Data.
86 modern inlet systems in Southeast North Carolina (flood tidal delta, inlet throat, and ebb tidal delta) show a much more homogenous mix of sandy material with very low silt content. Often lag deposits of shell hash and coarse sand are interspersed within the vertical samples. This transition in modern coastal environments from estuarine depositional regimes to tidal inlet regimes may explain the transition observed along the channel-like feature mapped in the vicinity of Area 2.
Figure 42 illustrates portions of four seismographs that depict a cross sectional view of the channel feature. The images shown in Figures 42A and 42B are views of the channel feature along Lines 145 and 149 respectfully. The high backscatter and multiple layering seen within the channel along these lines is indicative of highly variable alternating layers of muddy sand, sandy mud, clay, and fine sand. This portion of the channel is interpreted to be an estuarine
Environment or a tidal creek feeding into the estuarine system. In the images shown in Figures
42C and 42D, one can recognize the channel feature as it appears in Line 153 and 155 respectively, which is within the boundary of Area 2. The seismographs of the channel along these two lines exhibit far less layering and appear “cleaner”, which is a function of less backscatter. This channel segment is interpreted to be within the inlet complex, containing beach quality sand. Figure 43 shows a schematic drawing of the channel complex and the interpretation of the feature.
Area 3
Area 3 is located along cross section A – A’ between 0.7 km and 2.3 km offshore North
Topsail Beach (Figure 37). The boundary of Area 3 was established based on vibracore and sidescan sonar data. Three USACE vibracores, collected from within Area 3 contain 2 m thick
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Figure 42. Seismographs depicting the variability of the channel morphology observed throughout the subbottom data which bisects the channel associate with Area 2. Seismographs are from CPE seismic data collected along lines 145 (A), 149 (B), 153 (C), 155 (D). Note Line 153 and 155 are within area 2 where as Line 145 and 149 are landward of Area 2, and exhibit higher backscatter.
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Figure 43. Schematic drawing of the interpretation of the channel feature grading from an estuarine or tidal creek channel into an inlet complex. Note the location of the outline of Area 2 shown in yellow and the channel feature as mapped using the chirp seismic data.
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units of beach quality material. Two of the vibracores, TI03-VC-311 and TI03-VC-312, are
located along cross section A – A’ (Figure 17).
Interpretations of sidescan sonar data provided a means of delineating a number of rock
outcrops to the northwest (landward), southeast (seaward), and southwest of Area 3 (Figure 44).
These features were ground truthed in 2005 and 2006 and were confirmed as hardbottoms
(HAGUE, Personal Communication 2006). The boundaries between the sandy bottom and the
hardbottoms were used to refine the boundary of Area 3.
Concurrent with this research, Coastal Planning & Engineering (CPE) conducted a sand search on the shoreface off North Topsail Beach between 2005 and 2007 (FINKL et al., 2007).
CPE identified the area in and around Area 3 as a region of the shoreface that contained
significant quantities of beach fill material. A substantial amount of geophysical, geotechnical,
and diver survey data were collected within Area 3 that provided a robust data set for the
analysis of the sand deposit (Figure 44). The relatively close-spaced (90 m) survey lines
provided a means for detailed mapping of a channel-like feature that traversed Area 3 (Figure
44). The channel-like feature is approximately 3 km long and ranges from 0.1 km to 0.8 km in
width. At its deepest point the feature is greater than 9 m deep.
The location and orientation of the channel-like feature is unlike most documented channel-
fill and valley-fill features mapped in the area. Most channel-fill and valley-fill features that
have been mapped from New River Inlet south to Cape Fear have exhibited a shore normal
orientation. The feature mapped within Area 3 exhibits a nearly shore- parallel orientation. It
was not possible to determine if the feature continued landward and ultimately beneath the
barrier due to the landward limits of the chirp seismic data coverage.
One explanation for the origin of this sub-surface depression, other than one of fluvial origin,
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Figure 44. Map depicting Area 3. Note the location of Tracklines along which Chirp Seismic data were collected and the areal extent of the channel feature mapped using chirp seismic data.
91 was that it may be the result of a solution collapse. Smaller scale solution collapse features were
identified in portions of the study area (Figure 33 and 34). The apparent truncation of reflectors
seen along the margins of the feature (Figure 45) throughout most of the chirp seismographs do
not support a solution collapse origin.
Seismographs of the channel-like feature consistently exhibit a very “clean” (little to no
backscatter) seismic signature (Figure 45), indicating a homogenous deposit. Vibracore data
from samples taken within the channel-like feature show the material to be a homogenous, fine grained, quartz sand, with trace (< 10%) silt. Computation of composite values for vibracores
yielded a mean grain size value of 0.19 mm, percent silt values of 5.2 percent and, percent
carbonate values of 14 percent. These values are well within the tolerance set by the North
Carolina Technical Standards for beach fill projects.
Although the orientation of the channel differs from most that have been mapped in the area
it is possible that at some point during geologic time the topography of the land could have
produced such a channel alignment. The material into which the channel feature was incised is
assumed to be Oligocene (mid to upper River Bend Formation) in age. Therefore, the only age
control for the channel-like feature is that it was formed after deposition of the River Bend
Formation. Given the number of sea-level oscillations since the Miocene (MILLER et al., 2005),
it is not unrealistic that a channel could have an orientation other than shore perpendicular
(Figure 46). This same line of reasoning would explain the lack of a landward extension of a
major relict fluvial system expressed on the modern landscape. In examining the modern landscape of Topsail Island and the adjacent mainland, there does exist a small tidal creek that is in alignment with the channel fill feature. Although this is certainly not indicative of a fluvial system that could carve a 9 m deep channel, it does suggest a preferred drainage pattern that is in
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Figure 45. Annotated and unannotated chirp seismograph of a portion of Line 117 collected by CPE that depicts the channel feature that identified within Area 3. Note the truncation of the reflector and the difference in the amount of backscatter in the primary and secondary channel features.
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SEA LEVEL (M) -125 -100 -75 -50 -25 0 25 50 75 100 0
2
4 AGE (Ma)
6
8
10
Figure 46. Oxygen Isotopic-based sea-level estimates for the past 9 Ma after MILLER et al. (2005).
94
alignment with the sub-bottom channel-fill feature (Figure 47). The number of limestone
outcrops adjacent to the channel feature may also have played a role in the channel orientation in
that if the channel feature was established after the time in which these limestone units were
deposited and lithified, the resistance of these units relative to a more erosion-prone,
unconsolidated unit may have dictated the channel alignment (Figure 47).
As previously discussed, a secondary channel-like feature was incised within the primary
channel-like feature. Chirp seismographs of this feature exhibited high backscatter acoustic
returns (Figure 45), suggesting a more variable sediment type than the homogenous fine grained sand that comprises the material within the primary channel-like feature. Analysis of vibracore data obtained from sediment unit samples within this secondary channel-like feature indicates a variability of sediment type from fine sand, to gravelly sand, to sandy gravel. Also two samples contained fragments of wood and peat. Sieve analysis of layers of relatively clean fine grained sediment within the secondary channel-like feature yielded grain sizes relatively coarser than those within the primary channel-fill feature. As previously stated the composite grain size for the material within the primary channel-fill feature was 0.19 mm whereas the sand located in the secondary channel-fill feature yielded grain sizes between 0.34 mm and 0.44 mm. The relatively coarse grain size of the sand, coupled with the presence of layers of gravely sand and sandy gravel suggests a relatively higher energy environment in which the channel was infilled. This may be another area that is indicative of a historic tidal inlet feature. Although the nearly shore parallel orientation of the feature would not support this hypothesis, it is possible that during the
past 23 Ma years the orientation of the shoreline was conducive to formation of such a feature.
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Figure 47. Map depicting the primary and secondary channel feature in relation to the modern drainage pattern. Note aerial image is from Google Earth 2008.
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CONCLUSION
North Topsail Beach and Surf City, North Carolina are two of the most vulnerable coastal
communities in the State in regard to beach erosion and storm impacts. This is primarily due to
the relatively low topography of Topsail Island and the lack of sand available on the shoreface to
nourish the beaches. The vast majority of the shoreface is comprised of rock outcrops and
unconsolidated sediments interpreted to be Oligocene in age that are locally covered by a thin,
discontinuous veneer of Holocene sand and gravel. Although outcrops of limestone and
consolidated siltstone are common throughout the study area, the majority of underlying units are
unconsolidated sandy units. The relatively high silt and carbonate percentages within these
sandy units not only make them a poor candidate for beach fill material from an engineering
perspective but have also been determined to be non-compliant with newly implemented state
sediment criteria rules adopted by the North Carolina Division of Coastal Management.
A significant amount of sediment is being generated by the natural mechanical weathering
and bioerosion of the shoreface units. Samples of surficial sand collected throughout the area
have similar physical characteristics as the unconsolidated sandy units that comprise much of the
shoreface but are covered by discontinuous surficial Holocene sand sheets. Likewise surficial sand samples from the shoreface have correlative mean grain sizes, color, mineralogy, rounding,
and other physical properties to the sand fraction present in limestone and siltstone outcrops from
the shoreface. Large deposits of shell hash, shell fragments and lithic gravels found adjacent to
many of the offshore outcrops are also a testament to the active erosion of these features. Much
of the surficial samples collected away from rock outcrops also contained shell fragments or
lithic gravel fractions that could be attributed to the bioerosion and physical weathering of these features. It is well documented that a portion of this material is transported to the beach in times
97
of heightened wave activity (i.e. tropical storms and nor’easters). After such storm events, it is
common to find large cobble and boulder size clasts of rock on the beaches of North Topsail
Beach and Surf City that have broken off from the shoreface rock outcrops and been transported
onto the beach. Despite the evidence that significant quantities of sediment are being derived
from the offshore units and that a small fraction of the material migrates onto the beach it is not
feasible that a significant portion of the newly derived sediment migrates to the beach to stem the
0.6 to 1.1 m/yr (2.0 – 3.5 ft/yr) erosion rate for North Topsail Beach and Surf City.
Given the necessity of having a viable source of sand to nourish the beaches, the communities have sought help from UNCW, the USACE, and private engineering firms to locate potential sand sources. Much of these data were incorporated into this study to provide a detailed geological framework of the shoreface. The high density of data provided a level of resolution few other studies in the area have provided. This density of data allowed for the identification of three sub-surface features that contain deposits of sand with high potential to provide sufficient sand to be used to nourish portions of North Topsail Beach and Surf City.
The location of these deposits demonstrated the value of comprehensive reconnaissance data to locate sand deposits. It has been well documented that much of the shoreface off Topsail
Island contain little potential for significant quantities of beach quality sand; however this study suggests that there does exist isolated channel-like deposits of sand available for the needs of communities such as North Topsail Beach and Surf City. Given the relatively small footprint of these features, it is likely that additional similar features may exist but due to gaps in the data they have not been recognized.
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