facies model of a pro grading barrier island based on deposition of shakelford banks
TRANSCRIPT
Facies model of a prograding barrier island deposition of Shackelford
Banks, NC
Photo Courtesy of Elizabeth Clayton
By: Diana Baran
Abstract
Barrier islands are narrow strips of land that parallel the coastline and consist of a variety of fine sediments and particulate matter (Shumaker, 2008). They can be divided into different facies based on the environments energy potential and sediment grain size. Shackelford Banks, North Carolina is a barrier island that was observed based on its physical, chemical and biological characteristics. These characteristics were then used to interpret a facies model of a typical prograding barrier island. The preservation potential of each environment in the barrier island can be determined once the facies model is complete.
Introduction
Barrier islands are elongate accumulations of sand that are separated from
mainland by open water in the form of estuaries, bays or lagoons (NOAA, 2008). These
islands are dynamic ecosystems that migrate as a result of storms, sea level change,
and amount of available sediments. They are also more preserveable in the
stratigraphic record than other systems, particularly progradational near shore systems.
Controls on barrier islands include sea level change, wave and tidal energy, sand
availability, vegetation, and geologic framework. All barrier islands can fit into three
classifications transgresisve, regressive and inletfill. The islands can be broken dowin
into subenviornments (Fig.1) based on physical, chemical and biological aspects. Along
the coast of North Carolina, there is a system of barrier islands running its whole length
north to south. Within these islands there is a complex variability in the types of coasts
that occur. Riggs (1995) explains that the U.S. East Coast has barrier islands perched
upon pre-modern stratigraphic units that occur beneath and seaward of the shoreface,
and controls the shoreface morphology, modern beach dynamics, sediment composition,
and sediment fluxes. Shackelford Banks (Fig. 1) is one of these barrier islands. It is in
the southern portion of North Carolina's coast near the town of Beaufort. Shackelford is
relatively protected from storms because Cape Lookout extends south and Shackelford
is located on the inside of its cape. Barrier islands like Shackelford are easily preserved
in the stratigraphic record; particularly the progradational near shore systems and this
record can be viewed in a facies model. The purpose of this project is to create a facies
model with the observed characteristics of Shackelford Banks. A facies model is a
general summary of a specific sedimentary environment. The most effective way to
describe a depositional system is to observe a modern one in detail.
Figure 2) Map of Shackelford Banks in relation to the North Carolina barrier islands (NCNatural, 2002).
Methods
Shackelford Banks can be subdivided into many depositional environments. Of
these a tidal flat, sand flat, a maritime forest, a pond, interdune fields, washovers and the
shore face were studied (Fig. 3). At each depositional environment, the sedimentary
structures were observed and grain size, shape, sorting and composition were
determined. Also, the chemical characteristics of each geomorphic feature we studied
were observed and noted. The various types of animals and plants were recorded, and
at the swash zone and interdune environments, there were three trenches created to
interpret stratigraphic cross section.
Lithofacies Description
Each of the features on Shackelford Banks, NC will be described including its
physical, biological and chemical characteristics.
Aeolian Dunes
The aeolian dunes occur on three areas of the barrier island which include the
sound-side, ocean side and the middle of the island. They are the result of wind
reworking sandy sediments picking them up and depositing them into dune structures
along the island. The dunes can attain a height of several meters (Rigsby, 2008). The
dunes on the sound side of the barrier island on Shackelford Banks range from 2 to 5
meters tall and 3 to 6 meters in length. The aeolian dunes have laminated crossbedding
composed of medium, sub-rounded quartz sand, organic material and heavy minerals
(Fig. 4). There are two active sorting processes producing the forset laminae include
the larger particles are located along the outside of the laminae under the influence of
shear, and larger particles near the base of the laminae because of gravity (Brush,1964).
The thick layers which range between 10 to 20 cm and has ~90% medium quartz sand.
There are very thin laminated layers rich in heavy minerals like garnet, glucophane,
augite and illmanite. The top of the dunes have vegetation on top such as sea oats and
pennywart which can cause bioturbation.
Interdune
The interdune is an area on the barrier island surrounded by dunes covered by
grass and penny wart. A 30 cm trench (Fig. 5) has discontinuous laminations and
lenses of coarse pebbles and shell matter. While in the interdune area there were “ghost
trees” which is evidence of dune migration. Layers of fine to medium, subrounded
quartz sand and thin layers of heavy minerals that include garnet and augite or another
dark mineral. Along the top is a bioturbated organic rich layer.
Freshwater pond
A freshwater pond occurs in the interdune on Shackelford Banks and is called
Mullett pond. This pond is fed from the water in a freshwater lens in the water table,
there is approximately eight meters of freshwater below the surface, known from the
1:40 unit law. These ponds rarely get very deep; this one was about 15 to 20
centimeters deep. There is grassy vegetation growing at the bottom and with black
needle rush bushes and maritime forest trees surrounding it. Sediment was exposed
with a shovel from this area about 30cm deep showing two kinds of sediment. A top
layer of ~12% organic material and ~88% very fine subrounded quartz sand, then
another layer of fine quartz grains and shell fragments(Fig. 6)
.
Overwash fan
An overwash fan is the area of barrier islands where waves in large storm events
breach the dunes and deposit coarse material. Overwash fans are not always present
on a barrier island they are most likely to occur on a high energy island. A one meter
deep and fifty centimeter wide trench was dug in this overwash fan exposing the inner
sediments and layers (Figs. 7 & 8). There are coarse shells along the top and in the
middle of the trench. Between these shell layers light colored layers of fine grained,
subrounded, quartz sand and dark colored layers composed of ~5% heavy minerals that
include epidote and augite the other ~95% is fine grained quartz sand. These layers
made up three different units that are created by overwash events.
Tidal Flat
A typical tidal flat can be subdivided into five sections including tidal channel,
sand flat, mixed flats, mud flats, and salt marsh(Fig 9). On a tidal flat bedforms can
include flasier, wavy and lenticular bedding which are some of the common types.
Lenticular bedding forms if it is a mud dominated sediment with sand lenses, wavy
bedding forms when there is equal mud and sand, and flasier bedding forms if it is sand
dominated with mud drapes. The grain size trends tend to increase toward the open
sea. Biodiversity is high in this area which means there is extensive biotrubation from
worms, crabs and mollusks. This bioturbation can be in the form of burrows such as the
ophiomorphou burrows which are formed when shrimp pack balls of sand into the
burrow. Although often times on a tidal flat the rapid sediment movement inhibits much
bioturbation. Shackelford had local 2D, 3D and oscillating ripples. Many tidal flats
have meandering channels in the salt marsh through the mixed flats and if a core is dug
point bars and cut banks will be exposed.
Beachface
The beachface is the area on the barrier island from the edge of the aeolian
dunes to the seaward edge of the surf zone, and can be broken down into
subenviroments (Fig.10). Of these the backshore, foreshore, swash zone, surf zone
and lithofacies will be described.
Backshore
The backshore is the supratidal area between the foreshore and the aeolian
dunes. The bedding in this environment has parallel laminations and single sets of
crossbeds (Elliot, 1986). The backshore is typically separated by the forshore at the
berm which is generally the highest point on a “beach”. Although on Shackelford on
observation day the beach was in a dissipative form based on Dean’s parameter and
waves were washing up to and over the dunes with no visible berm.
Foreshore
The foreshore extends from the backshore to the start of the shoreface. Here
sediment is relatively coarse grained. A ridge and runnel is common here in low tides.
There can be 2D and 3D ripples that form here. This area can be subdivided into the
swash zone and the surf zone.
Swash zone
The swash zone is the area of the forshore that is intermittently covered and
uncovered by wave run-up and controls the evolution of beach morphology (Puleo et al.,
2002). The processes in this zone dominate the foreshore which is the intertidal part of
the beach face (Rigsby, 2008). A 13 meter trench was dug in the swash zone on
Shackelford Banks exposing the sediments and bedding (Fig. 11). There are coarse
layers of shell that became more dominate at the swash zone. There are also laminated
beds of heavy minerals and fine to medium quartz sands. Most visible in the trench is
the swash cross stratification which is low angle cross stratification, sub parallel to bases
of wedge-shaped sets, the stratification and set boundaries are formed parallel to
changing slope of beach face and dip gently seaward (McCubbin, 1982). The bedding
Surf zone
The surf zone is the region extending from the seaward boundary of wave
breaking to the limit of wave uprush (Smith, Jane M., 2003). Within the surf zone, wave
breaking is the dominant hydrodynamic process along with rip currents and longshore
currents. The surf zone is a high energy environment with coarse sediments deposited
here.
Shoreface
The shoreface can be defined as that area which extends from the surf zone to
the inner-continental shelf and is an area that remains poorly understood (Backstrom,
Joni T., 2007). In this area ephemeral fields of symmetrical and asymmetrical ripples
and laminated beds with varying amounts of bioturbation.
Offshore
Sediments offshore are typically very thinly and planar bedded laminations of fine
grained sediments. Small ripples form during storm events. The grain size trends show
that the further offshore the smaller the grain sizes. Orbital velocity from waves cannot
be felt on the bottom in deepwater and it becomes lower energy.
Processes of Lithofacies Description
The processes of lithofacies description an outline of how sediments are moved
and originated with the two major parameters being grain size availability but most
importantly, energy provided in relation to the subenvironment on a barrier island.
Energy potential of the environment is determined by storms, tides and distance from
wave action.
Dune Field
The lithofacies in the dune field is affected mostly by wind and storm events, but
affected very little by wave action.
Aeolian Dunes
The sediment deposition on aeolian dunes in the inner barrier island is affected
by winds, vegetation and storm events. The prevailing winds and storm enhanced winds
pick up sediments and deposit them on these dunes, and the winds also pick up the
sediments from these dunes and take them elsewhere. Vegetation like sea oats and
penny wart help anchor the sediments to keep sediment from being picked up by the
wind.
Interdune
The interdune mechanisms include both wind driven sedimentation and in
significant storm events sediment can be washed from elsewhere into the interdune.
Wave action does not reach the interdune.
Overwash fan
The overwash fan has units which represent different storm events. This area’s
primary source for sediment comes from storms. When a large storm hits the barrier
island and waves breach the berm and dunes and sediment gets deposited creates an
overwash fan. These areas are formed at high energy and coarse grained sediments
are deposited. There is high preservation potential in an overwash fan because
deposition is preserved.
Freshwater Pond
The freshwater pond is protected from winds and storm events by the
surrounding trees of the maritime forest. The ponds can change chemically when the
availability of freshwater decreases.
Tidal Flat
Tidal flats develop on coasts with a relatively high tidal range where enough
sediment is available and there is little wave action. The sediments that are deposited
on the tidal flat are some of the finest in the facies model. This area is not affected much
by storms or wind action. At high tide waters from the sound rise onto the tidal flat and
deposit fine sediments because the low energy of the rising water. Then during low tide
the waters recede and take some small sediment and create ripples. The tidal flat is
affected greatly by bioturbation, because this area is so low energy a variety of species
can live on or burrow into these sediments. The preservation potential of the tidal flat is
low because excessive amounts of bioturbation churn up the sediments bedding. . In a
very low energy spot on Shackelford’s tidal flat the oxygen is reduced and a biologically
mediated oxidation reaction takes place that comes from fecal matter and smells like
sulfur. Iron sulfide forms in this area.
Beachface
The sediment on the shore is the portion of the barrier island most affected
during storm events, wave action, tidal range, winds, and longshore transport.
Backshore
The backshore portion of the beachface is affected mostly by winds which pick
up and deposit sediment. Storm events affect this area by adding winds and possible
washovers from increased waves. Although wave energy generally has a minimal affect
on this area making less potential to remove sediment which allows coarser sediment to
accumulate here.
Foreshore
Swash zone
The swash zone portion of the beachface is intermittently affected by successive
waves that traverse this zone in a zig-zag fashion and can produce beach cusp. A
substantial quantity of longshore sediment transport also occurs in this zone (Dean and
Dalrymple, 2001). During storm events such as nor’easters and hurricanes there is high
wave energy which erodes sand from the swash zone and takes the majority of it to an
offshore bar and creates a dissipative beach profile. After these events if there is low
wave energy the waves pick up the sand from the bar and deposits it back in the swash
zone to build it back up and may become a modally reflective beach. The high wave
energy that the shore has makes the sediment here very well sorted and the coarsest on
the island. Storm events can cause rapid sediment erosion to occur in the swash zone.
The tidal range determines the position of sediment that is moved.
Surf zone
The surf zone is a high energy environment that is effected by wave action, tides,
longshore currents and storms. Most of the sediment transport in the surfzone is
associated with sediment reworking and bar migration in this area morphological
changes usually are large throughout the year (Klienhans, M.G., 2002).
Shoreface
Wave action, tidal range, winds and storm events all affect the transport and
deposition of sediment in this area. The internal dynamics are determined by slope-
dependent, wave-induced cross-shoreface transports, while the external driving factors
are lateral sediment supply and sea-level rise (Stive, M.J.F. and Vriend de, H.J., 1995).
Offshore
The processes of deposition offshore are only affected during storm events when
the wave orbital is large enough to feel the bottom which moves and deposits sediments.
The sediment is very fine grained here and there is a high rate of bioturbation. During
storm events small ripples can form.
Facies Model
Facies models are intellectual aids to the understanding of sedimentary
environments and the origin of ancient sedimentary rocks (Anderton, 1985). Anderton
(1985) also explained that many different models can be constructed to explain a given
set of data, depending on which aspect of the facies requires the most illumination.
Figure 12 shows the facies model developed for a prograding barrier island based on
processes and description of lithofacies on Shackelford Banks, NC. Changes in sea
level determine whether or not the barrier island is in a state of equilibrium. Donselaar
(1996) explains if the rate of relative sea level rise outpaces the vertical aggradation the
barrier coast will move landward by the processes of increased shoreface erosion, storm
overwash and flood tidal delta expansion, or the barrier coast will drown. In this
punctuated landward migration the subenvironments that occupy a low position in the
transgressive succession have the highest preservation potential. Lagoon, washover
fans and deltas, flood tidal deltas and the deeper part of the tidal inlet escape erosion,
whereas the sub-aerial barrier island, the beach front and ebb tidal deltas are usually
eroded by the shoreface ravinement (Donselaar, 1996).
Summary
Studying and observing Shackelford Banks a barrier island located off the coast
of North Carolina provides a basis of understanding for description and processes of the
lithofacies on a typical barrier island. The processes of depositon including storm
events, wind, wave action and others were determined. This information was used to
build a facies model for a prograding barrier island. To fully understand the environment
the chemical, physical and biological aspects were examined completely.
References
Anderton, R., 1985, Clastic facies models and facies analysis: The Geological
Society.. Facies models and modern sedimentary environments v.18 p.31-47.
NOAA. Coastal Services Center, 2008. 9 November, 2008
<http://www.csc.noaa.gov/beachnourishment/html>
Riggs, Stanley and Cleary, William and Snyder Stephen, Influence of Inherited
Geologic framework on barrier shoreface morphology and dynamics.: Marine Geology
v. 126. 23 November, 1994. 213-234.
NC Natural. “Barrier Island Dynamics” Guide to Coastal North Carolina. 2005. 9
November, 2008 < http://ncnatural.com/Coast/dynamics>
McCubbin, D.G., 1982, Barrier-island and strand plain facies IN, Scholle, P.A.
and Spearing, D., eds., Sandstone Depositional Environments: Amer. Assoc. Petroleum,
Geol., p. 247-280.
Puleo, Jack, Holland, K.T., and Slinn D., 2002, Numerical Modeling of swash
zone hydrodynamics: Storming Media, p.3.
Smith, Jane M., 2003, Surf Zone Hydrodynamics, Chapter 4, 11-4-1.
Rigsby, Catherine, 2008, Field Trip #2 - Creating a Model for a Progradational
Barrier Island System, p. 4 of 11.
Backstrom, Joani T., 2007, Short-term Shoreface Changes along a High-Energy
Headland-Embayment Coast: Journal of Maps, p. 12.
Klienhans, M.G., 2002, Sediment dynamics on the shoreface and upper
continental shelf, a review: Scientific American , 2.4.5.
Dean, Robert G., and Dalrymple, Robert A., 2001, Coastal Processes with
Engineering Applications. p. 114 of 446.
Stive, M.J.F. and Vriend de, H.J., 1995, Modelling shoreface profile evolution:
Marine geology, p. 235-248.
Shumaker, Dave, “How Barrier Islands Work” Geology News. 2008,<
http://www.liu.edu/cwis/cwp/library/workshop/citmla.htm>