Near Shore Depositional Environments Paper

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Transcript of Near Shore Depositional Environments Paper

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Near Shore Depositional Environments

12/2/2008

Justin Nixon

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Abstract

Shackleford banks is a migrating barrier island system, located off the coast of Beaufort, North Carolina.Barrier islands are geologically short lived and are, usually, difficult to preserve in the geologic record. By using physical, biological and chemical means to describe and identify a barrier islands lithofacies, they can be identified in a vertical sequence, long after the island has moved on.Because barrier islands are exposed on all sides, preservation potential of barrier islands is very low. With erosion acting on all sides, low sediment input, and biological processes shaping the island, the barrier islands depositional systems are difficult to preserve.After visiting shackleford banks, depositional and erosional environments are described to form a vertical sequence. Taking into consideration the biological and chemical aspects of the environments, the different sequences could be much easier distinguished from one another.

Introduction

Nearshore environments tell the story of changes in sea level, depositional versus erosional patterns, migration of beaches, and to a larger degree, the migration of barrier islands. Depositional and erosional features, fossils, and chemical components are often recorded in the geologic record. With these four key identification points, environments such as shorelines, dunes and inner-dune areas, can be identified.

Knowing how these depositional and erosional processes take place, geologists can better understand how barrier islands were developed, and how to identify different depositional environments in a vertical sequence.

Shackleford Banks, located just off the coast of Beaufort North Carolina, is one of North Carolina’s barrier islands (Fig 1). The North Carolina Barrier Islands formed during the Pleistocene, 18,000 years ago (Beyer, 1991). The Atlantic Coastal plain, which later helped to build the foundation of the barrier islands, was formed during the Mesozoic Era, 70 million years ago (Hutter 2004). As the ice ages began, sea level dropped. The lowering of the sea level revealed a sloping coastal plain composed of soft and easily eroded sediment (Hoel, 1986). Like all other barrier island’s located off the coast of North Carolina, it is migrating inland. The island is covered with diverse plant species and wildlife.

The objective of this paper is to develop a vertical facies model of the progradation of shackleford banks.

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Methods

(Reference Figure 1c for Locations)

At location 1, an Aeolian dune and coastal sands were examined. At location 2, a trench was cut in a swale and observed. A fresh water pond was studied at location 3. Closer to the ocean side of the island, at location 4, layering and biological processes were studied. At location 5, a trench was cut into an over wash fan and examined. On the ocean side of the island, at location 6, a trench was cut from the water’s edge of the coastal sand, to the edge of the Aeolian dunes.

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Samples were dug with a shovel and observed from the marsh and tidal flat at locations 7 and 8. All trenches were back filled. No samples were removed from the island. Laws prohibit the destruction and removal of sediment and wildlife of this national park.

Lithofacies

The lithofacies described in this section will provide information about the physical, biological and chemical components of each depositional environment present on the barrier island.

Sound side Coastal Sand

Located on the sound side of the island, at location 1 (fig. 1c), Aeolian dunes were examined on the backshore. The dunes are composed of predominately medium quartz sand. Planar bedding of quartz sand and heavy mineral bands, composed of garnet, are seen in the outcrop of the dune (fig 2). Sub rounded, very fine sand is located at the surface of the dune. Coarser sand is located on the lee side of the dune. Sea oats are randomly sorted over the surface of the dune. Salt meadow hay dominates the stoss side, or south side of the Aeolian dunes. The surfaces of the dunes are bio-turbated by roots.

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Dune Field

Swale

A swale is located in the center of the island, in the dune field (location 2, fig. 1c). Dunes are located on all sides of this area. Heavy minerals, such as garnet, formed dark laminations, 2-3 cm thick (fig 3a). Quartz rich, medium sand, dominated the remainder of the sequence. Coarse grained, felsic sand, mixed with coarse shell fragments are found in lenses in the sequence (fig 3b). Moving downward in the sequence, layers repeat themselves. The penny wort and salt meadow hay plants dominate the surface of the swale and are responsible for bio-turbation of the upper layers (fig 3a).

Mullet Pond

Located at a lower elevation in the dune field, a fresh water pond, by the name of mullet pond, was observed and studied (location 3, fig. 1c). Because the elevation of the swale is lower than the water table, the swale is covered by 50 cm of water, at its deepest point. Grain sizes are very similar to the ones found at the higher elevation of the swale. The sample shows a dark layer of

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sand which is composed of 2-3 % root mass from the juncus plant. The darker layer is underlain by a light colored sand of the same composition (Fig 4). The darker layer has a pungent smell of decaying organic matter. Black needle rush, along with the juncus plant, grow in the water. Surrounding Mullet pond are wax myrtles, and oak trees.

Closer toward the ocean side of the island, a 3.5 meter dune was examined (location 4, fig. 1c). Composed of light colored quartz sand, much like the inter dune areas, the dune was lined with sea oats and salt meadow hay on its surface. On the lee side of the dune, on the surface foreset bed, shell fragments and coarse grained sand is present (Fig 5). Just as the layering was present in the swale, layering could also be seen in the dune area if cut into. Heavy mineral bands separated from the silica rich sand. Sand lenses are also present, composed of about 3-4 % shell fragments. Sand lens layers tended to repeat themselves in the vertical sequence. Animals, such as the white ghost crab, were also responsible for burrowing into the substrate.

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Over wash Fan

Located just inland of the ocean, an over wash fan was examined (location 5, fig. 1c). Large shells peppered the surface of the fan. Large shells could also be seen in the layering (Fig 6). Separations between heavy minerals and quartz sand were present just as in the swale. Penny wort plants, salt marsh cord grass and sea oats littered the over wash fan.

Ocean side Coastal Sand

On the ocean side of the island, erosional processes are much stronger than the sound side (location 6, fig. 1c). The berm is characterized by the layering which was parallel to the beach and the layering that dipped back toward the island, interface. Coarse grained sediment such as sand and shells were located in the swash zone. The swash zone could also be identified in the vertical sequence by its composition but faded out as it progressed inland.

Salt Marsh

A marsh located on the sound side of the island is covered with oxygen reducing sediments and animal fecal pellets (location 7, fig. 1c). The deepest part of the marsh is covered with 18 cm of salt water low in oxygen. The sediment water interface is light in color. Underlying the surface

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sediment is reduced anoxic sediment. Feral horses are responsible for fecal piles found in the marsh. Crabs and snails were the main biological aspects of this environment (fig. 7).

Tidal Flat

The tidal flats are located adjacent to the marsh, on the sound side of the island (location 8, fig. 1c). Ripple marks are located all throughout the tidal flat (fig 8). The tidal flat is much more biologically active than the marsh. Tube worms, snails, crabs, scallop, starfish, and a variety of fish were just a few of the marine wildlife located in the flat. Other animals such as birds and feral horses roamed the tidal flats in search of food. Sea weed made up the majority of the plant life on the tidal flat.

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Processes

The processes section will discuss the processes active in each environment that are responsible for the formation of the lithofacies described above.

Sound Side Coastal Sand

Coastal sands are composed of typically four units: Aeolian dunes, Backshore, Foreshore, and the shore face. Aeolian dunes are deposited by wind energy while the other three are shaped by wave energy.

Dune Field

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A number of processes control the deposition of the dune field. Wind, rain, and storms shape this system. Wind travels over the stoss side of the dune, eroding sand and pulling it over the lee side depositing (fig 9). The dunes are constantly eroding and depositing in this fashion. Plants such as the sea oat and salt meadow hay slow this process by holding the sand in place.

Swale

During times of high erosion of the dunes, sediment is transported either by wind or water to the swale and deposited. Because of the high moisture content of the swale, sand particles have a tendency to flocculate, therefore promoting deposition. Where water is standing in the swale, wind transported sediment is deposited.

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Over wash Fan

Waves generated by storms and hurricanes have a large impact on wash over fans. Since 1585, North Carolina has had over 150 recorded storms -- an average of 1.64 storms per year -- and storm surges of up to 2.3 m above sea level have been recorded. Storm-related processes can move large volumes of sediment across the low-lying barrier islands and deposit that sediment in wash over fans. In addition, storms can redistribute coarse-grained sediment on the oceanside of the tidal inlets, deltas, and lagoons (Moslow and Heron, 1981).

Ocean Side Coastal Sand

Waves are the main force acting on the ocean side of the island. During the winter, coastal sand migrates off shore and is stored in bars because of high energy erosional waves. During the summer, low energy waves move the sand which was deposited in the winter, back onto the coast. Like the sound side coastal sand, the ocean side is composed of the same four units. Because wave energies are higher on the ocean side of the island, these units are larger and more distinguished. Aeolian dunes are usually much more eroded by over wash fans on the ocean side.

Salt Marsh

Tidal fluctuations are responsible for the deposition of salt marshes. Suspended sediment is moved into the salt marsh and deposited when the tide is high. When the tide falls, sediment is trapped in the marsh, and the water drains out.

Tidal Flat

The mean tidal range at Shackleford Banks is 0.89 m and tidal currents in excess of 115 cm s-1have been measured in the Outer Banks (Sarle, 1977). The tidal currents are responsible for the movement and deposition of the sediment in the tidal flats. Oxidized sediment is reworked daily from the salt marsh and moved to the tidal flat (fig. 10).

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Facies Model

The vertical sequence shows the grain size changes associated with each depositional feature.

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Preservation potential of all of the lithofacies discussed above, are different. Deposition rate and burial rate are the two main factors in determining the preservation of any depositional environment.

Transgressions in general are very erosive and therefore coastal sands, tidal flats and marshes are very difficult to preserve during a transgression and will most likely end up as poorly sorted, coarse sediment. The marsh in particular has a low chance of ever making it into the depositional record because of low sediment accumulation and extremely low rates of burial.

Different plants can sometimes offer good evidence of what depositional feature is being observed. The Black needle rush in particular, if preserved in the geologic record, would offer good evidence of standing water. Tree stumps preserved in cross stratified dunes would offer evidence of dune migration over a swale. The tree that was once present had to have grown on ground flat and stable enough for it to survive, representing a swale.

Animals such as bi valves, crabs, worms and snails, just to name a few, can also offer good evidence for certain environments if preserved.

The cross section below shows how the different lithofacies connect in a cross section of the island.

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Conclusion

Physical, Biological and Chemical aspects are shaping depositional environments as well as waves, tides and weather. These three aspects can also be used to identify lithofacies of a Barrier Island in a vertical sequence. When certain Physical, Biological, or Chemical components are preserved in the geologic record, lithofacies can be better interpreted.

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References

Beyer, F. (1991). North Carolina the Years Before Man: A geologic history. Durham: Carolina

Academic Press.

Catherine A. Rigsby, Department of Geology, East Carolina University, Greenville, NC 27858

Elizabeth Clayton 2008, Catherine A. Rigsby, Department of Geology, East Carolina University,

Greenville, NC 27858

Hoel, M. (1986). Land’s Edge: A natural history of barrier beaches from Main to North Carolina.

Newbury: The Little Book Publishing Company

Hutter, K. (2004). An Investigation into the Migration of North Carolina’s Barrier Islands

Laura L. Sarle, (1977) Processes and resulting morphology of sand deposits within Beaufort

Inlet, Carteret County, North Carolina

Moslow and Heron, (1981) Physical processes around a cuspate foreland:: implications to the

Evolution and long-term maintenance of a cape-associated shoal

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G. Kocurek, M. Townsley, E. Yeh, K. Havholm, and M.L. Sweet (1992) Dune and Dune-Field

Development on Padre Island, Texas, With Implications for Interdune Deposition and Water Table-Controlled - Accumulation

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