Field Trip #2 - Creating a Model for a Progradational Barrier Island System Field Area: Shackleford Banks, North Carolina

Introduction Nearshore systems are common in many parts of the geologic record. They are important economically -- foreshore sand bodies commonly contain heavy minerals in economic quantities -- and they record small-scale changes in sea level -- finding "the beach" in ancient strata allows us to map ancient shorelines that document sea level changes throughout geologic time. In order to exploit and understand ancient nearshore systems we must develop models that allow us to recognize these systems in the rock record. The best models are based on a thorough understanding of modern depositional systems and depositional processes. The primary goal of this project is to develop a model for a progradational nearshore system by analyzing the modern depositional environments and facies present on Shackleford Banks, North Carolina. The Study Area Shackleford Banks, located near the town of Beaufort in Carteret County, North Carolina, is one of North Carolina's Outer Banks (Figures 1 and 2a). The Outer Banks are a cuspate-shaped wave-dominated microtidal barrier island system. The cuspate shape of the system is controlled by a variety of features including the dominance of wave energy over tidal or fluvial processes, the sand-rich nature of the system, and the structure and topography of pre-Holocene erosion-resistant strata (Moslow and Heron, 1986; Susman and Heron, 1979).

In contrast to the northeast-southwest or general north-south orientation of the other islands in the cuspate system, Shackleford Banks is oriented in a northwest-southeast direction. This orientation provides the island with some protection from the highest energy waves (originating for the northeast) and makes it, while still wave-dominated, a lower energy island than most of the Outer Banks (Heron et al., 1984). The island is approximately 1.6 km wide and 14.5 km long and has a maximum elevation of 10.7 m above mean sea level. It exhibits parallel and recurved beach ridges, extensive dune fields, a back-barrier lagoon that is partially filled with salt marsh and tidal flat sediments, and a prominent tidal delta (Figures 2b and 3). The island's topography and morphology reflect the complex progradation and inlet migration patterns of the island's evolution (Heron et al., 1984). Although it was inhabited in the past, the island is essentially undeveloped and is currently part of the U.S. National Park system. FIGURE 1.A. shoreline morphology of the southeast U.S. Atlantic coast showing the major cuspate forelands of the North Carolina Outer Banks. B. Map of the Cape Lookout area showing the orientation of the Holocene barrier islands and the Late Pleistocene shoreline trends. Both maps are from Heron et al. (1984).

Fig. 1B

A. B.

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FIGURE 2.A. Shackleford Banks regional map. B. Diagram showing the geomorphic features of western Shackleford Banks. After Moslow and Heron (1989).

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B.

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FIGURE 3. Air photographs of portions of Shackleford Banks. A. High altitude, vertical air photograph of western Shackleford Banks, Beaufort inlet, eastern Bogue Banks and Back Sound (1=flood tidal delta; 2=sand wave field; 3 and 4=intertidal sand flats; 5=estuarine beach ridge). B. Oblique air photographs of eastern and coastal Shackleford Banks backbarrier. View is to the west. C. Oblique air photo of vegetative beach ridges and aeolian dune ridges on western Shackleford Banks. Back Sound is in foreground --view is to the south. All photos from Moslow and Heron (1989).

Coastal Processes and Morphology Sedimentary Processes The three dominant processes acting on all barrier island systems are tides, storms, and waves. In the Outer Banks -- a wave-dominated microtidal system -- waves are the dominant process affecting Holocene sedimentation. Although Shackleford Banks is relatively sheltered from northeast, east, and southeast waves (which account for approximately 75% of the total onshore wave energy flux in the Outer Banks), it is influenced by southerly onshore waves (which account for the remaining 25% of wave energy flux). Furthermore, waves generated by storms and hurricanes also have a significant impact on the study area. 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

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sediment in washover fans. In addition, storms can redistribute coarse-grained sediment on the ocean side of the tidal inlets, deltas, and lagoons (Moslow and Heron, 1981). The mean tidal range at Shackleford Banks is 0.89 m and tidal currents in excess of 115 cm s-1 have been measured in the Outer Banks (Sarle, 1977). These currents are responsible for the movement and deposition of sediment in the tidal inlets, tidal deltas, and lagoons. So, even though they are often overshadowed by wave processes, tides, and tidal currents make significant contributions to the sedimentary record. On Shackleford Banks, with its relatively low wave energy, tides are of particular importance in transporting sediment in the inlet and the sound. Depositional environments

As a classical wave-dominated micro-tidal barrier island shoreline, Shackleford Banks contains a variety of sedimentary environments. These environments include the offshore-transition zone, the shoreface, the foreshore, the backshore, the aeolian dunes, overwash fans, the backbarrier (including lagoon, tidal flat, and salt marsh), flood and ebb tidal deltas, and the inlet. Although the geographic position of most of these environments is clear, the subdivisions of the nearshore zone (the beach face and its components) are not always so evident. For this project, we will use the beach face terminology defined below (from Elliott, 1986) and as illustrated in Figure 4. Figures 5 and 6 show the geographic distribution of many of the barrier environments, the types of cross stratification common in those environments, and their characteristic processes and facies. Offshore-transition: the zone that extends from mean storm wave base to mean fairweather wave base. This zone is characterized by alternating high and low energy conditions. Shoreface: the zone that extends from mean fair-weather wave base to mean low water level. During fairweather conditions, oscillating and shoaling wave processes operate in the lower (outer) part of this zone and breaking wave/surf zone processes operate in the upper (inner) area. Foreshore: the intertidal part of the beach face. Swash zone processes operate in this zone during fairweather conditions. The foreshore is usually separated from the backshore by a low ridge or "berm". Backshore: the supratidal area between the foreshore and the aeolian dunes. This area is inundated only during storms. Aeolian dunes: backshore dunes. These dunes are the result of wind reworking of sand placed on the backshore by storm waves. They may attain a height of several meters. FIGURE 4. Beach face subenvironments, processes, and facies (Elliott, 1986).

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FIGURE 5. Schematic map and cross sections showing the major environments and facies of a barrier island-lagoon (McCubbin, 1982).

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FIGURE 6. Block diagrams showing common stratification types and associated bedforms in coastal-marine sands (McCubbin, 1982). See your lecture notes and handouts for variations in form that result from changes in grain size, flow conditions, and rates of deposition.

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Clifton, Hunter, and Phillips (1971) and Hunter, Clifton, and Phillips (1979) studied the processes, bedforms, and facies on high-energy coasts in detail and presented the most complete models for the offshore-transition zone and the shoreface (Figures 7 and 8). A brief summary of the nearshore facies presented in their work follows. Because our field study will not include the facies present beyond the swash zone, we will use the Clifton and Hunter model as a supplement to our work. As such, you will find the following summary and diagrams useful in completing the requirements of the field project.

OFFSHORE In this zone, waves begin to "feel" bottom and the landward velocity component of the

wave orbitals becomes important. The resultant primary sedimentary structures are small-scale asymmetric ripples. The small-scale ripples may be obliterated, however by intense biological activity (burrowing) and the zone is typified by bioturbated fine sand and silt.

NEARSHORE Outer Rough Facies

As waves move toward the beach, the interaction of the waves with the bottom increases and the landward velocity component increases. This facies is deposited in the zone of wave build-up just seaward of the breaker zone. It is characterized by shoreward-dipping lunate megaripples (3D dunes). Outer Planar Facies This facies is deposited in the breaker zone where high velocity shoreward-directed currents are dominant. Current velocity is high enough in this zone to be considered upper flow regime and upper flow regime planar bedding is the dominant sedimentary structure. Inner Rough This facies is deposited at the shoreward edge of the breaker zone where return flow dominates and offshore-directed bedforms are abundant. On complex and barred coastlines, this facies may contain both alongshore and offshore directed sedimentary structures. The upper (landward) portion of this facies -- called the toe of the beach -- usually contains the coarsest material in the nearshore zone. Inner Planar Shear sorting and high flow velocities is the swash zone result in the formation of the inversely graded, planar laminated sands that characterize this facies.

It is not uncommon to encounter some of these facies in the shallow subsurface when trenching in the foreshore and the backshore. Careful examination of our beach trenches will provide clues to the vertical sequence that is likely to occur as a result of a long-term change in sea level. FIGURE 7. Processes, bedforms, and facies on a non-barred high energy beach face (after Clifton, Hunter, and Phillips, 1971).

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FIGURE 8. Line drawings of the internal structure of the inner rough (A), outer rough (B), outer planar (C), and inner planar (D) facies (after Clifton, Hunter, and Phillips, 1971).

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The Project Logistics and Safety We will leave Greenville at 6am and arrive at the ferry dock in Beuafort by 8am. Our boat trip to Shackleford Banks will take about 30 minutes. ALL TRIP PARTICIPANTS MUST WEAR A LIFEJACKET WHILE THE BOATS ARE UNDERWAY. Because space may be cramped, please exercise caution while onboard and follow the boat captain's instructions at all times. We will traverse the western end of the island (Figure 2b) in the morning, making several stops along the way to examine the various sedimentary environments. At the southern end of our traverse, we will spend several hours trenching and describing the foreshore and backshore areas of the beach. We will return to our docking point in the afternoon by circling around the western end of the island. Depending on the tide level, we may make several additional stops on our return traverse. If time and the tide permit, we will stop to examine the flood tidal delta (on the mainland side of the sound) on our way back to Beaufort. Expect to be in Beaufort by about 5pm and back in Greenville between 7 and 9pm. It is important to remember that the island is part of the National Park System and must be treated with the utmost respect. We will leave no trace of our activities on the sound side or in the dune fields and will refill our trench on the beach. No one is to disturb the dune area in any way without prior approval and careful supervision. The dunes are delicate and the park Service is very serious about their preservation. We will dig only small holes in the inner dune areas and we will refill the holes and replace the grass cover (as undisturbed as possible) when we have finished our analysis. We will not dig in the active dunes. Bug spray, sunscreen, and foul weather gear are highly recommended. Requirements This project requires you to describe the physical and biological characteristics of the modern nearshore facies, to make observations and interpretations of the processes operating in the depositional environments of these facies, and to construct a facies model for a prograding nearshore system. The final result of this work will be a well-written, formal geological report that contains your observations, interpretations, and facies model. The report must conform to the report writing guideline you have been given in lecture, lab, and handouts. You may work in teams, but everyone must turn in individual reports and diagrams. A general outline for the report is attached. AT EACH STOP you will do the following:

1) plot the location on your field map (use Figure 2b for this purpose). 2) identify the depositional environment(s). 3) describe the facies characteristics -- either by direct observation of surface features

or by describing sediments exposed in a trench (trenching procedure is outlined below).

4) make a sketch or sketches in your field notes that illustrate(s) the important characteristics of the facies -- if a trench is dug, you should include a sketch of the trench in your notes. These sketches are important. You will include a sketch of each facies in your final report.

5) make observations and interpretations about the depositional processes operating in the environment.

The characteristics to be described include (but are not necessarily limited to) the following: grain size, sorting, texture, and composition; primary and secondary sedimentary structures; biologic structures; and biologic (fossil or modern) components (including shell and plant material). You are not required to identify biologic components at the genus and species level. The attached summary of common coastal flora and fauna (Appendix I) may be helpful. We will dig at least two trenches during the course of this project. A few general procedures should be followed to assure that we get a good exposure of the subsurface strata:

1) orient the trench wall so that it faces the sun 2) never walk of the wall side of the trench 3) when working in groups, be careful not to injure each other with the shovels

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Content of report

You are to write a report that details the methods and data used to develop a model for a progradational nearshore sequence and that describes that model fully. You are encouraged to use the attached general outline as a guide to report format and to review your notes and geowriting handouts. You are also encouraged to use a dictionary/spell checker and a good style manual to assure that your writing is technically sound. The due dates and editing deadlines for this project are posted on the course web site (http://core.ecu.edu/geology/rigsbyc/rigsby/Sedimentolgy/2008/writing.html and http://core.ecu.edu/geology/consult.html). Please take them seriously. The following data and interpretations are to be turned in as part of your formal report. Please note that all diagrams must be neat, legible, and properly labeled. Also note that this list contains only project-specific items. Other materials (location maps, reference lists, etc.) are also required. Your knowledge of fundamental necessities is assumed.

1) A map of the field area showing all sample and trench localities. 2) Complete descriptions of all facies shown on your map and included in your model. 3) Diagrams of each facies (internal structure of the sediments). 4) Interpretations of the processes that resulted in the described facies. 5) A discussion of the hypothesized vertical sequence of strata that would result from

progradation of this nearshore system (your model). 6) A vertical stratigraphic column that summarizes your model (see the lab handout) 7) A very brief discussion of the events that would be necessary for the deposition and preserve

the vertical sequence described by your model.

Shackleford Banks Report Outline

I. Title Page II. Abstract III. Introduction IV. Methods V. Lithofacies Descriptions

Describe the setting and the physical and biological characteristics of each lithofacies. -include diagrams of all of the lithofacies

VI. Processes of Lithofacies Deposition Discuss the processes active in each environment that are responsible for the formation of the lithofacies described above. Reference work that is not your own! -a diagram that shows all of the facies may be useful here

VII. Facies Model Discuss and describe your model for a progradational nearshore sequence based on the lithofacies you have presented above. Include a very brief discussion of the events that may cause such a sequence to be deposited and preserved in the stratigraphic record. -include a vertical stratigraphic column

-a block diagram illustrating the progradational nature of the facies may also be useful here

VIII. Summary and/or Conclusions IX. References Cited

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References Cited

Clifton, H.E., Hunter, R.E. and Phillips, R.L., 1971, Depositional models from a non-barred, high-energy

nearshore: Jour. Sed. Petrology, v. 41, p. 651-670. Elliot, T., 1986, Siliclastic shorelines IN, Reading, H.G., ed., Sedimentary Environments and Facies,

Blackwell Scientific Publ., pp. 113-154. Heron Jr., S.D., Moslow, T.F., Berelson, W.M., Herbert, J.R., Steele III, G.A., and Susman, K.R., 1984,

Holocene sedimentation of a wave-dominated barrier-island shoreline: Cape Lookout, North Carolina: Marine Geology, v. 60, p. 413-434.

Hunter, R.E., Clifton, H.E. and Phillips, R.L., 1979, Depositional processes, sedimentary sequences, and

predicted vertical sequences in barred nearshore systems, southern Oregon coast: Jour. Sed. Petrology, v. 49, p. 711-726.

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. Moslow, T.F. and Heron Jr., S.D., 1981, Holocene depositional history of a microtidal cuspate foreland

cape: Cape Lookout, North Carolina: Marine Geology, v. 41, p. 251-270. Moslow, T.F., and Heron Jr., S.D., 1986, Cape Lookout cuspate foreland depositional systems, SEPM

Fieldtrip Guidebook SEPM third Annual Midyear Meeting, Field Trip No. 12, p. 399-413. Moslow, T.F. and Heron Jr., S.D., 1989, Outer Banks depositional systems, North Carolina: Amer.

Geophys. Union, Field Trip Guidebook T171, 28th International Geological Congress, 28 pp. Sarle, L.L., 1977, Processes and resulting morphology of sand deposits within Beaufort Inlet, Carteret

County, North Carolina [unpubl. M.S. thesis]: Duke university, Durham, NC, 149 pp. Susman, K.R. and Heron Jr., S.D., 1979, Evolution of a barrier island, Shackleford Banks, Carteret

County, North Carolina: Geol. Soc. Amer. Bull., v. 90, p. 205-215.

Appendix I: Page 1 of 3

Appendix I: Some Common Coastal Flora and Fauna

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