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Home , Crevasse splay, Cut bank

Sedimentology Field Trip #1 Modern fluvial deposits of the Tar , eastern Pitt County, North Carolina

The Setting The modern along the Tar River provide excellent examples of the extreme lateral variability of lithofacies present within a single depositional system. These lithofacies record variations in local depositional conditions that result from the river's migration across its , its transport and of from upstream and local sources, and its response to regionally and locally controlled variations in velocity. The Tar River (figure 1) originates near the southern edge of the Piedmont Province and drains into the Pamlico Sound in the Outer Coastal Plain Province (at this point the river is called

Figure 1. Index map showing the location of the Tar River on the Outer Coastal Plain and of the study area in Pitt County, North Carolina. the Pamlico River; the Tar and the Pamlico are the same except in name). The river's is approximately 4300 mi2, and the system has an average annual outflow of about 5400 ft3/sec (Giese et al., 1985). In Pitt County, the river is a low-relief meandering with low to moderately low flow volumes and velocities. The flow volumes and velocities are strongly affected by regional, local, and seasonal precipitation patterns as well as ocean and wind tides. Ocean tides influence the river upstream to Greenville. Tides ranges near the mouth of the river are less and 0.5 foot. Near Washington, however (because of decreased dimensions), tide ranges approach 1 foot. Between Washington and Greenville, the tide ranges gradually decrease until they are near zero at Greenville. Winds and wind tides can be more important than either ocean tides or freshwater runoff in generating currents and in changing water levels on the river, even as far west as Greenville. Strong westerly winds (with or without a simultaneous ebb tide), for example, produce low water levels and faster downstream current velocities. Similarly, strong easterly winds result in higher water levels and local still water or, locally upstream flow.

The System As with all meandering fluvial systems, the extreme lateral variability in the Tar River system can be described by a finite set of geomorphic elements. These elements are defined below and they, along with other aspects of the meandering fluvial system, are illustrated in figure 2.

river bed: the major course of the river; this area is covered during -full flow. river channel: topographic low where flow is concentrated during normal flow levels. : a low, broad topographic high that separates the channel from the floodplain. are constructed by the river during times of and confine the subsequent flows to the channel except during very large . floodplain: low-lying, relatively smooth area adjacent to the river bed. are constructed by the river and are subaerial except during very large floods. : a wedge of sediment on the inner margin of the floodplain formed by rapid sediment deposition at the site of a breach in the levee during very large floods. : a sinuous curve or bend in the river's course produced by the river as it moves freely from side to side across its floodplain or shifts its course laterally toward the . cut bank: the steep bank on the outside of a meander formed by lateral of the stream. point : a low ridge of sediment deposited on the inside of a meander by lateral accretion during bar formation and channel migration. : the line connecting the lowest points along the ; the deepest part of the stream.

In meandering river systems deposition occurs by both lateral and vertical accretion. During normal flow conditions, flow is confined to the channel and sediment is transported in the channels via low-relief (ripples, , and minor bars) on the channel floor and on the . Because of the unique geometry of a meandering stream, deposition is not uniform

2 across or along the river channel. Flow velocity is highest on the outside of the meander bends – along the thalweg – causing enhanced erosion in that area, formation of a cut bank, and an asymmetrical channel profile (see figure 2, front panel of block diagram). The high flow velocity at the thalweg also assures that only the coarsest sediment (sand and gravel) will be deposited in that area. The shallower parts of the channel (on the insides of the ) are sites of lower flow velocities and of enhanced sand deposition. Thus, the meandering river channel is maintained by erosion on the outer banks of its meanders (at the cut bank) and deposition on the inner parts (on the point bar). The point bar is the primary depositional environment in the channel. It is formed primarily by lateral accretion; building both laterally and downstream across the flood plain. During flood conditions flow causes deposition on and upbuilding of the floodplain. As the river overflows its banks the flow velocity decreases rapidly and sediment is deposited in levees and floodplains. Generally, sand- and silt-size materials are deposited closest to the channel margins, forming the levees. The finer material settles out of suspension from pools of water left standing on the floodplain as the flood waters recede. This upbuilding of the levees and floodplain is termed vertical accretion. During rising flood stage, the levees can be breached and a crevasse splay can form. Crevasse splay deposits are commonly wedge-shaped and may contain some of the bedload portion of the river sediment.

Figure 2. Block diagrams showing major morphological elements (a) and flood plain (b) in meandering river depositional system. Modified after Walker and Cant (1979).

The Project The main purpose of this field project is to analyze, and make interpretations pertaining to, the variations in grain-size within this meandering fluvial system. To this end, we will map and take samples from a short segment of the Tar River, just downstream from Greenville, NC.

Logistics and Safety We will launch our canoes from the boat ramp on Port Terminal Road (off Highway 33, just east of Greenville, figures 3 and 4). As we paddle downstream, you will examine and map the geomorphic features that are observable from our boats. We will also make several stops to

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Figure 3. Soil survey map (from Karnowski et al., 1974) used as a base map for field work.

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Figure 4. Google Earth image of the field trip area showing the Tar River, its floodplain, and the surrounding geomorphic feature and roads.

5 sample channel and bar sediments before reaching the site of our most concentrated study and the end of our downstream journey – a meander bend approximately 2½ miles from the put-in site. Our extremely early departure (6:30 a.m. from in front of the Graham Building) should alleviate any danger from ill-mannered humans in speeding motor boats. Nevertheless, safety is of paramount importance and ALL FIELD TRIP PARTICIPANTS MUST EXERCISE EXTREME CAUTION AT ALL TIMES WHILE TRAVELING IN THE CANOES. Remain aware of all boat traffic – including the other canoes – and paddle defensively. Do not paddle ahead of the group or lag behind and wear your life jacket at all times. When we reach a stopping point, secure your boat and, where feasible, pull it as far out of the water as possible. Always make sure your boat is secure before leaving it! Remember, you are responsible not only for your own safety, but for the safety of the equipment. You will be required to replace any lost or damaged equipment, including canoes, paddles, and lifejackets. The on-the-water part of the day will probably take until early afternoon. Upon completion of our field work we will return to the lab and do the initial preparations that will allow us to begin detailed grain-size analysis next week during our regularly scheduled lab period.

Requirements In the field, you will examine and map the geomorphic features along the river and collect samples for the grain-size analyses you will perform in the lab. During the course of the field work you will take detailed notes that describe the samples you collect, the sample locations, the geomorphic features you observe and map along the river, and any observations you make about the relationship between sediment grain size and depositional environment. You will find it helpful to make sketches of sections of the river, of trench profiles, and of sample locations in your notes. Also, be sure that you mark each sample with an identification number and include that number in your notes. Upon returning to the lab, you will double check your samples and make sure the sample numbers and notes agree, make a quick estimate of the range of grain sizes present in the samples, and prepare the samples for grain-size analysis. I will assist with this process, but you should understand the basic procedures before we collect the samples. Make sure that you read the supplemental reading (from Folk, 1974, and Tucker, 1988) before leaving for the field. At each of the study locations, and while traveling along the river, make the observations asked for below. You should work in teams of 2 or 3 (boat-groups might work well), but remember that reports are required of everyone individually, so each team member should keep accurate notes of team findings.

ALONG THE RIVER

As we paddle slowly down the river, mark the location of observable geomorphic features (point bars, in-channel bars, cut banks, levees, etc.) on a copy of your base map. It may be helpful to keep the map in a large zip-lock bag and mark on the bag with a waterproof marker. To do this you must mark both the map and the bag so that they can be repositioned in the same way each time they are used. Or, you may prefer to laminate the map or to make a simple tracing of the river's course on a sheet of waterproof paper (mylar?) and make your observations directly on the sheet. Any of these ways is fine. Just be sure you end up with a readable field map that you can redraw into a clean diagram for your report.

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Procedures for detailed study

Be prepared to get wet at all stops along the river, possibly very wet. What we see in each studied location depends entirely on the water level. We will either remain near our canoes or ensure that the canoes are securely tied up at all times. If your boat is not tied up, be sure that you or your rowing partner are in physical contact with the floating canoe at all times.

(1) Mark the location of this stop on your field map. (1) Describe the geomorphic form that is present. Be sure to note the relief of the feature, its aerial extent, and its position on the river. Make a map (diagram) in your field note book. Note that, in some cases, identifying the specific geomorphic features may be difficult. Try to figure it out, but if you can't, ask for help. Be sure to describe all the feature completely. (2) Describe the sediment – completely. Make sure your description includes size, shape, and composition. (3) Collect samples of the sediment. The samples should be of sufficient size to assure adequate quantities are available for analysis. They should be sufficient in number to fully describe the grain-size variation across the geomorphic feature. (4) For features on dry land, use the shovels to dig shallow trenches that allow examination of internal features. Use a machete to smooth the trench sides so that bedding and sedimentary structure will be more visible. As a start, consider making river-parallel trenches. One (or a few trenches shared by the group will do. Remember to mark the location of the trench on your field map. (5) Make a diagram of all trench walls in your field notebook. Label all of the lithologies and that are present and indicate grain size trends on the diagram. Include a scale. (6) Take representative samples of the sediment that is exposed in the trench and elsewhere on this geomorphic feature. Label the sample locations on your diagrams and carefully label the sample bags. (2) Where trenching is not possible, use a shovel or an auger (which I will provide) to sample the sediment. (3) Before transferring each sample to a sample bag, describe it and any sedimentary structures preserved carefully. (4) Bag and label each sample.

At our most distal location, depending on the water level, we may have to wade on a 2' to 3' wide ledge of mudstone to get to our study location. The mudstone is extremely slippery and the water is very deep beyond the ledge. Use caution.

IN THE LAB

Because you will be unable to analyze all of your samples in the time allotted for this project, we will divide the samples and sample locations among the class (after we return to the

7 lab) and each person will be assigned a set of samples. Everyone will analyze their own sample(s) and then distribute their results in tabular form to the entire class for analysis.

We will spend about 2 hours in the lab after we return from the field. Our work there will include the following tasks:

(1) Distributing samples. (2) Examining the samples with the goal of deciding what type of analysis (wet sieving, dry sieving, pipetting, etc.) is appropriate. (3) Preparing the samples for the various analysis techniques. This could involve splitting the samples, separating fines from sands, and/or drying the samples. The exact procedures undertaken will depend on the type of samples we collect. Remember, you should be familiar with the various grain-size analysis techniques before we go into the field!

It is very important that you use good laboratory techniques during this stage of the project. Sloppy work now means sloppy results later. Sloppy results mean that the samples must be re- analyzed. This is not a happy thought. Take your time and use care!

Summary The basic complexities of meandering river depositional systems have been presented to you in class and you have described sediments and sedimentary rocks and analyzed grain-size data in lab. In addition, you will receive guidance in the lab while analyzing the samples we collect in the field. From your field work and from your analysis of the grain-size data, you are to make interpretations that relate sediment size in the river to flow conditions and fluid dynamics. to this end you are required to turn in the following data and interpretations in the form of a formal geologic report (using the report-writing guidelines you will receive in lab in the following weeks):

(1) A map of the river (in the field area) that shows all of the observable geomorphic features, your sample locations, and your trench locations. (2) Descriptions of all geomorphic features shown on your map. (3) Complete descriptions of all of the sediment samples. (4) Histograms and cumulative frequency curves of all grain size data. (5) Statistical analyses of the grain-size variations in each sample. You may use either graphical methods or the method of moments for your analyses. (6) An analysis of the grain-size variations among the samples. (7) A discussion about, and interpretations concerning, the grain-size trends (or lack thereof) that were revealed from your field and lab work. The interpretations should be set in the context of variations in flow velocity, depositional environment, and geomorphic features within the meandering river depositional system.

8 References Cited

Folk, R.L., 1974, Petrology of Sedimentary Rocks: Hemphill Publ. Co., Austin, TX, 182 pp.

Giese, G.L., Wilder, H.B., and Parker, G.G., Jr., 1985, Hydrology of major and sounds of North Carolina: U.S.Geol. Survey, Water Supply Paper 2221, 108pp.

Karnowski, J.R., Newman, J.B., and Meadow, J.A., 1974, Soil Survey of Pitt County, North Carolina: U.S. Dept. of Agric., Soil Conservation Service, 73 pp.

Tucker, M., ed., 1988, Techniques in Sedimentology, Blackwell Sci. Publ., Palo Alto, CA, 394 pp.

Walker, R.G. and Cant, D.J., 1979, Facies Models 3. Sandy fluvial System, IN Walker, R.G., ed., Facies Models: Geoscience Canada, Reprint Series 1, pp. 23-31.

9 Tar River Report Outline

Use this general outline and your handout on writing geologic reports to complete the Tar River Field Trip report.

I. Title Page

II. Abstract

III. Introduction Includes: Location of trip (include a location map) General regional setting Name and type of river system, etc. Scope and purpose of exercise

IV. Methods Describe exactly how your data were obtained

V. Results: data and observations 1) Text -describe geomorphic features -describe lithologies -describe grain size results (briefly) -your diagrams (see below) will help here -a table summarizing the statistical data will also be useful 2) Most of your diagrams will go in the results section; refer to them in your text as you describe your data Include: map of geomorphic features histograms showing your grain size data cumulative frequency curves sketches of the trench sections

VI. Discussion -Discuss the variations/trends in grain-size among and within the different geomorphic features -Include an analysis of the validity of your data (are the changes you see real?) -Interpret your results, making sure that your interpretations are supported by your data and using supporting evidence (both field and literature) 1) Discuss the relationship between grain size and flow velocity 2) Discuss how this is or isn't controlled by the local depositional environment 3) Discuss any observations variations in grain size within single depositional environments

VII. Summary Briefly reiterate the major findings and conclusions from previous sections. Do not include any new information or interpretations in this section.

VIII. Reference List Use the referencing format discussed in class and on the geowriting handout.

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