Sedimentology Field Trip #1 Modern Fluvial Deposits of the Tar River, Eastern Pitt County, North Carolina
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Sedimentology Field Trip #1 Modern fluvial deposits of the Tar River, eastern Pitt County, North Carolina The Setting The modern sediments 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 floodplain, its transport and deposition of sediment from upstream and local sources, and its response to regionally and locally controlled variations in current 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 drainage basin 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 stream 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 channel 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 bank-full flow. river channel: topographic low where flow is concentrated during normal flow levels. levee: a low, broad topographic high that separates the channel from the floodplain. Levees are constructed by the river during times of flood and confine the subsequent flows to the channel except during very large floods. floodplain: low-lying, relatively smooth area adjacent to the river bed. Floodplains are constructed by the river and are subaerial except during very large floods. crevasse splay: 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. meander: 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. cut bank: the steep bank on the outside of a meander formed by lateral erosion of the stream. point bar: a low ridge of sediment deposited on the inside of a meander by lateral accretion during bar formation and channel migration. thalweg: the line connecting the lowest points along the stream bed; 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 bedforms (ripples, dunes, and minor bars) on the channel floor and on the point bar. 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 meanders) 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 overbank 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 aggradation (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 3 Figure 3.SoilFigure map survey Karnowski (from et al. 4 , 1974) used as a base,asmap a 1974) used field work. for 1 Mile / / / 0 5000 4000 3000 2000 1000 0 5000 Feet Scale 1:15840 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.