Abstract

Cenozoic tectonism can be seen in a variety of different ways in the Coastal Plain of

North Carolina. Using high resolution Light Detecting and Ranging digital elevation models

(LiDAR), the geology of the coast can be analyzed to look for evidence supporting recent tectonism and disequilibrium. The in the Coastal Plain, as well as other features, provide much evidence to support this theory. Looking at the complicated geomorphology of the rivers, particularly the Cape Fear , gives insight to potential fault locations along the rivers. Most of the exhibit capture, another sign of disequilibrium, and the amount of stream capture was quantified on the Cape Fear River. There is a southwest migration trend in the major rivers of the Coastal Plain. While the Cape Fear Arch can explain this trend in the Cape Fear

River, it does not explain the southwest migration in other rivers in the north as well.

Knickpoints were found in all of the rivers in the Coastal Plain. While most of them can be associated with sea level fall or changes in underlying lithologies, there are many that are caused by tectonics. Wave-cut escarpments were once deposited horizontally in various places due to rises and falls in sea level. Elevation profiles along these scarps show a higher elevation in the north than in the south, which could be explained by uplift. This uplift would also account for the southwest migration trend in the rivers.

Introduction

As every geologist knows, everything on Earth is constantly changing. Mountains are being formed as well as destroyed, plates are moving, new crust is being formed as old crust is subducted and recycled, and rivers are moving. Our Earth is incredibly dynamic and is always changing. Even the coast of North Carolina, which was thought to be tectonically inactive, has changed drastically since the Pleistocene. Our coast is in disequilibrium, and active tectonics is the cause. Evidence of syndepositional and post-depositional tectonics can be seen in the geology of our coast, and more specifically, the rivers and paleoshorelines.

The sediments of the coastal plain were deposited during transgressive-regressive cycles caused by eustatic sea level fluctuations, which were partially caused by the expansion and recession of glacial ice caps (Soller). During an interglacial period, high sea level allowed marine sediment to be deposited, and falling sea level during the onset of glaciation caused regression, incision, and (Soller). These sequences occurred multiple times and are seen on the coast. During the maximum transgressions, erosional, wave-cut scarps were formed, marking the landward extent of a cycle’s deposits, or the paleoshores (Soller). Elevation differences, from north to south, in these scarps provide supporting evidence for uplift (Rowley).

There has been documented uplift occurring in North Carolina since the Cretaceous. The

Cape Fear Arch, located slightly northeast of the Cape Fear River, is the predominant upwarp in the coastal plain (Soller). The Norfolk arch in southern Virginia and northern North Carolina is also another location of upwarp, but is not as prominent (Soller). The Neuse Arch is in between these two arches and is also not as predominant (Rowley). The Cape Fear Arch has helped the

Cape Fear River to migrate in a southwestward direction. However, the Cape Fear River is not the only river that shows signs of southwest migration. In fact, all of the rivers in the coastal plain of North Carolina show a southwest migration trend. This could either be from regional uplift along the entire coastal plain, or faults (Bartholomew).

The Cape Fear River tells us a lot about what has been happening during and after deposition, especially because it is one of the oldest and clearest rivers in the coastal plain. The geomorphology of the Cape Fear has changed drastically from a confined channel, to a braided river system, to a wandering meandering river, to an entrenched river. The Cape Fear River has also migrated significantly from its initial position, providing evidence for uplift. Dunes deposited by the river show evidence of migration. As the transgresive-regressive cycles occurred, terraces were formed as the river migrated. These terraces can be dated and correspond with the fluctuations in sea level. Other key evidence associated with the Cape Fear River supporting disequilibrium in the coast includes stream capture and knickpoints in stream profiles.

Stream capture occurs essentially when a stream flows into a previous stream. Originally, the streams were moving in two different directions, but tectonics caused a shift in the river movement, allowing one stream to capture another. Knickpoints clearly indicate that the river gradient is in disequilibrium and is trying to reequilibrate.

The coastal plain gives us very clear evidence that the coast is not in equilibrium. An analysis on the scarp elevations and the change in the Cape Fear River will provide further evidence to support the hypothesis that the coast is in a state of disequilibrium. The area of interest can be seen below in Figure 1.

Figure 1. Map of North Carolina with area of interest, or the coastal plain, indicated by elevation data.

Methods

With non-consolidated rocks and few outcrops, the Coastal Plain is very difficult to analyze from the field. By using Light Detection and Ranging, or Lidar, data and the useful tools of ArcGIS, most of the important features on the coast can be observed and analyzed, providing supporting evidence for disequilibrium on the coast. Lidar data is extremely clear with a high resolution, providing an accurate and very detailed elevation map. All of the Lidar data was obtained through the North Carolina Department of Transportation, who obtained this data from the North Carolina Mapping Program. Elevation grids were available for all fifty counties in North Carolina with a twenty foot cell size. Only the counties located in the coastal plain were used and mosaicked together create a single map of the entire coastal plain of North

Carolina.

The general coastal geology as well as key structures can be clearly seen in the Lidar elevation grids. The paleoshorelines or scarps are clearly identified because of the elevation differences between the higher, older coastlines from high stands, and the lower sediments deposited as the shoreline was prograding. Using the 3D Analyst tools on ArcMap, these scarps can be traced, and the elevation along them can be plotted. Large elevation differences trending from north to south show how sediment that was originally deposited nearly horizontally, have changed and have been either uplifted or depressed, supporting the hypothesis that our coast is in disequilibrium. The most easily identifiable scarp would be the Suffolk Scarp. The Orangeburg

Scarp and the Surry Scarp produce very noisy results and will not be analyzed, except for one small section of the Surry Scarp.

The geomorphology of the rivers and how they have changed throughout time is also clearly expressed in the Lidar grids. The Cape Fear River has changed drastically over time from braided to meandering and has left its mark on the coastal plain. The sand dunes associated with the movement of the Cape Fear River as well as the different terraces associated with rise and fall in sea level can be seen very clearly. A history of how this key river has changed throughout time can be explained using the Lidar grid.

Figure 2. Map showing locations along the Cape Fear River on which cross sections were made.

The movement of the rivers through time can also be seen and quantified. The location of the Cape Fear River has changed drastically since the Pleistocene, and this shift or river movement can be identified. The Cape Fear River has moved southwestward, both on a long term scale across the coastal plain as well as a short term scale in the current flood plain. Both movement trends were quantified. The short term trend was quantified by using the 3D Analyst tools on ArcMap to plot cross sections of the flood plain every approximately 30,000 meters along the river (Figure 2). Some locations were moved due to complicated surrounding geology, such as , which would make the flood plain appear wider than it really is. Also, the floodplain is harder to see closer to the coast, so the cross sections do not extend all the way to the ocean. Using the cross sections, the width of the flood plain as well as the location of the river were calculated. The location of the river within the flood plain was then normalized to account for the difference in width of the flood plain along the river. Positions closer to zero indicated the most southwest position of the river in the floodplain, and values closer to one hundred indicate the most northeast position location of the river. The location of the river was then plotted against the distance, showing the movement across the current flood plain. To calculate the total distance the river has moved, the distance tool was used to calculate the distance between the initial river position and the position the river is currently in now. This was only done in the most exaggerated portions of the river, where the initial position was easily identifiable.

Stream capture is also clearly expressed in the Lidar grids with the right color ramp and elevation values. The amount of stream capture occurring along the Cape Fear River was quantified by basically figuring out how many tributaries coming off the Cape Fear River show signs of stream capture. Only tributaries that extend past the floodplain were used, as the tributaries that do not extend pass the flood plain are too short and immature to see how they behave around other tributaries and streams. Also, only the tributaries on the southwest side of the Cape Fear River were analyzed. The tributaries on the northeast side are not as long and are fewer in number. The tributaries on the southwest side are diverse and plentiful. Tributaries that branch in multiple directions were only accounted for as a single , even if multiple branches showed evidence of stream capture. The total number of all the tributaries was determined, as well as the qualifying tributaries that extend from the Cape Fear River. The qualifying tributaries were then split up into three categories: one showing no evidence of stream capture, one where there is clear evidence of stream capture, and one where the tributaries show evidence of stream capture, but have not been captured yet. The number of tributaries in each category was then put into a percentage to quantify the number of tributaries that show stream capture.

A stream profile of the Cape Fear River was created by using many tools in ArcMap, as well as data generated from the Shuttle Radar Topography Mission (SRTM), since the Lidar data was too large to analyze. Fill, Flow Direction, Flow Accumulation, and Flow Length were all performed first. Then using Model Builder, the tables to create the profile were generated using the setup seen in Figure 3. The tables were then used to create the stream profile. Using a program written by Jesse Hill, Ph. D student at the University of North Carolina, the stream profiles were plotted. The program also automatically selects knickpoints based on certain parameters. First, the program smooths out the profile. Then four different parameters, normalized steepness (ksn), smoothed slope, sum of the differences, and vertical drop. The normalized steepness is the slope divided by the drainage area raised to the concavity index. It finds the normalized steepness based on a concavity index of 0.45. The slope is fairly low for river profiles, but knickpoints have an increased slope. The vertical drop must be a greater than a certain amount for a knickpoint to occur, but also not too much. This part of the function gets rid of dams that look like knickpoints. The sum of the differences is a ratio of the elevations a certain distance upstream from the knickpoint over the elevations the same distance downstream from the knickpoint. The program then finds points that satisfy every condition. If one of the conditions is not met, then the program disregards that point. The ksn value used for generating these profiles was generally 110, although 280 was used in cases to analyze parts of the stream profile closer to the ocean. A slope of greater than 0.003 was used. The sum of the differences was greater than 1.7, and the vertical drop was greater than 2 meters since the Coastal Plain is relatively flat.

Figure 3. Image of the ModelBuilder tool used to create the stream profile.

Results

Figure 4. Five different sections of the Cape Fear River which were broken down according to geomorphologies.

The Cape Fear River can be broken down into five different sections based on the geomorphologies (Figure 4). The first section, Section A, is located in the most northern section of the river and does not show any large scale river migration trend like the rest of the river

(Figure 5). The floodplain is confined, and the river shows entrenchment. There is slight movement back and forth in the floodplain, but there are no significant trends. There are many large tributaries branching from this section of the river, the majority of which show stream capture. Section B shows a recent, shorter, northeast migration trend as well as few, long tributaries flowing in the same direction. The river shifts from a sporadic, thin, immature meandering river in the southwest to a mature, less sinuous meandering channel in the northeast

(Figure 5). Section C shows the typical southwest migration trend, as well as a shift from a complex braided river system to a much simpler meandering river system (Figure5). Section D shows a recent, strange migration trend that is very short and temporal (Figure 5). This part of the river also shifts from a braided river to a meandering river. The only long tributary joining this section of the river trends north east as well, showing that flow direction in that area is directed towards the northeast. Lastly, Section E shows a smaller southwest migration trend superimposed on the longer southwest migration trend and is characterized by sand dunes and terraces (Figure 5).

Figure 5. These sections correspond to the sections shown in Figure 4, but are enlarged to enhance geomorphologic features.

Figure 6. Map of the different scarps and surfaces that are plotted.

The Suffolk, Surry, and Orangeburg Scarps are clearly identifiable in the Lidar grids, as well as other surfaces that show elevation differences (Figure 6). The typical north to south trend in elevation is decreasing. There are noticeably higher elevations on the most north end of the scarps than the south end (Figure 7). There is strange noise in the elevation data, but after performing the tool multiple times, the elevation profile is correct. The profiles generated for the

Orangeburg and Surry Scarps were too noisy to analyze, except one section of the Surry Scarp which shows an elevation difference of about 20 feet (Figure 7). The elevation difference on the

Suffolk Scarp is roughly 20 feet. Rivers cutting through the cross sections produce noise due to the huge elevation difference in the river bed and the surrounding flood plain, yet the trend is still visible on the cross sections.

Figure 7. Cross sections showing the trend of decreased elevation from north to south among the Suffolk Scarp and a section of the Surry Scarp.

Figure 8. Image showing the southwest migration trend in the Cape Fear River.

The southwest migration trend in the river is clearly seen in the Lidar grids (Figure 8), but quantifying this trend is important. Over the short term migration, the river, for the most part, has migrated southwest (Figures 8 and 9). Besides the most northern portion and the odd part of the river that sticks out towards the northeast, the river has shown a southwest migration trend.

Only recently have parts of the river started shifting back to the northeast, as such trends are not seen in the old river beds. On the long term scale, the river has migrated at least approximately

100,000 meters but possibly as much as 130,000 meters as shown in Figure 10. Evidence showing that the river was previously further northeast and has migrated southwest would be the sand dunes that were deposited from the Cape Fear River. As the river moved southwest, it deposited more dunes parallel to the previous dunes and in lines parallel to river flow. This large scale migration trend occurs along the majority of the Cape Fear River in the coastal plain, shown in Figure 10.

River Position/Floodplan Width 100

80

60

40

River location River 20

0 0 100000 200000 300000 400000 500000 600000 700000 Distance (m)

Figure 9. Plot of the normalized river position in the floodplain. Values close to 0 represent the most southwestward location and values closer to 100 indicate the most northeast location.

Figure 10. Map showing the long term migration trend of the Cape Fear River, as well as its current floodplain and the extent of older floodplains.

Stream capture occurs frequently along the Cape Fear River among its one hundred and eighty-one tributaries. Only fifty-seven of these tributaries were used in quantifying stream capture because some were too short and did not extend past the flood plain. Out of the qualifying tributaries twenty, or 35.1 percent, showed stream capture, eighteen, or 31.5 percent, showed no stream capture, and nineteen or 33.3 percent, showed evidence of future stream capture (Table 1). Figure 11 shows examples of each scenario: prominently displayed stream capture, no stream capture, and evidence of future stream capture.

Tributaries Number Percent

Qualifying tributaries 57 31.5%

No stream capture 18 31.5%

Prone to stream capture 19 33.3%

Definite stream capture 20 35.1%

Table 1. Table showing the number of the different types of tributaries as well as the percentage of each type.

Figure 11. Images displaying the degree of stream capture. The first image shows definite stream capture, the second image shows potential stream capture, and the last image shows no stream capture.

Knickpoints occur in all of the rivers in the Coastal Plain (Figure 12). Most of the knickpoints occur in the higher elevated regions of the Coastal Plain further to the west. As seen in Figure 12, few knickpoints are located on geological contacts. There are several elevations at which multiple knickpoints occur. There are four occurring at elevations between 29 and 30 meters, eight occurring between 33 and 36 meters, six from 49 to 50 meters, five from 57 to 58 meters, seven from 63 to 65 meters, ten from 84 to 87 meters, 7 from 112 to 115 meters, five from 146 to 148, and five from 215 to 218.

Few knickpoints occur very close to the water. There are very few knickpoints in the area with underlying Cretaceous mud or clay around the Cape Fear River. More knickpoints are concentrated in the north than the south.

Figure 12. Map showing underlying geology contacts, streams in red, and knickpoints as dots. The colors of the dots represent the elevations at which the knickpoints are located. Gray is low elevation and purple is high elevation. The

Cretaceous mud and clay is the purple colored unit.

Discussion

There is a huge change in geomorphology from an entrenched river with a very narrow floodplain to a more sinuous, meandering river with a large long term floodplain that expands towards the ocean. Section D, the section that juts out to the northeast, is a very short and irregular trend in the river pattern, as it deflects far away from the original path, which appears to be more efficient. This can be explained by a small, localized fault that is impacting the flow of the river (Bartholomew). Flow is clearly redirected in the southeast direction, as seen in the tributary connecting to this strange bend. The shift from a braided river to a meandering river shows a decrease in energy over time. There is a very distinct southwest migration trend, both short term and long term, for the majority of the Cape Fear River. Overall, the Cape Fear River has migrated over 100,000 meters, which would not have occurred without reason. This can easily be explained by uplift from the

Cape Fear Arch; however, since this trend is apparent in all of the rivers, the uplift probably starts further north. The enhanced trend on the Cape Fear may be as such due to the combination of the Cape Fear Arch, as well as another source of uplift.

The majority (68.4%) of the tributaries on the southwest side of the Cape Fear River either clearly show stream capture, or show potential for stream capture. This is a fairly high number and is a clear indicator that the coastal plain, at least around the Cape Fear River, is in disequilibrium. The other rivers in the coastal plain do show evidence of stream capture, but were not quantified. Stream capture can occur from channel migration, tectonism (tilting, doming, etc), catastrophic avulsion from high stream flows, or aggressive headward erosion

(Bishop). While some of the places where stream capture occurs are from channel migration, catastrophic avulsion, or aggressive headward erosion, some of the places that show stream capture show no signs of any of the aforementioned, but might possibly have been generated due to tectonism in the area.

The stream profiles show many knickpoints in every major stream or river in the Coastal

Plain. The knickpoints are controlled by several factors, such as underlying geology, sea level fall, and tectonic activity. There are hardly any knickpoints in a Cretaceous aged unit in the southern part of the Coastal Plain as opposed to the Tertiary clay and mud deposits in the north.

The knickpoints that occur around the same elevations are most likely caused by a fall in sea level since the knickpoints would have migrated at the same rate. There may be some knickpoints in these areas caused by tectonics, although they are most likely correlated to a fall in sea level. There are still knickpoints leftover after the lithologies and sea level falls have been accounted for. These knickpoints are tectonically related and are caused by uplift or small faults in the area. Since the Coastal Plain is relatively flat, uplift and faults occur on small scales of less than ten meters, making them very difficult to identify. The knickpoints show that uplift or faults are occurring and that the coast is in disequilibrium.

Conclusion

There is substantial evidence supporting the hypothesis that the Coastal Plain of North

Carolina is in disequilibrium and is facing, at the very least, uplift associated with tectonics. The elevation difference from north to south along the scarps shows that there must be uplift somewhere in order for the elevation differences to occur. The Cape Fear River tells us a lot about what is occurring on our coast. The river has also changed drastically over time, mainly from braided to meandering, as the stream attempts to reach equilibrium. The Cape Fear River, as well as many of the other rivers in the coastal plain, has migrated in a southwest direction. The

Cape Fear River has moved at least 35,000 meters on a long term trend, which is caused by uplift in the north. The change in geomorphology along the Cape Fear and other rivers, such as abrupt sharp bends and channels that seem to take paths of higher resistance, provide evidence for faults around these river channels. Stream capture, another piece of evidence for disequilibrium shows that roughly two thirds of the tributaries on the southwest side of the Cape Fear River either clearly show stream capture, or appear to be prone to stream capture. Many other tributaries off of other rivers show stream capture as well. There are many knickpoints along the streams in the coastal plain. Some of the knickpoints occur along contacts where the lithology of the rocks change. Many are caused by a fall in sea level. These knickpoints occur at the same elevations across the coastal plain. There are leftover knickpoints that cannot be connected to sea level fall or change in lithologies. These knickpoints can be caused by uplift from the north, recent uplift over the past 65 million years in the Appalachian Mountains, or faults in the coastal plain. There is a lot of evidence supporting North Carolina’s dynamic coast has changed drastically since the

Pleistocene and is still currently changing.

References

Bartholomew, M. J., Rich, F. J., 2013. Pleistocene shorelines and coastal rivers: Sensitive

Potential Indicators of Quaternary Tectonism Along the Atlantic Coastal Plain of North

America. Geological Society of America Special Papers 493, pp. 17-36.

Rowley, D. B., et al., 2013. Dynamic Topography Change of the Eastern United States Since 3

Million Years Ago. Science 340, pp. 1560-1563.

Soller, D. R., Mills, H. H., 1991. Surficial Geology and Geomorphology. The Geology of the

Carolinas: The University of Tennessee Press, Knoxville, pp. 290-308