Abstract Cenozoic Tectonism Can Be Seen in a Variety of Different Ways In

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 rivers 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 River, gives insight to potential fault locations along the rivers. Most of the streams exhibit stream 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 erosion (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 Flood 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 tributaries, 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.

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