by Leslie R. Sautter, Co-science Lead, Department of Geology and Environmental Geosciences, College of Charleston
During the Windows to the Deep 2018: Exploration of the Southeast U.S. Continental Margin expedition, we will explore a variety of seafloor habitats. Many of these habitats are present as a result of the seabed’s geology, which ranges from vast, undulating expanses of sand, to nearly vertical rocks. To understand the foundation of the sites we’ll visit, let’s first look at the configuration of the entire region off the coast from North Carolina to Florida, referred to as the Southeast U.S. Continental Margin.
A continental margin is the continent’s enormous underwater portion that extends from the shoreline to water depths of over 4,000 meters (about 13,123 feet). Continental margins in the Atlantic Ocean are typically comprised of three provinces: the shelf, slope, and rise. Each province has distinctly different characteristics that are the result of its distance from shore, steepness, and water depth.
The continental margin off Cape Hatteras, North Carolina, exhibits a classic shelf-slope-rise sequence (Figure 1), beginning with a broad, flat continental shelf that reaches offshore to a depth of approximately 200 meters (656 feet). Globally, the continental shelf has been exposed to air many times throughout geologic history as sea levels have changed. During the most recent cold period in Earth’s climate 18,000 years ago when glaciers were expansive, sea level was lower by about 120 meters (394 feet), and the shoreline was located close to the edge of the continental shelf.
The continental slope is the steep transitional area between the shelf and rise, and lies between approximate depths of 200 and 2,500 meters (656 to 8,202 feet). Here is where deeply incised submarine canyons are found – some of which begin on the adjacent shelf (Figure 2). The rocky walls of these submarine canyons provide excellent habitat for deep-sea corals and other organisms that require hard surfaces on which to attach.
The continental rise is the base of the continental margin and leads to the deep ocean seafloor known as the abyssal plain. Most of the rise’s seafloor morphology, or shape, is the result of kilometer-thick deposits of gravel and sand-rich sediments that have been transported from the shelf to the deep sea by way of submarine canyons.
South of Cape Hatteras, between Cape Fear, North Carolina, and Florida, the continental margin morphology strays from the classic profile described above. The shelf’s width off Hatteras is approximately 30 kilometers (about 18.6 miles), but southward expands to more than 120 kilometers (74.6 miles) off the Georgia coast. In this region a huge, broad feature lies beyond the continental shelf, ranging in depth from approximately 300 to 1,200 meters (984 to 3,937 feet) (Figure 3). This feature is known as the Blake Plateau, which looks like a stepped-down terrace off the shelf. (A feature known as the Charleston Bump, explored in 2003, sits on the plateau, east of South Carolina.) The eastern edge of the plateau descends rapidly along the Blake Escarpment, sometimes as much as 3,500 meters (11,483 feet) over a distance of only 10 kilometers (32,808 feet)! Two adjacent features east of the escarpment are the Blake Spur and the sizeable Blake Ridge, the latter which includes massive mega-ripple bedforms of sand, as well as gas seeps.
The unusual continental margin profile of the southeast U.S. is the result of geologic formation and transformation. Previous studies suggest that over many millions of years, the entire plateau subsided, or "sank," from much shallower depths, in part due to the plate tectonic widening of the North Atlantic Ocean. As we continue to explore the seafloor using a remotely operated vehicle, high-resolution mapping, and other technologies, we expect to discover and better understand many new geologic features, seafloor habitats, and benthic communities on this unique and largely unexplored continental margin.