Introduction

In 1987, the U.S. Geological Survey (USGS) completed fieldwork on a Geologic LOng-Range Inclined Asdic (GLORIA) sidescan sonar survey of the U.S. Atlantic Exclusive Economic Zone (EEZ-SCAN 87 Scientific Staff 1991). The resulting GLORIA mosaic image from this survey covered the continental slope and rise off North Carolina, one of the best-studied and most intensely sampled margins of the world. Although a great number of studies have been conducted in this area, the GLORIA image presented the first truly integrated picture of the North Carolina margin that depicts the interplay of three different styles of slope and rise processes: (1) gravity-controlled processes including canyon, channel, and fan systems; (2) mass wasting processes including slumps, slides, and debris flows; and (3) contour current-controlled processes including the deposition of large sediment drifts and the scouring of large areas of seafloor. This chapter examines the GLORIA sidescan sonar image and the USGS seismic records to review the interplay of these systems in shaping the slope and continental rise of the segment of the margin off North Carolina. It reexamines the major rise features such as the canyons and debris slides and comments on previous studies that have been based on the interpretation of 3.5-kHz echo character.

Abstract

Large sections of the Blake, Florida, and Campeche Escarpments have been imaged by GLORIA, and this common data type allows a comparison of the morphology of these escarpments and inferences about the erosional processes that have shaped them. Four morphologic provinces have been identified as follows: (1) shallow valleys with tributary gullies, which coincide with areas of minimal erosion of the platform edge; (2) box canyons, which overlie areas of differential basement subsidence and fractured carbonate rocks; (3) straight terraced sections coinciding with areas of more uniform basement subsidence, but where varying lithologies exposed at the platform edge are being differentially eroded; and (4) straight unterraced sections where the lithologies of the carbonate rocks appear to be uniform. These different provinces are interpreted to be surficial expressions of processes that have shaped these escarpments through their histories.

Introduction

The escarpments at the edges of the Blake-Bahama, Florida, and Yucatan carbonate platforms are some of the largest cliffs on the surface of the earth. The Blake Escarpment extends along the eastern side of Florida and the northern Bahamas for about 450 km and has as much as 4,000 m relief. The Florida Escarpment extends along the western side of Florida for about 650 km and has about 2,000 m relief. The Campeche Escarpment rings the eastern and northern margins of the Yucatan Peninsula and has about 2,000 m relief (Figure 6–1).

The Gulf of Mexico Exclusive Economic Zone (EEZ) and parts of the northern Caribbean plate margin were surveyed using GLORIA (Geologic LOng-Range Inclined Asdic) during the U.S. Geological Survey EEZ-SCAN program. In the Gulf of Mexico, the first cruise was conducted in 1982 and three more were completed in the summer and fall of 1985. The survey of U.S. waters around Puerto Rico and the U.S. Virgin Islands was completed during a twenty-five-day cruise in the fall of 1985 as well. In addition, surveys were conducted in the Cayman Trough and north of Hispaniola during a transit in 1985 from the Gulf of Mexico to the Caribbean. In total, these surveys mapped approximately 600,000 km2 of the U.S. Gulf of Mexico and Caribbean EEZ along survey tracklines that were spaced 10 to 30 km apart. The data collected included digital GLORIA sidescan sonar images, 40- to 160-in3 airgun and 3.5-kHz seismic-reflection profiles, 10-kHz bathymetry profiles, and total magnetic field measurements.

The Gulf of Mexico is a small, geologically diverse ocean basin that can be divided into three distinct geologic provinces: a salt deformation province underlying the continental slope of the northern and western Gulf of Mexico, the Mississippi Canyon and Fan system in the central Gulf, and a carbonate province along its eastern and southern boundaries.

Introduction

The Caribbean region, south of Cuba (Figure 9–1A), forms one of the distinct lithospheric plates of the Earth's surface (Case and Holcombe 1980). Targets of a scale appropriate for GLORIA imaging are provided by tectonic disruptions of the seafloor along the plate's northern edge. We selected three areas to survey using GLORIA, which allows us to examine the variety of structures produced along this active plate boundary (Figure 9–1B). In the central Cayman Trough, plate motion and geometry cause extension, which creates a short spreading axis that is not connected to the world rift system; GLORIA is used to analyze the crustal structures that are created. Off northwestern Hispaniola, an irregularity in the plate boundary results in compressional motion, and GLORIA is used to analyze the accretionary wedge that is formed by sediments that are scraped off the North American Plate as it is forced against the Caribbean Plate. North of Puerto Rico, the plates appear to slide past each other with neither compression nor extension, yet, surprisingly, a major oceanic trench exists, which exhibits the world's greatest negative free-air gravity anomaly. Structural trends displayed by GLORIA and earthquake distribution are used to hypothesize the plate interactions that form the trench and analyze the response at a corner of a plate (the North American Plate) that is being overrun by another plate (the Caribbean Plate).

Tectonic setting of the Caribbean Plate

The Caribbean Plate is marked by clearly defined subduction zones to its east and west (Figure 9–1B).

The USGS and the British Institute of Oceanographic Sciences (IOS) first surveyed parts of the U.S. East Coast continental slope between Georges Bank and the Blake Escarpment using GLORIA during October and November of 1979. That early survey gave Twichell and Roberts (1982) a new look at the overall geomorphic pattern of submarine canyons on the mid-Atlantic continental slope. They could see that many canyons extend only partially across the mid or lower slope. Therefore, some submarine canyons can be created on the continental slope, without connection to the shelf edge and independently of shelf or shallow water processes. That insight inspired other studies and led to major conceptual advances in the understanding of submarine canyon origins and growth.

The EEZ-SCAN survey of the U.S. East Coast areas took place from February to May 1987, using a much superior GLORIA system, which recorded the data in digital form rather than as analog photographic records. The 1979 images had been as tantalizing as they were effective; they provided a new, broad view, but commonly lacked enough contrast to show the details clearly. The digital processing techniques available eight years later produced much clearer images that could be readily manipulated and combined with other data, such as bathymetry.

In the spring of 1987, survey operations proceeded aboard R/V Farnella during five periods, each about twenty-five days in length, beginning off Georgia and working northeastward.