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GIS Compilation of Data Collected from the Pulley Ridge Deep Coral Reef Region OF 2005-1089
OF       2005-1089

Disc       Contents

Introduction

Surficial Geology

GIS/Visualization

Data       Catalog

References       Cited

Acknowledgments

Contacts
Surficial Geology of Pulley Ridge
The southern part of Pulley Ridge has been imaged with high-resolution       seismic-reflection profiles, sidescan-sonar imagery, multibeam bathymetry,       bottom photographs and video of the seabed. These data have been used to       define the surficial and shallow subsurface geology of the area (Jarrett       and others, 2005). Seabed observations show deep-water coral of the family       Agariciidae, various algae, carbonate cobbles, and localized       outcrops of lithified beach rock (Halley and others, 2003; Cross and       others, 2004; Jarrett and others, 2005). The sidescan-sonar imagery and       multibeam bathymetry provide a regional context for the site specific       observations provided by the bottom photographs and video imagery. Because       of its depth (60-100 m), relatively few samples have been collected from       Pulley Ridge and the following interpretation is based entirely on the       remotely sensed data sets. These data have been incorporated into the GIS       within this publication, and this discussion is intended to provide a       preliminary overview of the geology of the area to aid users with the data       included in the GIS.
Figure             1. Location map.Pulley Ridge       trends roughly north-south for almost 200 km along the western edge of the       southwestern part of the Florida Platform in 60-90 m water depths. A       second ridge in the southern part of Pulley Ridge, lies to the east in       58-78 m water depths (Fig. 1). This ridge is the focus of this study because       of the presence of hermatypic corals covering its surface. The multibeam       bathymetry shows that the ridge trends north-northeast and resembles a       barrier island, the top of which is now submerged in       Figure             2. Shaded-relief       bathymetry.58-65 m of water (Fig. 2). The ridge is 4-7 km wide, approximately 34 km       long and stands as much as 20 m above the seafloor along its western side.       A well-defined central ridge separates the eastern from the western flank.       In the northern part of the study area the central ridge is less than 200       m wide and rises 2-8 m above the adjacent flanks (Fig. 3). At the southern end of the ridge, this central       ridge becomes more subdued in relief (1-4 m above the flanks) and       bifurcates into several low ridges that look much like recurved spits on a       modern barrier       Figure             3. bathymetry profiles.beach.       The central ridge is 2000 m wide at this southern end. West of the central       ridge, several low-relief ridges that trend roughly parallel to the       central ridge characterize the seabed(Fig. 2). West, beyond these low-relief ridges, the       seafloor is smooth with the transition between the ridges and the       surrounding seafloor marked by a small step in 75-78 m water depth.       Several sub-circular depressions interrupt the ridges along the northern 8       km of the western side of the ridge. These depressions are 30-200 m in       diameter and as much as 3 m deeper than the surrounding seafloor. The       eastern side of Pulley Ridge has a textured appearance on the       shaded-relief image that results from a series of discontinuous mounds and       depressions. Short ridges with varying orientations interrupt this       textured pattern. The seafloor along the eastern edge of the survey area,       beyond the limits of the ridge, is 68-69 m deep.       Figure             4. Sidescan sonar imagery.Eight       different acoustic signatures were identified in the sidescan sonar       imagery collected from the Pulley Ridge area. The sidescan image is quite       uniform in its backscatter intensity, and it is only when a strong       contrast enhancement is applied to the image that the backscatter       differences become apparent (Fig. 4). For this analysis, intensity values between 80       and 140 were spread across the entire 8-bit range (0-255). The       distribution of the different acoustic signatures is closely tied to the       morphology of the ridge, and thus the signatures will be described       relative to the ridge morphology (Fig. 5A).       Figure             5. Geologic interpretation.The       seafloor immediately surrounding Pulley Ridge has either a uniform,       moderate-backscatter signature (mod-backscatter) or a low-backscatter       signature with moderate intensity mottles on it (mottled-low). Areas       having a moderate-backscatter signature fringe the southern and eastern       sides of the ridge as well as the area northwest of the ridge (Fig. 5A, B). The low-backscatter signature with moderate       intensity mottles on it (mottled-low) fringes the central part of the       western side of the ridge and the northern end of the ridge (Fig. 5A, C). The moderate-backscatter signature area       south of the southern end of Pulley Ridge extends to the upper continental       slope. On the upper slope, alternating bands of high acoustic backscatter       (high-backscatter) and low acoustic backscatter (low-backscatter) that       trend parallel to the bathymetric contours characterize the seafloor (Fig. 5A).       
Five different acoustic signatures characterize the surface of Pulley       Ridge. Two of these acoustic signatures are found on the western side of       the ridge. One is the broad lineation signature (broad lines) in the       sidescan-sonar imagery (Fig. 5D), which coincides with low-relief linear ridges       that show in the multibeam bathymetry (Fig. 2). These topographic features are best imaged by       the sidescan in the northwestern part of the study area where these       low-relief ridges are larger than they are elsewhere. A second acoustic       signature that occurs only in the northwestern corner of the study area is       characterized by 30-200 m sub-circular depressions that have       low-backscatter floors (depressions-large; Fig. 5E). 

The crest of Pulley Ridge is characterized by two acoustic signatures;       a high-backscatter signature (high-backscatter) for much of its length (Fig. 5A), and alternating linear bands of low and very       high backscatter signature (terraces) along the northern part of the ridge       (Fig. 5A, F). These bands are interpreted to be ledges or       terraces, which lie along the flanks of the central spine that marks the       crest of the ridge.

The eastern flank of Pulley Ridge has an acoustic signature       characterized by a moderate backscatter surface interrupted by       higher-backscatter patches (mottled-moderate). This mottling (Fig. 5A, G) is the same scale and appears to be       associated with subtle mounds and depressions that were identified in the       multibeam bathymetry.
Figure             6. Details of pits and mounds.A       detailed inspection of the sidescan imagery shows that small pits and       mounds dimple much of the study area (Fig. 6A). The pits are mostly 1-5 m in diameter, and ROV       observations suggest they are less than 1-2 m deep (Fig. 6B). The mounds are 1-2 m in diameter, and ROV       observations indicate that they are 0.5-0.75 m high (Fig. 6C). Grouper and other large fish were observed in       the ROV video imagery consistently in or adjacent to the pits (Fig. 6B). Other studies have suggested that these pits       are excavated by fish and serve as protective habitat for them (Coleman       and Williams, 2002). The mounds in the bottom photographs and video       imagery comprise carbonate cobbles that are less than about 10 cm in       diameter. Most mounds have an opening at their bases. No fish were       observed entering these openings, however similar mounds have been       observed in the Caribbean and Bahamas and are constructed by sand tilefish       (Buttner, 1996). While no sand tilefish were observed near the mounds,       they are likely responsible for constructing these mounds. A systematic       mapping of the pits and mounds is being completed, and the geologic       controls on their distribution will be the topic of a future report.       
Bottom photographs and observations along ROV transects indicate that       much of the seafloor within the study area is covered either by carbonate       rubble, algae, or corals (Jarrett and others, 2005). These observations in       concert with the multibeam bathymetry suggest that some of the features of       the sidescan imagery are due to topographic effects while others are the       result of differences in the seafloor geology. The mottled eastern flank       of the ridge and the broad lineations that characterize the western side       of the ridge appear to be controlled by the fine-scale morphology of the       seafloor, as there are no obvious changes in the composition of the       seafloor observed in the ROV transects across these areas. 

By contrast, the floors of the large depressions and the       low-backscatter mottled areas do show a difference in seafloor sediment       type. In these areas the sediment is mostly sand, and the veneer of       carbonate rubble that characterizes much of the remainder of the study       area is absent or greatly reduced. The high-backscatter area along the       crest of the ridge is characterized by cobble pavement that in places       appears to be interrupted by outcroppings of rock. These rock outcrops       presumably are exposures of beach rock material that was recovered in       dredges from this area (Jarrett and others, 2005). The terraces along the       flanks of the ridge crest presumably represent outcrops of this beach       rock. 
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U.S. Department of the Interior | U.S. Geological SurveyURL: woodshole.er.usgs.gov/pubs/of2005-1089/htmldocs/geology.htmPage Contact Information: WHSC WebmasterThis page last modified on Monday, 14-Jan-2013 12:50:16 EST