Figure 39. Seismic profiles (1991) show shelf-margin features at The Elbow (see Fig. 27 for trackline locations). (A) Trackline crossed perpendicular to the margin (modified from Lidz et al., 1997a, b). Note V-shaped backreef trough filled with sediments. Also note four areas of low-relief features on the upper-slope terrace. Similar features occur elsewhere on the upper Keys terrace (Fig. 35), indicating the features are linear. They are interpreted to represent immature coral (outlier) reefs because of their characteristic irregular surfaces. Compare with similar irregular surfaces of known (Holocene) reefs in (B) and (C). (B) Trackline crossed the margin at an oblique angle south of The Elbow Light (just north of profile in A). Note elongation or distortion of features due to angle of crossing (compare with true seismic images in profile A). Difference between broad or narrow depression can be the angle at which the seismic line intersects the depression (or any other type of subsurface feature). Oblique angle will make a depression look broader; a crossing normal to the same depression will make it look narrower. Also note the patch reefs on small, localized bedrock high in Hawk Channel. 'Multiples' are an artifact common in seismic-reflection data. These reflections replicate those of existing, overlying, geologic surfaces and should not be regarded as representing any subsurface stratigraphic horizon. D wave = direct arrival (not a geologic surface). Latitude and longitude in degrees and decimal minutes based on GPS coordinates. Hours (military time) below coordinates serve as navigational correlation points along seismic line. (C) Interpretation of Pleistocene and Holocene surfaces in profile (B).