Echo character of off-platform carbonates

Abstract

A comprehensive study of the nature and distribution of echo types recorded on short-ping (2–5 msec), high-frequency (3.5 kHz) echograms combined with analyses of 100 bottom samples from the off-platform carbonate environment of Northwest Providence Channel (NWPC), Bahamas, has defined qualitative relationships between echo types, sedimentary processes and sediment types. Seven distinct echo types have been observed and their areal distributions mapped. These seven types fall into three larger groups: (I) Distinct; (II) Indistinct; and (III) Hyperbolic. A strong correlation of echo character with sediment type exists for the one distinct (IB) and two indistinct (IIA and IIB) echo types observed. Muddy, pelagic sediments occur in regions characterized by distinct, sharp, continuous, parallel, sub-bottom reflectors (Type IB). Pelagic sediments interbedded with coarse-sediment gravity-flow deposits or slope breccia are characterized by indistinct, discontinuous, intermittent sub-bottom reflectors (Type IIA). Coarse current-winnowed sands or thick biolithoclastic turbidites found near the surface correlate with regions characterized by an indistinct, prolonged echo with no sub-bottom reflectors (Type IIB). Hyperbolic echoes were observed from both erosional and constructional features. The most common echo type (IIIA) in NWPC consists of large, irregular, overlapping hyperbolae with varying vertex elevations above the sea floor which originate from areas of large-scale submarine erosional topography. This echo type is much more common on the slope south of Little Bahama Bank than north of Great Bahama Bank, indicating different sedimentary processes operative along these two deep carbonate bank margins. In contrast, echoes consisting of discrete single hyperbolae with approximately equal vertex elevations above the sea floor (Type IIIG) correlate with in-situ constructional deep-water bioherms (lithoherms). This study demonstrates the usefulness of 3.5-kHz profiles in mapping near-surface off-platform carbonate sediments. The relationships between echo types and sedimentary processes and/or sediment types described here in a region with good bottom-sample control may serve as a model to facilitate rapid mapping of off-platform carbonates in areas where there is less control.