Delineation of acoustic interfaces by seismic reflection profiling 

 is a reasonably accurate and straightforward procedure where the re- 

 flections are continuous or are interconnected by means of the survey 

 tracklines. Interpolations between survey lines or between gaps in a 

 line must be based on an assumption of continuity of slope or elevation 

 or on an assumed configuration which is geologically reasonable. Sub- 

 bottom reflecting horizons on the seismic records of south Florida are 

 frequently interrupted by absorption or scattering of the signal near the 

 bottom-water interface with consequent partial or total loss of subbottom 

 resolution. This sound absorption is especially noticeable in the old 

 reefs. For these reasons it is difficult to determine a regional reflec- 

 tor or to correlate over considerable distances with assurance that the 

 same horizon is being used. Therefore, wholly reliable qualitative sedi- 

 ment data exists only in the close vicinity of coring sites and only to 

 the depth of core penetration. In terms of acoustic interfaces, the bed- 

 rock delineation is considered to be reliable because the numerous expos- 

 ures provide a large number of data points and because the characteristic 

 irregular surface of bedrock provides a means of indirect identification. 



(2) Bedrock Morphology . Extension of the bedrock surface 

 between tracklines was facilitated by USC^GS chart notations, and bottom 

 morphology where rock is exposed on the bottom. Extension of the gross 

 outline and general elevations of the bedrock surface under sediments in 

 the "flats" (or plateaus) is considered reliable. This surface, although 

 not continuously definable is apparently step-like with principal levels 

 at -15 to -25 feet MLW; -50 to -60 feet MLW; and -80 to -90 MLW. Reef- 

 like features crop out on the seaward edges of these steps, and spill over 

 onto the next lower level. Lesser irregularities on this surface are of 

 localized and indeterminate form and cannot be extended in plan at the 

 existing trackline spacing. Generalized bedrock topography in the Miami 

 grid area is depicted in Figure 12. The density of subbottom profiles 

 elsewhere in the study area is not adequate for purposes of mapping the 

 bedrock surface. 



(3) Regional Subbottom Morphology . Because of sound ab- 

 sorption, scattering, and the relatively thin veneer of sediments covering 

 bedrock in the Miami grid area (and Section A in general) to water depths 

 of approximately -70 feet MLW, no regional reflector within the sediment 

 column would be discerned. However, seaward of this depth (the seaward 

 limit of the third plateau on the upper continental slope a thick blanket 

 of "modem" sediments overlies a regressive series of terraces extending 



to depths of approximately -300 feet MLW (Meisburger, 1968). Within this 

 sediment envelope are numerous and prominent sonic reflectors some in- 

 clined at a steep angle seaward and resembling fore-set beds (Figure 7a). 

 IVhether they are indeed fore-set beds representative of a relic shoreface, 

 or a fore-reef talus, is impossible to determine; they nevertheless are 

 interpreted as progradational marine features. The stratigraphically 

 highest of these deposits, extending from approximately -140 to -220 feet 

 MLW is terminated by a prominent sonic horizon indicative of a terrace 

 between -220 and -250 feet MLW. This terrace possibly relates to a 

 fluctuation of the Holocene transgression at approximately 11,000 years 

 B.P. (Curray, 1965). A deeper prominent reflector, dipping slightly from 



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