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ABSTRACT: Carbonate Slope and Platform Accumulations: Lower Florida Keys

E. A. Shinn, B. H. Lidz, A. C. Hine, J. L. Kindinger

Approximately 500 km of high-resolution seismic-reflection data off the lower Florida Keys reveal (1) a linear reef and trough seaward of the more shallow platform margin reefs and (2) possible late Pleistocene to early Holocene reef and beach-dune deposits 80-100 m below sea level.

The linear reef and sand-filled trough are an extension of a reef-and-trough system that extends more than 300 km along the southeast Florida reef tract. In the study area, the outer reef is shallow (-10 m at its top), has relief of up to 30 m, and is separated from the platform margin reef by a 0.5-km-wide, 30-m-deep sediment-filled trough. The outer reef trend is locally broken, and reefs vary in size. Farther north near Miami, the outer reef has lower relief, and the trough separating it from the platform margin is narrower. A 6-m-long rock core recovered from the crest of the outer reef trend in the lower Florida Keys, off Sand Key reef, reveals a Pleistocene massive coral facies that has a thin (<1 m) Holocene reef veneer.

Farther seaward, where the sea floor slopes into the Straits of Florida, thick (5-8 m) fringing-reef and barrier beach-dune deposits are buried beneath thin Holocene slope deposits 80-100 m below sea level. Beach-dune accumulations are distinguished from reef buildups by the presence of seaward and landward seismic reflections. Fringing-reef buildups, in contrast, are massive and lack reflectors. Both beach-dune and reef buildups are overlain by thin Holocene slope sediments. The beach-dune deposits are probably indicative of a paleoshoreline that existed between 9000 and 15,000 yr ago.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990