Geomorphology and Tidal-Bar Belt Depositional Model of Lower Florida Keys

Jack L. Kindinger

Changes in geomorphology from a coral reef island chain paralleling a linear platform margin to a series of oolitic islands perpendicular to the platform margin are the most striking feature of the lower Florida Keys. Rotary cores drilled from 6.1 to 17.7 m (20 to 58 ft) total depth were taken in critical areas of the Key Largo Limestone-Miami Oolite transition near Big Pine Key.

Morphology of the Pleistocene lower Keys oolite unit is similar to and probably of the same age as the Miami Oolite. The lower Keys have been recognized by many to represent a preserved tidal-bar system like those presently forming in the Bahamas but frozen in place by the lowering of sea level. The maximum elevation (1.5 m or 5 ft at Big Pine Key; 3.1 m or 10 ft at Key West) of the oolite unit in the Keys is lower than that (approximately 6.1 m or 20 ft) of the oolite unit to the northeast (Miami). The two units, located at opposite ends of the 100-mi-long (166 km) reef tract, are similar in shape.

Drilling showed the coral reef facies to be transitional with the oolite facies, occurring above a region-wide subaerial unconformity located between 3.1 and 7.6 m (10 and 25 ft) below present sea level. Oolite facies rest on the unconformity in the oolitic Keys, and coralline facies overlie it in the Pleistocene Key Largo Limestone.

It is proposed that upward shoaling of the Pleistocene Key Largo reef limestone restricted cross-Keys tidal flow, thus forcing strongest flow around the north and south ends of the reef. Increased tidal flow set up conditions favorable for oolite formation and tidal-bar deposition.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.