Rock exposed on Siesta Key, on the west-central coast of Florida, although resembling beachrock macroscopically, was cemented in a nonmarine environment. It is an example of meteorically-cemented beech sediment now exposed within the intertidal zone.

The “beachrock” is exposed for less than one kilometer along the coast from an elevation of about +1.0 m to almost -4.0 m, and is approximately two meters thick. The unit strikes along the same direction as the present beach and dips seaward at 0-12°.

The “beachrock” constituents resemble those of the surrounding beach; of mixed carbonate and siliciclastic sediments. The carbonate is composed of up to 80 percent medium-sized sand and gravel shell debris and up to 30 percent fine quartz sand. The cement is blocky and isopachous low-Mg calcite, and neomorphic replacement of shell debris by low-Mg calcite is common. Isotopic analysis indicates that the cement was derived from shell debris within the beach deposit, and diagenesis is interpreted to have taken place within a meteoric environment.

The age of the deposit is Holocene (about 2250-2450 yBP). Inclusions of “beachrock” within the stratigraphy of Siesta Key suggest that the “beachrock” was already lithified prior to much of the island building process. There is also some indication that the barrier island may have been more extensive in the past. Sea level reached an elevation approximately equal to present day at the time of deposition, but it is unclear whether sea level fluctuated during deposition or since that time.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah