Abstract: Holocene Dolomite Locality in Florida Bay

R. P. Steinen, Robert B. Halley, Shari L. Videlock

Dolomitic mud has been identified in the unlithified subsurface Holocene sediments recovered from Cluett Key in Florida Bay. The dolomite is in the lower one-third to one-half of the slightly greater than 3-m thick carbonate mud, sand, and peat accumulation. In this interval the dolomite is estimated to comprise up to 40% of the sediment and is present as dolomite grains and rhombs smaller than 2µ. The dolomite is poorly ordered and calcium-rich (Ca_0.56-0.60 Mg_0.40-0.44CO₃). A second type of dolomite is stoichiometric and is in small amounts (less than 5%) in some samples. This type of dolomite is believed to be similar to the detrital dolomite identified by earlier workers in Florida Bay.

Dolomite is common in cores from Cluett Key, but is absent or in insignificant amounts in the adjacent subtidal mud banks that surround the island. The dolomitic island mud is underlain by a peat with a C¹⁴ age of 3879 ± 70 years B.P. The dolomite is completely unlithified and lacks cementation features associated with supratidal dolomites from elsewhere in south Florida and the Bahamas. It appears in both supratidal and subtidal (subaqueous) sediments.

The origin of this dolomite is currently under investigation. Its distribution suggests a relation with some island process, most likely the formation and trapping of brackish and hypersaline groundwaters in the low-permeability (10 md average) island muds. Brackish groundwater (0 to 30 parts per thousand total dissolved solids) is present beneath the topographic highs (up to 0.5 m above sea level) of the island. This water supports a variety of hardwoods and grasses. Hypersaline groundwater (90 to 130 parts per thousand) is present beneath low elevations (within 0.1 m of sea level) where water is ponded intermittently. Bay water ranges between 30 and 50 parts per thousand during the year.

The areal distribution, stratigraphy, abundance, chemistry, and grain size suggest this dolomite to be of Holocene age. Although the complex hydrochemical setting requires further study, the presence of this dolomite mud may offer an alternative to dolomitization models requiring high permeabilities and rapid water movement.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC