Abstract: Holocene Freshwater Cements, Joulters Cays, Bahamas
Robert B. Halley, Paul M. Harris
Early meteoric cementation is recognized frequently as a first step in the preservation of porosity in limestones. The Joulters Cays, Great Bahama Bank, are three late Holocene cemented and uncemented ooid sand islands that provide a natural laboratory for the study of early meteoric-water cements. Water samples from ponds, several water wells, and boreholes on the largest island indicate that groundwater near the water table is generally fresh.
Seven radiocarbon dates establish the late Holocene age of the island and subsequent cementation of surface ooid sands. Two whole-rock dates give ages less than 1,000 years B.P. Five dates from the outer part of lightly lithified oolite are less than 600 years B.P.
Seven core borings were made on the largest island to explore the extent of these young cements. Four borings were in areas of seawater influence, three in freshwater areas. The borings penetrate (1) a cemented oolite zone 0 to 5 m thick; (2) a 3 to 6-m uncemented or very lightly cemented zone of muddy ooid and skeletal sand; and (3) Pleistocene limestone at a depth of 6 to 9 m. Measured water levels in the bore holes indicate no relation between the present water table and degree of cementation as measured by core recovery. The ooid sand appears equally well cemented above and below the water table. However, the style of cementation changes strikingly across the water table. Vadose cements are confined largely to grain contacts. Shallow phreatic cements are uniform grain coatings of calenohedral calcite. The thickness of cemented sand decreases with proximity to normal seawater; however, seawater by itself does not appear to be the most important factor in retarding cementation because some uncemented sands are present in groundwaters with salinities ranging from 1 to 3 parts per thousand. Hypotheses concerning the origin of freshwater cements will need to account for observations from these Holocene cemented ooid sands.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC