End_Page 535------------------------------

The carbonate rocks and sediments on the island of Bonaire in the Netherlands Antilles contain interesting examples of early dolomitization. A large flat area of Recent supratidal sedimentation exists on the south end of Bonaire, and in this area evaporation of sea water is depositing calcium carbonate and gypsum, which produces dense brines having large Mg/Ca ratios. Dolomite is found in most of the Recent supratidal sediments, and carbon-14 dates on the dolomite establish that the time since dolomitization has been less than 2,200 years. Textural evidence indicates that some of the dolomite was formed by replacing lime sediments.

The dense brines produced by evaporation tend to flow downwards into the permeable sediments, and an analysis of the chemistry and hydrology of a hypersaline lake chosen for detailed study shows that downward drainage of brine having a Mg/Ca ratio of about 30 must be happening today. Examination of marine Plio-Pleistocene rocks on the north end of Bonaire shows large areas of dolomitization whose boundaries cut across the bedding. The field evidence is consistent with the hypothesis that this dolomite has been produced by the flow of dense brines from a supratidal area. The time required to produce the estimated volume of dolomite found in the Plio-Pleistocene rocks would be of the order of 10⁵ years.

End_of_Article - Last_Page 536------------