SIVILS, DAVID J., Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM

ABSTRACT: A Hydrologic View of Mixing-Zone Dolomitization

Mixing-zone dolomitization has been proposed as a contributing mechanism to large-scale, ancient, platform dolomites. The current mixing-zone models have not satisfactorily addressed the mechanisms for (1) the supply of Mg{2+} and removal of excess Ca{2+}, and (2) maintaining a mixture passing through limestones over long periods of time. A confined coastal aquifer system along the margin of an epicontinental sea is likely to satisfy the constraints of chemistry, space, and time. In a confined system, the two most pertinent hydrologic factors affecting the mixing-zone are the hydraulic connection of the mixing-zone with the sea and the width of the mixing-zone. In a confined system, freshwater flowing to the sea must pass through an aquitard. The hydrologic boundary created by the aqu tard controls the location, response, and width of the mixing-zone. First, the mixing-zone isgenerally displaced several kilometers offshore. Second, the location of the mixing-zone migrates at a rate on the order of 1 km/10,000 years in response to changes in sea level. In addition recent studies suggest that hydrodynamic dispersion in a confined coastal aquifer enhances the width of the mixing-zone. The distance between the 25 and 50% isochlors can be on the order of one kilometer. The mixing-zone is maintained by vertical flowproviding a continuous supply of Mg{2+} and removal of excess Ca{2+}. The confined coastal aquifer system is a scenario in which a kilometer wide zone can be continuously flushed with dolomitizing waters for 10's of thousands of years.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.