Relationships Between Diagenesis and Regional Fluid Flow in Modern Regional Carbonate Aquifers

D. A. Budd, Brenton Johnson

Published reaction mass balances down potentiometric gradients in regional freshwater carbonate aquifers of Florida, the northern Great Plains, Atlantic coastal plain, and northern Mid-Continent all indicate similar suites of diagenetic reactions. Dissolution of all minerals present in the aquifer rocks occurs in the recharge zone, but calcite precipitation is generally indicated in down-gradient confined aquifer waters. This precipitation is driven by the addition of Ca to the pore waters due to dissolution of dolomite, gypsum/anhydrite, less stable phases of calcite, and/or Ca-bearing silicates like anorthite. Large amounts of authigenic calcite distributed widely through an aquifer is often indicated; volumes and a distribution pattern that are probably unreasonable, a d in some cases, unsupported by petrographic studies.

Regional fluid flow patterns explains the discrepancies between mass balance results and petrography. Fluid flow in many of these aquifers is complicated by upward discharge of deeply penetrating fresh waters. The large amount of calcite precipitation indicated by reaction mass balances is inherited from these upward discharging waters; little if any of the calculated mass transfers may actually occur at the depths from which waters are sampled. An illustration of this phenomena is the upper Floridan aquifer, where all the dolomite and gypsum dissolution signals and most of the calcite precipitation signal can be attributed to upward discharge. The source of these signals is probably calcitization of anhydrite and dolomite in a relatively narrow zone at the base of the aquifer. The am unt of upwelled water, as determined using an inverse mass-transfer model, increases down the potentiometric gradient from 0% to 95% of the total volume of water in the aquifer.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995