Alteration of Early Dolomite by Meteoric Waters: Examples from Devonian and Mississippian Carbonates in Montana and Idaho
DOROBEK, S. L., T. M. SMITH, and P. M. WHITSITT, Texas A&M University, College Station, TX
Regional studies of Devonian and Mississippian carbonates in Montana and Idaho illustrate meteoric alteration of early, near-surface dolomite. The early dolomites in these rocks formed in various near-surface settings (tidal flat, shallow reflux settings, subtidal environments). The best preserved early dolomites or remnants of
them are nonstoichiometric, isotopically heavy, and trace element enriched. These dolomites also generally predate compaction fabrics and early calcite cements.
Many of the early dolomites were altered by meteoric fluids in shallow subsurface environments (0-300 m burial depth); meteoric recharge occurred during long-term (10(6)-10(7) yr) episodes of regional subaerial exposure. Meteoric fluids caused partial to complete replacement of the early dolomite by later dolomite phases and extensive calcite replacement ("dedolomitization"). Petrographically, the altered dolomites exhibit complex dissolution and/or replacement fabrics if precursor dolomites were incompletely replaced; homogeneous microfabrics characterize dolomites that have undergone total meteoric recrystallization or replacement. Dolomites with the strongest meteoric signature occur closest to recharge surfaces and typically are the most stoichiometric, have the most negative O(18 values, and are trace element depleted. In some cases, meteoric flow paths can be mapped by contouring geochemical data from replacive dolomites. Depleted cerium contents ("negative Ce anomaly") also may characterize meteoric replacement dolomites. The negative Ce anomaly may develop because Ce(4+) is preferentially scavenged and incorporated into authigenic Fe-Mn oxyhydroxides in oxidizing near-surface diagenetic environments and, therefore, is not available for incorporation into the meteoric dolomite.
Retention of original fabrics and precursor geochemical signatures seems to be most prevalent in downflow regions, far from recharge surfaces. In downflow portions of paleoaquifers, groundwaters most likely were saturated with respect to dolomite because of extensive dolomite dissolution near recharge areas. Early, pore-filling calcite cements in downflow regions also may seal off porosity, thus preventing reaction of the early, more soluble dolomites with later meteoric waters.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)