Multiple Episodes of Dolomitization in Mississippian Castle Reef Formation, Northwestern Montana: Evidence for Meteoric(?) Dissolution and Precipitation of Dolomite

Philip M. Whitsitt, Steve L. Dorobek, Kathy M. Nichols

The Castle Reef formation (Osagian-Meramecian) in the Sawtooth Range of northwestern Montana is a massively dolomitized shallow-water carbonate sequence. The unit is capped by a major unconformity that developed sometime prior to deposition of overlying Jurassic strata. The time represented by the unconformity surface is approximately 180 m.y.

Four regionally correlative pre-Jurassic dolomite phases have been recognized based on their cathodoluminescence characteristics. Each dolomite phase is separated from successive phases by an irregular dissolution boundary. The first episode of dolomitization massively replaced the precursor carbonate. Subsequent dolomite phases partially to completely occlude intercrystalline and intracrystalline dissolution porosity within the earliest replacive dolomite. The second dolomite phase is most pervasive near the pre-Jurassic unconformity at the top of the Castle Reef formation. Pyrobitumens locally fill intercrystalline porosity; hydrocarbon migration postdates all phases of dolomite formation.

The petrographic data suggest that the massive replacement dolomite was susceptible to dissolution by meteoric waters originating at the unconformity surface and that an extensive network of dissolution porosity developed prior to precipitation of later dolomite phases. Ongoing geochemical analyses of individual dolomite generations will help to constrain the chemistry of dolomite-precipitating solutions.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.