Effects of Ancient Porosity and Permeability on Formation of Sedimentary Dolomites: Devonian Jefferson Formation (Frasnian), South-Central Montana

Tad M. Smith, Steven L. Dorobek

Petrographic and geochemical evidence indicates that multiple dolomitization and dolomite stabilization events affected the Devonian Jefferson Formation (Frasnian) in south-central Montana. Several types of dolomite occur, defined by cathodoluminescence: nonzoned, dully luminescent subhedral-anhedral mosaics (most common), euhedral nonzoned and zoned dolomites, zoned dolomite cements, and irregularly luminescent dolomites (dully luminescent with irregularly luminescent regions). The irregularly luminescent fabrics probably represent partial replacement of early dolomite phases with later dolomite phases. Electron microprobe and stable isotope analyses for the different dolomite types are (1) nonzoned subhedral-anhedral mosaics: 49.5 to 52.4 mole% CaCO₃; -12.9 t -3.3 ^pmil ^dgr¹⁸O (average -5.8) and -2.3 to +1.3 ^pmil ^dgr¹³C (average -0.4), PDB; (2) euhedral nonzoned and zoned dolomites: 49.7 to 56.8 mole% CaCO₃; Fe detectable only in rims of zoned crystals (up to 3,400 ppm); (3) zoned dolomite cements: 48.5 to 53 mole% CaCO₃ (rare analyses as high as 69 mole% CaCO₃); nonluminescent rims are Fe-rich (up to 4.0 weight% Fe) with local enrichments in Sr (400 to 840 ppm); and (4) irregularly luminescent dolomites: 51.5 to 55.8 mole% CaCO₃, with cores more Ca rich than rims; -5.5 to -1.6 ^pmil ^dgr¹⁸O (average -3.6) and +0.3 to +2.0 ^pmil ^dgr¹³C (average 1.0).

Nonzoned, Ca-enriched, euhedral dolomites occur in calcite-cemented, coarse-grained limestone layers. These permeable layers probably were conduits for early meteoric waters, that occluded porosity in the limestones and prevented later dolomite stabilization. Irregularly luminescent dolomites are interpreted as intermediate fabrics in the dolomite stabilization process. Later calcite cements which occlude intercrystalline porosity prevented further dolomite replacement. Total recrystallization of remaining dolomites and formation of final dully luminescent mosaics occurred prior to brecciation and stylolitization.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.