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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 8. (August)

First Page: 1350

Last Page: 1350

Title: Facies in Upper Part of Madison Group, Sawtooth Range, Northwestern Montana: ABSTRACT

Author(s): K. M. Nichols

Article Type: Meeting abstract


Portions of the Mississippian Madison Group are gas reservoirs in the plains adjacent to the Sawtooth Range, northwestern Montana, and are equivalent to Mississippian carbonates that are major gas producers in the Canadian Foothills. In the Sawtooth Range, three facies are recognized in the upper 125 m (410 ft) of the Madison Group; they comprise a carbonate shelf sequence that is several hundred meters thick, shoals upward, and is unconformably overlain by Jurassic strata. Economically significant porosity may occur in the upper part of the Madison Group, controlled by the eogenetic secondary dolomitization of a conspicuous crinoidal grainstone unit within it.

This dolomitized crinoidal grainstone unit (termed facies C) is the lowest of three facies in the upper part of the Madison Group, and it abruptly overlies lagoonal limestone that forms the major part of the group. Facies C is massively bedded and exhibits large-scale planar cross-stratification suggestive of its origin as a subaqueous dune field. Measured porosity in surface samples of the dolomitized grainstone of facies C is a maximum of 18% and consists of vuggy, intergranular, and intercrystalline pores. The upward transition from limestone to secondary dolomite commonly occurs in the lower part of facies C. The thickness of facies C ranges from 35 to 75 m (115 to 250 ft) and is inversely proportional to the thickness of the intertonguing and overlying facies B.

The uppermost two facies, termed B and A, reflect the upward transition from and open platform to a restricted platform environment. Facies B ranges in thickness from 25 to 75 m (80 to 250 ft) and is a nonporous, dolomitized mudstone and wackestone sequence generally containing some 1 m (3 ft) interbeds of porous dolomitized grainstone. This sequence is capped by < 10 m (30 ft) of intertidal rocks of facies A, which are thin-bedded, partly algally laminated, dense dolomites.

Locally, facies A and parts of B have been removed as a result of pre-Jurassic folding and erosion in the Sawtooth Range. However, all lateral thickness changes in facies C reflect its intertonguing with B. Although original facies patterns are greatly telescoped by thrusting, the porous grainstones of facies C are best developed in the vicinity of Blackleaf Canyon, Montana, the site of a recently developed commercial gas field in dolomite of the Madison Group.

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