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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 40 (1956)

Issue: 4. (April)

First Page: 789

Last Page: 789

Title: Paleotectonic Control of Carboniferous Sedimentation in Central Montana: ABSTRACT

Author(s): Phil A. Mundt

Article Type: Meeting abstract


In central Montana the accumulation of oil in the so-called "Heath" and "Amsden" formations is controlled by complex stratigraphic relationships. These relationships, however, are clarified by an analysis of the effect of tectonic elements upon depositional patterns. A revision of stratigraphic units is necessary in the process; such a revison is here proposed.

The "Amsden" formation of the central Montana area consists of three lithologic divisions--an upper light-colored cherty carbonate member, a middle brown ostracodal limestone unit, and a lower sequence of red shale and sandstone beds. The upper dolomite unit is lithologically, stratigraphically, and paleontologically equivalent to the carbonate portion of the Amsden formation at its type locality in northern Wyoming. The Amsden dolomite overlaps the underlying two units which pinch out southward and are not laterally continuous with Amsden beds in Wyoming.

The Amsden dolomite of Atokan age is unconformable with the underlying brown limestone unit which is probably of Chester age but may be very early Pennsylvanian, in part or altogether. This brown limestone was named the Alaska Bench formation by Freeman and his terminology is recommended for future use.

Since the "Heath" formation includes two lithologically distinct units which are separated by an unconformity, the term Heath is herein restricted to beds below the unconformity. The term Tyler is revived to apply to beds above the unconformity. The Tyler formation includes a lower sequence of dark gray shales and channel sands, a tongue of marine limestone, and an upper "red shale member." The channel sands in the lower part of the Tyler formation form excellent oil reservoirs at the prolific Northwest Sumatra and other central Montana fields.

The unconformity at the top of the restricted Heath formation is the result of tectonic instability at the end of Heath time; this instability is reflected in the depositional types of the Tyler formation. Paleogeologic maps illustrate that the "ancestral Sweetgrass arch" and its un-named counterpart on the south side of the Big Snowy trough were the principal effective tectonic elements. Where the Amsden dolomite onlaps the southern un-named element, excellent oil possibilities exist, and several oil fields have been discovered along the trend.

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Copyright 1997 American Association of Petroleum Geologists