Framework and Sedimentation Patterns of Tullock Member of Fort Union Formation, East-Central Powder River Basin, Montana and Wyoming

Janet L. Brown

Regional subsurface and outcrop studies of the fluvial Tullock Member of the Paleocene Fort Union Formation, in the east-central Powder River basin, Montana and Wyoming, indicate basin filling from the east and southeast by low-energy fluvial systems grading upward to high-energy stacked channel systems. Maps of sand-body geometry, lithofacies variations, and paleogeographic reconstructions were prepared from the analysis of approximately 200 well logs. Emphasis on subsurface data was necessary because of poor, inaccessible, or nonexistent outcrops in the study area. Where possible, subsurface models were field tested by matching paleocurrent data with inferred sand-body axes, as well as by comparing vertical and lateral grain-size variations with nearby well-log signatures.

In outcrops of the east-central Powder River basin, the lower part of the Tullock Member is composed of thin, lenticular sandstones, interbedded with mudstones, carbonaceous shales, and a few thin, impure coals. The carbonaceous shales display thin, laminated bedding, leaf fragments and impressions, and rooted texture. The upper part of the Tullock contains 20-80 ft (6-24 m) thick yellow, moderately well sorted, structureless or large-scale trough cross-bedded sandstones that have erosional bases and contain spherical iron-sulphide concretions.

In the subsurface of the study area, the Tullock Member is 200-600 ft (61-183 m) thick. The thickest sandstones in the study area are concentrated in a broadly elongate zone parallel to and slightly northwest of the eastern basin margin. Highest percent sand contours define an elongate northwest-trending zone superposed and generally coincident with isopach trends in the area. Well-log interpretation suggests fining upward, locally stacked, fluvial sandstone bodies adjacent to and inter-tongued with fine-grained overbank deposits. Stacked fluvial sandstones are 30-55 ft (9-17 m) thick and are separated by fine-grained intervals 10-20 ft (3-6 m) thick.

The orientation of framework sandstones shows westerly to northwesterly deposition of fluvial channel sandstones associated with inter-fluvial zones of flood plains and swamps. Local stacking of some higher energy channel sandstones in the upper part of the Tullock suggests the initiation of basin subsidence during the Laramide orogeny.

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