ABSTRACT: Complexities in Fluvial Stratigraphic Sequences in Medial to Distal Portions of a Developing Foreland Basin: An Example from the Lower Cretaceous of Wyoming and Montana

MAY, MICHAEL T., Indiana University, Bloomington, IN, LLOYD C. FURER, Indiana Geological Survey, Bloomington, IN, JAMES H. MEYERS, Winona State University, Winona, MN, LEE J. SUTTNER, Indiana University, Bloomington, IN, ERIK P. KVALE, Indiana Geological Survey, Bloomington, IN, and PETER E. DECELLES, Rochester University, Rochester, NY

Regional integrated surface and subsurface mapping of the nonmarine Early Cretaceous Gannett Group and Cloverly Formation in Wyoming, and the Kootenai Formation in Montana, reveal the stratigraphic and structural history of the early Rocky Mountain foreland basin. Sandstone body architecture, paleodispersal patterns, and stratigraphic positioning of chert-pebble sandstone bodies indicate that early basin history was characterized by: (1) subsidence of approximately 150 km wide foredeep in which up to 900 m of sediment accumulated, and (2) overfilling of this foredeep and development of northeastward-flowing trunk rivers carrying extrabasinally derived clasts in medial and distal portions of the foreland.

Lithologic correlation of surface sections to nearby well logs, together with regional litho- and chronostratigraphic correlation of over 750 subsurface logs, permits detailed isopach mapping of sandstone bodies. This, coupled with paleocurrent analysis, has revealed the existence of at least two northeast-flowing trunk rivers in central Wyoming. The trunk systems were approximately 5-10 km wide and were confined by subtle structural topography caused by recurrent differential subsidence along northeast-trending lineaments in Archean basement rocks. The lineaments are indicated in the subsurface by local thinning of the underlying Jurassic Morrison Formation and by magnetic anomaly patterns, and they coincide with dike swarms and faults mapped in basement rocks of Laramide uplifts in he Wyoming Province.

Distribution of the conglomeratic sandstones with respect to chronostratigraphic marker horizons defined on the basis of fission-track dating of ash horizons, magnetostratigraphy, and format correlation of well logs, indicates that the trunk rivers shifted progressively southeastward through time. Two river systems in the Wind River basin differ in age by 10 +/- 5 m.y. Other adjacent systems undoubtedly are closer in age. The southeastward shift of these rivers indicates that activation of the basement structures also migrated in this direction. Patterns of intraforeland deformation likely were related to tectonic activity along the western margin of Cretaceous North America, including terrane accretion, and/or changes in subduction patterns associated with counterclockwise rotation o the North American Plate relative to the Farallon Plate.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)