Sedimentation in Supradetachment Basins: The Influence of Extensional Folds on Stratigraphic Architecture, Grasshopper Basin, Southwest Montana

J. P. Matoush
Utah State University, Dept. of Geology, Logan, Utah

Existing facies and structural models of rift basins explain structural and stratigraphic spatial relationships in general, however, they are simplistic and fail to account for extensional folds. Existing models incorrectly predict the areal extent of footwall and hanging wall derived, reservoir quality alluvial fans, fan deltas, and the proximity of potential source rock lacustrine deposits with respect to border fault geometry. Prevailing cross sectional models through rift basins may underestimate the degree of stratigraphic lateral and vertical complexity.

The Eocene-Oligocene Grasshopper supradetachment basin is an east tilted half graben, containing a longitudinal anticline and a transverse anticline-syncline-anticline fold train. Preliminary field work, including 8+ measured sections, geologic mapping, and paleocurrent data, shows the basin to have been characterized by southward axial infilling analogous to basins within the East African rift system. A complex, time-transgression of fluvial-palustrine, fluvial-deltaic to lacustrine facies has been identified from north to south respectively, with carbonate mudflat, beach, and alluvial fan deposits restricted to the basin margins. Field studies show that significant sedimentationfold interplay did exist during basin evolution introducing a degree of stratigraphic complexity not previously documented in rift basin studies.

Stratigraphic and sedimentological field data will be placed into the context of several structural models currently being developed. By evaluating these basin-scale 3-D models (2D/3D Move from Midland Valley) with field data, a best-fit model relating structural geometry and stratigraphic architecture will be produced.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid