Tectonic Implications of Thermopolis Anticline, Southern Big Horn Basin, Wyoming
Surface geology interpreted from 1:24,000 scale Landsat Thematic Mapper images, in combination with topographic, borehole, seismic reflection, and field data were used to analyze the structure of the Thermopolis anticline, a Laramide Foreland structure in the southern Big Horn basin, Wyoming.
Our results demonstrate that the anticline is asymmetric and verges to the southwest, away from the basin axis. Subsurface fold geometry can be predicted in cross section using a concentric fold model constrained by surface geology and balanced by bed-length measurements. The steep southwest limb is cut by multiple northeast-dipping thrust faults that thin the stratigraphic sequence approximately 25%. In the footwall, beds are upturned against the fault zone providing structural closure and a possible exploration target. Northeast-trending compartmental faults cut both the hanging wall and footwall, and segment the anticline into blocks that deformed independently from one another. Northeast-trending, low-amplitude folds are superimposed on the anticline. Flexural-slip folding, thrust faulting, and conjugate faulting were dominant mechanisms operating on the sedimentary sequence during deformation by compression.
Seismic reflection data confirms predictions of subsurface geometry and indicate three stages of fault/fold development: (a) thrusting, initial brittle basement failure at low angles; (b) fold development, thrust fault propagation through the stratigraphic sequence inducing folding in sedimentary strata and cataclasis in subjacent crystalline basement; and (c) back-limb thrusting, culmination of fold development by backlimb thrusting of sedimentary strata.
Structural geometry is consistent with either: (1) multiple phases of Laramide compression, or (2) a single phase of Laramide compression with shear-zone deformation.
AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.