ABSTRACT: Gravity and Aeromagnetic Interpretation in Northwestern Wyoming Basins

Dolores M. Kulik

Detailed gravity and regional aeromagnetic data are used to interpret subsurface structures in northwestern Wyoming. Gravity and magnetic gradients are used to infer the continuation of the bounding fault on the eastern flank of the Big Horn basin where it is concealed by basin deposits. A high gravity anomaly indicates that high-density basement rocks or Paleozoic sedimentary rocks in an upwarp or fault splay extend into the southeastern part of the basin. The mapped axis of the basin diverges from the low gravity axis in the southern part of the basin. The data indicate that the structurally deepest part of the basin lies near the bounding Oregon basin fault and that low-density sedimentary rocks extend beneath the fault. The divergence of the surface and gravity axes s ggests that the basin axis migrated basinward with time and affected facies distribution and stratigraphic thicknesses. Gravity and magnetic data indicate the east-west-trending faults are present in the subsurface in the south-central part of the basin. A major gravity low is modeled by a sedimentary basin beneath the Absaroka volcanic rocks in the northwestern corner of Wyoming. The low correlates with the surface distribution of low-density volcanic rocks and is augmented by them; however, magnetic data show characteristic high-amplitude anomalies only over the volcanic rocks north of the South Fork of the ShoShone River. South of the river the magnetic values are flat where volcanic rocks are thin, over sedimentary rocks of the Box Creek downwarp, in the area of the Fish Lake and Win River basins, and in the area of the gravity minimum. This suggests that the volcanic rocks may also be thin over the gravity low and may be underlain by sedimentary rocks.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990