ABSTRACT

Major structural elements in the Centennial-Madison region of Southwestern Montana and adjoining Idaho are revealed by examination of the gravity maps of thearea. The gravity anomaly of the eastern Centennial Valley is almost 40 m/gals. and the southern Madison Valley has a comparable anomaly; whereas the Henrys Lake basin has a 25 milligal anomaly.

Modeling suggests there exists approximately 3500 feet of Cenozoic deposits in the Henrys Lake basin, less than half the depth of Cenozoic sediments estimated for the Centennial and Madison Valleys. A gravity low extends from the eastern end of the Centennial Valley toward the Madison Valley. The low is interpreted as a valley that previously connected the two valleys but is now hidden beneath several hundred feet of volcanic flows. The buried valley is aligned with the O'dell Creek fault, and appears to be controlled by the fault The east side of the O'dell Creek fault has been uplifted. The sediment thickness below the Upper Red Rock Lake is just 2000 feet, significantly less than the remainder of the Centennial Valley. Both the Centennial and Madison faults are normal faults that can be traced into the Henry Lake Basin.