Thrust faulting and oil possibilities in the plains adjacent to the Highwood Mountains, Montana

Abstract

Scope of paper. During May and June, 1926, the writer made a brief study of the geology in the plains adjacent to the High wood Mountains, in north-central Montana, the main purpose of which was the collection of data that would throw light on the origin of the thrust faults that nearly encircle the Bearpaw Mountains an isolated group about 60 miles northeast of the Highwood Mountains. In this investigation it was discovered that thrust faults similar to those adjacent to the Bearpaw Mountains are present north and east of the Highwood Mountains. In the regional study of these faults it became evident that the strata are undisturbed except within a narrow belt, commonly 600 to 700 feet wide, on the upthrown side of the faults, and additional data were obtained in support of the writer's belief that the thrust faulting typical of the region affects only the upper half of the Colorado shale and overlying formations. The principal purpose of this paper is to describe these two features of the faulting, inasmuch as they have an important bearing on the oil and gas possibilities of the area and, to judge from past drilling, have not been recognized by the geologists who have directed the drilling. Consequently, the report will be confined mainly to a description of the formations exposed and penetrated by the drill in the area, a description of the surface expression of the thrust faults, and presen-' tation of the evidence that the thrust faulting does not include the lower half of the Colorado shale and underlying formations, where oil is generally looked for in the region. Other phases of the geology of the region, especially its igneous rocks and Pleistocene history, have been the subjects of earlier investigations.1 The result of the regional study of the thrust faulting will be presented later in a Geological Survey publication.