Earth and Planetary Sciences ETDs

Publication Date

5-1971

Abstract

The Hogback Mountain area is located in the northern part of the Big Belt Mountains, twenty miles northeast of Helena, Montana.

Sedimentary rocks range in age from Precambrian to Recent, and have an aggregate thickness of about 16, 500 feet. The oldest stratified rocks are two conformable formations of the Precambrian Belt Series; the Newland Formation (oldest) and the Greyson Shale. These rocks have an aggregate thickness of about 10,700 feet.

Rocks of the Belt Series are unconformably overlain by about 1,200 feet of strata of Middle and Late Cambrian age separated into five conformable formation: the Flathead Sandstone (oldest), Wolsey Shale, Meagher Limestone, Park Shale, and Pilgrim Limestone (youngest).

The Pilgrim Limestone is disconformably overlain by the Jefferson Dolomite, 600-700 feet thick, of Late Devonian age, and the Three Forks Formation, 150-250 feet thick, of Late Devonian and Early Mississippian age.

Conformably above the Three Forks Formation is 2,100 feet of limestone of Mississippian age separated into two formations of the Madison Group; the Lodgepole Limestone (oldest) and the Mission Canyon Limestone. Disconformably overlying the Mission Canyon Limestone is the Big Snowy Group, 900-1,100 feet thick, followed disconformably by the Amsden Formation, 40 feet thick, and the Quadrant Formation 250-300 feet thick. The Big Snowy Group is Mississippian in age; the Amsden and Quadrant Formations are Pennsylvanian in age.

Small surficial deposits of Recent age complete the stratigraphic succession.

Intrusive rocks include two large sills of diorite, several small intrusive bodies of porphyritic andesite, and two small sills of intermediate composition. The intrusive masses are believed to be Late Cretaceous in age. Remnants of a once extensive Tertiary basalt flow cover an area of about one-half square mile.

The earliest recorded structural event was the development of the Beltian geosyncline and the Central Montana trough, which accumulated tremendous thicknesses of Belt strata during Precambrian time. This thick sedimentary wedge and the configuration of the unstable Central Montana trough strongly influenced Laramide tectonism in the Hogback Mountain area.

Most of the structure formed in response to northeast-southwest compression during the Laramide orogeny in Late Cretaceous and Tertiary time. Major thrusting, with yielding to the northeast, occurred first, followed by large-scale folding and more thrusting. The first thrust to form, the Scout Camp thrust, was intensely folded and overturned. The Moors Mountain thrust, was gently folded. Crustal shortening in the Hogback Mountain area due to Laramide thrusting and folding is estimated to be at least fifteen miles.

Intrusive bodies were emplaced before culmination of Laramide folding but the exact relationship between thrusting, folding, and plutonism is not known.

Post-Laramide deformation is limited to Cenozoic uplift with accompanying gentle northwestward tilting and minor high-angle faulting.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Lee A. Woodward

Second Committee Member

Sherman Alexander Wengerd

Third Committee Member

Albert Masakiyo Kudo

Language

English

Document Type

Thesis

Download

Included in

Geology Commons

Share

COinS