Doctoral Dissertations

Abstract

"Thirty-three measured sections, nine with thin sections and insoluble residues, were studied to determine the depositional history of the Allan Mountain Limestone and the lower member of the Castle Reef Dolomite of the Madison Group in northwestern Montana.

The Allan Mountain base is a fossiliferous limestone, herein named the Brownback Gulch Member, which thickens and becomes cherty to the southwest where the section at Lone Butte resembles that of the Bridger Range Waulsortian-type reefs. A crinoidal lag deposit marks a dis- conformity at the top. In the overlying silty limestone member a thin black shale (Unit 1) grades into normal marine, rhythmic laminated and dense mudstones (Unit 2). Silty limestone (Unit 3) formed as terrigenous sediment prograded the craton from the northwest. Dolomite and insoluble residues increase northwestward. The middle member changes upward from cherty, rhythmic laminated and dense spicular mudstones deposited below wave base to massive wackestones with reticulate chert deposited near wave base. A sponge spicule silica source is postulated.

Seven cycles with silty dolomite and cherty wackestone at the base and a bryocrinoidal grainstone top developed in the upper member and the lower member of the Castle Reef in response to water depth and salinity change. A northwestward change from massive carbonates to dark cherty carbonates suggests deepening water, possibly a shelf margin. Insoluble residue, dolomite, organic matter and chert increase to the northwest as the fossil content, crinoid grain size and member thickness decrease. A structurally positive area "Montana Island" and a northwestern terrigenous source are postulated. Silty dolomite marker beds formed early in hypersaline waters while others developed by seepage refluxion"--Abstract, page ii.

Advisor(s)

Spreng, Alfred C., 1923-2012

Committee Member(s)

Wixson, Bobby G.
Beveridge, Thomas R. (Thomas Robinson), 1918-1978
Grant, S. Kerry
Proctor, Paul Dean, 1918-1999

Department(s)

Geosciences and Geological and Petroleum Engineering

Degree Name

Ph. D. in Geology

Comments

The generous support of field expenses by the V. H. McNutt Memorial Foundation is also appreciated.

Publisher

University of Missouri--Rolla

Publication Date

1977

Pagination

xi, 117 pages

Note about bibliography

Includes bibliographical references (pages 111-116).

Geographic Coverage

Montana

Time Period

Mississippian

Rights

© 1977 Forest E. Haines, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Subject Headings

Geology, Stratigraphic -- MississippianSediments (Geology) -- MontanaGeology -- Montana

Thesis Number

T 4268

Print OCLC #

5997933

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