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Abstract: Precambrian Submarine Fan Deposits: Limestone-Rich Interval of the LaHood Formation, Southwestern Montana
Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers
The wedge-shaped LaHood Formation crops out along a narrow east-west belt which corresponds to the southern margin of the Precambrian Belt embayment. This 3,500 m thick drab-colored arkose and conglomerate deposit contains a distinctive limestone-bearing unit near the top. This 400 m thick limestone-rich “member” of the formation crops out in the Bridger Range and Horseshoe Hills of south western Montana and contains two types of depositional sequences which are repeated vertically through the section. These sequences reflect mass flow and suspension deposition in relatively deep water and are interpreted as submarine fan deposits.
The first type of sequence contains approximately 25 m of undeformed arkose beds which range between 1 m and 5 m thick and are interpreted as channel deposits of the lower-upper fan and middle fan of a submarine fan. Arkose beds in these sequences are amalgamated, and contain rip-up and floating clasts of siltstone, shale and limestone; both normal and inverse graded bedding; and basal conglomerates.
The second type of sequence contains approximately 30 m of both rhythmically and randomly interbedded limestone, siltstone, shale, and arkose in addition to intraformational slump structures and is interpreted as interchannel deposits of a submarine fan. Small-scale deformation features and their distribution in slump structures reflect open-cast slump processes and include: plastically deformed septarian concretions, molar-tooth structures, imbricate thrusts, and truncated decollement folds. The deformation reflects both bank undercutting by migrating distributary channels and possible faulting along the southern margin of the Belt embayment.
Acknowledgments and Associated Footnotes
1 Adrienne Bonnett-Nicolaysen: Gulf Oil Co.
© Wyoming Geological Association, 2015