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The AAPG/Datapages Combined Publications Database

Montana Geological Society

Abstract

MTGS-AAPG

Montana Geological Society: 1989 Field Conference Guidebook: Montana Centennial Edition: Geologic Resources of Montana: Volume 1
---, 1989

Pages 91 - 102

MARINE JURASSIC ROCKS OF WESTERN MONTANA NEW CONSIDERATIONS

Karen W. Porter, Consulting Geologist, Bozeman, MT 59715

ABSTRACT

Surface evaluation of the marine Jurassic Ellis Group in western Montana suggests that 1) this sequence can be understood in terms of the sea level fluctuations that produce both lowstand surfaces of erosion and transgressive surfaces of erosion; 2) the regional structural element Belt Island may have been comprised of second-order structural blocks with independent histories that influenced sedimentation and erosion; and 3) Swift Formation sediments probably derived locally on the Montana shelf rather than from a rising terrain to the west. Evaluation of unconformities and sedimentary sequences for this interval may shed light on early foreland basin history on the Montana shelf.

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