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Deposition, diagenesis, and porosity development in Mississippian Castle Reef formation, northwestern Montana

Conference · · Am. Assoc. Pet. Geol., Bull.; (United States)
OSTI ID:6962985
The Castle Reef formation forms the top of the Mississippian Madison Group in the Sawtooth Range of northwestern Montana and consists principally of dolograinstones, packstones, and wackestones that are disconformably overlain by Jurassic strata. The Caste Reef attains a maximum thickness of 145 m (476 ft) and can be subdivided into shoal, intershoal, shoal to open-platform, and open to restricted-platform facies representing an upward-shoaling sequence. Microprobe elemental analyses, cathodoluminescence and transmitted-light microscopy, and facies interpretation suggest primary intergranular porosities were destroyed soon after deposition by syntaxial overgrowths. Secondary moldic porosity with limpid equant and drusy cements developed along with destruction of primary porosity in the freshwater phreatic zone. Microprobe magnesium-calcium ratios of these cements and allochemical components range from 1:1.0 to 1:1.6, suggesting that mixing zone dolomitization occurred after cementation, probably by the Late Mississippian. Further cementation of porosity at depth after dolomitization was prohibited by hydrocarbon migration considered to be sourced from Devonian.
Research Organization:
Univ. of Idaho, Moscow
OSTI ID:
6962985
Report Number(s):
CONF-8609129-
Conference Information:
Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 70:8
Country of Publication:
United States
Language:
English