Graduate Thesis Or Dissertation
 

HoffmanHeidi1990.pdf

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xp68km63z

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  • Deposition of the Middle Devonian part of the Denay Limestone and Bay State Dolomite in the northern Antelope Range and Fish Creek Range, respectively, took place on and in front of a carbonate platform with a lime sand shoal at the shelf edge. In the Givetian part of the Denay Limestone, basin, slope, lime sand shoal, and open platform fades belts are repeated to form two upward-shallowing sequences preceded by deepening events. The first sequence begins in or below the ensensis conodont Zone. All four facies belts are represented in this sequence, indicating extensive progradation of the shallow-water carbonates after transgression. Abrupt drowning in the Denay occurred in the Middle uarcus conodont Subzone, characterized by deep-water basinal carbonates overlying the shallow, open-platform carbonates. The Bay State Dolomite is dominantly a restricted platform facies characterized by alternating laminated algal dolomite and Amphipora packstone. The Amphipora buildups of the Bay State Dolomite kept pace with rapid relative-sea-level rise and the event is not recorded there. After the second deepening event, progradation was not as extensive as in the previous sequence. An upper slope facies is as shallow as the second sequence exhibits. Both upward- shallowing sequences are recognizable in the western Canadian Middle Devonian. Diagenetic features in the Denay Limestone and Bay State Dolomite can be interpreted as representing depositional and postdepositional changes that took place in the following environments: (1) marine phreatic, (2) saturated meteoric phreatic, (3) mixing marine meteoric phreatic, and (4) burial. Conodont alteration index (CAI) values suggest that the Denay Limestone was never buried more than about 3,000 meters (10,000 feet). A major diagenetic change is the pervasive dolomitization of the upper portion of the lower upwardshallowing sequence. This dolomitization is clearly related to the paleogeography as it gradually becomes dolomitized up section to the top of the lower upward-shallowing sequence which is overlain by lime mudstones. The model proposed here for early dolomitization by marine meteoric mixing phreatic water. Silicification is also associated with this diagenetic environment. Other important diagenetic features include mosaic calcite cementation, aggrading neomorphism, calcite syntaxial overgrowths, and pressure solution.
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