Date of Award
Master of Arts (MA)
The upper Frobisher-Alida interval of western North Dakota was studied in order to determine the depositional envirornnents, bioturbation features, and the diagenetic history of these carbonate and evaporite rocks.
Detailed examination resulted in the identification of six distinct lithotypes: 1) crinoid, coral packstone-wackestone (LT-1); 2) brachiopod, skeletal wackestone (LT-2); 3) skeletal, burrowed mudstone-wackestone; 4) intraclast, peloid packstone-grainstone; 5) peloid, intraclast, ostracode packstone-wackestone; 6) nodular and bedded anhydrite (LT-6). Lithofacies generally follow a depositional sequence of thick units of sublittoral carbonates, followed by a thin sequence of littoral carbonates, and culminating in a thick unit of sublittoral evaporites. Sublittoral rocks dominate the basal lithologies throughout the study area. Depositional environments illustrate a regressive sequence: normal marine wackestones and packstones; marginally restricted wackestones; restricted wackestones and mudstones; littoral to sublittoral shoal complex with dominantly peloidal packstones; lagoonal mudstones; and littoral flats and ponds with anhydrite lithologies.
Early cementation of burrows inhibited compaction of burrowed carbonates. Rocks without evidence of early cementation of burrows would more readily compact and thus inhibit movement of pore fluids.
Diagenetic effects upon the rocks studied were varied and complex. Diagenetic effects which commenced prior to complete lithification included mechanical compaction, micritization and inversion of aragonite. Eogenetic diagenesis was the result of infiltration of hypersaline brines from overlying evaporate areas. Differential compaction, cementation and replacement were prevalent eogenetic diagenetic effects. Mesogenetic diagenetic effects were due to periods of pore fluid movements. Mesogenetic diagenesis included dolomitization, silicification, replacement, chemical compaction, pressure solution and neomorphism. Multiple dolomitization episodes were due to the movements of magnesium-rich pore fluids released during pressure solution.
Valvik, Jeffrey R., "Depositional environments, ichnology and diagenesis of the upper Frobisher-Alida beds of the Elkhorn Ranch and Roughrider fields of western North Dakota" (1990). Theses and Dissertations. 306.