Date of Award

1986

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geology

First Advisor

H.J. Fischer

Abstract

The Sherwood "bed" and the overlying Bluel! "bed" are two log-defined zones at the top of the Frobisher-Alida interval. The Frobisher-Alida interval, within either the Mission Canyon carbonates or the overlying Charles evap orites, is composed of a number (up to six) of log-defined zones. On the northeast flank of the Williston Basin the zones are separated by even, lithologically homogeneous argillaceous marker beds. Basinward, these marker beds terminate into less argillaceous carbonates.

The area studied covered four townships in southwest Renville·county, North Dakota. The section studied included the K-1 marker, the Sherwood bed (with the Sherwood argillaceous marker), and the Bluell bed (with the State "A" marker). About 290 m (940 ft) of core and 225 thin sections from 19 boreholes were examined. Depths varied from 1540 m (5000 ft) to 1840 m (6000 ft). Rocks were categorized into six lithotypes: 1 pisoid-ooid-intra clast wackestone to packstone; 2) mudstone to stromatolite boundstone; 3) massive anhydrite; 4) argillaceous silty dolomudstone; 5) ooid grainstone; and 6) sandy carbonates.

Lithofacies within individual zones form bands paral to regional strike. Deposition occurred on an arid, peritidal carbonate buildup bordered by a warm, restricted, shallow-basin sea to the southwest and a shallow, evapo rite-precipitating lagoon to the northeast. The argil laceous markers were probably deposited during a slight, basin-wide sea-level drop that drained most of the hyper saline lagoon and exposed a wide, flat, sabkha plain. Deposition or erosion of marker beds was controlled by groundwater capillary-fringe entrapment and deflation of higher areas. Marker-bed deposition terminated at the sea edge in intertidal or subtidal ooid shoals. Some evidence remains of drowned quartz and detrital-anhydrite sand bodies. Because progradation of the "continental"-type eolian sabkha was very rapid, the marker beds in the study area are considered quasi-litbochronozones.

Eogenetic diagenesis included cementation (calcite, anhydrite, and celestite), dissolution, micritization, and compaction. Matrix-selective pores were opened by early dissolution and are the predominant pore type in the study area. Mesogenetic diagenesis included cementation (calcite, dolomite, and anhydrite), minor amounts of silicification, high amplitude stylolitization, fracturing, and oil migra tion.

The development of most local structures in the study area was subsequent to the Mississippian-Triassic uncon formity. Precipitation of diagenetic anhydrite was a major cause of pore occlusion in much of the study area and is related, in part, to structural features.

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