--> ABSTRACT: Detailed Gravity Survey Over a Known Carbonate Reef (Devonian) in Williston Basin, by Stephen M. Braun; #91033 (2010)

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Detailed Gravity Survey Over a Known Carbonate Reef (Devonian) in Williston Basin

Stephen M. Braun

A detailed gravity study, conducted over the Shell Golden carbonate reef located in the Winnipegosis Formation (Devonian) of the Williston basin in north-central North Dakota, indicates a massive carbonate platform with several interconnected vertical accumulations, perhaps pinnacle in nature, from this platform. This reef is found at a depth of about 2,400 m (8,000 ft). Because elevations and north-south positions were surveyed to ±3 cm (0.1 ft) and ±1 m (3.3 ft), respectively, an accuracy of 0.01 mgal was obtained. Five profiles were made: three lines running east-west and two lines running north-south, forming a grid pattern over the reef. The distance between each line was 1.6 km (1.0 mi) with gravity-station spacing along each line being 0.4 km (0.25 mi).

The Golden reef and most reefs of this nature throughout the North Dakota portion of the Williston basin have been interpreted to be isolated pinnacles with physical dimensions about 60-75 m (200-250 ft) thick and 0.8 km (0.5 mi) in basal diameter. However, analysis of the residual Bouguer gravity anomalies (0.2-0.5 mgal) obtained from this study indicates this reef is more complex than previously thought. The maximum thicknesses of the complex are on the order of 120-185 m (400-600 ft) with compaction anticlines also contributing to the total gravity anomaly. The modeled reef complex extends in a northeast-southwest direction and probably extends beyond the study area along that line.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988