GAMMA RAY AND MAGNETIC SUSCEPTIBILITY PROFILES AS A TOOL FOR CORRELATION OF CARBONATE-EVAPORITE STRATA: A CASE STUDY FROM UPPER ORDOVICIAN RED RIVER FM., NORTH DAKOTA

Gamma ray and magnetic susceptibility (MS) profiles were examined from the entirely subsurface Upper Ordovician Red River Formation, North Dakota. We used data from a well in eastern North Dakota covering lower part of the formation, and three wells from western North Dakota representing the upper part of the formation. The carbonate facies analyzed include, from distal (basin-central) to proximal (basin-marginal): burrowed skeletal mudstone and wackestone (deeper subtidal), skeletal wackepackstone (moderately shallow subtidal), skeletal packstone and grainstone (high-energy, open-marine, well-oxygenated shoal), poorly fossiliferous lime mudstone (restricted subtidal), barren lime mudstone (restricted euhaline to mesosaline lagoon), thrombolite (shallow mesohaline subtidal), laminated dolomite (shallow mesohaline subtidal to intertidal), intraclast breccia-conglomerate (supratidal to intertidal). Non-carbonate facies include deeper-water shale and anhydrite (shallow penesaline subaqueous setting) that is restricted to the basin center. Generally, the mean MS values increase from the most distal towards the more proximal facies, suggesting that the MS signal preserved could be related to relative sea-level oscillations (MS being highest during sea level lows when siliciclastic impurities are delivered to the basin). Gamma ray values for carbonate facies are very low, and they poorly correlate with MS values of the facies; the only exception is shale with excellent positive correlation between the GR and MS values. The relationship between the GR and MS signal versus facies stacking pattern is analyzed in order to assess the possible significance of GR/MS variations as a proxy for high-frequency sea-level oscillations recorded in carbonate facies.