MAGNETIC SUSCEPTIBILITY PROFILES AS A TOOL FOR CORRELATION: A CASE STUDY FROM UPPER ORDOVICIAN RED RIVER FM., NORTH DAKOTA AND BIGHORN DOLOMITE, WYOMING

Magnetic susceptibility (MS) in carbonate rocks is commonly used as a proxy of paleoceanographic processes or to determine physical changes in depositional environment; it is also potentially useful and relatively inexpensive method for correlation between sections and/or cores. This study uses measurements from two Upper Ordovician sequences from western North America: Red River Formation, North Dakota, and Bighorn Dolomite, Wyoming. Measurements were made using the hand-held MS3 Bartington System. All readings were taken by holding the meter in direct contact with freshly cut flat surface of a sample. A total of 375 samples were analyzed, with an average stratigraphic spacing of 1 meter. Ten replicate measurements were made for each sample and then averaged.

The Upper Ordovician Red River Formation, North Dakota is a succession of mixed carbonate-evaporite sediments that formed in the intracratonic Williston Basin. Its upper part consists of three shallowing-and-brining upward sequences; each of the sequences is composed of the following members, from bottom to top: burrowed limestone, laminated dolomite, and anhydrite. The Upper Ordovician Bighorn Dolomite, Wyoming is a shallow-marine carbonate succession that formed further west along the passive continental margin of western North America; it contains no evaporites. Samples for MS were collected from five cores spanning the upper Red River Fm. in western North Dakota, including: Simpson #1 (Williams County), Federal #10-1 (Dunn County), Urlacher State Unit #1 (Hettinger County), Wunderlich #1 (McHenry County), and Klain #1 (McLean County). Magnetic susceptibility values vary from -2.53 to +36x10^-5 SI (mean +2.63x10^-5 SI). Mean MS values increase from anhydrite (0.75x10^-5 SI), to laminated dolomite (+1.46x10^-5 SI) and burrowed limestone members (+3.12x10^-5 SI). Several MS-peaks (positive excursions) can be traced between the cores, but generally there is no strong correlation between MS profiles of the studied cores. Samples for MS were also collected from the Horseshoe Mountain Member of the Bighorn Dolomite, Wyoming. Magnetic susceptibility values vary from -1.47 to +17x10^-5 SI (mean -0.09x10^-5 SI). The relationship between magnetic susceptibility values, facies type, porosity and relative sea-level changes is evaluated.