LARGE-SCALE CYCLICITY IN GAMMA-RAY AND MAGNETIC SUSCEPTIBILITY PROFILES OF THE UPPER MISSISSIPPIAN FAYETTEVILLE SHALE, NORTHERN ARKANSAS

The Fayetteville Shale is a major unconventional gas play in central and western Arkansas and eastern Oklahoma. Two road cuts south of Marshall, Arkansas on Highway 65 expose the middle and upper part of the Fayetteville and overlying Pitkin Limestone. The lower part of the Fayetteville is black shale interbedded with septarian nodules and thin irregular beds of organic-rich lime mudstone. A succession of thin to medium-bedded lime mudstone, 3 m (10 ft) thick, in the middle part is capped by a brachiopod packstone bed. The upper part is rhythmically bedded black to gray calcareous shale with regular thin to medium beds of organic-rich lime mudstone to wackestone. Overall exposed thickness is approximately 38.1 m (125 ft). The unit has been interpreted as accumulating in a deep-marine setting that gradually shallowed upward. We examined the Fayetteville Shale to compare magnetic susceptibility and natural radioactivity with lithofacies to characterize cyclicity in the unit.

Gamma-ray profiles confirm marked “cleaning-upward” successions in two major cycles. The four channels collected (TC1, KUT, UT, and T) showed strong cross-correlation throughout the interval. The magnetic susceptibility of the shales generally had an average value of 0.0186 x 10^-3 SI and limestones 0.025 x 10^-3 SI, but, in this study, readings ranged from 0.005 x 10^-3 SI to 0.025 x 10^-3 SI for shales, and the values for limestones were closely consistent in the rhythmic part of the Fayetteville. Magnetic susceptibility in limestones appears to be controlled by occurrences of iron-bearing silicates, pyrite, arsenopyrite, or possibly pyrrhotite. Inconsistency in the magnetic susceptibility in shales indicates changes in the composition over the cyclic interval. The exposure with a temperature of 105 °K-110 °K, field magnetic susceptibility measurements show that the magnetic fabric of the Fayetteville Shale is dominated by paramagnetic minerals like illite, chlorite, and smectite. This study shows magnetic susceptibility is an important tool for identifying subtle changes in mineralogy as it is expressed in the magnetic properties, but substantiating fifth-order Milankovitch cycles in rhythmic succession was below the sampling interval.