MULTIPLE SHEET MAGMA EMPLACEMENT IN THE MAIDEN CREEK SILL, HENRY MOUNTAINS, UTAH

Examination of a cliff face cross section on the Maiden Creek Sill in the Henry Mountains in Southeastern Utah, reveal a series of sub-horizontal sheets. Data includes Anisotropy of Magnetic Susceptibility, bulk susceptibility (K), and microstructures. Cm-thick shear zones separate meter-scale sheets which appear to be emplaced at different times. Newer magma sheets have much higher bulk K and are unfractured. Older sheets have lower bulk K and are highly fractured.

Data was collected using a grid system. The grid was set in 20 centimeter intervals making a total of 458 readings from a hand-held susceptibility meter. The area of data collection is approximately 6.4 m vertically and 3.6 m horizontally. Fractures and shear zones were documented throughout the work area. Changes in K coincide with visible changes in the outcrop; higher K values are found where the rock is mostly unfractured with euhedral phenocrysts and darker in color. Lower K values are found where the rock is highly fractured and bleached with no phenocrysts. AMS data taken from 13 cores that were drilled along a vertical traverse across the work area shows the orientation of the foliation and bulk K and correlates well with the hand-held K data.

Using the program ArcMap the magnetic susceptibility data was contoured using kriging (prediction and probability mapping) and inverse distance weighting. The contour maps of K illustrate two major sheets. The bottom sheet is about 1.5 m in thickness and the top sheet is about 1 m. Separating the sheets are 1-3 cm thick shear zones defined by shattered feldspar crystals and strongly aligned magnetic foliation. The dip of the shear zones are 42 degrees dipping into the sill. The drastic change in the magnetic susceptibility seems to be due to the dissolution of primary magnetite, which occurred as new magma sheets fractured the older ones and fluxed them with fluids.