MAGNETIC SUSCEPTIBILITY ANALYSIS SUPPORTS SHEET-LIKE EMPLACEMENT OF THE MAIDEN CREEK SILL, HENRY MOUNTAINS, UTAH
Using a hand-held susceptibility meter, data was collected on a grid system every 20cm for a total of 429 susceptibility readings. The area along the cliff where the data was collected was above and below a 1 cm thick shear zone within the sill that leads to a thin sliver of sandstone that separates the MCS into a top and bottom sheet. The area of data collection is approximately 3.2 m horizontally by 3.6 m vertically. Fractures and shear zones were also documented during data collection. Abrupt changes (approximately one order of magnitude) in K across the major shear zone, as well as 1 m below the major shear zone, support the hypothesis of multiple sheet emplacements. AMS data, which shows the orientation of the foliation and bulk K from 14 cores drilled across the shear zone, correlates well with the hand-held K data.
In ArcMap the K data was contoured using kriging and inverse distance weighting. The contour maps of K clearly illustrate the three sheets with the major shear zone as one contact. The middle sheet is approximately 1.4 m thick and has lower K values than the top and bottom sheet. Foliation within the middle sheet is moderately inclined to the southwest. Moving down the cliff face into the bottom sheet, the foliation becomes sub-vertical and eventually rotates again to become horizontal. Other fractures and shear zones were also plotted. Due to the abundance of brittle fractures in the middle sheet and cross-cutting features along the contact shear zone, the middle sheet appears to be older and the top and bottom sheet younger. Based on microscopy and preliminary SEM data, changes in K seem to be due to dissolution of primary magnetite in the older, middle sheet from fluids associated with the later sheets.