AMS AND IMAGE ANALYSIS DATA AT MULTIPLE SCALES SUPPORT A MULTIPLE SHEET EMPLACEMENT MODEL FOR THE MAIDEN CREEK SILL, HENRY MOUNTAINS, UTAH

Anisotropy of Magnetic Susceptibility (AMS) and image analysis data have been used to investigate possible contacts across magma sheets in the Maiden Creek Sill (MCS) from the Henry Mountains, Utah, USA. Thirty-one cores were drilled along an 8 m vertical traverse on the margin of the sill. The MCS is composed of at least two sheets which are locally divided by a thin sliver of sandstone. The AMS data illustrate there is an abrupt 10x change in magnetic susceptibility (K) across the sheet-on-sheet contact where the sandstone sliver is absent. This contact is also defined by a 1-2 cm thick solid-state shear zone. There are also several “drop offs” (~10x) in K possibly indicating several different sheets, or contacts at lower levels.

The magnetic foliation changes from subvertical to subhorizontal at the sheet-on-sheet contact. To further investigate the Shape Preferred Orientation (SPO) of the minerals, and to determine the controls on the magnetic foliation, image analysis was done using the Auto-Correlation Function (ACF) and Intercepts method. We used the 3^rd contour of the ACF image, which correlated with the finer grains, to calculate the SPO and we used cm-scale images to determine the SPO using intercepts. These SPO’s conflict with the AMS foliation data although we are continuing to collect more SPO data at different scales to determine which grain size carries the AMS foliation. The AMS foliation data are consistent with multiple sheets being emplaced and possibly reflects a bull-dozer track-like emplacement model. Further research on the AMS and image analysis is being conducted to test emplacement models.