MINERAL ORIENTATION, AMS DATA, AND IMPLICATIONS FOR MULTIPLE SHEET EMPLACEMENT OF THE MAIDEN CREEK SILL, HENRY MOUNTAINS, SOUTHERN UTAH

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

Foliation data taken from initial AMS readings show a varied range of mineral orientations. To further isolate, or narrow down, the angle of the mineral orientations image analysis was done using the auto-correlation/shape-preferred orientation (ACF/SPO) function of IMAGEJ. The data compiled using IMAGEJ showed near vertical orientations of the minerals ranging from approximately 115 degrees to 80 degrees from the horizontal. We used the 3^rd contour of the ACF image, which correlated with the finer grains, to calculate the orientation. The orientations are consistent with the proposed multiple sheet model and possibly reflects a bull-dozer track-like emplacement.

Preliminary SEM-EDS data shows two kinds of magnetite which corresponds petrographically with exsolved and non-exsolved magnetite. Low K, non-exsolved samples have minor amounts of titanium (Ti) representing Titanomagnitite. In contrast, the presence of exsolution textures may mean the Ti has exsolved and left a more pure end member, higher K, magnetite. This data may indicate later sheets were insulated and therefore magnetite had time to exsolve.

Taking in account the data provided by AMS, image analysis, and SEM-EDS; the model of multiple sheets at the Maiden Creek Sill is possible. Further research on the AMS is being conducted to test emplacement models.