STRAIN AND DEFORMATION ANALYSIS OF SANDSTONES SURROUNDING THE MAIDEN CREEK SILL AND MAGMA FINGERS, HENRY MOUNTAINS, UTAH

Strain and deformation analyses were performed on the Jurassic Entrada Sandstone surrounding the Maiden Creek Sill and attached magma fingers to be able to determine the volume producing mechanism(s) for the intrusion. 45 Entrada sandstone samples were collected above, below, and from the same horizon as the sill and protruding fingers. The Maiden Creek Sill is a 25 Ma shallow andesitic intrusion composed of a dominantly covered central portion and four protruding magma fingers that are well exposed. Analyses of the surrounding sandstone reveals that there is no uplift of surrounding rock adjacent to or above the fingers. Fingers vary in width between 10 and 80m and can be up to 30m in height. Fracturing is prevalent in the sandstone adjacent to these fingers and are found at various orientations but no or very little displacements are observed. There is a 50 cm thick brittle shear zone that separates the sandstone layers containing the sill and fingers to the sandstone layers above. Sandstone samples, collected as far as 55 m from the intrusion, show a wide variation in deformation intensity although almost all samples contain quartz grains that are crushed and fractured and locally up to 20% of grains are crushed. These deformed grains are not found along fractures. The crushed pieces are variably displaced into pore spaces. Samples collected above the magma fingers are also intensely deformed. There does seem to be a weak correlation between the distance from the intrusion and intensity of fracturing. Fry analyses have been conducted on 26 of the sandstone samples. One thin section from each sample was used for the strain analysis. Four areas from each thin section were measured using the program of Vollmer, F.W. (2018). Strain ratios are highly variable and do not seem to correlate with distance from the intrusion. There is no preferred orientation to the long axis of the strain ellipses.