DEFORMATION OF SANDSTONE AND EMPLACEMENT OF LACCOLITHS IN SOUTH-CENTRAL UTAH

The Trachyte Mesa, a laccolith intrusion along the flanks of the Henry Mountains in South-central Utah, was studied to determine deformation mechanisms that allowed for the addition of magma and accompanied loss of space in the host rocks. The Trachyte Mesa intruded into the flat lying shales and sandstones of the Colorado plateau. Since this region has never experienced any regional tectonic deformation, all of the deformation in the host rock around the Trachyte Mesa is due solely to the intrusional event. One particular bed in the Entrada Sandstone can be traced from where it was undeformed away from the intrusion to where it has been deflected up over the periphery and onto the top of the intrusion. While this bed has been translated and rotated, it was also thinned from 6.0m away from the intrusion to 4.7m on top of the intrusion. Observation and analyses of the Entrada Sandstone bed along the length of the section reveal that the deformation was accomplished through a combination of grain boundary sliding, pressure solution, fracturing and faulting. These deformation mechanisms produced a significant change in porosity, which allowed for the change in shape (attenuation).