ORIENTATION AND SPACING OF JOINTS IN DEFORMATION BAND SHEAR ZONES EAST OF MOUNT HILLERS, UTAH

Deformation bands are planar shear zones that accommodate fault offset in porous sandstones. They form by porosity collapse and cataclasis of sand grains, resulting in low porosity and permeability across the shear zone. Deformation bands form pervasively throughout deformed sandstone bodies and vary in thickness from 1mm to tens of meters, so they present significant barriers to fluid flow. In some cases, however, deformation bands host closely spaced joints that may serve as conduits for fluid flow across these impermeable shear zones. The relationships among deformation band orientation, joint orientation, deformation band thickness, and joint spacing have the potential to affect fluid flow through a porous sandstone reservoir, and are the subject of our study.

We examined deformation bands in Jurassic Entrada Sandstone east of Mt. Hillers, one of the Henry Mountains laccoliths in southern Utah. Deformation bands formed in Entrada Sandstone in response to Oligocene – early Miocene laccolith emplacement. The shear zones occupy two dominant orientations in the study area: a north-striking set and an east-striking set, both dipping nearly vertically. In deformation bands that contain joints, the relationship between deformation band thickness and joint spacing is roughly linear; narrow deformation bands develop closely-spaced joints, and joints in wider deformation bands are more widely spaced. East-striking deformation bands are consistently jointed throughout the study area, and display two perpendicular joint sets. In horizontal outcrops, these deformation bands display north-striking, vertical joints, and a set of horizontal joints is evident in east-striking deformation bands exposed on steep outcrop faces. However, most north-striking deformation bands are not jointed. Our data imply that the development of joints within deformation bands is dependent on deformation band orientation relative to the direction of stress responsible for jointing.

Future work will trace progressive changes in orientations of deformation bands and associated joints around the Henry Mountains, and will relate orientation changes to joint spacing variations within deformation bands.