Orientation and spacing of joints in
deformation band shear zones east of Mount Hillers, Utah
Hoefert,
Catherine M. and Tindall,
Sarah
E., 2004, Orientation
and
spacing
of
joints in deformation band shear zones east of Mount
Hillers, Utah [abs]: Northeastern Section (39th Annual) and
Southeastern Section (53rd Annual) Joint Meeting (March 25–27, 2004),
Geological Society of America Abstracts with Programs, Vol. 36, No. 2,
p. 102.
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.
