Abstract - Faults in high porosity, Lower Permian sandstones
from north-west England and south-west Scotland occur as millimetre-wide
bands of cataclastic deformation, across which shear displacement is limited
to a few millimetres or less. These cataclastic slip bands occur either
singly or in sets and may be associated with ordinary slip surfaces with
metre-scale shear displacements. Detailed petrography, including backscattered
scanning electron microscopy and cathodoluminescence imaging, shows the
cataclastic slip bands to be characterised by decreases of as much as four
grain size classes and complementary changes in sorting, with respect to
the host sandstones. Grain and overgrowth fracturing, together with minor
cementation, are the main processes identified. Crystal plastic deformation
mechanisms are not important. Core analyses show that textural changes
correlate with decreases in permeability and porosity of up to four orders
of magnitude across cataclastic slip bands. Although the cataclastically
deformed zones themselves are the sites of porosity loss, enhanced permeability
and porosity are often developed adjacent to and on both sides of these
zones. Dilation of the pore volume adjacent to the cataclastic slip bands
account for these increases. Removal of iron oxide grain coatings, together
with minor cementation, suggests that the fault zones may have been sites
of enhanced fluid flow during faulting. The area over which cataclastic
slip bands are developed is as much as 40m wide for aggregate displacements
of no more than a few metres. Such zones may be seriously detrimental to
potential reservoir quality through local reductions in permeabilityand
by partitioning of the sandstone body with respect to fluid movement. However,
the small aggregate displacement means that cataclastic slip bands cannot
be identified by seismic imaging. Further, although the fault orientations
follow the basin-wide trends of larger faults,their precise locations are
not so related, making prediction of their position difficult.
Marine & Petroleum Geology 11, 608-623, 1994.