Fault-Zone Deformation Mechanisms in the Cretaceous Limestones of South Texas
David A. Ferrill1 and Alan P. Morris2
1 CNWRA,
Southwest Research Institute, San Antonio, TX
2 University of Texas San
Antonio, San Antonio, TX
Normal faults exposed in Cretaceous (Glen Rose, Edwards, and Buda)
limestones along the southeastern margin of the Edwards Plateau and in the
Tertiary extensional province of West Texas, provide important analogs for fault
zone architecture and fault zone deformation characteristics in carbonate
reservoirs around the world. Mechanical layering, clay content, rock strength
characteristics, and depth at the time of faulting are fundamental constraints
on carbonate fault zones. Large planar faults with low displacement gradients
are developed in massive, strong (clay-poor) Edwards Group limestones. In the
more thinly bedded, lithologically variable Glen Rose, weak (clay-rich) beds
impede fault propagation, resulting in fault-related folding, and locally steep
bedding dips in fault damage zones. Faults in clay-poor massive limestones tend
to be steep (70° or steeper) whereas weaker, clay-rich limestones develop faults
with shallower (60° or gentler) dips. Faults cutting interlayered strong and
weak limestones tend to have refracted profiles and substantial vertical
variability in fault zone thickness. Refracted fault profiles have commonly
formed at shallow depths where low differential stress results in variable
failure angles due to changes in failure modes through the mechanically layered
sequence. Thin sections from a fault zone in the Edwards limestone show evidence
of cataclasis, cementation, deformation of cement by mechanical twinning and
pressure
solution
, and multiple generations of cement with differing degrees of
deformation, indicating cementation was contempoaneous with fault slip. Because
the fault-zone cementation occurred contemporaneously with fault slip, the
estimated "minus-cement" porosity does not reflect actual porosity of the fault
zone at any stage in development. This implies that while active, these faults
may have alternately behaved as (i) conduits for fluid movement after a slip
event during cementation, and (ii) barriers to fluid movement after cementation
was complete or nearly complete, prior to the next slip event.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005