2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 6
Presentation Time: 8:00 AM-12:00 PM


REED, Robert M. and GALE, Julia F.W., Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The Univ of Texas at Austin, Box X, University Station, Austin, TX 78713-8924, rob@alumni.utexas.net

Opening mode fractures in sandstone commonly show crack-seal texture in the quartz cement which lines and/or bridges the fractures. Crack-seal textures are most visible using SEM-based cathodoluminescence imaging (SEM-CL). These textures also turn out to be somewhat discernable in some cases using orientation contrast imaging (OCI) of forescattered electrons acquired by an electron backscatter diffraction (EBSD) system. Both types of imaging tend to show very complex but significantly different patterns of fracturing and growth within the fracture-lining quartz cement.

We used SEM-CL and OCI to image textures in the quartz cement. EBSD was used to measure the crystallographic orientation of both grains making up fracture walls and cement crystals in fractures. Sandstone samples examined are from the subsurface Cretaceous Travis Peak Formation of East Texas, the subsurface Cretaceous Frontier Formation of the Wind River Basin of Wyoming, and outcrop samples of the Cambrian Eriboll Group, NW Scotland.

Preliminary measurements confirm that grains in the fracture wall control much of the crystal orientation of cement in the fractures. EBSD has shown that grains with the fast growth direction of quartz (the C-axis) at a high angle to the fracture walls are more likely to have quartz overgrowth cement which completely bridges the fracture. However, twinning, complex intergrowth of crystals, quartz crystals growing into the plane of the thin section, and other crystallographic variations complicate the overall picture of what is going on in the fracture.

SEM-CL has provided many insights into these complexities, particularly relating to growth timing. Many of the features apparent in SEM-CL, especially growth zoning, do not correspond to features visible using EBSD and OCI. Only a few features visible using EBSD and OCI do not correspond to features seen with SEM-CL. Therefore, EBSD and OCI have proven to be more useful in aiding interpretation of SEM-CL than vice versa. Recognition of the composite nature of many bridges has simplified interpretation of complex crack-seal textures seen in SEM-CL. Although the most spectacular quartz bridges tend to be monocrystalline, many quartz bridges are amalgamations of cement growing from two or three neighbouring grains.