Mailing address update: As of 1/22/24, the Bureau will have a new mailing address. See our contact page for details.

Devine Test Site: Austin Chalk

Over most of its range, the Austin Chalk is dominated by two interbedding end-member rock types, chalk and marl (Figure 5). Tan to white chalk beds are composed of partly recrystalized, fragmented nannoplankton matrix having 5 to 25 percent foraminifers and whole or fragmented inoceramid pelecypods.

figure 5

Figure 5. Complex normal fault zone in upper Lower Austin Chalk at the SSC site, showing large normal faults (F) and smaller antithetic faults (A), mainly calcite filled. Almost 70 ft of throw across this zone has been interpreted from borehole data. Rollover monocline (R) is found on the far hanging wall. Water (W) was produced from only one small (3-ft throw) fault.

The chalk is relatively homogeneous compared with other carbonates. Its original low-Mg calcite composition has not been greatly altered by diagenesis. Marl beds are darker than chalk and consist of chalk mixed with 12 to 40 percent clay and as much as 3.5 percent organic carbon (Collins and others, 1992). Intermediate mixtures of chalk and marl (marly chalk) also occur. Locally, thin (<1 ft thick) bentonite (expansive clay) beds can be found, and basalt plugs penetrate the formation.

As indicated by fracture styles observed in exposures along the Balcones Fault Zone, chalks are generally brittle and display well-defined fractures, whereas marls are relatively ductile and may smear along fault surfaces and expand into fault apertures.

Legal Notes

Safety and Environmental: The Bureau and DGTS User shall each be responsible for the safety of their own respective employees, contractors, and agents while they are present upon or are in transit to or from the DGTS. DGTS User shall evaluate the hazards based on planned procedures and determine and provide appropriate protective equipment requirements for all personnel of User (e.g., ANSI-approved hard hats, safety glasses, safety shoes, gloves, etc.). UT/Bureau shall not be responsible for providing any worker's compensation insurance coverage for User's personnel on the DGTS and User shall not be responsible for providing worker's compensation insurance for the Bureau's personnel on the DGTS. User must submit to the Bureau a Safety and Environmental Plan prior to beginning any operations whatsoever on the DGTS. All rules and regulations promulgated by the Occupational Safety and Health Administration, Environmental Protection Agency, Texas Railroad Commission, Texas Natural Resource Conservation Commission, General Land Office, Texas Water Development Board, and other applicable Federal, State, or local regulations shall be followed by User in all operations on the DGTS.


© 2021 Bureau of Economic Geology | Web Privacy Policy | Web Accessibility Policy