The mechanical stratigraphy of the Upper Cretaceous Austin Chalk is established from study of fracture intensity along its outcrop trend from Dallas to San Antonio and westward to Langtry, Texas, and in the subsurface from study of cores and/or fracture identification logs from 30 wells. Three mechanical-stratigraphic units are recognized: (1) an upper, fractured massive chalk corresponding to the Big House Chalk Member; (2) a middle, ductile chalk-marl corresponding to the Dessau Chalk and Burditt Marl Members; and (3) a lower, fractured massive chalk corresponding to the Atco Chalk Member.

Representative samples from the three mechanical-stratigraphic units were experimentally shortened, dry, at 10, 17, 34, and 70 MPa confining pressure, at 24°C, and at a strain rate of 2.5 × 10^-4 sec^-1 to determine if the relative mechanical behavior observed at the surface could be extrapolated into the subsurface at different simulated burial depths. The experimentally determined ductilities parallel those determined from outcrop and subsurface studies. Multiple linear regression analysis indicates that porosity is most strongly correlated with fracture strength. Smectite-content has the second strongest correlation. For low-porosity specimens (9-13.5%), the strength of specimens with 4% smectite is reduced 30-42% compared to specimens with no smectite The coefficient of internal friction at 17 MPa confining pressure decreases from 1.66 to 0.61 as smectite content increases from 0 to 4%.

SEM photomicrographs of undeformed specimens show that smectite and other clays are distributed as large (30 µm), discrete, amorphous, concentrated masses throughout the chalk. They are comminuted along the induced fracture surfaces where their grain size is 0.5 µm or less. These observations suggest that smectite acts as a "soft-inclusion," localizing shear failure and correspondingly weakening the material.