MECHANICAL STRATIGRAPHY AND FAULT ZONE DEFORMATION IN THE AUSTIN CHALK IN TEN-MILE CREEK, DESOTO, TEXAS

The Austin Chalk is a rhythmically bedded sequence of chalk and marl that represents pelagic to hemipelagic carbonate deposition in the ancestral Gulf of Mexico during the Upper Cretaceous (Coniacian through Santonian). The Austin Chalk differs from other chalk deposits due to its relatively high abundance of clay and volcanic ash. Outcrops of the formation stretch from north-central Texas to west Texas and surface exposures mirror the subsurface trend of the Ouachita orogen. Deformation of the fractured Austin Chalk is commonly linked to normal faults associated with the Balcones Fault Zone.

Here, we present a structural analysis from two sections along Ten-Mile Creek in DeSoto, Texas, where deformation of the Austin Chalk is characteristic of normal faulting in platform carbonate sequences. Faults are identified by the presence of slickenlines and fault gouge, and are surrounded by a damage zone defined by synthetic faulting, fracturing, and folding. Deformation is concentrated near fault cores and decreases with increased distance. The mechanical properties of stratigraphic units are quantified using a Schmidt hammer. Fracture parameters, such as fracture spacing and intensity, are quantified using linear scanline surveys. Additionally, spectral gamma ray measurements are made in the field using a RS-230 spectrometer.

Lithologies present in the section include chalk, marly chalk, marl, clay-rich marl, and bentonite. Chalk and marly chalk lithologies show the highest Schmidt hammer values (R > 25), whereas marl and clay-rich lithologies show intermediate (R = 10-30) and the lowest (R < 20) Schmidt hammer values, respectively. Fractures in the formation are subvertical and results show a dominant fracture set with a northeast-southwest strike. Additional minor fracture sets with north-south and northwest-southeast strikes are also present. Seven linear scanline surveys ranging from 12 to 50 meters in length were completed in the two sections of Ten-Mile Creek. Linear scanline results give fracture spacing values ranging from 0.4 m to 0.9 m within the damage zones of normal faults, and spacing values ranging from 0.9 m to 2.9 m outside damage zones.