TOPOGRAPHIC PROFILE, MECHANICAL STRATIGRAPHY, AND INTERLAYER SLIP: EXPLORING COUPLED FOLD-FRACTURE EVOLUTION OF THE STILLWELL ANTICLINE, WEST TEXAS

We performed a study of Santa Elena limestone beds that define the Stillwell anticline, west Texas, to explore the roles that lithology, compressive strength, and interlayer slip have played in the accommodation of strain. The anticline is an asymmetric, NE-vergent fold with a shallow, SW-dipping backlimb, a subhorizontal middle limb, and a steep, NE-dipping forelimb. Previous field data and computer kinematic models suggest fold formation by fault propagation. These field and modeling studies also reveal maximum strain in the forelimb of the system, with minimum strain in the middle limb. Thus, we focused on the relatively undeformed middle limb to document anticline stratigraphy.

Throughout the exposed stratigraphy, we documented bed thickness, lithology, compressive strength (Schmidt hammer), fracture intensity, and joint orientations for each bed. We also divided the stratigraphy into 8 units defined by sharp changes in slope, since we hypothesized a connection between resistance to erosion and mechanical strength. However, our data reveal that nearly all beds were composed of a light tan to light grey, fine to medium grained limestone with very little variation in compressive strength throughout the exposed 56 m-thick stratigraphy. Instead, units with thinner average bed thickness are less resistant to weathering. In addition, the relative fracture intensities follow a power-law distribution, with thinner beds displaying significantly greater fracture intensity relative to thicker beds, suggesting that topographic profile is controlled by relative fracture intensity, which is in turn correlated with bed thickness. Fracture data throughout the stratigraphy reveal two dominant, steeply-dipping sets, striking NE and NW. However, similar to previous joint research, beds above and below a bed may not display the same joint set, and only rarely do fractures propagate across bed boundaries. Also, the dominant set in one of the 8 units may not be strongly represented in units above or below it. These data permit inference that inter-bed cohesion is low through the stratigraphy. Thus, we hypothesize that where shear strain is greatest, in the forelimb of the system, most strain is accommodated by interlayer slip with fracturing focused within the thinner beds, consistent with field observations.