THE ROLE OF MECHANICAL STRATIGRAPHY IN FAULT-PROPAGATION FOLD EVOLUTION: A CASE STUDY

Structural field measurements, slope-gradient analysis, and Schmidt hammer testing of the Santa Elena limestone were used to evaluate the relative importance of mechanical stratigraphy on the evolution of the Stillwell anticline, west Texas. The Stillwell anticline is an asymmetric, northwest-trending, northeast-vergent, Laramide-age fault-propagation fold that is best-defined by the relatively resistant Santa Elena limestone. The 200m-thick Santa Elena contains interbedded limestones (massive, 1 – 3 m thick) and shales (< 1 m thick) and has been hypothesized to represent deposition in a neritic shallow reef environment. To evaluate rigidity contrasts between beds within the Santa Elena limestone, we performed detailed Schmidt hammer testing of a nearly complete section of the unit, following the procedure outlined by Shackleton et al. (2005). This testing revealed significant variations in unit rigidity throughout the stratigraphic section. To complement these data, we also performed detailed slope-gradient analysis along a continuous topographic profile, using established qualitative relationships between slope and average rock-mass strength. Based on these analyses, we have developed a semi-quantitative mechanical stratigraphy for the entire Santa Elena limestone, permitting more rigorous examination of the accommodation of strain within the Stillwell anticline. Field observations of well-exposed cross-sectional outcrop views of the Stillwell anticline suggest that low strength units play an important role in strain accommodation, acting as barriers to fault propagation on the outcrop scale. Based on these observations, it is likely that these less rigid layers absorb a greater percentage of strain than the stronger, more massive beds. Thus, when reconstructing the link between fault propagation and fold evolution, we know that shear stress associated with the propagating fault tip will be reduced within mechanically weak layers, thus reducing the total accommodated slip. This reduction in slip must affect the geometry and amplitude of folding above the fault system and must be taken into account during future kinematic modeling of the system.