FORMATION MECHANISMS OF ARTESIA GROUP TEPEE STRUCTURES, SOUTHEAST NEW MEXICO AND WEST TEXAS

Tepee structures form anticlinal, chevron-like kinks within carbonate units. Because they form coeval with strata housing them, mechanisms for their formation are important to understand depositional conditions. Middle Permian Artesia Group tepees, located at the shelf-edge break of the Delaware Basin, provide excellent exposures and allow a test of formation mechanisms.

To assess a mechanism, we utilized field relationships at outcrop to map scale coupled with geochemical and petrographic data. Stratigraphic relationships demonstrate that botryoidal cement forms large masses within the core and bedding-parallel sheet cracks that thin into the interfold region. Lithologies present include pisoidal grainstone, dismicrite, and algal-fenestrae boundstone. Beds are steeply and sharply upturned at the anticline, forming a chevron-like structure and cannot be traced over the fold core. Map scale relationships show that tepees occur across the Tansill and Yates formations, but predominantly in the lower and middle units of the Tansill Formation. Hinge lines are generally southwest orientation but shift to the southeast in the Yates Formation. Samples that have undergone XRD analysis demonstrate that cement is altered from original aragonite mineralogy such that the lowest tepee sampled is entirely dolomite, whereas the upper tepee is a mixture of dolomite and calcite. Determination of high-Mg to low-Mg calcite is ongoing.

Based predominantly on field relationships, we present a mechanism for the formation of tepee structures where initial cracking results from thermal variation of the rock during exposure or shallow hypersaline conditions. Early marine cement forms in the crack and along bedding planes due to peeling of the strata during cracking. Uplift and formation of the anticline is facilitated by pressure exerted from cement growth in the core and along bedding planes within the interfold area.

This model suggests that Artesia Group tepee structures formed in the Delaware Basin near the shelf edge where periodic subaerial exposure from low magnitude sea level variation occurred. In contrast to competing models that invoke hydraulic pressure along confined aquifers, our results do not require a setting that would enable such a hydraulic head to exist.