STRUCTURAL COMPLEXITY WITHIN THE BLACK GAP SYNCLINE, WEST TEXAS

An initial field investigation of the Black Gap syncline in west Texas reveals greater structural complexity than previously documented. The syncline has been considered an open fold primarily defined by the present-day orientations of ~22 Ma trachybasalt flows. More than twelve of these flows unconformably overlie Cretaceous limestones and clays and range in thickness from ~2 m to more than 5 m, and the thickness and number of these flows rapidly decreases outside of the fold, to the east and west, where most Cretaceous and Miocene rocks are relatively flat-lying. The fold trends NNW, parallel to many Laramide and Basin and Range structures present in the area, and is hypothesized to have formed during the creation of a pull-apart graben during Early Miocene time, soon after the extrusion of the trachybasalt flows. The formation of the graben is thought to have been controlled by the reactivation of older Laramide-age structures.

Immediately to the south and to the north of Black Gap (a narrow E-W– trending valley that truncates both the eastern and western limbs of the fold), the syncline is well defined by flow orientations on east and west limbs. Further north, the east-dipping western limb is well-defined for nearly 5 km, but approximately 2.5 km north of Black Gap, the dip direction of both Miocene flows and underlying Cretaceous units on the eastern limb abruptly reverses from west-dipping to east-dipping. This rapid reversal in dip polarity suggests the presence of an undocumented transfer zone and a possible normal fault which has utilized weaknesses proximal to the hingeline of the fold. Instead of a simple fold, with basalt flows present on both limbs, it appears that the topography at this latitude has instead been created by repetition of section, with Cretaceous units exposed in the core of the structure due to displacement along an undocumented east-dipping normal fault.