STRATIGRAPHIC AND STRUCTURAL DEVELOPMENT OF A VERTICAL SALT WELD, LA POPA BASIN, NUEVO LEON, MEXICO

Structural and stratigraphic relationships at the southeast end of an exposed vertical salt weld in the La Popa basin add new insights into the development of this unique structure during latest Cretaceous-early Cenozoic time. The weld is a fault-like salt evacuation structure that trends ESE 12 km from its western end to a bend, and then another 12 km SE to its exposed end in the study area. Shallow marine siliciclastic strata of the Parras Shale (Campanian), Muerto Formation and lower part of the Potrerillos Formation (lower Maastrichtian) thin toward the salt weld, indicating syndepositional topographic expression of the weld. The Potrerillos Formation unconformably overlies the Muerto, which is conformable upon the Parras.

Structural features include two distinct fault sets and a NW-trending fold set. Normal to oblique-normal faults trend NW and right-lateral strike-slip faults trend NNW. The normal faults cut only the Parras and Muerto formations, whereas the strike-slip faults cut all three stratigraphic units. Post-depositional folds include a faulted anticline that plunges to the southeast over the tip of the salt weld and a syncline northeast of the anticline. Sub-parallel trends of the weld trace, folds and strike-slip faults suggest a kinematic link between crustal shortening and salt evacuation.

We infer a two-phase weld history from the above relations. An initial diapiric phase is recorded by thinning of strata onto the topographic expression of an elongate salt diapir. Normal faulting and diapir-flank folding accompanied the passive rise of the diapir during accumulation of the Parras and Muerto Formations. A later evacuation phase accompanied Hidalgoan (Laramide) shortening during the early Cenozoic. SW-NE shortening oblique to the trend of the elongate diapir resulted in approximately synchronous salt expulsion and oblique slip along the weld trace. Shortening in the vicinity of the weld was accommodated by salt evacuation and then transpressional slip along the resulting weld. Elsewhere in the La Popa basin, shortening was accommodated by detachment folding.