CHARACTERIZATION OF SYN-DEPOSITIONAL FAULTING OF A COMPOSITE TYPE B HALOKINETIC SEQUENCE ALONG THE LA POPA SALT WELD, LA POPA BASIN, NUEVO LEON, MEXICO

Halokinetic sequences are growth strata associated with passive diapirs and are bounded at the top and base by local angular unconformities. The unconformities are caused by drape-folding of the strata adjacent to a diapir as it rises topographically and are used to interpret the history of salt movement in a basin. La Popa salt weld in the La Popa basin, Nuevo Leon, Mexico is a vertical salt-evacuation feature and the strata adjacent to the weld exhibit halokinetic sequences formed during the diapiric phase of the salt structure. Two types of halokinetic sequences (A and B) have been identified in the strata adjacent to the weld. Type A is characterized by a thin beam of strata over the diapir creating a tightly drape-folded package of sediment at the salt-sediment interface; whereas, Type B is characterized by a thick beam of strata over the diapir generating a broadly drape-folded package with an onlap wedge of younger sediments against the folded beam.

The definition and attributes of Type B halokinetic sequences are based on outcrops along the northeastern side of the weld where the Maastrichtian age Muerto Formation forms the onlap wedge onto the thick beam of Campanian age drape-folded Parras Shale. Approximately 0.2 km from the weld at this location there is a syn-depositional fault that cuts through the sequence. Documented slickensides on the fault surface indicate a normal fault sense of movement. Detailed field mapping indicates faulting created a half-graben whose 3-D geometry is an elongate trough that parallels the trace of the weld. The hanging-wall strata is upper Parras Shale that is overlain by a growth stratal wedge of the Muerto Formation at the easternmost extent of the fault trace. This suggests that faulting was syn-depositional to the Muerto Formation and occurred late in the Type B halokinetic sequence formation as a response to extension associated with drape-folding of the thick beam of Parras Shale as diapirism progressed.