Mark G. Rowan1, Timothy F. Lawton2, Katherine A. Giles2, Kevin D. Hon2, Kyle S. Graff2
(1) Rowan Structural Consulting, Boulder, CO
(2) Institute of Tectonic Studies, New Mexico State University, Las Cruces, NM
ABSTRACT: Anatomy of an Exposed Vertical Salt Weld, La Popa Basin, Mexico
La Popa basin, near Monterrey, Mexico, contains a subvertical salt weld that is exposed for 25 km along strike. Outcropping strata on the southwestern, downthrown side include Maastrichtian to Eocene clastics; strata on the northeastern, upthrown side range in age from late Aptian to Eocene and include clastics as well as lenticular carbonate beds that developed on local bathymetric highs when salt was present. The weld is an analog for subsurface structures that play an important role in hydrocarbon migration and entrapment in the northern Gulf of Mexico.
Detailed traverses across the weld indicate significant strike-parallel variability of both weld structure and the geometry of adjacent strata. In some places the weld is a 2-4 m wide fault zone with brecciated and sheared siliciclastics, no remnant evaporite, and evidence of extensive fluid alteration. In other places, the weld contains up to 30 m of remnant gypsum, either massive or sheared, that locally contains inclusions of metaigneous rocks or Jurassic dolostone that were carried up in the diapir. The weld ends in the west at an outcrop of gypsum several hundred m in diameter that represents a remnant salt stock; in contrast, the weld trace is truncated in the southeast by an intra-Maastrichtian unconformity. In all cases, beds adjacent to the weld are fractured and exhibit local folding and erosional truncation resulting from passive diapirism. In addition, the southeastern half of the weld has been affected by regional shortening, so that beds dip away from the weld for up to 2 km in either direction.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado