FORELAND-BASIN DEVELOPMENT IN A SALT-INFLUENCED PROVINCE: SIERRA MADRE FORELAND, NORTHEASTERN MEXICO

Exposed structural relations and growth strata in Upper Cretaceous and Paleogene rocks of the La Popa and Parras basins of the Sierra Madre foreland in NE Mexico record two phases in the evolution of a foreland-basin system strongly influenced by salt. (1) From Santonian to mid-Maastrichtian time, the foreland region north of the Sierra Madre foldbelt, whose time-equivalent front remains imprecisely located, was a flexural foredeep with stratal patterns dominated in the south by orogenward thickening and in the north by localized depocenters created by salt withdrawal and diapirism. Growth strata adjacent to local salt stocks and a salt wall consist of halokinetic sequences separated by angular unconformities of pronounced discordance but limited lateral continuity, contain unambiguous diapir-derived fragments, and thicken monotonically away from the salt structures into adjoining salt-withdrawal minibasins. We infer that diapirs overlie basement steps created by faults inherited from a transtensional salt-basin history in the Late Jurassic. (2) From mid-Maastrichtian to approximately mid-Eocene time, the foreland was a depositional wedge top on which sedimentation patterns resulted from interplay of salt withdrawal and shortening-driven rise of periclinal to nearly concentric anticlines detached at different levels. Growth strata deposited during this phase of basin development likewise thin onto salt diapirs, some of which occupied the crests of folds; elsewhere, strata demonstrably thicken into contractional growth synclines that lack diapiric salt or a salt detachment. Provenance of growth strata during the second phase included the Sierra Madre foldbelt, a magmatic arc terrane in western Mexico, and the local diapirs. Rapid northward advance of the depositional wedge top probably resulted from detachment development within a weak salt layer that lies primarily beneath the northern part of the foreland-basin system. The northern salt layer is not continuous with a salt detachment beneath the Sierra Madre foldbelt, but the two detachments are kinematically linked via basement inversion structures. An increase in convergence rate between the Farallon and North American plates, which roughly coincides with the onset of the second phase, is likely of secondary importance to wedge-top advance.