Structure and Stratigraphy within the Evaporite Décollement of the Sierra Madre Oriental, Northeastern México

Structures preserved within evaporite décollements have important implications for the tectonic evolution of evaporite-detached fold belts. However, due to typically poor exposure, the kinematics of structural development in evaporite décollements is poorly understood. Upper Jurassic evaporites in the Galeana area of northeastern México reveal a diverse suite of well-exposed structures. Here, the décollement of the Laramide-age Sierra Madre Oriental fold belt has been exhumed by a late- or post-Laramide basement uplift. We have mapped (at 1:10,000 scale) an ~8 km long strip of outcrop that exposes the full thickness of the décollement. Our provisional stratigraphy (>1000 m thick) includes 11 regionally-persistent, mappable members that include five lithologically distinct carbonate packages. The remaining 6 members are composed of gypsum-anhydrite with varying proportions of thin (<5 m) carbonate interbeds. Macroscopic structural patterns are delineated by these carbonate intervals. North, north-northeast and northeast-trending macroscopic folds are the dominant structures, in places refolded by later, east-west trending folds. Most folds are isoclinal and overturned toward the east or southeast. Thinner carbonate interbeds are commonly boudinaged, and where boudinage is extreme a mélange texture is present, with dismembered carbonate blocks “floating” in an evaporite matrix. Thicker carbonate intervals (up to ~150 m thick) may show “formation-scale” boudinage, typically spatially associated with large volumes of void-filling sparry calcite. The dominant structure in evaporite lithologies is a mylonitic foliation which obliterates primary sedimentary structures. While foliation is locally developed in all evaporite-dominated members, the lowermost evaporite member is a ~100 m thick shear-zone with spectacular banded mylonites, porphyroclasts, and intrafolial isoclinal folds. These structures likely reflect two processes: heterogeneous bulk simple shear associated with the bedding-parallel displacement of the rocks above the décollement relative to the sub-décollement basement, and redistribution of décollement material in response to folding of the overburden.