47263-AC8
Evaporite Deformation in the Sierra Madre Oriental, Northeastern Mexico: Decollement Kinematics in an Evaporite-Detached Thin-Skinned Fold Belt
Field area and fieldwork strategy: We have spent 182 days in northeastern Mexico, carrying out fieldwork in the Galeana area. This area presents the best exposures of the Minas Viejas Formation that forms the evaporite décollement of the Sierra Madre Oriental fold belt. The décollement in this area has been exhumed by the late- or post-Laramide Potosí uplift. Our fieldwork has focused on geologic mapping (at a scale of 1:10,000) of a well-exposed, 9.75 km long outcrop strip on the west flank of the uplift. The strip of exposure varies between 0.8 and 3 km in width, and exposes the full décollement stratigraphy. Sub-décollement redbeds of the Huizachal Group are exposed over a distance of 2.6 km at east end of the strip, and were mapped in order to provide constraints on thick-skinned structure in the sub-décollement basement. To date, the mapping is ~75% complete. We selected this particular area after carrying out reconnaissance across the Potosí uplift, and we believe that the structures exposed within the mapping area are representative of the uplift as a whole. Provisional stratigraphy of the décollement interval: The Minas Viejas Formation in the Galeana area is 1000-1500 m thick. Our provisional stratigraphy includes 11 regionally-persistent, mappable members that include five lithologically distinct carbonate packages. The lower four carbonate members were previously considered to be repetitions or lateral facies equivalents of the same unit. One of these members (a shaly carbonate unit) contains ammonites that may allow the biostratigraphic age of the decollement rocks to be determined. The remaining 6 members are composed of gypsum-anhydrite with varying proportions of thin (<5 m) carbonate interbeds. The gypsum-anhydrite is often quite calcareous, with nodular, laminated or blebby textures. Lenses of volcanic and volcaniclastic rocks (previously described only briefly) occur in the middle part of the décollement interval, and include a previously unrecognized ignimbrite unit. A tuff sample collected from this ignimbrite contained abundant (almost 300) zircons, and we will analyze these zircons to obtain a radiometric U-Pb depositional age. This absolute age determination will represent the first for the widespread Gulf of Mexico province evaporites. Macroscopic structural geology: The evaporite exposures in the Galeana area of northeastern México preserve a diverse suite of well-exposed structures. So far, our fieldwork has focused on macroscopic structures mappable at a scale of 1:10,000. These macroscopic structural patterns are delineated by the five carbonate members, and by carbonate interbeds in evaporite-dominated members. The structural style is dominated by folds. Locally, boudinage is important in carbonate members, while some evaporite members are dominated by shear zones. Fold axis trends are highly variable, although north-south to northeast-southwest trends are most common. Locally, these structures are refolded by east-west trending F2 folds. Most folds are isoclinal and overturned toward the east or southeast. Plunges are typically moderate to steep, and plunge directions may reverse along the trend of an individual fold axis. Boudinage occurs in carbonate intervals of all scales, from mm- or cm-scale interbeds up to whole carbonate members as much as ~150 m thick (map-scale boudinage). Boudins are 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. We interpret these mylonites to have developed in evaporite shear zones. The lowermost evaporite member of the décollement is a ~100 m thick shear zone with spectacular banded mylonites, porphyroclasts, and intrafolial isoclinal folds. These folds consistently show top-to-the-east transport. Evaporite members higher in the décollement show only local, thin shear zones, and primary sedimentary textures are commonly well-preserved. Preliminary interpretations: The shear zones within the décollement likely reflect simple shear deformation associated with the bedding-parallel displacement of the rocks above the décollement relative to the sub-décollement basement. The top-to-the-east transport indicated by folds in the basal shear zone is compatible with Laramide kinematics. Furthermore, the field relations strongly suggest that this simple shear deformation was localized in the basal evaporite member of the décollement. The fold-dominated structure that characterizes the rest of the décollement section may represent redistribution of décollement material in response to folding of the overburden. Complex fold orientations and local refolding suggests that this redistribution was not a simple (i.e., plane strain) process. Future work: We will complete the fieldwork for this project over the next 6 months. We will address the following field objectives: 1) Complete mapping of the décollement strip; 2) Log stratigraphic sections, in order to refine and codify the provisional décollement stratigraphy and confirm map unit correlations across the mapping strip; 3) Analyze mesoscopic structures at targeted localities across the mapping area, to further characterize the structural chronology within the décollement interval.
