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Fieldwork area

We have spent 20 weeks in the field over the last year, carrying out further fieldwork in the Potosí uplift of northeast Mexico. This area presents the best exposures of the exhumed evaporite décollement (Minas Viejas Formation) of the Sierra Madre Oriental fold belt. This fieldwork has primarily focused on completion of geologic mapping (at 1:10,000 scale) of a well-exposed, 9.75 km long outcrop strip on the west flank of the uplift. This outcrop strip exposes the full décollement stratigraphy. This mapping is now nearly complete, with only three small areas to map. On the basis of reconnaissance observations across the Potosí uplift, we believe that the structures exposed within our mapping area are representative of the uplift as a whole.

Stratigraphy of the décollement interval

The completion of most of the geologic mapping over the past year has allowed us to further refine our stratigraphy for the mapping area. The Minas Viejas Formation in the mapped part of the Potosí uplift is ~850 m thick, somewhat thinner than originally estimated. Our stratigraphy includes 11 regionally-persistent, mappable members that include five lithologically distinct carbonate packages. These carbonate intervals make up ~40% of the formation. The remaining 6 members (~60% of the formation) are composed of gypsum (commonly calcareous) with varying proportions of thin (<5 m) carbonate interbeds. Ammonites collected from one of the carbonate members may allow the biostratigraphic age of the decollement rocks to be determined. Unfortunately, our attempts to obtain a radiometric age date for a tuff sample from a lens of volcanic rock in the middle part of the décollement have been unsuccessful. The sample yielded abundant zircons, but the U-Pb dates appear to have been affected by inheritance of older zircons.

Structural geology

The Potosí uplift evaporite exposures preserve a diverse suite of well-exposed structures. The macroscopic (mappable at 1:10,100 scale) structural patterns are delineated by the five carbonate members, and by carbonate interbeds in evaporite-dominated members. The structural style over most of the mapped area is fold-dominated, with locally important boudinage and faulting of carbonate members. Fold axis trends are highly variable, although north, north-northeast and northeast-trending 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 ~120 m thick (“map-scale” boudinage). The dominant structure in evaporite lithologies is a foliation which obliterates primary sedimentary structures. Some of these foliated evaporites may have developed during folding-related flattening, while others we interpret to be mylonites developed in evaporite shear zones. The lowermost evaporite member contains a shear zone up to ~55 m thick, with spectacular banded mylonites, cataclastically deformed inclusions, 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

We believe that the décollement deformation represents two distinct kinematic processes. Evaporite shear zones likely reflect décollement-parallel simple shear associated with displacement of the rocks above the décollement relative toward the foreland. Kinematic indicators in the basal shear zone (folds, lineations) indicate top-to-the-east transport, compatible with Laramide kinematics. Field relations suggest that simple shear deformation was localized in the basal evaporite member of the décollement, perhaps as a result of the complex folding of higher evaporite members. The fold-dominated structure in the rest of the décollement section cannot be easily explained by simple shear deformation, and instead likely reflects redistribution of décollement material in response to folding of the overburden. Variable fold orientations that include folds with axes parallel to the Laramide transport direction, together with local refolding suggest that this redistribution was not a simple (i.e., plane strain) process. The stratigraphic compartmentalization of these two kinematic processes and may reflect strain partitioning during décollement deformation.

Project completion

We will complete the fieldwork for this project by the end of the year. We will address two field objectives:

1)      Log a composite stratigraphic section of the Minas Viejas Formation, in order to codify the décollement stratigraphy deduced from map relations and field observations;

2)      Analyze mesoscopic (outcrop-scale) structures at targeted localities across the mapping area, to further characterize the strain distribution within the décollement interval.

Following completion of the fieldwork, we will complete the digitization of our maps, construct cross-sections and compile, represent and analyze our field data. We envision that at least 3 publications will stem from this project. These manuscripts will form the basis of the Ph.D. dissertation of graduate student Gareth Cross, with a scheduled completion date of August, 2010.