WEST TEXAS SILLS EMPLACED BY MAGMA INFLATION FOLDED IN POST-32.5 MA LARAMIDE FOLDS?

Relative timing relations of structures and isotopically-dated sills provide a key way of constraining regional deformation timing. This approach has proven controversial when applied to the Laramide Mariscal Mountain anticline in Big Bend National Park, West Texas. 1:12,000-scale geologic mapping and descriptive structural analysis in northern Sierra del Carmen (SDC), in eastern Big Bend National Park document sills were folded by Laramide folds. If sparse isotopic dates are applicable, local Laramide deformation continued after 32.5 Ma. SDC contains well-exposed Cretaceous carbonate and siliciclastic rocks intruded by felsic and mafic sills and 32.1 Ma McKinney Hills laccolith. A northern SDC sill was dated at 32.5 Ma (⁴⁰Ar/³⁹Ar on groundmass; Morgan and Shanks, 2008). Intrusions are part of the Trans-Pecos Magmatic province; isotopic dates range from 64 to 17 Ma (Henry and McDowell, 1986). SDC, within the easternmost Cordilleran orogen and Basin and Range province, contains two phases of Laramide folds cross-cut by high-angle Basin and Range faults and drag folds.

Data pointing to emplacement by magma inflation, not doming, include: a) sedimentary bedding is not warped or faulted adjacent to sills, b) adjacent Laramide folds remain consistent in orientation, c) contact aureoles contain lenticular boudins, and d) >579 m thickness of the Aguja Fm where intruded by felsic South Persimmon Gap sills greatly exceeds the 100 m thickness of the Aguja Fm to south (Lehman, 1989). Several lines of evidence indicate sills are pretectonic: a) one continuous, well-exposed sill displays a folded shape within a map-scale Laramide anticline and syncline, b) sills remain constant in thickness and do not thicken in hinge areas, c) sills do not reorient Laramide folds, and d) sills do not intrude along syn-folding joint sets.

The simplest sequence of events that fits current data is: 1) Sills intruded by inflation at 32.5 Ma, but did not reorient bedding, 2) Sills folded during Laramide orogeny, 3) Basin and Range faults cross-cut sills and Laramide folds. However, elsewhere in SDC unfolded 51-48 Ma Canoe Fm unconformably overlies a Laramide anticline (Lehman, 2004), raising the alternative that some northern SDC sills intruded before 48 Ma. A third hypothesis is that sills intruded after folding and mimic fold shapes extremely well.