Petrographic and geochemical characterization of the basal portion of the Capitan Formation, Patterson Hills, Guadalupe Mountains National Park, West Texas, U.S.A.
The following research project focuses on a dolomitic layer that forms the basal part of the Capitan Formation as exposed in the Patterson Hills area of the Guadalupe Mountains National Park, west Texas, USA. The purpose of this study is to better resolve and describe the dolomitic layer that forms the basal part of the Capitan Formation and which has been identified by Hughes (2011) as the marker bed between the Capitan reef complex and underlying basinal deposits of the Delaware Mountain Group (specifically the Rader Limestone Member) in the field and laboratory. Hughes (2011) does not provide any detailed characterization of this unit leaving a gap in knowledge regarding its formation, petrography, and geochemistry in the Back Ridge of the Patterson Hills. Field work included field description of textural types and collection 14 representative samples of the dolomite unit (approximately fist-sized or smaller), where sampling was possible. Hand samples were slabbed, thin-sectioned and stained for dolomite. Examination of the samples under a scanning electron microscope (SEM) was conducted for mineral identification through elemental mapping. Cathodoluminescence (CL) microscopy studies were conducted in order to observe and interpret diagenetic phases. Bulk samples were analyzed with handheld X-ray fluorescence in order to determine stoichiometry. The petrographic and geochemical characterization of the dolomitic contact of the Capitan Formation and Rader Limestone reveals that the contact is composed of mainly two types of sediment; silisiclastic sandstone and dolomite. The siliciclastic sandstones of the contact may be channel infills or sandstone beds of the Bell Canyon Formation (King, 1948; Reeckmann, 1985) that interfinger with the Rader Limestone above. The carbonate dominated samples of the contact generally consist of two types of dolomites. Dolomite A appears as fabric-preserving and the associated crystal size depends on the precursor limestone grain size. Dolomite B, on the other hand, is fabric-destructive and replaces both limestone and Dolomite A. The contact between the Capitan Formation and Rader Limestone preserves a number of diagenetic environments including shallow marine diagenesis, marine-burial diagenesis, deeper burial diagenesis, and uplift related diagenesis. Shallow-marine diagenesis of the contact includes isopachous fibrous and bladed marine cement formations. Fabrics related to marine-burial diagenesis include moldic porosity, fine calcite spar cementation, silicification, aragonite neomorphism, first generation of dolomitization, and evaporite cementation. Next, deeper burial diagenesis related textures include recrystallization of the early fabric-preserving dolomite. Finally, uplift-related diagenesis is marked by dissolution of evaporite cements. Tertiary uplift allowed meteoric water influx into the evaporite-cemented contact leading to kaolinization of feldspars, complete dissolution of evaporite cements, and precipitation of coarse blocky spars II and III.