Constraints on the Diagenetic History of the Capitan Shelf Margin (Upper Permian) from Magnetic and Diagenetic Studies

Abstract

Assemblages of magnetic minerals in sedimentary rocks are of depositional or diagenetic origin. Magnetic, diagenetic, and outcrop studies in rocks from the Capitan shelf margin (U. Permian), West Texas and New Mexico, show that the diagenetic history of these rocks was dominated by anhydrite cementation until uplift from Tertiary times onwards and that the magnetic minerals are of diagenetic origin. Diagenetic studies, including petrographic, cathodoluminescence and electron microprobe analysis, show that the diagenetic history can be divided into three main phases: (1) Pre-burial - cementation by calcium carbonate contemporary with deposition followed by anhydrite cementation and dolomitization. (2) Burial - continued cementation and replacement of some carbonate by anhydrite; accompanied by dolomitization. (3) uplift - minor calcitization of anhydrite accompanied by kaolinite formation. The mineralogy of magnetic particles and the age of remanence acquisition was deduced from thermal and alternating field demagnetization, despite the low magnetic mineral content of most samples. Pyrrhotite acquired remanence at around 190 Ma and from around 45 Ma to recent times. The formation of the oxides, hematite and geothite, from the oxidation of preexisting sulfides and by authigenic processes accompanied the dissolution of anhydrite and the precipitation of the calcite spar cements. These results shedmore » new light on the diagenetic history of the Capitan shelf margin and illustrate the value of magnetic studies of sedimentary rocks when magnetic data are integrated with accompanying diagenetic studies.« less