--> Abstract: Constraints on the Diagenetic History of the Capitan Shelf Margin (Upper Permian) from Magnetic and Diagenetic Studies, by G. Darke; #91004 (1991)
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Constraints on the Diagenetic History of the Capitan Shelf Margin (Upper Permian) from Magnetic and Diagenetic Studies

DARKE, GILLIAN, Shell Gapsa, Buenos Aires, Argentina

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 Previous HitanhydriteNext Hit 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 Previous HitanhydriteNext Hit cementation and dolomitization. These processes prevented much compaction in the back reef, reef, and upper foreslope facies. (2) Burial--continued cementation and replacement of some carbonate by Previous HitanhydriteNext Hit; accompanied by dolomitization, the development of dolomite overgrowth cements, and minor kaolinite formation. (3) Uplift--minor calcitization of Previous HitanhydriteNext Hit accompanied by kaolinite formation; followed by dissolution of Previous HitanhydriteNext Hit, brecciation and fracturing, and the precipitation of calcite spar cements.

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 Previous HitanhydriteTop and the precipitation of the calcite spar cements.

These results shed 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.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)