--> Abstract: Petrographic Documentation of Reactions Affecting Detrital Feldspars in Mudrocks during Burial, Frio Formation (Oligocene), South Texas, by K. L. Milliken; #91012 (1992).

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ABSTRACT: Petrographic Documentation of Reactions Affecting Detrital Feldspars in Mudrocks during Burial, Frio Formation (Oligocene), South Texas

MILLIKEN, KITTY L., University of Texas at Austin, Austin, TX

Back-scattered electron imaging of silt-sized detrital feldspars in polished mudrock thin sections reveals diagenetic reactions very similar to ones affecting detrital feldspars in associated sandstones. By burial depths of 3.5 to 4 km nearly complete removal of the primary detrital feldspar assemblage takes place. Dissolution, in some grains accompanied by brecciation and collapse, is the main process affecting detrital feldspars in Frio mudrocks during burial. Both plagioclase and K-feldspar are also replaced by albite. Calcite and Al-rich sphene are other important replacement phases. Very minor replacement phases include pyrite, Ti-oxide, and sphalerite. Complex replacement textures document a multistage reaction history for many grains that involves dissolution, replacement by mu tiple phases, and, possibly, subsequent removal of some replacement calcite by pressure solution.

As in sandstones, most replacement textures attest to the dominance of surface-reaction controlled rates for feldspar alteration in mudrocks. Reaction rims on a few grains, however, suggest increased influence of fluid mediated transport phenomena in controlling the distribution of replacement phases in these very low permeability rocks. The dissolution/precipitation character of feldspar reactions in Frio mudrocks suggests considerable mobility for a number of elements, some quite insoluble (e.g., Al, Ti), over minimum distances of hundreds of microns.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)