ABSTRACT: Pseudomatrix and its Precursors in Sands and Sandstones

Suzanne Kairo, Lisa Rhoades, Abhijit Basu

Pseudomatrix is the finely crystalline, phyllosilicate-rich, ductile mass that occupies the volume (may be oversized) between more competent framework grains. Pseudomatrix is commonly believed to be a diagenetic alteration product of labile rock fragments. Criteria for recognizing pseudomatrix in a thin section under an optical microscope include flame-like wisps of semihomogeneous altered and crushed lithic fragments, which extend into orifices between other grains and which may show an internal fabric of pseudoplastic flow conforming to the margins of confining grains as drape lines. Some pseudomatrix and detrital matrix may be indistinguishable from each other in extremely compacted and diagenetically altered sandstones. Whereas written descriptions and photomicrograph of typical pseudomatrix are usually published, no standard visual guide is available for recognizing pseudomatrix in thin sections.

We present a series of photomicrographs of altered crystalline rocks and rock fragments from (1) rock outcrops, (2) soils, (3) stream sands, and (4) sandstones demonstrating different degrees of diagenetic alteration. These photomicrographs not only depict the range of variations of pseudomatrix, but also aid in standardizing the visual criteria for its recognition. In our work we find that semihomogeneous, coherent, ductile, sand-size masses of altered minerals and rocks are produced in source areas, some of which are transported by streams and deposited as precursors of pseudomatrix in a sand body. We show that (1) not all pseudomatrix is diagenetic in origin, and (2) not only lithic fragments but monomineralic grains as well may produce pseudomatrix.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990