Paragenesis of Upper Proterozoic Coated Grains, Kwagunt Formation, Chuar Group, Arizona

Bonnie Bloeser

Silicified pisoid layers of the Walcott Member (approximately 850 Ma), uppermost Kwagunt Formation, locally crop out as laterally continuous, 15-60 cm, resistant ledges on the flanks of Nankoweap Butte, eastern Grand Canyon, Arizona. Three distinct areas of structureless cherty beds occur in a laminated cryptalgal limestone matrix. Pisoid replacement textures and compositions of cements, allochems, micrites, fracture fillings, and oomolds, record their progressive diagenesis from calcite to dolomite and iron-carbonate phases, ultimately to silica premineralization. Distinct mineralogic and chemical signatures, determined petrographically and from electron microprobe analyses, are recorded as goethite-hematite zoned dolomite rhombs, ferroan ankerite, siderite, mixed dolomi e-quartz, chalcedony, and euhedral quartz replacement phases. Filamentous endolithic(?) cyanobacteria, preserved in the outer pisoid margins, have been replaced by pyrite and goethite after pyrite in a distinct trichome cell-by-cell manner. Fracture-filling amorphous organic matter, which probably became mobilized during metasomatism, was ultimately replaced by goethite. These nondeformed spherical to oblate ellipsoidal solid pisoids range from 1.0 to 4.5 mm in diameter (x = 2.8 mm; N = 400), are well-packed, and possess a concentric (nonradial) fabric of alternating bands of light and dark laminae. Exfoliation, contact pressure solution, and geopetal features are easily distinguished in thin section. Exfoliation and/or spalling of the outermost pisoid rim, collapse, and compaction of in er cortical kerogen and clay inclusions has resulted in geopetal structures. Contact pressure solution, due to stress localization at pisoid boundaries, has led to outer margin localized dissolution. "Stylolitic" structures at point contact interfaces are composed primarily of detrital clays, organic matter, and iron-bearing carbonates with minor amounts of pyrite. These Proterozoic pisoids have a complex diagenetic history of carbonate and silica dissolution, replacement, and reprecipitation.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.