Abstract

Samples of the Paradox Formation from four localities in the canyon of the San Juan River (Honaker Trail, Raplee Anticline, Soda Basin, and Eight Foot Rapids) were examined to establish the diagenetic history of the upper Ismay unit. The paragenetic sequence includes: meteoric dissolution, precipitation of calcite spar and syntaxial overgrowths in carbonates, precipitation of calcite cements in sandstones, silica mobilization, pressure solution, and a late phase of dissolution. In addition, cores from the Aneth field, 25 mi (40 km) to the east of the outcrop localities, were examined. The diagenetic history is similar, although barite and flourite are also present. Porosity values average less than 1 percent. In the most porous zones in outcrop, porosity values range from 5-18 percent. Highest porosity values occur in the eastern, most basin ward, outcrops.

Meteoric, non-luminescent calcite spar, the dominant porosity inhibitor, has δ ¹⁸O values of-6.2 to −1.5 ‰ PDB and δ ¹³C values of −3.1 to 3.4 ‰ PDB (± 0.1‰). Electron microprobe analysis and staining of syntaxial calcite overgrowths on echinoderms show increases in ferroan calcite concentration as growth of the crystals proceeded (up to 0.13 wt percent FeCO₃ on the margins of crystals). Staining of blocky, pore-filling calcite spar also reveals elevated iron levels.

Siliciclastic material, incorporated within the carbonate packstones and in the litharenite units, increases to the west. In the siliciclastic units, precipitation of calcite cement occurred after point contact cementation of quartz grains and compaction of pores, which probably resulted from feldspar dissolution. Visual estimation of overburden by compaction of the grains and comparison with the burial history curve for the basin suggests that sandstone diagenesis began in the Middle to Late Permian.

Evaporites have not been previously recognized in the upper Ismay on the shelf. Quartzine (length-slow, fibrous quartz), indicative of silica-replaced evaporites, is found in in-filled allochem molds in nodules and in continuous beds capping shoaling upward mixed siliciclastic and carbonate sequences.