END-PERMIAN KARST STRATIGRAPHY AND GEOCHEMISTRY AT THE KAIBAB-MOENKOPI CONTACT AND ITS RELATIONSHIP TO THE PERMIAN-TRIASSIC BOUNDARY FOR NORTHERN ARIZONA

The top of the Permian Kaibab Formation at Vermillion Cliffs, Arizona, is a large chert rubble zone beneath the Triassic Moenkopi Formation, which might represent an End-Permian karst event strattling the Permian-Triassic Boundary. However, apparently similar chert breccias are also present lower in the Kaibab Formation. Therefore, lithology and stable isotope geochemistry on the upper chert breccia were investigated and compared with two breccias down section to clarify if karstification was active across the Permian-Triassic Boundary.

All measured sections contain cauliflower cherts, chert nodules, gypsum, quartz, and calcite-filled vugs within a dolomitic matrix, indicative of a sabkha environment. Lower breccias are 0.5 m thick, discontinuous, gypsiferous, and contain slumped and slightly fragmented chert nodules. These are interpreted as syndepositional breccias with cyclic precipitation and dissolution of gypsum and dolomite. The upper breccia is 4-6 m thick and laterally uniform. Chert nodules are highly fractured, zoned, slumped, and silicified with lenses of dolomite and calcite-filled vugs, which is suggestive of long-term subaerial exposure of the uppermost Kaibab Formation.

Both breccia types show large variations in δ¹⁸O and δ¹³C with separate trends on a δ¹³C-δ¹⁸O cross plot. Lower breccias exhibit strong variation in δ¹⁸O (-4.6‰ to –0.3‰) but not in δ¹³C (1.5‰ to 3.5‰), suggesting that no significant amount of ¹³C-depleted organic carbon was present during dissolution and re-precipitation. The upper breccia exhibits strong covariation between δ¹⁸O (-7.4‰ to 0.7‰) and δ¹³C (-1.7‰ to 2.3‰). Data for the lower breccias are consistent with syndepositional brecciation in a sabkha environment. Data for the upper breccia indicate humid climate karstification of a highly vegetated land surface.

It seems likely that karstification began after a eustastic sea level regression during the latest Permian and continued until the Sonoma Orogeny during the Early Triassic when fluvial channels in the basal chert pebble conglomerate of the Moenkopi Formation developed as a result of tectonic uplift. If geochemical tracers relevant to the End-Permian mass extinction were deposited across the boundary, they would most likely be present in the insoluble residues of the uppermost breccia.