Upper Pennsylvanian-Permian (Naco Group) paleosols (north-central Arizona): field and isotopic evidence

Abstract

Deposits of the upper Naco Group (Upper Pennsylvanian-Permian), north-central Arizona, include features consistent with multiple episodes of soil formation. The preserved paleosols are developed on shallowing-upward (regressive) carbonate cycles that accumulated on the margins of a shallow-shelf platform. Field evidence of paleosols consist of: (a) monopodial (taproot-like) and branching root and rootlet trace fossils, (b) cylindrical carbonate nodules (rhizoliths), (c) subspherical carbonate nodules that occur as discrete entities in the paleosol, and (d) an accumulation of red clay and quartz silt. Petrographic evidence consists of: (a) plasmic fabrics analogous to those found in some modern soils (cf., Brewer, 1976), (b) microscopic carbonate nodule accumulations, (c) soil nodule spar (crystallaria), and (d) alveolar-septal structures. Paleosol profiles occur as: (a) lithified, in situ, partly eroded clay-rich horizons that commonly grade into unmodified parent material at depth below the disconformity, and (b) reworked pedogenic-carbonate nodule horizons. Oxygen and carbon stable isotope ratios of pedogenic-carbonate are depleted relative to local marine carbonate. This suggests that isotopically light soil CO 2 and meteoric water were present during pedogenic-carbonate precipitation. The depleted stable isotope ratios are consistent with an interpretation of soil carbonate precipitated under near-surface conditions. The presence of well-developed calcareous rhizoliths and soil nodules in a red bed matrix, and desiccation-like crystallaria in host rhizoliths may qualitatively signify seasonally dry paleoclimatic conditions. Alteration of the underlying limestone units and development of multiple (albeit truncated) paleosols, record a cyclical shift between marine and terrestrial paleoenvironmental conditions near the Pennsylvanian-Permian boundary (∼280 m.y. ago).