A Soiled History: The Problem with Carbonates as Correlative Strata in Permian-Triassic Redbeds of the Midcontinent

Jonathon Knapp, Kathleen Benison, and James Zambito

Late Permian - early Triassic (?) red beds and evaporites of the midcontinent, including the Spearfish, Chugwater, and Nippewalla/Blaine strata, host minor thin carbonates. Despite limited study, these carbonates have been interpreted as indicators of marine transgressions and have been used to argue for marginal marine deposition of host red beds and evaporites. The carbonates have been used to correlate between and among surface exposures and subsurface sections over a large region. In contrast, we propose an alternate interpretation of calcretes. Our detailed observations of outcrops and cores include: (1) presence of Microcodium-like pseudospherules, (2) centimeter to tens of centimeter-thick wavy discontinuous bedding; (3) close association with paleosol features such as cracks and autoclastic breccia in red beds; and (4) association with anhydrite pseudomorphs after bottom-growth gypsum crystals. Additionally, the portion of these successions represented by carbonate features is much lower than suggested in past studies. For example, carbonates in the Amoco Rebecca K. Bounds core from Greeley County, Kansas represent less than 1% of the thickness of the Nippewalla Group. Such reinterpretation is possible because of the development over recent decades of comparative sedimentology, prolific investigations of paleosols, and a better understanding of the behavior of evaporite bearing rocks at the surface. Although questions remain about the exact processes and conditions which formed the rare carbonates, the new interpretation of a pedogenic origin is consistent with evidence for continental deposition for the host red beds and evaporites. The reinterpretation of this "common" feature has implications for Pangaean environments, geography, and climate. It calls for a re-evaluation of correlations and renewed continent-scale study guided by detailed petrographic and field observations of these enigmatic mid-continent successions. 

AAPG Search and Discovery Article #90176©AAPG Mid-Continent Meeting, Wichita, Kansas, October 12-15, 2013