Skeletal concentrations in upwelling records: greater sensitivity to hiatal duration than to paleo‑oxygenation in the Permian Phosphoria Rock Complex

The Permian Phosphoria Rock Complex (PRC) of Idaho, which is dominated by dark shales, commercial phosphorites, and cherts signifying exceptional productivity, includes a wide array of skeletal concentrations. These range from single-event beds in oxic, oligotrophic settings, dominated by shelly macrobenthos, to taphonomically and diagenetically more complex concentrations of shells, teeth, and other remains that bear evidence of repeated reworking, colonization, and post-mortem modification (hiatal accumulations) under periodically dysoxic and eutrophic conditions. Concentrations can be divided into seven taphonomic types that vary with host lithology (facies) but most strongly with the duration of associated stratigraphic hiatuses, which were inferred independently from discontinuity surfaces. Surfaces include bedding planes (single-event shell bed Types 1–2), bed-set boundaries (multi-event shell bed Types 3–4), flooding surfaces of parasequences (∼250–500 ky spacing) and, more rarely, laterally more extensive 3rd-order surfaces of maximum flooding or starvation (hiatal shell bed Types 5–7). The correlation of taphonomic complexity with hiatal duration suggests that many PRC skeletal concentrations are highly time-averaged, beyond that likely for assemblages from intervening depositional increments, which reflect the nutrient, oxygen, and energy conditions of a single habitat. During episodes of low or zero sediment accumulation, the final assemblage can (and here does) reflect the broader spectrum of skeletal input, taphonomic conditions, and early diagenesis that can arise locally (environmental condensation). The interplay of ancient nutrient regimes on oxygenation and their cascading effects on benthic communities were, somewhat surprisingly, secondary factors.