COMPARATIVE ANALYSIS OF PHOSPHORITE HARDGROUNDS FROM THE RECENT OF THE CALIFORNIA BORDERLANDS AND THE PERMIAN PHOSPHORIA ROCK COMPLEX OF IDAHO

This petrographic study investigates phosphatic nodules and hardgrounds from the recent of the California Borderlands (CB), offshore of San Diego, and the Permian Phosphoria Rock Complex (PRC) of Idaho to compare their sedimentology and depositional environments. Phosphatic grains form in a multitude of nutrient-rich environments and also contain data related to redox changes. This study assesses phosphorites from disparate times and settings, comparing similar deposits from a recent continental margin and ancient epeiric sea.

Macroscopically, samples from CB are biotically diverse and have distinct laminae, while those from the PRC have largely cryptic biotic features and laminae. Microscopically, both contain redox-aggraded and unconformity-bounded coated phosphatic grains, with accessory chert, glauconite, phosphatic intraclasts and peloids, and bioclasts. The CB samples are partially silicified, as are those from the uppermost PRC; the lower PRC samples are not silicified and are distinctive in their lack of glauconite. Many phosphorites from CB contain volcanic clasts and minerals, indicating a close volcanic association, while the PRC samples do not. Additionally, there is an apparent biotic paradox between the micro- and macroscales at each site. On a macroscale, CB samples contain a diverse array of bioclasts and trace fossils, but in thin section these samples have low benthic diversity and a high abundance of bioclasts, particularly large bone fragments. In contrast, the PRC samples have a paucity of bioclasts at macroscales, abundant borings, and a low abundance of diverse, primarily benthic fossils in thin section.

Phosphorites from CB and PRC are strikingly similar; both exhibit evidence of sedimentary condensation and likely similar benthic suboxic environments during phosphorite deposition. The PRC experienced a limited oxygen supply that allowed benthic organisms to survive but not proliferate, while the California margin seafloor was unsuitable for much benthic life but the overlying water was oxic enough to allow large organisms to thrive.