70-15 Environmental stress and faunal turnover of brachiopods and bivalves in the Permian Phosphoria Rock Complex, Idaho

Session: Paleontology, Biogeography/Biostratigraphy & Phylogenetic/Morphological Patterns (Posters)

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Abstract:

The Meade Peak (MP) Member of the Permian Phosphoria Rock Complex (PRC) hosts economically important phosphate deposits in Idaho. It records a range of nutrient-rich and low-diversity paleoenvironments, and age dating suggests that the upper MP may coincide with the Guadalupian-Lopingian boundary (GLB) biotic crisis. The MP is primarily composed of phosphatic mudstones and phosphorites, with rare carbonate and chert beds. These facies host a variety of macrofossils, dominated by brachiopods and bivalves, with subsidiary fish, gastropods, and ammonites. This study uses recently collected hand samples and previously published fossil occurrence data from over 40 localities to investigate spatiotemporal trends in MP fossils and to assess potential drivers of paleoecological change.

A key pattern at most localities in the basin depocenter is that brachiopods become rare or absent near the top of the MP, while the pectinid bivalve Streblochondria becomes abundant. This trend typically commences with an abrupt takeover by Streblochondria and other facultatively mobile macrofauna, followed by their gradual decline and the reappearance of many stationary brachiopods and infaunal bivalves above the MP.

Potential drivers of these faunal changes include (1) global extinction across the GLB, (2) facies changes, (3) taphonomic bias, or (4) local or basinwide environmental stresses independent of global drivers. This research evaluates the likelihood of these hypotheses. First, most MP brachiopod species did not go extinct during the Permian, but may have been extirpated from the basin. Second, Streblochondria and most brachiopods are found throughout many MP facies, indicating a low likelihood of facies-dependent fossils. Third, almost all of these fossils are preserved as molds, suggesting minimal impact of taphonomic biases.

These data suggest that the temporary faunal turnover near the top of the MP was driven by environmental stress such as anoxia, acidification, and/or higher temperatures, either locally or throughout the entire Phosphoria Basin, possibly caused by the same volcanic eruptions that led to the GLB biotic crisis. Further assessment of spatial data reveals that these trends are only observed in the MP depocenter of southeastern Idaho and northern Utah, supporting the hypothesis of local environmental stress. While brachiopods are typically resilient to stress, their sessile habits may have allowed facultatively mobile bivalves to thrive during periods of higher stress, potentially due to low oxygen or high temperatures. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9895

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