AN ANOMALOUS PHOSPHATIC SHELL BED: THE PAUCISPECIFIC AND WIDESPREAD 'FISH SCALE MARKER BED' FROM THE PERMIAN PHOSPHORIA ROCK COMPLEX, IDAHO

The ‘Fish Scale Marker Bed’ (FSMB) is a unique shell bed that accumulated within the Permian Phosphoria Rock Complex (PRC) in Idaho. It is a laterally extensive, thin phosphorite bed at the base of the economically important Meade Peak Member containing high concentrations of orbiculoid brachiopod shells. Due to its mineralogical and faunal composition and preservational style, the FSMB is anomalous globally and provides a condensed record of life during a transformative time in Earth’s history.

The FSMB at six localities ranging from nearshore to offshore settings is highly phosphatic with minimal siliciclastic sediment, and is entirely dominated by disarticulated and fragmented orbiculoids (median diameter of 2.26 mm). However, variation in fauna, bioclast sizes, and bedding is evident at the microscale. Nearshore FSMB localities are the thickest (10-14 cm) and also host fish bone, conodont elements, foraminifera, rare algae, and microborings. Shell fragment size is variable, though nearshore settings contain the largest bioclasts, which tend to densen and coarsen upwards. Central localities are of moderate thickness (6-8 cm) and also host fish bone, conodont elements, or abundant algae; close-packing and bioclast size trends are variable. The most offshore locality has less abundant fossils, with only loosely packed fish bone and orbiculoids, and is an outlier both lithologically (dolomitic) and as the thinnest FSMB (3 cm).

The high concentration of shell fragments relative to low siliciclastic sediment dilution indicates that the FSMB was deposited in a sediment-starved environment, likely during transgression. The pervasive shell fragmentation suggests both biogenic reworking and moderate energy at the seafloor. The phosphatic composition and abundant fossils strongly suggest high levels of primary productivity driven by coastal upwelling. The interplay of the above factors created “Goldilocks” shell bed conditions, allowing for the FSMB’s formation. Variation across localities is the likely result of spatiotemporal shifts in the intensity of these paleoenvironmental conditions in the Phosphoria Sea during a protracted interval of deposition.