STRATIGRAPHIC INSIGHTS FROM PERMIAN SHELL BEDS: COMPARISON OF FRANSON MEMBER DEPOSITS OF THE PERMIAN PHOSPHORIA ROCK COMPLEX, IDAHO

The Permian Phosphoria Rock Complex was deposited in a shallow epicontinental sea along the northwestern margin of Pangea. The black shales, phosphorites, and spiculites characteristic of this high-productivity system are punctuated by isolated limestone lenses (Franson Member). The Franson, which represents one of the few Permian-aged shell bed deposits globally, can provide insight into (a) modes of accumulation and preservation of skeletal concentrations during the transition from Archaic to Modern shell bed styles, and (b) biotic and paleoenvironmental changes in a system dominated by Paleozoic fauna prior to the end-Permian mass extinction. This study combines field observations, thin section analyses, and brachiopod δ13C of shell beds from two localities in Idaho spaced 80 km apart along the North-South paleoshoreline to investigate regional variability of the Franson.

In southeastern Idaho, the Franson hosts ≤ 38-m-thick packages of limestone with 10-50-cm-thick shell beds dominated by productid brachiopods and crinoids, many encased in blocky calcite cement and with borings infilled by phosphate. These densely packed and concordant accumulations represent multi-event composite concentrations. Farther north, the Franson hosts ≤ 1.6-m-thick silty micrites with 30-cm-thick shell beds dominated by brachiopods, with subsidiary bryozoan, echinoid plate, and fish fragments, many phosphatized. These loosely packed and chaotic accumulations reveal composite to hiatal concentrations.

Most characteristics of the Franson shell beds align with other Paleozoic examples. However, spatial variation in thickness, siliciclastic input, phosphatic replacement of fossils, and abundance of macrofauna between these two deposits was likely driven by proximity to a clastic source and fluctuation in nutrient cycling and associated deoxygenation. Positive shifts in brachiopod δ13C values at the base and top of the Franson at the southern locality correspond to sequence stratigraphic interpretations of subaerial exposure, which supports the interpretation of a highly productive setting briefly interrupted by a transgressive interval. Together, these Franson shell beds represent locally more normoxic oases among extensive sponge meadows and organic-rich, dysaerobic phosphorites and black shales.