T191 Stratigraphic Paleobiology: Working with the Nature of the Stratigraphic Record to Address Paleobiological Questions

Tuesday, 11 October 2022
1:30 PM - 5:30 PM
Mile High Ballroom 4B

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

Paper number: T191-10
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381333

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.