176-8 Food-rich and Sediment-starved: Fossil fates in condensed shell beds

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record

Presenting Author:

Author:

Abstract:

Shell beds are some of the most data-rich archives of paleoenvironments, and microstratigraphic analyses are key to deciphering small-scale taphonomic and sedimentologic processes that inform large-scale patterns. Within skeletal concentrations, high amounts of shelliness may reflect either abundant populations of shell producers or a lack of sediment. Additionally, skeletal concentrations deposited in nutrient-rich settings may provide a window into shell beds governed by exceptional biological productivity. Within such deposits, dilution by siliciclastic sediments can be highly variable, with examples from the Permian Phosphoria Rock Complex (PRC) of the western USA reflecting low dilution and the Jurassic–Cretaceous of the Morondava Basin, Madagascar reflecting much higher dilution rates. 

Both deposits host evidence of elevated nutrient availability and primary productivity, variably including abundant authigenic phosphate and glauconite, biogenic silica and phosphate, preserved organic matter, significant bioerosion, and generally low diversity fauna. The authigenic minerals not only precipitate as grains and cements, but also can coat, infill, or replace fossils. Euendoliths also preserve a range of early-diagenetic conditions. Skeletal concentrations associated with discontinuities and condensed beds are a common feature. While the PRC is generally interpreted as a paleo-upwelling system and serves as an excellent model for high-productivity and sediment-starved shell beds, Morondava remains understudied but reflects shell bed development and preservation in nearshore settings with abundant fluvial fluxes of both nutrients and siliciclastic sediments. Though the large-scale characteristics of these two deposits are distinct, at the micro-scale they share numerous authigenic and diagenetic features that suggest similar components to their taphonomic histories in nutrient-rich marine settings. Additional proxies such as trace metal and REE concentrations, stable isotope records, and microfossils are also being employed to elucidate pre- and post-mortem seawater and environmental conditions. This ongoing research emphasizes the importance of multi-scale and multi-proxy approaches to assessing skeletal concentrations, and consideration of a wider range of potential host lithologies than only typical upwelling facies when selecting and evaluating shell beds from high-productivity settings.

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

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.