139 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 304B

Understanding Stratigraphic Controls on Fossil Preservation in the Middle Cambrian of Montana

Paper number: 139-2
Program: Technical Sessions
Time: 1:50 PM – 2:05 PM

Authors

Abstract

The middle Cambrian of Montana is represented by the Flathead Sandstone, Wolsey Shale, and the Meagher Limestone; this succession spans the Wuliuan (509-504.5 Ma) and contains the Albertella, Glossopleura, and Ehmaniella Laurentian trilobite biozones. These units contain diverse depositional environments ranging from the intertidal to shelf settings below storm wave base. While these units have a long history of geologic study, the paleontology of these units has been largely understudied. These formations exhibit variable fossil preservation, ranging from locations lacking fossil material to others with soft-bodied preservation. Here, we provide the first steps in understanding the controls of fossil preservation through field-based observations, focusing on how fossils are hosted within their stratigraphic context. Multiple stratigraphic sections from across southwestern and central Montana provide a new detailed stratigraphic and sedimentological context for where these fossils are being found and what environmental conditions may be controlling their preservation. Most well-preserved fossils are found at the interface between the Wolsey Shale and Meagher Limestone, in carbonate-rich shales between limestone beds, with some additional mineralized material from the lower part of the Wolsey Shale, in areas with limited visible bioturbation or within laminated sandstone beds. Using this information, we can gain a better understanding of the role the physical environment plays in controlling the occurrence and quality of fossil preservation during this important period of Earth’s history. These sections also provide a framework for future mineralogical and geochemical analyses directed at understanding how paleoenvironmental conditions changed during deposition and correlate with fossil preservation.   

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

doi: 10.1130/abs/2025AM-414759

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