169-8 Phosphatic Fossil Assemblages from the early Cambrian of northwestern Canada

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

Presenting Author:

Prescott Vayda

Authors:

Vayda, Prescott¹, Evans, Scott², Strauss, Justin³, Xiao, Shuhai⁴, Zhao, Alyssa⁵, Lau, Kim⁶, Garber, Joshua⁷, Harding, Reina⁸, Cobbett, Rosie⁹

(1) Division of Paleontology, American Museum of Natural History, New York, NY, USA; Department of Geosciences, Virginia Tech, Blacksburg, VA, USA, (2) Division of Paleontology, American Museum of Natural History, New York, NY, USA, (3) Department of Earth Sciences, Dartmouth College, Hanover, NH, USA, (4) Department of Geosciences, Virginia Tech, Blacksburg, VA, USA, (5) Department of Geosciences, Virginia Tech, Blacksburg, VA, USA, (6) Department of Geosciences, Pennsylvania State University, University Park, PA, USA, (7) School of Earth and Environmental Sciences, University of St Andrews, St Andrews, Scotland, United Kingdom, (8) Department of Earth Sciences, Dartmouth College, Hanover, NH, USA, (9) Department of Energy Mines and Resources, Yukon Geological Survey, Whitehorse, Yukon, Canada,

Abstract:

The early Cambrian is notable for the abundant preservation of invertebrate fossils closely associated with phosphatic minerals, including apatite skeletonization, cementation, and replacement. These fossils, often called small shelly fossils, are useful for biostratigraphic correlation and for the preservation of small and soft-bodied taxa that are often not otherwise preserved in the fossil record. 

Here we describe two calcium phosphate-rich, fossil bearing carbonate units from the early Cambrian Ingta and Sekwi formations in Yukon territory, Canada. These units contain abundant phosphatized fossil material as well as phosphatic cements and laminae. Fossils from the Ingta Formation include internal molds of biomineralizing taxa such as Anabarites and phosphatic replacement of originally organic remains such as the tubular Rugatotheca. Additionally, these fossils are associated with microbially laminated phosphatized layers and intraclasts with phosphatic cements. Fossils from the Sekwi Formation include phosphatic internal molds of biomineralizing taxa such as archaeocyathids, phosphatic replacement of biomineralizing taxa such as helcionellids and trilobites, and originally phosphatic biomineralizers such as linguliformean brachiopods. Notably, the originally phosphatic brachiopods show evidence of secondary silicification, which is not observed in other phosphatized fossils from the deposit. This suggests that silicification may have happened before phosphatization of other materials or that the characteristics of the brachiopod shells made them more susceptible to silicification. The Sekwi Formation also bears apatite-cemented intraclasts but lacks microbially laminated phosphatic layers. The variety of materials preserved as apatite minerals including replacement textures, molds, and cements implies multiple phases of early diagenetic phosphatization. This inference is further supported by rare earth element data and uranium/lead ages for these phosphatic materials, which suggest their complex and protracted diagenetic and phosphogenesis history. The comparison of these two units from the same basin reveals how local and regional factors may affect this distinct taphonomic mode from the early Cambrian. 

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