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Hasiotis, Stephen T.,
A Brief Overview of the Diversity and Patterns in Bioturbation Preserved in the Cambrian–Ordovician Carbonate and Siliciclastic Deposits of Laurentia
Stephen T. Hasiotis1
1Department of Geology, University of Kansas, Lawrence, Kansas, U.S.A.
I thank Jim Derby for providing me the opportunity to assist him and other editors in the completion of this AAPG memoir honoring James Lee Wilson with this overview chapter on the Cambrian–Ordovician ichnology. This review was only possible because of the extensive research by our colleagues, past and present, who have dedicated their lives to the study of the uppermost Neoproterozoic to the lowermost Paleozoic ichnology and its implications to this important and unique interval in Earth history. I also thank Mike Pope and Kelly Dilliard for discussions on the Lower Cambrian stratigraphy and ichnological patterns in the Lower Cambrian Sekwi Formation in the Mackenzie Mountains, Northwest Territories, Canada. I also thank Bruce Lieberman for providing me the opportunity to work on the Lower Cambrian rocks and, hence, their ichnology, while collecting trilobites in the Mackenzie Mountains; B. Lieberman was funded by NSF-EAR 0106885. I thank John-Paul Zonneveld and Susan Longacre for their insightful reviews that greatly improved the manuscript. I am especially grateful to SEPM and Ron Blakey at Northern Arizona University for permission to use their figures in this chapter.
The diversity, abundance, distribution, and depth of trace fossils in the Cambrian–Ordovician deposits in Laurentia, from California and Nevada to New York (United States) and Quebec (Canada), are a series of biozones that record the early evolution and radiation of metazoans in shallow-marine environments. The Neoproterozoic–Paleozoic transition (NPT) plays a significant part in understanding the diversity, timing, rate, circumstance of first appearances and subsequent metazoan radiations, and trends in ecospace utilization through the Cambrian–Ordovician as recorded in carbonate and mixed carbonate-siliciclastic deposits. The first burrows with spreiten and complex branching, designated as the Phycodes (Treptichnus) pedum Zone, delineate the base of the Cambrian. This zone overlies the uppermost Neoproterozoic Harlaniella podolica Zone and is composed of relatively simple horizontal burrows. The Rusophycus avalonensis Zone, characterized by the occurrence of more complex burrow architectures, overlies the Phycodes (Treptichnus) pedum Zone and represents the last pretrilobite biozone. As recorded by ichnofabric through the Cambrian–Ordovician, trends in the depth and extent of bioturbation illustrate the spatial and temporal change in ecospace utilization. With the onset of the substrate (media) revolution across the NPT, animals adapted, and evolved new innovations to penetrate microbial-mediated sedimentary environments. This change reflects ongoing Cambrian–Ordovician evolution and radiation of metazoans from shallow inner-shelf environments to middle-shelf environments with increasing biogenic reworking through time. Nonetheless, a mixing depth of 6 cm (2.4 in.) was not surpassed until later in the Ordovician. This pattern is mirrored by the first appearances of trace-fossil ichnotaxa in shallow-water environments that later gradually moved offshore to shelf environments. The ichnological patterns are debated, however, as evidence of deep burrowing (i.e., 6 cm [2.4 in.]) has been described from the Cambrian and the Ordovician deposits in the Mackenzie Mountains (western Canada) and the Great Basin (western United States). Evidence for the early evolution of continental ecosystems does not exist in Laurentian deposits until the Late Ordovician, although some evidence for the invasion of land in the Early Cambrian and the Early Ordovician exists.
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