期刊
CANADIAN JOURNAL OF EARTH SCIENCES
卷 60, 期 7, 页码 897-911出版社
CANADIAN SCIENCE PUBLISHING
DOI: 10.1139/cjes-2022-0060
关键词
trace fossils; bioturbation; ichnodiversity; Ediacaran-Cambrian; outcrop bias
This study presents new ichnologic and sedimentologic data sets from four localities and evaluates the outcrop quality. It suggests that the base of the Cambrian can be determined by the first appearance of Treptichnus pedum or vertical trace fossils. Fortune Head, Grand Bank Head, and Lewin's Cove exhibit increases in ichnodiversity and bioturbation intensities, while Point May stands out with low ichnodiversity and inconsistent data on bioturbation intensities.
In 1992, the Chapel Island Formation at Fortune Head was selected as the Cambrian GSSP, which was placed at the first appearance of the ichnotaxon Treptichnus pedum. Although the transition from the Ediacaran to the Cambrian was well studied in Fortune Head and Grand Bank Head, it is also exposed at Lewin's Cove and Point May. Here, we report new ichnologic and sedimentologic data sets from the four localities. Ichnologic data sets consist of bioturbation intensities, burrow width and depth, ichnotaxonomic composition, and stratigraphic appearances. Sedimentologic data sets include sediment grain size, bed thickness and architecture, sandstone/mudstone ratio, and physical sedimentary structures. In addition, the quality of each outcrop is evaluated (i.e., accessibility, lateral and vertical continuity, stratigraphic completeness, and type of exposure). At the four localities, the base of the Cambrian can be placed at the first appearance of Treptichnus pedum or of vertical trace fossils of the T. pedum Ichno-Assemblage Zone. In addition, Fortune Head, Grand Bank Head, and Lewin's Cove display increases in ichnodiversity and bioturbation intensities that are concomitant with the first appearance of penetrative trace fossils. Point May stands out as a section with low ichnodiversity and an inconsistent data set on bioturbation intensities. Its limited data set is a consequence of poor bed surface exposures, which impact negatively on the preservation and visibility of horizontal trace fossils. This study demonstrates that evaluation of outcrop quality should be an essential procedure to any research addressing evolutionary trends that took place at this critical time of the history of life.
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