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Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana

Published online by Cambridge University Press:  15 June 2020

Ed Landing Open the ORCID record for Ed Landing [Opens in a new window] ,
Mark D. Schmitz ,
Gerd Geyer Open the ORCID record for Gerd Geyer [Opens in a new window] ,
Robin B. Trayler  and
Samuel A. Bowring
Ed Landing*
Affiliation:
New York State Museum, 222 Madison Avenue, Albany, NY12230, USA
Mark D. Schmitz
Affiliation:
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho83725, USA
Gerd Geyer
Affiliation:
Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für Geographie und Geologie, Bayerische Julius-Maximilians-Universität Würzburg, Am Hubland, 97074Würzburg, Germany
Robin B. Trayler
Affiliation:
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho83725, USA Present address: Department of Life and Environmental Sciences, University of California-Merced, Merced, CA95343, USA
Samuel A. Bowring
Affiliation:
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA02139, USA
*
Author for correspondence: Ed Landing, Email: ed.landing@nysed.gov
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Abstract

New U–Pb radioisotopic ages on early Cambrian volcanic zircons condition a high-resolution Bayesian age model that constrains the first occurrences and zonations of West Gondwanan archaeocyaths and trilobites in southern Morocco. The oldest archaeocyaths in the Tiout Member of the Igoudine Formation (519.71 + 0.26/− 0.35 Ma) are c. 6 Ma younger than the oldest Siberian archaeocyaths. The oldest Moroccan trilobite fragments, from the lower member of the Igoudine, are constrained to 519.95 + 0.43/− 0.40 Ma. The succeeding Issendalenian Stage (i.e. Hupetina antique – Eofallotaspis tioutensis – Fallotaspis plana – Choubertella – Daguinaspis trilobite zones) spans c. 1.5 Ma (519.78 + 0.26/− 0.37 Ma to 518.43 + 0.25/− 0.69 Ma). Identifiable Moroccan fallotaspidids and bigotinids, among Earth’s oldest trilobites, occur above a positive δ13C excursion dated with our age model at 520.27 + 0.59/− 0.57 Ma, and correlated with the IV excursion peak within the lower range of Siberian Atdabanian Stage trilobites (Repinaella Zone). This excursion is the best standard for a Cambrian Series 2 base. The oldest West Gondwana trilobite fragments are c. 1 Ma younger than those in Siberia and c. 0.5 Ma older than the oldest Avalonian trilobites (Callavia Zone). This diachrony means a trilobite first appearance datum is an inappropriate chronostratigraphic base for Cambrian Series 2. Taxonomic differences in the oldest trilobites between Cambrian palaeocontinents are in accordance with trace fossil evidence for the group’s appearance possibly as late as c. 530 Ma in the Cambrian Evolutionary Radiation. Coeval 519–517 Ma dates from Avalonia (cool-water siliciclastic shelf) and West Gondwana (tropical carbonate platform) sections with distinct macrofaunas emphasize these successions were latitudinally separate by the late Ediacaran Period.

Keywords

Lower CambrianU–Pb datesMoroccooldest trilobitespalaeogeography
Type
Original Article
Information
Geological Magazine , Volume 158 , Issue 2 , February 2021 , pp. 219 - 238
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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Footnotes

†

Deceased

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