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Tremadocian (Lower Ordovician) sea-level changes and biotas on the Avalon microcontinent

Ed Landing and Richard A. Fortey
Tremadocian (Lower Ordovician) sea-level changes and biotas on the Avalon microcontinent
Journal of Paleontology (July 2011) 85 (4): 678-694


The Chesley Drive Group, an Upper Cambrian-Lower Ordovician mudstone-dominated unit, is part of the Ediacaran-Ordovician cover sequence on the North American part of the Avalon microcontinent. The upper Chesley Drive Group on McLeod Brook, Cape Breton Island (previously "McLeod Brook Formation"), has two lithofacies-specific Tremadocian biotas. An older low-diversity benthic assemblage (shallow burrowers, Bathysiphon, phosphatic brachiopods, asaphid trilobites) is in lower upper Tremadocian green-gray mudstone. This wave-influenced, slightly dysoxic facies has Bathysiphon-brachiopod shell lags in ripple troughs. The upper fauna (ca. 483+ or -1 Ma) is in dysoxic-anoxic (d-a), unburrowed, dark gray-black, upper upper (but not uppermost) Tremadocian mudstone with a "mass kill" of the olenid Peltocare rotundifrons (Matthew)--a provincial trilobite in Avalonian North America that likely tolerated low oxygen bottom waters. Scandodus avalonensis Landing n. sp. and Lagenochitina aff. conifundus (Poumot), probable nektic elements and the first upper Tremadocian conodont and chitinozoan reported from Avalon, occur in diagenetic calcareous nodules in the dark gray-black mudstone. An upper Tremadocian transition from lower greenish to upper black mudstone is not exposed on McLeod Brook, but is comparable to a coeval green-black mudstone transition in Avalonian England. The successions suggest that late late Tremadocian (probable Baltic Hunnebergian Age) sea level was higher in Avalon than is suggested from successions on other paleocontinents. The Tremadocian sea-level history of Avalon was a shoaling-deepening-shoaling sequence from d-a black mudstone (lower Tremadocian), to dysoxic green mudstone (lower upper Tremadocian), and back to black mudstone (upper upper Tremadocian). Scandodus Lindstrom is emended, with the early species S. avalonensis Landing n. sp. assigned to the emended Family Protopanderodontidae. Triangulodus Van Wamel is considered a junior synonym of Scandodus. Peltocare rotundifrons is emended on the basis of complete specimens.

ISSN: 0022-3360
EISSN: 1937-2337
Serial Title: Journal of Paleontology
Serial Volume: 85
Serial Issue: 4
Title: Tremadocian (Lower Ordovician) sea-level changes and biotas on the Avalon microcontinent
Affiliation: New York State Museum, Albany, NY, United States
Pages: 678-694
Published: 201107
Text Language: English
Publisher: Paleontological Society, Lawrence, KS, United States
References: 114
Accession Number: 2011-066808
Categories: StratigraphyGeneral paleontology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: NSF Grant EAR-8805774
Illustration Description: illus. incl. strat. cols., 2 tables, geol. sketch map
N45°30'00" - N47°00'00", W61°30'00" - W59°30'00"
Secondary Affiliation: Natural History Museum, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, The Paleontological Society. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201137
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