• Medientyp: E-Artikel
  • Titel: Late Cambrian (middle Furongian) shallow-marine dysoxic mudstone with calcrete and brachiopod–olenid–Lotagnostusfaunas in Avalonian Cape Breton Island, Nova Scotia
  • Beteiligte: LANDING, ED; WESTROP, STEPHEN R.
  • Erschienen: Cambridge University Press (CUP), 2015
  • Erschienen in: Geological Magazine
  • Umfang: 973-992
  • Sprache: Englisch
  • DOI: 10.1017/s001675681400079x
  • ISSN: 0016-7568; 1469-5081
  • Schlagwörter: Geology
  • Zusammenfassung: <jats:title>Abstract</jats:title><jats:p>The common belief that organic-rich mudstones formed in quiescent, distal settings is further weakened by study of an upper Cambrian (<jats:italic>Leptoplastus</jats:italic>– lower<jats:italic>Peltura</jats:italic>superzones) succession in the Chesley Drive Group in Avalonian Cape Breton Island that is comparable to Alum Shale successions in Baltica. Dramatic sea-level (likely eustatic) changes are now recognized by punctuation of deposition of shallow, wave-influenced black mudstone with brachiopod (<jats:italic>Orusia lenticularis</jats:italic>) and olenid trilobite-bearing limestones by offlap and formation of a subaerially cemented calcrete-clast conglomerate. Subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with<jats:italic>Leptoplastus</jats:italic>Superzone (<jats:italic>L. ovatus</jats:italic>Zone) trilobite sclerites. Dynamic processes are shown by wave ripples in the mudstone and limestone, sorting and winnowing of fossil rudstones, and pre-compactional fracture of the conglomerate and rudstones.<jats:italic>Orusia</jats:italic>rudstones in the succession below the conglomerate are regarded as analogues of<jats:italic>Eoorthis</jats:italic>and<jats:italic>Billingsella</jats:italic>rudstones in the ‘biomere’ extinction intervals of the Laurentian basal Sunwaptan. The lowest<jats:italic>Orusia</jats:italic>-rich beds are no older than the<jats:italic>P. spinulosa</jats:italic>Zone but, as elsewhere in Avalonia, they range into the higher<jats:italic>Leptoplastus</jats:italic>(Cape Breton) and even the<jats:italic>Peltura</jats:italic>(Britain, New Brunswick) superzones. Rare agnostoid sclerites in lower<jats:italic>Peltura</jats:italic>Superzone (<jats:italic>Ctenopyge tumida</jats:italic>Zone) olenid rudstone resemble those traditionally assigned to<jats:italic>Lotagnostus trisectus</jats:italic>in Avalonian Britain and Sweden, and are distinct from Laurentian<jats:italic>L. americanus</jats:italic>. An<jats:italic>L. americanus</jats:italic>Zone cannot be identified in Avalonia or Baltica, and the first appearance datum (FAD) of purported ‘<jats:italic>L. americanus</jats:italic>’ is not suitable as a standard for the base of the highest Cambrian stage.</jats:p>
  • Beschreibung: <jats:title>Abstract</jats:title><jats:p>The common belief that organic-rich mudstones formed in quiescent, distal settings is further weakened by study of an upper Cambrian (<jats:italic>Leptoplastus</jats:italic>– lower<jats:italic>Peltura</jats:italic>superzones) succession in the Chesley Drive Group in Avalonian Cape Breton Island that is comparable to Alum Shale successions in Baltica. Dramatic sea-level (likely eustatic) changes are now recognized by punctuation of deposition of shallow, wave-influenced black mudstone with brachiopod (<jats:italic>Orusia lenticularis</jats:italic>) and olenid trilobite-bearing limestones by offlap and formation of a subaerially cemented calcrete-clast conglomerate. Subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with<jats:italic>Leptoplastus</jats:italic>Superzone (<jats:italic>L. ovatus</jats:italic>Zone) trilobite sclerites. Dynamic processes are shown by wave ripples in the mudstone and limestone, sorting and winnowing of fossil rudstones, and pre-compactional fracture of the conglomerate and rudstones.<jats:italic>Orusia</jats:italic>rudstones in the succession below the conglomerate are regarded as analogues of<jats:italic>Eoorthis</jats:italic>and<jats:italic>Billingsella</jats:italic>rudstones in the ‘biomere’ extinction intervals of the Laurentian basal Sunwaptan. The lowest<jats:italic>Orusia</jats:italic>-rich beds are no older than the<jats:italic>P. spinulosa</jats:italic>Zone but, as elsewhere in Avalonia, they range into the higher<jats:italic>Leptoplastus</jats:italic>(Cape Breton) and even the<jats:italic>Peltura</jats:italic>(Britain, New Brunswick) superzones. Rare agnostoid sclerites in lower<jats:italic>Peltura</jats:italic>Superzone (<jats:italic>Ctenopyge tumida</jats:italic>Zone) olenid rudstone resemble those traditionally assigned to<jats:italic>Lotagnostus trisectus</jats:italic>in Avalonian Britain and Sweden, and are distinct from Laurentian<jats:italic>L. americanus</jats:italic>. An<jats:italic>L. americanus</jats:italic>Zone cannot be identified in Avalonia or Baltica, and the first appearance datum (FAD) of purported ‘<jats:italic>L. americanus</jats:italic>’ is not suitable as a standard for the base of the highest Cambrian stage.</jats:p>
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