Late Cambrian (middle Furongian) shallow-marine dysoxic mudstone with calcrete and brachiopod–olenid–Lotagnostusfaunas in Avalonian Cape Breton Island, Nova Scotia

Abstract

AbstractThe common belief that organic-rich mudstones formed in quiescent, distal settings is further weakened by study of an upper Cambrian (Leptoplastus– lowerPelturasuperzones) 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 (Orusia lenticularis) 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 withLeptoplastusSuperzone (L. ovatusZone) 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.Orusiarudstones in the succession below the conglomerate are regarded as analogues ofEoorthisandBillingsellarudstones in the ‘biomere’ extinction intervals of the Laurentian basal Sunwaptan. The lowestOrusia-rich beds are no older than theP. spinulosaZone but, as elsewhere in Avalonia, they range into the higherLeptoplastus(Cape Breton) and even thePeltura(Britain, New Brunswick) superzones. Rare agnostoid sclerites in lowerPelturaSuperzone (Ctenopyge tumidaZone) olenid rudstone resemble those traditionally assigned toLotagnostus trisectusin Avalonian Britain and Sweden, and are distinct from LaurentianL. americanus. AnL. americanusZone cannot be identified in Avalonia or Baltica, and the first appearance datum (FAD) of purported ‘L. americanus’ is not suitable as a standard for the base of the highest Cambrian stage.