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

Abstract

The common belief that organic-rich mudstones formed in quiescent, distal settings is further weakened by study of an upper Cambrian (Leptoplastus – lower Peltura 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 (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 with Leptoplastus Superzone (L. ovatus 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. Orusia rudstones in the succession below the conglomerate are regarded as analogues of Eoorthis and Billingsella rudstones in the ‘biomere’ extinction intervals of the Laurentian basal Sunwaptan. The lowest Orusia-rich beds are no older than the P. spinulosa Zone but, as elsewhere in Avalonia, they range into the higher Leptoplastus (Cape Breton) and even the Peltura (Britain, New Brunswick) superzones. Rare agnostoid sclerites in lower Peltura Superzone (Ctenopyge tumida Zone) olenid rudstone resemble those traditionally assigned to Lotagnostus trisectus in Avalonian Britain and Sweden, and are distinct from Laurentian L. americanus. An L. americanus Zone 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.

The agnostoid arthropod Lotagnostus Whitehouse, 1936 (late Cambrian; Furongian) from Avalonian Cape Breton Island (Nova Scotia, Canada) and its significance for international correlation

“…MaS is located 0.5 km west of the mouth of Macintosh Brook (not MacDonald Brook as stated by Landing & Westrop, 2015, caption to fig. 2) and must be the same locality that was mentioned by Hutchinson (1952, p. 49) as being one-third of a mile (0.53 km) west of the mouth of the brook.…”

“…Section MaS (Fig. 1c; see Landing & Westrop, 2015, fig. 3 for a stratigraphic column with ranges of trilobites and agnostoids) comprises dark-grey to black siliciclastic mudstone with carbonate interbeds of the Chelsey Drive Group (see Landing & Westrop, 2015, p. 976 for discussion of the stratigraphic nomenclature), and yields Lotagnostus salteri sp.…”

Improved Geochronologic Accuracy and Precision for the ICS Chronostratigraphic Charts: Examples from the late Cambrian–Early Ordovician

“…Lotagnostus species taxonomy in Westrop et al, 2011;Tortello, 2014;Landing and Westrop, 2015). An alternative horizon was proposed as a somewhat a higher level for the base of Stage 10.…”

“…The succeeding subzone, that of Neoparabolina lobata, occurs some three meters above the dated ash (Howells and Smith 1997, p. 14) The Ogof Ddu ash lies within the upper part of the local range of specimens identified as Lotagnostus trisectus, which was originally described from Avalonia in the Malvern area of England (see Howells and Smith, 1997;Rushton, 2009). Lotagnostus trisectus has been regarded as a junior synonym of the Laurentian (Quebec) species L. americanus by Peng and Babcock (2005), although it is regarded as taxonomically distinct from L. americanus by Westrop et al, (2011), Tortello (2014), and Landing and Westrop (2015). Westrop et al (2011) limited Lotagnostus americanus to its topotype collection, which came from a limestone boulder in an Early Ordovician debris flow that accumulated on the Laurentian margin in southern Quebec.…”

“…Westrop et al (2011;also Landing and Westrop, 2015) noted that the species' restricted morphologic variability featured a low pygidial axis and subdued scrobiculation. Westrop et al (2011) also restricted L. trisectus to the diagenetically flattened, English topotype material, and questioned the appearance of L. trisectus in Baltica and in North American Avalonia (also Landing and Westrop, 2015).…”