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Trilobite-Based Recognition of the Early—Middle

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Trilobite-Based Recognition of the Early—Middle TRILOBITE-BASED RECOGNITION OF THE EARLY—MIDDLE CAMBRIAN (DYERAN—DELAMARAN) BOUNDARY IN THE LABRADOR GROUP, WESTERN NEWFOUNDLAND Douglas Boyce and Ian Knight Paleontological and lithological studies of mixed siliciclastic—carbonate, predominantly shallow-water successions of the Forteau and Hawke Bay formations (Labrador Group), ongoing since 1976 (34 measured sections; 434 fossil collections) have recently been reassessed. Until now, the succession has been broadly known to include late Early Cambrian to Middle Cambrian strata, but identification of the late Early Cambrian Dyeran Stage—early Middle Cambrian Delamaran Stage boundary has been elusive. Recent examination of data collected from measured sections exposed on the shores of Weymouth Cove and Bridge Cove, Chimney Arm, Canada Bay has highlighted a diverse fauna of previously unidentified trilobites, and indicates that the boundary is present in a thinly bedded, deeper water, fossiliferous succession of shale, siltstone, sandstone and lime mudstone to packstone. The condensed succession lies close to the base of a unit of thinly bedded quartzose sandstone and shale that was previously placed in the lower half of the Hawke Bay Formation in the area by Knight and Boyce (1987). This seems to concur well with data collected near Hawkes Bay and in Gros Morne National Park, where trilobites clearly indicate that the base of the Hawke Bay Formation must lie in the latest Early Cambrian. However, metre-scale parasequences of thick white quartz arenite and dark shale in these two areas likely precludes more biostratigraphic precision, making the Canada Bay section of special biostratigraphic significance along the length of Appalachia's Laurentian margin. O I O I 57 00 52 00 Belle LOCATION Carboniferous Isle basins BEC MAP É Taconian QU allochthons LABRADOR The Trilobite Taxa Proterozoic basement + Pistolet Bay supracrustal cover The fauna in Weymouth Cove (K86-120 and 122) is characterized by the following trilobite taxa (in ascending order): Strait of Belle Isle T HUMBER ZONE DH AUL x F Hare Bay Austinvillia virginica Resser, 1938b (1986F077) NEWFOUNDLAND LAKE Plum Pt. R432 olenellid gen. et sp. undet. (1986F077, 1986F147) 0 50 100 150 MILE Km MSM HPF Olenellus howelli in Meek White, 1874? (1986F076, 1986F078, 1986F140) TEN BER AINS Canada Bay SYMBOLS Hawkes Bay Thrustcontact................................... LR Bonnia columbensis Resser, 1936 (1986F078) StructuralbaseofTaconian TRF allochthon ......................................... BM Fault .................................................. Table Pt. Bristolia mohavensis in (Crickmay Hazzard and Crickmay, 1933) (1986F078) Basal Cambrian unconformity ........... Carboniferous unconformity .............. Drill holes........................................... DH x Y Periomma sp. undet. (1986F078) Forteau Formation Study Areas......... O I THERN PENINSULA 50 00 WHITE BA Alokistocare / Amecephalus sp. undet. (1986F075B) NOR LONG T RANGE MOUNT GREA Bonne Bay GM Alokistocare / Amecephalus sp. undet. also occurs in section K86-74 at Bridge Cove, 2km south of Weymouth Cove B LEGEND a y o f CARBONIFEROUS I s l (Knight and Boyce, 1987). a Sedimentary rocks nds SILURIAN-DEVONIAN O I 49 00 Granitic rocks GRAND Corner ORDOVICIAN-DEVONIAN Inboard Brook LAKE Sedimentary and volcanic rocks Port au Port CAMBRIAN-ORDOVICIAN Peninsula ALLOCHTHON Ophiolite complexes Sedimentary rocks/mélange Stephenville THRUST STACKS Transported shelf rocks Outboard AUTOCHTHON Shelf and foreland basin rocks CAMBRIAN-PROTEROZOIC 0 25 50 O I Internal Humber Zone 48 00 Km metasedimentary rocks PROTEROZOIC Crystalline basement O I O I 58 00 56 00 Geology map of western Newfoundland showing the primary geological terranes. Arrow points to study Deformed cranidium of Alokistocare area. GM - Gros Morne National Park. Cranidium of Alokistocare / / Amecephalus sp. undet. Amecephalus sp. undet. (1986F075B-002), 5 mm long. (1987F148-001), 7 mm wide. Cephalon of Olenellus howelli Meek Section K-86-74 Sections K-86-120, 120A, 120B, 120C in White, 1874? (1986F140-003), 24 Bridge Cove, Chimney Arm Weymouth Cove, Chimney Arm 10 mm long (clay cast). Cover 0m DELAMARAN 1986 F075, F075A, F075B 1986 F140 1986 F078, F078A 1987 F148 1986 F147 DYERAN 1986 F076 1986 F077 Cover Bridge Cove and Weymouth Cove composite sections, showing the Pygidium of Bonnia columbensis positions of the 1986 & 1987 trilobite collections, and the Early—Middle Cranidium of Periomma sp. undet. Resser, 1936 (1986F078-005), ~ 5 Cambian (Dyeran—Delamaran) boundary. (1986F078-001), 3 mm long. Fragmentary cephalon of Bristolia mm long. mohavensis (Crickmay in Hazzard and Crickmay, 1933) (1986F078- 016), 15 mm long (lateral border Global Trilobite Zones Series Series Stages Trilobite Zones Great Basin, USA Newfoundland and genal spine). and (Palmer 1998) (Palmer 1998) Sekwi Formation (Hollingsworth, 2011; Stratigraphy Stages (Fritz 1972b) Webster, 2007, 2011) Nephrolenellus multinodus 510 Ma ? ? Bolbolenellus euryparia Hawke Bay Peachella iddingsi Formation Bristolia insolens Bristolia mohavensis ? Arcuolenellus arcuatus Forteau Bonnia- Formation Olenellus Stage 4 Dyeran Not Zoned ? Bradore aucoban Formation W Juvenile cephalon of Olenellus 517 Ma Series 2 howelli Meek in White, 1874? Nevadella eucharis (1986F076-001), 2 mm long. Nevadia addyensis Nevadella Avefallotaspis maria Grandinasus patulus Esmeraldina rowei Montezuman Stage 3 Fallotaspis Fallotaspis Fritzaspis 521 Ma ? ? No Trilobites Begadean Stage 2 No stages designated Dyeran to Topazan biostratigraphy of western Newfoundland. Global and Laurentian series and stages of the Early Cambrian and early Middle Cambrian (from Hollingsworth, 2011). Fragmentary cranidium of Austinvillia virginica Resser, 1938b (1986F077-001), 4 mm wide. Significance and Laurentian Correlation of the Trilobite Taxa Most of the Chimney Arm taxa are of known late Dyeran age. Austinvillia virginica was originally described from the Shady Formation (Virginia, USA) and, subsequently, from late Early Cambrian boulder G-25 of the deep water Grosse Roches Formation, Québec. Bonnia columbensis occurs just below the disconformable 'top' of the type Bonnia–Olenellus Zone in the Sekwi Formation (Mackenzie Mountains, northwestern Canada). Bristolia mohavensis is the nominate species of the late Dyeran Bristolia mohavensis Zone in the Latham Shale (Great Basin, western USA). Olenellus howelli is recorded from the uppermost Dyeran Combined Metals Member of the Pioche Formation (Great Basin, Nevada). Alokistocare and Amecephalus, however (closely similar general) are characteristic of the Middle Cambrian, which post-dates the last occurence of olenellid trilobites in Laurentia. Alokistocare / Amecephalus sp. undet. suggests a correlation with the early Delamaran Amecephalus arrojoensis Zone. The underlying, earliest Delamaran Eokochaspis nodosa Zone appears not to be represented in the Chimney Arm sections..
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