Summary account of the Lower Ordovician (Arenig) graptolite biostratigraphy of the Cow Head Group, western Newfoundland
Cj T h e Cow H e a d G r o u p is an al lochthonous sequence of Middle Cambr ian to late Arenig sedimentary brec-cias, l imestones and shales deposi ted in a deep lower slope environment close to a continental margin. Im-bricate thrusting has resulted in repeated exposure of laterally equivalent &qu...
| Main Authors: | , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.555.480 http://2dgf.dk/xpdf/bull35_03-04-259-270.pdf |
| Summary: | Cj T h e Cow H e a d G r o u p is an al lochthonous sequence of Middle Cambr ian to late Arenig sedimentary brec-cias, l imestones and shales deposi ted in a deep lower slope environment close to a continental margin. Im-bricate thrusting has resulted in repeated exposure of laterally equivalent "proximal " to "distal " facies which may be correlated using graptolitic control in the interbeds. "Proximal " sections a re characterised by massive, coarse breccias with in terbedded l imestones and green/dark grey shales. M o r e distal ex-posures have fewer and th inner breccias and l imestones, while the green/grey shales a re replaced pro-gressively by red, non-graptolit ic ones. Al though the succession is by no mean unbroken or comple te, it furnishes one of the best and most con-tinuously graptolitic sections through the Arenig. A new zonal scheme is erected for the Cow H e a d G r o u p, which could prove suitable as a new Nor th Amer ican s tandard. Fur the rmore, several l imestones and siliceous shales have yielded exquisitely preserved isolated mater ia l, permit t ing integration of fine growth detail with complete flattened specimens. With the exception of the uppermos t Arenig U. austrodentatus Z o n e, Arenig graptoloids possess a p ro-sicular origin for thl1. The earliest graptoloids with a metasicular origin for the first theca appear in this zone, including Undulograptus, Cryptograptus and Paraglossograptus. This interval, equivalent to Dal of the Australasian scheme, therefore represents a h i ther to unrecorded major evolutionary s tep in graptoli te evolution. |
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