Summary account of the Lower Ordovician
(Arenig) graptolite biostratigraphy of the Cow Head Group, western Newfoundland
The Cow Head Group is an allochthonous sequence of Middle Cambrian to late Arenig sedimentary breccias, limestones and shales deposited in a deep lower slope environment close to a continental margin. Imbricate 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 are characterised by massive, coarse breccias with interbedded limestones and green/dark grey shales. More distal exposures have fewer and thinner breccias and limestones, while the green/grey shales are replaced progressively by red, non-graptolitic ones. Although the succession is by no mean unbroken or complete, it furnishes one of the best and most continuously graptolitic sections through the Arenig. A new zonal scheme is erected for the Cow Head Group, which could prove suitable as a new North American standard. Furthermore, several limestones and siliceous shales have yielded exquisitely preserved isolated material, permitting integration of fine growth detail with complete flattened specimens. With the exception of the uppermost Arenig U. austrodentatus Zone, Arenig graptoloids possess a prosicular 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 Dai of the Australasian scheme, therefore represents a hitherto unrecorded major evolutionary step in graptolite evolution.
