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Bulletin of the Geological Society of Denmark, Vol. 35/3-4, Pp. 259-270

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Bulletin of the Geological Society of Denmark, Vol. 35/3-4, Pp. 259-270 Summary account of the Lower Ordovician (Arenig) graptolite biostratigraphy of the Cow Head Group, western Newfoundland S. HENRY WILLIAMS AND ROBERT K. STEVENS U Williams, S. H. and Stevens, R. K.: Summary account of the Lower Ordovician (Arenig) graptolite bio­ stratigraphy of the Cow Head Group, western Newfoundland. Bull. geol. Soc. Denmark, vol. 35, pp. 259- 270, Copenhagen, July 1st, 1987. Cj The Cow Head Group is an allochthonous sequence of Middle Cambrian to late Arenig sedimentary brec­ cias, limestones and shales deposited 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 are characterised by massive, coarse breccias with interbedded limestones and green/dark grey shales. More distal ex­ posures have fewer and thinner breccias and limestones, while the green/grey shales are replaced pro­ gressively by red, non-graptolitic ones. Although the succession is by no mean unbroken or complete, it furnishes one of the best and most con­ tinuously 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 pro- 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 hitherto unrecorded major evolutionary step in graptolite evolution. S. Henry Williams and Robert K. Stevens. Dept. of Earth Sciences, Memorial University of Newfoundland, St. John's, Nfld, Canada A1B 3X5, August 29th, 1986. The Cow Head Group is composed of a series of slide breccias, while Johnson (1941) regarded the coarse sedimentary breccias or conglomerates sequence as a broken but relatively complete and with interbedded limestones and shales, together simple one. Oxley (1953) returned to the ideas of with subordinate silts, cherts and dolostones. It Schuchert and Dunbar, concluding the breccias ranges in age from late Middle Cambrian to late to be "thrust breccias" and that the intervening Arenig. The base is nowhere seen, but the unit is limestone-shale beds were large blocks within overlain by green sandstones of the Lower Head these units. Kindle and Whittington (1958) fi­ Formation (James and Stevens 1986), considered nally recognised the Cow Head Group as having to be of turbidite origin (Stevens 1970). a regular sedimentary origin with an age from Much discussion on the Cow Head Group has Middle Cambrian to Middle Ordovician. They been centred around the origin of the breccias. numbered the beds from 1 to 14, an informal sys­ Logan (1863) considered them to be part of a tem still used with the addition of a Bed 15 not simply bedded sedimentary sequence. Dawson exposed at Cow Head. The breccias are now (1883) and Walcott (1894) inferred that they re­ thought to have been deposited through a series sulted from the action of sea ice on a shallow wa­ of debris flows, bringing blocks of shallow water ter carbonate sequence. Schuchert and Dunbar carbonates into a deep water setting (James and (1934) offered an entirely different view; they Stevens 1986). They also eroded the underlying considered that some breccias were tectonic, deep water sediments to a greater or lesser extent while others were derived from the erosion of ad­ which now form the bulk of incorporated clasts, vancing submarine thrust sheets. Barrington especially in the more distal exposures. Brown (1938) compared them to sedimentary The Cow Head Group is considered allochtho- 260 Williams and Stevens: Summary of the Cow Head Group nous; it was probably transported at least 150 km History of graptolite research from the north-east during the Taconic Orogeny (Rodgers and Neale 1963; Stevens 1970 - see Graptolites were first collected from the Cow James and Stevens 1986 for discussion). It now Head Group, together with other fossils, by Ri­ rests in proximity to stratigraphically equivalent chardson in the 1860's during early work by the autochthonous shallow water carbonates of the Geological Survey of Canada, which were subse­ St. George and Table Head Groups. A second al- quently incorporated by Billings in Logan's "Ge­ lochthonous unit, the Curling Group, is also pres­ ology of Canada" (1863). These specimens were ent; it is considered more distal in origin and is among collections of Canadian material sent to mostly older than the Cow Head Group. The Lapworth at Birmingham University for identi­ Lower Paleozoic sediment pile is overlain fication. Although they were studied the results through thrust contact by Grenville gneiss and do not appear to have been used or published. granite which forms the core of the Long Range Schuchert and Dunbar (1934) made extensive fau­ Mountains (fig. 1). nal collections, but the first critical conclusions The imbricate nature of the Cow Head Group on biostratigraphy were based on graptolites col­ and overlying Lower Head Formation exposures lected by Johnson's field assistant Kindle. Rue- is of uncertain genesis. The sequence has never demann studied these in detail and incorporated been buried deeply; this is demonstrated by un- the results in his "Graptolites of North America" deformed macrofossils and low conodont colour published in 1947. He included extensive faunal alteration index of 2.5, making emplacement as a lists, together with a few illustrations and new subophiolitic accretionary prism extremely un­ taxa based on the Cow Head material. likely. The structure does, however, allow an in­ Kindle and Whittington (1958) were the first to sight into the transition of a sedimentary environ­ give a critical stratigraphical account of the grap­ ment from lower continental slope to toe of slope tolite faunas. They compared the succession with in an approximately west-east direction. Most the Australian standard and proposed a correla­ "proximal" sections, such as Lower Head and tion between the graptolitic and North American Cow Head, are characterised by massive poly- shelly zonal schemes. Our work has involved ex­ mict breccias, abundant limestone-shale rhyth- tensive recollecting through the Arenig at a lim­ mites and green/grey/black shale intervals. Dis- ited number of localities (fig. 1). This has per­ tally the breccias decrease in thickness, number, mitted critical taxonomic revision, incorporating clast size and variety of composition, while lime­ isolated, three-dimensional material, and the stones and green/grey shales become scarcer and erection of a new zonal scheme with eight divi­ red shale predominates. This suggests that the sions through the Arenig. oxygen minimum existed at some distance up- slope, oxygen content increasing with depth. Correlation of the sections is made possible Completeness of the Arenig graptolite through graptolites present in the interbeds, and succession conodonts in limestones. Coeval shallow water shelly faunas occur in limestone clasts in the brec­ The Cow Head Group contains possibly the most cias, permitting integration of several biostrat- complete and continuously graptolitic Arenig se­ igraphical schemes (Kindle and Whittington quence in the world. Both faunal and sedimen­ 1958). Previous faunal studies have concentrated tary breaks occur, but these are clearly recognis­ on the trilobites (e.g. Whittington 1963) and con­ able and well understood due to the number of odonts (e.g. Fåhraeus and Nowlan 1978). Ongo­ sections studied. The most obvious erosion oc­ ing research is centred on the conodonts (C. R. curs at the bases of several major breccia hori­ Barnes et al.) and graptolites (S. H. Williams, zons, notably the bases of Beds 10 and 12 at Cow R. K. Stevens and B.-D. Erdtmann) from the Head and Bed 14 at Lower Head, where the Upper Cambrian and Ordovician. cross-cutting relationship with underlying lime­ stone-shale lithologies is unambiguous. Less no­ ticeable, but equally important breaks occur throughout the interbeds. Some erosive intervals Bulletin of the Geological Society of Denmark, vol. 35 1987 261 are indicated by channelling and phosphorite well to bleaching treatment to reveal growth fu- conglomerate horizons, while others may be in­ sellae. The combination of abundant flattened or ferred through absence of lithological or biostrat- partially flattened non-isolated graptolites with igraphical intervals present at neighbouring lo­ flattened bleached specimens and three-dimen­ calities. Thus the thickness of Bed 11 on the sional unbleached ones has allowed an unique in­ Ledge section at Cow Head is only one half to tegration of overall form, growth structure and two thirds the total thickness of that at Jim's fine fusellar detail from the one stratigraphic Cove only a few hundred metres along strike. unit. Likewise beds representing the T. akzharensis Many lithologies are somewhat atypical for Zone thin from about 5 m at the eastern part of the graptolite preservation; the most fossiliferous lith- Cow Head peninsula to 0.5 m at the Ledge sec­ ology is a dark grey, slightly brown, silty shale, tion, a present distance of little more than one which is commonly interbedded with either dark kilometre. Successions in the down slope sections limestone or pale green shale. Black, very fine­ are probably more complete, but are characterised grained paper shales with a somewhat "soapy" by larger proportions of red and green non-grap- texture occasionally yield fragments but are mostly tolitic shales. non-graptolitic. They perhaps represent very slow The picture is complicated further by ubiquitous background deposition which was insufficiently but subtle tectonic deformation. Much cross-fault­ rapid to bury the organic skeletons prior to de­ ing and parasitic folding is obvious, but a large composition.
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