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S.B.Misra, Department of Geology, Memorial University of Newfoundland, St. Johnís, Newfoundland, Canada

Nature, v.220, no.5168, pp.680-681

Fossils found in the Pre-Cambrian Conception Group
of South-eastern Newfoundland

Anderson M.M. and MISRA S.B., (1968)

<<  Thesis Misra 1969a Anderson  & Misra 1969 >>

The Biscay Bay - Cape Race area of the southern end of the Avalon Peninsula, south-eastern Newfoundland (figs. 1 and 2), was mapped by one of us (S.B.M.) during the summer of 1967; it was found to be underlain by rocks belonging to the Conception Group and the overlying St. John's Formation of the Cabot Group, largely hidden beneath a cover of glacial drift except along the coast where the strata are well exposed in cliff sections. It has been found possible on a lithological basis to subdivide locally that part of the Conception Group which is present in this area into lower, middle and upper divisions. The lower division is chiefly cherts, the middle dominantly siliceous mudstones occasionally interbedded with sandstones and the upper of green and red shaly mudstones, more massive siliceous mudstones and graded sandstones (turbidites). These strata have a combined thickness of some 5000 feet which is less than Rose's1 estimate of 7000 feet for the thickness of the Group as a whole (a figure which Brueckner, personal communication, regards as probably an underestimate). The beds of the Formation are thrown into folds which trend north-east. Although the basal part of the Conception Group is apparently absent from the sequence in this area, the local upper division, which is the one that particularly concerns us here, undoubtedly lies within the actual upper part of the Conception Group as a whole, for the transition from Conception to Cabot Group takes place, only a little higher stratigraphically in the sequence, between Mistaken Point and Cape Race.

The Conception Group lies above, or is partly contemporaneous with (discussed later), the Harbour Main Group and it underlies the Cabot Group (or Hodgewater Group, its time equivalent in some western parts of the Avalon Peninsula) in a generally conformable sequence, and the latter is overlain unconformably by fossiliferous Lower Cambrian strata including the lowest recognized division of the Cambrian in the Atlantic Province, a pre-trilobite faunal zone characterized by hyolithids and inarticulate brachiopods (Hutchinson2). Lower Cambrian strata also overlap the Conception Group at several localities within the Peninsula. Stratigraphic relations thus indicate a Pre-Cambrian age for the Conception group. Rose1 considered the Conception Group as possibly mid Proterozoic, apparently using the term Proterozoic to cover the younger post -Archaean part of the Pre-Cambrian. More recently an Rb/Sr isochron age of 574 +_ 11million year has been determined by McCartney et al.3 for the Holyrood granite which intrudes the Harbour Main Group of acid and basic volcanic rocks and associated sediments in the north-eastern part of the Avalon Peninsula. It should therefore be possible on the basis of the relationship between the Harbour Main and  Conception Groups to give a more definite age for the latter. Unfortunately the relationship of the Conception Group to the Harbour Main volcanics and to the Holyrood granite is still controversial.

Some authors like McCartney et al.3 regard the Conception Group as succeeding the Harbour Main Group and therefore definitely post-Holyrood granite in age, whereas others like Rose1 and Brueckner believe the two groups are partly contemporaneous and interfingering., which would make the lower part of the Conception Group pre-Holyrood granite in age, the middle part roughly contemporaneous with the emplacement and consolidation of the Holyrood granite, and the upper part post-Holyrood granite in age. In either case the upper part  of the Conception Group would have been laid down assuming the isochron age is correct, less that 574+_ 11 million yr ago, which places its time of sedimentation very close to that for the transgression of the Cambrian sea if the figure of 570 million yr estimated for the base of the Cambrian by Cowie5 is used. McCartney et al.3 proposed a tentative maximum age of 560+_11 million yr for the base of the Cambrian in this region, a figure which is in reasonable accord with that of Cowie5 and the figures given in the time scales of several other authors reviewed by Glaessner6. Taking the margin of error of +_11million yr for each figure, there would be, as Rose7 has pointed out, a severe foreshortening of Pre-Cambrian time in the area, for the following events must have taken place in the time interval between emplacement of Holyrood granite and the transgression of the Cambrian sea: unroofing and erosion of the Holyrood granite, the deposition and preservation of some17000 feet of sediments including at least the upper part of the Conception Group and the whole of the Cabot Group (within which there are disconformities), the folding and faulting of this assemblage followed by its uplift and erosion prior to the gradual subsidence which enabled the Cambrian sea to transgress over the eroded surface of all these rocks. McCartney et al.3 allowed 15 million yr for these events, a period which seems to be quite insufficient, and the authors therefore question the validity of the isochron age. Consequently until more absolute dates are available to enable a reassessment of the age of the Conception Group to be carried out, it can only be said that the group is definitely late Pre-Cambrian.

The upper Conception mudstones in the cliffs west of Mistaken Point were found to be fossiliferous. This is the first record of Pre-Cambrian fossils within the conception Group, and excluding the doubtful A spidella terranovica Billings of the St. John's Formation, which Matthew8 believed to be a slicken-sided mud concretion striated by pressure, it is the first definite evidence of Pre-Cambrian animal life in the rocks of Newfoundland. It is also an important addition to the small number of world localities at which undoubted Pre-Cambrian invertebrates, as distinct from Pre-Cambrian plants and ichnofossils, have been found, and represents only the second such locality on the North American continent, the other being in Arizona where a single medusa imprint is known from the Late Pre-Cambrian Nankoweap Formation (Bassler9,VanGrundy10).Medusa like impressions have been found at another horizon in the Grand Canyon by Alf11, and worm-like bodies from the Huronian Lorrain Formation of the Canadian Shield have been described as possible metazoa by Frarey and McLaren12, but the organic origin of these fossils is questionable. Glaessner13 considers that the former could be impressions of gelatinous sheaths of colonies of Cyanophyta on bedding planes, while Barnes and Smith14 believe that latter are probably mud-crack infill on ripple-marked surfaces even though the authors had dismissed such an origin. It is, however, most unlikely that such highly developed organisms as worms existed 1600 or more million yr ago. The Pre-Cambrian age of thin shelled brachiopods and other fossil remains found in the Shaler Group on Victoria Island in arctic Canada by McNair15 remains to be substantiated. Fossil Protozoa have been recorded from Pre-Cambrian localities in North America, but whether any of these microscopic bodies actually represent one-celled animals is still uncertain.

The fossils of the Conception Group, which are soft-bodied representatives of the Metazoa, occur as impressions on ripple-marked bedding surfaces and have been observed so far at five horizons within a relatively small thickness of Conception beds; the lateral extension inland of these fossiliferous beds is not known. The impressions on the bedding surfaces of the lower part of the cliffs have become worn and indistinct as a result of wave action, but higher in the cliff where they are less exposed many of them are still quite sharp and detailed. The assemblage is a small one as far as the number of different forms present is concerned, but judging from the large numbers of individuals of each type preserved in the small area available for study these forms were all extremely abundant, and one form in particular is very common (Fig. 3) and represented by hundreds of individuals ranging in size from juveniles about 6 cm long to adults 30 cm or more long. This obviously once flourishing fauna appears to be unlike any known fauna. Whether it is related to the Pre-Cambrian Ediacara fauna of South Australia and its extensions in other parts of the world described by Glaessner16, 17, or is a distinct and possibly slightly older fauna remains to be determined.

Unfortunately the hardness of the siliceous mudstone beds bearing the fossil impressions, their thickness and the fact that they are partly cleaved and highly fractured makes it virtually impossible to collect these fossils intact so that work on them has to be done in situ using casts and peels. The work of describing the fauna is in progress and the details will be reported elsewhere.

M. M. Anderson
S. B. Misra

Department of Geology
Memorial University of Newfoundland,
St. John's
Newfoundland, Canada

Received July 24, 1968


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