| | S.B.Misra, Department of Geology, Memorial University of Newfoundland, St. John’s, Newfoundland, CanadaNature, v.220, no.5168, pp.680-681Fossils found in the Pre-Cambrian Conception Group
of South-eastern Newfoundland Anderson M.M. and MISRA S.B., (1968)
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 tothe 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 thestrata are well exposed in cliff sections. It has been found possible on a lithological basis to subdivide locally that part of the Conception Group whichis present in this area into lower, middle and upper divisions. The lowerdivision is chiefly cherts, the middle dominantly siliceous mudstonesoccasionally interbedded with sandstones and the upper of green and red shalymudstones, more massive siliceous mudstones and graded sandstones (turbidites).These strata have a combined thickness of some 5000 feet which is less thanRose's1 estimate of 7000 feet for the thickness of the Group as a whole (afigure which Brueckner, personal communication, regards as probably anunderestimate). The beds of the Formation are thrown into folds which trendnorth-east. Although the basal part of the Conception Group is apparently absentfrom the sequence in this area, the local upper division, which is the one thatparticularly concerns us here, undoubtedly lies within the actual upper part ofthe Conception Group as a whole, for the transition from Conception to CabotGroup 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 (discussedlater), the Harbour Main Group and it underlies the Cabot Group (or HodgewaterGroup, its time equivalent in some western parts of the Avalon Peninsula) in agenerally conformable sequence, and the latter is overlain unconformably byfossiliferous Lower Cambrian strata including the lowest recognized division ofthe Cambrian in the Atlantic Province, a pre-trilobite faunal zone characterizedby hyolithids and inarticulate brachiopods (Hutchinson2). Lower Cambrian strataalso overlap the Conception Group at several localities within the Peninsula. Stratigraphic relations thus indicate a Pre-Cambrian age for the Conceptiongroup. Rose1 considered the Conception Group as possibly mid Proterozoic,apparently using the term Proterozoic to cover the younger post -Archaean partof the Pre-Cambrian. More recently an Rb/Sr isochron age of 574 +_ 11millionyear has been determined by McCartney et al.3 for the Holyrood granite whichintrudes the Harbour Main Group of acid and basic volcanic rocks and associatedsediments in the north-eastern part of the Avalon Peninsula. It should thereforebe possible on the basis of the relationship between the Harbour Main and Conception Groups to give a more definite age for the latter. Unfortunately therelationship of the Conception Group to the Harbour Main volcanics and to theHolyrood granite is still controversial.
Some authors like McCartney et al.3 regard the Conception Group as succeedingthe Harbour Main Group and therefore definitely post-Holyrood granite in age,whereas others like Rose1 and Brueckner believe the two groups are partlycontemporaneous and interfingering., which would make the lower part of theConception Group pre-Holyrood granite in age, the middle part roughlycontemporaneous 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 iscorrect, less that 574+_ 11 million yr ago, which places its time ofsedimentation very close to that for the transgression of the Cambrian sea ifthe figure of 570 million yr estimated for the base of the Cambrian by Cowie5 isused. McCartney et al.3 proposed a tentative maximum age of 560+_11 million yrfor the base of the Cambrian in this region, a figure which is in reasonableaccord with that of Cowie5 and the figures given in the time scales of severalother authors reviewed by Glaessner6. Taking the margin of error of +_11million yr for each figure, there would be, as Rose7 has pointed out, a severeforeshortening of Pre-Cambrian time in the area, for the following events musthave taken place in the time interval between emplacement of Holyrood graniteand the transgression of the Cambrian sea: unroofing and erosion of the Holyroodgranite, the deposition and preservation of some17000 feet of sedimentsincluding at least the upper part of the Conception Group and the whole of theCabot Group (within which there are disconformities), the folding and faultingof this assemblage followed by its uplift and erosion prior to the gradualsubsidence which enabled the Cambrian sea to transgress over the eroded surfaceof all these rocks. McCartney et al.3 allowed 15 million yr for these events, aperiod which seems to be quite insufficient, and the authors therefore questionthe validity of the isochron age. Consequently until more absolute dates areavailable to enable a reassessment of the age of the Conception Group to becarried 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 werefound to be fossiliferous. This is the first record of Pre-Cambrian fossilswithin the conception Group, and excluding the doubtful A spidellaterranovica Billings of the St. John's Formation, which Matthew8 believed tobe a slicken-sided mud concretion striated by pressure, it is the firstdefinite evidence of Pre-Cambrian animal life in the rocks of Newfoundland. Itis also an important addition to the small number of world localities at whichundoubted Pre-Cambrian invertebrates, as distinct from Pre-Cambrian plants andichnofossils, have been found, and represents only the second such locality onthe North American continent, the other being in Arizona where a single medusaimprint 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 Formationof the Canadian Shield have been described as possible metazoa by Frarey andMcLaren12, but the organic origin of these fossils is questionable.Glaessner13 considers that the former could be impressions of gelatinous sheathsof colonies of Cyanophyta on bedding planes, while Barnes and Smith14 believethat latter are probably mud-crack infill on ripple-marked surfaces even thoughthe authors had dismissed such an origin. It is, however, most unlikely thatsuch highly developed organisms as worms existed 1600 or more million yr ago.The Pre-Cambrian age of thin shelled brachiopods and other fossil remains foundin the Shaler Group on Victoria Island in arctic Canada by McNair15 remains tobe substantiated. Fossil Protozoa have been recorded from Pre-Cambrianlocalities in North America, but whether any of these microscopic bodiesactually represent one-celled animals is still uncertain.
The fossils of the Conception Group, which are soft-bodied representatives ofthe Metazoa, occur as impressions on ripple-marked bedding surfaces and havebeen observed so far at five horizons within a relatively small thickness ofConception beds; the lateral extension inland of these fossiliferous beds is not known. The impressions on the bedding surfaces of the lower part of the cliffshave become worn and indistinct as a result of wave action, but higher in thecliff where they are less exposed many of them are still quite sharp anddetailed. The assemblage is a small one as far as the number of different formspresent is concerned, but judging from the large numbers of individuals of eachtype preserved in the small area available for study these forms were allextremely abundant, and one form in particular is very common (Fig. 3) andrepresented by hundreds of individuals ranging in size from juveniles about 6 cmlong to adults 30 cm or more long. This obviously once flourishing fauna appearsto be unlike any known fauna. Whether it is related to the Pre-Cambrian Ediacarafauna of South Australia and its extensions in other parts of the worlddescribed by Glaessner16, 17, or is a distinct and possibly slightly older faunaremains to be determined.
Unfortunately the hardness of the siliceous mudstone beds bearing the fossilimpressions, their thickness and the fact that they are partly cleaved andhighly fractured makes it virtually impossible to collect these fossils intactso that work on them has to be done in situ using casts and peels. Thework of describing the fauna is in progress and the details will be reportedelsewhere.
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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