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In the map area, the Conception Group and the overlying St. Johns' Formation of the Cabot Group constitute a continuous, conformable succession of Precambrian sedimentary rocks about 8000 ft. thick, beneath an overburden of glacial drift; the base of the sequence is unexposed. Varied rocks of the Conception Group are predominantly green and purple and the St. John's shales are various shades of gray. The Conception Group is divisible into three lithologic units: Drook, Freshwater Point, and Cape Cove Formations, in order of decreasing age. The Drook Formation is composed mainly of chert, siliceous argillite, and siltstone; the Freshwater Point Formation is sliiceous argillite with minor sandstone; and the Cape Cove Formation is composed of Cyclic beds that grade form graywacke at the bottoms through siltstone to argillite at the tops. The Cape Cove Formation contains, in its upper part, imprints of soft-bodied coelenterates represented by polyps as well as Medusae.

$Precambrian fossil impressions on the fractured and jointed rocks of the Cape Cove Formation.$

Spindle-shaped colonial organism showing bilateral symmetry.

Medusoid organism exhibiting alternate lobes and depressions.

These constitute a newly described fauna, the environment of which can be inferred from the sediments that enclose them. During late Precambrian time, deposition of the Conception Group began in isolated basins that subsequently joined to form a shallow marine environment. The sea continued to deepen and became deepest during deposition of the middle part of the Cape Cove Formation.

Turbidity currents Played an important role in the deposition of the Conception Group and the St. John's Formation, and reached a maximum during deposition of the Cape Cove Formation. After a substantial thickness of the Conception Group had been deposited, the sea became shallow again, perhaps during deposition of the uppermost part of the Cape Cove Formation, and remained shallow duringpost-Conception times. During deposition of the St. John's Formation, the intensity of turbidity currents decreased. Also, mild volcanism, here reported for the first time, occurred during early St. John's time. [TOP]

Introduction

Fossils of late Precambrian organisms in the rocks ofConception Group are the most significant aspect of geology of the BiscayBay-Cape Race area (Anderson and Misra, 1968; Misra, 1969a, 1969b). The fossilsare well-preserved imprints (Figs. 1, 2) of soft bodied coelenterates ofuncertain habitat. The organisms possibly were floating and colonial (Misra,1969b). There are no direct evidences of bathymetry norof salinity. Therefore, a detailed study of various other aspects of the geologysuch as stratification and sedimentation are necessary, in order to understandthe ecologic conditions in the Conception sea The information provided in the following pages will serve thispurpose in part, until more detailsare available.

Prior to the present work, geologic studies in theAvalon Peninsula have been conducted in the Torbay (Rose, 1952), Harbour Grace(Hutchinson, 1953), Whitbourne (McCartney, 1967), and St, John's (Brueckner,1971) areas. These studies, however, concern only the northern part of thepeninsula; the present study is the first in the peninsula south of theWhitbourne map area (Fig. 3). The area is accessible by road from St. John's, 100 miles north of Biscay bay, or from the Trans-CanadaHighway via Highways 6 and 7. Within the area, there is a secondary gravel roadalmost parallel to the shoreline. Inland, the area is barren and easy to traverse in all directions. Results of mapping completed in the summer of1967 are summarized in a geological map on a scale of 1:50,000 (Fig. 4) [TOP]

Stratigraphy

The oldest rocks in the Avalon Peninsula are those ofthe Harbour Main Group intruded by the Holyrood Granite (Rose, 1952; McCartney,1967). These volcanic and plutonic rocks supplied most of the detritus composingthe successively overlying Conception and Cabot Groups. The latter groups aregradationally conformable in most parts of the peninsula, except in the Torbaymap area where Rose (1952) reported a disconformity between them. In the BiscayBayCape Race area, these groups comprise a conformable succession about 8000 ftthick with base unexposed. The Conception Group consists of cherts, argillites,siltstones, and graywackes and the St. John's Formation of the Cabot Groupconsists of shales. The St. John's Formation outcrops on either side of theConception Group, at Cape Race to the east and in Biscay Bay to the west,indicating that the map area constitutes a major anticlinorium with its axis trending northeast and passing through Freshwater Bay. This regional structure is referred to as Drook Anticlinorium. [TOP]

Conception Group

The Conception Group, in the Torbay map area to thenorth, was defined by Rose (1952) as a thick sequence of sedimentary rocksoverlying the Harbour Main Group and underlying the St. John's Formation of hisCabot Group. He divided the Conception Group into the "Conception slate" andthe "Torbay slate" but did not propose a formal classification. In theHarbour Grace area, Hutchinson (1953) recognized a formal division, the HibbsHole formation, and McCartney (1967) applied this name to a correlative unit inthe Whitbourne map area. In the Biscay Bay-Cape Race area, the Conception Groupcan be divided into three formations (table 1) here named in order of decreasingage: Drook, Freshwater Point, and Cape Cove Formations. Although these rockunits are separated by gradational boundaries, each has characteristics thatdistinguish it form the overlying and underlying formation.[TOP]

Drook Formation

The "Drook Formation" is the name proposed forabout 2500 ft. of well-bedded, gently folded, hard siliceous siltstones andcherts, oldest rocks of the area (Fig. 5A). The type area is Drook, 3 misoutheast of Portugal Cove South (Fig. 4). Apart from the coastal exposures, theformation crops out only along Drook River. many of the chert beds aresilicified siltstones varying in thickness from a fraction of an inch to 2 in.Each bed has its characteristic color, usually a shade of green that is moreaccentuated on the exposed surface. Qualitative x-ray analysis of the chertsand argillites of the Drook /formation shows that the most common constituentsare quartz, albite, chlorite, and sericite; minor constituents are epidote,sphene, and leucoxene. Clay minerals are absent. Calcite is found only asconcretions.

The base of the formation is not exposed, and thecontact relation between the conception Group and the underlying rocks isunknown; the oldest beds of the formation exposed along the Drook River arealmost pure chert. The gradational upper boundary of the formation is marked by agradual increase in the argillaceous and arenaceous material and decrease insilicification. [TOP]

FreshwaterPoint Formation

The name "Freshwater Point Formation" is proposedfor about 1500 ft of predominantly argillaceous beds exposed in coastal areas onthe western and eastern limbs of the Drook anticlinorium. the western coastalsection near Daly's Point is interrupted by a boulder beach, but the easterncoastal section is complete. The formation is composed mainly of gradedsiliceous argillites and siltstones, but minor amounts of fine-to medium grainedsandstones occur at the base of some graded units. The sandstones are made up ofsub-angular to sub-rounded grains of quartz, feldspar, and rock fragments set in amatrix of chlorite, sericite, epidote, leucoxene, and sphene. The argillites arecommonly composed of quartz, albite, chlorite, and sericite. Clay minerals areabsent. The fine-grained sediments of the formation are green, the sandstonesare gray, and the weathered rocks are whitish.

Although the main rock type of the Freshwater PointFormation is argillites, the upper-part includes graded beds composed ofsandstone at the bottom through siltstone to argillite at the top. In thesegraded beds, the basal sandy part makes up less than 20 percent of the gradedunits - a criterion applied to draw the upper boundary of the formation. [TOP]

Cape Cove Formation

The name "Cape Cove Formation" is proposed forabout 2900 ft of graded beds that are distinguished form the Freshwater Pointand St. John's Formations based on color, cleavage, and graywacke percentage,with emphasis on change of color from purple to gray at the upper boundary. Thetype section is exposed along the northern coast of Cape Cove near Cape Race. Inaddition to coastal section, the formation is also exposed along Portugal CoveBrook and Briscal Cove River. The main part of the Cape Cove formation comprisesgraded beds of graywacke and well-cleaved, green argillite. The beds are as muchas 10 ft thick and maintain a uniform thickness along strike for hundreds offeet. The graywacke in the graded beds makes up as much as 60 percent of themiddle part of the Cape Cove Formation. The upper part of the formation includespurple argillites and graywackes that are separated form the overlying St.John's Formation by a transition zone of about 400ft. In addition to gradedbedding, the sedimentary structures occurring in the formation include ripplemarks, sole marks (Fig. 6B) , convolute bedding, and locally small-scalecross-stratification.

Stratification in the rocks of Freshwater Point Formation. Thin sandstone beds are weathered more easily due to differential weathering.

Graded bedding in the Cape Cov Formation. The hammer is resting on the sharp contact between 2 graded units.

Interbeds of shales and sandstones in the St. John's Formation

The most striking petrographic feature of thesandstones is their bimodal size distribution: abundant matrix separates largegrains and rock fragments. All rock fragments are of rock types similar to thosenow exposed in the Harbour Main Group and the Holyrood plutonic series. Nometamorphic rock fragments were recognized. The composition determined by modalanalysis indicates that detrital quartz forms 22 to 38 percent of the rock. Thematrix comprises 40 to 55 percent, and is composed of chlorite, biotite, sphene,epidote, leucoxene, pyrite, and apatite. Mineral constituents of the siltstonesand argillites are quartz, feldspar, chlorite, and sericite. Rock fragments areabsent in the siltstones and argillites, but these finer-grained rocks have ahigher proportion of opaque minerals.

In the costal exposures near Mistaken Point, theformation contains organic imprints that represent the fauna of the Conceptionsea during late Precambrian time. Organic origin of the imprints is evident(Misra, 1969b; Anderson and Misra, 1969) based on: (1) mode of preservation andorientation (Fig. 1) , (2) size variation which may be ontogenic, (3) symmetry(fig. 2B), and (4) resemblance to known Precambrian fossils. The organisms aregrouped into four categories (Misra, 1969b): spindle-shaped, round lobate,leaf-shaped, and dendrite-like. [TOP]

CabotGroup

The Cabot Group defined outside the area to the north(Rose, 1952) includes three conformable sedimentary formations which inchronologic order are the St. John's, Signal Hill, and Black Head Formations.Only the St. John's Formation is exposed in the map area.

St.John's Formation

The formation was named "St. John's slate" byJukes, 1843, "Aspidella slates" by Murrayand Howley, 1881 (Both inMcCartney, 1967), and "Momable slates" by Walcott (1899). The name "St,John's Formation" was proposed by Rose (1952) for the sequence overlying theConception Group and underlying the Signal Hill Formation of his Cabot Group.

The St. John's Formation in the map area Consists ofabout 1100 ft of well-cleaved (Fig. 6D) dark- to light-gray shale that overliescoastal exposures: the first from Cape Race to Shingle Head and the second fromBiscay Bay to portugal Cove south. Apart from these coastal exposures, theformation is found in several outcrops along back River and also along severalbroks near Cape Race. the basal part of the St. John's Formation is gray,well-cleaved, thin-bedded shale inter calated with sandstone laminae (Fig. 5D).Sandstone beds in this part of the formation, and also in the transition zoneimmediately below, commonly contain large euhedral crystals of pyrite.Disseminated pyrite if ubiquitous. The baseal part of the formation near ShingleHead includes a layer of volcanic tuff about 2 ft thich that isstratigraphically only a few hundred feet above the fossil-bearing horizon inthe underlying Cape Cove Formation. This is the first report of Volcanism in theSt. John's Formation.

Ellipsoidal calcareous sandstone nodules in banded cherts of the Drook Formation. The greater part of each nodule lies above the general surface of the underlying bed.

Sole marks on the undersurface of a graded bed in the Cape Cove Formation.

Intraformational conglomerates in the siliceous argillites of the Freshwater Point Formation

Well cleaved St. John's shales showing deflection of cleavage as it passes from one layer to other.

The main part of the formation is predominantlythin-bedded, gray shale with intercalated sandy streaks (Fig. 6D). Fracturecleavage is very pronounced and obscures bedding in some places; but in otherplace, the relation between cleavage and bedding is distinct (fig. 6D). primaryfeatures of the formation include cross-stratification and slumping. The slumpstructures (figs. 7A, 7BV, 7C and 7D) were produced by contortion of beds whichwas governed, among other things, by the relative competence of the towlithologies in juxtaposition at the time of slumping . chert, clayey sediments,shale, and calcareous sediments give rise to better developed slump structuresthat do silty argillite and graded sandstone which are almost devoid of suchfeatures. The rocks are medium to fine grained; microscopic examination revealsquartz. feldspar, mica, chlorite, and pyrite. The accessory minerals are thesame as in the Conception argillite, except that pyrite is more common andcalcite more frequent. In site replacement of quartz by calcite isindicated by parchy extinction in fine-grained sandstone and indistinguishableboundaries between calcite and quartz grains.[TOP]

Correlation and Age

Precambrian-Cambrian boundary in Newfoundland isdefined by Hutchinson (1962) at the first disconformity in the stratigraphicsequence below the Callavia zone. Rocks of the Harbour Main (a predominantlyvolcanic sequence), Conception, and Cabot Groups lie stratigraphically below thisboundary (Hutchinson, 1953; McCartney, 1967). However, radioactive-agedeterminations (Fairbairn and other, 1966; McCartney and others, 1966) do not indicate their Precambrian age.Nevertheless, the evidence by stratigraphic correlation is convincing, sincelower Cambrian strata "unconformably overlap" the rocks of the ConceptionGroup at many localities in the Avalon Peninsula (Rose, 1952). The bed rock inthe map area (Fig. 4) clearly correlates (Misra, 1969a) with the ConceptionGroup and the St. John's Formation of Rose (1952) although it is covered withPleistocene glacial deposits. The coelenterates reported from the ConceptionGroup (Misra, 1969b) are represented by polyps (Fig. 2B) as well as Medusae(fig. 2A) . M. F. Glaessner (1968, personal commun. ) interpreted thespindle-shaped organisms of the fauna as "a new floating colonial Hydrozoan ofthe order thecata." In any case, this fauna is older than Coleoloidesand Hyolithes whcih were succeeded by Calavia fauna.[TOP]

Recumbent and overturned slump folding in the St. John's formation

Pseudo nodule produced by slumping in the St. John's Formation.

Depositional History of the Area

The sediments of the Conception Group and the St.John's Formation were derived from a complex terrane consisting of volcanic,igneous, and sedimentary rocks, situated to the northeast of present exposures.The conclusions regarding the source area are drawn from rock constituents andcurrent directions obtained from slumping and cross stratification. Thediscussion that follows indicates that the environment of deposition of theDrook and Freshwater Point Formations was initially shallow, but became deeperduring deposition of the upper part of the Freshwater Point Formation, remaineddeep during Cape cove time, and became shallow again during St. John's time . Asimilar sedimentary model is suggested by McCartney (1967) in the Whitbourne maparea.

Sedimentation of the Conception Group in the BiscayBay-Cape Race area probably started in isolated basins bounded by volcanic rocks of the earlier Harbour Main Group. Such isolated basins are envisaged by McCartney (1967), in the case of the sedimentaryrocks that he included in the Harbour Main Group and that are strikingly similarto those of the Conception group. These sediments assigned to the Harbour MainGroup are possibly the beginnings of the Conception Group deposition becauselocally the underlying volcanic rocks and Conception Group rocks are inter-beddedin transition zones. Subsequently, the isolated basins probably joined to forma shallow-water marine environment. Such an environment of deposition isindicated for the rocks of the Drook and Freshwater Point Formations by highcontent of silica in the sediments, calcareous nodules (Fig. 6A), megaripplemarks, beds exhibiting waviness (Rich, 1951), and intraformational structures(Fig. 6C). In the Freshwater Point Formation, graded bedding, grain size,thickness of beds, and lithology suggest that energy of the sedimentary systemhad increased; the environment of deposition gradually became deeper, and by theclose of deposition of the Freshwater Point formation, the sea had become deep enough for turbidity currents of large magnitude.

Rocks of the Cape Cove Formation were probably deposited by turbidity currents. Themain arguments in favor of this are : (1) lack of evidence of tidal action, (2)presence of sole marks (Fig. 6V), (3) large volume of graded beds, and (4) consistency in the thickness of beds and direction of supply. Flute casts(Fig. 6B) indicate that the current hugged the bottom and was not the surfacecurrent (Kuenen and Migliorini, 1950). Furthermore, (5) the great thickness ofsome graded beds indicates a current of abnormal energy. Finally, (6) the sandstones at the base of the graded bedscontain abundant matrix which indicates that deposition of fine material hastaken place simultaneously with that of large particles. After deposition of asubstantial thickness of the Cape Cove Formation, the seaward slop upon newlydeposited detritus increased progressively. During this increase, submarineslumping may have been initiated by agents such as wave action during heavystorms, earthquakes, or volcanism, or by abundant supply of sediments. Slumpstructures (Fig.s 7A, 7B, 7C and 7D) associated with some beds in the lower partof the St. John's Formation indicate that deposition was still taking place on asloping surface and that currents which originated in shallow water probablyflowed down a slope to the deeper areas. However, siltstone laminae of the St.John's Formation (fig. 5D) could not be the result of separate turbidity flows.This type of lamination can be explained by traction transport that started as theturbidity currents became more dilute and contained relatively more mud thanlarge particles. Small-scale cross-bedding and ripple lamination, which are insome cases associated with flame structures, were formed by bottom traction andnot by turbulence. The presence of pyrite together with dark gray color of the shales suggest that the St. John's Formation was deposited in a reducingenvironment.[TOP]

Acknowledgements

This study was conducted as part of my graduate work atMemorial University of Newfoundland, and was financed for the most part by W.D.Brueckner's National Research Council (Canada) Grant No. 7882 and partly by hisGeological Survey of Canada grants. I am grateful to Professor Brueckner forfinancial assistance, field supervision, and for suggesting the area. I amthankful to E.R.W. Neale, H. Williams, and M.M. Anderson of Memorial Universityand to J.A. Donaldson of Carleton University, Ottawa, for reading the paper andmaking helpful suggestions. Field assistance of Donald J. Fitzpatric and Paulthompson of Memorial University is gratefully acknowledged. Thanks are due to W.Marsh of Memorial University for help in photography. [TOP]

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