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

Geological Society of America Bulletin, v. 82, p.979-988, 7 figs., 8 pls., April 1971

Stratigraphy and Depositional History of Late Precambrian Coelenterate-bearing rocks, Southeastern Newfoundland.

 
ABSTRACT
INTRODUCTION
STRATIGRAPHY
- CONCEPTION GROUP
   - Drook Formation
   - Fresh Water Point Formation
   - Cape Cove Formation
- Cabot Group
   - St. John's Formation
DEPOSITIONAL HISTORY
CORRELATION AND AGE  
ACKNOWLEDGEMENTS
REFERENCES CITED
Plate Section
<<  Misra 1969b

 Misra 1981 >>


Abstract

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 during post-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 of Conception Group are the most significant aspect of geology of the Biscay Bay-Cape Race area (Anderson and Misra, 1968; Misra, 1969a, 1969b). The fossils are well-preserved imprints (Figs. 1, 2) of soft bodied coelenterates of uncertain habitat. The organisms possibly were floating and colonial (Misra, 1969b). There are no direct evidences of bathymetry nor of salinity. Therefore, a detailed study of various other aspects of the geology such as stratification and sedimentation are necessary, in order to understand the ecologic conditions in the Conception sea The information provided in the following pages will serve this purpose in part, until more details are available.

Prior to the present work, geologic studies in the Avalon 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 the peninsula; the present study is the first in the peninsula south of the Whitbourne map area (Fig. 3). The area is accessible by road from St. John's, 100 miles north of Biscay bay, or from the Trans-Canada Highway via Highways 6 and 7. Within the area, there is a secondary gravel road almost parallel to the shoreline. Inland, the area is barren and easy to traverse in all directions. Results of mapping completed in the summer of 1967 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 of the Harbour Main Group intruded by the Holyrood Granite (Rose, 1952; McCartney, 1967). These volcanic and plutonic rocks supplied most of the detritus composing the successively overlying Conception and Cabot Groups. The latter groups are gradationally conformable in most parts of the peninsula, except in the Torbay map area where Rose (1952) reported a disconformity between them. In the Biscay BayCape Race area, these groups comprise a conformable succession about 8000 ft thick with base unexposed. The Conception Group consists of cherts, argillites, siltstones, and graywackes and the St. John's Formation of the Cabot Group consists of shales. The St. John's Formation outcrops on either side of the Conception 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 the north, was defined by Rose (1952) as a thick sequence of sedimentary rocks overlying the Harbour Main Group and underlying the St. John's Formation of his Cabot Group. He divided the Conception Group into the "Conception slate" and the "Torbay slate" but did not propose a formal classification. In the Harbour Grace area, Hutchinson (1953) recognized a formal division, the Hibbs Hole formation, and McCartney (1967) applied this name to a correlative unit in the Whitbourne map area. In the Biscay Bay-Cape Race area, the Conception Group can be divided into three formations (table 1) here named in order of decreasing age: Drook, Freshwater Point, and Cape Cove Formations. Although these rock units are separated by gradational boundaries, each has characteristics that distinguish it form the overlying and underlying formation. [TOP]

Drook Formation

The "Drook Formation" is the name proposed for about 2500 ft. of well-bedded, gently folded, hard siliceous siltstones and cherts, oldest rocks of the area (Fig. 5A). The type area is Drook, 3 mi southeast of Portugal Cove South (Fig. 4). Apart from the coastal exposures, the formation crops out only along Drook River. many of the chert beds are silicified 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 more accentuated on the exposed surface. Qualitative x-ray analysis of the cherts and argillites of the Drook /formation shows that the most common constituents are quartz, albite, chlorite, and sericite; minor constituents are epidote, sphene, and leucoxene. Clay minerals are absent. Calcite is found only as concretions.

The base of the formation is not exposed, and the contact relation between the conception Group and the underlying rocks is unknown; the oldest beds of the formation exposed along the Drook River are almost pure chert. The gradational upper boundary of the formation is marked by a gradual increase in the argillaceous and arenaceous material and decrease in silicification. [TOP]

Freshwater Point Formation

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

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

Cape Cove Formation

The name "Cape Cove Formation" is proposed for about 2900 ft of graded beds that are distinguished form the Freshwater Point and 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. The type section is exposed along the northern coast of Cape Cove near Cape Race. In addition to coastal section, the formation is also exposed along Portugal Cove Brook and Briscal Cove River. The main part of the Cape Cove formation comprises graded beds of graywacke and well-cleaved, green argillite. The beds are as much as 10 ft thick and maintain a uniform thickness along strike for hundreds of feet. The graywacke in the graded beds makes up as much as 60 percent of the middle part of the Cape Cove Formation. The upper part of the formation includes purple argillites and graywackes that are separated form the overlying St. John's Formation by a transition zone of about 400ft. In addition to graded bedding, the sedimentary structures occurring in the formation include ripple marks, sole marks (Fig. 6B) , convolute bedding, and locally small-scale cross-stratification.

 

Detail of bedding in the Drook cherts 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 the sandstones is their bimodal size distribution: abundant matrix separates large grains and rock fragments. All rock fragments are of rock types similar to those now exposed in the Harbour Main Group and the Holyrood plutonic series. No metamorphic rock fragments were recognized. The composition determined by modal analysis indicates that detrital quartz forms 22 to 38 percent of the rock. The matrix comprises 40 to 55 percent, and is composed of chlorite, biotite, sphene, epidote, leucoxene, pyrite, and apatite. Mineral constituents of the siltstones and argillites are quartz, feldspar, chlorite, and sericite. Rock fragments are absent in the siltstones and argillites, but these finer-grained rocks have a higher proportion of opaque minerals.

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

Cabot Group

The Cabot Group defined outside the area to the north (Rose, 1952) includes three conformable sedimentary formations which in chronologic 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" by Jukes, 1843, "Aspidella slates" by Murrayand Howley, 1881 (Both in McCartney, 1967), and "Momable slates" by Walcott (1899). The name "St, John's Formation" was proposed by Rose (1952) for the sequence overlying the Conception Group and underlying the Signal Hill Formation of his Cabot Group.

The St. John's Formation in the map area Consists of about 1100 ft of well-cleaved (Fig. 6D) dark- to light-gray shale that overlies coastal exposures: the first from Cape Race to Shingle Head and the second from Biscay Bay to portugal Cove south. Apart from these coastal exposures, the formation is found in several outcrops along back River and also along several broks 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 zone immediately below, commonly contain large euhedral crystals of pyrite. Disseminated pyrite if ubiquitous. The baseal part of the formation near Shingle Head includes a layer of volcanic tuff about 2 ft thich that is stratigraphically only a few hundred feet above the fossil-bearing horizon in the underlying Cape Cove Formation. This is the first report of Volcanism in the St. 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 predominantly thin-bedded, gray shale with intercalated sandy streaks (Fig. 6D). Fracture cleavage is very pronounced and obscures bedding in some places; but in other place, the relation between cleavage and bedding is distinct (fig. 6D). primary features of the formation include cross-stratification and slumping. The slump structures (figs. 7A, 7BV, 7C and 7D) were produced by contortion of beds which was governed, among other things, by the relative competence of the tow lithologies in juxtaposition at the time of slumping . chert, clayey sediments, shale, and calcareous sediments give rise to better developed slump structures that do silty argillite and graded sandstone which are almost devoid of such features. The rocks are medium to fine grained; microscopic examination reveals quartz. feldspar, mica, chlorite, and pyrite. The accessory minerals are the same as in the Conception argillite, except that pyrite is more common and calcite more frequent. In site replacement of quartz by calcite is indicated by parchy extinction in fine-grained sandstone and indistinguishable boundaries between calcite and quartz grains. [TOP]

Correlation and Age

Precambrian-Cambrian boundary in Newfoundland is defined by Hutchinson (1962) at the first disconformity in the stratigraphic sequence below the Callavia zone. Rocks of the Harbour Main (a predominantly volcanic sequence), Conception, and Cabot Groups lie stratigraphically below this boundary (Hutchinson, 1953; McCartney, 1967). However, radioactive-age determinations (Fairbairn and other, 1966; McCartney and others, 1966) do not indicate their Precambrian age. Nevertheless, the evidence by stratigraphic correlation is convincing, since lower Cambrian strata "unconformably overlap" the rocks of the Conception Group at many localities in the Avalon Peninsula (Rose, 1952). The bed rock in the map area (Fig. 4) clearly correlates (Misra, 1969a) with the Conception Group and the St. John's Formation of Rose (1952) although it is covered with Pleistocene glacial deposits. The coelenterates reported from the Conception Group (Misra, 1969b) are represented by polyps (Fig. 2B) as well as Medusae (fig. 2A) . M. F. Glaessner (1968, personal commun. ) interpreted the spindle-shaped organisms of the fauna as "a new floating colonial Hydrozoan of the order thecata." In any case, this fauna is older than Coleoloides and 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 and current directions obtained from slumping and cross stratification. The discussion that follows indicates that the environment of deposition of the Drook and Freshwater Point Formations was initially shallow, but became deeper during deposition of the upper part of the Freshwater Point Formation, remained deep during Cape cove time, and became shallow again during St. John's time . A similar sedimentary model is suggested by McCartney (1967) in the Whitbourne map area.

Sedimentation of the Conception Group in the Biscay Bay-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 sedimentary rocks that he included in the Harbour Main Group and that are strikingly similar to those of the Conception group. These sediments assigned to the Harbour Main Group are possibly the beginnings of the Conception Group deposition because locally the underlying volcanic rocks and Conception Group rocks are inter-bedded in transition zones. Subsequently, the isolated basins probably joined to form a shallow-water marine environment. Such an environment of deposition is indicated for the rocks of the Drook and Freshwater Point Formations by high content of silica in the sediments, calcareous nodules (Fig. 6A), megaripple marks, 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 system had increased; the environment of deposition gradually became deeper, and by the close 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. The main 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 surface current (Kuenen and Migliorini, 1950). Furthermore, (5) the great thickness of some graded beds indicates a current of abnormal energy. Finally, (6) the sandstones at the base of the graded beds contain abundant matrix which indicates that deposition of fine material has taken place simultaneously with that of large particles. After deposition of a substantial thickness of the Cape Cove Formation, the seaward slop upon newly deposited detritus increased progressively. During this increase, submarine slumping may have been initiated by agents such as wave action during heavy storms, earthquakes, or volcanism, or by abundant supply of sediments. Slump structures (Fig.s 7A, 7B, 7C and 7D) associated with some beds in the lower part of the St. John's Formation indicate that deposition was still taking place on a sloping surface and that currents which originated in shallow water probably flowed 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 the turbidity currents became more dilute and contained relatively more mud than large particles. Small-scale cross-bedding and ripple lamination, which are in some cases associated with flame structures, were formed by bottom traction and not 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 reducing environment. [TOP]

Acknowledgements

This study was conducted as part of my graduate work at Memorial 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 his Geological Survey of Canada grants. I am grateful to Professor Brueckner for financial assistance, field supervision, and for suggesting the area. I am thankful to E.R.W. Neale, H. Williams, and M.M. Anderson of Memorial University and to J.A. Donaldson of Carleton University, Ottawa, for reading the paper and making helpful suggestions. Field assistance of Donald J. Fitzpatric and Paul thompson of Memorial University is gratefully acknowledged. Thanks are due to W.Marsh of Memorial University for help in photography. [TOP]

References Cited

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  • ANDERSON, M.M.  and Misra, S.B., (1968) Fossils found in pre-Cambrian Conception Group of Southeastern Newfoundland. Nature, v. 220, no. 5168 pp. 680-681.
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