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

Stratigraphy and Depositional History of Late Precambrian Coelenterate-Bearing Rocks, Southeastern Newfoundland

ABSTRACT mained shallow during post-Conception times. During deposition of the St. John's In the map area, the Conception Group Formation, the intensity of turbidity currents and the overlying St. John's Formation of decreased. Also, mild volcanism, here re- the Cabot Group constitute a continuous, ported for the first time, occurred during conformable succession of Precambrian sedi- early St. John's time. mentary rocks about 8000 ft thick, beneath an overburden of glacial drift; the base of the INTRODUCTION sequence is unexposed. Varied rocks of the Fossils of late Precambrian organisms in Conception Group are predominantly green the rocks of Conception Group are the most and purple and the St. John's shales are significant aspect of geology of the Biscay various shades of gray. The Conception Bay-Cape Race area (Anderson and Misra, Group is divisible into three lithologic units: 1968; Misra, 1969a, 1969b). The fossils are Drook, Freshwater Point, and Cape Cove well-preserved imprints (Figs. 1, 2) of soft- Formations, in order of decreasing age. The Drook Formation is composed mainly of chert, siliceous argillite, and siltstone; the Freshwater Point Formation is siliceous argil- lite with minor sandstone; and the Cape Cove Formation is composed of cyclic beds that grade from 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. 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 Con- ception 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 thick- ness of the Conception Group had been de- posited, the sea became shallow again, per- Figure 1. Precambrian fossil impressions haps during deposition of the uppermost on the fractured and jointed rocks of the Cape part of the Cape Cove Formation, and re- Cove Formation.

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

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Whitbourne map area (Fig. 3). The area is accessible by road from St. John's, 100 mi 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).

STRATIGRAPHY The oldest rocks in the Avalon Peninsula are those of the Harbour Main Group in- truded by the Holyrood Granite (Rose, 1952; McCattney, 1967). These volcanic and plu- tonic rocks supplied most of the detritus composing the successively overlying Con- ception and Cabot Groups. The latter groups are gradationally conformable in most parts Figure 2A. Medusoid organism exhibiting alternate lobes and depressions. of the peninsula, except in the Torbay map area where Rose (1952) reported a discon- formity between them. In the Biscay Bay- Cape 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 anti- clinorium with its axis trending northeast Figure 2B. Spindle-shaped colonial organ- ism showing bilateral symmetry. and passing through Freshwater Bay. This regional structure is referred to as Drook Anticlinorium. bodied coelenterates of uncertain habitat. The organisms possibly were floating and colonial (Misra, 1969b). There are no direct CONCEPTION GROUP evidences of bathymetry nor of salinity. The Conception Group, in the Torbay Therefore, a detailed study of various other map area to the north, was defined by Rose aspects of the geology such as stratification (1952) as a thick sequence of sedimentary and sedimentation are necessary, in order to rocks overlying the Hatbour Main Group understand the ecologic conditions in the and underlying the St. John's Formation of Conception sea. The information provided his Cabot Group. He divided the Conception in the following pages will serve this purpose Group into the "Conception slate" and the in part, until more details are available. "Torbay slate" but did not propose a formal Prior to the present work, geologic studies classification. In the Harbour Grace area, in the Avalon Peninsula have been conducted Hutchinson (1953) recognized a formal divi- in the Torbay (Rose, 1952), Harbour Grace sion, the Hibbs Hole Formation, and McCart- (Hutchinson, 1953), Whitbourne (McCart- ney (1967) applied this name to a correlative ney, 1967), and St. John's (Brueckner, 1971) unit in the Whitbourne map area. In the areas. These studies, however, concern only Biscay Bay-Cape Race area, the Conception the northern part of the peninsula; the present Group can be divided into three formations study is the first in the peninsula south of the (Table 1) here named in order of decreasing

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age: Drook, Freshwater Point, and Cape Cove Drook Formation Formations. Although these rock units are separated by gradational boundaries, each The "Drook Formation" is the name pro- has characteristics that distinguish it from the posed for about 2500 ft of well-bedded, overlying and underlying formations. gently folded, hard siliceous siltstones and

48°

47°

Figure 3. Map of the Avalon Peninsula of Newfoundland showing the mapped area (dotted part). The other mapped areas of the peninsula are also shown (from Misra, 1969b).

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TABLE 1. TABLE OF FORMATIONS

Age Group Formation Lithology

Pleistocene Glacial drift Cabot St. John's Well-cleaved gray shales with 1100 ft sandstone laminae and sandy streaks Gradational boundary Conception Cape Cove Graded beds of graywackes, 2900 ft siltstones, and well-cleaved green argillites; purple argillites and graywackes in the upper part of the formation Gradational boundary Freshwater Point Green siliceous argillites and 1500 ft siltstones with some graywackes Gradational boundary Drook Banded cherts with silicified 2500 ft siltstones and argillites

cherts, oldest rocks of the area (Fig. 5 A). The is complete. The formation is composed type area is Drook, 3 rni southeast of Portu- mainly of graded siliceous argillites and silt- gal Cove South (Fig. 4). Apart from the stones, but minor amounts of fine- to medium- coastal exposures, the formation crops out grained sandstones occur at the base of some only along Drook River. Many of the chert graded units. The sandstones are made up of beds are silicified siltstones varying in thick- subangular to subrounded grains of quartz, ness ftom a fraction of an inch to 2 in. Each feldspar, and rock fragments set in a matrix bed has its characteristic color, usually a of chlorite, sericite, epidote, leucoxene, and shade of green that is more accentuated on sphene. The argillites are commonly com- the exposed surface. Qualitative x-ray analy- posed of quartz, albite, chlorite, and sericite. sis of the cherts and argillites of the Drook Clay minerals are absent. The fine-grained Formation shows that the most common sediments of the formation are green, the constituents ate quartz, albite, chlorite, and sandstones are gray, and the weathered rocks sericite; minor constituents are epidote, are whitish. sphene, and leucoxene. Clay minerals are Although the main rock type of the Fresh- absent. Calcite is found only as concretions. water Point Formation is argillites, the upper The base of the formation is not exposed, part includes graded beds composed of sand- and the contact relation between the Con- stone at the bottom through siltstone to ception Group and the underlying rocks is argillite at the top. In these graded beds, the unknown; the oldest beds of the formation basal sandy part makes up less than 20 per- exposed along the Drook River are almost cent of the graded units—a criterion applied pure chert. The gradational upper boundary to draw the upper boundary of the formation. of the formation is marked by a gradual in- crease in the argillaceous and arenaceous ma- Cape Cove Formation terial and decrease in silicification. The name "Cape Cove Formation" is pro- posed for about 2900 ft of graded beds that Freshwater Point Formation are distinguished from the Freshwater Point The name "Freshwater Point Formation" and St. John's Formations based on color, is proposed for about 1500 ft of predomi- cleavage, and graywacke percentage, with nantly argillaceous beds exposed in coastal emphasis on change of color from purple to areas on the western and eastern limbs of the gray at the upper boundary. The type section Drook andclinorium. The western coastal is exposed along the northern coast of Cape section near Daly's Point is interrupted by a Cove near Cape Race. In addition to coastal boulder beach, but the eastern coastal section sections, the formation is also exposed along

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Portugal Cove Brook and Briscal Cove River. ring in the formation include ripple marks, The main part of the Cape Cove Formation sole marks (Fig. 6B), convolute bedding, and comprises graded beds of graywacke and locally small-scale cross-stratification. well-cleaved, green argillite. The beds are as The most striking petrographic feature of much as 10 ft thick and maintain a uniform the sandstones is their bimodal size distribu- thickness along strike for hundreds of feet. tion: abundant matrix separates large grains The graywacke in the graded beds makes up and rock fragments. All rock fragments are of as much as 60 percent of the middle part of rock types similar to those now exposed in the Cape Cove Formation. The upper part of the Harbour Main Group and the Holyrood the formation includes purple argillites and plutonic series. No metamorphic rock frag- graywackes that are separated from the over- ments were recognized. The composition lying St. John's Formation by a transition determined by modal analysis indicates that zone of about 400 ft. In addition to graded detrital quartz forms 22 to 38 percent of the bedding, the sedimentary structures occur- rock. The matrix comprises 40 to 55 percent,

Figure 5. A. Detail of bedding in the Drook Cove Formation. The hammer is resting on the cherts. B. Stratification in the rocks of Fresh- sharp contact between 2 graded units. D. Inter- water Point Formation. Thin sandstone beds beds of shales and sandstones in the St. John's are weathered more easily due to differential Formation. weathering. C. Graded bedding in the Cape

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and is composed of chlorite, biotite, sphene, Hill, and Black Head Formations. Only the epidote, leucoxene, pyrite, and apatite. Min- St. John's Formation is exposed in the map eral constituents of the siltstones and argil- area. lites are quartz, feldspar, chlorite, and sericite. Rock fragments are absent in the siltstones St. John's Formation and argillites, but these finer-grained rocks The formation was named "St. John's have a higher proportion of opaque minerals. slate" by Jukes, 1843, "Aspidella slates" by In the coastal exposures near Mistaken MurrayandHowley,1881(both/#McCartney, Point, the formation contains organic im- 1967), and "Momable slates" by Walcott prints that represent the fauna of the Con- (1899). The name "St. John's Formation" ception sea during late Precambrian time. was proposed by Rose (1952) for the se- Organic origin of the imprints is evident quence overlying the Conception Group and (Misra, 1969b; Anderson and Misra, 1969) underlying the Signal Hill Formation of his based on: (l) mode of preservation and Cabot Group. orientation (Fig. l), (2) size variation which The St. John's Formation in the map area may be ontogenic, (3) symmetry (Fig. 2B), consists of about 1100 ft of well-cleaved (Fig. and (4) resemblance to known Precambrian 6D) dark- to light-gray shale that overlies fossils. The organisms are grouped into four the Cape Cove Formation. There are two categories (Misra, 1969b): spindle-shaped, coastal exposures: the first from Cape Race round lobate, leaf-shaped, and dendrite-like. to Shingle Head and the second from Biscay Bay to Portugal Cove South. Apart from these Cabot Group coastal exposures, the formation is found in The Cabot Group defined outside the area several outcrops along Back River and also to the north (Rose, 1952) includes three con- along several brooks near Cape Race. The formable sedimentary formations which in basal part of the St. John's Formation is chronologic order are the St. John's, Signal gray, well-cleaved, thin-bedded shale inter-

Figure 6. A. Ellipsoidal calcareous sand- tional conglomerates in the siliceous argillites stone nodules in the Drook Formation at Drook. of the Freshwater Point Formation. D. Well- B. Sole marks on the undersurface of a graded cleaved St. John's shales showing deflection of bed in the Cape Cove Formation. C. Intraforma- cleavage as it passes from one layer to the other.

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calated with sandstone laminae (Fig. 5D). tion of beds which was governed, among Sandstone beds in this part of the formation, other things, by the relative competence of and also in the transition zone immediately the two lithologies in juxtaposition at the below, commonly contain large euhedral time of slumping. Chert, clayey sediments, crystals of pyrite. Disseminated pyrite is shale, and calcareous sediments give rise to ubiquitous. The basal part of the formation better developed slump structures than do near Shingle Head includes a layer of vol- silty argillite and graded sandstone which are canic tuff about 2 ft thick that is strati- almost devoid of such features. The rocks graphically only a few hundred feet above are medium to fine grained; microscopic ex- the fossil-bearing horizon in the underlying amination reveals quartz, feldspar, mica, chlo- Cape Cove Formation. This is the first report rite, and pyrite. The accessory minerals are of volcanism in the St. John's Formation. the same as in the Conception argillite, ex- The main part of the formation is pre- cept that pyrite is more common and calcite dominantly thin-bedded, gray shale with more frequent. In situ replacement of quartz intercalated sandy streaks (Fig. 6D). Fracture by calcite is indicated by patchy extinction in cleavage is very pronounced and obscures fine-grained sandstone and indistinguishable bedding in some places; but in other places, boundaries between calcite and quartz grains. the relation between cleavage and bedding is distinct (Fig. 6D). Primary features of the CORRELATION AND AGE formation include cross-stratification and Precambrian-Cambrian boundary in New- slumping. The slump structures (Figs. 7A, foundland is defined by Hutchinson (1962) 7B, 1C, and 7D) were produced by contor- at the first disconformity in the stratigraphic

Figure 7. A. Recumbent and overturned Slump folding and faulting in the St. John's slump folding in the St. John's Formation. B. Formation near Cripple Cove. D. Pseudo nodule Slump folds near Fisher's Point. Axes of the produced by slumping in the St. John's Forma- folds are almost vertical and normal stratifica- tion. tion is distinctly seen in the background. C.

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sequence below the Callavia zone. Rocks of bour Main Group and that are strikingly the Harbour Main (a predominantly volcanic similar to those of the Conception Group. sequence), Conception, and Cabot Groups These sediments assigned to the Harbour lie stratigraphically below this boundary Main Group are possibly the beginnings of (Hutchinson, 1953; McCartney, 1967). How- the Conception Gtoup deposition because ever, radioactive-age determinations (Fair- locally the underlying volcanic rocks and bairn and others, 1966; McCartney and others, Conception Group rocks are interbedded in 1966) do not indicate their Precambrian age. transition zones. Subsequently, the isolated Nevertheless, the evidence by stratigraphic basins probably joined to form a shallow- correlation is convincing, since lower Cam- water marine environment. Such an environ- brian strata "unconformably overlap" the ment of deposition is indicated for the rocks rocks of the Conception Group at many of the Drook and Freshwater Point Forma- localities in the Avalon Peninsula (Rose, tions by high content of silica in the sedi- 1952). The bed rock in the map area (Fig. 4) ments, calcareous nodules (Fig. 6A), mega clearly correlates (Misra, 1969a) with the ripple marks, beds exhibiting waviness (Rich, Conception Group and the St. John's Forma- 1951), and intraformational structures (Fig. tion of Rose (1952), although it is covered 6C). In the Freshwater Point Formation, with Pleistocene glacial deposits. The co- graded bedding, grain size, thickness of beds, elenterates reported from the Conception and lithology suggest that energy of the sedi- Group (Misra, 1969b) are represented by mentary system had increased; the environ- polyps (Fig. 2B) as well as Medusae (Fig. ment of deposition gradually became deeper, 2A). M. F. Glaessner (1968, personal and by the close of deposition of the Fresh- commun.) interpreted the spindle-shaped or- water Point Formation, the sea had become ganisms of the fauna as "a new floating deep enough for turbidity currents of large colonial Hydrozoan of the order Thecata." magnitude. In any case, this fauna is older than Coleoloides Rocks of the Cape Cove Formation were and Hyolithes which were succeeded by Calavia probably deposited by turbidity currents. The fauna. main arguments in favor of this are: (l) lack of evidence of tidal action, (2) presence of DEPOSITIONAL HISTORY sole marks (Fig. 6B), (3) large volume of OF THE AREA graded beds, and (4) consistency in the thick- The sediments of the Conception Group ness of beds and direction of supply. Flute and the St. John's Formation were derived casts (Fig. 6B) indicate that the current from a complex terrane consisting of vol- hugged the bottom and was not the surface canic, igneous, and sedimentary rocks, situ- current (Kuenen and Migliorini, 1950). Fur- ated to the northeast of present exposures. thermore, (5) the great thickness of some The conclusions regarding the source area graded beds indicates a current of abnormal are drawn from rock constituents and current energy. Finally, (6) the sandstones at the directions obtained from slumping and cross- base of the graded beds contain abundant stratification. The discussion that follows in- matrix which indicates that deposition of dicates that the environment of deposition fine material has taken place simultaneously of the Drook and Freshwater Point Forma- with that of large particles. After deposition tions was initially shallow, but became deeper of a substantial thickness of the Cape Cove during deposition of the upper part of the Formation, the seaward slope upon newly Freshwater Point Formation, remained deep deposited detritus increased progressively. during Cape Cove time, and became shallow During this increase, submarine slumping again during St. John's time. A similar sedi- may have been initiated by agents such as mentary model is suggested by McCartney wave action during heavy storms, earth- (1967) in the Whitbourne map area. quakes, or volcanism, or by abundant supply Sedimentation of the Conception Group of sediments. Slump structures (Figs. 7A, in the Biscay Bay-Cape Race area probably 7B, 7C, and 7D) associated with some beds started in isolated basins bounded by vol- in the lower part of the St. John's Formation canic tocks of the earlier Harbour Main indicate that deposition was still taking place Group. Such isolated basins are envisaged by on a sloping surface and that currents which McCartney (1967), in the case of the sedi- originated in shallow water probably flowed mentary rocks that he included in the Har- down a slope to the deeper areas. However,

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siltstone laminae of the St. John's Formation W. H., Jr.; and Hurley, P. M. Whole (Fig. 5D) could not be the result of separate rock age and initial 87Sr/86Sr of volcanics turbidity flows. This type of lamination is underlying fossiliferous lower Cambrian in explained by traction transport that started the Atlantic Provinces of Canada: Can. J. as the turbidity currents became more dilute Earth Sci., Vol. 3, p. 509-521, 1966. Hutchinson, R. D. Geology of Harbour Grace and contained relatively more mud than large map area, Newfoundland: Can. Geol. particles. Small-scale cross-bedding and rip- Surv., Mem., No. 375, 43 p., 1953. ple lamination, which are in some cases Hutchinson, R. D. Cambrian stratigraphy and associated with flame structures, were formed trilobite fauna of southeastern Newfound- by bottom traction and not by turbulence. land: Can., Geol. Surv., Mem., No. 88, The presence of pyrite together with dark 42 p., 1962. gray color of the shales suggest that the Kuenen, Ph. H.; and Migliorini, C. I. St. John's Formation was deposited in a re- Turbidity currents as a cause of graded ducing environment. bedding: J. Geol., Vol. 58, p. 91-127, 1950. McCartney, W. D. Whitbourne map area, ACKNOWLEDGMENTS Newfoundland: Can., Geol. Surv., Mem., No. 341, 135 p., 1967. This study was conducted as part of my McCartney, W. D.; Poole, W. H.;Wanless, graduate work at Memorial University of R. K.; Williams, H.; and Loveridge, Newfoundland, and was financed for the W. D. Rb/Sr age and geological setting most part by W. D. Brueckner's National of the Holyrood Granite, southeast New- Research Council (Canada) Grant No. 882 foundland: Can. J. Earth Sci., Vol. 3, p. and partly by his Geological Survey of 947-957, 1966. Canada grants. I am grateful to Professor Misra, S. B. Geology of the Biscay Bay-Cape Brueckner for financial assistance, field super- Race area, Avalon Peninsula, Newfound- vision, and for suggesting the area. I am land: M.S. thesis, Memorial Univ. of Newfoundland, St. John's, Canada, 139 thankful to E.R.W. Neale, H. Williams, and p., 1969a.* M. M. Anderson of Memorial University and Misra, S. B. Late Precambrian(?) fossils from to J. A. Donaldson of Carleton University, southeastern Newfoundland: Geol. Soc. Ottawa, for reading the paper and making Amer., Bull., Vol. 80, p. 2133-2140, 1969b. helpful suggestions. Field assistance of Rich, John Lyon. Three critical environments Donald J. Fitzpatric and Paul Thompson of of deposition, and criteria of rocks deposited Memorial University is gratefully acknowl- in each of them: Geol. Soc. Amer., Bull., edged. Thanks are due to W. Marsh of Vol. 62, p. 1-20, 1951. Memorial University for help in photography. Rose, E. R. Torbay map area, Newfoundland: Can., Geol. Surv., Mem., No. 265, 64 p., 1952. REFERENCES CITED Walcott, C. D. Precambrian fossiliferous for- mations: Geol. Soc. Amer., Bull., Vol. 10, Anderson, M. W.; and Misra, S. B. Fossils p. 199-224, 1899. found in the pre-Cambrian Conception *Available on interlibrary loan Group of southeastern Newfoundland: Nature, Vol. 220, No. 5168, p. 680-681, 1968. Anderson, M. M.; and Misra, S. B. Criteria MANUSCRIPT RECEIVED BY THE SOCIETY MAY for recognizing pre-Cambrian fossils: 1, 1970 Nature, Vol. 223, No. 5210, p. 1076, 1969. REVISED MANUSCRIPT RECEIVED AUGUST 10, Brueckner, W. D. Geology of the eastern part 1970 of the Avalon Peninsula: Amer. Ass. Petrol. AUTHOR'S PRESENT ADDRESS: DEPARTMENT Geol., Bull., 1971 (in press). OF GEOLOGY, LUCKNOW UNIVERSITY, Fairbairn, H. W.; Bottinc, M. L.; Pinson, Luc KNOW, INDIA

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