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• MISRA, S.B.,(1969a). Geology of Biscay Bay-Cape Race area, Avalon Peninsula, Newfoundland .
  
• ANDERSON M.M..& MISRA S.B., (1968). Fossils found in pre-Cambrian Conception Group of Southeastern Newfoundland 
  
• ANDERSON, M.M..& MISRA S.B., (1969). Criteria for recognizing pre-Cambrian fossils.
  
• MISRA, S.B., (1969b). Late Precambrian(?) fossils from southeastern Newfoundland
  
• MISRA, S.B.,(1971). Stratigraphy and Depositional History of Late Precambrian Coelenterate-bearing rocks, Southeastern Newfoundland
  
• MISRA, S.B., (1981) Depositional Environment of the Late Precambrian Fossil-Bearing rocks of Southeastern Newfoundland, Canada
  

S.B.Misra, Department of Geology, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada

Abstract

Introduction

The map area lies on the eastern part of a two sided Appalachian systemof Williams (1964) and its geology has been described by Misra (1969a; 1969b)and Williams and King (1976). The age of the rock (Anderson and Misra, 1968;)Anderson, 1972 is Precambrian. a short account of the stratigraphy and thedepositional history has been given by Misra (1971) and this paper is intended to deal with the ecological conditions in whichthe animals lived and died in that basin.

The 3200 meter thick sequence of sedimentary rocks is separable into alower Conception Group consisting of banded cherts, green argillites, greywackesand siltstones and an upper St. John's Formation of the Cabot Group (Rose, 1952;McCartney , 1976; Misra, 1969b) comprising shales with sandstone laminae (TableI, Figure 1). The Conception Group of the area was further subdivided (Misra,1969a, 1971) into three formations: (1) Drook Formation, (2) Freshwater PointFormation and (3) Cape Cove Formation, in the chronological order. Williams andKing (1976) have reclassified the rocks of the Conception Group into fiveFormations which in the chronological order are: (1) Mall Bay, (2) Gaskiers, (3Drook, (4) Briscal and (5) Mistaken Point. The first two formations are notexposed in the map area and their stratigraphic position too is doubtful.Williams and King (1976) themselves state that the Gaskiers and Mall BayFormations could be time equivalents of part of the Harbour Main Group. It is therefore preferable to take the base of the Conception Group atthe base of the Drook Formation and the Mall Bay and Gaskiers Formations can betaken as part of the Harbour Main Group. The top of the Conception Group wasdefined by Misra (1969a) at the transition from purple to grey beds. This criterion has been taken by Williams and King (1976) as well. Theyhave, however, subdivided the Cape Cove Formation into : (1) Briscal, (2)Mistaken Point and (3) Trepassey Formations (Table I). Thus., the arbitraryformational boundaries in the gradational succession may vary depending on theparameters chosen but the sequence of depositional events will remain unchanged.

Figure 1. Stratigraphic column of the Biscay Bay-CapeRace Area, Newfoundland

[TOP}

Drook Formation

TheDrook Formation comprises a 1000 meter thick sequence of very hard, compact,uniformly banded cherts and siliceous argillites. The most common constituentsas identified from X-Ray diffraction are quartz. albite, chlorite and sericite(Table II). The secondary constituents are epidote, siderite and leucoxene.Calcite is found only in the form of ellipsoidal, concretionary, flat-bottomnodules (Fig. 6A of Misra, 1971). Large- scale current ripple marks are seen inthe exposures near Drook. [TOP}

Freshwater Point Formation

Overlying the Drook Formation with a gradational boundary are siliceousand chloritic argillites and siltstones with minor proportions of medium to finegrained greywackes similar in composition to the underlying Drook Formation(Table II). The greywackes are composed of sub-angular to sub-rounded grains ofquartz, feldspar and rock fragments set in a matrix of chlorite, sericite,epidote, leucosene and sphene. Syn-sedimentary floundering is locally indicatedby the ellipsoidal and other fragments in the siliceous argillites (Fig. 6C ofMisra, 1971) and pull aparts of sandstone layers within the argillites is alsoseen.[TOP}

Group	Formation		Lithology	Sedimentary Structures
	Williams and King (1976)	Misra (1971)
Labot (St. John's)	Fermuse	St. John's	well cleaved, grey  shales with sandstone laminae and sandy streaks	slump structures convolute bedding, graded and cross bedding, cone-in cone
	Trepassey	Transition	laminated, graded and cross bedded shales	graded and cross bedding
Grad.Contact
	Mistaken Pt	Cape Cove	graded beds of greywackes, siltstones and green argillites, and greywackes at top	graded bedding, sole marks, fossil imprints, current ripples
Conception		Freshwater Point	green, siliceous argillites and siltstones with some greywackes	pull aparts, floundering and graded bedding
	Drook		banded cherts with silicified siltstones and argillites	parallel lamination marked by the chert bands, thixotropic structures and calc. nodules
	Gaskiers	Drook not the area of study	grey tillite with red mudstone and red agglomerate	drop stones glacial striations
	Mall bay		siliceous siltstone, argillites, grey sand stone, chert, tuff and agglomerate; minor limestone
Grad.Contact
Harbour Main			silicic tuff, agglomerate, rhyolite and basalt

Note; All the formational contacts are gradational with passage beds andhave arbitrary.

Cape Cove Formation

Overlying the Freshwater Point Formation with a gradational boundary is a1200 meter thick sequence of uniformly graded greywackes, siltstones andargillites. The proportion of greywackes in the graded units reaches a maximumof about 60 per cent in the middle part of the formation; decreasing graduallyin the overlying beds. The graded units on their top surfaces near MistakenPoint contain imprints or rich Precambrian fauna (Fig. 2A). The bottom of gradedunits are characterized by load casts, flute casts, prod marks, groove casts andgraded bedding (Fig. 2C). The flute casts indicate the current direction towardssouthwest and the prod marks taper out in a S30W direction.

Micrometric analysis shows that the sub-angular to subrounded quartz(Fig. 3C) constitutes 2 to 38 per cent of the greywackes. Albite grains aresimilar in shape

Table II. Variation in the proportion of quartz with respect to other minerals in the argillites as determined by X-ray diffraction. Ratiosare based on peak lengths. Calcite has clearly developedat the expense of quartz.

Formation Sample No. Q/A Q/Chl. Q/Cal Chl./Seri St. John's MCR-26 3.25 3.40 - 1.55 " MCR-18 3.25 3.25 - 1.78 " MCR-2 1.99 2.36 3.20 1.90 " MSH-4 2.61 3.65 1.58 1.59 Cape Cove MCC-25 3.89 3.70 - 1.69 " MLB-20 3.50 4.36 - 1.29 " MLB-46 3.20 3.23 - 1.26 " MFP-2 3.12 3.26 - 1.12 Freshwater Point MCC-3 4.40 4.26 - 1.30 " MPC-24 2.05 2.85 - 1.16 Drook MPC-14 2.90 2.40 - 1.48 " MPC-8 4.00 5.56 - 1.50

Note A = Albite Q = Quartz Chl. = Chlorite Cal. = Calcite Seri. = Sericite

but smaller in size. Some plagioclase grains are sericitized and clouded. Rockfragments are of chert, rhyolite, microgranite and basic volcanics. The matrixconstitutes 40 to 55 per cent of the greywackes and is composed of chlorite,biotite, sphene, epidote, leucoxene, pyrite and rarely apatite. [TOP}

St. John's Formation

The formation in the lower part comprises thin bedded, grey shales interstratifiedwith cross bedded sandy laminae (Fig. 2D), which in some cases are graded withshale tops (Fig. 2F). The formation in its upper part is grey to dark greyshales (Fig. 2E) with sandy streaks. Reduction in the thickness of beds comparedto those of the Conception Group indicates that the energy of the system haddecreased. The alternation of cross bedded with the graded units suggests thatturbidity currents were still in operation. Measurements on crossstratification, sole markings and slumping King (1976) also suggest a transportdirection from northeast of the St. John's faults, slump breccia, slump nodules,sand rolls produced as a result of contortion of beds. Convolute bedding (Fig.3A) and other slump features are more common in the fine grained rocks. Someother noteworthy features are alignment of minerals (fig. 3E) and cone-in-conestructures observed on microscopic scale (Fig. 3F).

The rocks under microscope exhibit quartz, feldspar, mica, chlorite andpyrite. The quartz grains in some calcareous sandstones gradually merge withcalcite in contacts, indicating an incomplete replacement of quartz by calcite.The accessory minerals are normally the same as in the Conception argillites(Fig. 3D) except that pyrite is more common and calcite more frequent. Theresults obtained by X-ray diffraction of the shales reveal that illite andmontmorillonite are absent.[TOP}

EXPLANATION OF FIGURE 2

Precambrian fossils occurring as imprints of jelly fish (near hammerhead), spindle shaped Hydrozoans and other forms, on the top surface of anargillite bed in the Conception Group near Mistaken Point.

Siliceous argillites (light coloured) with intercalated sandy beds (darkcoloured) in the Fresh-water Point Formation. The irregular sandstone layer ispossibly the result of load casting.

2C. Sharp contact between two consecutive graded units in the Cape CoveFormation.

2D. Small - scale cross stratification overlying the shales in the basal part of the St. John's Formation.

2E. Small-scale slump folds on a weathered surface of the shales in the St. John's Formation Pen for scale.

2F. Interbeds of shale and sandstone in the basal part of the St. John'sFormation near Cape Race.

EXPLANATION OF FIGURE 3

3A. Convolute bedding in the calcareous shales of the St. John's Formation.

3B. Flute casts in the greywackes of the cape Cove Formation. Currentdirection from left to right. The scale is same as for 3A.

Photomicrograph of the Cape Cove greywacke showing rock fragments (topleft) and quartz grains (centre ) × 32 (approx).

3D. Photomicrograph showing a sharp contact between two consecutive gradedbeds in the Cape Cove Formation . × 32 (approx).

3E. Photomicrograph showing internal organization in silty shales in the St.John's Formation . Pyrite grains are arranged (bottom) along the bedding plane.×32 (approx) under crossed nicols.

3F. Photomicrograph of a thin calcareous layer showing cone -in -conestructure and stylolites in the St. John's Formation . ×32 (approx.) undercrossed nicols.

DEPOSITIONALHISTORY OF THEAREA

Thehistory of sedimentation requires the probable source of the sediments, theirtransportaion and environment of deposition. The rock fragments and mineralconstituents of the rocks indicate that the sediments were derived from acomplex terrain consisting of volcanic, sedimentary and igneous rocks situatedto the northeast[TOP}

McCartney(1967) and Brueckner (1969), the transport directions of the sediments indicatean area of high relief to the northeast of the present exposures where from thesediments were derived. The slope of the depositional basin as inferred from thesole markings and slump structures was to the southwest.

Sedimentation of the Conception Group started probably in isolated basinsbound by volcanic rocks of the earlier Harbour Main Group. This situationexisted at least in the eastern part of the Avalon Peninsula and similarisolated basins are envisaged by McCartney (1967) in the case of the sedimentsthat he included in the Harbour Main Group. These sediments assigned to theHarbour Main Group are possibly the beginning of the Conception Group depositionbecause locally the underlying volcanic rocks and Conception Group rocks areinterbedded in the transition zone. The rocks of the mall Bay Formation (williamsand King, 1976) reported from outside the area, seem to have this transitionalcharacter. The glacial conditions represented by the tillites of the GaskiersFormation mark an important time plane (Anderson and Bruecknerm 1971) and may bethe cause of the extensive greencolour of the sediments.

The present study is confined to the rocks younger than the GaskiersFormation. The presence of limestone in the lower part of the Conception Group (Mccartney,1967) and the calcareous nodules (Misra, 1971) indicate warm, clear water ofdeposition. The presence of pillow basalt within the Drook Formation and aproximal volcanic terrane (Williams and King, 1976) during the deposition of thelower Conception Group suggest near shore conditions for the Drook Formation.The other evidences of a shallow marine environment are penecontemporaneousdeformation, calcareous nodules, mega ripple marks, thixotropic deformation(Misra, 1969a, 1971) and enormous silica precipitate. Even if it is concededthat some of the silica beds are silicified siltstone, most of the siliceoussediments (Fig. 5A of Misra, 1971) comprising a thickness of over 100 meters hadtheir origin in the underlying volcanic rocks of the Harbour Main Group. Uniformparallel lamination in the cherts expressed by the colour bands suggeststectonic stability.

The depositional framework of the Freshwater Point Formation was not muchdifferent from that of the Drook Formation except that the influx of theterrigenous sediments increased considerably towards the close of the formation.A shallow water environment for the Freshwater Point Formation is inferred fromthe type of lithology, bedding characters (Fig. 2B) and synsedimentary features(Table I). In the upper part of the formation, graded bedding, grain size,thickness of beds and lithology suggest that the energy of the depositionalsystem had increased. The environment of deposition gradually became deeper andby the close of the deposition of the Freshwater Point Formation, the sea hadbecome deep enough for turbidity currents of large magnitude. This change in thestability of the depositional basin corresponds with the Freshwater Point andCape Cove boundary and has possibly been taken by Williams and King (1976) asthe base of the Briscal Formation.

The rocks belonging to the Cape Cove formation were deposited byturbidity currents in a basin whose northeasterly trending axis paralleled thepresent strike of the beds. The main arguments in favour of this deduction are:

• Absence of wave causeddisturbances,
• Presence of sole marks includingflute casts (fig. 3B) and prod marks formed by unidirectional flow,
• Deposition dominantly from suspensiongiving rise to graded bedding (fig. 2C),
• Uniformity in the direction ofsupply,
• Prevailing muddyness andsimultaneous deposition of fine and coarse material resulting in greywackes(Fig. 3C).
• Large volume and areal spread ofthe graded beds indication high energy of the depositional system,
• Thickness of individual bedsindication high energy of depositional system, and
• Well marked,uniform, repeated bedding of greywackes with sharp contacts
  (Fig. 3D).

After a substantial thickness of the Cape Cove Formation was deposited,the seaward slope upon the newly deposited detritus increased progressively.During this period of progressive increase in the slope of the basin, submarineslumping could have been initiated by agents like deep reaching wave actionduring heavy storm, minor earthquakes, vulcanism (Misra, 1969a) or by abundantsupply of sediments during floods. Large-scale slump structures in the lowerpart of the St. John's Formation (Misra, 1971) indicate that deposition wastaking place on a gently sloping surface. The laminated siltsones (Fig. 2F)about 10 layers per foot, ofseparate composition in the St. John's Formation indicate that the load wascarried by traction currents. Small-scale cross lamination (Fig. 2D) and crossripple laminations were formed by the currents having relatively high lutitecontent and small amounts of large particles. The presence of pyrite (fig. 3E) and the grey colour of the sediments of the St. John'sFormation suggest that the depositional environmentwas and the cone-in-cone structures (Fig. 3F) are of syn-diagenetic nature orlater.[TOP}

ECOLOGICALCONDITIONS OF THE ANIMALS

Withinthe above frame work of the sedimentary basin and environmental condition, theexistence of the Precambrian animals of the Conception sea can be discussed .Imprints of animals are found on the top surfaces of the graded beds in theupper part of the Cape cove Formation (Mistaken Point Formation) and thesediments enclosing them are of turbidite origin. The purple colour of thesediments is characteristic and may indicate a humid tropical climate asinterpreted elsewhere by Wlaker (1974).

The animal imprints were grouped by Misra (1969b) into four categories:(1) spindle-shaped organisms with bilateral symmetry (Fig. 2A), (2) leaf-shapedorganisms with stalk and hold-fast. (3) round lobate (jelly fish) forms (Fig.2A) and dendrite like organisms. The fossils are believed to be the imprints ofsoft-bodied Metazoans (Coelenterates and others) represented by Polyps as wellas Medusae. The Polyps include both colonial Hydrozoans and Pennatulid corals.the round lobate organisms are jelly fishes and are represented by severalextinct species. The Mistaken Point and continue throughout the formation. Inmost of the cases, they are covered by a thin film of volcanic tuff (Misra,1969a,p. 102) suggesting that the volcanoes was responsible, at least partly,for the death of the organisms. Since the fossils are found in a turbidite sequence of graded greywackes,siltstones, and argillites containg load casts, flute casts, prod marks andconvolute bedding, the animals must have lived in a moderately deep environment(Misra, 1969b). They were living probably at the bottom of the sea during theintervening quiescent period between two successive turbidity currents and liveduntil they were buried by sediments brought by suffocating turbidity currents.Submarine volcanism was also an important cause of their death. In any case, thesurface of mud on top of the graded units provided the ideal conditions forpreserving the imprints.

Since the fossils are found only as impressions, no part of their bodypreserved, it suggests that they were soft bodied Metazoans that constituted aflourishing fauna during the Late Precambrian time in the Conception sea. Thefauna is supposed to be older than the Ediacara fauna of Australia. The fossilsare preserve in situ asindicated by the preservation of the soft bodied animals in entirety. Besides,the presence of bent or curled animal imprints and segmented forms in the faunatogether with the organisms with the hold fast (Misra, 1969b) are evidences oftheir bottom dwelling habit.[TOP}

ACKNOWLEDGEMENT

Iam grateful to late Prof. W.D. Brueckner of the Memorial University ofNewfoundland for financial assistance and guidance, to Prof. M.M. Anderson forproviding some of the recent literature and to Prof. K.S.Valdiya of KumaunUniversity, Nainital, India for critical reading of this paper. [TOP}

References

(Received:May 3, 1980; Revised form accepted: Mar. 26, 1981)

Journal of the Geological Society of India, Vol.22,No.8,1981.