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S.B.Misra, Department of Geology, Memorial University of Newfoundland, St. Johnís, Newfoundland, Canada
The map area lies on the eastern part of a two sided Appalachian system of 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 the depositional history has been given by Misra (1971) and this paper is intended to deal with the ecological conditions in which the animals lived and died in that basin.
The 3200 meter thick sequence of sedimentary rocks is separable into a lower Conception Group consisting of banded cherts, green argillites, greywackes and siltstones and an upper St. John's Formation of the Cabot Group (Rose, 1952; McCartney , 1976; Misra, 1969b) comprising shales with sandstone laminae (Table I, Figure 1). The Conception Group of the area was further subdivided (Misra, 1969a, 1971) into three formations: (1) Drook Formation, (2) Freshwater Point Formation and (3) Cape Cove Formation, in the chronological order. Williams and King (1976) have reclassified the rocks of the Conception Group into five Formations which in the chronological order are: (1) Mall Bay, (2) Gaskiers, (3 Drook, (4) Briscal and (5) Mistaken Point. The first two formations are not exposed in the map area and their stratigraphic position too is doubtful. Williams and King (1976) themselves state that the Gaskiers and Mall Bay Formations could be time equivalents of part of the Harbour Main Group. It is therefore preferable to take the base of the Conception Group at the base of the Drook Formation and the Mall Bay and Gaskiers Formations can be taken as part of the Harbour Main Group. The top of the Conception Group was defined by Misra (1969a) at the transition from purple to grey beds. This criterion has been taken by Williams and King (1976) as well. They have, however, subdivided the Cape Cove Formation into : (1) Briscal, (2) Mistaken Point and (3) Trepassey Formations (Table I). Thus., the arbitrary formational boundaries in the gradational succession may vary depending on the parameters chosen but the sequence of depositional events will remain unchanged.
Figure 1. Stratigraphic column of the Biscay Bay-Cape Race Area, Newfoundland
The Drook Formation comprises a 1000 meter thick sequence of very hard, compact, uniformly banded cherts and siliceous argillites. The most common constituents as 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-bottom nodules (Fig. 6A of Misra, 1971). Large- scale current ripple marks are seen in the exposures near Drook. [TOP}
Freshwater Point Formation
Overlying the Drook Formation with a gradational boundary are siliceous and chloritic argillites and siltstones with minor proportions of medium to fine grained greywackes similar in composition to the underlying Drook Formation (Table II). The greywackes are composed of sub-angular to sub-rounded grains of quartz, feldspar and rock fragments set in a matrix of chlorite, sericite, epidote, leucosene and sphene. Syn-sedimentary floundering is locally indicated by the ellipsoidal and other fragments in the siliceous argillites (Fig. 6C of Misra, 1971) and pull aparts of sandstone layers within the argillites is also seen.[TOP}
Note; All the formational contacts are gradational with passage beds and have arbitrary.
Overlying the Freshwater Point Formation with a gradational boundary is a 1200 meter thick sequence of uniformly graded greywackes, siltstones and argillites. The proportion of greywackes in the graded units reaches a maximum of about 60 per cent in the middle part of the formation; decreasing gradually in the overlying beds. The graded units on their top surfaces near Mistaken Point contain imprints or rich Precambrian fauna (Fig. 2A). The bottom of graded units are characterized by load casts, flute casts, prod marks, groove casts and graded bedding (Fig. 2C). The flute casts indicate the current direction towards southwest 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 are similar in shape
Table II. Variation in the proportion of quartz with respect to other minerals in the argillites as determined by X-ray diffraction. Ratios are based on peak lengths. Calcite has clearly developed at the expense of quartz.
but smaller in size. Some plagioclase grains are sericitized and clouded. Rock fragments are of chert, rhyolite, microgranite and basic volcanics. The matrix constitutes 40 to 55 per cent of the greywackes and is composed of chlorite, biotite, sphene, epidote, leucoxene, pyrite and rarely apatite. [TOP}
The formation in the lower part comprises thin bedded, grey shales interstratified with cross bedded sandy laminae (Fig. 2D), which in some cases are graded with shale tops (Fig. 2F). The formation in its upper part is grey to dark grey shales (Fig. 2E) with sandy streaks. Reduction in the thickness of beds compared to those of the Conception Group indicates that the energy of the system had decreased. The alternation of cross bedded with the graded units suggests that turbidity currents were still in operation. Measurements on cross stratification, sole markings and slumping King (1976) also suggest a transport direction 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. Some other noteworthy features are alignment of minerals (fig. 3E) and cone-in-cone structures observed on microscopic scale (Fig. 3F).
The rocks under microscope exhibit quartz, feldspar, mica, chlorite and pyrite. The quartz grains in some calcareous sandstones gradually merge with calcite 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. The results obtained by X-ray diffraction of the shales reveal that illite and montmorillonite are absent.[TOP}
EXPLANATION OF FIGURE 2
Precambrian fossils occurring as imprints of jelly fish (near hammer head), spindle shaped Hydrozoans and other forms, on the top surface of an argillite bed in the Conception Group near Mistaken Point.
Siliceous argillites (light coloured) with intercalated sandy beds (dark coloured) in the Fresh-water Point Formation. The irregular sandstone layer is possibly the result of load casting.
2C. Sharp contact between two consecutive graded units in the Cape Cove Formation.
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's Formation 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. Current direction from left to right. The scale is same as for 3A.
Photomicrograph of the Cape Cove greywacke showing rock fragments (top left) and quartz grains (centre ) ◊ 32 (approx).
3D. Photomicrograph showing a sharp contact between two consecutive graded beds 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 -cone structure and stylolites in the St. John's Formation . ◊32 (approx.) under crossed nicols.
DEPOSITIONAL HISTORY OF THE AREA
The history of sedimentation requires the probable source of the sediments, their transportaion and environment of deposition. The rock fragments and mineral constituents of the rocks indicate that the sediments were derived from a complex terrain consisting of volcanic, sedimentary and igneous rocks situated to the northeast[TOP}of the present exposures (Misra, 1971). Although the palaeogeography of the Avalon Peninsula of Newfoundland is only vaguely known from the works of Rose (1952),
McCartney (1967) and Brueckner (1969), the transport directions of the sediments indicate an area of high relief to the northeast of the present exposures where from the sediments were derived. The slope of the depositional basin as inferred from the sole markings and slump structures was to the southwest.
Sedimentation of the Conception Group started probably in isolated basins bound by volcanic rocks of the earlier Harbour Main Group. This situation existed at least in the eastern part of the Avalon Peninsula and similar isolated basins are envisaged by McCartney (1967) in the case of the sediments that he included in the Harbour Main Group. These sediments assigned to the Harbour Main Group are possibly the beginning of the Conception Group deposition because locally the underlying volcanic rocks and Conception Group rocks are interbedded in the transition zone. The rocks of the mall Bay Formation (williams and King, 1976) reported from outside the area, seem to have this transitional character. The glacial conditions represented by the tillites of the Gaskiers Formation mark an important time plane (Anderson and Bruecknerm 1971) and may be the cause of the extensive green colour of the sediments.
The present study is confined to the rocks younger than the Gaskiers Formation. The presence of limestone in the lower part of the Conception Group (Mccartney, 1967) and the calcareous nodules (Misra, 1971) indicate warm, clear water of deposition. The presence of pillow basalt within the Drook Formation and a proximal volcanic terrane (Williams and King, 1976) during the deposition of the lower Conception Group suggest near shore conditions for the Drook Formation. The other evidences of a shallow marine environment are penecontemporaneous deformation, calcareous nodules, mega ripple marks, thixotropic deformation (Misra, 1969a, 1971) and enormous silica precipitate. Even if it is conceded that some of the silica beds are silicified siltstone, most of the siliceous sediments (Fig. 5A of Misra, 1971) comprising a thickness of over 100 meters had their origin in the underlying volcanic rocks of the Harbour Main Group. Uniform parallel lamination in the cherts expressed by the colour bands suggests tectonic stability.
The depositional framework of the Freshwater Point Formation was not much different from that of the Drook Formation except that the influx of the terrigenous sediments increased considerably towards the close of the formation. A shallow water environment for the Freshwater Point Formation is inferred from the 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 depositional system had increased. The environment of deposition gradually became deeper and by the close of the deposition of the Freshwater Point Formation, the sea had become deep enough for turbidity currents of large magnitude. This change in the stability of the depositional basin corresponds with the Freshwater Point and Cape Cove boundary and has possibly been taken by Williams and King (1976) as the base of the Briscal Formation.
The rocks belonging to the Cape Cove formation were deposited by turbidity currents in a basin whose northeasterly trending axis paralleled the present strike of the beds. The main arguments in favour of this deduction are:
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, submarine slumping could have been initiated by agents like deep reaching wave action during heavy storm, minor earthquakes, vulcanism (Misra, 1969a) or by abundant supply of sediments during floods. Large-scale slump structures in the lower part of the St. John's Formation (Misra, 1971) indicate that deposition was taking place on a gently sloping surface. The laminated siltsones (Fig. 2F) about 10 layers per foot, of separate composition in the St. John's Formation indicate that the load was carried by traction currents. Small-scale cross lamination (Fig. 2D) and cross ripple laminations were formed by the currents having relatively high lutite content and small amounts of large particles. The presence of pyrite (fig. 3E) and the grey colour of the sediments of the St. John's Formation suggest that the depositional environment was and the cone-in-cone structures (Fig. 3F) are of syn-diagenetic nature or later.[TOP}
ECOLOGICAL CONDITIONS OF THE ANIMALS
Within the above frame work of the sedimentary basin and environmental condition, the existence 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 the upper part of the Cape cove Formation (Mistaken Point Formation) and the sediments enclosing them are of turbidite origin. The purple colour of the sediments is characteristic and may indicate a humid tropical climate as interpreted 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-shaped organisms 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 of soft-bodied Metazoans (Coelenterates and others) represented by Polyps as well as Medusae. The Polyps include both colonial Hydrozoans and Pennatulid corals. the round lobate organisms are jelly fishes and are represented by several extinct species. The Mistaken Point and continue throughout the formation. In most 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 and convolute bedding, the animals must have lived in a moderately deep environment (Misra, 1969b). They were living probably at the bottom of the sea during the intervening quiescent period between two successive turbidity currents and lived until they were buried by sediments brought by suffocating turbidity currents. Submarine volcanism was also an important cause of their death. In any case, the surface of mud on top of the graded units provided the ideal conditions for preserving the imprints.
Since the fossils are found only as impressions, no part of their body preserved, it suggests that they were soft bodied Metazoans that constituted a flourishing fauna during the Late Precambrian time in the Conception sea. The fauna is supposed to be older than the Ediacara fauna of Australia. The fossils are preserve in situ as indicated by the preservation of the soft bodied animals in entirety. Besides, the presence of bent or curled animal imprints and segmented forms in the fauna together with the organisms with the hold fast (Misra, 1969b) are evidences of their bottom dwelling habit.[TOP}
I am grateful to late Prof. W.D. Brueckner of the Memorial University of Newfoundland for financial assistance and guidance, to Prof. M.M. Anderson for providing some of the recent literature and to Prof. K.S.Valdiya of Kumaun University, Nainital, India for critical reading of this paper. [TOP}
(Received: May 3, 1980; Revised form accepted: Mar. 26, 1981)
Journal of the Geological Society of India, Vol.22,No.8,1981.