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S.B.Misra, Department of Geology, Memorial University of Newfoundland, St. Johnís, Newfoundland, Canada
M.S.Thesis, Memorial University., Newfoundland. Canada, p.139., 1969
Geology of Biscay Bay-Cape Race area, Avalon Peninsula,
|1||CHAPTER I : INTRODUCTION|
|2||CHAPTER II : GENERAL GEOLOGY|
|3||CHAPTER III : PRIMARY SEDIMENTARY STRUCTURES|
|4||CHAPTER IV : SECONDARY STRUCTURES|
|5||CHAPTER V : PRECAMBRIAN LIFE|
|6||CHAPTER VI : DEPOSITIONAL HISTORY OF THE AREA|
|<< Chapter I||Chapter III >>|
The Biscay Bay - Cape Race area is underlain by a thick sequence of Precambrian sedimentary rocks which can be separated, even though there is a gradational boundary, into two main divisions: a lower division consisting of banded cherts, argillites, greywackes, and siltstones corresponding to the Conception Group (Rose, 1952); an upper division consisting of shales with sandstone laminae and sandy streaks corresponding to the St. John's Formation (Rose, 1952). The rocks of the lower division are mainly green and purple. Those of the upper division are various shades of grey.
|Late Precambrian||Cabot||St.John's 1100 ft.||Well cleaved grey shales with sandstone laminae and sandy streaks|
|Conception||Cape Cove 3000ft.||
Graded beds of greywackes, siltstone, and well cleaved green argillites; purple argillites and greywackes in the upper part of the formation
|Freshwater Point 1500 ft||
Green siliceous argillites with some greywackes
Banded cherts and silicified argillites and siltstones.
Brief visits to the area were made by Professors Neale, Brueckner, Williams, Kennedy, King, and Anderson. All agree that the proposed division is well founded and that the Lithological correlation of the divisions with the Conception Group and the St. John's Formation is clear and definite.
The Conception Group 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 the Cabot Group. He divided the Conception Group into two rock units, the 'Conception Slate' and the 'Torbay slate'. However, a formal classification was not proposed. In the adjoining Whitbourne map-area, McCartney (1967) made no attempt to subdivide the Conception Group. In the Biscay Bay - Cape Race area, the Conception Group can be divided into three formations here named: the Drook Formation, the Freshwater Point Formation, and the Cape Cove Formation in order of decreasing age. Although the rock units are separated by gradational boundaries, each formation has its own characteristics that distinguish it from overlying and underlying formations. Rocks of the Conception Group throughout the Avalon Peninsula have previously been referred to as slaty siltstone, slates, argillites, mudstones etc. Although these lithologies are not easily compared there is some suggestion of facies change from fine sediments in the west to coarser in east.
The name Drook Formation is proposed for a sequence of banded cherts and highly siliceous argillites which are the oldest rocks of the area. The type area is Drook, three miles east of Portugal Cove South.
Rocks of the Drook Formation occupy the core of a regional anticlinorium with axis trending northeast through Freshwater Bay. The western boundary of the formation lies just east of Pigeon Cove Point and the eastern boundary lies half a mile west of Freshwater Point (Plate 2-3). Apart from coastal exposures the formation is found in numerous outcrops along Drook River, which runs in a valley along the axis of the anticlinorium.
The thickness of the formation estimated from exposures of the western flank of the anticlinorium is about 2500 ft.
The Drook Formation comprises well bedded, gently folded banded cherts and siliceous argillites (Fig. 2-2). Many of the chert beds are silicified siltstones and each bed has its characteristic colour, usually containing shades of green that are more accentuated on the exposed surface. The thickness of the beds varies from a fraction of an inch to two inches.
The cherts and siliceous argillites are exceptionally hard and compact. In places, the rocks are fractured but cleavage is rarely well developed. They break with conchoidal fracture and sharp edges on the fresh surface, but when crushed or jointed, weather into cubic, rhomboidal, or irregular fragments. The cherts are easily crushed and brecciate even along minor faults. Nevertheless, they are very resistant to weathering and form the areas of greatest relief near Drook.
The most common constituents of the rocks, as identified from X-Ray diffraction are quartz, albite, chlorite, and sericite (Table I). The other constituents are epidote, siderite (?), and leucoxene. Clay minerals are absent. Calcite is found only in the form of concretions, such as ellipsoidal nodules, concretionary silty blocks and concretionary, limy chert etc.
The base of the formation is not exposed in the area and the contract relation between the Conception Group and the underlying rocks is unknown. However, the oldest beds of the formation exposed along Drook River are almost pure chert.
The upper boundary of the formation is gradational with a gradual increase in the argillaceous and arenaceous material and decrease in silicification. Near Pigeon cove Point the transition zone, between the Drook and Freshwater Point Formation, includes two layers of pseudo oolitic concretionary chert of about 2 thickness.
|Q= quartz, A= Felspar, Chl. = Chlorite, Seri. = Sericite|
The Freshwater Point Formation is proposed for a sequence of predominantly argillaceous beds underlain by the Drook Formation and overlain by the Cape Cove Formation.
The Freshwater Point Formation is exposed in coastal areas on the western and eastern limbs of the anticlinorium where it overlies the Drook Formation with a gradational boundary. The western coastal section near Daly's Point is not complete because of a boulder beach along that part of the coast. The eastern coastal section is complete and stretches for more than one mile.
An estimated minimum thickness of the formation is about 1500 ft.
The Freshwater Pointy Formation is composed of siliceous argillites and siltstones, locally with minor proportions of medium to fine gained sandstone found at the base of the graded beds. The sandstones have a composition similar to those of the overlying Cape Cove greywackes (Table II), but are better sorted. The sandstones are composed of subangular to subrounded grains of quartz, felspar, and rock fragments, exhibiting a bimodal texture. The matrix is composed of chlorite, sericite, epidote, leucoxene, and sphene etc.
The diffraction patterns of some of the fine grained rocks reveal that quartz, albite, and chlorite and sericite are the common constituents. Clay minerals are absent. Chlorite which is chiefly responsible for the green colour of the rocks, is derived from more than one source, but mainly from alteration of basic rock fragments and clay minerals. The accessory minerals include epidote, leucoxene and sphene.
The fine grained sediments of the Freshwater Point Formation are green, the sandstones grey and the weathered product whitish. Weathered fragments are slab like and parallel fracture cleavage planes. Fracture cleavage is common but rarely well developed. The rocks are also cut by numerous joints parallel with or normal to the bedding planes.
Thus the boundaries of the Freshwater Point Formation with the underlying Drook Formation and the overlying Cape Cove Formation are gradational.
The name Cape Cove Formation is proposed for a sequence of graded beds that overlies the Freshwater Point Formation and underlies the St. John's Formation. The type section is exposed along the western side of Cape Cove, near Cape Race.
An estimated thickness of the formation is 2700 ft., which is overlain by a transition zone of about 400 ft.
The upper part of the formation included graded beds of greywackes, siltstones and purple argillites. These greywackes are darker in colour and finer in grain size than those in the lower party of the formation. The purple colour in the argillites is probably due to a change in the environment during sedimentation of these rocks (see page 128).
Several bedding features occur on top as well as on bottom surfaces of the graded units. They locally include ripple marks and organic markings on the top of the beds and sole markings on the undersurfaces of the graded units. The contacts of successive graded beds are, however, well defined. Sharpness of a contact can be seen even in a thin section (Fig. 2-7).
Nine specimens of the Cape Cove greywackes wee examined in thin section. The compositions, as determined by micrometric analysis are plotted in Fig. 2-1. In these specimens detrital quartz forms 22 to 38 per cent of the rock (Table II). Quartz grains are chiefly subangular to subrounded. Felspar grains are similar in shape to the quartz but in general are smaller. The felspar is sodic plagioclase; in part untwined. Many plagioclase grains are sericitic or cloudy; others are clear and show no signs of alteration. Rock fragments consist of chert, rhyolite, microgranite and basic volcanic rocks (Figs. 2-4, 2-5 and 2-6). The matrix generally comprises 40 55 percent of the rock, and is composed of chlorite, biotite, sphene, epidote, leucoxene, pyrite, and very rarely apatite.
Mineral constituents of the siltstones and argillites, as determined from X-Ray diffraction, are quartz, felspar, chlorite, and sericite. The rock fragments are absent as they break into individual minerals. Also, finer grained rocks have a higher proportion of opaque minerals.
|Sp. No.||Quartz grains||Felspar grains||Rock Frag.||G.M.(matrix)|
The St. John's Formation in the map area consists of a sequence of well cleaved dark to light grey shales that overlies the Cape Cove Formation. There are two coastal exposures, the first from Cape Race to Shingle Head, and the second from Biscay Bay to Portugal Cove South. The second exposure continues westward to the adjoining Trepassey area. Apart from these coastal exposures the formation is found in several outcrops along Back river and also along other unnamed brooks near Cape Race.
An estimated thickness of that part of the formation exposed in the area is about 1100 ft.
Near Shingle Head (Plate 2-2), the basal part of the St. John's Formation includes a layer of volcanic tuff of about 2 ft. thickness. The rock consists of quartz, plagioclase, calcite, sericite, etc. with a tuffaceous texture portrayed by shards, volcanic matrix and subhedral grains of felspar etc. The shards are clear and plentiful (Fig. 2-9) . This is the first record of volcanism in the St. John's Formation.
The main part of the formation is predominantly thin bedded, grey shales, with intercalated sandy streaks. Fracture cleavage is very pronounced and obscures bedding in some places. In other places, the relation between cleavage and bedding is distinct (Fig. 2-11). The weathered surfaces are masked by rust derived from weathering of pyrite, which is common in the St. John's Formation.
Near Cape Race and also at Portugal Cove Point, the formation includes dens, dark grey, calcareous layers of 2-3 inches thickness, some of which in thin section exhibit cone -in-cone structure. In the same association one finds calcareous, cherty, and sandstone nodules locally showing a concentric internal structure caused by slumping. Well developed slump structures and associated features are found throughout the formation.
The rocks are commonly composed of quartz, felspar, mica, chlorite, and pyrite. Quartz grains in some calcareous sandstones gradually merge with calcite grains, indicating an incomplete replacement of one mineral by the other. Furthermore, some of the fine grained calcareous sandstones exhibit a patchy extinction caused by partial replacement of quartz by calcite, which must have taken place during or after digenesis of the rocks, as most of the quartz grains are diagenesis in origin.
The accessory minerals are normally the same as in the Conception argillites, except that pyrite is more common and calcite more frequent. The results obtained from X-Ray diffraction of the shales reveal that clay minerals are absent.
|Fig. 2 - 2||Photograph showing southerly dipping beds of the Drook Formation west of Drook Point. Note the bedding, joints and the pattern of weathering in the cherts.|
|Fig. 2 - 3||Photograph showing ripple marked joints and the fractured surface of the purple argillites that contain fossil impressions near Mistaken Point. For location see plate 2 - 2|
|Fig. 2 - 4||Photomicrograph of Cape Cove greywacke showing quartz grains and rock fragments. Note the bimodal texture of the rock and undulatory extinction of the quartz grains. X32 (approx.) under crossed nicols.|
|Fig. 2 - 5||Photomicrograph of Cape Cove greywacke showing altered rock fragments with their outline diffused with the groundmass (matrix) producing thereby an indistinct or fuzzy texture. X32 (approx.) under crossed nicols.|
|Fig. 2 - 6||Photomicrograph showing basic volcanic rock fragments in the Cape Cove greywacke. X32 (approx.) under crossed nicols.|
|Fig. 2 - 7||Photomicrograph showing a sharp contact between two consecutive graded beds of the Cape Cove Formation. Note the dark grains of pyrite in the argillite. X32 (approx.) under crossed nicols.|
|Fig. 2 - 8||Photomicrograph of a thin calcareous layer showing cone-in-cone structure. Note the cleavage in calcite. X32 (approx.) under crossed nicols.|
|Fig. 2 - 9||Photomicrograph of volcanic tuff of the St. John's Formation showing devitrified glass (shards). X32 (approx.) under crossed nicols.|
|Fig. 2 - 10||Well-cleaved shales with interbedded sandstone laminae in the basal part of the St. John's Formation near Cape Race.|
|Fig. 2 - 11||Well cleaved St. John's shales with intercalated sandy streaks. Note the relation between cleavage and bedding.|
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