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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|
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Depositional History of the Area
The discussion that follows indicates that the environment of deposition of the Drook and Freshwater Point Formations was shallow marine area which became deeper during deposition of the upper part of the Freshwater Point Formation, remained deep during deposition of the Cape Cove Formation and became shallow again during deposition of the St. John's Formation.
A shallow water marine environment for the rocks of the Drook, Freshwater Point, and St. John's Formations is inferred from features exhibited by sedimentary accumulation (package), type of lithology, and type of bedding. Less reliance is placed on grain size, ripple marks, and current bedding as all of these features can be produced either by shallow water deposition or in deep water by turbidity currents (Kuenen, 1953). A deep water environment of deposition for the Cape Cove Formation is also based on the type of lithology and the nature of bedding but with special emphasis on greywacke composition and graded bedding.
A significant feature, as mentioned above, in deciding conditions of deposition is the presence of graded bedding in the Cape Cove Formation. In fact Bailey (1936) goes so far as to say that graded bedding and cross bedding are typical of two different environments of sedimentation. Kuenen and Migliorini (1950) arrived independently at the conclusion that the most important types of graded bedding appear to have been produced by the action of turbidity currents of high density on the sea floor. One of them (Migliorini) was engaged in the investigation of graded rocks encountered in the field and the other (Kuenen) had studied artificial turbidity currents of high density in the laboratory. They suggested four possible explanations of graded bedding: Tsunami waves, slumping, mud flows, and turbidity currents of high density.
The environment can also be considered in terms of tits spatial position in a sedimentary basin. Rich (1951) defined three critical environments in a sedimentary basin: UNDA, the region of a basin above the wave base; CLINO, the sloping surface extending from wave base down to the generally flat water body called FONDO environment. Rocks of the Drook, Freshwater Point and St. John's Formations were deposited in an environment similar to an UNDA environment. All the features typical of a CLINO environment can be recognized on or inferred from the rocks of the Cape Cove Formation; these include a slightly inclined surface of clino form having freedom from wave caused disturbances of the water, prevailing muddiness, great and often repeated variation in sediment supply, deposition dominantly from suspension, density currents periodically flowing down the slope, gravity sliding and/or intra or interstratal flowage. Based on these and some other criteria mentioned earlier, the sedimentary history of the rock sequence is narrated in the following pages.
The sedimentation of the Conception Group in the Biscay Bay-cape Race area presumably 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 sediments which he included in the Harbour Main Group and described as practically indistinguishable from the Conception sediments. The sediments assigned to the Harbour Main Group are possibly the beginnings of Conception Group deposition and locally the underlying volcanic rocks and Conception Group rocks are interbedded in the contact zones. Subsequently these isolated basins probably joined to form a shallow water marine environment characteristic of lower Conception times.
The percentage of precipitated silica in cherts of the Drook Formation is now known, but this 2500 ft. thick siliceous deposit is believed to have been laid down in shallow water conditions. Moreover, the occurrence of limestone in the lower part of the Conception Group is reported from Whitbourne map-area (McCartney, 1967) and ellipsoidal nodules of calcareous sandstone occur also locally in the map-area. These lithological considerations together with mega wave ripples and the type of bedding indicate shallow water conditions. Furthermore, deposition of the rocks of the Drook Formation took place probably under more stable tectonic conditions than those which accompanied deposition of the younger units of the group. This conclusion is based on the fine grained and thinly laminated character of lithology which is present almost throughout the formation.
Freshwater Point Formation marks an increase in the proportion of argillaceous material and decrease in siliceous matter as compared to the underlying Drook Formation. The presence of graded bedding and structures resembling pull apart together with the type of lithology suggest that the environment of deposition was gradually becoming deeper, and thus by the end of the Freshwater Point Formation the sea had become deep enough to produce turbidity currents of large magnitude.
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 the favour of attributing the deposition of the Cape Cove Formation to the action of turbidity currents are absence of tidal action and symmetrical ripple marks, presence of sole marks formed by unidirectional flow, large volume of graded beds, and uniformity in the direction of supply.
In this sequence of graded beds in the Cape Cove Formation the aerial extent is so large that it cannot represent the accumulation of a river bed. Moreover, the volume of thicker beds is too large to be accounted for by deposition in one season, even if there were seasons. The sole marks and the very coarse size of many graded beds together demonstrate that the current hugged the bottom and was not a surface current. Furthermore, the large volume of some graded beds indicates a current with a large suspended load and hence with abnormal density. Finally the sandstones occurring at the base of the graded beds are muddy sediments or greywackes that indicate that deposition of finer material has taken place simultaneously with that of larger particles. This type of deposit is best explained by turbidity current formation.
After a substantial thickness of the Cape Cove Formation was deposited the seaward slope upon newly deposited detritus increased progressively. During this period of progressive increase in the slope of the basin, submarine slumping could have been initiated by relatively insignificant agents such as deep reaching wave action during heavy storms, minor earthquakes, or volcanism, or abundant supply of sediments during floods. Thus large-scale slump structures associated with some of the beds in the lower part of the St. John's Formation indicate that deposition was sill taking place on a sloping surface and probably the currents originated in shallow water and flowed down a slope to the deeper areas.
The basal part of the St. John's Formation is composed of laminated siltstones and shales about 10 layers per foot. Such a large number of more or less separate flows each with separate composition are inconceivable. The explanation for this type of lamination is that the load was carried forward by traction along the bottom. This bottom traction started as the turbidity currents became more dilute with a relatively high lutite content and a small amount of larger particles. The traction currents were of lesser density and had ceased to carry sand and silt only in suspension but started to move this load by traction. The small scale cross lamination and cross ripple lamination found in the St. John's Formation were produced by the bottom traction and could not develop by sedimentation from a turbulent current carrying its load only in suspension.
The presence of pyrite and the dark grey colours of the shales in the St. John's Formation suggest that the depositional environment was closed to water circulation and was reducing in nature. It is probable that towards the end of Conception time the depositional conditions changed and the environment became shallow again as suggested by presence of the purple beds. This shallowing of the environment may be attributed either to infilling of the depositional basin or else to epeirogenic movements. This shallow water deposition was characteristic of post Conception time (McCartney, 1967)
To summarize the sedimentary history of the area, the sedimentation of the Conception Group started in the isolated basins which subsequently joined to form a shallow marine environment. The environment continued to become deeper and was the deepest during deposition of the middle part of the Cape Cove Formation. Turbidity currents were also of maximum magnitude during this period. 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 (McCartney, 1967). The intensity of turbidity currents became feebler. Deposition of rocks during early St. John's time witnessed mild volcanism and probably earthquakes and/or epirogenic movements that produced slumping.
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