Geology of the Trepassey area, Avalon Peninsula, Newfoundland

This thesis presents the results of the first detailed study of the geology of the Trepassey area on the south coast of the Avalon Peninsula, southeastern Newfoundland, and includes a geological map on a scale of 1:8,000. -- The area is underlain by a thick sequence of late-Precambrian rocks, belonging to the Conception Group and to the St. John's Formation of the Cabot Group, and at least one bed of tuff indicating contemporaneous volcanic activity. Although these rocks are well exposed along the coast, inland they are largely hidden beneath ground moraine of late Pleistocene age. -- The main structural features of the area are a synclinorium and a major fault cutting its western flank. Most of the subsidiary folds of the synclinorium, the associated fracture cleavage, and many of the faults have a north-northeasterly trend although some folds and faults are cross-cutting; folds generally plunge gently to the south-southwest. An analysis of the joint pattern in the area shows its tectonic origin. -- The Conception Group consists mainly of graded beds in which the coarser element is lithic or feldspathic greywacke, containing many volcanic rock fragments, and the finer-grained element is mudstone similar in composition to the greywacke. The succession also includes fine-grained very siliceous rocks, mudstones and colour-banded cherts. Conception beds are grey to green or purple; the colouring agents are dark rock fragments, iron sulphide, chlorite and hematite. -- The rocks of the St. John's Formation are predominantly fine-grained and although the succession includes, in its lower part, thin graded beds resembling those of the uppermost part of the Conception sequence, it consists mainly of mudstone (shales) interbedded with laminae of siltstone forming a repetitive sequence thousands of feet thick; the silty beds are subgreywackes. Slump zones are characteristic of the succession. Pyrites is universally present and probably responsible for the grey colour of these beds; also noteworthy is the common occurrence of authigenic carbonate. -- The sediments laid down in Conception and St. John's Formation times appear, from their composition, to have come from the same M northern source. A turbidity current origin is favoured for the S Conception Group graded beds, hence the Trepassey area is believed f to have been a deep-sea area during the greater part of Conception times. Shallowing began towards the close of Conception times and continued into St. John's Formation times when turbidity currents generally ceased. Comparison with other areas suggests that folding and faulting of these sediments took place before the close of the Precambrian although the major fault may be post-Ordovician. The Trepassey area provides no evidence of subsequent geological events until Pleistocene times apart from the presence of an upland surface (500-600 feet) believed to represent late Tertiary peneplanation of the region. During the Pleistocene the Trepassey area was probably glaciated more than once but the effects of the last, or Wisconsin glaciation, masks the effect of earlier glaciations. Ice movements modified the form of the land and when the ice melted at the close of the Pleistocene it left a blanket of boulder clay over the entire area. Vegetation soon established itself and the growth and decay of swamp plants in bogs over thousands of years has given rise to extensive peat deposits. \n