Regional geology of the Topsail-Foxtrap area

Dawson, James M. (1963) Regional geology of the Topsail-Foxtrap area. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis is concerned with an area of Precambrian volcanic, sedimentary and intrusive rocks located south of the east side of Conception Bay in the north-central part of the Avalon Peninsula, Newfoundland. Physiographically the region can be divided into a southeastern, gently rolling, plateau-like surface about 600 feet above sea level, and a northwestern sector in which the dominant features are north- northwest-trending ridges and valleys apparently carved by glaciers travelling in this direction. -- The oldest major rock group occurring here is the Harbour Main group which is mid-Proterozoic or older in age. The base of this group is not exposed, however, the visible portion comprises a lower, rhyolitic division and an upper andesitic division, both of which consist of lavas and pyroclasics. To the east of the volcanic rocks lie greenish sandstone, siltstones and cherty sediments of the Conception group, part of which is intertongued with the andesitic rocks of the Harbour Main group. -- The Harbour Main and Conception rocks are intruded by small podlike bodies of felsite, which are particularly prevalent in the zone where andesitic and sedimentary rocks are intertongued. The two main groups mentioned above are intruded by the Holyrood "granite" batholith which consists mainly of adamellite but contains smaller bodies of quartz- feldspar porphyry and aplite. Within the granite an isolated roof pendant of meta-diorite is found. This may be related to the andesites of the Harbour Main group or may represent an older or younger intrusive mass. A small number of basic dikes invade the granite and the Harbour Main group. However, in the rhyolites several sheared basic dikes are found which are possibly older than the granite. -- Lying unconformably upon the granite and volcanic rocks are small, isolated, remnants of interbedded graywacke and slate or shale, herein called the Black Hill sequence. -- Shortly before the emplacement of the batholith, the Harbour Main rocks and intertongued sediments were deformed and subjected to low grade regional metamorphism. This resulted in overall schistosity and cleavage, and in chloritization and epidotization of the andesites. Late plutonic hydrothermal solutions altered the sheared fragmental acid volcanics, in the vicinity of the granite-rhyolite contact, to quartz schist, quartz-pyrophyllite-sericite schist and smaller amounts of almost pure pyrophyllite schist. -- Such folds as can be discerned in the Conception group seem to trend north-northeast and are quite tight, becoming more open to the east. They have probably formed at the same time as the schistosity prevalent in the Harbour Main group as both have the same trend. Folds formed later within the Black Hill sequence have an approximate east-west trend. -- There are many faults in the area, a large proportion of which belong to two sets: (a) north to north-northeast- trending, longitudinal faults and (b) northwest-trending transverse faults. The major longitudinal faults seem to have a diagonal displacement whilst the transverse faults are normal faults. Displacement at most faults is rather small. Most, if not all faults are younger than the Black Hill sequence. The largest of the longitudinal faults, the Topsail Fault, is of post-Ordovician age. -- The main deformations can be placed in two orogenic periods of Precambrian age. The older of these began at some time after the Harbour Main volcanics had been formed and ended with the intrusion of the granite. The later one occurred in post-Black Hill time, folding these rocks as well as other late Precambrian sediments to the east of the area. Apart from some faulting, the rocks do not seem to have been deformed during Paleozoic and younger time. -- The only mining carried out in the area is concerned with the pyrophyllite deposits occurring at Johnny's Pond, about 2 miles south of Manuels. At present, approximately 20,000 tons of 75 per cent-pure pyrophyllite are exported.

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