Relationship among rock groups within and beneath the Humber Arm Allochthon at Fox Island River, western Newfoundland

Schillereff, Herbert Scott (1980) Relationship among rock groups within and beneath the Humber Arm Allochthon at Fox Island River, western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Middle Cambrian to Middle Ordovician sedimentary and igneous rocks comprise three separate structural slices and two distinct melange zones within the southern part of the Humber Arm Allochthon at Fox Island River, western Newfoundland. These overlie a Cambro-Ordovician platformal carbonate sequence in a stratigraphic-structural succession as follows: -- a) autochthonous Lower Cambrian to Middle Ordovician carbonate sequence, with a prominent Middle Ordovician carbonate breccia and shale unit at its top; -- b) autochthonous Middle Ordovician clastic flysch unit; -- c) Basal Melange, consisting of sedimentary blocks in a shaly matrix; -- d) allochthonous Middle Cambrian to Middle Ordovician sediments and structurally associated mafic volcanics which collectively comprise the lowermost slices of the Humber Arm Allochthon; -- e) Medial Melange, consisting of sedimentary, volcanic and gabbroic blocks in a shaly and tuffaceous matrix; -- f) allochthonous Lower Ordovician and older mafic volcanics which form a small medial slice; -- g) allochthonous Upper Cambrian ophiolitic rocks which form the uppermost slice of the allochthon. -- These rocks are covered by Carboniferous sediments and Pleistocene glacial deposits. -- The structural succession is interpreted according to the model of a stable Atlantic-type continental margin destroyed by ophiolite obduction and westerly transport of rock units during the Taconic Orogeny. A three-part history of obduction, uplift and final gravity-sliding is recognized. The components and age of easterly-derived flysch units and the melange zones suggests that during emplacement, the uppermost volcanic and ophiolitic slices began final westward gravity-sliding first, but that the lowermost sedimentary slice assemblage was detached soon thereafter and was first to arrive in the map-area. -- Pre-emplacement deformation consists of metamorphic tectonism within and fragmentation at the base of the ophiolitic slice. Emplacement-related deformation consists of phacoidal melange cleavages and west-facing and verging recumbent folds, which diminish in extent and intensity away from the base of the allochthon. Cleavage in the matrix of the Basal Melange and folds in the uppermost autochthon are shown to be geometrically related. Devonian upright folds which are strongly developed throughout the autochthon, diminish upwards to mild warps in the ophiolitic slice. Post-Carboniferous high-angle faults offset each of the Paleozoic rock units. -- An isolated carbonate sliver at Fox Island River is interpreted as a post-emplacement thrust klippe, though it may be part of the assembled allochthon, incorporated during latest emplacement. -- Volcanic rocks of the allochthon are subalkaline and tholeiitic, and are interpreted as seamounts which grew at or near an ancient continental margin. They were displaced during earliest ophiolite obduction. The volcanic rocks correlate lithologicaliy and structurally with the Skinner Cove Formation of western Newfoundland, although there are significant chemical variations between the rock groups.

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