The Humber Arm allochthon at South Arm, Bonne Bay, with extensions in the Lomond area, west Newfoundland

Quinn, Louise Anne (1985) The Humber Arm allochthon at South Arm, Bonne Bay, with extensions in the Lomond area, west Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Lomond area displays all the main tectonic elements present in west Newfoundland, from Grenvillian basement, through autochthonous rocks to sedimentary and ophiolitic slices of the Humber Arm Allochthon. -- Autochthonous rocks adjacent to the Humber Arm Allochthon include limestones of the Table Head Group which are overlain by chromite-bearing flysch of the Sandbar formation. This passes westward into the north-south trending Gadds Point melange, which marks the base of the allochthon. The melange is discordant with allochthonous sedimentary units. -- Rocks of the lower structural slices of the Humber Arm Allochthon are assigned to the Bonne Bay group which includes four formations. From east to west they are the Mitchells (quartzites), Barters (shales and quartzites), McKenzies (shales and limestones, siltstones), and Sellars (greywackes) formations. These are correlatives, respectively, of the Summerside, Irishtown, Cooks Brook/Middle Arm point, and Blow me Down Brook Formations of the Curling Group at Humber Arm. The rocks are unfossiliferous and highly deformed, but relative stratigraphic relationships among the Mitchells, Barters and McKenzies formations appear similar to those between their lithic equivalents in the Curling Group. The Sellars formation is structurally isolated at a higher level than other allochthonous sediments. Petrographic data for the Sellars formation suggest that it has been derived from a cratonic source. This is in contradiction to the interpretation of the equivalent Blow me Down Brook Formation as an transgressive flysch derived in Ordovician times from an allochthon to the east. It is here suggested that the Sellars/Blow me Down Brook sandstones are more closely compared with older (Precambrian or Cambrian) rift related sandstones and now occur at an anomalously high structural level within the allochthon. -- The Bonne Bay group is overridden by mafic volcanic rocks of the Crouchers formation. Relationships between this unit and other allochthonous volcanic rocks are poorly understood. Higher slices are represented by igneous rocks of the Little Port and Bay of islands Complexes in the west of the area. The Little Port Complex consists of polydeformed and metamorphosed ophiolitic and volcanic rocks. It is adjacent to the Bay of Islands Complex which contains ultramafic and gabbroic segments of a classic ophiolite suite complete with a polydeformed metamorphic sole.

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