A parallochthonous group of sedimentary rocks unconformable overlying the Bay of Islands ophiolite complex, North Arm Mountain, Newfoundland

Abstract

This paper reports and documents the existence on the North Arm Mountain massif of a parallochthonous group of sedimentary rocks that overlie the Bay of Islands ophiolite complex with marked angular unconformity. Some of these rocks are those previously regarded as oceanic sediments lying conformably on top of the igneous rocks of the ophiolite complex, and some were mapped as part of the igneous rocks themselves. The name Crabb Brook Group is proposed for this sedimentary sequence and three new formation names are also put forward. Massive sedimentary breccias directly overlie and contain clasts from all units of the ophiolite complex. Evidence of intense predepositional weathering of the clasts and the underlying bedrock is common. Bedded shales and their disrupted equivalents in olistostromes containing sedimentary and igneous blocks overlie the breccias. Two fossil localities within the shales yield acritarchs of Llanvirnian age and inarticulate brachiopods probably not older than this. A small remnant of coarse red arenite caps the section. The sequence appears to overlie some thrust faults and to be cut by others. It is tightly folded on a large scale along with the thrusts and the ophiolite complex. We interpret the whole folded stack, including some underlying allochthonous sediments, to be truncated by gently inclined thrusts formed in the later stages of allochthon emplacement. The lithologies, facies, structural and tectonic relationships, and fossil age of this sedimentary group indicate that it is parallochthonous in the context of the regional geology. It was deposited on the Bay of Islands ophiolite complex during its obduction and cannot therefore be used to infer the tectonic environment of the spreading ridge and transform fault that formed the Bay of Islands complex.