Geology of the Humber Arm area, west Newfoundland

The area investigated includes the type section of the Humber Arm series along Humber Arm, as well as an area east of Blow Me Down Brook on the south shore of the Bay of Islands, the south shore of Middle Arm, and Woods and Eagle Islands. Five newly named formations can be recognized in the sedimentary part of the sequence. Graptolites found in the third youngest formation indicate a Tremadocian age and the series as a whole probably ranges in age from Middle Ordovician to Lower Cambrian. The youngest rocks are volcanic flows and associated basic and ultrabasic intrusions, which form the Bay of Islands Igneous Complex. They were not investigated. The Humber Arm series has been much deformed. The main folded structure is the Humber Arm synclinorium. Associated with this fold are dislocations ranging in magnitude from major reverse faults or thrusts, to strain-slip cleavage. High angle faults cut these structures. The stratigraphy is locally disrupted by zones of chaotic deformation which also mark the junction between the Humber Arm series and the main carbonate sequence to the east of the area. This sequence is a contemporaneous facies of the Humber Arm series but underlies it. The best explanation of the geological setting of the Humber Arm series is that it is allochthonous, being emplaced as a klippe during Taconic times, thus accounting for the superposition of older Humber Arm rocks on the youngest rocks of the main carbonate sequence, the facies contrast between the Ordovician parts of the two sections, and the geology of the Rattler Block. This block is interpreted as an anticlinal window through which a lower tectonic slice of the Humber Arm series is seen. The chaotic zones are interpreted as rubble zones produced during the gravitational gliding of the klippe. Three structural units within the klippe are recognized; a lower slice seen through the Rattler window, the main Humber Arm slice and a slice containing the Bay of Islands Igneous Complex with some associated sediment. Differential movements within the klippe are thought to have produced these units