Geological relationships in northwestern Fogo Island and their implications for the timing of orogenic events. Newfoundland Department of Natural Resources, Geological Survey, Report 2015-1, 27-42.

• Main Authors: Donaldson, C., Sood, R., Barth, A., Christie, H., Kerr, A. C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Newfoundland and Labrador Department of Natural Resources Geological Survey 2015
• Subjects: 02 - Geodynamics; Geophysics and Tectonics; 05 - Petrology - Igneous; Metamorphic and Volcanic Studies; Brimstone; Brimstone Head; Change Islands; Fogo; Fogo Harbour; Fogo Island; Newfoundland
• Online Access: http://eprints.esc.cam.ac.uk/3581/; http://eprints.esc.cam.ac.uk/3581/1/plate.png; http://eprints.esc.cam.ac.uk/3581/2/Donaldson-2015.pdf; http://www.nr.gov.nl.ca/nr/mines/geoscience/publications/currentresearch/2015/Donaldson-2015.pdf

Sedimentary and volcanic rocks in northwestern Fogo Island are traditionally assigned to the Silurian Botwood Group, and are subdivided into the Fogo Harbour Formation (dominated by siliciclastic sedimentary rocks) and the Brimstone Head Formation (dominated by pyroclastic rocks). These two formations have long been considered to form a continuous, con- formable, homoclinal sequence. Detailed mapping shows that the contact between the Fogo Harbour and Brimstone Head formations exhibits a complex geometry and that the volcanic rocks sit upon different lithostratigraphic units within the under - lying sedimentary rocks in different places. New observations also indicate that sandstones sitting beneath the basal contact of the Brimstone Head Formation are locally downward-facing. These results suggest that the contact between the two for- mations is a cryptic angular unconformity across which there could be a significant time gap. It appears that the Fogo Har- bour Formation experienced recumbent folding, uplift and erosion prior to the extrusion and deposition of the Brimstone Head Formation. The Brimstone Head and Fogo Harbour formations are juxtaposed locally by reverse faults associated with sig- nificant penetrative deformation. The lowermost volcanic formation of the Botwood Group (the Lawrenceton Formation) has previously been reported to sit unconformably upon folded rocks of the older Badger Group on nearby Change Islands. The new information from Fogo Island suggests that there may also be a second unconformity within the sequence assigned as part of the Botwood Group, unless the Fogo Harbour Formation is actually an unrecognized part of the older Badger Group. In either interpretation, geochronological studies in northwestern Fogo Island could provide important constraints on the timing of Silurian and Devonian orogenic events in the Newfoundland Appalachians.