JUL 18TH 2020


Brimstone Head is located on Fogo Island, it lies within the Dunnage Zone which is the 2nd zone out of the 4 it is located next to the Gander Zone which is an hours drive south of Fogo Island. 

Techtonostratigrapic Zones (East to West)

1. Avalon Zone

2. Gander Zone

3. Dunnage Zone

4. Humber Zone

The island was once apart of the Iapetus Ocean (Southeast Gondwana), it represents past island arcs and sedimentary basins. The ocean was once located in the southern hemisphere between the past continents of Laurentia, Baltica and Avalonia. The ocean is around 600 to 400 million years in age, from the Neoproterozoic and early Paleozoic. 

The Siluro-Devonian high-level bimodal Fogo Island Batholith is around 7 km thick but acts almost like a sill unit, this is because of how thin the intrusion is. Stratigraphy of the area suggests that the batholith was intuded without strong deformation of the surrounding host rocks. below the sill tntusuion large composite gabbro-microgranite dikes which were most likely the feeders to the Fogo Island Batholith. The batholith was again intuded by a heterogeneous mafic unit which is slightly younger. 

The Fogo island Batholith/Intrusion lies within a curved northwest-trending fault slice which is dominated by mid-Ordovician to Silurian strata and has been affected by more than one phase of deformation (polyphase deformation). Within Fogo Island, folds are open with limb dips rarely exceeding 40°. Unlike the islands to the south where the strata dips steeply in a northeast-trending direction, the southern islands are dominated by Northwest-verging structures which dominate the outcrop pattern.



Rock Layers

All the rock layers present in Fogo Island are associated with the Fogo Island Intrusion, this includes the younger sedimentary sediment along with volcanic rocks. The sedimentary rocks are classed as the Fogo Harbour Formation while the volcanic rocks are placed under the brimstone Head Formation. Both formations have been tilted so they lie left to right of each other, the Brimstone Head Formation is located in between two sections of the Fogo Harbour Formation at Brimstone Head.  

The Fogo Harbour Formation is truncated by the Hare Bay Granite which is a high-level component of the Fogo Island Batholith Intrusion and lies south of the town of Fogo. All the visible rock formations within the Brimstone Head park are apart of the Botwood Group, this includes the Brimstone Head Formation, the Fogo Harbour formation and the Fogo Island Intrusion which is the only layer with age data.   

The Fogo Island Intrusion's age data is not perfect, as the Zircon age data puts it at 420 million years old while the Titanite age data puts the Intrusion at 408 million years old. This puts the Fogo Island Intrusion in the Silurian or the Devonian age.

Fogo Harbour Formation

The bedding in the Fogo Harbour Formation is moderate, it dips to the Northwest with beds tilted right-way-up in the majority of the visible areas. The Fogo Harbour Formation has a consistent thickness of 1000 to 1300 meters, the rocks in the formation were once shallow marine sediments with conglomerate intervals which are attributed to slumping during deposition. Slumping in terms of geology is a form of mass wasting when a coherent mass of loose slow-moving material moves downslope a short distance at a time. 


The Fogo Harbour Formation contains Grey-Green and Brown siltstone along with Sandstone, the rock types have been laminated and contain grading, ripple marks and cross-bedding. Conglomerate forms in lenses with rounded siltstone cobbles within a homogenised siltstone matrix which occupies erosive channels up two 2 meters thick which are the remnants of past stream systems.

Traces of volcanic materials are visible in the formation, a few beds contain 2-10% by volume of pink feldspar lapilli. Feldspar crystal fragments up to a centimetre in diameter are also present within the formation but are not very common. Volcanic bombs are also visible but only within beds located in Rogers Cove.   

The sedimentary rocks are intruded by granites, felsic dykes and composite mafic-felsic dykes that are associated with the Fogo Island Intrusion. 


A fault is visible along Brimstone Head, it is located along the contact of the Fogo Harbour Formation and the Brimstone Head Formation close to the town of Fogo. The fault-related movement is local and only occurs in that area of the island and isn't connected to any fault zones. 


Brimstone Head Formation

The Brimstone Head Formation lies above the sedimentary Fogo Harbour Formation, the contact between the Brimstone Head Formation and the lower Fogo Harbour Formation is only visible around the town of Fogo.

The formation contains fine-grained dark and glassy rock, it is a mix of flattened pumice and other volcanic materials which are visible in lenses within the Brimstone Head Formation. Quartz Phenocrysts are also rather common in the formation, phenocrysts are early forming crystals that are larger than the other grains in the formation.


The rocks in the formation are described as Indurated Tuffs which is a mix of volcanic ash fragments comprised of previously formed rocks. Flow-Banding is rare but visible within the formation and occurs as layers within the visible rocks. 


The Brimstone Head Formation contains Lapilli which is a classification of tephra and the size ranges from 2 - 64 millimetres. The lapilli has been flattened and that makes determining the accurate bedding rather difficult, the volcanic rocks seem to dip towards the northwest and overlie the sedimentary Fogo Harbour Formation in some areas. The contact between the Volcanics and the Sedimentary rocks are far more complex and relatively not well understood. 




Currie, K. (2003). Emplacement of the Fogo Island Batholith, Newfoundland. Atlantic Geology, 39(2). 

Donaldson, C., Sood, R., Barth, A., Christie, H., & Kerr, A. C. (2015). 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. Report, 15(1), 27-42.