Rocks at Cape Tribulation

The striped and layered rocks that underlie the promontory at Cape Tribulation look like sedimentary strata. They were just that at one time in the past but they have been  altered and partly metamorphosed into metasedimentary rocks. They belong to a group known as the Hodkingson Formation and are the result of transformations brought about as two tectonic plates of the earth’s crust collided, with one plate then sliding under the other (subduction), generating tremendous heat and pressure that altered existing sedimentary rocks, promoted magma movements below the crust (leading to intrusive igneous rocks), and stimulated surface volcanic activity (making extrusive igneous rocks).

Rocks with Copper at Bunmahon

Colour and texture in quartz with copper minerals

Beautiful green-blue stained rocks are frequently found in stone walls at Bunmahon in southern Ireland. The small  village was at one time home to a successful copper mining industry. The copper is thought to have formed 354 million years ago at the beginning of the Carboniferous Period but possibly even earlier. The village is now the centre of the Copper Coast GeoPark and has a lovely roadside rock garden illustrating the geological history of the area. The copper mineral chalcopyrite (copper-iron sulphide) occurs as veins in white crystalline quartz and alters to copper carbonate forms such as green malachite and blue azurite.  Weathered stones show these colourful blue-green variants of the mineral, with the rusty patches representing the iron component. Stones of this composition are found in walls all around the area.

Silurian Rocks at Arisaig, NS

Silurian rocks from Arisaig, Nova Scotia, Canada.

The Northumberland Strait shoreline of Arisaig Provincial Park in Nova Scotia, Canada, is described as one of the best sections of Silurian rock in the world. The strata are shales, sandstones, and siltstones from the Arisaig Group which was deposited in the early Silurian Period dating from about 443 to 424 million years ago.

I was fascinated by the way that some of the rocks were made up hundreds of extremely fine layers that were breaking up very easily. As far as I understand it, these darker shale layers were the result of deposits created in the coastal waters of the time by storm events rather than by tides or currents; and they are known as tempestites.

Hickman Hild and Barr (2015) say that the uninterrupted accumulation of fine-grained sediment during the Silurian Period, exposed here along a continuous 5 kilometre stretch, suggests that the area was tectonically quiet for at least 20 million years.

REFERENCES

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp 50-53

Fault Zone Rocks at Clarke Head Part 2

More pictures of rock textures and patterns seen on the shore at Clarke head, near Parrsboro, Nova Scotia. At this site the fault zone sedimentary rocks include Blomidon Formation Triassic red sandstone and siltstone with strata laid down in repeating cycles, and preserved water ripple marks much in evidence. Igneous North Mountain Formation basalt from Jurassic period rift volcanism is present high in the cliff and not shown here for lack of accessibility. Light grey sedimentary Windsor Group Carboniferous limestone strata is also present. Large blocks of Precambrian metamorphic rock have been brought up from deep down by the faulting. These blocks, sometimes huge, are found in the breccia and include garnet-grade schist. Gypsum is common in the breccia, as boulders and as matrix. The boulders from the mega-breccia weather out from the cliff deposits and lie together with numerous smaller boulders, shards, and fragments littering the beach, varying in colour and composition as you walk along the waterline.

REFERENCES

Nova Scotia Field Guide, Arthur D. Storke Memorial Expedition, Department of Earth and Environmental Sciences, Columbia University in the State of New York, August 23 to September 2, 2012.

Fault Zone Rocks at Clarke Head Part 1

A mélange of rock textures from the fault zone at Clarke Head, near Parrsboro in Nova Scotia, Canada. The geology here is extremely complex and I have only just begun to unravel what is going on. Key research papers with precise details are not easily accessible. Others are a bit too generalised to enable me to identify exactly each rock type that I photographed….for the moment. I will update when I can be sure I have accurate identifications. The variety was wide and included igneous, sedimentary and metamorphic rocks. It is the same place that I photographed the satin spar gypsum. The colours, textures, and patterns are amazing.

Rocks at Moulin Huet Part 2

Contrasting rock textures at Moulin Huet Bay

The metamorphosed igneous rocks of Moulin Huet Bay in the Channel Island of Guernsey, the Icart Gneiss, are traversed by later intrusions of molten volcanic rock that filled spaces where fractures opened up in the gneiss. These intrusive rocks are sheet-like formations of varying extent and thickness, appearing on the weathering rock surface as narrow bands of contrasting colour and texture, and they are known as dykes. Dykes are igneous rocks that can be composed of different combinations of minerals, and they can also be metamorphosed later into yet more compositions. On Guernsey there are apparently six different types of basic (as opposed to acid) dyke and it is difficult to distinguish between these types when just observing in the field. According to the simple guide written by Pomerai and Robinson (1994) most of the dykes at Moulin Huet are made of dolerite, as shown in the examples illustrating this post. [There are also a couple of lamprophyre dykes which I will show in a separate post]. I am aware that this identification as dolerite may be an over simplification but will investigate further.

The photographs here show the contrasting textures and colours of the rocks, with the relatively fine-grained, smooth, and homogenous grey-green dolerite dykes within the coarse-grained Icart Gneiss and its large, squashed pink-orange feldspar crystals. In some instances, there are pieces of the Icart Gneiss within the dolerite, these having broken off the sides of the bedrock and become incorporated into the molten lava as the dyke was formed – these inclusions are called xenoliths.

REFERENCE

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8.

Rocks at Marble Bay 1

There is no marble at Marble Bay in the Channel Island of Guernsey! It looks as if there is but really there is none. The name is thought to be due to the massive vein of white quartz that crosses the beach. Equally, the name may have arisen from the phenomenon of encrusting bio-films of various types (algae, bacteria and lichens) that coat the rocks with vivid coloured patches of red, orange, yellow, and black.

The main bedrock in the bay is in fact Icart Gneiss with its large squashed pink-orange feldspar crystals (as found in the nearby Moulin Huet Bay on the other side of the Jerbourg Peninsula). This metamorphosed type of granite is riven by a single massive 2-3m thick vein of quartz in a fault zone that extends right across the peninsula so that the same vein reappears at Petit Port adjacent to Moulin Huet. Smaller branching veins of quartz also appear in the Icart Gneiss. What seems to be a large dolerite dyke with grey fine-grained texture and smooth surface additionally crosses the beach. The true appearance of each of the rock types is mainly masked by the bio-films and larger seaweeds attached to the rocks. Inter-tidally, however, some outcrops remain clear of growth, and the location of the wave-cut notch at the base of the cliffs is especially good for viewing the Icart Gneiss natural pattern and texture.

REFERENCE

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8.