Meteorite impact deposit, Stoer Group (Torridonian)

From Stac Fada, Bay of Stoer

Outcrop

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
This rock type occurs as a single sheet, several metres thick, showing no bedding or layering. It is a resistant rock forming a promontory that projects out to sea. The main part of it can be seen in the left centre of this view forming dark, purplish vertical faces. Below it, to the right, are bedded sandstones, and above (top left) are thinly layered limestones laid down in a lake. The rock (which can be seen at several locations spread over 50 km north to south) has previously been described as a volcanic mudflow - the only volcanic feature in the Stoer Group - but recently Ken Amor from Oxford has found quartz grains in it showing the type of shock metamorphism found at meteorite impact sites. Chemical studies also support the new interpretation that this is part of the blanket of debris thrown out of an impact crater that is probably now buried beneath younger rocks out to sea.

A number of features suggest that the rock was forcefully emplaced as a warm, fast-moving flow. Towards the bottom right of the photo there are several large slabs of light brown sandstone that have been torn up from the rocks beneath, tilted and folded.

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
This is a more detailed view of the lower part of the deposit, where it appears to have been injected, from left to right, between the layers of light brown sandstone.

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
This close-up view of the rock shows angular fragments of a greenish material that resembles a glassy volcanic rock, suspended in purple muddy sandstone. Also suspended in the rock are a couple of fragments of red-brown siltstone. The hammer head is about 15 cm across.

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
A number of features suggest that the rock was hot when deposited. In this view, a vein of pink feldspar shows the path along which hot water escaped upwards, precipitating its dissolved mineral content, as the deposit settled and cooled.

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
Another indication of high temperatures: this is a detail of one of the folded blocks of sandstone picked up by the flow. It contains many tube-shaped holes, parallel to its bedding, each about 2cm in diameter. It is possible that these were formed and filled by steam boiled out of the sediment's pore water.

Hand specimen

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
This hand specimen consists of angular fragments of dark green rock of volcanic appearance and sand grains (quartz and white feldspar) in a purplish-brown matrix. There is no layering - the rock has no bedding. The material caught up in it is a mixture of altered glass (interpreted as bedrock melted at the impact site) and recycled sedimentary grains.

Thin section

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
At first sight in thin section, the rock looks like a sandstone. However, the grains are very angular and poorly sorted, and there is no bedding. The larger fragments are of a yellowish-green rock that is poorly crystalline and sometimes shows a banding: it is an altered glassy rock, solidified from a melt, and its banding was produced by flow just before it solidified. The smaller angular grains are cloudy feldspar and clear quartz: they will have been recycled from other Torridonian sediments.

Plane polarized light, field of view 7 mm across.

Meteorite impact deposit, Stoer Group (Torridonian), Bay of Stoer
This is a detailed view of one of the igneous fragments. Natural glass (such as obsidian) will eventually begin to crystallize, even at low temperature. The new crystals are extremely small (the textures can be called microcrystalline or cryptocrystalline), and in some cases many crystals begin to grow at the same point and build a spherical radiating cluster of needle-like crystals. Most of this field of view is crystallized glass. The round objects are radiating aggregates of green chlorite (a hydrated iron-magnesium silicate), and the colourless material in between them is microcrystalline quartz and feldspar.

Plane polarized light, field of view 3 mm across

D.J. Waters, Department of Earth Sciences, June 2008