Geology of Durness

The Eilean Dubh Formation belongs to the lower parts of the Durness 'Limestone' and is characterised by pale-grey to white coloured dolostones. Alternating beds of silty (some porcelain-like), massive, pale-grey dolostones and much lighter grey rocks showing vague ripple-lamination and abundant stromatolite (algal mound) features dominate the lower half of the unit. These then become more stained and chert-rich towards the top.

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Towards the middle of the succession at Balnakeil Bay, flaggy dolostone beds are pink-peach to pale yellow in colour with deeply pink coloured stained surfaces. Younger beds contain less stromatolitic laminae and fenestrae which contain partial sandy infills. Highly pitted, domed stromatolitic bed tops are abundant in the oldest beds and small pink-creamy coloured nodular chert lenses begin to appear here, more abundant towards the top of the formation. These form chert beds containing the original lamination of the dolostones, indicating a diagenic nature.

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Convolute, soft sediment deformation features are also seen here and shallowing cycles (typically 2.5m thick) become less easy to find. Multicoloured very fine grained, well-laminated beds showing abundant sedimentary features are common towards the top of the unit. These fine beds also overly scarlet-red stained laminae containing very fine concentric wavy rings.

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The lower parts of the unit differ slightly in that abundant and diverse stromatolites vary in size and appearance with gentle hummocky, digitate and close / distal laterally linked forms seen (some up to 0.5m wide). Outcrops around Eilean Dubh also show highly pitted surfaces due to small centimetre-wide, columnar stromatolites where they have been weathered out. In cross-section, variations in grainsize between the laminae can also be seen. Larger scale, domal to bun-shaped stromatolite mounds are also found (some over 1m wide). These stromatolites commonly cap thin, often red-stained interbedded breccias typically up to 0.1m thick, which in turn cap pale grey massive to cross-laminated beds.

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Depositional environment: Shallowing cycles within this Formation is indicative of gradual sea-level changes in a warm, marine carbonate platform, with frequent intraclast breccias being indicative of frequent storms breaking up stromatolitic structures, along with scour marks. Deposition upon old microbialites indicates ‘rapid’ stromatolite growth, forms indicating excellent growing conditions, possibly allowing exposure and hence desiccation-cracks within a likely sheltered environment. (e.g. behind coral reef). Lenticular, pinch-out bedding also implies tidal influences.