Geology of Durness

The Balnakeil Formation belongs to the middle parts of the Durness 'Limestone' and is characterised by very dark, organic rich dolostones that contain high levels of muddy sediments (calcareous muds). The section at Balnakeil bay is also well known for its large stomatolite features.

​

Middle parts show mm-thick red stained ribbons on weathered surfaces of the blueish-grey limestone with blocks surrounded by a dolomitic matrix seen forming clotted mounds, capped by laminated dolostones and beige limestones. Occasional brown coloured, lens shaped dolstones beds become more abundant in younger layers before the sequence is gradually dominated by argillaceous carbonates.

​

Cyclic wackestone-mudstone sequences become abundant towards the top of the formation with mud-flakes and ‘bivalve’ burrows. Abundant metre-scale laterally-linked stromatolites show weathered detrital grain pits and some partial chert recrystalisation, as is lenticular bedding within muddier sediments. Pale grey bioherms overlain by lighter, thin wavy limestones and dolostones were also found.

​

The upper section of the formation consists of interbedded dark grey-blue limestones and thin intraclast breccias with erosional bases cutting into grey-beige, stromatolite-poor grainstones with peloidal shaped pits on weathered surfaces. The very top has dark bioturbated beds showing white vein-like burrows that gradually dominate the sequence, with maroon coloured chert horizons common.