Eriboll Formation (ERSA)

Parent:	Ardvreck Group (ARDV)
Daughter(s):	Pipe rock Member (PPR) Basal Quartzite Member (BAQ)
Age:	Lower Cambrian
Age range (Ma):
Overlying unit:	An t'Sron Formation (ASR)
Underlying unit:	unconformity
Thickness:	150 - 225 m
Lithologies: main lithology FARREN feldspathic-arenite main lithology QUAREN Quartz-arenite main lithology QZITE Quartzite main lithology SDST Sandstone subsidary lithology CONG Conglomerate trace lithology MDST Mudstone
Density:	2.25 - 2.65 kg/m3
Porosity:	xx
Type area:	NC 449 584 - NC 451 584 NC 228 246 - NC 235 244
{{#display_map:Eriboll | width=300 | height=220 | zoom=7 |label=Eriboll}}

Author(s): M Krabbendam (BGS)

Summary

The Eriboll Formation is the lowest formation of the Ardvreck Group(ASR), a sequence of quartz-rich sandstones rocks of Cambrian age. The Formation comprises two members, the Basal Quartzite Member (BAQ) and the overlying Pipe Rock Member (PPR): both are dominated by quartz arenite. The formation occurs in the North West Highlands as a thin band stretching from Durness to Skye. Very similar rocks occur in East Greenland, Svalbard and Newfoundland, confirming the geological link between Scotland, Greenland and North America. Together, these landmasses formed the ancient continent of Laurentia.

Distribution

The Eriboll Formation forms a narrow belt, extending along the northwestern side of Scotland from the Durness-Eriboll area, through Assynt, Dundonnell, Kinlochewe and Kishorn, to Skye. The formation occurs undeformed in the Caledonian foreland, but also occurs within thrust sheets in the Moine Thrust Zone.

Stratigraphical position

Definition of Lower Boundary: The base of the formation is an angular unconformity above either Lewisian Gneiss Complex (L) or the Neoproterozoic Torridon Group (TC).

Definition of Upper Boundary: Defined at the boundary with the overlying Fucoid Beds Member (FUB) of the An ‘t-Sron Formation (ASR). This contact is conformable. The An ‘t-Sron Formation and the Erriboll Formation together make up the Ardvreck Group(ARDV). This group is followed by the carbonates of the Durness Group(DNG).

Lithological description

The Eriboll Formation is divided into the older, pervasively cross-bedded Basal Quartzite Member (BAQ) (75–125 m thick), and the overlying Pipe Rock Member (PPR) (75–100 m thick). Both members range in composition from sub-arkoses to quartz arenites; beds are typically 20-80 cm thick.

The Basal Quartzite Member is generally white to pale yellow quartz arenite to subarkose and contains abundant planar and herring-bone cross-bedding – as its former name ‘False-bedded Quartzite’ implies. Cosets are typically 10-30 cm thick. The Basal Quartzite rests upon a remarkably flat unconformity. A 0.1 – 1 m thick pebble [http://www.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=CONG conglomerate] with rounded clasts of quartz and feldspar is commonly present at the base of the Member.

<img src="http://bgsintranet/asset-bank/action/directLinkImage?assetId=81074&width=300&height=200">	<img src="http://bgsintranet/asset-bank/action/directLinkImage?assetId=84787&width=300&height=200">	<img src="http://bgsintranet/asset-bank/action/directLinkImage?assetId=85822&width=300&height=200">
Cross-bedding in Basal Quartzite Member. Ben More Assynt.P530546.	Skolithos trace as seen on a bedding plane. P531629.	Pip Rock with Skolithos trace fossils in section (white) in dark-pink stained quartz arenite. Skiag Bridge. P531881.

The Pipe Rock Member, which varies in colour from white to dark pink, is characterized by ubiquitous presence of burrow trace fossils (‘pipes’) Skolithos and, less commonly, Monocraterion (‘trumpet pipes).

Although cross-bedding occurs locally, most sedimentary structures are generally destroyed by the extensive burrowing. Pipes are commonly white against the pink to red host rock, or the other way around; this effect is probably caused by differential iron staining during diagenesis.

Skolithos trace fossils occur as dimples some 5-20 mm across on bedding planes, whilst in section they are typically 10 cm long, although specimens up to 100 cm have been seen (Davies et al. 2009).

The Monocraterion (‘trumpet pipes’) trace fossils are less common. They are funnel- or trumpet shaped, with the top up to 40 mm across, narrowing to 5-10 mm lower down, and have been seen up to 30 cm long. The open top maybe caused by sand collapse at the burrow opening (Hallam & Swett, 1966).

It is possible that Monocraterion and Skolithos are the trace fossils of the same species (Hallam & Swett 1966). The variable length of the pipes is in part related to the sedimentary regime, e.g. the extent of available recently deposited soft sand, and the time available for growth before the next sand body was deposited, or part of the sand eroded by storms (Davies et al. 2009).

The top few metres of the Pipe Rock Member become more clay-rich followed by an abrupt change to the distinctive yellow-brown dolomitic siltstones of the Fucoid Beds Member (An t-Sròn Formation; 12-27m thick).

On the basis of colour, abundance and shape of trace fossils, the Pipe Rock Member has been subdivided into five zones (Peach et al. 1907; Geological Survey 1923).

V. Large pipes, massive quartzite

IV. Abundant nomral pipes, pink and white streaked quartaite, pipes red or white

III. Trumpet pipes, flaggy white quartzite

II. Normal pipes, massive quartzite

I. Small pipes; massive white quartzite

Whilst this subdivision can be applied to the Assynt area, due to lateral facies changes it cannot be applied to the entire outcrop of the Pipe Rock Member (Hallam & Swett 1966).

Palaeontology – fossil content

Trace fossils (Hallam & Swett 1966, Davies et al. 2009):

Monocraterion and

Skolithos

Trace fossils:

Acritarchs (needs confirming)

Palaeoenvironmental interpretation

The Eriboll Formation was deposited onto a coastal shelf fringing the ancient continent of Laurentia. The start of deposition of the Eriboll Formation heralds a major transgression (flooding) onto the Laurentian margin. This transgression is part of the global Cambrian transgression(sea level rise). The basal unconformity across Scotland, but also in East Greenland and Newfoundland, is remarkably flat, indicating a very flat coastal plain onto which the Eriboll Formation was deposited. The Eriboll Formation was thus deposited on a very shallow shelf. The herring-bone cross-bedding in the Basal Quartzite Member indicates a tidally-influenced beach deposit; locally wave ripples show exposure to waves (McKie 1990). Cross-bedding is locally present within the Pipe Rock Member, but this is generally obliterated by bioturbation (burrowing by organisms), suggesting a similar depositional environment for the Pipe Rock Member. The succession from the base of the Eriboll Formation to the top of the Fucoid Beds Member represents an overall trend of sea-level rise. Herring-bone cross bedding and vertical burrowing trace fossils in the Eriboll Formation suggest tidally dominated shelf sedimentation in the Eriboll Formation. A sea-level rise probably resulted in to background sedimentation below fair weather wave-base, as represented in the Fucoid Beds Member (McKie, 1990).

Detrital zircon from the Eriboll Formation are of Later Archaean age (3000–26000 Ma) or Mid-Palaeoproterozoic age (1800–1600 Ma), consistent with Laurentia as a source area for the sediments (Cawood et al. 2007).

The sequence of quartz-arenite followed by carbonate rocks also occurs in Newfoundland, Svalbard and East Greenland (Swett & Smit 1972), indicating that the entire eastern magin of Laurentia was a very flat, shallow margin.

Physical properties

Density: No density measurements have been taken by BGS, but in common with other quartzites, the density is expected to be in the order of 2.5–2.65 kg/m³.

Intact rock strength / Hardness: No intact rock strength measurements are available. Compressive strength of typical quartzite elsewhere lie in the range of 90–140 N/m² (www.mineralszone.com/). Schmidt Hammer Hardness tests in the field have yielded results in the range of R = 55–60 (McCarrol et al. 1995; Krabbendam & Bradwell, 2010). The rock is hard but brittle.

Jointing: Joint spacing of subvertical joints is typically in the order 10–80 cm (Krabbendam & Bradwell, 2010). Combined with bedding-parallel joints, this results in a cuboid blocks 10–80 cm across.

<img src="http://bgsintranet/asset-bank/action/directLinkImage?assetId=102671&width=200&height=150">

Frost-shattered debris forming a blockfield on the summit ridge of Ben More Assynt.P516915.

<img src="http://bgsintranet/asset-bank/action/directLinkImage?assetId=2146&width=200&height=150">

Oblique aerial photo of Arkle. Eriboll Formation forms most of the mountain. Active scree slope mask the unconformity with Lewisian Below. P000848.

Weathering and mass movement

The Eriboll Formation quartz arenite is virtually impervious to chemical weathering as shown by the excellent preservation of glacial striae on many outcrops (e.g. Lawson, 1996). However, Eriboll Formation quarts arenite is highly susceptible to frost weathering and extensive block fields occur on high summit plateaux and ridges (Ben More Assynt) typically above 600-700 m above sea level (Ballantyne 1995). Frost weathering was particularly strong during glacial periods, but may well continue to this day. On mountain slopes, the supply of frost weathered blocks results in extensive scree slopes, many of which are active (Arkle see Photo, Foinaven).

Previous names

False Bedded Quartzite for Basal Quartzite Member (BAQ)

Type sections

Type section 1: Eastern slopes of ridge separating the Kyle of Durness from Loch Eriboll, Durness area, north coast of Scotland. Grid reference provided refers to location where basal conglomerate is well exposed. NC 449 584–NC 451 584

Type section 2: Skiag Bridge by Loch Assynt displays a good reference section, showing gradational boundary with the overlying Pipe Rock Member. NC 228 246–NC 235 244

Other key localities:

Links to BGS/Google Book collection (pre-1980) - Description in 1907 Memoir. CLICK

See also

North-west Highlands

• Eriboll Formation

• Moine Thrust

• Knockan Crag (key locality for Moine Thrust)

• Loch Borralan