Review on Eri Boll Geology

The Eriboll region is located in the highlands of northwest Scotland (Figure 2). The general topographic structures seen in this area include mountainous terrain, such as Ben Arnaboll which stands at 232m above sea level. The area has been shaped by glaciers with steep hillsides (Figure 1), and U-shaped valleys (Mitchell et al., 2015), although the relief along the north coast is more subdued than other areas in the highlands. Unusually, in the context of the rest of the United Kingdom, the terrain is uncultivated and subsequently, it is one of the least inhabited areas in the country. Loch Eriboll, to the west of the mapping region, is a flooded glacial valley which carried glacial ice obliquely towards the northwest, this is backed up by striations and numerous erratics seen in the area (Mitchell et al., 2017).

The climate of Loch Eriboll and most of northwest Scotland is mild, although such high latitudes would normally give rise to low temperatures such as Labrador in Canada which is at the same latitude and yet experiences frozen seas and temperatures down to -31°C (Scotsman, 2013). The reason for northwest Scotland’s mild climate is because it is warmed by the gulf stream. In the summer the weather can be changeable often clear and sunny, sometimes cold and wet, but overall summers are mild. Changes in weather in this region can occur very rapidly as weather systems come over from the North Sea.

The geological context of the Eriboll region is significant, it is a renowned site for British geology with rocks dating back near the age of the Earth. This district exhibits outcrops with superior exposure, which has led to this area and the wider region to become a ‘mecca’ for geologists. The readily accessible and relatively compact outcrop areas provide excellent exposures and examples of thrust faults and folds. The Eriboll region is most notable for the discovery of the Moine thrust zone which runs submerged through Loch Eriboll. Here, it was discovered that late-Proterozoic metasediments were thrust northwest on top of two successions of sedimentary strata, the Archean basement and its sedimentary cover (Mitchell et al., 2017). This then underwent major faulting and intrusion - many textbook examples of faulting are observed here, in fact, the term ‘thrust’ was first coined here in 1884 (Leeds, n.d.).

Early mapping in this region showed that older rocks such as the Lewisian gneisses, were resting on younger Cambro-Ordovician sediments, this observation ultimately led to the concept of thrust tectonics. Over 100km of thrusting occurred along the Moine thrust which led to the formation of many features that are observable in the field at present, such as, imbricate faulting, duplexes, mylonites and rock outcrops that are typically associated with major thrusting (Fossen, 2016). The imbricate faulting caused repeated alternations of the main sedimentary units, namely the saltarella grit and fucoid beds. The rock units that were stacked up in the thrusts and can be observed here are very distinctive and varied from the far-traveled Lewisian Gneiss to Cambrian quartzites and imbricated sediments.

The Moine Thrust continues to run all the way to Sleat on the Isle of Skye from Scotland. These large-scale thrusts are a result of the Caledonian Orogeny, which happened around 430 million years ago. Before the Caledonian Orogeny, the majority of Scotlands geology was formed by metamorphic processes, the remains of which can be seen as the Lewisian Gneiss complex observed at the summit of Ben Arnaboll, this unit is colored in blue to the east of the map. The metamorphic gneiss material was carried by the Moine Thrust. The center of the map shows a number of alternating layers of Fucoid and Salterella Grit which are a result of this thrust faulting. Their constituent ramps and flats can be observed in many locations across this map.

The western side of the map is dominated by Basal Quartzite which is of Cambrian age, this unit is bordered by the Durness Limestone which is the youngest unit mapped, it is of Cambrian/Ordovician age. The Limestone generally dips to the east. Alternating bands of Fucoid and Salterella Grit can be seen in the center of the map, stretching roughly north-south. This is where thrust and piggyback faulting has occurred which led to the repeated exposure pattern observable today. The Fucoid and Salterella beds generally dip to the south-east. The eastern area of the map mainly consists of the oldest unit, the Lewisian Gneiss. The Gneiss caps the summit of Ben Arnaboll (Figure 1.5), two million years ago an ice age engulfed this region and glaciers have significantly weathered many outcrops such as at the peak of Ben Arnaboll. The foliations within the gneiss are dipping towards the south-east. Finally, pipe rock is seen in the north-west corner of the map and dips to the east.

Results:

The main characteristic feature of unit one is the appearance, it is fine grained and shows foliation banding between grey and white layers, much of the foliation was also folded. The thickness of this unit is undefined. The white layers showed coarser grains and the grey layers contained specs of a darker mineral. Under a hand lens, it was visible that the mineral flakes were aligned with their long side parallel to the banding in the rock. Some outcrops were also pink in color.

Unit two is a very hard, whiteish grey rock mainly composed of rounded grains set within a finer-grained matrix. The entire unit is approximately 70m thick. Multiple bedding planes are often visible at outcrops (Figure 4). Prominent cross bedding is visible. There are clear straight bedding planes around 40cm apart and between these bands are curved, more closely spaced planes - this observed structure is called cross-bedding and was visible on many of the outcrops visited. This unit overlies the bed below it on an angular unconformity.

Unit two grades into unit three which is approximately 85m thick. Unit three is lithologically similar to unit two except some areas are bioturbated by burrows perpendicular to the bedding surface, The lengths of the pipes varied. The vertical white pipes stand out against the stained purple/brown matrix of the rock. The grains of the matrix are coarse. A horizontal bedding plane is visible, there is also a white coloration at the bottom of the unit which transitions to pink as it moves towards the top of the unit.

Unit four is approximately 15m thick and can be quite variable in appearance. Some outcrops were rich in carbonate and effervesced under dilute hydrochloric acid. Other outcrops were darker in color and very soft and flaky like a slate or shale. Most outcrops had a characteristic ‘rusty’ orangey brown color, fine to medium grain size and relatively soft - they would scratch under a steel knife. Upper surfaces on outcrops showed pits caused by acid rain dissolving away the carbonate. The unit contains small spiral shells, roughly 2mm in diameter. The bedding planes are all parallel and the horizons of this unit are rich in quartz.

Unit five is approximately 10m thick. This unit is fairly coarse-grained grey, a poorly sorted, gritty quartzite rock. This unit is not as hard as unit two and has small calcified worm tubes, some of which have been weathered out, this would suggest there is carbonate in this units cement. This unit weathers to a slightly brown color but fresh surfaces are a dull grey.

Unit six was only observed at 30m thickness but extends much further. This unit has a fine-grain size, the surface of this unit was a dull grey and leaden color and lichens nor heather grew on top of it because of the lime-rich soil caused by the chemistry of this unit. This unit is aphanitic and medium to dark grey on fresh surfaces, the texture of these surfaces was smooth. Rainwater seemed to have dissolved the surface of the outcrops, this unit also effervesced with a dilute hydrochloric acid, which would suggest the presence of carbonate. Many of the outcrops were fractured due to fluids passing through the cracks. Trace fossils such as feeding burrows and ooids can be seen in this unit. Planar laminations were observed.

Tectonic features within the mapping area: The main feature in this mapping area is the series of low angle reverse faults, otherwise known as thrust faults. These thrust faults can be seen stretching north-south in the center of the map, there are alternating bands of unit five and unit four. The surface exposure forms complex imbricate structures. The hanging wall of the faults is to the east.

Thrust faults are formed in compressional regimes, subsequently, the thrusts seen in the center of the map were likely formed in an orogeny, the older unit four beds have been thrust onto of the younger unit five beds. The thrusts climb the stratigraphy and form a ramp-flat geometry of folds, otherwise known as ‘piggy-back’ thrusting, this is what is responsible for the repeated beds seen on the map. The tectonic activity responsible for the thrust faulting will have also led to some folding, this is when the rock undergoes plastic deformation but hasn’t reached its critical yield - beyond which brittle deformation takes place and faulting occurs.

It is also worth noting that an angular unconformity can be seen at the base of unit two, located in the southeast corner of the map. Overstepping is displayed, where the unit cuts across the boundaries of the younger units such as unit six. There is also a difference between the dips of the unconformity and beds below it - unit two generally has shallower dips than unit six.

Discussion:

My unit 1 is equivalent to the Lewisian Gneiss complex. The Lewisian Gneiss was formed during the late-Archean, as plutonic suites intruded into lower layers of the crust. It is likely that the protoliths of the Gneisses formed at active plate margins (Mendum et al., 2009). The Lewisian Gneiss has likely come from a mid-depth crustal area, most probably formed in an Andean-type margin or island arc. Island arcs were very common at the time of formation and were the prime areas for crustal growth. The island arcs also led to the formation of cratons which are an amalgamation of numerous micro-plates (Mendum et al., 2009). The Lewisian Gneiss’ are the oldest rocks that can be found in the British Isles, and indeed the oldest in the world, the Lewisian complex has a long and extensive history of study by geologists such as James Hutton, “the father of modern geology”.

With recent technology the radiometric age of the Lewisian Gneiss can be accurately determined, this has considerably revised the understanding of Scottish Archean crust and disproved the idea that previously stood where the Lewisian represented a continuous crustal block with the same geological history. Eriboll’s Lewisian gneiss is younger at 1.7 billion years old than some found elsewhere in northwest Scotland such as Assynt which is 3 billion years old, this highlights the different origins, in fact, it is not until 1740Ma ago that they fully share a common history (Mitchell et al., 2017). As the Lewisian gneiss complex has been reworked in the thrust belt it is difficult to unravel the geological history. Foliations, folds, and shear zones are visible and these would have occurred under ductile deformation.

Originally these rocks were probably granitic, then volcanic heat and pressure caused metamorphism and altered the structure. This is the reason for the large variations within this unit’s layers are displayed, ranging from white to pale grey to dark grey. The pale bands in banding most likely contain feldspar whilst the darker bands are dense mafic minerals, perhaps biotite mica or hornblende (Mendum et al., 2009). The intense heat and pressures from metamorphosis and burial have allowed for mineral growth within the unit (Heb, 2014).

My unit two is equivalent to Basal Quartzite. Cross-bedding is a common feature of many sandstones, this is because sediments tend to settle in relatively flat layers, this is called the principle of original horizontality (Dawes, 2011). The principle of original horizontality is one of the most important principles for determining relative geological age, however, not all sedimentary beds are horizontal, to begin with. Cross-beds begin as inclined beds, which are formed by sediment piling up on the slopes of ripples in the sediment. Therefore cross-bedding is due to being down-current. This could suggest a shallow marine environment, such as an alluvial area.

In the case of unit two, the clean quartz sand may have been deposited in a tidal near-shore flat environment and the crossing beds present the sloping sides of sandbars against which the layers of sand were laid down (Waters, 2003). Outcrops in the Eriboll district showed extensive cross-bedding. The depositional origin of Basal Quartzite is likely of similar marine origin to unit three.

My unit three is equivalent to the pipe rock formation. Outcrops were often purple in color due to staining by iron and manganese oxide (Mitchell et al., 2017), the vertical burrows observed in the pipe rock are useful as they act as natural strain markers, which can be used to assess thrust-transport directions (Mitchell et al., 2017). This unit is mature, heavily bioturbated, with long, vertical Skolithos burrows which likely belong to filter-feeding organisms. The abundant cylindrical burrows are orientated perpendicular to the bedding plane. These trace fossils are very useful when inferring the unit's environment of deposition, this organism would have existed in a near-shore, subtidal shallow marine and high-energy environment where food was delivered to the burrowing organism by the currents and tides and sunlight would be abundant for the suspended algae and nutrients to thrive.

My unit four is equivalent to the Fucoid beds, outcrops were particularly rich in potash which has been attributed to diagenesis after the deposition of the Durness limestone. The environment of deposition was likely an off-shore shallow marine environment due to the presence of shallow marine fossils such as Skolithos. A range of fossils can be observed in the Fucoid unit, including trace fossils formed by burrowing deposit-feeding invertebrates. The presence of these fossils allows us to infer a shallow marine, perhaps lagoonal environment of deposition. The low-angle planar laminated and graded beds may also indicate a storm-dominated tidal environment (Trewin, 2018).

My unit five is equivalent to the Salterella Grit, the presence of calcified trace fossils of Salterella worm tubes, in addition to the Skolithos burrows that are also seen in the pipe rock unit suggests a marine environment of deposition. The worms would have burrowed into the silt on the seafloor of a shallow sea. This unit perhaps represents a regressive episode during which sandboxes migrated over a muddy shelf and were succeeded by sand sheets indicative of renewed transgression (McKie 1990).

My unit six is equivalent to the Durness Limestone formation, only 30m of this unit was observed but it continues for over 1km in thickness. Fossils within the Durness Limestone are very rare, this is partly because of recrystallization from the original Limestone rock, but those still found such as small cephalopods, gastropods and sponges were exceptionally complex for their time. Some trace fossils such as feeding burro traces and ooids can be observed, now as rounded mineralized bodies, usually composed of calcite or aragonite. The sequence is extremely abundant in algal structures such as stromatolites. This life increased oxygen levels in Earth's atmosphere through photosynthesis, encouraging the evolution to continue. This unit indicates a shallow marine depositional environment, because of the planar laminations were seen in the outcrops (Scott, 2016). This type of environment would have created the conditions necessary for the organisms now fossilized to form. Some slumping was also observed at the base of the limestone unit which could indicate an unstable reef slope palaeoenvironment. The deposition of the carbonate that makes up this unit, usually only occurs in environments where there is a lack of siliciclastic input in the water, siliciclastic input increases the cloudiness of the water which prevents light from entering. Silicate minerals are much harder than carbonate which means the carbonate minerals would be eroded by the silicates. Carbonate deposition usually requires warm water because this increases the abundance of carbonate secreting organisms.

Tectonic features within the mapping area: Features indicative of the Caledonian orogeny include shearing between beds and thrusts between the pipe rock and limestone. The cross sections in my map are also useful for illustrating these thrusts and imbricate faulting. After the orogeny, deformation took place in the form of extensional faulting, evidence for this extensional regime are slickensides on fault planes.

The geological history of the Eriboll region is extensive, stretching from the Archean to the Holocene and outcrops provide a rich diversity of rock types. The basement of this region is the Lewisian gneiss, found in the southeastern corner of the map, this unit was the first to form, some 3 billion years ago. Small outcrops of Lewisian Gneiss can also be found in various other locations in the Eriboll region, but these small outcrops are likely to be drop stones from glacial processes. The protolith gneiss likely underwent high-grade metamorphism, then later, high-temperature reworking which formed migmatite (Scott, 2016). This was then followed by the formation of the Moine Thrust which created a highly sheared unconformity. The sandstone protolith was clay-rich, likely deposited in a shallow marine environment. The protolith then underwent low-grade metamorphism which created a clearly defined cleavage that was perpendicular to compression, the compression was likely due to an orogenic event (Scott, 2016). The gneiss may have been formed as part of a volcanic island arc during an event of crustal growth around 2.7Ma ago (Mendum et al., 2009). The first member of the Cambrian units to be deposited on top was Basal Quartzite, the second youngest rock on the map, found in the southwest corner, this unit was likely deposited in an alluvial environment, perhaps as sand on a beach. This was then likely followed by a marine transgression which accommodated the deposition of the Pipe Rock unit. The Fucoid and Salterella grit were then deposited on top of the Pipe Rock which indicates another marine transgression but this time then followed by a regression (Butler, 2010). This marine regression led to the deposition of many carbonate materials which contributed to the formation of the Durness Limestone unit. The lower section of the Limestone unit formed with low siliciclastic input, perhaps in a tropical region, this is because generally, carbonate deposition only occurs in environments where there is a lack of siliciclastic input in the water, this is because siliciclastic input increases the haziness of the water which prevents light from entering, also because silicate minerals are much harder than their carbonate counterparts which means the carbonate minerals would be eroded by the silicates. Carbonate deposition usually requires warm water because this increases the abundance of carbonate secreting organisms.

Many thrusts can be seen in the center of the map. These thrusts were a result of the Moine thrust which caused a compressional regime during the Caledonian orogeny. The thrusts carried the older units onto of the younger units, namely the fucoid on top of the Saltarella. The Archean Lewisian Gneiss was carried back to the surface and then traveled westwards by thrust faults, it can, therefore, be seen truncating the younger sedimentary beds such as the Fucoid in the southeast of the map.