Draft dated 3rd Nov 2006
A report on the geology of
Fladdabister and Quarff, Shetland
Sidney Sussex College
2. Rock types and stratigraphy
The area investigated spans from Cunningsburgh to Brindister, south-east Mainland, Shetland. The region comprises Dalradian metasediments thrust against a tectonic melange covered with Old Red Sandstones.
The metasediments originated as sea-floor sediments in an intra-continental basin, and were emplaced upon its closure, linked to the closure of the Iapetus. [REF]
They are thoght to correlate the latest Middle Dalradian (REF). The melange consists of metamorphosed blocks sourced from locally exposed rocks and others, and of granite; the overlying ORS consists of a subrounded conglomerate topped by channel and lacustrine deposits.
The terrain is low moorland with excellent exposure at the coast and at places inland, but with the larger hills poorly exposed.
There has been no previous mapping at this resolution, with maps rarely agreeing on details and generally grouping the rocks of the area into five or fewer units (REFS of maps).
2. ROCK TYPES AND STRATIGRAPHY
2.1 Clift Hills Succession (CHS)
This unit consists of steeply dipping, well sorted mudstones, with occasional calcareous, sandy, and quartzite layers, and contains mainly planar beds – occasionally (e.g. [qn]) some large scale cross bedding is evident; climbing ripples and tool marks appear at [rd] ((Fig rd2 – tool marks). (Palæocurrent needs interpreting). Whilst most beds are very fine grained, some (e.g. [wc]) are pebbly, gravelly or sandy. (REF) suggests that turbidite facies are present though I did not have time for in-depth facies analysis of this unit, which was rarely well exposed.
It is lightly metamorphosed to a silver-cyan colour occasionally with small (⅛mm) black crystals visible, with abundant (<40%) Muscovite.
Minor deformation is indicated by occcasional quartz veins and kink bands in bedding.
At [wk], cyclic repetitions of bed thicknesses and colours are observed every few cm, which could be interpreted as Milankovic cycles, – suggesting a very slow deposition rate; as seasonal variations (e.g. varves), suggesting a very high deposition rate; or as variations in basin chemistry and oxygen levels – which fails to explain the cyclic nature.
2.2 Tectonic Melangé (TcM)
This is a band of juxtaposed metamorphosed rocks of widely varying grade, with granites prevalent in the Quarff area. Each rock type may outcrop for ~5m²-2500m². It does not obviously contain any members of the ‘Quarff Succession’, although (REF) suggests that it contains metamorphosed fragments of both this and the Clift Hills Succession, as well as rocks from unknown sources. Whilst this interpretation would explain the apparent thickness variation, the failure to identify a CHS block with a TcM member to its west makes me suspicious of its validity. Also a far greater proportion of the Tectonic Mélange was calcareous than the local sediments and indeed the eastern succession hypothesised by (REF) as a potential source, suggesting a different provedance – possibly of units now eroded.
A typical TcM member is represented by the schist at [be] (Stack of Okraquoy) (Figure 2.*). Its crenulation does not seem related to the north-south axis expected due to the shape of the TcM unit; the wide spread of deformation directions in all members suggests either that they were rotated after initial deformation, or the presence of a highly chaotic deformation field.
A nearby marble, the ‘Ukinsetter Veined Marble’, is described at [?] and its high degree of veining is indicative of a high degree of stress; the ‘Speckled Schist’ [?] nearby has clearly been metamorphosed to a far higher degree than anything else in its locality, suggesting that the TcM may draw on members far deeper than the surrounding rocks – or at least that high P&T were present during its formation.
The boundaries of the TcM are locally rough but clearly faulted.
2.3 Quarff Succession
The Quarff Succession (after Mykura 1976), the basement to the east of the Tectonic Mélange, consists of a three conformable sequences of metasediments, increasing in grade to the north.
2.3.1 Hevda Quartzite (Qte)
Generally poorly sorted, this unit usually consists of planar beds, with occasional large-scale cross-bedding (Fig ca), and some channel cross-bedding (Fig ua).
It is compositionally mature – around 95% quartz, with some Fe2O3 in places.
It is probably a relatively deep water deposit close enough to land to occasionally be moulded by channel-forming flows.
Or – could it be flood-plains and levée deposits? Let’s check the log!
2.3.2 Trumba Gritstone (ORB in field, now TrG)
This overlies – seemingly conformably – the Hevda Quartzite at [ch] (Trumba).
It consists of interbedding on a cm scale, with beds varying in clast presence and size distribution, and in matrix calcareous content. Most beds are parallel but some are truncated (Figs uc).
(REF) describes the grit as predominantly white quartz with acid plagioclase, although I also identified with some calcite. It is angular to subangular, sizes ranging from 1mm to 2cm, locally 5cm. Some gritty beds are grain supported, the matrix – which varies compositionally from quartzite to marble - generally found to be continuous with the non-gritty bed above. Clast long axes (Figure * - we need to rotate data with bedding but they’re clearly grouped with a palæocurrent of about 200° ) lie in the bedding plane and suggest a flow direction of 030° or 210°, but short axes (Figure *) are only poorly centred upon the vertical, suggesting a weak velocity gradient (Rees 1968).
The more calcareous beds erode deeply (up to 10cm in situ erosion product) and with a karstic nature, resulting in a ‘honeycomb sandwich’ effect.
Report upon log.
2.3.3 Fladdabister Bay Limestone (FbL)
A fine grained limestone with blue, pink and yellow beds, with the degree of calcareous material highly variable, leading to a high-relief weathering profile.
Cross bedding (Fig vd) and herringbone structures (Fig vd) are visible in places.
Calcite nodules and venulets, and chert nodules, are present in places. This is interbedded with the Trumba Gritstone for a distance of 200m to the south, and with a dark bluish-grey, poorly sorted, slightly micaceous sandstone to the north. This sandstone contains herringbone cross-bedding (Fig vb) as well as channel fills.
2.3.4 Fladdabister Bay Schist (FbS)
This is seperated from the limestone by a fault, and comprises a light pink-grey metasediment, with very strong foliations obliterating original bedding, these planes being lineated – the lineations possibly resulting from the intense small-scale chevron folding seen in places ([fv], Fig fv). Grains appear aligned. The rock is very porous.
2.3.5 Brindister Basement Gneisses (BBG)
A sequence of permeation gneisses (Miller & Flinn 1966), some strongly banded and isoclinally folded, in places (kp, nf) cut, and partially molten (Fig kp), by sills and dykes of varied composition
2.4 Old Red Sandstone sequence
The ORS sequence lies unconformably, with an uneven base (Fig ma), upon the Quarff Succession.
2.4.1 Basal Breccia (BFC)
I would call the Basal Breccia (after REF) a conglomerate due to the sub-rounded nature of its cobble-sized clasts, which are supported in a grey slit-sandstone matrix, containing occasional lenses of Brindister flag, oriented with long axes at ~15° to bedding. See Figures
The nature of the intraclasts reflects the underlying basement – they have barely been transported.
This unit is highly resistant to erosion and thus forms prominent ridges and hills, the current valleys presumably once filled with ORS.
2.4.2 Brindister Flag (ORS)
This unit, named according to Finlay (1926), after Peach + Horne (1879), consists mainly of a well sorted purple-red sandstone of 0.1mm grains of quartz (65%), mica (20%), and [ms6091] ¿pyroxene/olivine? (15%), which forms parallel beds typically a millimetre thick, up to 2cm in places. It contains evidence for higher energy events, for example occasional pebbly beds showing pronounced fining-up (Fig ez)., and mudstone rip-up clasts (at [si]).
It splits readily along bedding surfaces into flags, resulting in large flat ledges.
Cross-bedding (Fig ba, sa) and ripple marks (Fig aa, ac, xb), with inferred palæocurrent direction being south-southeasterly, are occasionally visible; dubious dessication cracks are visible locally (Fig sh).
North of Okraquoy bay, this unit is interbedded unconformably with Basal Breccia. It is probable that up to three facies are represented in this unit – see section 4 (sedimentology).
Due to the young landscape and absence of rivers, the abundant drift is peat (where it has not been harvested), reaching 3m thickness in places; however the Burn of Laxdale is large enough to have deposited a small region of alluvium which is notable in places. Till is mostly absent but noted in Fladdabister, where topography may have trapped it and prevented its remobilisation.
Faults separate the region into four main regions, shown on Figure 3.1:
· To the west, The Clift Hills (CHS), highly folded and bounded by the Tectonic Mélange (TcM);
· To the east, at Aithsetter and Brindister, barely deformed sandstones (ORS);
· In the central east, an uplifted section where Basal Breccia overlies a Dalradian near-shore sequence and a metamorphic basement.
3.1 The Clift Hills
The Clift Hills comprise a nappe sequence of deep ocean sediments isoclinally folded to form steeply dipping beds.
3.2 The Tectonic Mélange
The Mélange is bounded on its west side by an eastwards-dipping faulted contact of dip ~15°, calculated by the degree of veeing apparent in the Quarff gap. On the east, the boundary is almost independent of topography and thus must be almost vertical. The wedge-shape thus generated can be resolved in two manners; either the top is open (Figure 3.2a) or closed (Figure 3.2b).Given the size of the blocks and the difficulty in continuing the wedge downwards, the open approach looks unlikely. The simplest way to close the system is to assume it’s lens-shaped (as Figure 3.2b), perhaps caused by a kink in the original fault (c.f. NW’s interpretation of fault breccia in that Gill in Cumbria, REF[ms6092] , on a larger scale.)
Whilst the fault would be required to have a degree of vertical motion this would not limit its lateral motion.
The outcrop pattern has been interpreted as the onshore half of a tight syncline (REF), which would agree with the suggestion that alluvial fans were sourced from the east. However the near or total disappearance of the TcM near Clepps is difficult to fit in this pattern and thus faulting is difficult to rule out; were it a fold would be appear to be a type three fold (REF, check) but with a much wider nose (south shore of Okraquoy Bay) than one would expect.
3.3 Brindister Flag in north and south
The Aithsetter and Brindister regions comprise largely undeformed Brindister Flag which presumably overlies Dalradian or older basements as exposed between Burland and Okraquoy. They indicate that the region has been tectonically quiet since the Devonian.
3.4 Dalradian window
The area from Okraquoy to Fladdabister is a right-way-up, conformable sequence of shallow marine grit-, sand- and lime- stones presumably deposited comteperaneously to the Clift Hills Succession on a nearby continental slope, and merely tilted by the ocean closure event, without significant folding or faulting. Its uplift relative to Aithsetter / Brindister may result from partial reactivation of the Tectonic Mélange bounding fault, or east-west faulting cutting through all units.
3.5 Metamorphic basements
The metamorphic basements – FbS, schists, and BBG, gneisses – presumably underlie the Dalradian sediments. The change in degree of metamorphism may be due to greater burial depth or overloading (hence pressure) or due to chemical considerations – both probably contribute.
These are overlain unconformably by Basal Breccia, and thus must have been uplifted to around their present positions – with most uplift in the north - before the Devonian deposition, detailed in Section 4.
3.6 The Quarff Gap
The ‘Quarff gap’, a 180m deep valley crossing the mainland at Quarff, is a striking feature of the region, with two competing hypotheses for its formation.
Most of Shetland is characterised by its prominent north-south glacial valleys exploiting preferentially-eroding limestone units. It may seem unusual to suppose that a small east-west valley would arise.
It may have been the result of a deep valley from an early glaciation whilst the current valleys were unformed, the modern valleys merely the easiest gouged by a waning phase of glaciers.
The valley does have a perceptable ‘U’ shape – but no other east-west features of this scale are seen on the isles.
A river may have originally carved the valley. Perhaps the Clift Hills formed a dam to a lake topping the limestone beds which was breached here (which would imply a west-east flow direciton – but the catchment area in this direction is small, and the watershed lies well east, perhaps suggesting an initial east-west gradient?). The lack of fluvial deposits, and the ‘U’ shape, could be explained by a pre-existing depression being amplified by glaciers; this would explain the paucity of other such channels (also found less developed in Voe and Yell (REF), though neither were as impressive or obvious (personal observation) (MAP)).
(REF) suggests that the top of Devonian sediments follow a veeing topography under the East Voe of Quarff, which are covered with Triassic sediments, seeming to confirm this hypothesis and constraining the date of incision.
- Comments on deductions around Fladdabister Bay
- Comments on stereoplots re. [BFC]
- Comments on CHS, including possibility of Milankovic cycle record
- Palæocurrent on ORS, including [ig]’s ‘concentric ripples’
The Okraquoy-Fladdabister sequence was deposited upon a metamorphic basement, then forming a continental shelf, in the Dalradian.
Simeltaneously the Clift Hills Succession (CHS) was forming as a near-shelf deep sea deposit on the Iapetus Ocean floor.
The closure of the Iapetus resulted in the napping of the CHS against the Okraquoy-Fladdabister sequence, a large fault zone bounding them. Sediments from the accretionary wedge may have been incorporated into the fault zone to contribute to the Tectonic Mélange.
The continental basement bay have been uplifted during or after this event, raising the Fladdabister Schists (FbS) and the Brindister Basement Gneisses (BBG) to the surface. These gneisses were injected by igneous activity on several occasions, possibly by back-arc processes and possibly due to depth-related partial melt.
A (broadly) north-south valley was filled with sediements during the Devonian, with the Basal Breccia forming fans on the slopes, and a river – occasionally becoming blocked, possibly by landslip-induced dams, to form shallow lakes – flowing down its centre from north to south, its deposits forming the Brindister Flag. This river may have connected lakes to the north (Bressay) and south (Exnaboe, Sumburgh) (Ref), and on a larger scale to the Orcadian basin (REF); as subsidence continued the valley may have become completely filled.
A river flowed from east to west, forming the Quarff gap, this was amplified by glaciers during the Pleistocene.
Isostatic rebound has raised the isles to their present position
- Hevda Quartzite is channel fill, which is overlain by Fladdabister limestone when the region becomes submerged and thus lacustrine. See [va]. Then the introduction of tides [vb] – so must in fact be shallow sea.
Mykura, W. 1976. Orkney and Shetland. British Regional Geology. Edinburgh: HMSO
I am very grateful for the interest and support support of Peter Friend and Hazel Chapman, and the advice and assistance of Neils Hovius and Nigel Woodcock.
7. MORE FLOATING PIECES OF INFORMATION TO INTEGRATE
Quarff Succession [defn: BBG, FbS, Fladdabister Grits, Qte] overrode the CHS – to give TcM
Flinn (date) suggests TrG correlates with a slice near Rova Head – other side of TcM, rest of succession correlates with the top of the East Mainland Succession.
My doubts: All seemed conformable; some literature (Flinn, 1967) states that it’s not found elsewhere on Shet. I think that the literature (the ~1977 revision of Shetland Stratigraphy??) states that the correlation with the Laxfirth Limestone of the FbL is unwise.
TrG / ccd are distinct from the rest of Shetland.
Grit in TrG is mostly Quartz, with some acid plag.
Miller & Flinn (1966) – Phases of metamorphism
TcM contains units from above and below it – as well as some of unknown origin.
Predates the lamprophyres as one cuts it.
Dating suggests that the Quarff Succession is significantly older than the CHS – but the accuracy of this is debatable. Could the age refer to the date of the metamorphism? Stratigraphically it’s suggested that the QfSccn is younger than the CHS.
My view – nothing to forbid them from being simeltaneous.
BBGs are called ‘permeation gneisses’.
Upper Dalradian age due to shift to deep water facies, and correlation to mainland Scotland. But is this wise? Later (Flinn et al 1977?) moved to Mid Dal.
P A Allen 1981 – Sed. Geol 29:31-66
Sediments and processes on a small stream-flow dominated Devonian alluvial fan, Shetland Islands
(Set at Rova Head).
Similar to BFC?
Allen and Marshall 1981 – Scot. J. Geol.
BFC referred to as Basal Breccia; red siltstone to very fine sandstone as occasional matrix – infiltrated after deposition
My thought – helped washing and rounding of clasts?
Millburn valley’s lenticular fabric suggests fluvial deposition; some grading.
Dating problems occur as there are no mainland correlates after the Eday flags; also poor miospore preservation (Marshall 1980) – can place near Frasnian boundary.
Finlay (1926), Peach + Horne (1879)
Named the Brindister Flags – including those occuring at Aithsetter.
Harris et al, 1994. In T. G. S. – A revised Precambrian correlation
Clift hills are Upper Dalradian: Lower Southern Highland group.
R. V. Fisher – features of __ fluids and their deposits. J. Sed. Pet. 1971
Debris flows move over surfaces that would easily be eroded by normal traction flow (i.e. streams).
Can come to rest with very steem sides and fronts.
Does this suggest that they could have been rather wide?
The production of preferred orientation in a concentrated dispersion of elongated and flattened grains – Rees, A. I., J. of Geol. (Calif) 1968 pp457-
Grains align with the flow boundaries if they collide. Short axes point to maximum velocity gradient.
Interesting question of why so much scatter. Maybe they didn’t move very far?
[ms6091]SCALE! On Ripple Marks at aa