Letters to the Editors – Lyell Collection

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Letters to the Editors

AGE OF THE CAMAS MALAG FORMATION, SKYE

SIRS
Nicholson (1978) has lucidly described in detail hitherto enigmatic beds
found at the base of the Liassic succession in Loch Slapin (Skye). Field work
carried out in the area (Amiri-Garroussi 1978) suggests that the beds described by
Nicholson represent a locally developed palaeokarst topography in the Durness
Group which was buried as a result of an advancing Liassic sea. Although attempts
to find fossils in the described beds proved to be unsuccessful, there exists very
strong circumstantial evidence for suggesting an Early Liassic age for them.

Liassic sedimentation took place under non-oxidising, warm and humid (?trop-
ical at times) climates i.e. the type of euxinic condition likely to result in the
deposition of dark, pyritic, laminated sediments. Indeed the matrix of the sedi
ments described by Nicholson shows a striking compositional similarity to the
semicostatum Zone shales and siltstones present in the area, which are highly
micaceous and also contain some strained and polycrystalline quartz fragments,
which probably resulted from
the erosion of Precambrian-Palaeozoic schists,
gneisses and granites in the immediate vicinity.

During Triassic (New Red Sandstone) times arid to semi-arid, warm climatic
conditions represented by continental facies red-beds, caliche and fluvial conglom
erates existed in north-west Scotland (Steel 1974a,b). Triassic “rhythmites” are
proposed by Nicholson (1978), in order to explain the local colour and composi
tional changes of the sediments. The evidence is interpreted as suggesting rapid
changes of climatic conditions in the area, together with lake deposits showing
alternating small scale cycles or sequences attributable to chemical or temperature
stratification of the waters. Such sediments are not found elsewhere in the Triassic
of Scotland.

Conglomerates not belonging to Triassic strata but clearly overlying them are
quite common elsewhere in the Skye area [NG 630164 and 585181]; this was
“inconsistent conglomeratic beds of
pointed out by Peach et al. (1910):
quartz pebbles in a calcareous and sandy matrix develop frequently in the lowest
part of the Lias and are not always easy to distinguish from
the Triassic
conglomerate . . .. the Liassic limestone near Boreraig changes laterally into con
glomerate of quartz and sometimes limestone”. These together with three other
distinctly recognisable conglomeratic beds can be found in the Skye area.

The earliest shallow marine Mesozoic transgression extended over the Mull
area in Rhaetian time, but did not reach the Skye region until late Hettangian (?)
and early Sinemurian, when semicostatum Zone silty micaceous shales overlap the
underlying beds to lie directly upon older Palaeozoic formations. During this time,

Scott. J. Geol. 18, (2&3), 247-249, 1982

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248

LETTERS TO THE EDITORS

in the hinterland affected

the sedimentation
periodic short-lived disturbances
pattern (Amiri-Garroussi 1977). Although protected marine and quasi-marine
conditions were created, true lacustrine deposits were absent in north-west Scotland
during Liassic times. It is possible that during this time, shallow, stagnant, isolated
bodies of water (?ponds) developed in the chemically eroded Durness Limestone
and were periodically disturbed by pulses of sediment b r o u g ht into t h em from the
erosion of nearby topographically higher areas. T he cross-stratified, orthoquartzi-
tic, angular pebble wedges within the dark silts and micaceous silty shales s h ow a
dominant
to have been
deposited concurrently w i th the semi-indurated siltstones and shales, as evidenced
by the presence of lenticular shale clasts contained within t h em and also by the
existence of flame-features at the interface of the t wo lithologically distinct beds.
Most of the sediment seems to be infilling hollows and near vertical fissure systems
up to 1 m long and 20 cm wide. In places large deformed and rotated blocks of
Durness Limestone are found in the dark siltstone matrix.

the south west. T h ey appear

transport direction

to

In the Loch Slapin area, the interstratal solution of the Durness Limestone
carbonates caused by climatic change probably produced a m o d e st palaeokarst
landscape during Early Liassic times; this was preserved as buried karst w h en
inundated by the turbid waters of later Liassic seas. Eroded surfaces of upstanding
Durness Limestone “pillars” were preserved within the dark muscovite-rich Liassic
(?angulata-semicostatum Z o n e) siltstone matrix; these together w i th some Durness
Limestone fragments produced “paraconglomerate” beds containing lenticular and
rounded clasts. T he formation of breccia and rock slides due to tropical karstifica-
tion, together w i th pseudostructural fissure caves and sinkholes as a result of
chemical erosion could also have produced the observed features. T he develop
ment of palaeokarst surfaces within the Durness G r o up in n o r th west Scotland has
described recently by Palmer et al. (1980).

Summarising, it is suggested here that the beds described by Nicholson (1978)
are of Liassic age (Pangulata-semicostatum Zone), representing the infilling of a
karst-type topography and shallow stagnant,
the
Durness G r o up Limestones of the Skye area. T he sediments were carried into these
features by a slowly transgressing Liassic sea and w e re also locally derived under
turbulent conditions.

isolated ponds developed

in

I thank D r. R. W. Cleverly for showing me the relevant outcrops in 1975. D r.
N. G. T. Fannin and an a n o n y m o us reviewer provided helpful c o m m e n ts on an
earlier draft of the manuscript.

REFERENCES

AMIRI-GARROUSSI, K. 1977. Origin of montmorillonite in the early Jurassic Shales of

NW Scotland. Geol. Mag. 114, 281-90.

1978. Sedimontological and palaeoenvironmental studies in the Broadford Beds
(Hettangian-Sinemurian) of north west Scotland. Univ. Oxford D. Phil. Thesis (unpubl.).

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LETTERS TO THE EDITORS

2 49

N I C H O L S O N,

R.

interbedded
C a m as Malag
R h y t h m i t e / c o n g l o m e r a te sequence of probable Triassic age, Loch Slapin, Isle of Skye.
Scoff /. Geol. 14, 3 0 1 – 9.

Formation:

1978.

T he

an

P A L M E R, T. J., M c K E R R O W, W. S. and C O W I E, J. W. 1980. Sedimentological

evidence for a stratigraphic break in the Durness G r o u p. Nature 287, 720-2.

P E A C H, B. N ., H O R N E, J., W O O D W A R D, H. B ., C L O U G H, C. T ., H A R K E R, A.

and W E D D, C. B. 1910. T he geology of Glenelg, Lochalsh and southeast Skye. Mem.
Geol. Surv. G.B.

S T E E L, R. J. 1974a. N ew Red Sandstone p i e d m o nt and floodplain sedimentation in the

Hebridean province of Scotland. /. Sediment Petrol. 4 4, 336-57.

1974b. C o r n s t o ne (fossil caliche)-its origin, stratigraphic and sedimentologic impor

tance in the N ew Red Sandstone, western Scotland. /. Geol. 82, 361-9.

Department of Geology and Mineralogy,
Parks Road,
Oxford OX1

3PR

K. A M I R I – G A R R O U S SI

MS. accepted for publication 12th January 1981