Upper Melby fish beds
Clumpers. Cross-bedded sandstone.
Upper Melby fish beds
Ness of Melby. Bioturbated sandstone.
Huxter and Melby
Significance:
Sediments within the Middle Devonian (Old Red Sandstone) Melby Basin. The Melby
basin is separated from the older Walls basin by the Melby (also known as the
St. Magnus Bay) Fault, which has been interpreted both as a transcurrent fault
and as a reverse fault.
The Melby Formation, with an exposed thickness of about 760m, lies within the
Melby Basin and can be divided into two lithological groups. The lower group is
exposed as a 600m thickness of buff and red sandstones, pebbly sandstones and
sandy siltstones. Near the base of this group two lacustrine siltstone and shale
beds, (the Lower and Upper Melby Fish Beds) containing plant and fish remains,
intercalate with the sandstones. These sandstones were laid down on alluvial
fans and plains by rivers flowing from the WNW but on at least two occasions
were submerged by a lake which blanketed the sandstones with silts and muds.
Correlation of the Melby fish-bed fauna with those of Caithness and Orkney
suggest that it was the large Lake Orcadie that transgressed the area.
The upper group of the Melby formation consists of thick beds of pink
feldspathic sandstone containing plant remains, volcanic clasts and thick beds
of bioturbated purple sandy siltstone. Currents flowing from the ENE deposited
these sandstones; the change in direction from the lower group was probably due
to the formation of volcanic highlands to the north-east, which diverted the
older drainage system. The bioturbated siltstone may have been laid down in
localised lakes formed by ponding of the run-off from the newly formed
highlands. Large pieces of plant debris among the pebbly sandstone may indicate
that these newly formed volcanic areas were providing fertile soils for the
establishment of large plants.
These new highlands may well have been formed by the earliest part of the
volcanic activity which produced the volcanic rocks of Papa Stour, and later,
Eshaness. The two groups of rhyolite flows (exposed below the church and on the
Ness of Melby) may be equivalent to those of Papa Stour placing the Melby
sandstones stratigraphically below the volcanics of Papa Stour.
Locality 1. Clumpers. HU 174 575 to HU 177 576. Lower Group Strata below the Lower Melby Fish Beds. Lower Melby Fish Beds.
The strata below the Lower Melby
Fish Beds are well exposed in a NE traverse along the shore and can be divided
into two sandstone units. The Lower Melby Fish Beds are exposed in the cliffs in
the south-west end of this section then make a short incursion inland to
reappear again in the east end of the section, a small headland at HU 177 575.
The sandstones of the lower unit are mainly of a reddish-brown, medium to coarse
grained, poorly graded, arkose representing channel fill deposits. Beds of finer
grained siltstone representing overbank deposits have mostly been eroded away by
migrating channels of straight or braided rivers.
The upper unit is a pale buff to ochre-weathered, medium to coarse grained, well
graded arkose. These pass upward into finer grained sandstone alternating with
beds of silty shale and sandy siltstone. The beds of the upper unit were laid
down by slower moving currents of the rivers as they spread out over the
alluvial plain occasionally flooding to leave overbank deposits.
The dip of the foresets of the cross-bedded sandstones and trend of trough
cross-bedded units is to the ESE and SE indicating current flow from the WNW and
NW.
The Lower Melby Fish Beds can be divided into three units. The lower unit is
laminated pale grey siltstone and silty mudstone with thin sandy layers and
includes sand filled desiccation cracks and plant remains. This unit represents
shallow water mud-flat deposits that periodically dried out in the sun.
The middle unit is a pale to dark grey, fissile, shale containing bituminous
bands which alternates with a mudstone that contains carbonate-rich ribs and
nodules (concretions). Ribs are up to 35mm thick and 2m long; the nodules 25mm
thick and 75mm long. Fish remains are mainly from the nodules that have formed
around them. This unit represents a change to deeper water (lacustrine)
conditions when thinly laminated sediments with alternating carbonate-rich and
bituminous muds were deposited.
The upper unit consists mainly of shale with fine to medium grained lenses of
sandstone with abundant plant debris. This unit with its aligned plant debris
may represent a river and stream delta environment during a period when the lake
was becoming shallower.
Locality 2. Pobie Skeo to Matta Taing. HU 168 566 to HU 165 560. Lower Group.
Strata between Lower and Upper Fish Beds. Upper Melby Fish Beds.
The Upper Melby Fish Beds are
exposed along the cliff-tops at Pobie Skeo above fluvial sandstones and
siltstones. The lithology and interpretations here are similar to that at
locality 1.
A deep section has been cut through the Upper Melby Fish Bed and adjacent strata
at Matta Taing and has been interpreted as a transgression-regression cycle. A
narrow zone within the fish bed contains small (5-20mm), zoned sulphide nodules
containing pyrite, chalcopyrite, covellite, bornite, sphalerite, galena and
tennantite. The complex formation of these nodules was probably due to chemical
changes in the groundwater as the lake transgressed and regressed.
Locality 3. Ness of Melby HU 186 580 to HU 184 577. Melby Rhyolite. Upper
Group fluvial sediments.
The rhyolite varies in colour on
weathered surfaces from pink to brown and is purple on fresh surfaces. Scattered
pink phenocrysts of feldspar occur throughout the rock and many surfaces show
marked flow banding, especially in the cliffs on the north-west corner of the
ness. Flow banding, or 'streaking out' of layers of slightly different
composition occurs in highly viscous lavas and is due to slight variations in
the composition and texture of the original lava becoming accentuated as the
flow congeals and crystallises.
The base of the rhyolite is brecciated and very uneven consisting of blocks and
tongues which have penetrated and distorted unconsolidated sediments beneath.
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Source Material and Detailed Work on the Area.
Hall A.J. and Donovan R.N. - 1978. Origin of complex sulphide nodules related to
the diagenesis of lacustrine sediments of Middle Devonian age from the Shetland
Islands. Scottish Journal of Geology, 14, (4), 289-299.
Mykura W. and Phemister J. - 1976. The Geology of Western Shetland. Memoir of
the Geological Survey of Great Britain. HMSO, Edinburgh.
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