Devonian of SE Shetland

Significance: during the upper Devonian (around 375 Ma) the south-east Shetland basin was a NNW-SSE elongated basin which lay about 100km north of the larger Orcadian basin. It is likely that this was a defunct rift valley that contained a series of small lakes joined by major rivers flowing to the SSE. Sedimentation into the valley may have been tectonically controlled during its early life when coarser fluvial sediments were laid down but most of the sediments now exposed represent a period of quiescent infill of the basin. Source rocks for most of the sediments were the schists and gneisses of the Caledonian Mountains, which now form the deeply eroded basement beneath the sediments. The highly arkosic nature of the sandstones indicate erosion in an arid or semi-arid environment followed by rapid transport, deposition and burial. Erosion exposed the Aith-Spiggie intrusive complex as well as granites of the type now seen in the Northmaven area.

Locality 1. East Voe of Quarff. HU 432 353 to 440 358. Basal breccia.

The unconformity between coarse basal breccia-conglomerate and the underlying Quarff Nappe metasediments. Outcrops of the dark grey psammitic metasediments with occasional pegmatite intrusion and outcrops of very coarse breccia. This coarse breccia was formed as scree slopes on hills and mountains composed of what is now the underlying metasediments. The largest outcrop shows breccia consisting of very coarse and angular clasts cutting steeply dipping metasediments of the Quarff Nappe – a ‘fossil’ scree slope.

Locality 2. Gulberwick. HU 447 387 to HU 450 385. Braided stream environment.

      The medium to very coarse sandstone displaying trough and planar cross bedding above channel lag pebbles were laid down in active channels of braided streams. Fine grained red siltstones and mudstones with desiccation cracks are overbank deposits from flood events. Thicker beds of fine-grained sandstone formed from suspension in abandoned channels.

Locality 3. Shingly Geo to Broken Brough. HU 406 124 to HU 409 135 Lake shoreline and lacustrine environments (including Exnaboe fish beds). Aeolian  sandstones.

            Shingly Geo north-east along the shore at The Cletts to the Point of Blo-geo. The three main lacustrine facies recorded in this area are:

1) Horizontally stratified fine dark grey sandstones with inter-bedded siltstones and rare thin dark mudstone. Some low angle planar cross-stratification and ripple cross-lamination in the sandstone. Common convolute lamination, slump and load structures. These have been interpreted as lake marginal environments dominated by the high energies in the surf zone but just lake-ward of the breaker zone.

2) Ripple cross-laminated and horizontally-laminated siltstones with plant remains, sub-aqueous shrinkage cracks and convolute lamination. This has been interpreted as an intermediate environment characterised by continuous to intermittent wave agitation.

3) Laminated and un-laminated calcareous siltstone and mudstone with no evidence of current or wave action. These sediments are interpreted as having been deposited offshore below storm wave base. Fish fauna have been recorded in the darker grey laminated beds at the head of Shingly Geo. The preservation of organic matter and the laminated nature of the beds suggest deposition in an anoxic environment below a thermocline. The un-laminated beds suggest that the thermocline was not permanent.

The main non-lacustrine facies in this area are:

1) About 250m north-east of Shingly Geo at The Cletts, and stratigraphically below the lacustrine facies, there are good exposures of a pebbly sandstone displaying horizontal and some high angle cross-stratification of braided streams.

2) At Swart Skerry, just above the Exnaboe fish bed facies, is about 10m. of coarse grained flat bedded sandstone with current scour and fill troughs, and some high angle planar cross-sets. These have been interpreted as being deposited close to a zone of high sediment supply, i.e. a river outflow.

            Cliffs show excellent exposures arkosic sandstone displaying very large scale cross-bedding with sets of 3 to 5m. These are interpreted as having been sand dunes, mostly transverse and barchanoid, formed by prevailing southerly winds.