Notes on Stratigraphy and Structure of

Marloes Sands and Neighbourhood

The ages of the rocks in this region range from Late Ordovician to Late Silurian, and they include volcanic assemblages of the Skomer Volcanic Group, shallow marine sedimentary rocks of the Coralliferous and Grey Sandstone groups, and desert deposits of the Old Red Sandstone. Most of these rocks were deformed during the Variscan orogeny, in the Late Carboniferous, showing folds, cleavage, veins and other tectonic structures.

Skomer Volcanic Group

The Skomer Volcanic Group (SVG) is a c. 1000m thick pile of basaltic and rhyolitic lavas, pyroclastic deposits and an array of sedimentary rocks, which accumulated in a rift (the Skomer Basin) through Pembrokeshire during the Late Ordovician and Early Silurian. Lower parts of the pile were deposited under continental conditions but the volcanic terrain became progressively submerged by a shallow marine shelf. That shelf was situated on the northern flank of a landmass, Prettania and passed northwards into deeper waters of the Welsh Basin. The volcanic rocks are best exposed on Skomer Island itself and in the cliffs of the neighbouring Deer Park peninsula, but are largely inaccessible except by kayak and climbing rope. Conditions permitting, we will look at the volcanic rocks in Martin’s Haven but will spend most of our time on the section at Marloes Sands.

Martin’s Haven

We may visit Martin’s Haven briefly after seeing the Marloes Sands section. This little bay, the Skomer Island ferry port, provides outcrops of basalt, volcaniclastic deposits and beach deposits. They are mostly sea-weed covered but clean rocks are seen at the foot of the sea-cliff. Take care on the slippery rocks.

In the basalt, keep a look out for the following.

· White, well-shaped feldspar phenocrysts.

· Consider the significance of phenocrysts in the lava cooling history.

· Vesicles recording gas bubbles.

· Football-sized masses fringed by reddened material (??pillows, but unconvincing!).

Pyroclastic intercalations, look out for the following.

· Rock fragments: their shape; composition; proportion of matrix; ?mode of emplacement .

Rock unit on which the ferry slip-way is built.

· Form of rock bodies (?massive, ?bedded).

· Rock type (?igneous, ?sedimentary).

· Rock texture.

· Compositions of clasts.

· Shapes and size distribution of clasts.

· Arrangements of clasts (proportion of matrix? touching?).

· Environment of deposition.

Marloes Sands

At Marloes Sands, we will see rocks of the SVG at Localities B, C & D. Despite its name, the group in this section comprises predominantly sedimentary rocks, an association of conglomerate, sandstone, siltstone and mudstone that were deposited in a shallow sea fringing the main volcanic centres. Much of the sediment was derived from the volcanoes and some from older Lower Palaeozoic and basement rocks in the vicinity.

The shallow marine, warm water setting of these rocks is revealed by the uncommon presence of Lingula, Leptaena and rhynchonelids and other brachiopods, corals and crinoid debris

The sequence includes layers of tuff, which are not readily distinguished from the sandstones. This is ash, composed rock- and mineral-fragments which were erupted from the volcanoes and settled out onto the seafloor, or volcanic materials reworked by currents around the volcanoes.

Basalt lava flows are intercalated with the sedimentary and volcaniclastic rocks at Locality B. They are distinguished from the other rocks by their darker colour, lack of layering, their spotty (porphyritic) and vesicular (holey) textures. Parts of them are distinctly red.

As an exercise, spend a bit of time at Locality B looking for the following features.

· How many flows?

· Thicknesses?

· Why are bases more vesicular than interior parts?

· Are the red parts located at the base of flows, tops or middles?

· What caused the red colouration?

· What is the palaeogeographic significance of the reddening?

Coralliferous Group

This group (CG) comprises sedimentary rocks without substantial volcanic components. Along the Marloes section it is said to rest unconformably on sedimentary rocks of the SVG, with an angle of 5-10⁰ (see if you can find the unconformity – then help me!). At Renney Slip (not visited), near Martin’s Haven, the SVG is described as reddened at the contact, probably by weathering before the CG was deposited. The top of the SVG is interpreted from fossils to be late Aeronian (early Llandovery, very early Silurian) whilst the base of the CG is dated as late Telychian (late Llandovery, early-mid Silurian), a time gap of a few million years. The discordance is interpreted to be a result of the SVG in this part of Pembrokeshire being tilted against a big fault, uplifted, red-weathered and eroded, before re-submergence as the shallow sea was re-established. This tectonic event raised Prettania in South Wales, sending sediment northwards along the axis of the Welsh Basin, notably in the form of turbidites and submarine fans: the Aberystwyth Grits are famous results of this event.

At Renney Slip, the lowest part of the CG consists of conglomerate (containing fragments of the SVG and basement), cross-bedded mature (quartz-rich) sandstones and mudstones which were deposited in river channels. That is, the region was still emergent.

A shelf-sea then encroached southwards onto the Prettania margin, covering the early CG river deposits at Renney Slip and the eroded top of the SVG at Marloes. The sediments deposited in shoreline settings (`shoreface’) are again conglomerates, sandstones and mudstones but contain shallow marine fossils. The fossils include brachiopods, gastropods and crinoids, commonly in the form of beds of skeletal `hash’. Burrows are also common, particularly Planolites and Skolithos. The general picture is one of rather tranquil conditions under which the finer sediment accumulated and shelly faunas thrived, interrupted by stormy weather that spread coarser grained tempestites across the shelf .

At times the shelf deepened and the seafloor subsided to levels below fair-weather wavebase but above storm wavebase (`distal shelf’). Muddy siltstones with fine sandstones and some conglomerates were deposited. Shell beds (`coquinas’) are common, marked by layers rich in brachiopods and solitary corals; the coarser layers are probably tempestites, the shell debris winnowed by currents. The mudstones are burrowed by sand-lined, mud-filled Palaeophycus and Chondrites that liked the open marine conditions.

Grey Sandstone Group

This group (GSD) succeeds the CG without a break, and is dated by fossils as Wenlock (middle Silurian). In the Marloes section, the boundary is rather hidden by fallen blocks. We will look at the CG exposed in the cliffs at Locality J, between where the footpath from the car park reaches the shore and the Mathews Slade embayment: out of stratigraphic sequence because of faulting but nicely exposed.

The succession comprises mainly sandstones, siltstones and mudstones that were deposited in shallow marine, commonly tidally influenced settings (`littoral’) which separated the Prettania landmass from the Welsh Basin. Relative sea-level fluctuated several times during deposition (5 times according to Hillier, 2000) due to global changes, local tectonics or a combination of both. During periods of high sea-level (`highstands’), the sea encroached southwards and spread shallow marine conditions across the region. Sand and mud accumulated in mainly sub-tidal settings dominated by wave-action. During periods of low sea-level (`lowstands’), the region became one of mudflats and estuaries, incised by channels and strongly affected by tidal changes.

Several valleys have been identified in the region, cut many metres deep into mudflats and several hundred metres across. The types of sediment, sedimentary structures and fauna of the deposits which accumulated in the valleys are complex and governed strongly by tidal current activity.

Tide-dominated conditions are recorded mainly by rhythmically alternating beds of mudstone and sandstone (`bundles’) . The sands were deposited during tidal current surges (ebb and flow), whilst the mud accumulated during slack tide. The sands are commonly cross-bedded and ripple-marked. Herring-bone cross-bedding occurs in some beds, recording deposition during ebb, then flow conditions.

Keep a look out for the following features.

· Ripple marks and cross-bedding. (Suggest current directions. Work out top and bottom of beds).

· Sedimentary breccia. (Origin?).

· Phosphate concretions (black). (Origin?).

· Burrows.

· Shell beds, with crinoid stems, corals, brachiopods. (Depth of water, climatic conditions?)

Lower Old Red Sandstone

The Lower Old Red Sandstone in Pembrokeshire ranges from Upper Silurian to Lower Devonian. Lower parts of the succession are assigned to the Milford Haven Group, which comprises the Red Cliff Formation at the base (Ludlow Stage of the Upper Silurian), the Albion Sands Formation and the Sandy Haven Formation (Přίdolί Stage of the Upper Silurian), and Gelliswick Bay Formation at the top ( Lochkovian Stage of the Lower Devonian). The Red Cliff Formation is now regarded as conformable on the Grey Sandstone Formation. We will look at the Red Cliff and Albion Sands formations at the eastern or western ends of the Marloes Sands section, depending on circumstances.

The Red Cliff Formation is in transitional contact with the underlying Grey Sandstone Formation. The older formation comprises shallow marine, estuarine, inter-tidal grey sandstones and mudstones. Towards the top, there are intercalations of red mudstone interpreted as soil horizons formed during intermittent emergence. Further up the succession, red mudstones with calcrete horizons (fossil soils) predominate and are intercalated with sandstones deposited by flash floods on an alluvial plain. The region was fully emergent and the climate had changed from sub-tropical to semi-desert conditions with wet and dry seasons.

The Albion Sandstone Formation consists mainly of sandstones, thickly bedded, yellow, with interbeds and lenses of conglomerate and red mudstone. The sandstones and conglomerates include fragments of locally reworked mudstone (`intraclasts’) and a wide variety of igneous rocks derived from distant sources (`exotic’). These sediments were deposited from very vigorous rivers during flash floods, probably in a narrow E-W trough that was guided active faults.

At outcrop, keep a look out for the following features of the Albion Sandstone Formation.

· Channels.

· Pebble types (intraclasts and exotic; palaeogeographic significance).

· Pebble shapes and sizes (significance?).

· Mudstone beds (origin of green spots?).

Structure

This part of Pembrokeshire was caught up in the northern edge of the Variscan orogen, an E-W belt of folds, thrust faults and cleavage caused by continental collision in the Late Carboniferous-Early Permian.

Marloes Sands

In this section the Silurian strata are situated in the southern limb of the regional-scale Marloes Anticline. The strata strike E-W, are nearly vertical and becoming younger southwards. Their younging direction is shown on a small scale by ripple marked tops of beds and by truncated cross-bedding, and on a large scale by the ages of the formations decreasing southwards.

Cleavage is widely developed in the finer grained rocks. It is steep, strikes E-W and is axial planar to the Marloes Anticline. In places, the fossils have been flattened in the cleavage. Similarly, the originally spherical green spots in the red mudstones have been squashed into ellipsoids.

Veins of quartz are very common along the section. Many of them are in the form of tension gash arrays, structures produced by brittle failure. Bright green epidote is found in veins in the basalts.

At the western end, near the Horse Neck, the tension gash arrays accompany shear zones produced by ductile deformation, so the structures were formed close to the brittle-ductile transition within the crust.

Look out for the following features of quartz vein arrays.

· En echelon arrangement.

· Work out sense of shear.

· Conjugate patterns in places.

· Work out orientations of max., min. & intermediate stresses.

· How do sigmoidal tension gashes form?

Look out for the following features of ductile shear zones.

· Shapes and orientations of detrital grains flattened in the shear zone cleavage.

· Change of shape and orientation from centre to edge of shear zone.

· Work out sense of shear.

· Work out orientations of max., min. & intermediate stresses.

St Anne’s Head, Cobbler’s Hole

Here we can see a profile through a large-scale anticline-syncline pair, E-W structures produced by the Variscan deformation. Look out for these additional structural features.

· Steep cleavage. Is it axial planar to the folds?

· Small white quartz tension gash arrays.

· Offset of bedding by small faults. Type of fault? Relationship of faults to folds?

More Information

Aldridge, R.J. 2000. Marloes, Llandovery SSSI. In British Silurian Stratigraphy, R.J. Aldridge & 6 co-editors. Geological Conservation Review Series, 19, Joint Nature Conservation Committee, Peterborough, 96-101. (Freely available on JNCC website)

Barclay, W.J. 2005. Albion Sands and Gateholm Island, Pembrokeshire SSSI. In The Old Red Sandstone of Great Britain, W.J. Barclay & 5 co-editors, Geological Conservation Review Series, 31, Joint Nature Conservation Committee, Peterborough,281-284. (JNCC website)

Bassett, M.G. 1982. Silurian rocks of the Marloes and Pembrokeshire Peninsulas. In Geological excursions in Dyfed, south-west Wales, M.G. Bassett (ed.). National Museum of Wales, Cardiff, 103-122.

George, G.T. 2008. Marloes Sands. The Geology of South Wales. A Field Guide. Privately published, 120-126.

Hancock, P.L., Dunne, W.M. & Tringham, M.E. 1982. Variscan structures in South-West Dyfed. In Geological excursions in Dyfed, south-west Wales, M.G. Bassett (ed.). National Museum of Wales, Cardiff, 215-248.

Hillier, R.D. 2000. Silurian marginal marine sedimentation and the anatomy of the marine – Old Red sandstone transition in Pembrokeshire, SW Wales. In New perspectives on the Old Red Sandstone, P.F. Friend & B.P.J. Williams (eds), Special Publication Geological Society, London, 180, 343-354.

Hillier, R.D. 2002. Depositional environment and sequence architecture of the Silurian Coralliferous Group, southern Pembrokeshire, UK. Geological Journal, 37, 247-268.

Hillier, R.D. & Williams, B.P.J. 2006. The alluvial Old Red Sandstone: fluvial basins. In The Geology of England and Wales, P.J. Brenchley & P.F. Rawson (eds). The Geological Society, London, 154-171.

Knipe, R.J. & White, S.H. 1979. Deformation in low grade shear zones in the Old Red Sandstone, S.W. Wales. Journal of Structural Geology, 1, 53-66.

Siveter, D.J. 2000. Marloes, Wenlock SSSI. In British Silurian Stratigraphy, R.J. Aldridge & 6 co-editors. Geological Conservation Review Series, 19, Joint Nature Conservation Committee, Peterborough, 227-231. (JNCC website)