Merioneth Series, Cambrian, Wales
From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.
In the area of Harlech and Llŷn, the period of mud deposition at the end of St David’s Series times was terminated by the third major incursion of mainly fine- to medium-grained turbiditic sand. The incursions overstep progressively to the north so that between Nantlle and Bethesda the sandstones considered to be at the base of the Merioneth Series (Upper Cambrian) lie directly on the Llanberis Slates Formation (Comley Series) (P916150). On St Tudwal’s peninsula, the lowermost coarse-grained, turbiditic sandstones, with rounded pebbles of St David’s Series limestones, grade up into thinly bedded, finer grained turbiditic sandstones with interbedded shales containing the trace fossil Nereites?, which indicates a relatively deep-water environment. A similar upward gradation is repeated in the Harlech district and is thought to reflect a late St David's Series regression on the adjacent shelf.
In the Harlech district, the Maentwrog Formation is assigned to the lower part of the Merioneth Series. It comprises grey silty mudstone interbedded with coarse siltstone to fine sandstone turbidites, with thin reworked sandstone laminae and predominantly mudstone above, and passes upwards into the Ffestiniog Flags Formation. The trilobite Olenus, associated with agnostids, occurs at several levels, and indicates at least three subzones of the early Merioneth Olenus Biozone. To the north, between Nantlle and Bethesda, these beds appear to pass laterally into the thickest expression of turbiditic sandstones, which have been given various local names by the quarrymen but are now grouped into the Bronllwyd Grits Formation. Palaeocurrent indicators in the sandstones determine dominant derivation from the north-west. The formation includes manganiferous shales in its lower part, and many of the sandstone beds have conglomeratic bases, with rounded quartz and quartzitic pebbles, up to 20 mm, and silty mudstone flakes, up to 0.25 m across. It is well exposed, through a series of steeply inclined folds in the steep north-facing slope on the south side of Llyn Peris. From Llŷn to Arfon, these variable turbiditic sandstones are overlain by massive or poorly bedded silty mudstone, thinly bedded quartzose siltstone and fine-grained sandstone with ripple marked bedding planes, low-angle cross-lamination and plane parallel lamination (Marchllyn and Ffestiniog Flags formations). The sandstones were deposited mainly as turbidites derived from the north but were modified by wave and current action. This formation is host to the Coed y Brenin porphyry copper deposit just north of Dolgellau. In the area about Nant Ffrancon, a thicker, quartzose sandstone, the Carnedd y Filiast Grit Member (P916150), occurs near the top of the Marchllyn Formation, and forms a distinctive feature in the back wall of Cwm Graianog, where the ripple-marked top is separated from the slightly discordant base of an Ordovician sandstone by a thin band of siltstone (P662397). The occurrence of the trace fossils Cruziana, Rusophycus and Skolithus indicate deposition in a shallow-water setting with current flow towards the south-west; infilled mudcracks indicate temporary emergence.
Around the Harlech Dome, the Ffestiniog Flags Formation grades up through sandy and silty beds with abundant brachiopods (Lingulella davisii) into dark grey to black, carbonaceous, pyritic and locally uraniferous mudstone (Dolgellau Formation) at the top of the Cambrian sequence. The mudstone, up to 150 m thick, contains laminae, less than 1 mm thick, of disseminated pyrite and siltstone, which define bedding. These beds reflect slow accumulation, mainly in a pelagic setting, in poorly oxygenated bottom waters with restricted circulation. They contain relatively rich trilobite and brachiopod faunas, which indicate the presence of about 12 subzonal assemblages from the P. spinulosa to Acerocare zones (Table 2). Two beds of reworked volcanic rocks, one near the top of the Dolgellau Formation and the other almost coincident with the Dolgellau-Doly-cyn-afon transition, have given radiometric ages of 491 Ma and 489 Ma, respectively.
In south Wales, the upper part of the Cambrian sequence, the Lingula Flags Formation, crops out both in Pembrokeshire and in the Llangynog inlier, south-west of Carmarthen. In both localities, siliceous siltstone and micaceous mudstone are a distinctive component, and cross-laminated, siliceous sandstones indicate deposition in shallow water. The high mica content may reflect local shoreline erosion of Precambrian metamorphic rocks at the southern margin of the Welsh basin. In Pembrokeshire, the sequence is some 700 m thick near St David’s, and the lowest beds contain the earliest Merioneth Series trilobite, Agnostus pisiformis, representing a zone that has not yet been proved in north Wales. Conversely, the higher zones recognised in north Wales cannot be clearly identified, but the abundance of the brachiopod Lingulella davisii supports a broad correlation. To the east of St David’s, the entire Upper Cambrian sequence is overstepped by lower Ordovician (Arenig) strata near Hayscastle, but at Llangynog, black shales, which are lithologically and faunally similar to those of the Dolgellau Formation in north Wales, contain olenid trilobite faunas representing four or five Olenid subzones.
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