St David’s Series, Cambrian, Wales
From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.
In earliest St David’s Series (Middle Cambrian) times, a regressive episode about the Harlech Dome is reflected in the thin manganese-bearing shales at the base of the Hafotty Formation. The contact is well exposed at the Hafotty Mines, on the hillside above Llanaber, where coarse-grained, thickly bedded turbidites of the Rhinog Grits Formation pass abruptly into grey, thinly bedded, fine-grained, cross-bedded sandstones and siltstones with distinctive concentrations of manganese, which has been worked extensively in the vicinity. Above, the formation consists mainly of banded siltstone, mudstone and fine-grained sandstone, possibly distal turbidites. One finely laminated ore bed contains 12.3 per cent manganese. The manganese was precipitated in a carbonate form with silica, and is quite unlike the spheroidal manganese nodules on modern sea floors. It has been postulated that the manganese was derived from the intense weathering of a gneissic landmass. Conversely, a diagenetic origin of the manganese has been proposed, and such a model would rule out the need for a manganese-rich provenance. However, it is difficult to envisage a shallow marine basin in which reducing conditions could develop within a sequence that mainly reflects more dynamic processes. Manganese-rich shales and sandstones (Trwyn y Fulfran Formation) occur in the equivalent position overlying the Hell's Mouth Grits Formation on St Tudwal’s peninsula, Llŷn.
Subsequently, incursions of turbiditic sands occurred in both the Harlech area and in Llŷn, although their direction of transport was contradictory, to the north-west and south-west, respectively. The coarse pebbly turbiditic sandstones (Barmouth Formation and Cilan Formation) are similar to the earlier submarine fan deposits; they grade up through thin turbiditic sandstone with interbedded purple and green shale (Gamlan Formation, Ceiriad Formation), with persistent indication of manganese precipitation, into black, pyritous mudstone with lenses and laminae of sulphides and few fine-grained sandstones (Clogau Formation, Nant-y-big Formation). The black mudstone of the Clogau Formation forms a ‘slack’ feature at outcrop and is easily traced around the flanks of the dome. The pyritous character suggests quiescent accumulation in a reducing environment, and it contains the lowest rich fossil fauna in Wales, which is dominated by the trilobite Paradoxides with the small eyeless agnostid trilobites. In addition, the mudstone hosts the main concentration of gold, copper, lead and zinc vein mineralisation through the Dolgellau Gold Belt on the north side of the Mawddach estuary. North of Llŷn and Harlech there is no evidence of St David’s Series strata.
In Pembrokeshire, the series is represented by the Solva and Menevian groups. The Solva Group comprises green and purplish sandstone, siltstone and mudstone, which are well exposed in Solva Harbour and on the west side of Caerbwdy Bay. Apart from a possible unconformity in Caerfai Bay, the sequence conformably overlies the Caerbwdy Sandstone Formation. The sedimentary structures and the trace fossils indicate a shallow marine environment and the trilobites, inarticulate brachiopods, and acritarchs indicate the Paradoxides oelandicus Biozone and possibly the Ptychagnostus gibbus Biozone (Table 2). The abrupt appearance of the trilobite fauna at the base of the Solva Group is related to a change in facies. The Menevian Group is transitional with the underlying Solva Group and consists of thin sandstones and mudstones overlain by dark grey shale with thin beds of lenticular sandstone and fine-grained, pyroclastic debris and, at the top, coarse-grained, massive sandstones with interbedded shales and thin turbiditic sandstones. The dark grey shales are equivalent to those of the Clogau Formation in the Harlech district, and suggest accumulation in a quiescent, marine, oxygen-poor environment, probably associated with deepening of the basin during continued transgression. The shales contain an extensive fauna that indicates the zones of Tomagnastus fissus, Hypagnostus parvifrons and Ptychagnostus punctuosus. The uppermost coarse sandstone contains the brachiopod Billingsella hicksii. The overlying Lingula Flags Formation (see below) consists of alternating beds of siliceous siltstone and micaceous mudstone. It is difficult to correlate, with unconfirmed records of ‘Paradoxides’ and ‘Concoryphe’ that suggest a Mid Cambrian age for at least the lower part of the formation.
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