Cambrian Stratigraphy of St Tudwal's Peninsula, Gwynedd, Northwest
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Geo!. Mag. 131 (3), 1994, pp. 335-360. Copyright © 1994 Cambridge University Press 335 Cambrian stratigraphy of St Tudwal’s Peninsula, Gwynedd, northwest Wales T. YOUNG*, F. MARTINI, W. T. DEAN* & A. W. A. RUSHTON| ‘ Department of Geology, University of Wales College of Cardiff, P.O. Box 914, Cardiff CF1 3YE, UK t Département de Paléontologie, Institut royal des Sciences naturelles de Belgique, rue Vautier 29, B-1040 Brussels, Belgium I British Geological Survey, Keyworth, Nottingham N G I2 5GG, UK (Received 14 October 1993; accepted 17 January 1994) Abstract - Lithostratigraphic units of early to late Cambrian age established by T. C. Nicholas in 1915 in the St Tudwal’s Peninsula are revised. They comprise, in ascending order: Hell’s Mouth Formation (> 190 m); Trwyn y Fulfran Formation (37 m); Cilan Formation (400 m); Ceiriad Formation (40 m seen); Nant-y-big Formation (> 110 m seen); Maentwrog Formation (in part, 50 m seen, an estimated 250 m concealed); Ffestiniog Flags Formation (in part, c. 120m seen). The ‘calcareous grit’ at the top of Nicholas’s Nant-pig Mudstones spans the unconformable boundary between the Nant-y-big and Maentwrog formations. Previously described limestone clasts in the ‘ grit ’ are probably erosional remnants of an in situ bioclastic limestone bed ; their contained trilobites include genera and species found in the Andrarum Limestone (late Middle Cambrian) of Scania, Sweden. Acritarchs are documented and compared particularly with those from eastern Newfound land. Those from the highest part of the ‘calcareous grit’ include Cymatiogalea sp. and are of late Cambrian age. One new species, Heliosphaeridium ? llynense Martin, comes from the lower part of the Nant-y-big Formation (middle Middle Cambrian), where it appears a little earlier than the Adara alea Biozone. under NERC contract F60/G2/28 awarded to the 1. Introduction(T.Y., W.T.D., F.M.) Department of Geology, University of Wales College St Tudwal’s Peninsula and adjacent small islands of Cardiff. All map references refer to National Grid form the southeastern tip of the Llÿn Peninsula, at the square SH. northern end of Cardigan Bay, northwest Wales (Fig. Macrofossils are generally scarce in the Cambrian 1). Cambrian rocks and fossils were recognized there of St Tudwal’s, and apart from the discovery of late during the nineteenth century, but the first detailed Lower Cambrian trilobites in the highest Hell’s Mouth account was that of Nicholas (1915, 1916), which Formation in 1960 (Bassett & Walton, 1960; Bassett, formed the basis of subsequent interpretations. The Owens & Rushton, 1976) there have been no significant relatively small Cambrian outcrop is important in additions to the collection made by the late T. C. Welsh Lower Palaeozoic stratigraphy as it lies near Nicholas and reviewed both by Rushton (1974) and the northwest margin of the Welsh Basin. The herein. Most of the macrofossils are Middle Cambrian succession (Fig. 2), broadly comparable with that of trilobites, particularly agnostids or eodiscids, of which the Harlech Dome, 30 km to the east, is some 1400 m the former provide zonal indices for most of the thick but the base is not seen. The Harlech Grits classic ‘reference’ sections in Sweden (Westergárd, Group (Lower-?Middle Cambrian) consists largely of 1946) and were widely distributed, though with a arenaceous turbidites; mudstones and siltstones are distinct preference for deeper marine environments. A dominant in the overlying Ceiriad and Nant-y-big few Upper Cambrian trilobites at one level in the formations (Middle Cambrian). Above an uncon Maentwrog Formation indicate, probably, the Olemts formity the Maentwrog and Ffestiniog Flags form Biozone; the lower part of the Parabolina spinulosa ations (Upper Cambrian) correspond to a shelf Biozone is possible but unlikely. environment, with indicators of storm-dominated A pioneer study on the Cambrian acritarchs of depositional processes. The highest parts of the North Wales by T. L. Potter in a Ph.D. thesis of 1974 succession are not well seen at St Tudwal’s, but north remains unpublished except for a short note (Potter, of the Sarn-Abersoch Fault, Tremadoc (Ordovician) 1974). The present palynological investigation permits strata crop out, represented by the Dol-cyn-afon a partial comparison with results obtained in eastern Formation. Revised terminology of the Cambrian N ew foundland (M artin & D ean, 1981, 1983, 1984, rocks used in the BGS Pwllheli Memoir (Young, 1988), where biostratigraphic control based on tri Gibbons & McCarroll, in press) and followed here lobites is often better and macrofossils, which have forms part of the mapping of Sheet 134 (Pwllheli) many species in common with those of Scandinavia 336 T. YOUNG AND OTHERS Ui* I ||0p-Á-l|D8n UA mJX 92 uj * or Figure 1. Outline maps of North Wales (inset), St Tudwal’s Peninsula and adjacent islands, showing locations of sections mentioned in the text. Geology after British Geological Survey (BGS) 1:50000 Sheet 134, Pwllheli (in press). Course of faults inland partly uncertain. Section number as follows: (1) type section for Hell’s Mouth Formation; (2) type section for topmost Hell’s Mouth Formation, Trwyn y Fulfran Formation and base of Cilan Formation; (3) type section for body of Cilan Formation and base of Ceiriad Formation; (4) fault bounded type section for middle of Ceiriad Formation; (5) base of Nant- y-big Formation; (6) type section for Nant-y-big Formation and section through lower part of Maentwrog Formation; (7) isolated section in Maentwrog Formation; (8) section through part of Ffestiniog Flags Formation. and Wales, are more readily collected. A few selected and Sara, 6 km northwest of St Tudwal’s, the trilobites and acritarchs are published elsewhere by succeeding Gwynedd Supergroup (Ordovician, Young, Gibbons & McCarroll (in press). Present Arenig) rests unconformably on the late Precambrian illustrations of macrofossils are confined to material Sara Complex. In the Harlech area, 30 km to the east, from Porth Ceiriad, in the so-called ‘calcareous grit’ the base of the Dyfed Supergroup was seen in the of Nicholas (1915), which spans the boundary between Bryn-teg borehole, where the Dolwen Formation lies the Nant-y-big and Maentwrog formations, and at a unconformably on the Bryn-teg Volcanic Formation higher level in the Maentwrog Formation. Charac of probable late Precambrian age (Allen & Jackson, teristic acritarchs illustrated here range from the 1985). upper part of the Hell’s Mouth Formation at Trwyn The base of the Gwynedd Supergroup was defined y Ffosle to the Maentwrog Formation at Porth (Woodcock, 1990) as the base of the transgressive Ceiriad, and from the lower part of the Ffestiniog succession of Ordovician (Arenig) age over most of Flags Formation outcrop on St Tudwal’s Island East. North Wales, but locally as the base of the Rhobell Palaeontological specimens with prefix SM are Volcanic Complex. It is marked in the current area by from the Sedgwick Museum, Cambridge, and those the unconformity below the Nant Ffrancon Group with the prefix IRScNB from the Institut royal des (Arenig). The latter group rests on the Dol-cyn-afon Sciences naturelles de Belgique, Brussels. Formation (Ordovician, ?lower Tremadoc) to the north of St Tudwal’s Peninsula near Abersoch, and on the peninsula itself, on progressively older parts of the 2. Dyfed Supergroup (T. Y.) Cambrian succession from the Ffestiniog Flags Form The Cambrian succession at St Tudwal’s forms part of ation in the east to the Hell’s Mouth Formation in the the Dyfed Supergroup (Woodcock, 1990), the base of west. The unconformity becomes more angular from which is not seen here. In the area of Mynydd Rhiw east to west. Cambrian stratigraphy, St TudwaVs Peninsula 337 Comparison with the sequence in the Harlech area THIS PAPER NICHOLAS, 1915 suggests that a considerable thickness of Cambrian strata (equivalent to the lower Rhinog, Llanbedr and TOP NOT SEEN Ffestiniog Flags Ffestiniog Beds Dolwen formations, totalling 1500 m at Harlech) Fm. (c. 120m seen) (107m seen) might be concealed beneath the sea in the Hell’s Mouth area. gap in section gap in section 12.8m s e e n 2.b. Hell’s Mouth Formation M aen M a en u n e x p o se d u n e x p o s e d twrog' tw rog (est. 250m) (est. 213m) Fm. B e d s 2.b.l. Lithostratigraphy (T.Y.) 34.5m s e e n 30.5m. seen The unit, equivalent to the ‘Hell’s Mouth Grits’ of Nicholas (1915), comprises laterally persistent, coarse- to medium-grained, graded green sandstone beds Nant-pig Msts. (67m) Nant-y-big Fm. (<4m thick), interbedded with thinner (<0.5m ) (>ll0m seen) U. C aered M st Caered Msts. green and grey siltstones. Some 190 m of the Hell’s C eiriad _ _Caered Flags & F lags i not s e e n Mouth Formation are seen, but the base is not F m .(4 0 m )— ' _ not s e e n____ (>152m ) L. C aered Mst. exposed and the oldest visible beds are those near sea- level on the west side of St Tudwal’s Peninsula. Type Section. At Trwyn y Ffosle, where a relatively Cilan Grits (305m) Cilan Fm. (c. 400m) complete succession can be traced from sea-level [2868 2482] to the manganese trials marking the line of outcrop of the overlying Trwyn y Fulfran Formation HARLECH [2894 2464], The upper part of this section is not well GRITS Trwyn y Fulfran Fm. Mulfran Beds (137m) GROUP (3 7 m ) exposed, so the section above Trwyn Carreg-y-tir (part) [2876 2402] has been used to define the base of the Hell's Mouth Fm. Hell's Mouth Grits Trwyn y Fulfran Formation (Fig. 3). (>190m ) (>213m ) Description.