Post on 21-Apr-2015
Embed Size (px)
Journal of the Geological Society A laminated hemipelagic facies from the Wenlock and Ludlow of the Welsh BasinA. J. DIMBERLINE, A. BELL and N. H. WOODCOCK Journal of the Geological Society 1990; v. 147; p. 693-701 doi:10.1144/gsjgs.147.4.0693
Email alerting service Permission request Subscribe
click here to receive free email alerts when new articles cite this article click here to seek permission to re-use all or part of this article click here to subscribe to Journal of the Geological Society or the Lyell Collection
Geological Society of America on 9 July 2010
1990 Geological Society of London
Journal of the Geological Society, London, Vol. 147, 1990, pp. 693-701, 6 figs. Printed in Northern Ireland
A laminated hemipelagic facies from the Wenlock and Ludlow of the Welsh BasinA . J . D I M B E R L I N E , A . BELL & N . H . WOODCOCK Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Abstract: A hemipelagicfaciesfrom the Siluriansuccessionin Wales consists of finely laminated, dominantlysilt-gradesediment. It canbeinterbeddedwith sand- or mud-grade turbidites, or can occur as an independent facies. Its occurrence and character was controlled mainly by low bottom wateroxicity.Thepelagic fauna of the laminatedhemipelagitesupportsthis view. Many of the silt-grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton. The fine laminationis thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin. By comparison with modern examples this alternation was likely to have been annual or seasonal.
The Silurian Welsh BasinThe Welsh Basin was a Cambrian to Early Devonian zone of relatively rapid subsidence on the Avalonian microcontinent (Fig. l ) , bordered to the northwest and southeast by shallow or emergent platforms. Recent reviews of its history are given by Woodcock (1984, 1990) and Kokelaar (1988). During most of Ordovician time, represented by rocks of the Gwynedd Supergroup, it was a volcanically active marginal basin, but most volcanism ceased in late Caradoc time. A basin-wide unconformity in the early Ashgill (Pusgillian) marks tectonic activity along the basin-bounding fault systems andtheonset of deposition of the Powys Supergroup (Ashgill through Lower Devonian). During this interval the basinwas in anon-volcanicactive margin or collision zone setting. The northern and central to southern parts of the Welsh Basin had differing histories during the time of Powys Supergroup accumulation. In Ashgill and Llandovery times, slowhemipelagic depositionin thenorth contrasted with repeatedandvoluminous influxes of turbiditycurrentsin mid-Wales. The majority of these sediments were deposited under oxic conditions, which encouraged burrowing and grazingorganisms andthedestruction of anyhemipelagic lamination. During earliest Wenlock time there was a marked decrease in the oxicity of the basin bottomwaters.This anoxic event can be traced throughout the basins bordering the Iapetus Ocean (Kemp 1985, 1990). The early Wenlock also saw increased subsidence and clastic supply in the northern Welsh Basin. A turbiditedepocentredeveloped, termed the Denbigh Trough (Cummins 1957), which remained a discrete sub-basin throughout Wenlock and Ludlow times, largely barred from thecontinuing mid-Wales depocentre, the Montgomery Trough (Fig. 1). Inversion of the Welsh Basin probablybeganduring Ludlow time and climaxed in the Acadian shortening late in the Early Devonian. The inversion was accompanied by a rapidtransitionfrommarine to non-marine facies in the basin and on its Bounding platforms. 693
Distribution of laminated hemipelagitePalaeogeographical distributionLaminated hemipelagite occurs over the whole area of the Welsh Basin where Wenlock and Ludlow rocks are preserved (Fig. 1; Cummins 1 9 5 9 ~ Warren et al. 1984). It ; laps some way onto the basin slopes but is not preserved on the platforms. Laminated hemipelagite in not restricted to those basinal area inundated by the contemporary sand turbidite systems. Two areas exemplify this relationship. On the southeast basin margin, northerly-directed Wenlock turbidity flows were confined laterally by a fault-controlled slope, whilst hemipelagic sediment was deposited on basin floor and slope alike (Dimberline & Woodcock1987). On the palaeohigh between the Montgomery Denbigh and Troughs, the Derwen Ridge of Cummins (1957), Wenlock turbidites are almost absent yet a continous laminated hemipelagite sequence was deposited. The distribution of Fig. 1 is compatible with turbidity flows hugging depositional lows and with vertical fallout forming laminatedhemipelagite indiscriminately on lows, slopes and highs throughout the basin.
Stratigraphical distributionLaminated hemipelagite occurs in the basin in strata from the centrifugus Biozone(lowest Wenlock) to the incipiem Biozone (lower Ludlow) (Fig.2). However, the character of the resulting sediment is controlled stronglyby temporal fluctuations in the proportion of interbedded turbidites and in the bottom water oxicity. At the base of the Wenlock Series (centrifugus Biozone) laminatedhemipelagite is interbedded with turbiditesbut can form >50% of the sequence. Examples of this facies are the Benarth Formation Wales the in north and murchisoni Nant-y-Sgollen Shales in mid-Wales. In the through linnarsoni biozones, turbidites dominate the intrabasinal troughs, forming the Denbigh Grits Group. By contrast,laminatedhemipelagitedominatesthesequences
A . J . DIMBERLINE E T A,!,.
Fig. 1 Map of Wales showing outcrop . of Wenlock and Ludlow rocks, selected
palaeogeographical features and depthrelated fauna1 communities for midWenlock time (rigidus Biozone); modified after Hurstet al. (1978). Facies boundaries are conjectural away from outcrop control. LHP, laminated hemipelagic facies.
on the intervening Derwen Ridge palaeohigh, forming 100% of the sequence Llangollen. near The magnitude and frequency of the turbidites in the troughs decrease in the linnarsoni Biozone, so thatlaminatedhemipelagitemakes up about 60% of the sequence. This facies, typical of the NantglynFlags Group (ellesae to nilssoni biozones), has a distinctive striped or ribbon-bandedappearance due to interbedding of 5-40 mm laminatedhemipelagitepackets with 5-20 mm silt-mud turbidites. Turbidite deposition increasingly affected the Derwen Ridge during late Wenlock time and the basal Ludlow Glyndyfrdwy Group is very similar to the Nantglyn Flags Group. There are two intervalsin the NantglynFlags Group, within the ludemis and nassa biozones respectively,which preserve little laminated hemipelagite but contain a benthic infauna as well as epifauna. More oxic bottom waters allowed burrowers and grazers to destroy any lamination that formed. The two oxic episodes have been linked to a continental regression coincident with the development of the Wenlock Limestone the on Midland Platform (McKerrow 1979; Kemp 1990). Both may be a consequence of a drop in sea-level, allowing reef build-up on the shelf and the breakdown of stratification in the basin. Laminated hemipelagite is rarely preserved rocks in younger than those of nilssoni Biozone age. In north Wales this can be explained by the increasing volume of turbidites supplied to the Denbigh Trough. In mid-Wales the laminated hemipelagite deficiencymay have resulted from the breakdown of basin-water stratification, since inter-
bedded silt and fine sandbeds show structuresconsistent with astorm influence(Tyler & Woodcock1987). Storms would lead to mixing, to increase of bottom-water oxicity, matter onthe and in turn to the oxidation of organic sea-floor. Despitethis,somelaminatedhemipelagitehas been described from the storm-influenced Bailey Hill & Woodcock 1987), the Formation (Tyler Tyler 1987: Ludlow turbidites of Cummins (1959b). This laminated hemipelagite could have resulted from the re-establishment of water stratification and oxygen depletion of bottom waters between major storm events. Alternatively the storm mobilized sediments could have been deposited rapidly onto organic rich sedimentsthathadnot had time to oxidize, isolating the laminated hemipelagitethe from now oxygenated surface waters. The youngest laminated hemipelagite in north Wales is found in the Bont-UchelFormation of the Elwy Group (Warren et al. 1984), in the incipiens Biozone.
General description and interpretation Lithology, structure and sequenceThe laminated hemipelagite consists of silt laminae alternating with organic carbon-rich laminae on a scale of three to four carbon-rich laminae per mm (Cummins 1959a; Warren et al. 1984). The lamination is discontinuous laterally on a 1-5 cmscale inlamination-normalsections (Fig. 3a). The silt laminae contain silt and clay aggregates
HEMIPELAGIC FACIES OF THE WELSH BASIN
Fig. 2. Representative stratigraphical columns for the Silurian of:2, Denbigh Trough; north flankof Berwyn High (Llangollen Syncline);4, 1, Montgomery Trough; 5, Midland Platform. DC, Denbigh Grits Group; GDG, Glandyfrdwy Group; LNF, Lower Nantglyn Flags Group; LS,
Ludlow shale; PG, Pen-y-Clog Group; UNF, Upper Nantglyn Flags Group; Wenlock Limestone; WS, Wenlock Shale; LHP, laminated WL hemipelagic facies. Sections parallel the cut to up to 2 mm in diameter. lamination show a distinctive mottled appearance (Fig. 3b) due the to presence of these aggregates. The detailed description and interpretation of the aggregates is covered in a later section. Interbedding of laminatedhemipelagite with turbidites in the SilurianWelshBasin hasbeen well describedand illustrated by Cave (1979) and Cave & Hains (1986). Logs through the Wenlock of mid-Wales (Fig. 4)