Telychian Stage Research Articles

Metabentonites from the Silurian inliers of the southern Midland Valley of Scotland: distribution and geochemistry

Synopsis A survey of eight Silurian inliers in the southern Midland Valley of Scotland has identified four new metabentonite occurrences within the Telychian Stage of the Llandovery, in the Bavelaw Castle, Hagshaw Hills, Lesmahagow and Girvan Main inliers respectively. The bulk geochemical composition of the metabentonites highlights two broad associations: an intermediate/mafic, slightly alkaline group represented by the Lesmahagow samples, and a rhyodacitic group comprising samples from the other inliers surveyed. A potential correlation is proposed between a metabentonite from the Wether Law Linn Formation of the North Esk inlier and one from the west end of the neighbouring Bavelaw Castle inlier. The unique character of the Lesmahagow metabentonites will assist in further correlation studies of Upper Llandovery successions.

The 1997 core drilling through Ordovician and Silurian strata at Röstånga, S. Sweden: Preliminary stratigraphic assessment and regional comparison

A core drilling at Röstånga, the first such drilling ever undertaken in this classical Lower Paleozoic outcrop area in W-central Scania, penetrated an approximately 96 m thick succession of Lower Silurian-upper Middle Ordovician marine rocks. The drilling was stopped at a depth of 132.59 m in an interval of crushed rocks, probably a prominent fault zone, that proved impossible to drill through. The core contains a stratigraphical sequence from the basal Upper Llandoverian (Telychian Stage) to the upper Middle Ordovician (Harjuan Stage). The following units are recognized in descending stratigraphic order (approximate thickness in parenthesis): Kallholn Formation (35 m), Lin-degård Mudstone (27 m), Fjäcka Shale (13 m), Mossen Formation (0.75 m), Skagen Formation (2.5 m), and Sularp Shale (19 m+). Except for the Skagen Formation, the drilled sequence consists of shales and mudstones with occasional thin limestone interbeds and is similar to coeval successions elsewhere in Scania. There are 11 K-bentonite beds in the Kallholn Formation, 2(3?) in the Lindegård Mudstone, 1 in the Mossen Formation, 7 in the Skagen Formation, and 33 in the Sularp Shale. The core serves as an excellent Lower Silurian-upper Middle Ordovician reference standard not only for the Röstånga area but also for southern-most Sweden in general because the cored sequence is the stratigraphically most complete one known anywhere in this region.

Metabentonite and tuff occurrence in Silurian greywackes at Grieston Quarry, Innerleithen, Southern Uplands

Grieston Quarry lies 1.6 km WSW of Innerleithen, Peeblesshire \[NT 3130 3618\] (Fig. 1). The rocks in the quarry comprise greywackes with intercalated pelitic beds which dip 60–65° NW and belong to the Gala Group, Telychian Stage. This succession has experienced prehnite-pumpellyite facies metamorphism ([Oliver et al. 1984][1]) which has transformed the pelitic beds into fissile flagstones, once used as roofing material. The quarry is important for yielding type specimens of the graptolite Monoclimacis griestoniensis Nicol, 1850 ([Toghill and Strachan 1970][2]). Approximately 7.5 m NW from the extreme southerly exposure of the main quarry face, a composite band 65 mm thick, comprising alternating coarse and fine layers, occurs within the greywacke sequence. The lowermost 5 mm of the band is made up of reddish grit, followed by 15 mm of friable fine-grained grey claystone, which shows no evidence of grading within it and has sharp upper and lower contacts. The next 45 mm are made up of alternating fine dark mudstone and coarse tuff laminae. Inspection of the tuff layers in thin section shows fragments of partly corroded euhedral feldspar crystals, fragments of quartz (some showing pyramid terminations) and rare zircon grains, all set in a mica and carbonate matrix. The grey claystone was sampled and prepared for XRD study. The clay fraction (<2 μm) was separated and analysed dry and after glyeolation. Chlorite and mixed-layer illite/smectite with 15% of expandable layers were found ([Reynolds 1980][3]). The non-clay fraction contains quartz, albitic plagioclase, chlorite, K-micas and traces of calcite. The claystone . . . [1]: #ref-7 [2]: #ref-10 [3]: #ref-8

Worldwide correlation of Telychian (Upper Llandovery) strata using graptolites

Abstract The formal boundaries to the Telychian Stage have been stated to correlate with the bases of the Monograptus turriculatus Biozone at the bottom and the Cyrtograptus centrifugus Biozone at the top. The evidence for this, however, is far from conclusive. The Telychian has been divided, mainly using monograptids, into eight or more biozones, although fewer have been recognized in Britain due to the general paucity of recent studies of the graptolite faunas of this age and to the existence of ‘barren beds’ high within most British Telychian sections. Each biozone has a duration of only a few hundred thousand years, but most are, at present, difficult to recognize in the absence of the index species or where this species’ stratigraphical range extends beyond the limits of its biozone. Intercontinental correlation of Telychian strata using graptolites is hampered to a limited extent by provincialism and water mass specificity, and considerably by taxonomic problems. A thorough taxonomic revision of published species would provide a firm foundation for future biostratigraphical subdivision and correlation of the Telychian Stage. Knowledge of Telychian graptolite biostratigraphy would also be enhanced by restudies of two classic sections, the Trannon river section in Wales, and the Bajiaokou section, Ziyang, China.

A fault-controlled depositional model for the Aberystwyth Grits turbidite system

AbstractThe Aberystwyth Grits Group was the first in a series of southerly-supplied sandstone-rich turbidite systems that invaded the Welsh Basin during the Telychian Stage (late Llandovery; Silurian). Structural and stratigraphical evidence indicates that deposition of the sand-dominated parts of the system was largely restricted to the east by a zone of syndepositional faulting, now marked by a major vergence divide. Downfaulting and reverse drag along the fault hangingwall created a bathymetric low, which focused deposition of high-density, sand-carrying turbidite flows and flow components, allowing substantial thicknesses of sediment to accumulate. In contrast, low-density, mud-rich turbidites were deposited over a wider area, enhancing the topographic relief generated by uplift above the fault footwall, to form a feature which restricted the spread of the coeval easterly-supplied turbidites of the Devil's Bridge Formation.

Biostratigraphy of late Llandovery (Telychian) and Wenlock turbiditic sequences in the SW Southern Uplands, Scotland

ABSTRACTIn the SW part of the Southern Uplands of Scotland the relatively thin Moffat Shale Group (late Ordovician–early Silurian) is succeeded by a thick development of Silurian greywackes, of variable turbiditic facies. This includes late Llandovery (Telychian) quartzose greywackes with interbedded thin graptolitic shales of the turriculatus and crispus biozones, in the upper part of the Gala Group, a sequencewhich is laterally equivalent to the basal part of the Hawick Group. The age of the finer-grained calcareous Hawick Group, which here includes 1. the Ross Formation, ranges from late Llandovery (turriculatus Biozone) to early Wenlock (riccartonensis Biozone). The Riccarton Group, which contains thick units of thinly-bedded siltstones and mudstones, is of Wenlock age (riccartonensis to lundgreni biozones). Within this sequence, all the biozones of the standard graptolite zonal scheme have been recognised in the area, with the exception of the crenulata Biozone of the late Llandovery (Telychian Stage) and the murchisoni and ellesae biozones of the Wenlock (Sheinwoodian Stage). Details of the graptolite biostratigraphy areclosely comparable with those of the markedly thinner sequences of northern England. Acritarchs occur throughout the sequence but are most numerous and best preserved in the Gala Group. Poorly preserved chitinozoa and spores are also present, the former occurring sporadically throughout the succession but the latter become common only in the Riccarton Group.

Depositional environments in the Telychian Stage (Silurian) of the central Oslo region, Norway

AbstractThe Telychian succession in the central Oslo region was previously interpreted as deposited in a relatively deep, calm environment showing a continuous transgressive development. A new analysis of fossil assemblages shows that sediments in the central districts were deposited at varying water depths in an environment marked by intense infaunal activity in soft substrates. Two small‐scale fluctuations in sea level are attributed to eustasy. The maximum deepening events occurred during the Monograptus turriculatus and Monoclimacis crenulata graptolite zones.Previous depositional models include development of a trough to the north of the Oslo region in early Telychian time, which formed due to isostatic loading caused by an advancing thrust front of the Caledonian Orogeny. Bathymetric analysis shows a deepening in the Ringerike district in the latest Aeronian, which here is interpreted as a distal effect of the loading to the north. The local deepening led to reversal of the epicontinental slope between the northern and western districts and the central districts and the development of an east–west elongate positive area further south. The thrust front acted as a source of siliciclastic material; and when the front halted in mid‐Telychian time the trough was filled in gradually by a prograding coast or delta. The diachronous Vik Formation is viewed as a distal development of this progradation. When the east–west oriented positive area subsided the Vik Formation was deposited in Skien to the south.