The Silurian System of rocks was named by Roderick Murchison in July 1835, deriving derived the term Silurian from the name of an Iron age tribe, the Silures, who lived in the Welsh Borderlands. Of the eight stratotypes that define the bases to the Silurian stages, seven are in Britain, making the British Silurian of global importance.
Graptolites and conodonts have yielded highly refined Silurian
zonations, but analogous schemes exist for other groups, notably
brachiopods,
acritarchs and chitinozoans. The Silurian stage boundaries have been chosen with reference to the bases of specific graptolite biozones in the boundary stratotypes or in nearby sections.
The Iapetus Suture, trending NE–SW close to the England–Scotland Border, marks the former site of the Iapetus Ocean between Laurentia to the north-west and Avalonia to the south-east and provides a subdivision of Britain on which Silurian
geological history was overlain. The Silurian witnessed the demise of marine Silurian sedimentaion over most of Britain, and an extensive succession of non-marine sediments, the ‘Old Red Sandstone’, was laid down on the newly
consolidated continent during the Devonian.
The generalized British Silurian
lithostratigraphies show a lack of lithological similarity between regions at most times, a variability resulting from the contrasting palaeogeographical, tectonic and sedimentological settings of these regions during the rapidly changing British Silurian. Consequently, there are numerous lithostratigraphical schemes in use. Five l
ithostratigraphic columns summarize this variability, representing the major palaeogeographical provinces during Silurian time, and the following succession of events link these lithostratigraphies together.
During the
early to mid-Llandovery deep-marine mudstones (
Moffat Shale Group) deposited on the Iapetus oceanic crust were overlain by turbidite sandstones (
Gala Group) as the crust neared the trench. Periodically segments of this sedimentary succession were accreted to the base of the thrust stack on the Laurentian margin. Trench turbidites were supplied in part from an absent southern segment of the Midland Valley. Farther north, the preserved part of the Midland Valley received clastic debris from both the south and the north. On the Avalonia Midland Platform, the early Llandovery
regressive sediments (a remnant of the late Ordovician glaciations), are overlain by shallow marine mudstones and limestones, often with transgressive sandstones at the base (e.g.
Kenley Grit–Pentamerus Beds–Hughley Shales). In the basinal areas, the same transgression is recorded by a blanket of carbonaceous mudstones (e.g.
Cwmere and Skelgill Formations)
Late Llandovery to early Ludlow: Offscraping of deep-marine turbidites at the Laurentian margin continued from late Llandovery (
Hawick Group) into late Wenlock time
(Riccarton Group). Shallow marine environments in the upper Llandovery of the Scottish Midland Valley gave way to subaerial alluvial fans in the Wenlock. On Avalonia, the Midland Platform accumulated an upper Llandovery to
Ludlow succession, dominated by mudstones (e.g.
Coalbrookdale Formation). Shorter periods of
limestone deposition (
Woolhope,
Buildwas and
Much Wenlock Limestone formations) coincide with lowstands in eustatic sea-level. The Welsh Basin received large volumes of SW-derived turbidite sand (Cwmystwyth Grits Group and Penstorwed Grits Formation of the Tranearth Group) from late Llandovery time onwards (probably sourced from uplifted areas in the Laurentia–Avalonia collision zone. The Lake District Basin accumulated mostly mudstones until early Wenlock time (e.g.
Brathay Formation), receiving the first substantial turbidite sands into its southeastern part in the mid-Wenlock (Birk Riggs &
Coldwell Formations), with the main turbidite influx during early Ludlow (
Coniston Group).
Late Ludlow to Pridoli: As Avalonia and Laurentia were driven ever more tightly together, the subsidence of most of their marine basins was curtailed, which began to create new uplands, shedding increasing amounts of sediment into the remaining basins. Deep marine environments were replaced by
shallow marine and continental conditions. Remnants of non-marine clastic sediments suggest a scatter of small, locally sourced alluvial basins controlled by faulting. The Lake District (
Bannisdale,
Underbarrow and
Kirkby Moor formations) and Welsh Basins were rapidly filling with sediment and the marine shoreline was moving quickly south-eastward. By the end of the Pridoli, the basins were emerging above sea level and were starting to be eroded. River systems took the sediment southwards to be deposited first in a marginal marine embayment and then on alluvial plains (
Downton Castle, Ledbury Formations) on the site of the Midland Platform (extracts from Palmer et al. 2000). Field guides are available to silurian (
1,
2) in southern uplands.
MWH
Further Information
Palmer, D., Siveter, D.J., Lane, P., Woodcock, N. & Aldridge, R., (2000),
British Silurian Stratigraphy , Geological Conservation Review Series, No. 19, Joint Nature Conservation Committee, Peterborough, 542p.
Robin, L. Cocks, M., Fortey, R.A. & Rushton, A.W.A. (2010).
Correlation for the Lower Palaeozoic.
Geological Magazine,
147,171–180.