ChapterPDF Available

Pitch Coppice: GSSP for the base of the Ludlow Series and Gorstian Stage

Authors:
  • Halliburton & University of Birmingham
Pitch Coppice: GSSP for the base of the
Ludlow Series and
Gorstian Stage
Alan T. Thomas & David C. Ray
A. T. Thomas — Earth Sciences (GEES), University of Birmingham,
Birmingham, B15 2TT, UK; a.t.thomas@bham.ac.uk
D. C. Ray — Neftex Petroleum Consultants Ltd, Abingdon, England, OX14
4RY, UK; daveray01@yahoo.com
Age: Late Homerian to early Ludlow. Pitch Coppice was proposed as the
standard section for the coincident base of the Ludlow Series and its lower
(then Eltonian) stage. This was later endorsed by the Silurian
Subcommission (Holland, 1980a, b; Holland et al. 1980; Martinsson et al.
1981).
Lithostratigraphic units: Topmost Much Wenlock Limestone Formation
and basal Lower Elton Formation. Pitch Coppice is the type section for the
base of the Lower Elton Formation.
Locality: Old quarry 35 m south of Wigmore Road (Mortimer Forest Stop
3), ~4 km SSW of Ludlow, Shropshire (SO 472 730). Two further small
quarries (Mortimer Forest Stops 1 and 2), within the Much Wenlock
Limestone Formation, occur immediately west of Pitch Coppice (SO 472
730 and SO 471 730) (Fig. 29).
80
Lithology and fossil constituents
Pitch Coppice and the two stratigraphically lower quarries (Mortimer Forest
Stops 1 and 2) contain the upper 14 m of the Much Wenlock Limestone
Formation. Sedimentation here occurred in an extension of the depositional
setting represented by the off-reef tract recognised on Wenlock Edge, and
the lithology comprises poorly fossiliferous, carbonate mudstones,
wackestones and silty mudstones. In detail the lowest beds consist of blue-
grey carbonate mudstones separated by thin silty mudstone partings (84
cm). Above is a highly distinctive interval (~4.8 m) of rhythmically bedded
carbonate mudstones (beds and nodules) and silty mudstones of equal or
greater thickness. Towards the top of the section at Mortimer Forest Stop 1
the silty mudstone content is reduced, resulting in a predominance of
carbonate mudstone beds. These give way in the overlying quarry
(Mortimer Forest Stop 2) to nodular limestones with thin silty mudstone
partings and a single thin bentonite (PC1). Pitch Coppice quarry (Mortimer
Forest Stop 3) exposes the topmost 3.35 m of the Much Wenlock Limestone
Formation and 2.8 m of the overlying Lower Elton Formation (Fig. 29;
Siveter in Aldridge et al. 2000, p. 331). The Much Wenlock Limestone
Formation here comprises grey, nodular limestone with thin silty mudstone
partings. Additional features include two thin bentonitic bands (PC2 and
PC3) and a prominent silty mudstone band (18 cm), 70 cm above the oor
of the quarry. The Formation yields a sparse but varied macrofauna
including bryozoans, brachiopods (Atrypa, Gypidula, Leptaena, Resserella,
Sphaerirhynchia, Strophonella), crinoid ossicles, corals (Favosites,
Heliolites, Thecia), gastropods, and rare trilobites (Dalmanites, Proetus).
The corals and relatively large brachiopods are characteristic of the unit
here. The basal Lower Elton Formation comprises shaly siltstones with
impersistent limestone beds and nodules. Thin (1-2 cm) crinoidal beds are
also observed and indicate episodes of storm sedimentation. A bentonite
(PC4) occurs 0.25 m above the base. The fauna of the Lower Elton
81
Figure 29. Location, sedimentary log, sequence stratigraphic interpretation and
carbon isotope record of the succession exposed in Pitch Coppice (Mortimer Forest
Stops 1 to 3). SM, silty mudstone; M, carbonate mudstone; W, wackestones; P,
packstone; G, grainstone.
82
Formation here is mostly fragmentary, with many species common to the
underlying unit, although small brachiopods are particularly characteristic
(Holland et al. 1963; Lawson & White, 1989).
Biostratigraphy
The only graptolites recorded from Pitch Coppice are fragments possibly
belonging to Saetograptus varians and Neodiversograptus nilssoni from
0.03 m and 0.23 m above the base of the Elton Formation respectively
(White, 1981). These records support the notion that the base of the Lower
Elton Formation lies at or close to the base of the nilssoni Biozone (Holland
et al. 1969; Lawson & White, 1989; Bassett et al. 1975; also see discussion
in Ray et al. 2010). No widely correlatable changes in microfaunas occur at
the boundary (see Siveter in Aldridge et al. 2000 for review).
Sequence stratigraphy
Mortimer Forest Stops 1 to 3 contains parasequences PS9 to PS12, as
observed from Wenlock Edge and the West Midlands (Ray et al. 2010).
PS10 is the thickest of the parasequences (~12 m) and represents an abrupt
shallowing toward the top of the Much Wenlock Limestone. Above, a
prominent silty mudstone band marks the ooding surface of PS11 and the
gentle onset of relative sea-level rise. As at the Lea Quarry South and
Wren’s Nest Hill stops, the base of the Lower Elton Formation here
coincides with the base of PS12. This marked deepening is the local
reection of a eustatic change (Highstand 6 of Johnson, 2006). The
sequence stratigraphic signicance of this and other sea-level changes of
about this age are now beginning to be explored in detail (e.g. Ray et al.
2010; Ray & Butcher, 2010).
83
Isotope stratigraphy
Nineteen carbon isotope samples (Mortimer Forest Stops 2 and 3) have
been taken stratigraphically above and below the GSSP and the Much
Wenlock Limestone and Lower Elton formation contact (unpublished data,
Cramer NSF# EAR-0948277). δ13Ccarb values from the Much Wenlock
Limestone Formation range from 0.44‰ to 1.25‰ and are generally higher
than those of the Lower Elton Formation (-1.82‰ to 0.99‰). This negative
shift across the Wenlock-Ludlow boundary has also been reported from
Pitch Coppice and several other locations across the Midland Platform and
Welsh Basin by Coreld et al. (1992) and probably correlates with the
declining limb of the upper peak of the well-known Homerian positive
carbon isotope excursion.
Bentonites
Five bentonitic horizons are observable from the three quarries (Mortimer
Forest Stops 1 to 3). All are extremely thin (0.5 to 1.0 cm) and are
identiable as blue-grey or rusty orange bands that appear slightly more
clay rich than the surrounding strata. The thickest of the bentonites (PC4)
occurs 0.25 m above the base of the Lower Elton Formation and has been
sampled for heavy minerals. Zircons are present, but unfortunately were of
insufcient quality and number for radiometric dating (pers. comm. Cramer
2011).
84
... The low point between the two peaks of the δ 13 C carb excursion appears to mark the onset of the recovery interval and δ 13 C carb values began to rise into the second peak somewhere near the base of the Ho3 Stage Slice (Cramer et al., 2011(Cramer et al., , 2012 and the base of the Djupvik Member of the Halla Formation. Carbonate carbon isotope values return back to baseline and end the second peak prior to the base of the Ludlow Series (Thomas and Ray, 2011;Cramer et al., 2012;McAdams et al., 2019). ...
Article
High-resolution paired analyses of δ¹³Ccarb and δ¹³Corg from a new drill core from Gotland, Sweden, demonstrate asynchronous positive change in the carbon isotope records during the onset of one of the major Silurian biogeochemical events known as the Mulde Event or “Big Crisis”. The detailed carbon isotope record presented here provides Δ¹³C (the difference between δ¹³Ccarb and δ¹³Corg) and allows the calculation of changes in organic carbon burial (forg) throughout the late Wenlock. The paired data suggest a ~ 38% increase in forg during the peak of the positive δ¹³Ccarb excursion and the high-resolution record reveals several short-lived inflections in Δ¹³C that have not been previously identified. When combined with sedimentological and sequence stratigraphic data from multiple paleocontinents, the new data presented here provide strong evidence for a transient global decrease in CO2, in support of previous interpretations of regression and global cooling coinciding with the Mulde Extinction Event.
... New carbon isotope data are presented for five of the sections-four from across the Indiana-Ohio border and one from Nashville, Tennessee, from which new conodont data are also presented. A sequence The onset of the Mulde carbon isotope excursion is within the upper part of the lundgreni graptolite Zone, and the end is below the base of the Ludlow Series at the type area in Shropshire, England (Thomas and Ray, 2011;Cramer et al., 2012). The excursion was first recognized in the West Midlands, England, and immediately associated with the graptolite extinction (Corfield et al., 1992); it is now documented globally and associated with many other biological events . ...
... (GSSP) at Pitch Coppice in the UK(Thomas and Ray, 2011), and a flat line of stable isotope values in the uppermost Homerian is interrupted by a transient shift to lower values immediately across the base of the Ludlow Series. In the Schlamer #1 Core, the interval of stable +2‰ isotope values in the upper St. Clair Formation is followed by a brief decline in the basal Moccasin Springs that matches the pattern seen in the UK. ...
Article
The Illinois Basin of southwestern Illinois and southeastern Missouri contains one of the most extensive Silurian records in North America and is a potentially critical area for global sea-level and paleoclimatic/ paleoceanographic studies. Unfortunately, the Silurian stratigraphy regionally remains understudied and often can only be correlated globally at series level. Silurian strata from southwestern Illinois and southeastern Missouri are currently assigned to the Sexton Creek Limestone, Seventy-Six Shale, St. Clair Limestone, and Moccasin Springs Formation. The latter three units are variously considered members of the Bainbridge Formation or formations within the Bainbridge Group. We advocate the formalization of the term Bainbridge at the group level throughout its area of use, composed of (in ascending order) the Seventy-Six Formation (raised in rank and geographically expanded herein), the St. Clair Formation (stratigraphically expanded herein), and the Moccasin Springs Formation with four members: the Greens Ferry, Sheppard Point, Lithium (proposed herein), and Randol members. The Bainbridge Group is unconformably underlain by the Silurian Sexton Creek Formation and conformably overlain by the ? Pridoli-Devonian Bailey Formation. Herein, additional nomenclatural changes are presented to the Cason Formation of Arkansas as well. Integrated carbon isotope (δ¹³C), conodont, and graptolite biochemostratigraphy of three localities in the southwestern Illinois and southeastern Missouri area indicate the presence of the early Sheinwoodian "Ireviken," Homerian "Mulde," and Ludfordian "Lau" positive carbon isotope excursions in the Bainbridge Group (the first published consecutive occurrence from North America). The stratigraphic revisions presented in this study provide a consistent and widely applicable lithostratigraphic nomenclature for the Bainbridge Group throughout its areal extent. This study also provides the first detailed integrated biochemostratigraphic record of all three major Wenlock-Ludfordian δ¹³C excursions from this important Laurentian basin, and it enables the first precise correlation of the Bainbridge Group into the global Silurian chronostratigraphic scheme.
... As Goggin Road represents the most distal of the sections, in situ carbonate production will likely reflect the lighter 13 C carb values more typical of the open ocean, while carbonates derived from shallower-water carbonate production will be somewhat heavier. A particularly notable feature of PS11 at Goggin Road, and to a lesser extent Pitch Coppice (Thomas and Ray 2011), is the variability of 13 C carb values. Such variations may reflect pulses of platform-derived carbonate deposited during storm events. ...
Article
Full-text available
The Wenlock-Ludlow series boundary (Silurian) has been recognized as a time of pronounced sea-level rise and the end of a globally recognized Late Homerian Stage (Mulde) positive carbon isotope excursion (CIE). However, the precise timing and synchronicity of the end of the excursion with respect to the Wenlock-Ludlow boundary is debated. Within the type Wenlock and Ludlow areas (UK), high-resolution δ13Ccarb isotope data are presented across the Wenlock-Ludlow boundary, and within a range of carbonate platform settings. Correlation between sections and depositional settings has been based upon the characteristics of high-order sea-level fluctuations (parasequences). Comparisons between parasequence-bounded δ13Ccarb values reveal clear spatial variations, with lighter values recorded from more distal settings and heavier values from shallower settings. Temporal variations in the δ13Ccarb values are also documented and appear to reflect local variations in carbonate provenance and productivity in response to sea-level rise. While δ13Ccarb values converge in all sections towards the Wenlock-Ludlow boundary, the apparent end of the Mulde CIE appears diachronous and is progressively older within more distal settings. © 2016, National Research Council of Canada. All right reserved.
Chapter
The Silurian Period (443.1–419.0 Ma) was a time of general convergence of continental plates, strong fluctuations in global sea level, and the early stages of colonization of land. The base of the Silurian System is defined at the level of the first appearance of the graptolite species Akidograptus ascensus at Dob’s Linn, Scotland. Silurian time can be finely resolved using integrated graptolite, conodont, and isotope biochemostratigraphy. The Silurian time scale is based on a CONOP9 composite of graptolite range data derived from 837 stratigraphic sections and 2651 graptolite taxa, with interpolated radioisotope dates, spanning the Ordovician into the Lower Devonian. There is a succession of at least seven globally recognizable positive carbon-isotope excursions, most of which are associated with important bioevents and environmental changes indicated by other geochemical proxies. These data show that the Silurian was a time of dramatic changes in climate, ocean chemistry, and biodiversity.
Article
Full-text available
New δ¹³Ccarb data from the most stratigraphically extensive graptolitic sections of Homerian age in the study area are reported from Wenlock Edge and the Ludlow Anticline (UK). These sections, situated upon the Midland Platform (Avalonia), were key in establishing the Homerian graptolite biozonation used within the type Wenlock Series, and are consequently of international importance. Based upon 162 δ¹³Ccarb samples from four outcrops (Eaton Track, Longville–Stanway Road Cutting, Burrington Section and Mortimer Forest Stop 1), new graptolite collections and a re-evaluation of the original graptolite collections, the onset of both lower (older) and upper (younger) peaks of the Homerian Carbon Isotope Excursion have been calibrated to a revised graptolite biozonation (lundgreni – nassa – praedeubeli-deubeli – ludensis biozones). In addition, high-resolution correlation between the Ludlow Anticline and Wenlock Edge has been achieved by bio-, chemo- and sequence stratigraphic techniques. These correlations suggest a uniformity of depositional rates across the study area and indicate minor diachroneity at the base of the Much Wenlock Limestone Formation. Finally, correlations of the Midland Platform Homerian Carbon Isotope Excursion have allowed for better comparisons with other sections from which high-resolution graptolite and carbon isotope data are available. Such comparisons highlight the pan-regional synchronicity of the Homerian Carbon Isotope Excursion.
Chapter
Rapid and sustained biotic diversification (“Ordovician radiation”) to reach highest diversity levels for Paleozoic; prolonged “hot-house” climate punctuated by “ice-house” intervals and oceanic turnover; strong fluctuations in eustatic level, global glaciation, and mass extinction at end of period; appearance and evolution of pandemic planktonic graptolites and conodonts important for correlation; moderate to strong benthic faunal provincialism; re-organization and rapid migration of tectonic plates surrounding the Iapetus Ocean; migration of South Pole from North Africa to central Africa, all characterize the Ordovician period. HISTORY AND SUBDIVISIONS Named after the Ordovices, a northern Welsh tribe, the Ordovician was proposed as a new system by Lapworth in 1879. It was a compromise solution to the controversy over strata in North Wales that had been included by Adam Sedgwick in his Cambrian System but which were also included by Murchison as constituting the lower part of his Silurian System. Although it was initially slow to be accepted in Britain, where it was instead generally called Lower Silurian well into the twentieth century, the Ordovician was soon recognized and used elsewhere, such as in the Baltic region and Australia.
ResearchGate has not been able to resolve any references for this publication.