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Content uploaded by David C. Ray
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All content in this area was uploaded by David C. Ray on Dec 01, 2016
Content may be subject to copyright.
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).
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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 floor
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
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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.
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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 flooding 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
reflection of a eustatic change (Highstand 6 of Johnson, 2006). The
sequence stratigraphic significance 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).
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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 Corfield 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
identifiable 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
insufficient quality and number for radiometric dating (pers. comm. Cramer
2011).
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