Exploration Insights March 2020 | Page 28

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Exploration Insights | 29
Ragleth Hill
Caer Caradoc
with Baltica. Much of the overlying Silurian strata
(Wenlock and Ludlow epochs) are associated
with ridge-and-vale topography; the resistant
shallow-marine limestones form the ridges, and
the intervening deep-marine shales form the
broad low-lying vales (Ape Dale looking west, and
Hope Dale and Corve Dale looking progressively
east) (Figure 8).
The Lawley
Longmynd
Figure 7> Looking west from Wenlock Edge, over an attenuated succession of Silurian to Precambrian rocks.
reefs, which are a notable feature of the Much
Wenlock Limestone (Wenlock Epoch) and
the Wenlock Edge escarpment. On a clear
day, fantastic views across large parts of the
Shropshire Hills allow the close relationship
between the local geology and topography to be
appreciated (Figure 7).
Caer Caradoc, and The Lawley. To the far north is
the Wrekin.
In front of these hills, the succession consists
of Cambrian and Ordovician sedimentary rocks,
which dip eastward beneath the Silurian strata
that occupy Ape Dale. The Silurian succession
begins with Late Llandovery sandstones,
which contain numerous oil shows and suggest
the generation of petroleum from overlying
Wenlock and underlying Ordovician shales
(Selley 1992). The stratigraphic gap between
the Late Ordovician and Silurian is the Shelvian
Unconformity (Figure 1), which was created by
regional uplift in response to the closure of the
Tornquist Ocean, as eastern Avalonia collided
Looking west, the skyline is marked by a
flat-topped hill, called the Longmynd, which
is composed of late Precambrian clastic
sedimentary rocks. Three prominent hills in the
middle distance are situated along the Church
Stretton Fault System, and are composed of late
Precambrian Uriconian volcanics and intrusions.
Going from south to north, these are Ragleth Hill,
North-west
South-east
Wenlock Edge
Ape Dale
Hope
Bowdler
Ticklerton
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V
Alte
V
rna
V
Uriconian
Volcanics
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Hope Dale
Munslow
Eaton
rmit
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Corve Dale
Downton
Series
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Elton
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nlock
Sh
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Coa
Lime ales
Pent
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P
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F
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Shale was
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Beds
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Late Precambrian
Devonian
Pridoli
Muc
Ludlow
© 2020 Halliburton
Late Ordovician
Llandovery
Wenlock
Continental sediments Shallow-marine shale Shallow-marine limestone
Shallow-marine sandstones Deep-marine shale Igneous (volcanics and intrusive)
Figure 8> Schematic cross-section from Hope Bowdler to Corve Dale, showing the ridge-and-vale topography of the Silurian. Modified
from Toghill and Chell, 1984, Figure 1.
Based on global comparisons, these
transgressive-regressive cycles appear to be
eustatic, and have been linked to the waxing
and waning of ice sheets on Gondwana.
Superimposed on this eustatic signal is a longer-
term, upward shallowing trend that reflects the
collision of Western Avalonia and Laurentia,
and closure of the Western Iapetus Ocean. This
Acadian Orogeny resulted in the deposition of
continental sediments (Old Red Sandstone facies)
toward the close of the Silurian, and an absence
of much of the Devonian.
Key Discussion Points:
» Summarize the day’s learning,
emphasizing the regional context of the
Late Precambrian and Lower Paleozoic
stratigraphy.
» Highlight the potential Silurian source
rock interval, and the remaining major
tectonic events that affected the wider
succession (Figure 1).
SUMMARY
To summarize the excursion, discussions
can focus on petroleum plays, tectonics, and
sedimentology. Regarding the central question
of the source of the Tar Tunnel oil, the most
likely answer is the Carboniferous Pennine Coal
Measures Group (Stop 2), owing to the temporal
and spatial proximity of the likely source rock and
reservoir (Figure 3). The Lower Paleozoic source
rocks have undergone several tectonic events
that may have caused petroleum generation prior
to the presence of the Carboniferous reservoir.
This may be evidenced by the numerous oil
shows within Llandovery sandstones.
STILL TIME LEFT TO EXPLORE?
You could round off the trip by investigating the
highly fossiliferous Much Wenlock Limestone
via the geology trial accessible from the nearby
National Trust cark park at Presthope (UK
grid reference SO 5833 9754). Extra days of
fieldwork could easily be filled by investigating
the succession along Wenlock Edge, around
Church Stretton and the River Onny, and at
Ludlow.
ACKNOWLEDGMENTS
Jamie Higton is thanked for his insights into the
geology and tectonics of this area.
REFERENCES
Craig, J., F. Gerali, F. Macaulay and R. Sorkhabi 2018. The
history of the European oil and gas industry (1600s–2000s).
Geological Society of Greece - Special Publication, v. 465, no.
1, p. 1.1-24. (XURBB_643115).
Didyk, B.M., B.R.T. Simoneit and G. Eglinton 1983. Bitumen
from Coalport Tar Tunnel. Organic Geochemistry, v. 5, no. 3, p.
99-109. (XURBB_539005).
Earp, J.R. and B.A. Hains 1971. British Regional Geology The
Welsh Borderland (Third Edition). Her Majesty’s Stationery
Office, 1-118 p. (XURBB_643210).
Selley, R.C. 1992. Petroleum seepages and impregnations in
Great Britain. Marine and Petroleum Geology, v. 9, no. 3, p.
226-244. (XURBB_236905).
Toghill, P. 1994. Earth Science Teachers’ Association Annual
Course and Conference, September 16-18th 1994. Field
Workshop Handbook. University of Birmingham, 62 p.
(XURBB_643208).
Toghill, P. and K. Chell 1984. Shropshire Geology - Stratigraphic
and Tectonic History. Field Studies Council, 59-101 p.
(XURBB_643207).
Wright, A.E., I.J. Fairchild, F. Moseley and C. Downie 1993.
The Lower Cambrian Wrekin Quartzite and the age of its
unconformity on the Ercall Granophyre. Geological Magazine,
v. 130, no. 2, p. 257-264. (XURBB_483395).
AUTHORS
David Ray, Stratigraphy Advisor, Halliburton
Landmark
David joined Neftex 2008, and has held a number
of roles, mostly focused on biostratigraphy
and the development of the Neftex ® Sequence
Stratigraphic Model. David holds a BSc degree in Geology
with Paleontology from Imperial College, U.K. and a PhD in
Event Stratigraphy from the University of Cincinnati, USA. He
is a Research Fellow at the University of Birmingham, UK,
and a member of the International Subcommission on Silurian
Stratigraphy.
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