Structure and evolution of the East Midlands region of the Pennine Basin : subsurface memoir
2011
sedimentary
history of the eastern part of the English
Midlands (hereafter referred to as the region) emphasising
the structure and evolution of the Carboniferous
basins. It is the fourth in the Subsurface Memoir series of
the British Geological Survey, following the Northumberland–
Solway Basin (Chadwick et al., 1995), the Craven
Basin (Kirby et al., 2000) and south-west Pennine Basin
(Smith et al., 2005), and forms a sequel to a previously
published subsurface study of the Mesozoic basins of
England and Wales (British Geological Survey, 1985a).
The region includes areas of high urban population
density, contrasting with pastoral countryside and
fenland (Figure 1). The cities of Nottingham, Leicester,
Lincoln and Peterborough are the principal conurbations,
but much of north Nottinghamshire and South
Yorkshire bear the legacy of industrialisation following
development of the concealed Yorkshire–Nottinghamshire
coalfield. The geology of the region (Figure 2) is
superficially simple. Carboniferous strata of the southeastern
part of the Pennine Basin dip eastward beneath
a cover of Permian and Mesozoic strata. This description
belies the complex structure and stratigraphy of the
Carboniferous basins however; rapid stratigraphical variation
occurs where the Carboniferous strata onlap onto
the Anglo–Brabant Massif in the south of the region.
Early Palaeozoic strata have very limited outcrop, and
are mainly known from deep boreholes. Deformation
of the rocks occurred during a number of tectonic
episodes, producing structures that control their nature
and distribution. The surface and near-surface geology
have been described in the British Geological Survey
1:50 000 Series maps and memoirs (Appendix 1). This
account examines the deeper parts of the Permian–
Mesozoic and, in particular, the Carboniferous successions,
in greater detail than in earlier publications. Over
2000 seismic reflection profiles have been interpreted,
and some 500 deep boreholes studied during the course
of this work, and the results are synthesised in the accompanying
1:625 000 scale structure contour and isopach
maps (Appendix 3) and associated text figures. This
account provides both a regional review and an explanation
of these maps and figures.
The Carboniferous rocks of the region have long been
of economic significance. Coal has been worked for
centuries. Prior to the 1960s, only limited investigation
of the concealed geology was possible, mainly through
coal exploration and extraction, and oil exploration. The
National Coal Board (NCB) established a sophisticated
system for coal seam identification and correlation using
geochemical methods. This was combined with biostratigraphical
information from marine bands to yield a
very detailed lithostratigraphical correlation of the Coal
Measures. The region has experienced several phases
of hydrocarbon exploration: immediately following the
First World War; just before and after the Second World
War, when drilling was mainly on the edges of structural
highs recognised from gravity anomalies; in the
1950s and 1960s, with the widespread application of
seismic techniques; in the 1970s and 1980s, using 2D
reflection seismic data of increasingly high fold; and
since 1995, using 3D reflection seismic. The 2D data are
variable in fold of stack (12 to 60) and data quality (poor
to good). The acquisition of the hydrocarbon exploration
seismic data, in combination with Coal Authority
high resolution seismic data over more localised areas,
has resulted in an extensive network of seismic reflection
profiles (Figure 3), except across the Anglo–Brabant
Massif (Figure 4) and in the main conurbations. This
account relies heavily on these data. The 3D data are
now becoming available through release, but were not
extensively used during the mapping programme. A few
examples of these data are however included as figures.
All phases of exploration have involved the drilling of
deep boreholes which, together with surface exposures,
provide stratigraphical calibration.
Depth conversion of interpreted seismic data
to produce the structural maps was complicated by a
number of factors, including the variable age of the
data and the quantity and quality of the data. Horizons
mapped from the seismic data in two-way-travel-time
(TWTT) were converted to depth using velocity functions
based on depths and times in key boreholes. Particular
attention was paid to the depth conversion of the
early Carboniferous interval, due to the known variation
of Tournaisian and Visean lithologies (and thus interval
velocities) at crop and in boreholes in surrounding
basins (i.e. platform carbonates and basinal mudstone
facies)
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