Page 8 - An Atlas of Carboniferous Basin Evolution in Northern England

Chapter 1

Introduction

Rationale

Why an atlas of the Carboniferous in northern England? There can hardly be a
more researched system in the whole of the British Isles, given its widespread
distribution at outcrop and annual appearance in numerous PhD theses
(including our own). But perhaps all we really know about the Carboniferous is
no more than skimming the surface. In this atlas, using modern multifold
seismic and borehole data collected by the oil industry in its search for
petroleum accumulations, we can start to look beyond the surface exposures
and gain some new insights into the structure and stratigraphy of the
subsurface (and surface) Carboniferous. The main elements of this atlas are: (i)
a series of regional seismic lines crossing all the basinal areas in northern
England illustrating the Carboniferous in section, and (ii) a set of palaeofacies
maps describing the evolution of the system in map view.
The unique appeal of this atlas of seismic sections is that it is based on data
from onshore UK. That is, we can walk the seismic lines at outcrop and in
many cases compare exposure to the seismic data and palaeofacies maps. For
example, stand on top of Mam Tor in Derbyshire and look eastwards over
Hope valley towards Castleton (see frontispiece). Here, we can look out over a
Carboniferous basin fill. On the right is an exhumed Dinantian platform
margin and, ahead, Namurian Edale Shales form the valley floor. On the left,
the north side of the valley is composed of basinal and slope turbidites of the
Kinderscoutian, and in the distance, the prominent "gritstone" escarpments are
composed of younger Marsdenian fluvial strata. But what of the basin
geometry and the structural-stratigraphic relationships? Now take the Edale
Gulf seismic line in this atlas (see frontispiece). The valley marks the position of
a Dinantian half graben, and the carbonate platform margin is localized over
the footwall of a down-to-the-north normal fault zone. The Namurian
mudstones and turbidites onlap the Dinantian carbonates and are post-rift
sediments, initially infilling the sediment-starved half graben with post-rift
mudstones and then prograding from the north, once the basin had shallowed
sufficiently, to establish fluvial sandstones across the area.
The atlas presents a unique collection of onshore seismic data from BP's
once-extensive onshore database. Although these lines were originally shot as
small segments targeting individual prospects and trends, they have been
spliced together to produce a series of basin-scale regional lines which should
be of value to academic researchers and industry alike. Unfortunately, since
this seismic was collected and compiled, much of the database has been
fragmented as BP made the commercial decision to withdraw from active
exploration in northern England in the early 1990s. As well as providing a
paper record of these data, the atlas can be used as a tool for teaching aspects
of stratigraphy, basin analysis and linking outcrop and subsurface studies in
one of the classic areas of UK geology. Moreover, much of the data presented
in this atlas was collated as part of a major re-assessment of the petroleum
potential of northern England. We have therefore included in Chapter 5 a
description of the main hydrocarbon play systems in northern England,
providing a dataset for teaching the key aspects of play fairway analysis as
applied by the oil industry.
Previous palaeogeographic and lithofacies reconstructions for the northern
England Carboniferous have relied heavily on surface outcrop mapping
supplemented by shallow borehole and gravity data (e.g. Wills 1951; George
1958; Johnson 1960; Kent 1966; Leeder 1974; Miller & Grayson 1982; Fig. 1. Summary map of northern England Dinantian structural elements (after Fraser & Gawthorpe 1990; Corfield et al 1996).