Page 11 - An Atlas of Carboniferous Basin Evolution in Northern England

4 Chapter 1

Gawthorpe \981a; Gutteridge 1987; Guion & Fielding 1988; Cope et at. 1992). consistent regional chronostratigraphic framework within which to interpret varying degrees of confidence in interpretation both within and between maps.
In contrast, the contents of this atlas are primarily based on subsurface (seismic the structural and stratigraphic evolution of the basins and to assess the Finally, Chapter 5 provides a summary of the main hydrocarbon play fairways
and borehole) information. The borehole data have provided the main control hydrocarbon habitat. in the northern England Carboniferous.
on subsurface depositional facies; however, seismic facies analysis has been Within this tectono-stratigraphic framework and where borehole and
invaluable in facies interpretation in areas where there are few deep well outcrop data are sufficient, depositional sequences and systems tract mapping
penetrations. The seismic data have also provided a hitherto unattainable can provide further constraints on palaeogeography, facies relationships and
insight to the Carboniferous basin geometries. The earlier structural the controls on stratigraphy (e.g. Church & Gawthorpe 1994). In fact, given the Database
interpretation of "gulfs' and "blocks' with poorly constrained basin margins very high resolution framework provided by regionally extensive and
(Kent 1966), has now been integrated into a coherent structural framework. biostratigraphically diagnostic marine bands, the Carboniferous of northern The area covered by this atlas incorporates all the half graben identified in the
With respect to the Carboniferous of northern England, the availability of England has the potential to test the concepts underlying sequence northern England Carboniferous rift system (Fig. 1). Hence, the early
seismic reflection data has provided a further tool for intra- and inter-basin stratigraphic models (e.g. Church & Gawthorpe 1997). Palaeozoic Southern Uplands and Wales-Brabant Massif represent the
stratigraphic correlation, in addition to the traditional use of lithostratigraphy northern and southern geological limits of the study area respectively. The
and biostratigraphy. Vail & Mitchum (1977) suggested that a seismic reflection western and eastern boundaries of the study area extend into the Irish Sea and
is generated by a time-synchronous stratal surface, that may pass through all southern North Sea
facies belts. Seismic data, therefore, can potentially identify chronostrati- Atlas layout The well and seismic database used to compile this atlas is summarized in
graphic units through the definition of seismic sequences (Vail & Mitchum Figures 2 & 3. A total of 270 key wells which have penetrated Lower
1977). This principle provides the basis for the sequence-stratigraphic Following this introduction, in Chapter 2, the structural setting of the northern Carboniferous strata in northern England have been incorporated. The wells
subdivision of the Carboniferous presented herein. England Carboniferous is placed in its regional plate-tectonic context in order have been used to control palaeogeographic maps, generate isopachs and as
An ideal sequence-stratigraphic framework for correlation would divide the to constrain the external controls on basin development. Having established fence posts in seismo-stratigraphic work in the Widmerpool Gulf and
stratigraphy into depositional sequences and their component systems tracts, how and when the basins were formed, Chapter 3 describes the stratigraphic fill Gainsborough Trough. Additional biostratigraphic studies were carried out
bounded by sequence boundaries, regressive surfaces, transgressive surfaces, and structural style of the depocentres with reference to a series of interpreted on key wells, such as Hathern-1 in the East Midlands, which proved to be
maximum flooding surfaces and their correlative condensed intervals (e.g. Van regional seismic profiles, tied to available well and surface geological data. The critical in the dating and correlation of sequence boundaries between half
Wagoner et al. 1988, 1990). Given sufficient well data and seismic resolution, regional tectono-stratigraphic framework established in Chapter 3 provides the graben and with the numerous outcrop-based studies that exist in the literature
this procedure is relatively straightforward and the resultant stratal packages basis for the generation of a series of palaeogeographic maps, which are for the Carboniferous. Much of this well data, including summary biostrati-
can be considered the fundamental units for stratigraphic correlation. In the described in Chapter 4. These palaeogeographies have been compiled using a graphic data, is now in the public domain and can be accessed from the BGS
northern England Carboniferous, however, depositional sequences and their combination of borehole and outcrop information combined with published Hydrocarbons Unit, Murchison House, West Mains Road, Edinburgh via the
component systems tracts are below seismic resolution and thus a complete and BP in-house literature and structural, seismic, gravity and magnetic data. Department of Trade and Industry. Similarly, some of the seismic reflection
breakdown of the stratigraphy in this manner on seismic is generally The datasets were correlated using a mixture of proprietary BP and published data can be accessed through the UK Onshore Geophysical Library.
impractical. biostratigraphic data. It is important to realise that the detail and quality of the
The approach described in this atlas adopts the tectono-stratigraphic palaeogeographies varies according to the availability of biostratigraphic and
methodology of Hubbard et al. (19850, b) and utilizes seismic data tied to lithostratigraphic data. For example, the late Devonian map is very poorly
outcrop and wells in order to construct an integrated, regional tectono- constrained, whereas the database for the late Carboniferous reconstructions is
stratigraphy for the Carboniferous of northern England. Although this vast by comparison. Similarly, more prolific subsurface data are available for
approach does not define depositional sequences sensu stricto, it provides a the East Midlands than for basins to the north and west. This introduces