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An Atlas of Carboniferous Basin Evolution in Northern England

By A. J. Fraser and R. L. Gawthorpe

Abstract

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 appearances in numerous PhD theses (including those of the authors). 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 unique appeal of this atlas of seismic sections is that it is based on data from onshore UK. 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. With this atlas, we can walk the seismic lines at outcrop and in many cases compare exposure to both the seismic data and associated palaeofacies maps.

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    Abstract

    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

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    Abstract

    The pre-Permian geology of northern England and Wales (Wills 1973, 1978; Whittaker 1985; BGS 1985) is divided by lineaments and faults into a series of major terranes (Fig. 4). Turner (1949) and Wills (1973, 1978) realized that old Caledonian faults had exerted considerable influence on the geometry and orientation of subsequent tectonic features and recognized the presence of a triangular shaped platform with a thin, flatlying undeformed Palaeozoic cover underlying the English Midlands (Figs 4 and 5). The platform is bounded on its southern side by the Variscan thrust front, on its northwestern side by the Longmynd Fault and to the northeast by a NW-SE-trending lineament which Turner (1949) and Wills (1973) placed in different locations. The Longmynd Fault, in the NW, separates the platform from the deformed and cleaved Palaeozoic sediments of the Welsh Caledonides. The NE boundary represents a major lineament which separates the platform from a hidden East Midlands Caledonide belt, again with deformed and cleaved Lower Palaeozoic sediments. In this study, the boundary of Turner (1949) was found to be the more appropriate. This triangular platform is now referred to as the Midlands Microcraton (Pharaoh et al. 1987).

    Turner (1949) compared the northern triangular apex of the microcraton with the Hindu Kush and suggested that the NE Caledonian trend in Wales could be traced in an

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    Abstract

    The Carboniferous basin development of northern England is illustrated in a series of regional seismic lines presented in this chapter. Each of the major syn– rift basins will be described in turn, using representative seismic lines (Fig. 10) that have been tied to well and outcrop control. In addition to the seismic data, depth converted geological interpretations for each of the seismic lines are presented to illustrate the development of these tectono–stratigraphic sequences across the province. Particular attention is paid to the Widmerpool Gulf because the combination of seismic quality, well penetrations and the presence of exposure of the syn–rift along–strike in Derbyshire allows us to discuss the tectono–stratigraphic sequences in detail.

    The depth converted geological interpretations are based on corrected sonic logs taken from nearby boreholes, where available. Elsewhere, and for the deeper Dinantian section on most lines, seismic stacking velocities have been applied. The errors inherent in this latter method could result in errors in the depth section of as much as ±10%. Typical velocities used in the depth conversion of the Eakring and Welton sections in the East Midlands and the Northumberland Trough (Kimbell et al. 1989) are shown in Table 1.

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    Abstract

    Having discussed the broad-scale tectono-stratigraphic subdivision of the north of England Carboniferous in the previous section, we now use the megasequences and tectono-stratigraphic sequences to determine the spatial and temporal evolution of depositional systems using sequence palaeogeo- graphies.

    By late Devonian times, rifting had begun in northern England with sedimentation occurring in incipient half graben under an arid climate. The remnant Caledonian mountain belt to the north acted as a major sediment source (e.g. Gilligan 1920; Leeder 1988; Gawthorpe et al. 1989) and, in the study area, Caledonian structures were reactivated and also acted as local sediment sources.

    The northward drift of European Pangaea during the Dinantian led to a change to humid climatic conditions by the late Dinantian (Duff, 1980). This, together with regional transgression, caused a change from red-bed style deposition to fluvio-deltaic deposition in the north of the area, close to the major sediment source, and predominantly carbonate depositional systems in the south of the area, particularly on footwall highs starved of clastic sediment. The development of high-frequency cyclicity in late Dinantian times (e.g. Walkden 1987; Leeder & Strudwick 1987) signifies the growing importance of glacio-eustasy as a control on stratigraphic development; a control which became dominant in the Silesian.

    There is general agreement that northern Britain occupied an equatorial position during the Namurian (Scotese et al. 1979; Smith et al. 1981), and the occurrence of coal and bauxitic soil horizons in Scotland indicates a humid, tropical

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    Abstract

    Integrated structural and sequence stratigraphic analysis, such as that described in this Memoir (specifically Chapters 3 and 4), represents a powerful tool for analysing a petroleum system by identifying hydrocarbon plays and constraining the regional distribution of the key elements of a play. A play is a combination of reservoir, source and seal facies which, together with a trap, may lead to hydrocarbon accumulations at a specific stratigraphic level. The geographic area over which a play is thought to extend is known as the play fairway and is usually determined by the depositional and erosional limits of the reservoir. This need not always be the case, however; play fairways based on the regional extent of a hydrocarbon source rock system or particular structural style are equally valid.

    Play fairway analysis is essentially an assessment of exploration risk at a basin scale. In the past the petroleum industry has applied the concept of risk mainly at a prospect-specific level. On a larger scale, applying risk analysis to the play fairway level in frontier basins permits channelling of exploration effort into the most prospective parts of a basin. Furthermore, by combining the risks for individual plays within a basin, different basins can be ranked, allowing exploration to be focussed towards particular basins. In more mature areas, the technique can highlight new plays in under-explored parts of the basin or, equally, provide an indication that the basin has very little remaining prospectivity and that it may be time to

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    Abstract

    The structures that controlled the Carboniferous tectonic and stratigraphic development of northern England were inherited from the earlier Caledonian orogeny which imparted a strong NW-SE and NE-SW tectonic grain that is evidenced on surface and subsurface data throughout northern England. The subsequent Variscan plate cycle, which involved the closure of the Rheic and Rheno-Hercynian oceans, controlled the development of syn-rift, post-rift and inversion megasequences from late Devonian to early Permian times. Regionally extensive, seismically resolvable sequences developed within the Carboniferous were controlled by episodic rifting and fault reactivation, with eustatic sea-level changes providing a high frequency control on depositional sequences that are generally below seismic resolution.

    In the south of the province during the Dinantian, carbonate environments were extensively developed in syn-rift basins starved of terrigenous clastics. The north of the region was dominated from early Dinantian times onward by a southward prograding terrigenous clastic delta system. The carbonate environments in the south were finally drowned in the early post-rift (Pendleian) when the supply of terrigenous clastic sediments outpaced subsidence for the first time. Following this, rapid southward progradation of the delta systems occurred as a series of pulses largely controlled by eustatic sea-level changes, but with important local controls, namely antecedent rift physiography. By the early Westphalian, delta top conditions had been established over most of northern England. These conditions were progressively disrupted from late Westphalian C times by inversion related to the Variscan orogeny.

    Over 70 years of exploration in northern England has resulted in the discovery of

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