Page 34 - An Atlas of Carboniferous Basin Evolution in Northern England

Carboniferous basin development 19

to the north by this time (e.g. Collinson 1988; Fraser et al 1990). Facies and the sequence ranges into the Westphalian C where it was terminated by the ships are represented by the Arundian-Holkerian (EC3) and a very thin late
thickness of this advancing delta system were strongly influenced by the onset of Variscan inversion in the area. The initial phase of inversion is Brigantian-early Pendleian (LC1a).
existing fault-produced bathymetry. Backstripping of regional seismic data interpreted to have occurred between the Anthraconauta phillipsi and A. tennis The overlying Silesian post-rift (LC1-LC2) has been described in detail by
suggests a water depth of about 300 m in the Widmerpool Gulf during the zones of the Upper Carboniferous (Wills 1956). Steele (1988). The thickening of the Namurian sequence into the Gainsborough
initial stages of LC1, an interpretation supported by the predominance of deep Trough is ascribed to three processes: (i) the infill of existing pre-Namurian
water conodont faunas. However, for most of the Namurian and early Correlation. A major shift from lower delta plain to upper delta plain bathymetry, (ii) differential compaction, and (iii) localized Namurian tectonic
Westphalian, northern England was dominated by laterally extensive 'sheet- conditions throughout the northern England Pennine Basin, approximating to activity. The earliest Namurian sediments deposited in the half graben were
like' deltas with little differentiation into shelf and deep water basinal the Amaliae Marine Band, has been described by Guion & Fielding (1988). organic-rich mudstones deposited in relatively deep water. Steele estimates
environments. The entire seismic dataset for the East Midlands resolves only This event is thought to have regional significance throughout northern some 300 m of bathymetry existed at the end Dinantian in the centre of the
one example of clinoforms associated with the delta system prograding into a England. The periodic tectonic activity evidenced on several of the major basin. The subsequent fill of the basin has been one of progressive advance of
deep water environment — from the Ashover delta (LClc) advancing along the basin-bounding faults during the early part of the post-rift had almost ceased the Kinderscout, Ashover and Chatsworth-Crawshaw shallower water delta
axis of the gulf. Clinoform height suggests basinal water depths of 150-200 m by early Westphalian A times. This also coincided approximately with the systems which infilled the trough from the north (Steele 1988). The culmination
and is supported by borehole and outcrop studies by Jones (1980). cessation of igneous activity in the East Midlands (Kirton 1984). During LC2, of this process was the deposition of extensive coal deposits in the Westphalian
upper delta plain, coal swamp conditions were established over most of which, on seismic data, are represented by strong, laterally continuous high-
Age. Late Brigantian to early Westphalian A. The base of the LC1 sequence is northern England, forming an internally drained basin north of the Wales- amplitude reflectors (Figs 12 and 16).
no older than the G. girtyi collinsoni Zone, suggesting a late Brigantian age. Brabant Massif (Guion & Fielding 1988). In the late Namurian (LClc), mild reactivation of extensional faults
Regional evidence indicates that the sequence ranges into the early occurred particularly in the eastern Gainsborough Trough. Here some 200 m
Westphalian A. of relief developed on the NE-SW-trending Scampton Fault (Fig. 16)
Gainsborough Trough establishing a strong footwall unconformity and thick (150 m) sandstone
Correlation. The inversion at the end of EC6 is recorded at outcrop on the deposition (Rough Rock equivalent) in the hanging-wall. Localized inversion
nearby Derbyshire carbonate platform. This inversion led to the creation of The Gainsborough Trough forms the northern of the two main depocentres features are recorded along the Askern-Spital and Nettleham Faults (Figs 11
late Brigantian intrashelf basins (Gutteridge 1987), such as the Welbeck Low that comprise the East Midlands province (Fig. 11). It comprises a major half and 16) forming the West Firsby, Glentworth, Nettleham and Welton oilfields
(Fig. 11). The Welbeck Low formed between the Anston-Manton, Ladybrook graben controlled by the Askern-Spital Fault in the north which downthrows (Fig. 11). In the West Firsby area, where a thick LC2 (Westphalian A/B)
and Eakring inversion anticlines present along its north, south and eastern the Dinantian section to the south (Figs 11 and 12). A four-fold increase in sequence is preserved, an intra-LC2 tectonic event is evidenced by folding of
margins respectively. An easterly-dipping carbonate ramp developed during thickness of the Dinantian section (EC1-EC6) is evidenced across the fault the pre-LC2b sequence and onlap of the subsequent deposits (LC2b) in the
the early part of LC1. Similar intrashelf basins are described from outcrop in (Fig. 12). The Gainsborough Trough contains a more complete Namurian and hanging-wall of the Askern-Spital Fault (Fig. 16). This may represent either an
Derbyshire (Gutteridge 1987). Elsewhere, over the majority of the East Westphalian section than the Widmerpool Gulf to the south. Within the early compressional pulse of the Variscan orogeny or a late extensional faulting
Midlands Platform, shelf carbonates continued to accumulate during late depocentre there are no borehole penetrations below the top Dinantian (EC6). phase evidenced in the Beckingham area (Fraser & Gawthorpe 1990, their fig.
Brigantian times. The overlying clastic sequences represent the progressive infill The major control on the stratigraphy is provided by the Grove-3 borehole, 8). Aitken et al. (1999) have recognized similar intra-LC2 tectonic event and
of the basin by the deposits of increasingly shallow water delta systems. which penetrated a complete, but condensed, Dinantian section sitting above major erosion at the level of the Wooley Edge Rock (mid-late Westphalian B).
?Charnian metamorphic basement on the southern flank of the half graben
(Fig. 12).
LC2 sequence Major rifting occurred in the Gainsborough Trough during EC1 (late Edale Gulf
Devonian-Courceyan) and EC3 (Arundian-early Holkerian) times. Both the
Seismic character. The LC2 sequence is thickest on the northern and southern EC1 and EC3 sequences exhibit classic wedge-shaped geometries, typical of The Edale Gulf forms the third of the major half graben within the East
margins of the Widmerpool Gulf. It is absent over much of the central part of syn-rift sedimentation. The present deep burial of the early syn-rift sediments Midlands province (Figs 11 and 17). It lies to the north of the Derbyshire
the basin due to inversion, uplift and erosion in late Carboniferous times (Fig. of late Devonian-Courceyan age in northern England generally precludes any carbonate platform and is buried beneath the Upper Carboniferous of the
12). The sequence is characterized by laterally persistent, high-amplitude valid seismic facies analysis interpretation. However, the regional seismic line Central Pennine Basin. The basin was originally thought to be part of a large
reflectors that overlie LC1 with subtle onlap. compiled for the East Midlands and Gainsborough Trough provides some Dinantian trough that extended northwards to the Askrigg Block and was
indication of the nature of the earliest syn-rift sediment package (Fig. 12). The connected to the Bowland Basin (George 1958; Ramsbottom 1969). Lee (1988)
Facies. The sequence comprises interbedded grey carbonaceous mudstones, hanging-wall of the Beckingham Fault contains a series of broadly prograda- using 3D, 2D and residual gravity modelling, and Smith et al. (1985) have
siltstones, sandstones and coal seams deposited in an upper delta plain tional downlapping reflectors interpreted as alluvial fans or fan deltas that suggested that the Edale Gulf comprises a half graben controlled by a major
depositional setting (Fig. 14) (e.g. Guion & Fielding 1988). prograded into a lake or marginal marine gulf. These probably form part of a east-west-trending fault that underlies the northern margin of the Derbyshire
series of similar coalescing footwall-sourced fans extending some 4-5 km into carbonate platform. Lee (1988) also identified a residual gravity high
Biostratigraphy. The boundary between the LC2 sequence and the underlying the basin. underlying the Holme area some 15 km to the north of this fault. The regional
LC1 sequence is marked by a pronounced 'bulge' on the sonic log, equating to The seismic data illustrate major backstepping of the southern bounding seismic profile (Fig. 17) confirms the Edale Gulf to comprise a half graben with
the Amaliae Marine Band in the Strelley, Ilkeston and Ironville-5 boreholes fault of the basin between EC1 and EC3 rifting. At the onset of EC3 rifting, the main bounding fault underlying the northern margin of the Derbyshire
(Fig. 14). fault control shifted southwards some 7 km from the Beckingham Fault to the carbonate platform. The hanging-wall dipslope rises northwards to a basement
Clarborough Fault. This is a common occurrence in evolving rift basins and high underlying the Holme area. Evans & Kirby (1999) show that the Holme
Interpretation. By the end of LC1 times, shallow-water deltaic conditions were has been particularly well demonstrated from the East African rift valley by High is fault bounded along its northern margin by the east-west-trending
widespread over the Widmerpool Gulf. The LC1/LC2 boundary reflects a Rosendahl et al. (1986). In contrast to the Widmerpool Gulf, there is very little Holme Fault.
marked change in depositional facies from lower delta plain to upper delta evidence for a well-developed EC5 (late Asbian-early Brigantian) rift phase.
plain. The continuous high-amplitude reflectors that are characteristic of LC2 The syn-rift facies shown on the geological interpretation (Fig. 12) are The Alport and Edale boreholes provide data on Dinantian sedimentation in
seismic facies are generated by the many coal seams developed throughout the largely inferred from seismic character. In the Gainsborough Trough both the Edale Gulf. These were drilled on the Alport and Edale anticlines lying
seismic sequence. footwall and hanging-wall shelf margins are well preserved along the Askern- some 12 km and 3 km north of the Derbyshire carbonate platform. The
Spital and corresponding antithetic faults respectively. The seismic data Dinantian stratigraphy of these boreholes has been described by Hudson &
Age. Early Westphalian A to late Westphalian C. The base of the LC2 suggest the development of shelf margin complexes during the Chadian (EC2), Cotton (1945a, b). More recently, the Wessenden borehole, drilled on the crest
sequence lies within the early Westphalian A. Regional evidence indicates that late Holkerian-mid Asbian (EC4) and Brigantian (EC6). Onlapping relation- of the Holme High, penetrated Chadian and Arundian shallow-water