Page 30 - An Atlas of Carboniferous Basin Evolution in Northern England

Carboniferous basin development 17

emergent EC4 carbonate shelf. By the end of EC5 times shallow-water VF/NC miospore Zone boundary of Neves et al. 1972). The base of the NC
Age. Late Holkerian to mid-Asbian. The oldest age for the sequence is defined conditions were developed across the Widmerpool Gulf as sedimentation Zone (Neves et al. 1972) is marked by the appearance of a number of taxa
by the presence of the TC Zone (Neves et al. 1972) at the base of EC4. The exceeded subsidence. including Bellespores nitidius, Reticulatisporites carnosus, Convolutisporites
youngest age for the sequence is defined by the top of the G. girtyi Subzone, P. varicosus and Schopfipollenites ellipsoides. Assemblages continue to be
commutatus Zone of Metcalfe (1981). Age. Late Asbian-early Brigantian. The oldest age of EC5 is well constrained characterized by elements consistently recorded in the underlying sequence,
by the youngest age of the underlying EC4 sequence. The record of namely L. pusilla, R. nigra, R. magnificus, R. fracta, R. knoxii, S. nux,
Correlation. Similar high-angle shelf margins to those developed along the palynofloras belonging to the NM Zone (Neves et al. 1972), ME Subzone Triquitrites marginatus, T. trivalvis and species of Schukospora (Ebdon et al.
Hoton Fault are exposed nearby at Castleton in Derbyshire. In this case the (Clayton et al. 1977) is consistent with the late Asbian age assigned to the base 1990). The EC6 sequence belongs exclusively to the NC Zone (Neves et al.
Asbian grainstone shelf margin forms the northern margin of the Derbyshire of the sequence. The upper age limit is less well constrained, being 1972), the top of the sequence falling within the biozone.
Dome and supplied carbonate sediment northward into the Edale Gulf. approximately equivalent to the YF-NC miospore zone boundary of Neves
Gutteridge (1987) concluded that in the region of the present-day Derbyshire et al. (1972). Clayton et al. (1977) equates the base of the NC Zone to a Interpretation. High on the hanging-wall dipslope, EC6 is considered to
Dome a major carbonate shelf developed across earlier basement topography. position within the P2 goniatite Zone. comprise carbonate grainstones. Carbonate production was re-established
The strong parallel reflectors seen across the Widmerpool Gulf in sequence following EC5 and a carbonate ramp prograded southward into the basin from
EC4 are believed to represent interbedded hemipelagic carbonates and Correlation. Sequence EC5 ties in well with a late Asbian-early Brigantian the hanging-wall dipslope. The laterally and vertically accreting clinoforms
calciturbidites deposited basinward of the evolving carbonate shelf margins, phase of tectonic activity in the Bowland Basin (Gawthorpe 1986, 1987a). observed along the hanging-wall shelf margin suggest an evolution from a
comparable to the time-equivalent succession described from the Bowland Basaltic lavas on the Derbyshire Dome (the Lower and Upper Miller's Dale carbonate ramp to a grainstone rimmed shelf. Clinoform height suggests that
Basin (Gawthorpe 19870). Lavas) lie along strike from, and are stratigraphically equivalent to, the water depths basinward of the shelf margin were in the order of 300 m. This
volcanic centres developed along the hanging-wall margin of the Widmerpool interpretation is further supported by conodont faunas recorded from the
Gulf (Macdonald et al. 1984). This volcanism is interpreted to be associated basinal sediments. The margins have not been penetrated by wells. Volcanic
with reactivation of extensional faults. Gutteridge (1987) records two periods rocks, of EC5 and EC6 age, extend out into the basin as lava flows. They are
EC5 sequence of emergence and karstic erosion on the Derbyshire carbonate platform in the believed to have generated shallow water depths around the basin margins and
late Asbian and early Brigantian. These are believed to correspond to two enhanced the development of the grainstone margin (Fig. 12). The volcanics
Seismic character. The EC5 sequence, like the EC1 and EC3 sequences, diverges pulses of extension in the area during EC5 times, leading to footwall uplift and are represented seismically by very high-amplitude, low-frequency, laterally
and thickens into the Hoton Fault and thins northwards onto the hanging-wall erosion. Horizons of debris flows, carbonate breccias and sedimentary slides continuous reflectors.
dipslope (Fig. 12). The base of the sequence shows progressive onlap onto EC4. occur in the Bowland Basin associated with this phase of extension, as with
Internally the sequence consists of low amplitude, high-frequency, laterally earlier phases, e.g. EC3 (Gawthorpe 19870). During EC5 times fine terrigenous Age. Early to mid-Brigantian. The top and base of the sequence are well
continuous parallel events that progressively onlap the hanging-wall dipslope elastics related to the advance of major delta systems from the north reached constrained biostratigraphically. The oldest age for the base of EC6 is
(Fig. 12). the Bowland Basin. constrained by the base of the NC Zone (Neves et al. 1972) which Clayton et al.
(1977) correlated to the middle of the P2 goniatite Zone (Bisat 1928). The age
Fades. In the basinal setting borehole evidence indicates that EC5 consists of of the top of EC6 is constrained by the top of the G. bilineatus conodont Zone,
calcareous mudstones and thin dolomitic limestones. In contrast, shallow- placed by Metcalfe (1981) within the P2b goniatite Subzone.
marine shelf limestones accumulated on the East Midlands Platform and EC6 sequence
Hathern Shelf (Strank 1987). Volcanic centres along the Cinderhill Fault were Correlation. The carbonate grainstone shelf margin interpreted along the
active during EC5 times (e.g. Strelley-1 borehole, Fig. 14). Seismic character. EC6 is thickest along the Cinderhill Fault with a carbonate hanging-wall dipslope in the subsurface of the Widmerpool Gulf lies along
shelf margin developed basinward of the EC5 volcanic centres (Figs 12 and 14). strike from the Wirksworth carbonate grainstone margin in Derbyshire
Bio stratigraphy. Conodont faunas are impoverished towards the base of the The base of the sequence exhibits downlap onto EC5 along the hanging-wall (Walkden 1982). The Wirksworth grainstone shelf margin lies above a volcanic
sequence where assemblages are indeterminate and become progressively richer dipslope, becoming sub-parallel to EC5 in the basin centre. The top of the horizon, the Lower Matlock Lava, which is believed to be of early Brigantian
and more diverse at the top of the sequence. In boreholes, the top of EC5 sequence is marked by progressive onlap of the overlying sequence LC1 (Fig. age (Walkden 1982). The grainstone margin-volcanic rock association on the
occurs within the G. bilineatus Zone (Metcalfe 1981). Assemblages are 12). Along the hanging-wall dipslope, EC6 comprises a series of complex northern margin of the Widmerpool Gulf adds further credibility to the
characterized by G. bilineatus, G. homopunctatus, P. commutatus and G. girtyi sigmoidal and oblique, high-amplitude, low-frequency clinoforms (Fig. 12). interpretation made from the seismic data. Mounded features mapped on the
(Ebdon et al. 1990). Mestognathids, including Mestognathus beckmanni, These change character along the hanging-wall dipslope to hummocky Hathern Shelf are interpreted to be analogous to the carbonate buildups of the
characteristically occur at the top of the sequence and indicate a shallowing clinoforms on parallel seismic lines, and pass southwards into high-amplitude, Coalhills complex in Derbyshire (Walkden 1982). In contrast, the Bowland
of the system with time. Towards the base of the sequence the indeterminate high-frequency parallel events in the basin (Fig. 12). In addition, mounded Basin at this time was dominated by deep water pro-delta mudstones and
faunas include fragmented conodonts, foraminiferal casts, isolated brachiopod features up to 0.5 km across have been identified on the Hathern Shelf. basinal turbidites (Bowland Shale and Pendleside Sandstone) (e.g. Collinson
spines and goniatite spat. 1988).
Palynofloral assemblages from the sequence can be assigned to the VF zone Fades. In the basin EC6 comprises a monotonous series of thinly bedded, dark
of Neves et al. (1972). Assemblages typically contain species of Lycospora and grey, calcareous mudstones and brown, dolomitic, muddy limestones. Volcanic
Densosporites which are common, in association with Schulzospora species, centres along the Cinderhill Fault continued to be active during EC6 times as LC1 sequence
Spelaeotriletes arenaceous, Raistrickia nigra and Remysporites magnificus evidenced by the basalts and tuffs penetrated in the Strelley-1 borehole (Figs 12
(Ebdon et al. 1990). and 14). Seismic character. The LC1 sequence is thickest within the basin just north of
the Hoton Fault and thins northward and southward onto the hanging-wall
Interpretation. The pronounced thickening of the sequence into the major Biostratigraphy. The top of the sequence is well constrained by the top of the G. and footwall margins respectively (Fig. 12). The lower part of the LC1
basin-bounding fault (the Hoton Fault) indicates that EC5 was deposited bilineatus conodont Zone of Metcalfe (1981). Conodont assemblages are rich sequence internally comprises high-amplitude, high frequency, continuous
during a phase of active rifting. This was accompanied by footwall rotation, and diverse throughout and resemble those present in the underlying sequence. parallel reflectors which display subtle downlap onto EC6 within the basin, and
which generated both uplift and erosion. Contemporaneous volcanic rocks G. bilineatus, G. homopunctatus, G. girtyi and P. commutatus continue to typify onlap onto the EC6 carbonate platform margins.
were extruded from centres of igneous activity aligned along the Cinderhill the microfauna, in association with Neoprioniodus scitulus, Neoprioniodus
Fault. Microfaunal evidence indicates that the sediment during the early stages singular is, Hibbardella milleri and Ozarkodina delicatula (Ebdon et al. 1990). Fades. The lower part of the sequence comprises interbedded dark grey,
of EC5 was derived from a mixed source, suggesting a contribution from the The EC5/EC6 seismic sequence boundary is approximately coincident with the pyritic, carbonaceous mudstones and thin turbiditic sandstones. Thin