Page 25 - An Atlas of Carboniferous Basin Evolution in Northern England

16 Chapter 3

Interpretation. The high-amplitude, laterally continuous reflectors in the basin Age. Late Chadian-late Holkerian. The age of the lowermost part of EC3 is
are believed to represent carbonates. During EC2, aggradational to prograda- based on analogy with sections in the Rowland Basin (e.g. Gawthorpe 1987a).
tional carbonate ramps and rimmed shelves developed. This interpretation is The age of the upper part of the sequence is defined by the identification of the
based on the thickening of EC2 across the hanging-wall dipslope and the TC Zone of Neves et al. (1972). The base of the TC Zone is in the late
tentative identification of hummocky downlapping clinoforms. The lack of Holkerian (Ebdon et al. 1990).
thickening and reflector divergence into the Hoton and Thringstone-Sileby
Faults suggests that EC2 was a phase of tectonic quiescence. Correlation. In the Bowland Basin there was a period of enhanced tectonic
activity during the late Chadian-early Arundian (Gawthorpe 1987a). At
Age. Chadian, possibly as old as late Courceyan, based on correlation with outcrop this is marked by horizons of debris flow, carbonate breccias and
outcrop and well data. sedimentary slides which unconformably overlie calciturbidites of the
preceding EC2 sequence. This is believed to correlate approximately with the
Correlation. EC2 has been penetrated by Hathern-1 on the footwall of the onset of the EC3 rifting in the Widmerpool Gulf. On the East Midlands
Hoton Fault. Here the sequence is developed as shelf limestones (Llewellyn & Platform a widespread Arundian shale horizon marks the cessation of
Stabbins 1968, 1970), which are dated as no older than the CF4 Zone of Conil carbonate growth within the platform and is believed to represent the
et al. (1979). On the adjacent East Midlands Platform, EC2 comprises shelf maximum extent of the EC3 transgression.
limestones and is the oldest of the Dinantian sequences preserved. Comparison
with Strank (1987) suggests that EC2 is no older than late Courceyan in the
shelf setting. At Dovedale, on the Derbyshire carbonate platform along strike EC4 sequence
from the Widmerpool Gulf, elongate Waulsortian buildups are reported by
Bridges & Chapman (1988). These buildups developed on a ramp to the SW of Seismic character. The sequence is thickest at the margins of the basin, along
an evolving carbonate platform during Chadian times. the Hoton and Cinderhill fault systems, and thins into the basin centre (Fig.
12). The base of the sequence shows downlap onto the underlying EC3
sequence along the hanging-wall dipslope, becoming subparallel with EC3
towards the basin centre. The top of the sequence is marked by progressive
ECS sequence onlap of the overlying sequence (Fig. 12). Internally, in the basin, the sequence
comprises high-amplitude, low-frequency, laterally continuous reflectors.
Seismic character. The EC3 sequence thickens significantly into the Hoton Along the Hoton Fault EC4 comprises high-amplitude, low-frequency, subtle
Fault and thins progressively northwards onto the hanging-wall dipslope. In high-angle reflectors that represent vertically and laterally accreting clinoforms
footwall locations, major truncation of older sequences is observed (Fig. 12). that prograde northwards into the basin (Fig. 12). Along the hanging-wall
At the base of the sequence there is progressive onlap onto the underlying EC2 dipslope the sequence is largely characterless, but in places high-amplitude,
sequence. The upper boundary of the sequence is marked by apparent low-frequency, hummocky clinoforms are identified, prograding southwards
truncation of reflectors (Fig. 12). Internally the sequence consists of low- into the basin and downlapping onto EC3.
amplitude, high-frequency, laterally discontinuous events which progressively
onlap onto the hanging-wall dipslope (Fig. 12). Fades. In the basin the high-amplitude reflectors are interpreted to represent
an increase in high-velocity carbonates within an otherwise monotonous
Fades. Where penetrated in the basin, the sequence comprises a monotonous sequence of mudstones. The clinoforms identified along the basin margins are
series of variably calcareous, carbonaceous, pyritic, dark grey mudstones thinly believed to represent the progradation and aggradation of carbonate
interbedded with grey, dolomitic limestones (e.g. Ratcliffe-on-Soar-1, Fig. 14). platforms. Boreholes penetrating the area of topset reflectors up-dip of the
The seismic character of EC3 exhibits no lateral variation, reflecting the clinoforms (e.g. Ironville-5, Figs. 12 and 14) prove the presence of shallow-
monotonous character of the facies across the Widmerpool Gulf. On the water marine limestones interpreted as shelfal carbonates.
adjacent East Midlands Platform the sequence is represented by shelf
limestones. Biostratigraphy. The age of the sequence is constrained biostratigraphically
through borehole data tied to seismic sections. Diverse conodont faunas
Biostratigraphy. Only the top part of the sequence has been penetrated in the include Paragnathodus commutatus, Gnathodus homopunctatus, Gnathodus
basin. Miospore assemblages recovered from the sequence include abundant girtyi girtyi, Cavusgnathus unicornis and Apatognathus sp. in association with
Lycospora pusilla in association with Schulzospora campyloptera, Stenozono- bivalve spat, ostracodes and brachiopod spines (Ebdon et al. 1990). The
triletes coronatus, V. baccatus, C. aculeata, P. tesselatus and Waltzispora sequence equates to the G. girtyi Subzone of the P. commutatus Zone (Metcalfe
planiangulata (Ebdon et al. 1990). The miospore assemblages recovered from 1981). The upper boundary of the sequence coincides with the boundary
the upper part of the interval are broadly equivalent to the TC Zone of Neves between the G. bilineatus and P. commutatus zones (Metcalfe 1981).
Fig. 13. Dinantian-early Namurian seismo-stratigraphy of the Widmerpool Gulf after
Ebdon et al. 1990). Gt ps is the Gnathodus pseudosemiglabar Subzone. Lithology et al. (1972). Although conodont analysis has been undertaken, no Palynofloras are broadly equivalent to the NM Zone and the upper part of
symbols as in Figure 9. biostratigraphically significant assemblages have been recovered. the TC Zone (Neves et al. 1972).

Interpretation. The geometry of the sequence, particularly the significant Interpretation. Sequence EC4 is interpreted as having developed during a phase
Fades and bio stratigraphy. The EC2 sequence has not been penetrated by thickening into the Hoton Fault, indicates that deposition of EC3 was initiated of limited tectonic activity during which carbonate deposition was re-
boreholes in the Widmerpool Gulf. The upper part of the sequence was by rejuvenation of the major basin-bounding fault. This phase of extension was established along the basin margins and low rates of subsidence prevailed.
penetrated on the southern margin of the gulf in the Strelley-1 borehole (Fig. associated with rotation of the fault blocks and significant footwall erosion. This enabled the carbonate platforms to become established and prograde both
14). Here the interval is represented by a series of shelf margin, carbonate Carbonate shelf margins which developed during EC2 on the footwall blocks southward and northward from the hanging-wall and footwall margins
grainstones. were eroded, those located on hanging-wall dipslopes were drowned. respectively.