Foraminiferal biostratigraphy of the Woo Dale Borehole, Derbyshire and the age of the Dinantian-Basement unconformity

Detailed micropalaeontological analysis of the Woo Dale Borehole shows that Dinantian strata of Holkerian and Arundian age lie unconformably on top of the pre-Carboniferous basement beds. The Chadian and Tournaisian are missing. The chronostratigraphy here recognised is compared with that proposed by Cope (1973) and the differences in the two interpretations explained in the light of recent palaeontological and Midlands regional research.


INTRODUCTION
Only two deep boreholes, namely Eyam and Woo Dale, have penetrated the complete Dinantian :succession on the Derbyshire Dome. During the Dinantian, North Derbyshire was a shallow shelf province depositing thick richly bioclastic limestone sequences off the northern shores of St:. George's Land (Fig. 1). To the east of the area a basinal embayment known as the Edale Gulf penetrated the Midlands shelf, depositing shales, sandstones and muddy limestone bands. The area was pulsed by a series of shallow marine events depositing sediments across the submerged areas. A series of local basement highs coupled with differential subsidence resulted in local non sequences caused by non deposition or erosion. The very thick Dinantian succession at Eyam (1803. 25 m) contrasts markedly with that proven in the Woo Dale 13orehole (273.60m), 11 km to the W.S.W. illustrating, the consequences of different local conditions within an extensive shelf environment.
The Woo Dale Borehole drilled by I.C.I. in 1947-48 was situated on an anticlinal crest at the confluence of Woo Dale and Wye Dale 5 km east of Buxton (SK 4099 372,6) and was spudded into Woo Dale Limestone. The borehole was cored throughout having a diameter of 0.2 m at the top reducing to 0.04 m at the ba;se.
The surviving core is housed in the Geology Departments at Keele, Manchester and Reading Universities with the remainder in the care of Professor F. W. Cope. Much of the limestone to 243.20 m is heavily dolomitised and most of the original texture and fossils hiave been destroyed by recrystallisation. Plentiful corals., brachiopods, foraminifera and algae occur in the less severely dolomitised horizons. Below 243.20 m dolomitisation is only slight and from 266.91 m to the top of the breccia at 271.16m the limestones are less altered (Cope, 1973).
The brecciated horizons contain waterworn fragments 1973). A possible deepening of basement from Woo Dale (273.60m) to Eyam (1803.25m) corresponds to a descent in the gravity anomaly over the area (Cope, 1973). Cope interprets this as a condensed sequence, a view he supports by the presence of clay band breaks in the succession, the intense dolomitisation and the fabric of the non dolomitised limestones (Cope, 1973). He concludes that differential subsidence of the North Derbyshire basement during the Dinantian is the mechanism responsible for the thickness change.
Cope's (op.cit.) biostratigraphical analysis of the macrofauna suggests S, (Holkerian) strata to a depth of 110.20m, C2SI (Arundian-Chadian) strata to 243.20m and a probable ? C , Zone (Tournaisian) to 273.60m. This paper presents a revised interpretation of the biostratigraphy together with palaeoenvironmental interpretation to explain the thickness changes between Eyam and Woo Dale.

BIOSTRATIGRAPHY
The microfossil data presented result from examination of thin sections cut throughout the borehole. This includes a collection of slides donaited by Professor F. W. Cope for examination by the author. The macrofaunal occurrences recorded are taken from the detailed logs compiled by Professor Cope during the sinking of the borehole and from recent examination of coral sections by Mr. M. Mitchell (B.G.S., Keyworth). No palynological information is available.
135.94-273.60m : Arundian The upper greyibrown dolomitised limestones of this interval are characterised by a paucity of microfauna but some diagnostic corals including Dorlodotia briarti Sale6 (135.94m), Lithostrotion minus, L . martini? Edwards & Haime and Syringopora sp. have been found. Together these indicate an Arundian age. Lower down in this succession the less dolomitised limestones contain a rich microfaunal assemblage including Glomodiscus sp., Eoparastaffella simplex (Vdovenko, 1953) and E . restricta Postojalko & Garini, 1972, confirming an Arundian age. Associated taxa include Brunsia spirillinoides, Palaeospiroplectammina mellina (Malak hova), Glomospiranella sp. and Dainella sp. together with the algae Koninckopora inflata and Solenopora sp. The lowest diagnostic Arundian taxonGlomodiscus occurs at 258.47 m. The occurrence of Koninckopora inflata lower down at 268.98 m and ?Koninckopora at 269.9 m indicates that Chadian strata have not been penetrated at these depths. The lowest 3.7 m of grey/blue limestone and brecciated limestone in the Dinantian succession contain a sparse undiagnostic fauna. From the absence of Chadian microfauna, the lowest 3.7 m are assumed to be of Arundian age.
The overall palaeoenvironment represented is that of a shallow shelf. The general paucity of fauna may reflect periodically unfavourable conditions for a foraminifera1 habitat. The original diversity of the fauna is difficult to assess due to the destruction by dolomitisation of many taxa.
273.60-312.00m (T.D.) : Pre Carboniferous These lavas, volcanic breccias and pyroclastic rocks have been dated by KlAr techniques as Devonian or older (i.e. 383 Ma or older) by Cope (1979). No refinement of this age determination is yet available.

DISCUSSION
those given by Cope (1973).
The biostratigraphical results differ markedly from Foraminifera1 biostratigraphy of the Woo Dale borehole Iepth/ lithe-1 Lithology I r / Chronom Stratigraphy  With Holkerian foraminifera recorded down to 115.97m and Arundian corals at 135.94m, the Arundian-Holkerian boundary has been placed between these two depths (see Fig. 3).

Stratigraphy
The major difference in the two biostratigraphical schemes is below the Arundian. Cope records the C,S, zone (Arundian and Chadian) and the underlying ? Tournaisian. Strank (this paper) can find no evidence of limestones older than Arundian. One reason for this discrepancy may be the records of Thysanophyllum pseudovermiculare (McCoy) at 135.94 m and obscurely at 156.59m. The specimen at 135.94m has been reidentified by Mr. M. Mitchell as Dorlodotia briati, a species diagnostic of Arundian strata (Poty, 1975(Poty, & 1981. Dorlodotia, a fasciculate species is distinguished from the cerioid species Thysanophyllum by the presence of a columella. The taxon Thysanophyllum pseudovermiculare is thought to be characteristic of Chadian rocks. At the time of Cope's (1973) publication, these two taxa had not been distinguished and all such specimens were recorded as Thyr;anophyllum pseudovermiculare suggesting a Chadian age.
Cope's record of Tournaisian strata is based on the tentative correlation of Solenopora cf. garwoodi clasts with the Solenopora Subzone of Garwood in northern England. Solenopora is not however restricted to Tournaisian strata. The rich foraminifera1 assemblage in these beds confirms their Arundian age.

CONCLUSIONS
The brecciated limestones, lava and pyroclastics in the basal Dinantian sequence indicate deposition of Arundian limestones onto a basement high land area, of pre-Carboniferous age. There is no evidence that any pre-Arundian limestones were deposited in this area. During the early Arundian, a shallow, warm water shelf environment was established in which 137.66 m of richly bioclastic limestones were deposited. This shelf environment persisted throughout the overlying Holkerian when over 11 5.97 m of limestones accumulated. Evidence from surrounding wells and outcrop sections (Aitkenhead et al., in press) suggests that Asbian and Brigantian limestones were originally deposited in the Woo Dale area and have since been eroded away.
In the nearby Eyam district the basement was engulfed by a marine event much earlier in the Dinantian than at Woo Dale. Thick limestones accumulated throughout the Lower Carboniferous from the Courceyan to the Brigantian. Approximately 490 m of pre-Arundian sediments were deposited at Eyam before the Arundian marine incursion transgressed the Woo Dale area. At Eyam, much thicker Arundian (593.38 m) sequences were deposited compared with Woo Dale.