Basal Dinantian miospores from the Cockermouth area, West Cumbria

Miospore assemblages are described from basal Dinantian argillaceous strata lying below and above the Cockermouth Lavas, and within the Seventh Limestone in the area to the north east of Cockermouth, West Cumbria. The strata enclosing the Lavas are assigned to the Schopfites claviger-Auroraspora macra miospore zone, indicating the Ivorian stage (Tournaisian, Tn3). The Lavas are thus shown to be of similar age to the Solway Plateau Lavas to the north which represent an early stage in the formation of the Solway Basin. Shales from below the Seventh Limestone are assigned to the Lycospora pusilla miospore zone and from this and other fossil evidence are presumed to be of Chadian or early Arundian age. Shales from within the Seventh Limestone are assigned to the Perotrilites tessellatus – Schulzospora campyloptera miospore zone (Asbian stage) It is concluded that the Ivorian-Asbian sequence in this area is greatly attenuated compared with its development on the northern margin of the Solway Basin.


INTRODUCTION
Miospore assemblages have been examined from the Basement Conglomerate and from strata above the Cockermouth Lavas and associated with the Seventh Limestone in the area of West Cumbria lying to the nortlh east of Cockermouth, on the northern flanks of the Lake District (Fig. 1). 'The strata (c. 30 metres) below the iavas a r e termed the E{asemient Conglomerate (Eastwood et al., 1968) and lie with marked unconformity on the Skiddaw Slates. The), are well exposed in Tommy Gill, Redmain (Grid Ref.: N Y 138335). T h e Cockermouth Lavas die out eastwards and in the Uldale and Fellside areas the Base-men1 Conglomerate lies unconformably o n Eycott Lavas and is thicker, but seldom exposed. In both areas it consists of coarse conglomerates interbedded with grits, sandstones and mudstones, ranging in colour from grey-green t o yellow and red, and sometimes with a calcareous cement.
The Cockermouth Lavas, olivine basalts o f Dalmeny type, reach up to 100 metres in thickness. They overlie the unfossiliferous Basement Conglomerate and are succeeded by about 20 metres of interbedded shales, limestones, and calcite mudstones which, together with the overlying Seventh and succeeding limestones, are assig:ned t o the Lower Chief Limestone G r o u p (Easfwooci et ul., loc. cit.). These strata crop out in Gill Beck ( N Y 149345), south of the Ghyll Yeat Inn in Blindcrake, which drains southwards into the River Derwent. Shackleton (1973), Butcher (1974) and Welsh (1980) give details ot the section between the Cockermouth L a w s and the base of the Seventh Lime-stone, and there is a brief description by Eastwood et al. (loc. cit.) who record a shelly fauna in the uppermost shales. T h e 20 metres o r so of exposed strata comprise mudstones with occasional shales and with three harder horizons of calcite mudstone containing Modiolus and ostracods, and forming small waterfalls. T h e highest calcite mudstone horizon is overlain by a metre of limestone breccia with quartz pebbles which makes a convenient marker horizon.
The earliest diagnostic fossils recorded in these Dinantian strata are the brachiopods and corals occurring in black shales immediately below the Seventh Limestone and in the limestone itself. According t o Eastwood el ul. (1968, pp. 160-161) these indicate t h e Nemutophyllum minus (S2) zone and fossils normally restricted t o the LowerDibunophyllum Zone ( D 1) occur at the top of the Seventh Limestone. George et al. (1976) Table I1 the higher part of the Cockermouth Lavas is included in the Holkerian stage.) Mitchell (1 978,p. 173) states that corals recorded by earlier workers from low horizons in the Seventh Limestone at Beckermet t o the south west and Berrier to the east of the present area a r e now identified as the late Chadianearly Arundian species Thysanophyllum pseudovermiculare ; they a r e associated with typical

MAT ERIA L STU DI ED
Six wmples yielded reasonable miospore separations when macerated by standard procedures, i.e. removal of carbonates and silicates by hydrochloric and hydrofluoric acids and oxidation of t h e organic residues with fuming nitric acid.
Samples o f fine-grained sediments in the Basement Conglomerate directly underlying the Cockermouth l>avas in l'ommy Gill, Kedmain were non-productive.

MIOSPORE ASSEMBLAGES
N~e s el al. (1972) defined eight concurrent range zones based on miospore distributions in the British concluded therefore that all of the Seventh Limestone samples in the present study (iv, v and vi) are from the TC rone, Dinantian, and five of these were recognised in the Midland Valley of Scotland and in northern England by Neveset a1 (1973). Clayton et al. (1977) modified the limits of the zones and applied them to western Europe generally. Further modifications (Clayton et al., 1978) followed from comparative studies of miospore, conodont and foraminifera1 distributions in the British Isles and the Ardennes.
Thie samples from the Basement Conglomerate (i) and the lowest part of the Lower Chief Limestone Group (ii) contain miospore assemblages dominated by Punctatisporites spp. and with significant numbers of Retusotriletes incohatus Sullivan, Crassispora trychere Nevt:s & Ioannides and Schopfites claviger Sullivan ; the presence of the last two species together with the absence oSLycospora spp. indicates that the samples are almost certainly of Schopfites claviger -Auroraspora macra (CM) zone age. Other species present include Rugospora polyptycha Neves & Ioannides, Colatisporites deco , 'us Wi I I i a m s, Grarzdispora echinata Hacquebard and A . mucra Sullivan. The higher assemblage (ii) contains a more diverse m i c r o f 1 o I a i n c 1 u d i n g Ve r r u c 0 s isp o rites n i t idus (Naumova) Playford, Convolutispora spp., Pulvinisporu quusrlabruta Higgs, P. scokcophora Neves &: Ioannides and Spelaeotrilete,s pretiosus (Playford) Neves & Belt.
The assemblage yielded by the shales above the limestone breccia in Gill Beck (iii) differs from those of samples (i;) and (ii) in the absence of most of the species listed above and in the presence of high riumbers of Proljcospora rugulosa (Butterworth & Spinner) Turnau and some Lycospora noct,uina Butterworth & Williams and 1,. pusilla (Ibrahim) Somers. It is concluded that this horizon lies in the Lycosporu pusilla (Pu) zone as intensive searching yielded no evidence of ( h s s i s p o r a uculeatu Neville, Schulzospora spp. or Perotrilites tes.sel1atu.s ( S t a p h ) Nevilllc which characterise the overlying P. tessellatus -S . campyloptera (TC) zone.
The prlesence of abundant lycospores, common densmpores, Schulzospora campylopteru (Waltz) Potonie & Kremp and Dictyotriletes plumosus Butterworth & Spinner in sample (iv) from Wood Hall, probably in the lower part of the Seventh Limestone, indicates the T C zone. !Samples (v) and (vi) from the Bryozoa Band in Park End Beck, higher in the Seventh Limestone sequence, yielded similar but less varied assemblages; Pro1.ycospora rugulosa is common in the lower of the two and Triquitrites cornptus Williams appears in the higher sample. CRayton et ul. (1977) indicate that Triquitrites spp. make their first appearance in the Raistrickia nigra -Triquitrites murginutus (N M) zone but Williams (in Neves et ul., 1973)

THE AGES OF THE MIOSPORE ASSEMBLAGES
In their survey of Dinantian strata in Britain, George et ul. ( 1 976, Table I) indicate the relationships between their stages and the miospore zones of Neves ei al. (1 972). Ramsbottom & Mitchell (1980) suggest that the lowest (Courceyan) stage should be replaced by the Belgian Hastarian and Ivorian stages, representing lower (Tn 1 b pars and Tn2) and upper (Tn3) parts of the Tournaisian series respectively; in their Table I and Clayton et al. (1978) indicate that the Pu zone extends through the Chadian, Arundian and Holkerian stages with the overlying TC zone having a common base with the Asbian stage.
It follows from the present investigations, therefore, that the C M zone samples from the Basement Conglomerate (i) and from just above the Cockermouth Lavas (ii) arc Ivorian in age. The Pu zone sample (iii) from above the limestone breccia in Gill Beck could be of any age from Chadian to Holkerian.
The presence of high numbers of Prolycospora rugulosa does not help to shorten this possible range; similar abundances are present in black shales sampled in Tarras Water, Langholm, from the highest part of the Lower Border Group (M.A.B., unpublished data) which George et al. (1976) assign to the upper part of the Courceyan stage (Ivorian stage of Ramsbottom & Mitchell, 1980), and Butterworth & Spinner (1967) record high numbers from horizons in the Chadian and Holkerian stages in addition to the Lewisburn type locality of presumed Asbian age. P. rugulosa thus has a long stratigraphic range and its local abundance must be due to unknown ecological factors.
The T C zone sample (iv), (v) and (vi), from shales within the Seventh Limestone, have miospore assemblages consistent with an Asbian age.

DISCUSSION
The miospore assemblages recorded from the Basement Conglomerate and the lower part of the Lower Chief Limestone Group suggest that these strata, and the included Cockermouth lavas, are older than the Arundian age previously suggested (George et al., 1976). They are of a similar (Ivorian) age to other basal Carboniferous strata flanking the Lake District (Holliday, Neves & Owens, 1979;Welsh, 1980 (1897) and later believed by Leeder (1974) t o represent a n early stage in the formation of the Solway Basin.
T h e Pu zone assemblage recorded from the black shales (iii) above t h e limestone breccia in Gill Beck could b e from the Chadian, Arundian o r Holkerian stages. However, the presence of the late Chadianearly Arundian coral Thysanophyllum pseudovermiculare and Arundian archaediscid foraminifera recorded at similar o r slightly higher horizons elsewhere in the area (Mitchell, 1978, p. 173) suggests that the shales overlying the breccia a r e of Chadian o r early Arundian age.
T h e TC zone (Asbian) age of miospore assemblages from black shales within t h e more massive parts of the Seventh Limestone (samples iv, v and vi) a r e presumably from higher horizons than the beds containing the Holkerian brachiopod Davidsonina carbonaria which occurs near t o its base (George et al., 1976). T h e age based o n miospores is in accord with t h e presence of D, (Asbian) fossils recorded by Eastwood et u l . (1968) at the top of the Seventh Limestone.
It follows from tlie above that the strata representing t h e Chadian, Arundian a n d Holkerian stages in this area a r e greatly reduced in thickness compared with their development to the east a n d north where extensive sandstone deltas were forming along t h e northern margin of the Solway Basin.