A chert microfauna from the Gramscatho Groupof the Lizard Peninsula, Comwall

J. A. G. Cooper

Department of Geology and Applied Geology, University of Natal, King George V Avenue,Durban 400}, South Africa

1. INTRODUCflON

The age of rocks of the Gramscatho Grou~ ofCornwall has been a subject of debate for some time.This is largely due to the paucity of fossil remains. Awide range of ages recorded from limestone clasts inthe Meneage Formation (middle Gramscatho) wasreported by Barnes (1983), the youngest beingGivetian. He therefore considered the MeneageFormation to be Givetian in age , continuing into theFrasnian.

Using palynological evidence , Le Gall , Le Herisse& Deunff (1985) assigned a late Devonian (Fam­menian?) age to the Meneage Formation. This wasbased on the occurrence of Frasnian fossil spores andacritarchs in pelitic sediments of the PortscathoFormation (lower Gramscatho) of the west coast ofthe Lizard peninsula. Limestone clasts in the MeneageFormation in the Roseland area containing pre­Fammenian conodonts (Hendriks, House & Rhodes,1971) have been interpreted as reworked earlierDevonian fragments (Barnes, 1983).

New micropalaeontological evidence. is presented. inthis communication for a probable middle Devomanhorizon in the Meneage Formation.

2. EAST LIZARD FOSSILS

Published records of fossil remains in the Meneagearea are few. Stubblefield (1939) noted brachiopods ofLower Emsian age from a grey-green siltstone atMudgeon and of Lower Devoni~n age in calcareousshale at Gillan Creek. These horizons were not foundduring this study nor by Lambert (1965). Lang, (1929)records plant remains identified as Dadoxylonhendriksi from Polnare Cove to which a probablemiddle Devonian age is ascribed . In addition Green(1904) records an orthocone cephalopod from NareCove which is of undeterminable age (Stubblefield ,1939) and a possible SerpuJites longissimus also ofunknown age from black shales at Fletchings Cove.Middle to upper Eifelian ages at Came aredocumented by Sadler (1973).

3. CHERT MICROFAUNA

During a visit to the east coast of the Lizard peninsu~a

in 1983, continuous black chert layers, up to 60 em In

75

thickness , were noted in Nelly's Cove, north ofPorthallow (Fig. 1). These dip moderately steeply tothe east and are thought to represent original bedding.They are interbedded with sediments composed ofnumerous irregularly shaped lithic fragments ran­domly orientated in a mudstone matrix. The chertbeds are of black , brecciated chert cemented bynumerous quartz veins.

A 1 kg sample of this chert was etched in 4~%

hydrofluoric acid (Schallreuter, 1982), and the dn.edinsoluble residue inspected. All calcareous remainswere converted to CaF. A microfauna was recoveredwhich included a number of indeterminate shellfragments , bryozoa, octactin sponge spicules, twoostracods and a single foramin ifer.

The foraminifer is unilocular and tubular with aperforate, radial, hyaline test. It is provisionallyassigned to the subfamily Nodosariinae (Ehrenberg,1838).

One of the two ostracods was identified as a leftvalve of Schmidtella sp. (Ulrich , 1892), while thesecond , known from a solitary right valve, is assignedto Sulcella (Postsulcella) sp. subgenus (Adamczak,1968). It has a smooth lateral surface as in S.(P.) testis(Adamczak, 1968) but the out~ine i~ different. Thespecimen probably represents a juvenile.

Schmidtella sp. has a stratigraphic range of

Fig. 1. Locality map of East Lizard.

76 J . A . G . COOPER

mid-Ordovician to mid-Devonian while Sulcella(Postsulcella) testis is lower Eifelian in age. The chertsample from which these were extracted is thereforenot younger than middle Devonian.

4. CONCLUSIONS

This scant microfauna suggests that the MeneageFormation may be , in part , as old as middleDevonian.

Further inspection of these chert beds may yield amore diverse fauna and aid in correlation within the

Gramscatho Group. The advantage of this microfaunais that it is not recovered from a reworked clast as theconodonts of Roseland (Hendricks et al. , 1971) andrepresents a direct method of dating rocks of theMeneage Formation in the Eastern Lizard.

ACKNOWLEDGEMENTS

Thanks are extended to Rob Orr for identification ofthe ostracods and to Dr. T . R. Mason for reading themanuscript. Material was collected while the authorwas an undergraduate at Oueen's'University, Belfast.

References

ADAMCZAK, F. 1968. Palaeocopoda and Platycopa(Ostracoda) from Middle Devonian rocks in the HolyCross Mountains, Poland. Stockholm Contr. Geol., 17,1-109.

BARNES , R. P. 1983. The stratigraphy of a sedimentarymelange and associated deposits in South Cornwall,England . Proc. Ceol. Ass. , 94,217-29.

EHRENBERG, C. G. 1838. Uber dem blossen Augeunsichtbare Kalktheirchen und Kieselth ierchen als Haupt­bestandtheile der Kreidegebirge . K. preuss. Akad. WzSs. ,3, 192-200.

GREEN, U. 1904. On the Discovery of Silurian Fossils ofLudlow Age in Cornwall . Geol. Mag.• 1, 289-90.

HENDRIKS, E. M. L. , M. R. HOUSE & F. T. RHODES.1971. Evidence bearing on the stratigraphical successionsin south Cornwall. Proc. UssherSoc. , 2, 270-5.

LAMBERT, J. L. M. 1965. A reinterpretation of thebreccias in the Meneage crush zone of the Lizardboundary, south-west England. Q. JI. geol. Soc. Lond.,121, 339-57.

LANG, W. H. 1929. On fossil wood (Dadoxylon hendriksi,n. sp.) and other plant remains from the clay slates ofsouth Cornwall . Ann. Bot., 43,663-81.

LE GALL, B., A. LE HERISSE & J. DEUNFF. 1985. Newpalynological data from the Gramscatho Group at theLizard Front (Cornwall) : palaeogeographical and geodyn­amical implications. Proc. Geol. Ass., 96,237-53.

SADLER, P. M. 1973. A proposed stratigraphical successionfor the Roseland area of S. Cornwall. Unpubl. Ph.D.thesis, Univ. Bristol.

SCHALLREUTER, R. E. L. 1982. Extraction of ostracodsfrom siliceous rocks. In (Bate, R. M., J. E. Robinson & L.M. Sheppard, Eds) Fossil and Recent Ostracods. BritishMicropalaeontology Society Series, Chichester, 169-76.

STUBBLEFIELD, C. J . 1939. Some Devonian andsupposed Ordovician fossils from South-West Cornwall .Bull. Ceol. Surv. Ct. Br., 2, 63-71.

ULRICH, E. D. 1892. The Lower Silurian ostracoda ofMinnesota. In. The Palaeontology of Minnesota, 3,Geological Survey of Minnesota. 475-1081.