Pergamon Journal of South American Earth Sciences, Vol. 7, No. 1, pp. 35-44, 1994 Elsevier Science Ltd & Earth Sciences & Rests Institute Printed in Great Britain. All rights reserved 0895-9811/94 $6.00 + 0.00 0895-9811(94)E0004-K Angular unconformity between the L6pez de Bertodano and La Meseta Formations (Campanian-Maastrichtian and Eocene), Cockburn Island, northern Antarctic Peninsula R.P. BARNES 1 and J.B. RIDING2 IBfitish C~ologiealSurvey,MurehisonHouse,WestMains Road, EdinburghEH9 3LA.UK 2British GeologicalSurvey, Keyworth, NottinghamNG12 5GG, UK (Manuscript received February 1993; Revisions accepted December 1993) Abztraet--A thick, largely shallow marine succession of Cretaceous and Lower Tertiary strata, deposited in the James Ross Basin, is well exposed on an archipelago of islands at the northern end oftbe Antarctic Peninsula. This succession was thought to be relatively continuous, partieuharly on Seymour Island where a thick Upper Cretaceous succession passes up into Paleocene and Eocene strata. Several onconformities present in the succession were thought to be relatively minor and have been ascribed to sea level changes. Equivalent sedimentary deposits of the L6pez de Bertodano and La Meseta formations, Campanian/Maastrichtian and Lower Eocene respectively, are exposed on Coekburn Island, less than 10 km from Seymour Island. The contact between the two units is an unconformity with an angular discordance varying from 20* to 55". Rotation of the L6pez de Bertodano Formation corresponds with syn-sedimentary disruption elsewhere in the basin throughout the Late Cretaceous and points to persistent iosta- bility. The ages of the strata on either side of the onconformity have been dated on the palynologieal content of five samples and correlation with the Seymour Island succession suggests that at least 900 m of Cretaceous (650 m) and Lower Tertiary (250 m) strata are missing from, or were not deposited at, Cockbum Island. Reworked Maastrichtian palynomorphs in a breccia bed within the La Mnseta Formation confirm that Upper Cretaceous strata were eroded although there is no evidence for reworking of Pale- ocene sediments. We conclude that Cockburn Island includes a continuation of the Eocene unconformity of Seymour Island, extending the trend for the base of the La Meseta Formation to onlap onto progressively older strata. This unconformity may mark a major phase of basin inversion, with tilting and uplift possibly affecting much of the James Ross Basin at this time. Reaumen--La grnesa sucesi6n C.retlcico y Tereiario inferior, dominantemente marino poco profundo de la cuenea de James Ross eata bien expneata on un archipielago de ishas en el extremo norte de la peninzula Antartiea, particularmente en la isla Seymour donde la gruesa sucesidn de el CretAcico superior pasa a los ostratos de el Paleoceno y Eoceno. Las varias ineonformidades pre- sentes en la sucesi6n son concideradas menores y atribuidas a camhius de el nivel del mar. Los dep6sitos equivalentes de las For- maciones L61~z de Bertadano y La Meseta, Campaniano/Maaatrichtiano y Eocene inferior respeetivamente estan expuestas en la isla Coekburn a menos de 10 km de de la isla Seymour. El eontacto entre las dos unidades es una ineonformidad angular entre 20 ° - 55 °. Per rotagi6n de la Formagi6n L6pez de Bertodano corresponde con una discontinuidad sin-sedimentaria de finales de el Cret~ico a trav6s de la cnenea. Dataeiones palinol6gicas de los estratos de cada hado de la inconformidad y correlaei6n con ha sucesidu de la ~ Seymour sugiere qne al menns 900 m, 650m de Cretheico y 250 m de Terciario estan desaparecidos o fuer6n no depositaries eta la isla Cockburn. Aunqne no hay evideneia de retrabajamiento de sedimentos Paleocenos, la presencia de palino- morfos de edad maaatrichtiana retrabajados en una breeha dentro de la Formaci6n La Meseta confirma que estratos de el Cret(wico superior fner6n erodadns. Coneluimns queen la iala Conekburn hay una cot3tinuidad de la inconformidad Eocena de la isla Seymour, extondiendo el tren a la base de ha Formaci6n La Meseta transgrediendo estratos progresivamente m~ antfguos. Eata inconfonnided puede marear ona fase mayor de inversi6n, con inclinaei6n y levantamiento afectando pnsiblemente la mayor parle de ha euenca de James Ross durante este tiempo. INTRODUCTION CO KBURN ISLAND is one of a group of islands off the northern tip of the Antarctic Peninsula (Fig. 1) where sedi- mentary outcrop is protected by a cover of Pliocene-Pleis- tocene volcanic rocks. These islands expose the Cretaceous to Eocene sedimentary fill of the James Ross Basin (del Valle et al., 1992). the northern sub-basin of the mo~e extensive Larsen Basin of Macdonald et aL (1988). The majority of the outcrop comprises basin marginal And shallow marine shelf Cretaceous sedimentary deposits with a gentle regional dip towards the southeast. At the base of the exposed sequence, the largely Lower Creta- ceous, coarse-grained, Gustav Group only crops out in the northwest of James Ross Island (Ineson et al., 1986). Above this, the richly fossiliferous, mainly fine-grained, Upper Cretaceous Santa Marta and L6pez de Bertodano formations (Marambio Group) crop out on James Ross, Vega, Humps, Cockbum, Seymour and Snow Hill islands with varying stratigraphic levels being exposed as a result of faulting (Crame eta/., 1991). The youngest Cretaceous sediments are preserved in two relatively complete sec- tiros, ou Vega Island (Cape Lamb, Pirrie et al., 1991) and on Seymour Island where the succession extends into the Address all correspondence and reprintrequeststo R.E Barnes, Telephone+ 44 031 667 1000; Telefax:+ 44 031 667 2683; Telex: 727343 SEISED G. 35