ANGULAR-UNCONFORMITY BETWEEN THE LOPEZ-DE-BERTODANO AND LA-MESETA FORMATIONS (CAMPANIAN-MAASTRICHTIAN AND EOCENE), COCKBURN ISLAND, NORTHERN ANTARCTIC PENINSULA

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 of the Antarctic Peninsula. This succession was thought to be relatively continuous, particularly on Seymour Island where a thick Upper Cretaceous succession passes up into Paleocene and Eocene strata. Several unconformities present in the succession were thought to be relatively minor and have been ascribed to sea level changes. Equivalent sedimentary deposits of the Lopez de Bertodano and La Meseta formations, Campanian/Maastrichtian and Lower Eocene respectively, are exposed on Cockburn Island, less than 10 km from Seymour Island. The contact between the two units is an unconformity with an angular discordance varying from 20-degrees to 55-degrees. Rotation of the Lopez de Bertodano Formation corresponds with syn-sedimentary disruption elsewhere in the basin throughout the Late Cretaceous and points to persistent instability. The ages of the strata on either side of the unconformity have been dated on the palynological 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, Cockburn Island. Reworked Maastrichtian palynomorphs in a breccia bed within the La Meseta Formation confirm that Upper Cretaceous strata were eroded although there is no evidence for reworking of Paleocene 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.