New evidence of a Campanian age for the Cretaceous fossil-bearing strata of Cape Marsh, Robertson Island, Antarctica

Abstract

Cape Marsh, located on the eastern end of Robertson Island to the east of the Antarctic Peninsula, exposes an isolated outcrop of Late Cretaceous sedimentary strata. The outcrop is approximately 120 km southwest of the much better-studied exposures of similar age on and around James Ross Island (JRI); as such, its remoteness has complicated both logistical access to the site and hindered geologic correlations on a regional scale. Here we present the results of fieldwork conducted in 2016 that yielded a more diverse invertebrate fossil assemblage than had been previously recognized, in addition to new detrital zircon (U-Pb) and magnetostratigraphic data. The invertebrate fauna, particularly the ammonites and inoceramids, support a biostratigraphic correlation of the upper Cape Marsh strata to Ammonite Assemblage 7 previously established on JRI. U-Pb analysis conducted on a sandstone sample from the same strata indicates a maximum depositional age of 74.2 ± 1.1 Ma, and magnetostratigraphic interpretation of the lower strata suggest a normal magnetochron. These results are all consistent with a Campanian age for the deposition of the upper strata at Cape Marsh, and deposition during magnetochron C33N for the lower layers. However, a slight age inconsistency between the biostratigraphic correlation and the detrital zircon maximum depositional age may imply that the fossils are reworked. Regardless, these new data allow us to correlate the strata at Cape Marsh to the Santa Marta and Rabot formations (or possibly the lower part of the Snow Hill Island Formation) in the northern part of the James Ross Basin.

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