BIOSTRATIGRAPHIC AND DETRITAL ZIRCON GEOCHRONOLOGICAL AGE ASSIGNMENT OF LATE CRETACEOUS SEDIMENTARY EXPOSURES ON ROBERTSON ISLAND, ANTARCTICA

Cape Marsh is a seasonally ice free region hosting Late Cretaceous sedimentary strata on the southeastern end of Robertson Island. The exposures at Cape Marsh are generally considered to be part of the much better studied units in the James Ross Basin ~100 km to the north. Due to its isolation from other similar exposures, and its more general inaccessibility, the locality has been rarely examined. The most recent studies (Medina and del Valle, 1980; del Valle and Medina, 1985) are over 30 years old, but have provided a thorough sedimentary and locale-specific stratigraphic description at Cape Marsh. Here we present results from two days of fieldwork conducted in 2016 during which both detrital zircon and paleomagnetic samples were collected. In addition, several bivalve and cephalopod genera new to Robertson Island, but described from elsewhere in the James Ross Basin, were recovered and allow for improved biostratigraphic correlations. Previous Campanian age assignments are confirmed for the unit, but here we further constrain the age to ~74 Ma (late Campanian), and correlate the upper part of the Cape Marsh exposures to ammonite assemblage Zone 7 of Olivero (2012). Specifically, fragments of very large inoceramid clams (Antarcticeramus (?) rabotensis (?)) and moderately well-preserved kossmaticeratid ammonites (including Neokossmaticeras (?) redondensis (?)) provide strict ties to the James Ross Basin. Taken together, new data from Cape Marsh provide links to the Santa Marta and Rabot formations exposed in the northern part of the James Ross Basin, although the rocks and fossils at Cape Marsh are much more altered. The discovery of many new fossil groups may reflect increasing exposure of new strata due to melting of permanent ice cover during the last half century.