Geochronology and sedimentary provenance analysis of Upper Cretaceous strata in the Antarctic Peninsula, with a focus on the Santa Marta Formation

The James Ross Basin of the Antarctic Peninsula preserves the only known exposures of Upper Cretaceous sedimentary strata from anywhere in Antarctica. In 2024, a six-week NSF/British Antarctic Survey-supported expedition (G-074-E) to James Ross Island, in the Antarctic Peninsula, resulted in the collection of a diverse assemblage of plant and vertebrate macrofossils from the Santonian–Campanian Santa Marta Formation, along with supporting geological samples for age dating. The primary aim of this study is to conduct the first radioisotopic dating of these deposits and several other Upper Cretaceous units on James Ross, Vega, and Snow Hill islands. Additional project goals include investigating aspects of the sedimentology and sedimentary provenance of this Late Cretaceous back-arc basin. To achieve these goals, sixteen volcanic tuff and volcanoclastic sandstone samples were collected from throughout the stratigraphy for U–Pb zircon geochronology via laser ablation inductively coupled mass spectrometry (LA-ICPMS) and petrographic and geochemical analysis. In addition, 19 well-preserved invertebrate shells from the Hidden Lake and Santa Marta formations were collected for Sr-isotope stratigraphy. A small suite of tuff and detrital zircon samples, and Sr-isotope samples have already been analyzed. Preliminary results indicate alternating periods of syndepositional magmatic activity in the adjacent volcanic arc, along with periods of magmatic quiescence, and that provenance and sediment routing into the basin are primarily from erosion and transport from the arc itself, west of the basin. Several of the volcanic tuffs are surprisingly zircon poor, limiting their potential for U–Pb dating and suggesting alternating bimodal (mafic and felsic) magmatism in the arc. Initial dating results for the upper Santa Marta Formation at two key localities reveal a minor, but persistent population of syndepositional detrital zircon grains that yield similar maximum depositional ages between 75–79 Ma for the top of the unit, which closely matches our preliminary Sr-isotope stratigraphy results for the same intervals.