The Seal Islands broken formation: an enigmatic Cenozoic volcanoclastic forearc succession, South Shetland Islands, Antarctica

ABSTRACT The Seal Islands, at the northwestern edge of the South Shetland microplate, preserve a poorly constrained Cenozoic sedimentary record within an active forearc setting influenced by subduction, strike-slip tectonics and glacial processes. This study integrates sedimentological, petrographic and whole-rock geochemical data to constrain the depositional environment, provenance and tectonic setting of these successions. Two units were identified, one carbonate and one siliciclastic. Within the latter, two distinct sandstone petrofacies were identified: quartz–feldspar-rich (QFR) and plagioclase–volcanic lithic-rich (PVR). Modal and geochemical data indicate that QFR sandstones were derived from felsic plutonic and recycled sources, whereas PVR sandstones reflect erosion of intermediate volcanic arc rocks. Weathering indices suggest moderate to low chemical weathering under semiarid conditions. The siliciclastic succession records turbiditic and debris-flow deposition within a submarine fan system, with features consistent with glacial influence, while the carbonate succession reflects shallow marine platform conditions during a highstand phase. Biostratigraphic data constrain deposition to the Miocene–Pliocene. These results indicate sediment supply from a composite arc–basement system and deposition in a forearc basin affected by arc exhumation, strike-slip tectonics and glacio-eustatic fluctuations, providing new constraints on Cenozoic forearc evolution in the South Shetland Islands.