Abstract

The South Georgia Island segment of the North Scoria Ridge is interpreted as having once been adjacent to Tierra del Fuego, South America. The upper Mesozoic graywackes, mudstones, and tuffs of South Georgia (Cumberland Bay and Sandebugten strata) and Tierra del Fuego (Yahgan Formation and Tekenika Beds) are the infill of a marginal basin that formed between the South American continent and an active calc-alkalic arc. The former arc site is now occupied by the Patagonian batholith; ophiolites represent the former basin sea floor.

Sediment gravity flow fabrics, sedimentary structures, and bedding styles and relatively deep-water trace fossils (Phycosiphon, Helminthopsis, Taenidium, Zoophycos, Chondrites, Scalarituba?, Lophoctenium?) indicate deposition on submarine fans. Paleocurrent and petrographic analyses indicate bilateral infilling of the basin. The only known possible sources for the Sandebugten sandstones are silicic volcanic and interbedded sedimentary rocks of the Jurassic Tobifera Formation of South America; this evidence indicates former juxtapositioning of South Georgia and Tierra del Fuego. The Sandebugten-type sandstones and the Jurassic volcanic rocks have abundant quartz and plagioclase, uncommon potassic feldspar, and essentially no pyroxene and amphibole. Lithic fragments in these sandstones have identical counterparts in the Jurassic volcanic and sedimentary rocks. The Yahgan and Cumberland Bay clastic rocks were eroded chiefly from calc-alkalic volcanic rocks positioned south of their depositional areas. The latter sandstones are made up dominantly of andesite and dacite tuff and flow fragments. Plagioclase is common; quartz, ferromagnesian minerals, and mafic volcanic fragments are uncommon. Basin closure and deformation occurred during the Late Cretaceous Andean orogeny when the rocks were metamorphosed to prehnite-pumpellyite grade. South Georgia was translated relatively eastward, probably as the result of collision of the Drake Passage spreading zone with the continent during Oligocene to Miocene time.

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