Sedimentology of the Section Peak Formation (Jurassic), Northern Victoria Land, Antarctica.

Abstract: Models of Gondwana reconstruction agree in linking South Africa and Antarctica in Devonian to Triassic times, with an overall slow accumulation of shallow-water and alluvial deposits on a craton. In the Transantarctic Mountains, this sedimentary context, with the Gondwanide uplift, is well represented up to the Triassic by the rocks of the Beacon Supergroup; later similar alluvial deposition is recorded in northern Victoria Land (Priestley and Campbell Valleys) by the ‘Beacon-like’ sandstones of the Section Peak Formation, which interfingers with and is capped by basalt flows of the Ferrar Group. In this study, sedimentological observations of these sandstones, previously ascribed to the upper part of the Beacon Supergroup, are described. The Section Peak Formation shows variously stratified alluvial facies associations with aeolian trough cross- bedding. Several sections have been measured and studied, leading to interpretations of the vertical evolution of the deposits, the lateral variations of the sedimentary environments and the provenance framework of clastic sediments and their palaeogeographical interpretation. A sandy braided-stream environment is envisaged, composed of channels with dunes and foreset macroforms associated with both slip-faces and lee sides with descending dunes and rare plane beds. The abundance of thin sandstone beds with aeolian sedimentary structures indicates common aeolian reworking. Thicker sandstone bodies can be interpreted as the deposits of relatively persistent fluvial channels. An aeolian environment, different from that described above, has been observed locally in the upper part of the area studied; large-scale, cross-bedded, fine-grained sandstones show foresets close to the angle of repose. The palaeogeographical evolution, from Triassic to Middle Jurassic, indicates a low-gradient slope with mean- dering channels, grading into a more complex river system with braided streams (Section Peak Formation). Sandstone petrology proves that it drained mostly high-grade granulite-facies metamorphic and plutonic rocks of the basement (Wilson Terrane), but also intermittently reworked volcanic debris. These were eroded from coeval basaltic lava flows, erupted in the Section Peak basin during the initiation of lithospheric stretching, which preceded the Gondwana break-up.