Depositional Environments of the Byrd Group, Byrd Glacier Area: A Cambrian Record of Sedimentation, Tectonism, and Magmatism

The geology of the Byrd Group immediately south of Byrd Glacier records a major sequence of geologic events, beginning with the development of a carbonate platform (Shackleton Limestone) during the early Atdabanian (approximately 525 Ma), followed by a transitional interval of siliciclastic deposition and volcanism (Starshot Formation) during the late Botomian (approximately 512 Ma), and ending with a coarse cover of clastic molasse deposition (Douglas Conglomerate), no younger than plutonism at 492 ±2 Ma. Thus, the Byrd Group was deposited during a span of less than 33 myr, as a sequence of passive shelf-margin sedimentation through active uplift and erosion related to the Ross Orogeny. The newly subdivided Shackleton Limestone records at least two major depositional cycles, interrupted by a significant karst event. A layer of volcanic ash overlain by a thick layer of argillite in the uppermost Shackleton Limestone records the timing of a conformable carbonate to siliciclastic transition, accompanied by basalt volcanism of the Starshot Formation. Continued clastic deposition of the Starshot Formation coarsens upwards into the Douglas Conglomerate, where the primary sources of clastic detritus are derived from the Shackleton Limestone and possibly much of the Starshot Formation itself.