STRATIGRAPHY OF THE CAMBRIAN BYRD GROUP, ANTARCTICA, A TECTONICALLY-ACTIVATED CARBONATE TO CLASTIC TRANSITION

Recent field study of the Byrd Group immediately south of Byrd Glacier adds to our understanding of this tectonically important suite of Cambrian rocks in the Transantarctic Mountains. Folded and unmetamorphosed, the Byrd Group is comprised of Early Cambrian Shackleton Limestone (interpreted as a shallow-water, carbonate shelf), and Dick Formation and Douglas Conglomerate (overlying clastic units with polymict clasts including Shackleton Limestone). A ~2,000-m composite of 8 measured sections of Shackleton Limestone for the first time provides a stratigraphic reference section for much of the formation. A distinctive, “butterscotch-weathering” unit in the middle of the formation, composed of cm scale rhythmic alternations of green shale and pink limestone, provides an excellent marker horizon for structural mapping. The discovery of karsted surfaces, including terra rossa soils and breccias, at several horizons within the formation demonstrates that the carbonate shelf was repeatedly emergent. The collection of an ash-bed near the top of the Shackleton may provide the first direct isotopic measurement of the age of the formation. The contact with Dick Formation is apparently conformable, as recently reported from the Holyoke Range, 180 km to the south (Pope et al, 2000). In our area, however, the transition is marked by pillow basalts and interfingering of clastic and carbonate units, rather than a sharp carbonate to clastic change. Likewise, the transition from Dick Formation to Douglas Conglomerate appears to be interfingering. At Mt. Hamilton contacts of Douglas over Shackleton over Douglas were previously interpreted as two N-dipping thrust faults (Rees et al, 1987). Based on our mapping, we reinterpret the upper contact as Douglas unconformably overlying eroded Shackleton, and the lower contact as a SW-dipping normal fault. Recognition of previously unreported, second generation clasts of conglomerate in Douglas demonstrates that the basin was feeding on itself. Newly mapped occurrences of greenschist-grade Dick and Douglas extend the clastic basin eastward to the Ross Ice Shelf. Relationships indicate complex tectonic interactions of deposition, erosion, and redeposition during a deformational episode with both extensional and contractional components.