WAVE-MODIFIED TURBIDITES: RECORD OF COMBINED FLOWS IN CAMBRIAN DEPOSITS OF STARSHOT FORMATION, ANTARCTICA

Sandstone tempestite beds in the Starshot Formation, central Transantarctic Mountains were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that they were deposited by wave-modified turbidity currents. Such currents are combined flows in which storm-generated waves overprint flows driven by excess-weight forces (density-induced flow). The interpretation of the tempestites as wave-dominated turbidites rests on a number of criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes and thick divisions of climbing ripple lamination. The ripple lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those experimentally produced in current-dominated combined-flows in clear water. Finally, paleocurrent data also support a turbidity-current component of flow. Asymmetric folds in abundant convolute bedding reflect gravity-driven failure and hence their orientations provide a down-slope direction at the time of deposition. The vergence direction of these folds parallels paleocurrent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, thus indicating that the flows were in fact driven down slope by gravity. The wave component of flow is indicated by the presence of small- to large-scale hummocky cross-stratification and small 2d ripples. Wave-modified turbidity currents form in modern environments by a combination of storm waves and downwelling flows or as a result of oceanic floods in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. The Starshot tempestites may have formed from hyperpycnal flows since this syntectonic unit is a more distal time-equivalent of coarse fan-delta deposits of the Douglas Formation.