EFFECTS OF HYDRAULIC JUMPS ON HIGH-DENSITY TURBIDITY CURRENTS AND THEIR DEPOSITS

Enigmatic strata were observed intercalated with otherwise ‘classic' turbidites in the Isaac Formation of the Neoproterozoic Windermere Supergroup. To understand these medium-bedded, coarse-tail graded, structureless sandstone deposits, experiments were conducted to replicate the conditions during deposition. A first series of experiments with sustained currents with initial volume-fraction sediment concentration of 0.20 to 0.35 produced parallel lamination, even under aggradation rates up to 4mm/s. A second series of experiments with similar currents but a hydraulic-jump design produced deposits similar to the enigmatic strata in the field. Turbulence generated in the hydraulic jump entrained coarse- and fine-grained sediment from the bed and maintained them in temporary suspension. Here we present results from a detailed comparative analysis of sediment concentration profiles between runs with and without a hydraulic jump. The major effects of a hydraulic jump on vertical profiles in turbidity currents are: (1) an almost-constant volume-fraction sediment concentration, (2) a smaller decrease in median grain size, and (3) smaller reduction in the concentration of different grain sizes (i.e. less well sorted). These effects are manifest in the deposited sediment, and accordingly, should be easily recognized in outcrop and core.