Small-scale hummocky cross-stratification in turbidites: a form of antidune stratification?

Small-scale hummocky cross-stratification occurs in Upper Cretaceous calciclastic turbidites exposed in the western Basque Pyrenees; facies associations and microfossil assemblages indicate slope to base-of-slope (bathyal) depositional environments. It is developed in the fine-grained portion of beds and displays spacings mostly between 0.2 and 0.7 m. The beds fine upward with no sharp grain size breaks or mud partings, suggesting that deposition occurred during a single flow event. Hummocky intervals are 0.1–0.8 m thick and consistently grade laterally and vertically into flat, planar laminations of Bouma B divisions suggesting that deposition occurred under upper-flow-regime conditions. They have wave-like geometries with laminae continuous across ‘crests’ and ‘troughs’ and display a ratio of ‘wavelength’ to estimated underflow thickness of 11.3–12.8. Combining the above observations and inferences, these examples of small-scale hummocky cross-stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner, denser underflow (main body/tail of the turbidity current) and an overlying thicker, low-density layer. This occurrence is further evidence that small-scale hummocky cross-stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment.     

Hydraulics of sediment-laden sheetflow and the influence of simulated rainfall

Laminar sheetflows, transporting sediment at their capacity rates, both with and without rainfall disturbance, were investigated. Values of flow depth and relative submergence were very small. In the flows without rainfall, measured velocities exceeded the predictions of the smooth-surface, clear-water laminar model by an average of 12 per cent. Reduced flow resistance due to high sediment concentrations may explain this result. Velocities in the rainfall-disturbed flows were not significantly different from the predictions of the smooth-surface, clear-water model, and the velocity reduction due to rainfall was about 12 per cent. Although the uniformity of rainfall intensity under the single-nozzle rainfall simulator is high, variation of momentum and kinetic energy fluxes along the 1-5 m long flume was significant. The rainfall angle of incidence was highly correlated with deviations from expected flow velocities in the upper and lower sections of the flume.     

Vibration of linear structures due to jump-discontinuous,non-interrupted,stochastic processes

The solution of the problem of vibration of linearly elastic structures, caused by stochastic excitation of non-interrupted, jump-discontinuous character, is considered in the paper. Two types of excitation are examined. They form an enhancement of the continuous model which is most often used. The solution in the form of expected values, correlation functions and spectral densities of the deflection of the structure is formulated. Examples illustrating the solution are presented.