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.