Sedimentary structure of inferred cyclic-step bedforms in submarine volcaniclastic slope deposits, Cuatro Calas, south-east Spain

Abstract

Cyclic steps are widespread on submarine slopes of many modern insular volcanoes. This paper provides the first detailed description and interpretation of the sedimentary structures and depositional architecture of cyclic-step deposits of such bedforms formed on the submarine slope of an ancient volcano. The partially depositional cyclic steps are preserved in a 67 m thick coset of 1 to 12 m thick cobble-based units of middle Miocene submarine volcaniclastics, exposed along a cliff outcrop in south-east Spain. The main structure in the units is unidirectional crude low-angle cross-bedding passing upward to centimetre to decimetre-scale diffuse stratification more or less parallel to the unit bounding surfaces. The depositional architecture produced by inferred sinuous to straight-crested cyclic steps is compared with deposits of crescent-shaped cyclic steps formed in confined settings. With a novel method, a maximum cyclic step height and length of 22 m and 460 m, respectively, have been calculated. The architecture of some of the thicker cyclic-step units is complicated by structures that were formed as cyclic-step trough-fills, by superimposed cyclic steps or downstream migrating antidunes. These structures possibly reflect adaptation processes of the bedform morphology to a lower strength of the hydraulic jumps and related density flows. In the upper, less well-exposed part of the succession more steeply inclined gravel backsets that probably represent deposits of crescent-shaped cyclic steps accreted in a more energetic, confined setting proximal to the coastal source of the density flows. A facies model of straight-crested cyclic steps is presented that may aid in the identification of similar bedforms in submarine volcaniclastic environments and comparable non-volcanic settings.