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.

中文翻译：

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西班牙东南部库阿特卡拉斯海底火山碎屑岩斜坡沉积中推断的循环阶跃床形的沉积结构

在许多现代岛状火山的海底斜坡上，循环台阶很普遍。本文对古代火山海底斜坡上形成的这种岩床的周期性阶梯沉积物的沉积结构和沉积结构进行了首次详细描述和解释。部分沉积的循环步骤保存在西班牙东南部沿悬崖露头裸露的中新世海底火山碎屑岩中67 m厚的伴生岩群中，该伴生群为1至12 m厚的中卵石基单元。单元的主要结构是单向粗低角度交叉层，该层层向上或越过厘米达到十亿度尺度的扩散分层，或多或少地平行于单元边界表面。将推断出的蜿蜒至直顶的周期性台阶所产生的沉积构造与在有限的环境中形成的月牙形周期性台阶的沉积物进行了比较。使用一种新颖的方法，已经计算出最大循环步长分别为22 m和460 m。一些较厚的循环步骤单元的体系结构由于通过叠加循环步骤或下游迁移反沙丘而形成为循环步骤槽式填充物的结构而变得复杂。这些结构可能反映了床形形态对水力跃迁和相关密度流较低强度的适应过程。在上层中，暴露程度较低的部分，砾石后倾陡峭，可能代表了新月形循环台阶的沉积物，这些台阶以更有活力的方式积聚，靠近沿海密度源的密闭环境。提出了直顶循环阶跃的相模型，该模型可以帮助识别海底火山碎屑环境和可比较的非火山环境中的相似地层。