The fringe of fine‐grained deep‐marine systems often exhibits complex sedimentary facies and facies associations, because the presence of clay promotes the development of transient turbulent flows with complex depositional properties. Relatively little is known about the variation of current‐induced sedimentary structures found within these facies. This study provides the first comprehensive description and interpretation of mixed sandstone–mudstone bedforms observed in the fringe of the mud‐rich submarine fan that makes up the Aberystwyth Grits Group and Borth Mudstone Formation (Wales, United Kingdom). Using textural and structural descriptions, 158 bedforms in sediment gravity flow deposits were characterized into three main types: ‘classic’ sandy current ripples, large current ripples and low‐amplitude bed‐waves. The sandy current ripples comprise clean sandstone, with average heights and lengths of 11 mm and 141 mm, respectively. The large current ripples are composed of mixed sandstone–mudstone and possess greater dimensions than the sandy current ripples, with an average height of 19 mm and an average length of 274 mm. The low‐amplitude bed‐waves are long thin bedforms composed commonly of mixed sandstone–mudstone, with an average height and length of 10 mm and 354 mm, respectively. The large current ripples and low‐amplitude bed‐waves are strikingly similar to experimental bedforms produced under decelerating mixed sand–mud flows and are interpreted to form beneath transitional flows with enhanced and attenuated near‐bed turbulence, respectively. From the fringe to the distal fringe of the fan, the dominant bedform type changed from sandy current ripples, via large current ripples, to low‐amplitude bed‐waves, suggesting that the flows changed from turbulent to increasingly turbulence‐modulated. It is proposed that the flow Reynolds number reduced, reflecting this flow transformation, from a combination of constant or decreasing flow height, flow deceleration from sediment deposition, and increasing flow viscosity due to the shear‐thinning nature of clay‐rich suspensions. Large current ripples and low‐amplitude bed‐waves are likely to be common in the fringe of other submarine fans. The presence and spatial trends in mixed sand–mud bedform types may be an important tool in interpreting fan fringe environments.

中文翻译：

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海底扇边缘瞬态湍流产生的混合砂泥床形态：流动转变的指标

细粒深海系统的边缘通常表现出复杂的沉积相和相组合，因为粘土的存在促进了具有复杂沉积特性的瞬态湍流的发展。关于在这些相中发现的电流诱发的沉积结构的变化知之甚少。本研究首次全面描述和解释了在构成阿伯里斯特威斯砂砾岩群和博斯泥岩组（英国威尔士）的富含泥浆海底扇边缘观察到的砂岩-泥岩混合床形态。使用质地和结构描述，沉积物重力流沉积物中的 158 种床型被表征为三种主要类型：“经典”砂流波纹、大电流波纹和低幅度床波。砂质流波纹由洁净砂岩组成，平均高度和长度分别为 11 毫米和 141 毫米。大电流波纹由砂岩-泥岩混合组成，比砂质电流波纹尺寸更大，平均高度为19 mm，平均长度为274 mm。低振幅基波是细长的砂岩-泥岩混合岩床，平均高度和长度分别为10 mm和354 mm。大电流波纹和低振幅床波与在减速的砂泥混合流下产生的实验床形非常相似，并被解释为分别在具有增强和减弱的近床湍流的过渡流下方形成。从扇子的边缘到末梢边缘，主要的床型类型从砂质电流波纹，通过大电流波纹，变为低幅度的床波，表明流动从湍流变为越来越受到湍流调制。提出了流动雷诺数降低，反映了这种流动转变，这是由于流动高度恒定或降低、沉积物沉积引起的流动减速以及由于富含粘土悬浮液的剪切稀化性质而增加的流动粘度的组合。大电流波纹和低幅度的床波很可能在其他海底扇的边缘很常见。砂泥混合床型的存在和空间趋势可能是解释扇缘环境的重要工具。表明流动从湍流变为越来越受到湍流调制。提出了流动雷诺数降低，反映了这种流动转变，这是由于流动高度恒定或降低、沉积物沉积引起的流动减速以及由于富含粘土悬浮液的剪切稀化性质而增加的流动粘度的组合。大电流波纹和低幅度的床波很可能在其他海底扇的边缘很常见。砂泥混合床型的存在和空间趋势可能是解释扇缘环境的重要工具。表明流动从湍流变为越来越受到湍流调制。提出了流动雷诺数降低，反映了这种流动转变，这是由于流动高度恒定或降低、沉积物沉积引起的流动减速以及由于富含粘土悬浮液的剪切稀化性质而增加的流动粘度的组合。大电流波纹和低幅度的床波很可能在其他海底扇的边缘很常见。砂泥混合床型的存在和空间趋势可能是解释扇缘环境的重要工具。由于富含粘土的悬浮液的剪切稀化性质而增加流动粘度。大电流波纹和低幅度的床波很可能在其他海底扇的边缘很常见。砂泥混合床型的存在和空间趋势可能是解释扇缘环境的重要工具。由于富含粘土的悬浮液的剪切稀化性质而增加流动粘度。大电流波纹和低幅度的床波很可能在其他海底扇的边缘很常见。砂泥混合床型的存在和空间趋势可能是解释扇缘环境的重要工具。