The spatial and temporal distribution of grain-size breaks in turbidites

<p>Grain-size breaks are surfaces where abrupt changes in grain size occur vertically within deposits. Grain-size breaks are common features in turbidites around the world, including ancient and modern systems. Despite their widespread occurrence, grain-size breaks have been regarded as exceptional, and not included within idealized models of turbidity current deposition. This study uses <em>ca</em> 100 shallow sediment cores, from the Moroccan Turbidite System, to map out five turbidite beds for distances in excess of 2000\&nbsp;km. The vertical and spatial distributions of grain-size breaks within these beds are examined. Five different types of grain-size break are found: <em>Type I</em> \textendash in proximal areas between coarse sand and finer grained structureless sand; <em>Type II</em> \textendash in proximal areas between inversely graded sand overlain by finer sand; <em>Type III</em> \textendash in proximal areas between sand overlain by ripple cross-laminated finer sand; <em>Type IV</em> \textendash throughout the system between clean sand and mud; and <em>Type V</em> \textendash in distal areas between mud-rich (debrite) sand and mud. This article interprets <em>Types I</em> and <em>V</em> as being generated by sharp vertical concentration boundaries, controlled by sediment and clay concentrations within the flows, whilst <em>Types II</em> and <em>III</em> are interpreted as products of spatial/temporal fluctuations in flow capacity. <em>Type IV</em> are interpreted as the product of fluid mud layers, which hinder the settling of non-cohesive grains and bypasses them down slope. Decelerating suspensions with sufficient clay will always form cohesive layers near to bed, promoting the generation of <em>Type IV grain-size breaks</em>. This may explain why <em>Type IV grain-size breaks</em> are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere. Therefore, <em>Type IV grain-size breaks</em> should be understood as the norm, not the exception, and regarded as a typical feature within turbidite beds.</p>