Abstract
Abdul Salpm Khan & Gilbert Kelling write: In their interesting recent paper, Dimberline et al. 1990 have drawn attention to the prevalence of a finely laminated silt and mud lithology within the Silurian sequences of north and central Wales, and in the Lake District. They conclude that this lithofacies is the product of predominantly hemipelagic ‘background’ deposition in the anoxic bottom waters of a stratified oceanic water-mass. They argue that the lamination represents seasonal-to-annual alternations of sediment supplied respectively by vertical fallout from planktonic sea-surface blooms (for the organic-rich laminae), and by extremely dilute density flows augmented by vertical fallout of faecal pellets, largely produced in the water column (creating the silt laminae).
While we are in broad accord with the ascription of this delicately laminated facies to hemipelagic processes, our detailed examination of these sediments and possible modern analogues leads us to place a somewhat different emphasis on the various factors involved in the complex sedimentological scenario necessary for the formation of this unusual lithofacies. These differences rest mainly on: (i) the homogeneity or heterogeneity of the silt–clay fractions in the laminated hemipelagic units and the interleaved mud-turbidites: (ii) the origin of the aggregate structures that make up the bulk of the silt laminae in the laminated hemipelagic facies; (iii) the origin of the organic-rich laminae: (iv) the duration and causes of the periodicity implied by the lamination.
Dimberline et al. correctly have pointed out that the laminated hemipelagic units commonly occur independently of authentic sand, silt or
