Discussion on a sequence-stratigraphic interpretation of a mudstone-dorninated succession: the Lower Jurassic Cleveland Ironstone Formation, UK

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Abstract
S. P. Hesselbo writes: Macquaker & Taylor (1996) have provided a well-illustrated account of sediment micro fabrics in the Cleveland Ironstone Formation, and have clarified the occurrence of phosphate within part of the succession at Staithes. It may be useful, however, to discuss some of their broader conclusions, particularly those relating to sequence stratigraphy. A principal theme of the paper is that subtle trends in grain-size distribution, revealed though microfacies analysis, leads to a re-assessment of the sequence stratigraphy of the Cleveland Ironstone in particular, and ‘mudstone-dominated’ successions in general. It should be noted that both the number of samples (37), and the size of the samples (?< 1 cm 2 ) are small. Trends must therefore be highly susceptible to bias resulting from small-scale sediment inhomogeneity, which is undoubtedly present in the succession (e.g. as described for this interval by Greensmith et al. 1980 ). The authors' results are novel in that they document: (a) a fining-upward trend over c elm above the Avicula Seam, and (b) three separate coarsening-upward cycles between the Avicula Seam and the Raisdale Seam. The problem is that these features rely on three key samples (ST5, ST14, and ST19), without any of which the grain-size arguments are significantly weakened. Because the grain-size plots are based on visual estimates, the 5% change in silt content between ST18 and ST19 is not very convincing, especially in view of the theoretical problems of deriving a 3D quantity (grain-size distribution) from a 2D data set (polished section) Furthermore, the plot in