Size versus Sorting in Some Caribbean Sediments

The relationship between average size (phi median) and sorting (phi percentile deviation) in sediments is explored empirically by means of statistical analysis of data from over twelve hundred individual samples taken from Miocene and Oligocene horizons in southern Trinidad, the Eocene Scotland series from Barbados, and the Pliocene Superficial Gravels of British and Dutch Guiana. The association between size and sorting is estimated by means of the correlation coefficient, and the relationship is reduced to linear equations of the form PDφ = bMdφ + a. Deviations from the trends are measured in terms of the standard error of estimate (Sy). On the basis of the statistics it is shown that, in water-deposited sediments, size varies with sorting over the range oΦ (I.oo mm.) to 6Φ (0.0156 mm.) in median diameter and 0-4.5Φ in phi percentile deviation. Best-sorted sediments (PDΦ < IΦ) possess median diameters around 2.5Φ (0.177 mm.), and sorting becomes poorer with both increasing and decreasing size from the fine-sand range (2-3Φ). Departures from the trend of over 1 Sy unit indicate that in finer-grained sediments (clays and fine silts) flocculation had taken place during grain-size analysis; whereas in coarser-grained sediments (very fine sands and coarse silts) the departures imply events of geological significance which interfered with normal sedimentation.