Figure 2.4   (a) Hypothetical and minimalist graphic correlation based on the stratigraphic distribution of the fi ve apparent chronospecies of the Silurian brachiopod Eocoelia, in ascending order: E. hemisphaerica, E. intermedia, E. curtisi, E. sulcata and E. angelini; the fi rst four range through the middle and upper Llandovery whereas the last is characteristic of the lower Wenlock. The ranges of these species are given from two artifi cial sections with the fi rst appearances of each species plotted on both sections as x and y coordinates. The straight line fi tted to the points allows a precise correlation between each part of the two sections. In this simple example all the points fi t on a straight line; in practice a regression must be fi tted to the scatter of data points.

(b) Seriation of biostratigraphic data. The fi ve Eocoelia species were collected from fi ve horizons in a stratigraphic section; the data were collected and plotted randomly as a range chart. Seriation seeks to establish any structure, usually gradients, within the matrix by maximizing entries in the leading diagonal. The seriated matrix reveals the stratigraphic succession of Eocoelia species that is widely used for the correlation of Lower Silurian strata. Most seriations are based on much larger and more complex data matrices where any non-random structure, if present, is initially far from obvious.

(c) The RASC method predicts the solution most likely to occur in the next section based on previous data. Three sections (13) are presented and, based on a majority vote, the RASC solution is constructed; since the fi rst two sections are similar they win over the third slightly different section. This is different to the maximum range solution that may be constructed by other methods. (c, based on Hammer & Harper 2005.)

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