Interpretation of Complex Lithologic Successions by Substitutability Analysis

Abstract

Many stratigraphic successions are characterized by repetitive patterns of lithologies. These patterns are most apparent if lithologies are grouped into relatively few categories, and become increasingly obscure as rock types are classified into finer subdivisions. Most cyclothems and megacyclothems, for example, are patterns composed of only four or five distinctive lithologies. Unfortunately, the gross classification necessary to reveal a cyclic pattern results in lithologic categories which yield meager environmental information.

A section through supposedly cyclic lower Pennsylvanian rocks in eastern Kansas was examined and the lithologies classified into 17 states. Although this degree of subclassification is typical of lithofacies studies, the variety of rock types conceals any cyclicity that might be present. Seemingly different lithologies appear at common positions within cyclothems, obscuring the repetitive pattern in the sequence. These lithologies “substitute” for one another in successive cycles, but may be identified by substitutability analysis, a classification procedure that groups states on the basis of their context in a sequence. States with common high conditional probabilities on subjacent and superjacent states are considered equivalent. Results suggest that lithologies must be combined into fewer than eight states before a cyclic pattern emerges. Analyses also suggest that the lower Pennsylvanian cyclothems studied represent interaction of two depositional processes rather than a single megacyclic process.