Abstract

The Lower Ordovician El Paso Group contains several “third-order” depositional sequences which consist of higher-frequency “fifth-order” depositional cycles. Cycle stacking patterns of cycle type, cycle thickness and subfacies components provide a semi-quantitative representation of systematic shifts in third-order cycles of accommodation. In the Franklin Mtns, two complete El Paso Group third-order sequences (each approximately 2 myr in duration and 200-450 ft thick) have been described in detail by Goldhammer et al (1993). In light of the integration of cyclostratigraphy with sequence stratigraphy, it is apparent that high frequency cycles are regarded as the fundamental attribute which best characterizes or describes the vertical and lateral structure of carbonate platforms.

In this study, an empirically-based classification of the architecture of platform carbonates is introduced, invoking the “cycle” as an elemental unit of measurement. In this scheme, carbonate systems can be assigned to one of three classes of “behaviour” with regard to the empirical organization of architectural elements. Drawing an analogy to all natural systems, we classify the behaviour of carbonate systems, as deterministic through chaotic to stochastic, and intepret such differences as a function of secular variation (icehouse vs greenhouse dynamics), tectonic setting and platform geometry. This attempt at an empirical classification de-emphasizes process and origin and places the focus on detectable structure.

Application of geo-statistical routines (runs tests, Durbin-Watson) to stacks of contiguous platform cycles from the Franklin Mtns suggests long-term (third-order) non-random behaviour. This behaviour is best described as chaotic, wherein third-order sequences provide a stratigraphic example of non-resonant ‘quasi-periodicity’. The utility of fractals as applied to describing the El Paso Group structure is investigated.