Stratal order in Pennsylvanian cyclothems
Interpretation of continental to shallow-marine sedimentary sequences is commonly based on a perception of the presence of order in the upsection recurrence and/or arrangement of one or more rock types. These patterns of stratal organization span a spectrum of scales, ranging from thick transgressive-to-regressive “sequences” within passive continental margins, to thin upward-shallowing “parasequences” reported from many peritidal carbonate successions. However, in spite of the widespread acceptance of this idea, little quantitative assessment has been conducted to ascertain the relative dominance of stratal order and disorder in sedimentary sections. Because perception of high-frequency stratigraphic repetition is especially prevalent in Pennsylvanian “cyclothemic” terrestrial to marine successions generally coeval with Gondwanan glaciation, hypotheses of stratal order have also been widely linked to possible periodic changes in global sea level. Because of the general importance of implied relations between stratal order and glacioeustasy, lithofacies thicknesses, recurrences, and transition frequencies were examined in outcrops of west-central Illinois which comprise the type region of “cyclothem” deposition in order to evaluate the incidence of depositional cyclicity at this rather intermediate scale of consideration. Data on compositions, thicknesses, and vertical stratigraphic arrangement of 1031 units were derived from measured sections of Wanless, these comprise 11 distinct lithofacies types that span 868 m of section exposed at 44 localities. Philosophically, stratigraphic cyclicity could be represented as expectation in lithofacies thickness, lithofacies recurrence, or as some regular pattern in lithofacies ordering. Frequency distributions of lithofacies thicknesses and stratigraphic separations between recurrent lithofacies are exponential in form; these distributions of stratigraphic intervals would result if horizons of lithologic change or recurrence occurred independently (randomly) in sedimentary successions. Stratigraphic cyclicity manifested as lithofacies ordering is evaluated by considering frequencies with different rock types succeed each other in stratigraphic sections; presumption of cyclothemic order requires that lithofacies9 transitions exhibit some manifestation of statistical predictability. Markov chain analyses indicate that most high-frequency transitional order exists at underclay-to- coal horizons. Because underclay development may, in part, reflect a post-depositional (diagenetic) process that was associated with plant growth and coal formation, underclay-to-coal couplets may exist as one lithofacies association (as opposed to two genetically unrelated superjacent units). Few other transitions exhibit statistically significant predictability that cannot be ascribed to linkages between gradational rock types or to low-frequency (formation scale) variation in the bulk rock composition of the aggregate Illinois succession. In addition to these statistical attributes, cyclothemic associations designated in numerically generated random sequences that have the same lithofacies attributes as do the Illinois sections exhibit virtually identical thicknesses and abundances of lithofacies elements as those recognized by Wanless. Thus, we conclude that the classic Pennsylvanian successions in west-central Illinois exhibit no compelling evidence of high-frequency cyclic order, either as regularity in the recurrence or in the compositional ordering of lithofacies elements. On the basis of these considerations, we suggest that thicknesses, recurrences, and degrees of stratal order among Pennsylvanian lithofacies reflect largely uncorrelated durations, intensities, and styles of sediment deposition along the western margin of the Illinois Basin. Moreover, portrayal of cyclothemic order in the Illinois sections was found to be distinct from the rather obvious alternation of marine and terrestrial units that characterize these and many other Carboniferous successions. Presumption of recurrent change in sea level is an assumption that is largely unrelated to the presence or absence of cyclothemic scales of stratal ordering. If these Illinois sections are at all typical of other Carboniferous successions, then “cyclothemic” order, manifested as recurrent and potentially periodic accumulation, may be much less evident than is generally perceived to be the case during this important interval of Earth history.