MEASURED SECTIONS TO TRACK VARIOUS STRATAL ARCHITECTURES—A KEY TO ANALYZING CONODONT MORPHOGENESIS

Conodonts were a rapidly evolving fossil group that produced widely distributed microfossils found in Paleozoic and Triassic marine rocks. Conodonts can be used biostratigraphically to differentiate middle Pennsylvanian through early Permian cyclothems, which are generally recognized to record a glacial-eustatic cycle of sea-level rise and fall on the order of 100,000-400,000 years. Such a rapid pace of potential taxonomic turnover indicates that conodont morphogenesis should be expected through any composite third-order sequences. Note that cyclothems are now commonly treated as high-frequency sequences.

The occurrences of different conodont taxa are often closely associated with distinct lithofacies. This has been used to interpret conodont paleoecology and, conversely, to use that understanding to interpret the parameters of some lithofacies. For example, the genus Gondolella and closely related taxa are typically limited to the core shale of cyclothems (the maximum flooding unit), where their richness can exceed a hundred per kilogram of rock. However, they are seldom recovered from the surrounding strata, even as a rare fragment.

The resulting situation is that any detailed analysis of conodont morphogenesis must track the appropriate lithofacies through any given section. Though clearly objective, standard measured sections can commonly miss a critical lithofacies wherein the taxon under study occurs abundantly. Sequence stratigraphy has forced us to recognize that sediments do not always accumulate vertically across a broad depositional swath, but rather that stratal architectures may be discontinuous—with progradational or retrogradational geometries that deflect lithofacies considerably off-section. This paper will propose a system to clearly communicate sampling data that tracks lithofacies through non-standard sections to consistently account for different stratal architectures. Concepts will be demonstrated with examples using distal shelf and slope carbonates from the Permian of west Texas, and with example Pennsylvanian cyclothems from the Midcontinent.