Parasequence Expression in Epicontinental Organic-Rich Mudstones: Examples from the Middle-Late Devonian of North America
Differentiating mudstone-dominated, organic-rich successions into a sequence stratigraphic framework requires the recognition of the basic building blocks of a depositional sequence, the parasequence. Identifying genetically related beds and bedsets bounded by flooding surfaces has proven to be a challenging task due to the apparent lack of facies contrast in fine-grained units. Furthermore, the ability to observe very subtle changes in composition, diagenetic overprint, sedimentary textures and fabric, as well as bioturbation intensity and diversity often requires significant petrographic and microscopic analyses. However, when these heterogeneities are adequately documented, a predictable genetic model can be developed and utilized for projecting depositional elements away from data control. For this study, parasequences from Middle-Late Devonian organic-rich mudstones from the Appalachian, Illinois, and Western Canadian basins were analyzed to provide temporal and spatial relationships from same-aged mudstones deposited in adjacent epicontinental basins of North America. Parasequences are distinct and show predictable changes in composition, organic-richness, bed-thickness, and bioturbation. Although deposited in different basins 100’s of kilometers distant, similar facies associations and stacking patterns are observed and support the notion that mud can be transported and distributed far from its source areas through a myriad of depositional processes. Furthermore, compositional changes appear to be associated with lateral extent, where siliceous and argillaceous mudstones appear to be widespread with high lateral continuity and carbonate-rich mudstones tend to show limited spatial extent and are generally restricted to 10's of miles away from the carbonate factory. These relationships are supported by flume experiments on clays and clay/carbonate mixtures that suggest that mud composition may be a major control on bedload transport efficiency. Parasequences can be correlated across the basin and in many instances show onlapping terminations against structural (i.e., faults) and depositional (i.e., carbonate buildup/reef) elements, suggesting that bedload transport dominated these intervals and bottom-currents were actively distributing mud across the basin. Using these relationships, a robust sequence stratigraphic framework can be developed and is critical for understanding unconventional reservoir heterogeneity and facies dispersion. Results from this study highlights the importance of incorporating data from adjacent basins and outcrops to provide a regional context that allows predictability for exploration.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019