--> --> Advances in Sequence Stratigraphy—Insights From 35 Years of Studying the Other 80% of the Stratal Record: Mudstones

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Advances in Sequence Stratigraphy—Insights From 35 Years of Studying the Other 80% of the Stratal Record: Mudstones


A fundamental question for sequence stratigraphy in general, and for mudstones in particular, is whether the stratigraphic record can be understood as “sand/grainstone” and “not sand/grainstone” with mud serving only as passive ‘filler’, or do various grain-size categories each have their own inherent geometry and the stratal record is the resultant of the vigorous interaction among the grain sizes. Observations accumulated over the last 35 years, accelerated by the ‘shale-gas revolution’, indicate that mud is a quite active component at many scales, from being essential to many sediment-transport mechanisms to forming distinctive types of parasequences and complex stratal geometries at depositional-sequence to sequence-set scales. A wide variety of sedimentary structures occur in mudstones that indicate a comparably wide range of transport mechanisms, in many of which the presence of mud alters the fluid and transport properties of the flow. At the parasequence scale, most marine shelfal mudstone strata appear to have accumulated as one of three end-members that can be differentiated quantitatively and related to depositional regimes dominated by storm waves, river floods, or tidal currents through characteristic modes of sediment transport and accumulation, as well as variations in benthic-energy and oxygen levels. At the depositional-sequence scale, most marine biogenic-rich mudstones tend to occur in one of three physiographic settings (constructional shelf margin, platform/ramp, continental slope—basin), each of which has a commonly recurring pattern of biogenic enrichment distinctive from the other settings. At the depositional-sequence-set scale, all major shale-gas plays can be grouped into four main families, based on repeated patterns of stratal stacking of depositional-sequence-scale biogenic-rich physiographic settings. Although these observations imply that most mudstones accumulate discontinuously, they still preserve detailed records of paleoenvironmental conditions and depositional history, especially in microbially mediated authigenic products. The sequence-stratigraphic approach is particularly useful for organizing all these discontinuities and varying rock properties into a hierarchy of nested scales. The resulting sequence-stratigraphic framework is essential for integrating the wide range of physical, biogenic, and chemical attributes of mudstones into a comprehensive understanding of Earth history and hydrocarbon systems.