Despite recent advances in understanding the complex dynamics of mud deposition, it remains a challenging task to characterize the grain size, origin of different components, and sedimentary textures of mudstones through detailed petrographic analysis. In this study, the Tununk Shale in Utah has been examined by optical and scanning electron microscopy (SEM) to determine how variations in petrographic characteristics (e.g., composition, texture) of this shelf mudstone succession reflect changing depositional environments. In the context of the general depositional setting, detailed petrographic studies indicate that most mud in the Tununk system were transported in bedload as silt- to sand-size mud-dominated composite particles (MCPs), rather than specific components (e.g., clay minerals, silt grains, fossil fragments) of smaller size (micrometers to tens of micrometers). Three types of MCPs in the Tununk Shale can be identified and distinguished from each other. These include fecal pellets, altered volcanic rock fragments, and shale lithics. Two other types of MCPs, namely floccules and soft mud rip-up clasts, likely contributed significantly to the formation of the precursor mud matrix of the Tununk Shale. Due to their water-rich nature, however, floccules and mud rip-up clasts suffer significant compaction. Except in fortunate circumstances, they are therefore no longer discernible in the rock record.

MCPs and their role in the formation of fine-grained sedimentary successions has largely gone unnoticed in previous studies. The recognition criteria, as well as petrographic characteristics of each type of MCP in different depositional environments of the Tununk Shale, are summarized here, with the intent that they may benefit future studies of other mudstone successions. The complex variability in the characteristics of different types of MCPs illustrated in this case study, however, highlights the need for additional systematic petrographic studies (integrating both optical and SEM) in order to develop and refine the current recognition criteria of MCPs in fine-grained sedimentary rocks. Detailed petrographic examination of mudstones, though labor intensive, can yield critical information regarding their provenance and depositional setting, as well as provide general insights into the underlying causes for mudstone heterogeneity.

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