Detailed Petrographic Studies of the Late Cretaceous Tununk Shale Member of the Mancos Shale Formation: Prevalence and Types of Mud-Dominated Composite Particles in Mudstones

Abstract

Despite recent advances in understanding the complex and dynamic processes of mud deposition, detailed petrographic characterization of mudstones has far lagged behind. Within a developed depositional framework, this study analyzed the Upper Cretaceous Tununk Shale of the Mancos Shale Formation through detailed petrographic methods integrating optical and scanning electron microcopy (SEM), in order to determine the variations in petrographic characteristics (e.g. composition, texture) of this shelf mudstone succession in response to changing depositional environments. Results of this study indicate that most muds in the Tununk system were transported in bedload as silt/sand-sized mud-dominated composite particles (MCP), rather than discrete particles. Based on combined optical microscopy and SEM analysis, three types of MCP that can be distinctly identified and distinguished from each other in the Tununk Shale include planktonic fecal pellet, altered volcanic rock fragment (VRF), and shale lithic (i.e. shale fragment derived from older mudstone/shale strata). In the context of the general depositional setting, two other types of MCP including floccule and mud rip-up clast probably also 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 would suffer significant degree of compaction, and therefore are visually indistinguishable in the rock record.

The prevalence of MCP and their roles in the formation of fine-grained sedimentary successions appear to have been largely overlooked in the literature. The recognition criteria, as well as petrographic characteristics of each type of MCP within different depositional environments of the Tununk Shale are summarized, which can be applied to recognize similar types of MCP in analogous mudstone succession. The complex variability in the characteristics of different types of MCP revealed in this case study, however, highlights the need of more systematic petrographic studies (integrating both optical and SEM) in order to develop and refine the current recognition criteria of MCP in the fine-grained sedimentary rocks. This study reinforces the notion that detailed petrographic examination of mudstones is of significant importance, as it can yield important information regarding their provenance and depositional setting, and provide valuable insights into the essential causes for heterogeneity in mudstones.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018