The significance of composite particles in the transport and deposition of fine-grained sediments: Insights from multiple offshore mudstone units in the Late Cretaceous Western Interior Seaway

Although the roles of composite particles on the transport and deposition of fine-grained sediments (muds) have long been investigated in modern sedimentary environments, the significance of composite particles remains under-appreciated when interpreting depositional conditions of ancient mudstones. To develop a more comprehensive understanding of the types and abundance of composite particles in different offshore depositional environments, this study examined mudstones deposited from the proximal delta to the distal shelf environment in the Cenomanian-Turonian Western Interior Seaway. Polished thin sections and ion-milled samples made from core and outcrops samples were examined through optical and scanning electron microscopy to characterize the composition and grain size of different offshore mudstone units at the particle scale.

Mudstones deposited from the proximal to distal offshore settings consist of a wide range of particle types and are highly heterogeneous in texture. Most muds were likely transported and deposited as coarse silt to sand-sized composite particles (aggregates) by traction currents instead of suspension fallout. These composite particles can be both allochthonous and autochthonous in origin. Allochthonous composite particles include various types of lithic fragments, such as fine-grained sedimentary rock fragments, volcanic rock fragments, and low-grade metamorphic rock fragments. Autochthonous composite particles include flocs, mud rip-up clasts, and biogenic fecal pellets. Regardless of the origin or type, allochthonous composite particles generally decrease in size offshore to the distal prodelta environment. Further offshore, the proportion of autochthonous composite particles in mudstones significantly increases. However, characterizing the true grain-size distribution in distal shelf mudstones becomes more challenging because water-rich autochthonous composite particles suffer significant compaction. By way of fluid dynamics, many offshore mudstones likely started out as an accumulation of sand-sized particles. Understanding the composition and grain size of fine-grained sediments at the particle scale is essential for characterizing the dynamic nature of surface environments and interpreting their ancient counterparts.