Abstract

Mudstone layers in the tide-dominated Bluesky Formation (i.e., the “mud drapes”) have enormously variable sedimentary characteristics: they range from 0.1 to 20 cm thick, can be homogeneous or internally stratified, and can have sharp or gradational upper and lower contacts. Based on recent flume studies, this diversity is interpreted to reflect the wide range of suspended-sediment concentrations (SSC; < 1 g L⁻¹ to > 100 g L⁻¹) and flow velocities (slack water to several meters per second) that are typical in tidal environments. Four recurring mudstone types are present, each of which is interpreted to have formed under distinct depositional conditions: (1) unstratified mudstone Type 1 (UM1): “classic” mud drapes generated by slow settling from suspensions with SSC values less than 1 g L⁻¹; (2) stratified mudstone Type 1 (SM1): cross-stratified mudstone layers deposited by turbulent or transitional turbulent flows with appreciable current speeds (> 0.2 ms⁻¹) and moderate SSCs (1–10 g L⁻¹); (3) stratified mudstone Type 2 (SM2): horizontally laminated mudstone layers formed under conditions of transitional plug flow with moderate to high SSCs (1–100 g L⁻¹) and appreciable currents (> 0.2 ms⁻¹); and (4) unstratified mudstone Type 2 (UM2): thick mudstone laminae and beds (> 2 mm thick), with no internal lamination and common soft-sediment deformation, deposited by unstable plug flow or quasi-laminar plug flow, with moderate to high SSCs (1–1000 g L⁻¹) and current speeds ranging from slack water to 1–2 ms⁻¹. UM2 and SM2 mudstone types are equivalent to what recent studies have interpreted as the lithified product of fluid mud and may have been deposited dynamically, under current speeds above the threshold of mud erosion in clear-water flows. This indicates that, in high-SSC settings, “mud drapes” could form over significant portions of the tidal cycle (i.e., not only at slack water) and perhaps continuously over multiple tidal cycles if fluid-mud layers persist. Thus, tidal rhythmites might be poorly developed in areas with high SSC values.