Tidally-influenced deposition and microfacies sequences of fluid muds: Early Cretaceous McMurray Formation, Alberta, Canada

Fluid muds are common in estuarine environments, but their ancient examples have rarely been documented due to poor comprehension of their depositional processes and characteristics. Mudstone layers in the tidally-influenced channel sequences of the middle McMurray Formation are examined in detail through microscopic observations and interpreted on the basis of recent advances in the understanding of flow dynamics of high mud-concentration flows. The mudstone layers, < 1 to 25 mm thick, are classified into three microfacies. Structureless mudstone (Microfacies 1) consists mainly of clay particles with randomly dispersed coarse grains (coarse silt to fine sand). It represents cohesive mud flows with sufficient cohesive forces to support coarse grains (quasi-laminar plug flow). Silt-streaked mudstone (Microfacies 2) is similar to Microfacies 1 in texture, but contains discontinuous streaks of coarse-silt to very-fine-sand grains. It is interpreted as being also deposited by cohesive fluid muds. The silt streaks are, however, suggestive of the presence of weak turbulence under the cohesive plug (upper transitional plug flow). Heterolithic laminated mudstone (Microfacies 3) is characterized by alternations of very thin, silt and clay laminae, which are either parallel or low-angle cross-laminated. It is interpreted as the deposits of low-amplitude bed-waves formed in lower transitional plug flows. These microfacies reflect a range of flow phases of fluid muds, which changed as flow velocities and suspended sediment concentrations fluctuated with tidal cycles. Repeated vertical changes of microfacies are suggestive of an ideal sequence of fluid muds formed during tidal acceleration and deceleration. The sequence comprising microfacies 3, 2 and 1 in ascending order represents deposition from lower transitional plug flows through upper transitional plug flows to quasi-laminar plug flows as suspended sediment concentrations increase with flow deceleration. That of the reversed order of microfacies reflects the reversed change in flow types during acceleration. These results provide the basis for recognizing fluid-mud deposits and tidal signatures in ancient estuarine sequences.