| Mud-dominate point bars are common features and are widely accepted as showing deposition by either tidal influence, bar tails, or counter point bars. Less understood, but still observed, are point bars that lack these depositional characteristics and yet, are still mud dominate. The goal of this study was to determine processes by which normal-muddy point bars form. Late Cretaceous fluvial strata of the Dinosaur Park Formation in the Steveville badlands of Dinosaur Provincial Park, Alberta are targeted to address this issue. Strikes and dips, paleocurrents, panoramas, and stratigraphic columns were collected to determine accretion trajectories and lithologic trends. This point bar was typically between 8 to 10 meters thick with altering layers of sand and mud, with mud being the predominate sediment and comprising over 50% of the point bar. Flood plain deposits stratigraphically confined the point bar, allowing for the determination that it was a single complete point bar. Typically, the mud layers are silty mudstones and the sand layers started as medium grained at the base of the point bar and then fined up to layers of lower fine sand at the top of the point bar. Mud layers and sand layers varied between thin to medium bedded. Mud layers within this point bar are thick-bedded and have current ripples indicative of deposition by transport. This suggests that the mud layers were deposited by active accretion events and are not simple drapes. This point bar also consists of accretion packages with differing orientations. Accretion packages commonly were between 1 to 3 meters thick and 10 to 15 meters long. Sand content shifts between packages, and alternating packages have sand content more consistent with typical sandy point bars. The sand and mud packages are present throughout the point bar and do not appear to reflect location within the bar. These data suggest that the muddy deposits of the muddy-normal point bar reflect changes in trajectory of the bar and sudden and temporary adoption of accretion orientations not conducive to sand deposition, and do not record either late stages of growth in the overall bar formation process, deviations from fully fluvial drivers, or counter point bar patterns. An analysis of strikes and dips of major accretion surfaces and the sediment associated with each surface revealed that sand surfaces are typically oriented to the north, mud to the south east, and 50/50 in between the two. These data, when compared to the channel flow direction, implies that as the river expanded sand was deposited and as the river translated mud was deposited. This river pulsated between these two phases allowing geometries of a normal point bar to form during times of expansion and geometries similar to a counter point bar during times of translation, overall providing the environment for a muddy-normal point bar to form. |
