ALTERNATE MODELS FOR THE COMMON POINT BAR: ORIGIN AND STRATIGRAPHY OF THE FRAGMENTARY AND THE MUDDY-NORMAL POINT BAR

Traditional models record point bar preservation as continuous accretionary bodies with bounding surfaces extending along the entire bar face. Point bars are also traditionally modeled as predominantly sandy, unless counter or tidal bars. Although both these models occur, alternative fragmentary and muddy-normal point bars are also prevalent. A 3D architectural-element analysis of point bars in the Late Cretaceous Dinosaur Park Formation from the Steveville area of Dinosaur Provincial Park, Alberta, Canada reveals fragmentary bars with a fining upward trend from basal lower medium to fine sand, upward into siltstone, mudstone, as well as peaty clay on the bar top. Mapped accretion surfaces could not be traced either in down dip or strike direction for more than a 10¹ meters before being truncated by younger accretion surfaces. Point bar deposits consist of centimeter to meter vertical-scaled and imbricately stacked unit bars. Consecutive unit bars are commonly reshaped by erosion and/or depositionally draped. This highly fragmented accretion style results from changes in orientation of the accretion face between discrete accretion events by as little as two degrees. Results for the muddy-normal bar found altering intervals of sand and mud, with mud comprising over 50% of the bar. Mud layers are thick-bedded and have current ripples indicative of deposition by bed transport rather than drapes. Accretion packages commonly were between 3 to 4 meters thick and 10 to 15 meters long. The sand and mud packages are present throughout the bar and do not appear to reflect location within the bar. Surfaces in sand and mud sets are at differing orientations, suggesting changes in trajectory of the bar drive mud vs sand deposition intervals. Both normal and fragmentary bars reflect the high importance of bar accretion vector in controlling bar stratigraphy.