Unsuccessful cut offs – origin and partial preservation of enigmatic channel fills encased within a large-scale point-bar deposit – The McMurray Formation type section, Alberta, Canada

This study describes and attempts to interpret processes leading to the formation and preservation of eight channel-fill deposits encased within an interpreted large scale (40-m-thick) point-bar deposit. The studied channel fills are superbly exposed along the McMurray Formation type section, Alberta, Canada. Exposures were mapped in detail as part of an outcrop study extending over multiple seasons. The apparent emplacement of channel fills into vertically continuous point-bar deposit is difficult to explain by common facies models for meandering rivers. Very deep incisions (15 to 35 m) and sharp erosional contacts of channel bases are indicative of high-magnitude flood events and/or prolonged bifurcation of stream flow that would deepen channel over time. Channel-fill truncation and the presence of younger overlying thick and extensive lateral-accretion deposits suggest point-bar reactivation. The spatial relationship of channel incisions and hosting point-bar reactivation allows for interpreting the order of geologic events for each encased channel-fill: (i) a flooding event and associated increasing discharge causing a channel incision into an existing point-bar; (ii) channel plugging by sediments (due to reduced discharge); and (iii) reactivation of the hosting point bar and/or meander avulsion with associated erosion of the upper part of incised channels, followed by burial (encasement) of channel remnants at the bottom of the incision. The proposed model suggests that all eight encased channel-fills are only minute keels of significantly larger channel incisions that did not ultimately undergo cut off or avulsion. Although these deposits and interpreted sedimentary processes are expected to be common in both modern and ancient meandering river systems and particularly those that have experienced major floods, this phenomenon does not appear to have been described in the scientific literature. Implications are numerous and potentially significant. Proposed depositional models may be useful for distinguishing encased channel deposits from vertically continuous point-bar deposits in subsurface studies. Sand-dominated encased channel deposits may contribute to increased reservoir connectivity.