MODERN SCALING RELATIONSHIPS AS A MEANS TO EVALUATE DOWNSTREAM POSITION IN ANCIENT STRATA: THE MCMURRAY CONUNDRUM, LOWER CRETACEOUS OF THE ALBERTA FORELAND BASIN

Studies of modern river morphodynamics show that as a river transitions from normal flow to its backwater reach, predictable changes occur in channel migration rates and point-bar morphology. Dimensionless scaling relationships that express morphological changes—for example, point-bar width-to-thickness ratios—can be used in ancient systems to evaluate paleogeomorphology. Under normal flow conditions, outside of the backwater reach, predictive sand-body width-to-thickness ratios are 70–300:1; conversely, sand-body width-to-thickness ratios may be as low as 10–30:1 within the lower backwater reach where estuaries are found. As a corollary, much of the valley width in upstream reaches consists of channel-belt sands, whereas downstream within the backwater reach, only a small proportion of the valley consists of channel-belt sands, with the majority consisting of floodplain muds and crevasse splays.

The Lower Cretaceous McMurray Formation, in the Athabasca region of the Alberta foreland basin, provides an opportunity to use modern analogs to test alternative hypotheses. Classic interpretations of trace fossils in the middle McMurray at Athabasca suggest an estuarine system that is tidally influenced. An alternative hypothesis argues the Athabasca McMurray represents a giant fluvial system that is well upstream from any possible brackish influence, and is not an estuarine deposit.

Using modern analogs to evaluate the McMurray requires locally available seismic data to measure point bars, or well logs to calculate the percent valley widths occupied by channel-belt sandstones. This research has composed a modern calibration dataset that quantifies the characteristic downstream evolution of channel-belt width-thickness and percent coverage in river valleys, as normalized to backwater length. Measurements are ongoing, but preliminary data show the Athabasca McMurray characteristics are consistent with a river in the normal flow to backwater transition, 100s of km upstream from any brackish or marine influence.