Abstract

This paper examines facies and stratigraphic relationships at the tip-out of a sequence boundary along the fluvial-to-marine transition zone of the Cretaceous Ferron Notom delta, Utah. Correlation of 11 measured sections shows that the sequence boundary underlies amalgamated fluvial deposits in more proximal parts of the valley, whereas the valley fill splits into a pair of fluvial channel facies separated by transgressive shoreface deposits in the transition zone. This indicates at least two episodes of downcutting and valley filling, and suggests a compound valley system. Thick sandy deltaic deposits are interpreted as the falling-stage and lowstand deposits farther downdip, fed by the rivers that cut the valley. Paleohydraulic calculations indicate that the valley-forming trunk channels were on the order of 5.5 m deep, and carried coarse sand with rare pebbles over a slope on the order of 0.0005. Corresponding backwater lengths were on the order of 10 km. Recent models show that channels in the backwater zone should experience greater levels of aggradation, versus rivers landward of the backwater, which may show greater lateral migration or incision, and these predictions are matched by our observations. In addition, tidal backwater effects are predicted to be on the order of a few kilometers. The valley-fill shows increasing aggradation and tidal facies in sections a few kilometers from the mapped shorelines, which suggests a short backwater length. The relatively short backwater also allows the rivers to bring coarser sand and gravel close to the shoreline.