Siliciclastic systems are lumped into wave- or tide-dominated classes, and many detailed facies and sequence-stratigraphic analyses interpret pronounced changes to a shift from one to the other caused by major (commonly external) perturbations. To explore fundamental assumptions of some of these studies, Legler et al. document distal deposits near the pinchout of the widely studied lower Sego Sandstone in the Book Cliffs, western Colorado, a unit generally regarded as the type example of an ancient tide-dominated delta. The analysis of facies and stratigraphic architecture suggests that coeval waves and storms were equally prominent in controlling its deposition. The re-interpretation of the lower Sego Sandstone as a mixed, tide- and wave-influenced delta is important in three ways: (1) the prevailing, widely used archetype of an ancient tide-dominated delta should be viewed as recording a wider mixture of depositional processes; (2) the recognition that wave- and tide-dominated deposits can be coeval; and, as a result, (3) sequence stratigraphic framework of the lower Sego Sandstone and other ancient tide-dominated/influenced, regressive strata may need to be re-assessed.
Facies relationships and stratigraphic architecture of distal, mixed tide- and wave-influenced deltaic deposits: lower Sego Sandstone, western Colorado, U.S.A. by Berit Legler, Gary J. Hampson, Christopher A.L. Jackson, Howard D. Johnson, Benôit Y.G. Massart, Marcus Sarginson, and Rodmar Ravnås