Abstract

The facies characteristics and stratigraphic architectures of ancient mixed, wave- and tide-influenced deltas are poorly documented, despite the abundance of analogous modern shorelines. This paper presents the first facies analysis and stratigraphic-architectural characterization of the distal part of the Cretaceous lower Sego Sandstone (Book Cliffs, Colorado, U.S.A.), whose correlative proximal part is arguably the most comprehensively documented example of ancient deltaic strata that exhibit evidence for the activity of tides and waves.

The distal lower Sego Sandstone consists of tide-dominated and wave-dominated deposits arranged within four regressive–transgressive tongues bounded by flooding surfaces. The proximal part of each regressive–transgressive tongue contains tide- dominated sandstones that sharply to erosionally overlie, and laterally interfinger with, wave-dominated deposits in the distal part of the tongue. Tide-dominated sandstones are fine to medium grained, and comprise wavy bedded and cross-bedded sandstones with abundant mud drapes, reactivation surfaces, and bidirectional, ebb- and flood-tide-directed paleocurrents. They define active tidal distributary channel-fill and nonchannelized tidal-bar deposits of similar lithologic character, which represent the proximal part of the delta front. Abandoned tidal distributary channel fills comprise intensely bioturbated mudstones. The outer, paleoseaward parts of tidal bars contain direct evidence for mixed tide and wave influence in the form of hummocky cross-stratified, very fine-grained sandstone beds intercalated with cross-bedded, fine- to medium-grained sandstones containing tidal indicators. These deposits locally pass paleoseaward into low-angle and hummocky cross-stratified sandstones with grain size and texture similar to tidal-bar deposits, implying reworking by storm waves. Wave-dominated deposits consist of coarsening-upward successions of mudstone and hummocky cross-stratified sandstone beds that represent the distal part of the delta front. Localized rhythmic variations in the wavelength of hummocky cross-stratification in some deposits may record tidal modulation of water depth and storm-wave velocities.

These facies components and their architectural arrangement suggest that tides and waves operated together throughout deposition of the “lower Sego Delta,” but that tide-dominated and wave-dominated deposits were spatially partitioned into the proximal and distal parts of each regressive–transgressive tongue. Thus, tides and waves were effective in transporting and reworking sediment in different water-depth ranges and at different geographic locations. Mixed tide- and wave-influenced deposits are generally preserved only at the down-dip pinchout of regressive–transgressive tongues, because they are removed by tidal scour during delta progradation in more proximal locations. The spatial partitioning of time-equivalent tide-dominated and wave-dominated deposits is readily explained in a single depositional model, which requires the interpretation of neither sequence boundaries nor temporal changes in process regime across the sharp-to-erosional juxtaposition of tide-dominated sandstones above wave-dominated deposits in a regressive–transgressive tongue.