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Rocks of coastal to shallow‐marine origin are challenging to interpret owing to the complex interplay of various depositional processes. This study reevaluates the relative roles of fluvial, tidal and wave processes in the Upper Cretaceous Sego Sandstone (and subordinately in the underlying Buck Tongue) of the Book Cliffs, USA, a well‐studied ancient coastal to shallow‐marine succession. Detailed sedimentological and ichnological analyses were used to interpret a previously underemphasised riverine signature, consisting of centimetre‐ to decimetre‐thick alternations of sandstone and heterolithic beds inferred to represent flood–interflood periods of variable river discharge. Recognition of a widespread fluvial‐dominated signature across the studied units better agrees with other sedimentological and regional observations in the study area, such as high sandstone–mudstone ratios, largely unidirectional and seaward‐oriented palaeocurrents, and modelled weak tidal conditions in the basin. When considering all of the sedimentological, ichnological and stratigraphic observations together with its regional depositional context, the Sego Sandstone/Buck Tongue system is better explained using a mixed‐energy but fluvial‐dominated deltaic model. This highlights an historical over‐interpretation of tidal processes and subordinate wave processes in the Sego Sandstone and likely in similar units. The widely used approach that emphasises only certain sedimentary features in discerning the process regime from analysis of rocks of inferred coastal to shallow‐marine origin is unrefined and may therefore underrepresent the actual complexity of these systems. [ABSTRACT FROM AUTHOR]

The 2 to 5 km thick, sandstone‐dominated (>90%) Jura Quartzite is an extreme example of a mature Neoproterozoic sandstone, previously interpreted as a tide‐influenced shelf deposit and herein re‐interpreted within a fluvio‐tidal deltaic depositional model. Three issues are addressed: (i) evidence for the re‐interpretation from tidal shelf to tidal delta; (ii) reasons for vertical facies uniformity; and (iii) sand supply mechanisms to form thick tidal‐shelf sandstones. The predominant facies (compound cross‐bedded, coarse‐grained sandstones) represents the lower parts of metres to tens of metres high, transverse fluvio‐tidal bedforms with superimposed smaller bedforms. Ubiquitous erosional surfaces, some with granule–pebble lags, record erosion of the upper parts of those bedforms. There was selective preservation of the higher energy, topographically‐lower, parts of channel‐bar systems. Strongly asymmetrical, bimodal, palaeocurrents are interpreted as due to associated selective preservation of fluvially‐enhanced ebb tidal currents. Finer‐grained facies are scarce, due largely to suspended sediment bypass. They record deposition in lower‐energy environments, including channel mouth bars, between and down depositional‐dip of higher energy fluvio‐ebb tidal bars. The lack of wave‐formed sedimentary structures and low continuity of mudstone and sandstone interbeds, support deposition in a non‐shelf setting. Hence, a sand‐rich, fluvial–tidal, current‐dominated, largely sub‐tidal, delta setting is proposed. This new interpretation avoids the problem of transporting large amounts of coarse sand to a shelf. Facies uniformity and vertical stacking are likely due to sediment oversupply and bypass rather than balanced sediment supply and subsidence rates. However, facies evidence of relative sea level changes is difficult to recognise, which is attributed to: (i) the areally extensive and polygenetic nature of the preserved facies, and (ii) a large stored sediment buffer that dampened response to relative sea‐level and/or sediment supply changes. Consideration of preservation bias towards high‐energy deposits may be more generally relevant, especially to thick Neoproterozoic and Lower Palaeozoic marine sandstones. [ABSTRACT FROM AUTHOR]