Mixed Energy Deltas: the rule rather than the exception

River deltas are usually interpreted as fluvial-, wave- or tide-dominated based on the process regime dominance and although this classification is logical and extremely useful most of the modern and ancient deltas show a... [ view full abstract ]

River deltas are usually interpreted as fluvial-, wave- or tide-dominated based on the process regime dominance and although this classification is logical and extremely useful most of the modern and ancient deltas show a mixture of these processes. The relative influence of processes on the behavior of deltaic shorelines is controlled by unsteady external forcing conditions over long time periods or by autogenic process changes over relatively short temporal and spatial scales (hundreds to thousands of years; km to 10s of km). The modern large Holocene deltas stratigraphic responses were autogenic because late Holocene external forcing was relatively steady (constant rates of relative sea level rise, and sediment supply). Responses over longer time intervals would likely be allogenic due to the increased probability of unsteady external forcing over longer time scales. Such changes are better exemplified in ancient delta deposits.
The dominant depositional process can change down depositional dip as the delta progrades across the shelf and into slightly deeper water, or laterally because of shifting fluvial discharge or because of oceanographic differences between distributaries. The three most common types of process change seen in Holocene deltas are from fluvial- to wave-dominance (Mississipi and Danube deltas) from tide- to wave-dominance (Mekong Delta) or from fluvial- to tide-dominance (Mahakam Delta). Particular segments of large delta complexes can also show these changes.
The pervasive and rapid process changes seen in Holocene deltas suggest that process regime variability was also common in ancient deltas, expressed by changes in the character of the deltaic succession, especially on the delta front, the regime-defining segment of the delta. Such changes should be considered more as a rule than an exception. Campanian and Maastrichtian delta deposits from the Western Interior Seaway and Laramide Washakie Basin show clear evidence of such changing process regimes. Maastrichtian Fox Hill deltas in Washakie Basin, Wyoming show facies of variable (fluvial, wave, tide) processes between successive vertically stacked parasequences which were controlled by allogenic forcing and within the same delta sequence that reflects autogenic forcing. A reasonably good time framework in the rock record is needed to aid discussion of whether observed stratigraphic responses are autogenic or allogenic, and it is important to make this distinction because of local or regional implications for stratigraphy. A common theme seen in shelf-delta stratigraphy is the occurrence of spatially extensive flooding surfaces bounding deltaic complexes of 1-300 ky duration, but with great lateral, between-lobe variability within such units. The stratigraphic interval bounded by the flooding-surface , , records the overall cross-shelf regression and transgression of the delta complex (allogenic response), whereas the great internal variability reflects autogenic spreading of the lobes during the cross-shelf transits.