Facies models of tide-influenced river deltas are less developed than models for river- and wave-dominated types because the interaction of river and tidal currents on deltas is complex, flow and sediment transport over bedforms on tide-influenced river deltas has not been extensively studied, and relatively few studies have recognized ancient deposits of tide-influenced deltas. Tide-influenced deltas occur in shallow seas or adjacent to broad shallow shelves, conditions less common in the modern, following early Holocene sea-level rise, than during other times of Earth history. Tides change patterns of deposition on river deltas by increasing rates of river and basin water mixing, elongating sand-body trends perpendicular to the coastline, making deposits more heterolithic by oscillating depositional current speeds and building up tidal flats in interdistributary areas. Mouth bars deposited as flows decelerate out of distributaries are reworked into more elongate sand bars on the subtidal delta top and delta front. Distinguishing facies and the hierarchy of cross strata formed in tide-influenced fluvial and distributary channels, shallow marine subtidal delta-top and delta-front sand bars, and erosion related to fluvial-channel and tidal-current incision are key to interpreting proximal-distal changes in deposition and water-depth variations from deposits of tide-influenced deltas.
Facies patterns in tide-influenced deltas are complex, because patterns of erosion and deposition can change significantly from areas actively fed by the river to areas temporarily abandoned where tidal currents continue to rework sediments. Where rivers supply abundant sand, rapidly prograding delta fronts develop steeply inclined beds containing tide-winnowed sands, whereas areas of slower progradation are extensively reworked by migrating tidal bars. Sandy deposits of tide-influenced deltas commonly have basal erosion surfaces that record scouring of prodelta muds, as well as internal erosion surfaces that record local tidal reworking of sediments following distributary-channel switching. Evidence of significant erosion and changes in the basinward extent of sands do notnecessarily reflect changes in water depth. Significant erosion and sediment reworking can also occur during transgression. Distinguishing sequence stratigraphic divisions depends critically on recognizing large-scale shiftsin facies belts that define key allostratigraphic surfaces, but this can be difficult in tide-influenced deltas,where local changes in sediment supply and tidal-current strength can produce pronounced facies variations and significant erosion.
Although there has been a tendency for all tide-influenced deposits to be interpreted as transgressive estuarinevalley-fill deposits because they commonly overlie extensive erosion surfaces, it is increasingly recognized that erosion at the base of sand facies is the norm in most strongly tide-influenced depositional settings. Outcropping deposits interpreted to have formed on tide-influenced deltas show a wide variety of facies patterns and stratal architecture. Although there are too few well-documented examples to define general trends, the characterof tide-influenced deltaic systems is interpreted to change with proximal-distal position within deltaic successions, whether deltaic deposition is confined or broadly distributed, and with the sequence stratigraphic setting.Changes in deposits of tide-influenced-deltas associated with expansion of deposition into a basin and varying rates of shoreline progradation may be as pronounced as those that define sequence stratigraphic divisions.
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River Deltas–Concepts, Models, and Examples
Deltas are amongst the most environmentally and economically important coastal sedimentary environments. Studies of deltas lag behind research in both fluvial and deep-water depositional systems, as well as more geomorphologically oriented land studies. This knowledge lag reflects both a reorientation of the petroleum industry in the last two decades toward deep-water systems, as well as the difficulty of working across the shoreline with the traditional tools used for oceanographic or land-based work. However, deltaic studies are experiencing a renewed focus, because of their global importance in environmental and other societal concerns. This volume stems from a special session: Deltas: Old and New, held at the Annual Geological Society of America conference in October 2002, that was convened to highlight these new directions in deltaic research.