Deltas

Deltas are discrete shoreline protuberances formed where a river enters a standing body of water and supplies sediments more rapidly than they can be redistributed by basinal processes, such as tides and waves. In that sense, all deltas are river-dominated and deltas are fundamentally regressive in nature. The morphology and facies architecture of a delta is controlled by the proportion of wave, tide and river processes; the salinity contrast between inflowing water and the standing body of water, the sediment discharge and sediment caliber, and the water depth into which the river flows. The geometry of the receiving basin (and proximity to a shelf edge) may also have an influence. The simple classification into river, wave and tide-dominated end-members must be used with caution because the number of parameters that control deltas is more numerous.

Other depositional environments, such as wave-formed shorefaces or barrier-lagoons can form significant components of larger wave- influenced deltas, but conversely smaller bayhead or lagoonal deltas can form within larger barrier-island or estuarine systems. As deltas are abandoned and transgressed they may also be transformed into another depositional systems (e.g. transgressive barrtier lagoon system or estuary). Delta plains also contain distributary river channels and their associated floodplains and bays, which can be equally classified as both fluvial and deltaic environments.

Tens to hundred meter thick, sharp-based blocky sandstones within many ancient mid-continent deltas have been routinely interpreted in the rock record as distributary channels, although many of these examples are now reinterpreted as incised fluvial valleys. Distributary channels may show several orders of sizes and shapes as they bifurcate downstream around distributary mouth bars. Bifurcation is inhibited in strongly wave-influenced deltas, resulting in relatively few terminal distributary channels and mouth bars flanked by extensive wave-formed sandy barriers or strandplain deposits. In shallow-water fluvial dominated deltas, tens to hundreds of shallow, narrow and ephemeral terminal distributary channels can form intimately associated with mouth bars that form larger depositional lobes. Tides appear to stabilize distributary channels for hundred to thousands of years, inhibiting avulsion and delta switching. As deltas prograde they form upward coarsening facies successions, as sandy mouth bars and delta front sediments build over muddy deeper water prodelta facies. Deltas display a distinct down-dip clinoform cross sectional architecture. Many large muddy deltas show separate clinoforms, the first at the active sandy delta front and the second on the muddy shelf. Along-strike facies relationships may be less predictable and depositional surfaces may dip in different directions. Overlapping delta lobes typically result in lens-shaped stratigraphic units that exhibit a mounded appearance.

All modern deltas grade updip from marine into non-marine environments, and Walther’s Law predicts that deltas should show a marine to nonmarine transition as they prograde. However, in many low accommodation settings, topset alluvial or delta plain facies can be removed or reworked by wave or tidal erosion during transgression, resulting in top-eroded deltas. Historically, some of these top- eroded deltas have been interpreted as distal shelf deposits, not related to shoreline processes. Sequence stratigraphic concepts, however, allow facies observations to be placed within a larger context of controlling allocyclic mechanisms which allow the correct interpretation of larger delta systems of which only small remnants may be preserved.