Delta Environments Nile Delta Mississippi Delta

Discrete shoreline protuberances, partly subaerial and partly submarine, built by rivers into a body of permanent water

Delta plain: mostly subaerial part of delta complex (dominated by fluvial processes) Distributary channels Interdistributary bay Delta front: shallow subtidal part of delta, above wave base (mix of fluvial and wave/tide processes) Prodelta: deeper subtidal part of delta, below wave base

Progradation of relatively steep delta front (1-10°) produces a type of bed geometry called a clinoform Topset Foreset Toeset Bottomset Delta front foresets

Prodelta (sub-wave base) generally resembles fine-grained offshore facies Slumps may occur due to steeper prodelta slopes and rapid sedimentation rate

May have characteristics of wave or tide influence Delta front sediments occur as coarsening-upward succession over prodelta May have characteristics of wave or tide influence Transgressive lag Delta Plain Delta Front Delta Front Prodelta P

Hypopycnal plume Density < seawater Relatively continuous Deposition from suspension (=hemipelagic sedimentation) Hyperpycnal plume Density > seawater Episodic, lasts hours-days Deposition from suspension, modified by traction

Waxing/waning flow Inverse-normal grading Hemipelagic sediments Finely laminated or bioturbated mud Suspension deposition Normal grading

Delta plain: mostly subaerial part of delta complex (dominated by fluvial processes) Distributary channels Interdistributary bay Delta front: shallow subtidal part of delta, above wave base (mix of fluvial and wave/tide processes) Prodelta: deeper subtidal part of delta, below wave base

By definition, fluvial influence is important in deltas Waves or tides can also rework delta plain deposits

Fundamental sedimentological unit of a delta is the distributary mouth bar, formed where sediment is rapidly deposited after rivers enters basin

High sedimentation rates during river flood events Traction deposition with significant contribution from suspension settling Climbing ripples

Interdistributary bay Atchafalaya River, Louisiana Distributary channel Interdistributary bay

Cycles of interdistributary bay filling and abandonment, with crevasse-splay type deposits, organic-rich muds, rippled fluvial channel sands, coal Delta plain sediments may be finer-grained (coal, interdistributary bay mudstone, etc.) than delta front sands

In wave-dominated deltas, mouth bars modified by wave action into linear beach ridge sand bodies Tiber delta, Italy

Tide-dominated deltas contain mouth bars that are molded into elongate, shore-perpendicular sand bodies by tidal currents Fly River delta, Papua New Guinea

Abandoned subsiding lobe “Lobe switching” cycles may be autocyclic (inherent due to delta processes) rather than allocyclic (due to external forces like base level) Abandoned subsiding lobe Active delta lobe