Grain Size Controls on the Facies Sequences and Clinoforms of River-Dominated Deltas

Four external variables are thought to control delta geometry and stratal architecture--grain size, waves, tides, and river discharge--but teasing apart the relative contributions of each has been difficult. The number of distributary channels covering a delta is significantly correlated with maximum monthly discharge, but part of this correlation could be due to the fact that rivers with higher discharges (e.g., Amazon, Mekong, and Orinoco) deliver finer-grained sediment to their deltas. Coarse-grained sediment is thought to produce fan deltas with more continuous sandbodies and steeper clinoforms, but these deltas often occur in regions of higher wave power. Here we investigate the affect of sediment size on the shapes, number of active distributaries, internal facies, and clinoform architecture of river-dominated deltas using Delft3D v. 3.28, an engineering-quality, morphodynamic model. Thirty experiments simulate self-formed delta growth from a sediment-laden river entering a standing body of water devoid of waves, tides, and buoyancy forces. At the inlet boundary a steady, uniform discharge of 1000 m³ s^-1 carries varying size distributions of both cohesive silt and noncohesive sand grains, each in equilibrium concentrations. Results show a variety of self-formed deltas are generated by the same three processes observed in field-scale deltas: 1) channel bifurcation around stagnant river mouth bars; 2) subaqueous dissection of the mouth bar and the levees; and 3) subaerial channel avulsion. Furthermore, the partitioning of discharges down distributary arms by subtle bed adjustments at bifurcations produces discharge ratios between bifurcate pairs that are similar to field deltas. A finer sediment feed produces bird’s foot-like deltas with rugose shorelines and rough floodplains because levees aggrade rapidly and confine the flow, thereby promoting rapid channel progradation. Delta clinoforms show a lower range of dip direction and lower dip angles. Sand bodies are disconnected shoe strings encased in mud. A coarser-grained, less cohesive sediment produces fan-like deltas with smooth shorelines and floodplains because levees are easily incised and the flow is more uniformly distributed over the delta topset. Clinoforms show a higher range of dip direction and higher dip angles. Sand bodies are coarsening-upwards continuous sheets. Clinoform data from the Cretaceous Ferron Last Chance delta are consistent with these trends.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California