Sedimentology, Hydrology, and Climatic Environment of Alluvial Fans on Earth and Mars

A global survey doubles the number of previously identified alluvial fans and deltas on Mars. Alluvial fans are far more widespread than previously reported but are not uniformly distributed. Fans within craters show a favored N-S orientation, but there is a lack of correlation between fan morphometry and local geography or topography. An investigation into the sedimentology of the alluvial fans in Saheki crater provides insights into the climatic environment during deposition. Fan forming discharges consisted of the channelized transport of primarily sand- and gravel-sized sediment as bedload coupled with extensive overbank mud-rich flows depositing planar beds of sand-sized or finer sediment. A comparison with fine-grained alluvial fans in the Chilean Atacama Desert constrains the interpretation of processes responsible for constructing the Saheki crater fans. Flow events are inferred to have been of modest magnitude (probably less than ~60 m3/s), of short duration, and probably occupied only a few distributaries during any individual flow event. The most likely source of water to form the martian fans is inferred to be snowmelt released after annual or epochal accumulation of snow in the headwater source basin on the interior crater rim. Crater count derived ages of the fans in Saheki crater yield an age of early Amazonian, and a vast majority of fans across Mars superpose Hesperian or Amazonian terrains.

A new landform evolution model is presented that simulates the formation of fine grained alluvial fans. The model generalizes the hydrodynamics of flow and sediment transport while incorporating multiple sizes of sediment, channelized and overbank sediment deposition, and multiple active distributaries. The sediment depositional model is based on interpretations of alluvial fans in Saheki crater and the Chilean Atacama Desert. When subjected to variant boundary conditions such as input discharge and sediment load, the model responds in a similar manner to natural alluvial fans, indicating that it can provide qualitative and quantitative insights into linkages between alluvial fan processes, morphology, and stratigraphy.

A series of previously unstudied fans along the Hilina Pali escarpment on the Island of Hawaii are characterized by their steep slopes, coarse grain sizes, and lobate surface morphology. On the basis of exposed stratigraphy, geologic setting, and calculated flow discharges the fans are interpreted to be forming predominately from sieve lobe deposition, a style of bedload transport that has fallen out of use in the alluvial fan literature due to a perceived lack of field examples.