A subaqueous fan is a fan-shaped deposit formed beneath water (similar to deltas or terrestrial alluvial fans), and are commonly related to glaciers and crater lakes.
Subaqueous fan deposits are generally described as coarse to fine gravel and/or sand, with variable texture and sorting. Underflows (meltwater denser than lake water) tends to produce subaqueous fans with channels and levees.
References
Subaqueous fan Wikipedia(Text) CC BY-SA