For high-porosity siliciclastic sediments, we find that the controls on velocity and formation factor are surprisingly different from what has been previously assumed based on low-porosity sediments. It is well known that velocity depends on both lithology and porosity at low porosities. However, well log data from Ocean Drilling Program Leg 156 to the Amazon Submarine Fan show that velocities of these high porosity (30-60%) sediments fit a single velocity/porosity relationship, regardless of lithology. The only lithology effect on velocity is indirect, associated with the different porosities of sandy and muddy sediments.