ABSTRACT

The steep, narrow (0.5 to 4 km wide) shelf off the Rio de la Plata on the north coast of Puerto Rico is a site of equilibrium shelf sedimentation. Recent sand and mud can be readily distinguished from relict material on the basis of color, texture, and carbonate fraction physical condition. Recent material contains abundant rock-fragments; relict sand is made up of worn calcareous particles. Boundaries between sediment types are very sharp. They usually occur within a 200-m-wide transition zone. No mixing of the relict calcareous central-shelf sand with the recent rock-fragment beach sand is evident.

Fluvial sediment is contributed by river-mouth bypassing to this allochthonous shelf during floods only. Sand travels and is deposited in a band across the shelf to a depth of about 20 m and is then moved laterally both east and west. River sediment is also contributed to a beach-innershelf band east and west of the river mouth. Mud travels across the shelf to depths beyond 40 m, whereupon it either moves laterally on the outer shelf or moves seaward into a canyon system. The inner shelf is a zone of sand deposition and mud bypassing. The bypassing process is termed "mud hopping." Mud is initially deposited after major floods as a thin blanket across the shelf and is subsequently moved seaward by smaller storms in a series of hops.