A strongly dendritic submarine canyon system with four major canyons occupies a 30-km indentation in the insular shelf off the south coast of Puerto Rico between Guanica and Ponce. Each canyon has several headward branches at depths of 100 to 1,100 m. Each of the five major rivers that reach the coast between Guanica and Ponce is opposite a canyon head, and off each of the rivers a channel 20 to 30 m deep is incised into the insular shelf. The entire canyon system is 40 km long and terminates at a depth of about 3.3 km; no fan or other constructional feature appears to have been formed. A 410-km continuous seismic profiling survey shows that three unconformable stratigraphic units underlie the canyon area. One sample of indurated clay of probable Miocene age was dredged from a canyon-wall outcrop
of the middle stratigraphic unit. The three stratigraphic units may be roughly correlative with (in ascending order) the Juana Diaz Formation,
the Ponce Limestone, and an unnamed upper section found in a nearby drill hole ashore. Canyon-axis sediment at depths of 700 to 2,200 m is entirely of pelagic origin, indicating that sediment is not now being transported down the canyons.
