PUERTO RICO BEACH SYSTEM

The coastal zone of Puerto Rico is remarkably diverse. Unlike the shorelines of many major continents, there are no long interrupted stretches of basically similar beach. The beaches of Puerto Rico are relatively short and are divided into separate and distinct beach systems that have restricted communication with one another. Each is a closed or semi-closed unit receiving its supply of sediment from limited local sources and transmitting little of its longshore moving sand to another beach system. In analyzing the sand and beach systems to determine the extent of isolation, mineralogy, metal content and biogenic constituents have been studied; also the actual physical parameters of separation have been analyzed.

The sources of beach sand are relatively limited and the total available volume of sand that can be added to the system is not very large. These sources include:

offshore sands

erosion of land biogenic material

DRAINAGE BASIN AND BEACH COMPOSITION

The beaches of Puerto Rico contain sand grains derived from several major sources. Erosion of land areas and transport of rock material by rivers to the beach supplies terrigenous sand grains. The composition variances are a function of the source area where the material is eroded and added to the river sand system. Areas of river drainage underlain by basaltic type rocks supply dark minerals and dark igneous rock fragments. Granite rock outcrops supply quartz and feldspar to the beach. Feldspar is a relatively unstable mineral that breaks down to clay minerals. The presence of feldspar in a beach system is indicative of a geologically young environment.

Calcium carbonate is supplied to the beach by the shoreward transport of the shells of marine organisms. The composition changes in calcium carbonate and terrigenous material are a function of available supply and transport system.

Along parts of the coast containing major rivers, there is an increase in terrigenous content in the beach sands. There is also a shift toward terrigenous beaches where an offshore carbonate source is lacking. By analyzing the availability of supply we can draw conclusions about the transport system and beach dynamics. Larger concentrations of calcium carbonate can indicate shoreward transport. In an area such as Guanajibo Beach (Mayagüez ) the absence of carbonate grains in the beach sands coupled with the presence of reefs just offshore shows that shoreward transport of beach material is negligible. This is part of the reason for the erosion on this relatively protected beach environment.