Beachrock characterization and historical shoreline changes in selected locations along the coast of Puerto Rico

Abstract

Beachrock is characterized by rapid cementation, is exposed mainly in the intertidal zone, and it can provide a record of sea level fluctuations, episodic events, and serve as an archeological proxy. The presence of beachrock has been primarily reported along the northern coast of the island of Puerto Rico. Most studies on the beaches of the island have not considered the history and evolution of the exposures nor their petrographic character. Samples from the intertidal zone (swash zone), as well as unconsolidated sediment associated with the beachrock, were collected at seven locations in the island. Petrographic characterization of the beachrock was done using thin sections and the mineralogical composition was described. Also, the geospatial distribution and historical changes in the evolution of beachrock exposures were studied using aerial photography for an interval close to 80 years. The dominant mineral composition of the beachrock was calcite. The terrigenous minerals in the rock are attributed to the presence of rivers systems and their inputs into the beach environments. Micritic, blocky and rim cement morphologies are similar to other studied locations from tropical areas (i.e., Bahamas). The cements present in our samples suggest beachrock formed under marine vadose and/or marine phreatic conditions. Well cemented beachrock in Punta Borinquen seems to have been formed or altered under phreatic conditions. The uppermost layer in the Steps beachrock, Rincón has evidence of subaerial diagenesis in the form dominant vadose morphologies. The geospatial analysis of the surface area of exposed beachrock through time suggests the morphodynamical patterns and rate of exposure of the beachrock are not related. The presence of archeological artifacts in Jayuya beach and wood in Escambrón samples demonstrate rapid cementation and recent beachrock formation. Our analysis suggests that beachrock can occur in either high or low energy environments and that submerged paleo-shorelines can be identified along the coast. Understanding the cemented beach helps to gather information on coastal and drainage basin changes, archaeology and coastal management.