A comprehensive study of beachrock in Puerto Rico: morphology, texture, cementation and its implications in coastal processes, including coastal erosion.

Abstract

Beachrocks in Puerto Rico are millennial-scale geologic features formed in coastlines by cementation of heterogeneous coastal sediments (sand and/or gravel). The factors and mechanisms of beachrock formation and its cementation process are not completely understood. Metastable carbonate phases (high Mg-calcite and/or aragonite) serve as the primary cementing agents in Puerto Rican beachrocks. They can, however, also be formed by the precipitation of low Mg-calcite, and in rare cases, oxide cements. This research provides new geological, stratigraphic, and diagenetic information on beachrocks that can help identify regional similarities and/or differences in its characteristics and dynamics between Puerto Rico and the rest of the world. To address this goal, this research (1) produced descriptions of beachrock formation, providing geological information concerning to formation mechanisms, (2) considered their impacts (erosion/protection) on the Puerto Rican coastal zone, and (3) explored the role of beachrocks as a coastal natural barrier along the Puerto Rican shoreline. The occurrence of beachrock may contribute to coastal environmental problems by altering beach morphodynamics, modifying sediment transport processes and promoting ecological and geomorphological changes in the coastal zone. Understanding this information and the relationship between beachrock occurrence and coastal environmental problems is an important element in developing a foundation for future studies and establishing sustainable management and mitigating approaches against the impacts of coastal erosion and seal-level rise on coastal communities.<br /> <br /> For this study, a literature review on beachrocks in Puerto Rico was carried out. In addition, surveys, and applications for data collection in the field were developed using Survey 123 and FieldMaps from ESRI. We used an integration of photogrammetry and GIS (Geographic Information Systems) tools to evaluate high-resolution aerial images and analyze beachrock outcrops. Integrated techniques were applied to evaluate samples with optical petrography, sedimentological descriptions, and radiometric and isotopic analysis. The most important contributions of the study are: (1) the development of an updated map of &ldquo;hotspots&quot; of beachrock occurrences around the globe based on a literature review, (2) the generation of an updated beachrock outcrop distribution map for Puerto Rico and outlying islands, and (3) the identification of 804 outcrops distributed along the backshore, foreshore, and nearshore for submerged beachrocks. Additionally, our geospatial and sedimentological analysis of beachrock reveals the position of 366 paleoshorelines, and the geomorphological subdivision of beachrock outcrops into the inner, central, and outer face (classification based on the geomorphic classification created by Bezerra et al., 2005). According to our compositional analysis, beachrocks contain 34.81 % terrigenous material and 65.19 % of carbonate material. The petrographic evidence suggests a multi-model for the beachrock formation in Puerto Rico in which (1) direct cement precipitation, (2) mixing of marine and metric waters, and (3) formation by biological activity interact simultaneously along different sections of the outcrop. This work establishes that beachrock formation in Puerto Rico is a relatively young geologic diachronic process. This research also proposes, for the first time the ages of beachrocks at Rincón (Steps), Isabela (El Pastillo), Camuy (Peñón Amador), and Hatillo (Hatillo del Mar) beaches as 3,030 ±30, 1,660 ±30, 1,240 ±30 and 600 ±30 years B.P., respectively, placing them in the Meghalayan Age of the Holocene Epoch, based on radiometric ages of scleractinian corals and mollusks.<br /> <br /> Beachrocks analyzed in this work reveal that in some cases they can have a “negative” impact on beach morphodynamics. Our study provides additional evidence to suggest that the effect of beachrock exposure on beaches is diverse and varies according to the sedimentological and oceanographic conditions of each coastal site. This study confirms that, in some cases, beachrocks mitigate coastal erosion, whereas paleo-beachrocks and hardbottoms promote wave breaking and wave energy dissipation. Further on, outcropping beachrocks may affect the free fluctuation of the sediments, “locking” the beach profile and reducing the ability of the littoral cell to incorporate new material into the beach.