Revisiting Beachrock Cementation Processes

Abstract

This project aims to produce a conceptual model of cementation in beachrock formed in high energy, high precipitation, normal salinity coastlines. Early cementation of beachrock provides a protective layer against erosion while preventing cementation of adjacent and underlying units, potentially preserving primary porosity and permeability. Beachrock, although a common modern cementation phenomena, is rarely identified in the rock record. A better understanding of beachrock geometries, and cement distribution, could be key to understanding and identifying ancient analogs and potential reservoirs. Three sites along the northwestern coast of Puerto Rico have been chosen for in-depth analysis in the cities of Aguadilla and Hatillo in the north, and Aguada on the west. The sands in these sites consist of mixed carbonate-siliciclastic grains. Groundwater seepage at the land/sea interface is expected at all these sites since they lay on the rainy side of Puerto Rico. These sites and respective core samples are being used to conduct geochemical analyses, petrographic analyses, and geophysical mapping of deposits with the goal of defining the fluid conditions conducive to modern cementation of beachrock. This work also examines the biota present at the sites to better understand their influence on the cementation process. The chosen sites exhibit a diverse range of biota with a notable shift between green algae dominance landward and red algae dominance seaward. Bioerosion by organisms such as sea urchins, gastropods, and chitons is common, producing rounded pits that sometimes preserve modern artifacts (e.g. glass, concrete pebbles or blocks, metallic strike plates) in recently cemented sand. Preliminary geochemical analysis shows a mixed system with varying marine influences. Beachrock most proximal to sea has strongest marine influence, and freshwater influence increases with depth. Beachrock generations vary at a centimeter scale as opposed to the classically perceived, quarter meter to half meter scale beds. Four cement types are dominant: fibrous, dentate, micritic, and pelleted organic rich. Many grains have multiple generations of cement, most commonly the generations are of the same type, but occasionally generations are of different cement types (i.e. pelleted organic rich followed by fibrous cement, micritic followed by fibrous). Cementation is generally most prevalent at the top of the section, and decrease with depth.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017