Formation of Beachrock Horizons of the Nicoya Peninsula, Costa Rica
Author:
Andrew BarnhartMentor:
Jeff Marshall, Professor of Geological Sciences, California State Polytechnic University PomonaThe Nicoya Peninsula of Costa Rica is a tectonically active convergent margin along the Pacific Rim. The subducting Cocos plate moves under the Carribean plate at a rate of 9 cm/year, generating earthquakes. Large earthquakes result in sudden changes in coastal land-level that lifts the coastline and all subsequent tides. On the coast, this uplift is evident by geomorphic formations such as wave-cut terraces, incised river channels, and on the coast, beachrock. Beachrock is a tabular horizon dipping gently seaward that is composed of carbonate-cemented beach fragments. Fluid in the groundwater excursion zone between high and low tide flows through buried beach sediment resulting in the precipitation of carbonates in pore space. Current hypotheses indicate that water mixing and evaporation of CO2 saturated fluids cause the precipitation of cementing minerals. On many sites, beachrock is observed in close proximity to tide-controlled estuary systems. It is believed that estuaries and ocean water represent end members of the local hydrologic system responsible for beachrock formation. During field-studies in the summer of 2012, two types of samples were collected. The first type was a rock sample, used for radiometric dating and for thin-section preparation. Thin-sections are viewed with a petrographic microscope, giving clast material information and cement textures, two important characteristics to determining formation methods. The second type was a water sample, collected from ocean water, estuarine water, and any seepage sites along the current beach. Chemical analysis such as alkalinity, cation and anion content, and isotope ratios can help to show fluid origin as well as the mixing relationships essential to understanding the mechanics governing beachrock formation in Costa Rica. The Nicoya Peninsula coastline undergoes uplift and subsidence with each earthquake. Once the origins of beachrock have been understood, we can use suitable outcrops as indicators to better track vertical uplift.