VOLCANIC ORIGIN OF HOLOCENE BEACH RIDGES ALONG THE CARIBBEAN COAST OFCOSTA-RICA

The formation of beach ridges along the Caribbean Coast of Costa Rica, ranging in age between about 100 and 5000 years, appears to be relate d to discontinuous sand supply by volcanic eruptions in the Costa Rica n Central Cordillera. This is indicated by consistent and statistical significant differences in chemical composition between sediments of i ndividual beach ridges, which cannot be explained by textural differen ces. The differences are thought to be caused by variations in magma c omposition of different eruptions in the volcanic hinterland. SiO2 con tents of the ridges vary between about 52% and 59%, other elements sho w variations corresponding to magmatic differentiation. Petrographical ly this is expressed in variations in the amount of andesitic rock fra gments and pyroxene grains. Chemical composition of beach ridge sedime nts is alike in composition to the erupted products. However, in spite of the removal of part of the mobile elements and mixing with weather ed sediments, original differences between the erupted products appear to remain reflected in the resulting beach ridge sediments.