The objective of this research was to characterize and determine the provenance of beach sands from selected locations on the island of Guam, by comparing their composition with the geological structure and stream network of the hinterland. This comparison was based on analyses of twelve beach sand samples and seven samples of potential source rocks using X-ray fluorescence (XRF), X-ray diffraction (XRD), petrographic and reflected light microscopy and on GIS-based drainage network maps.
Based on the results, the samples were divided into three distinct groups, with their provenance related to the geological setting, surface drainage network, and sediment supply from recent coral reefs. The carbonate group comprises predominantly bioclastic sediments collected in the northern and southeastern parts of the island, where the hinterland consists of soluble carbonate bedrock and lacks surface streams, thus limiting terrigenous material input and making the adjacent coral reefs the main sediment source. The terrigenous group comprises volcanic sediments collected in the southern part of the island, where their composition reflects the local volcanic and volcaniclastic rocks, including andesites, basalts, and tuffs, as well as the influence of a well-developed surface-stream network that transports terrigenous material to the coast. The mixed group consists of two samples collected in the southwestern part of the island, where carbonate material from coral reefs and terrigenous material from volcanic and volcaniclastic rocks converge along the coast. These samples therefore represent a mixed sedimentary environment between the carbonate and terrigenous groups. Local hydrodynamic sorting was also identified in the swash zone, where grains are separated according to their density, size, and shape, potentially resulting in the accumulation of denser minerals, particularly magnetite and pyroxenes, as well as foraminiferal tests.
These findings improve the understanding of beach sediment provenance, distribution, and compositional controls along Guam’s coast, and may also help interpret coastal sediments on similar islands.
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