Composition and provenance analysis of beach sands in an almost isolated sedimentary system – A field study of the Galápagos Archipelago

Abstract

The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.