Beachrock formation in temperate coastlines: Examples in sand-gravel beaches adjacent to the Nerbioi-Ibaizabal Estuary (Bilbao, Bay of Biscay, North of Spain)

Beachrocks are coastal sedimentary formations resulting from a relative rapid cementation of beach sediments by the precipitation of carbonate cements. These lithified structures are not usually observed at temperate settings. The present work is focused on the occurrence of a significant intertidal cementation in sand-gravel beaches formed among 43 °N latitude coastline, close to the Nerbioi-Ibaizabal estuary (Bilbao, Bay of Biscay, North of Spain). Raman micro-spectroscopy combined with SEM-EDX analyses and petrographic descriptions have been applied for the determination of the cement generations and the cemented materials compositions of the beachrock outcrops. In general terms, the cements described were: Cement Generation 1 (CG 1, aragonite, high-magnesium calcite and silicate mixtures), Cement Generation 2 (CG 2, aragonite) and Cement Generation 3 (CG 3, mixtures of CaCO3 polymorphs and iron oxides). The rest of the interstitial porosity of the rocks appeared either empty or filled with heterogeneous cemented mixtures of previously reworked compounds. The mineralogy, the regular distribution and the isopachous character of the carbonate cements together with the accurate cementation at advanced seaward bands propose a possible marine-phreatic context for the beachrock formation. However, the impure cements and the materials covering the interstitial porosity seem to be the result of both, the weathering actions consequences and the surface alterations of specific grains. Moreover, the presence of modern cemented materials (e.g. slag, bricks and pebbles) suggest a recent formation of the phenomenon.

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► Unusual setting: 43 °N latitude coastline.
► Raman spectroscopy, SEM-EDX and petrographic analyses.
► Cements described: (1) aragonite, HMC and silicate mixtures; (2) crystalline aragonite and (3) CaCO3 polymorphs with iron oxides.
► Cemented materials identified: magnetite, lepidocrocite, limonite, quartz, hematite, goethite, rutile, etc.
► Modern materials incrusted in the beachrock formations suggest a recent formation of the phenomenon.
► Accurate cementation at advanced seaward bands propose marine-phreatic context for the beachrock occurrence.