"Beachrocks" de La Palma, Islas Canarias

Resumen

The "beachrocks" studied crop out in the southwestern part of the island of La Palma (Canary Islands), specifically in the Puerto Naos, Charco Verde, Las Zamoras, Chica and Echentive beaches. All these beaches are developed on the lavas of the Cumbre Vieja volcano, corresponding to different eruptions with ages from 20.000 to less than 30 years. The beachrocks are up to 1.5 m in thickness, attain some tens of metres in width and consist of several decimetric-thick horizons, dipping 2-15s seaward. The beachrocks are rudstones and arenites, with volcanic clasts as main components. The porosity of the beachrocks is intergranular and locally intragranular, both partially occluded by cements, neomorphic texture and locally by internal sediment. The beachrocks are cemented by different types of aragonite (fibrous, microfibrous and botroidal) and HMC (micritic and peloidal) cements. The HMC micritic cement constitutes the first cementation phase. The aragonitic cements, with a very high content of strontium (with a mean of 10.000 ppm) and a high content of sodium (with a mean o f2.000 ppm), constitute the second phase of cementation. The peloidal cement is the third phase of cementation. The oxygen isotopic composition of the aragonite cement ranges from -4.2 to -2.4 %o PDB and the carbon isotopic composition of the fibrous aragonite cement varies from +4.0 to +4.9 %o PDB.