Beachrocks from the island of La Palma (Canary Islands, Spain)

摘要

Beachrocks on La Palma Island developed on platform-forming lavas of the Cumbre Vieja volcano. Some of these lavas are related to the 1585 (Puerto Naos), 1677 and 1971 (Echentive) eruptions. Radiocarbon dating of the Charco Verde beachrock gives a conventional age of 33 330 +- 490 BP, while that at Playa Chica beach gives a calibrated age of 14940 +- 525 BP. The beachrocks, up to 1.5 m thick and some tens of metres wide, consist of several decimetre-thick horizons dipping 2-15 deg seaward. Petrographically, they can be classified as rudstones and arenites, with volcanic clasts as their main component. The original porosity of the beachrocks was intergranular (and occasionally intragranular) and was partially occluded by cementation and locally by internal sediments. The main cements are fibrous aragonite and micrite high-magnesium calcite (HMC). Spar aragonite, peloidal HMC and microbotryoidal HMC are scarce. The elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters. The evolution of beach deposits, and especially the beachrocks in La Palma island, follows three stages: (1) beach deposition, (2) beachrock formation, and (3) beach retrogradation and/or erosion. The studied beachrocks prompt us to make some important considerations. (1) The mean tidal range in the Canary Islands has not varied over the last thousand years. (2) The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years. (3) Volcanic activity supplies the sediment that forms the beaches. (4) A dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6-11 h/day) favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches. (5) The presence of beachrocks in the La Palma beaches prevents the total disappearance of the beaches.