Abstract

CANTERA-KINTZ, Jaime R.. Bioerosion patterns in Tertiary rock cliffs of the Pacific coast of Colombia (Eastern Tropical Pacific). Rev. acad. colomb. cienc. exact. fis. nat. [online]. 2016, vol.40, n.155, pp.288-299. ISSN 0370-3908.  https://doi.org/10.18257/raccefyn.286.

The degree of internal Bioerosion of five tertiary sedimentary cliffs was examined in three tidal levels (high, medium and low) located in two bays of the Central region of the Pacific coast of Colombia, Tropical Eastern Pacific. The objective was to estimate bioerosion patterns and understand the principal factors determining spatial and temporal variations. The cliffs are composed by layers of soft sedimentary rocks (limestone) alternated with hard sedimentary rocks (shale, sandstones). Plate tectonics, high seismic risk, and processes occurring in the coastline heavily influence geology and geomorphology of the central region of the Pacific coast of Colombia. The region consists of Quaternary alluvial sediments of Pliocene and tertiary cliffs of sedimentary rocks (sandstones, mudstones and shales) of the Mayorquín and the Naya formations. Due to the high slopes and the high fracturing of the rock, non-biologic erosion is caused primarily by runoff and waves, which produces planar mass movements in different scales. It is also common the occurrence of falling blocks, mainly in the coastal cliffs. Main borers of soft rocks were bivalves of the family Pholadidae: Cyrtopleura crucigera, Pholadidea spp. and the crustacean Upogebia spp. Hard rocks were bored by mytilids species: Lithophaga aristata, L. plumula, and by the sipunculid Phascolosoma sp. Major factors controlling bioerosion are in order of importance: hardness and nature of rocks, tidal level, abundance and distribution patterns (zonation) of benthic organisms associated to cliff fauna, and wave action. Total internal bioerosion displayed high values in low tidal levels and soft rocks of cliffs with moderate wave action. Bioerosion declines in externally located hard rocks of the two studied bays despite their exposure to higher hydrodynamics. Higher population densities of borer species were recorded in cliffs located in the internal parts of the bays and in the lower tidal levels. Bored volume was significantly different among tidal levels but not among stations. This was positively correlated with number of species and individuals. Measurements of rocky material loss during the 12-month study period showed that cliff retreat varied between 4.2 cm y^-1, in a hard-rock cliff located on an island in the outer region of the Malaga Bay to 13.2 cm y^-1 in a mixed-rock cliff of the outer region of the same bay. These rates are high compared to other estimates based on erosion by organisms but are far lower than rates obtained in broad scale studies. Temporal variations would be attributed to local oceanographic conditions, particularly extreme tidal range, strong sea currents and intensity of wave action.

Keywords : Benthic fauna; bioerosion; rocky cliff retreat; Tropical Eastern Pacific; zonation.

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