along 3 km of Azuaje Ravine in the volcanic island of Gran Canaria (Canary Islands, Spain). The
systems show a clear evolution from travertine to tufas along the Ravine. Coated grains have
been grouped into ooids and oncoids based exclusively on their characteristics
Ooids show regular concentric smooth envelopes and 0.1 mm-2 mm in size. Three main types of
cortex have been distinguished: (i) radial fibre or acicular crystals comprising the whole cortex
diameter and crosscutting the lamination, (ii) banded-radial arrangements with alternating
layers of radial fibres and micritic layers, and (iii) micritic. Nuclei are intraclasts, peloids,
spherulites, small plant part moulds, or may absent by dissolution, or undistinguishable from the
cortex. Ooids are commonly spherical to ellipsoidal. Frequently two or more individual coated
grains agglutinate and form compound grains. Mineralogy varies between aragonite and
aragonite-calcite.
Oncoids are 0.4 mm to several millimetres and exceptionally tens of centimetres (nuclei of palm
tree leaf moulds). They display two main types of coatings: (i) thin, irregular, dense micritic
laminae, and (ii) generally thicker porous laminae usually made of dendritic, shrubby, or
columnar (branched) crystal aggregates, containing alternating lamination. Lamination is
slightly wavy to mammillated. Nuclei are similar to those of the ooids, being intraclasts and
plant moulds (leaving moldic porosity) the most common. Shapes are varied and generally
irregular depending strongly on the shape of nuclei. Mineralogy varies from aragonite, to
aragonite-calcite mixtures, and to exclusively calcite in tufa.
Diagenetic changes are widespread, being apparently more intense in travertine than in tufa
facies: aggrading recrystallization, aragonite inversion, dissolution, and cementation are the
most common processes.
δ13C-δ18O plots from each individual deposit show positive covariant trends. Isotope signals of
coated grains are indistinguishable from other of the same deposit. δ13C decreases as the
amount of calcite increases which suggest: a) that primary aragonite facies are heavier in
carbon than those composed of calcite, and ii) diagenetic transformations deviate the primary
signals to heavier 13C values.
Ooids are restricted to travertines. Oncoids are ubiquitous, but changing strongly their features
in downstream direction. Tufa deposits placed downstream only content oncoids with intraclasts
and large plant moulds as nucleus, as well as thicker and more irregular coatings, than those
found in upstream travertines.
Coated grains formed with low to no displacement, as is suggested by their high diversity and
the uncommon occurrence of deposits with mixed grains. Scarcity of detrital clasts as nuclei of
coated grains suggests low clastic input from upstream but also from ravine walls. These may
be related to slope stability, but also with the absence or attenuation of floods.
Restricted distribution of ooids, sharp changes in oncoid type, mineralogy, isotopes and
diagenesis suggest an abrupt change downstream in environmental conditions reflected in
changes in vegetation and CaCO3 precipitation rates and mineralogy.This study was funded by Ayuda Grupos de Investigación-2014 GR3/14 granted to Research
Group 910404-Petrología Aplicada al Análisis de Cuencas y a la Conservación del PatrimonioPeer reviewe
