Significado paleoambiental de los ooides y oncoides de las calizas del Barranco de Azuaje (Gran Canaria)

Abstract

Varied types of ooids and oncoids have been found in travertines and tufas along 3 km of Azuaje Ravine in the volcanic island of Gran Canaria. Ooids show spherical to ellipsoidal shapes, 0.1 mm->2 mm in size, and regular concentric smooth cortices. Oncoids are 0.4 mm to several millimetres in general, with slightly lamination, and generally irregular shapes. Mineralogy varies from aragonite and aragonite-calcite in travertine, to calcite in tufa. Nuclei in all cases are intraclasts, plant part moulds, or they are undistinguishable from the cortex. Coated grains layers commonly pass laterally to stromatolites, but also to other facies (rafts, hanging coated stems, bryophytes, etc.). In general terms, and considering the characteristics of nuclei and cortices, facies associations, and their distribution, coated grain deposits would represent several stages of: (a) availability of loose particles that serve as nuclei, (b) formation of coatings, (c) immobilization of the grains and (d) fossilization of the deposit as a consequence of crusting on it. All these stages would be related to specific physical, chemical, and biological conditions, and their variations, responding to the sedimentary dynamics of the ravine.