Fluvial Carbonates in high energy rivers: an example of cool wáter continental carbonates

Abstract

Some of the high energy mountain rivers draining carbonate catchments in the Pyrenees show carbonate coatings on the bedload and also as framestone patches, which can be included within the term tufa. However, the size of coated clasts (several dm) and the high energy of the waters make them different to the commonly described tufa or fluvial carbonates. The studied carbonates come from Orós and Oliván streams, all tributaries from the East of the Gállego River, and draining carbonate catchments within the Spanish Pyrenees. In many areas of the rivers water velocities are higher than 1 m/s. Temperatures vary along the year from 4 to 20ºC and pH is around 8. At present the coarse clastic dominated deposits of the streams are formed by pebble to boulders sourced from the Eocene Flysch. The carbonate precipitates within these streams occur as: 1) Irregularly laminated carbonate coatings (oncoids), 1-15m thick, around pebbles to boulders and phytoclasts; 2) Framestone patches formed by “in situ” coated plants occur in relatively protected areas and also in cascades; 3) Rudstones of coated phytoclasts joined together by carbonate. All the carbonate is LMC. Diatoms and vegetal debris are common in all the deposits. The laminated coatings of oncoids, framestones and phytoclastic rudstones are composed by coarse calcite and/or micrite-microspar laminae. Coarse calcite laminae/crystals are relatively thick (1 mm) and composed either of: a) bladed to fibrous crystals about 1 mm long and from 0.5 to 0.1 mm wide, b) irregular bands formed by fans of crystals containing micritic filaments. Micrite laminae constitute the first coating of any nuclei, they are dark and relatively homogenous with low porosity. Micrite to microspar laminae are very porous, the crystals are arranged following irregular lines and leaving high porosity. Micritic filaments are common. Carbonate precipitation in these mountains cold-high energy rivers is not uncommon, but rarely studied because the dominant coarse detrital sedimentation and the reduced amount of carbonate precipitates in comparisons with tufas. Carbonate sedimentation in these environments is favoured by the suitable pH and the high concentration of calcium and carbonates sourced from the hinterland. Although CO2 mechanical degassing due to turbulence is the main driving force for precipitation microbial biofilms also contributed either actively (photosynthesis) or acting as templates. The alternation of different type of laminae reflects changes in environmental conditions either seasonally or on a longer term. The location in a mountain area and wide range of water temperature along the year, very low in winter very low, suggests that these carbonates could be considered as cool water continental carbonates. Our study widens the scope of carbonate fluvial deposition and provides clues to better understand some features of the sedimentary record, such as some carbonate coatings and matrix found in fluvial coarse deposits, allowing so more precise palaeoenvironmental interpretations.