Marta Vázquez-Urbez

Chapter 3 Fluvial and Associated Carbonate Deposits

Abstract Fluvial and associated carbonate deposits are common nowadays and in the Quaternary record, but are less abundant further back in time. At present, they develop under climate conditions ranging from cool temperate to arid. A complex interaction among climate, tectonics, topography, hydrology and hydrochemistry controls their development. Their formation is primarily related to calcium- and bicarbonate-rich waters, the presence of aquatic vegetation and sufficient water recharge from carbonate aquifers. Physico-chemical and biological factors control calcite precipitation. Mechanical CO2 outgassing is enhanced under various sedimentary conditions. Texture, sedimentary structures and geometry allow distinction of a wide variety of carbonate facies, as well as asssociated fine and coarse clastic and organic facies. Their associations in simple vertical sequences record the evolution of the several fluvial, lacustrine and palustrine subenvironments through time that results from aggradation, progradation, retrogradation and lateral migration processes. Two broad environmental contexts encompass the formation of these deposits. These are (1) low-gradient, non-stepped fluvial and fluvio-lacustrine conditions that include low-sinuosity and meandering river styles, wide floodplains and ponds or lakes; rudstones of oncoids and rudstones and boundstones of coated plants can be the most abundant facies, and give evidence of shallow channels, lake shores and extensive palustrine environments and (2) high-gradient and stepped fluvial conditions, with generally narrow transverse profiles and a limited extent of floodplains. Depositional environmental settings comprise: springs, waterfalls, cascades, barrages, caves, gently sloped, low-sinuosity river stretches, dammed areas and palustrine zones. Spectacular moss buildups form in waterfalls and cascades related to barrages and topographic breaks. Stromatolitic facies are typical of the fast-flowing water stretches between topographic breaks, whilst lacustrine facies are dominant in areas of low current velocity between barrages. Both situations can exist in a single system, although one is usually dominant. Carbon- and oxygen-isotope compositions of fluvial and associated carbonate deposits provide environmental information concerning temperature, humidity, soil and vegetation, as well as information about the CO2 source in carbonates, characteristics of the catchment areas and hydrological features of underground and surface-water systems. Most deposits have a wide range of δ13C and a narrower range in δ18O, and therefore covariance between these is low. Better covariant trends are related to a longer residence time of water, e.g. in ponds and lakes. Some laminated facies record periodic variations in δ18O related to seasonal changes in temperature, and generally are not parallel to δ13C variations.

Benthic bacterial diversity from freshwater tufas of the Iberian Range (Spain)

Aiming to characterize the bacterial diversity of modern tufa systems of the Iberian Range (Spain), we surveyed the 16S rRNA gene sequence diversity from 24 sites within three rivers (Añamaza, Mesa and Piedra). These tufas record substantial calcareous growth under different physicochemical conditions and are part of an important, regional landscape-building system. The bacterial community structure and composition, richness and diversity were quantified from denaturing gradient gel electrophoresis fingerprints. Retrieved DNA sequences could be assigned to 10 bacterial phyla and included a variety of phototrophic and heterotrophic groups. Cyanobacteria, mainly filamentous taxa, constituted 43% of all the retrieved sequences, followed by Firmicutes (11%), Gammaproteobacteria (10%), Alphaproteobacteria (7%), Acidobacteria (6%), Bacteroidetes (5%), Betaproteobacteria (4%), Planctomycetes (4%), Actinobacteria (3%) and Deltaproteobacteria (2%). Diatom and Xanthophyceae chloroplast sequences were also detected. Physicochemical variables measured at each site were modelled with multivariate statistics. Principal component analyses yielded the highest variance for salinity-related variables (conductivity; Na(+) , Cl(-) and SO4(2-) concentrations), which correlated negatively and significantly with diversity indices. However, the highest variance explained by individual principal components was relatively low (< 34%). Overall, we show that these young fluvial tufas are inhabited by a large variety of bacteria in diverse and widespread communities.

The holocene tufa barrage system of Las Parras de Martín (Iberian range, Teruel)

RESUMEN. En el valle del rio Las Parras (Cuenca alta del rio Martin, Cordillera Iberica turolense) aparecen importantes registros tobaceos holocenos. El perfil longitudinal del rio muestra dos fuertes rupturas de pendiente coincidiendo con la salida de sendos estrechamientos conocidos como los hocinos de las Palomas y del Pajazo. En estos puntos se encuentran las acumulaciones tobaceas mas potentes, 49 m en el Hocino de las Palomas y 26 m en el Hocino del Pajazo, con facies que indican un crecimiento rapido en un sistema de cascadas y barreras que se desarrollan vertical y lateralmente aguas abajo. El resto del tramo estudiado presenta pendientes suaves (2%) con depositos tobaceos fluvio-lacustres, de morfologia aterrazada de gran desarrollo longitudinal y menor potencia (hasta 8 m en El Batan), asociados con represamientos. El conjunto morfosedimentario corresponde a un sistema fluvial escalonado compuesto por cascadas y areas represadas. Los datos cronologicos indican que las tobas holocenas del rio Las Parras se formaron entre 9.9 y 3.1 ka, con un periodo de mayor desarrollo entre 5.5 y 3.1 ka. En los ultimos 2000 anos se produce un descenso brusco en la actividad tobacea en la zona que coincide con un periodo de minima insolacion estival y probablemente con una menor disponibilidad hidrica en la zona. The Holocene tufa barrage system of Las Parras de Martin (Iberian Range, Teruel)