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Sedimentary facies in carbonate spring and fluvial environments in volcanic settings. Is it possible to predict porosity distribution?


Spring and fluvial carbonates (i.e. travertines and tufas) are common in continental basins worldwide and volcanic settings. These deposits have usually low preservation potentials being more identifiable in Neogene and Quaternary basins. Although there is not a common consensus in the use of the terms travertine versus tufa, a “classical” approach consider that tufas have higher porosity than travertines. The use of these terms is not easy due to two main reasons: 1) the downward change of many hot-spring carbonate deposits (travertines) to fluvial-like carbonates (tufas), and 2) diagenetic processes can take place rapidly in these deposits modifying the initial porosity distribution.

The volcanic Gran Canaria Island (Spain) contains some ravines with excellent outcrops in which the transition from hot water spring (travertines) to fluvial carbonates (tufa) can be analysed, providing so a good analogue for porosity evolution along these systems. These sedimentary environments are very discontinuous in space and they contain some components (spherulites, shrubs, coated bubbles,…) that are very specific and usually not considered in the standard limestone/porosity classifications. In addition many of the facies may occur in different subenvironments.

In the ravines of Gran Canary Island the spring/fluvial carbonate systems have three zones: proximal, medial and distal. The proximal zones are constituted by crystalline aragonite crusts deposited by feeding channels, and by aragonite + calcite waterfall deposits constituted by speleothems, framestones and laminated microfabrics. Porosity in these carbonates varies between 5% (the crusts) to 40% (the framestones). The medial areas include also waterfall and pool deposits. Coated bubbles, rafts, shrubs, spherulites and coated grains mostly of aragonite formed in quiet to agitated pools. These components leave between them high interparticle and intraparticle porosity, which ranges between 10 to 60%. Distal areas are constituted by cascade, pool and channel deposits all calcitic. Bryophyte and crystalline stem framestones deposited in cascades. Laminated mudstones deposited in quiet pools and micritic stem framestones developed in channels. Mean porosity of the sediments of these areas is 20%, being higher in the framestones.

The most common porosity types observed are: interparticle, intraparticle, moldic, fenestral, growth-framework, intercrystal (fabric selective), fracture and vug (not fabric selective), following the Choquette and Pray (1970) classification.

Identification of primary facies is key to characterise porosity evolution, because early diagenetic processes tend to occlude primary porosity mainly by cementation. Aragonite travertines have higher porosity than their calcitic counterparts. However, calcite cementation and aragonite to calcite transformation leads to a decrease in primary porosity. Other common processes such dissolution and fracturation generates secondary porosity.

The downflow general trends in this kind of systems are a) changes in mineralogy from travertine (aragonitic and calcitic-aragonitic) to tufa (mainly calcitic) and a subsequent, b) changes in primary facies from crystalline-dominated (travertine) to plant mould-dominated (tufa), c) changes in the effects of early diagenesis, from strong (travertine) to slight (tufa) diagenesis, and d) changes in type of porosity, with tufa being dominated by moldic and growth framework porosity.

Our work shows the high variability of facies and its porosity along these travertine-tufa deposits, which makes difficult to obtain an accurate model for the prediction of porosity-permeability. However the study case can be used as a good analogue for these types of deposits which are not very well know in terms or porosity predictability.