3-D modelling of a fossil tufa outcrop. The example of La Peña del Manto (Soria, Spain)

Classical studies of tufas lack quantitative outcrop descriptions and facies models, and normally do not integrate data from subsurface in the stratigraphic and evolutive analysis. This paper describes the methodology followed to construct one of the first digital outcrop models of fossil tufas. This model incorporates 3-D lines and surfaces obtained from a terrestrial laser scanner, electric resistivity tomography (ERT) profiles, and stratigraphic and sedimentologic data from 18 measured sections. This study has identified seven sedimentary units (from SU-1 to SU-7) which are composed of tufa carbonates (SU-1; 3; 5; 6) and clastics (SU-2; 4; 7). Facies identified occur in different proportions: phytoherm limestones of bryophytes represent 43% of tufa volume, bioclastic limestones 20%, phytoherm limestones of stems 12%, oncolitic limestones 8%, and clastics 15%. Three main architectural elements have been identified: 1) Steeply dipping strata dominated by phytoherm limestones of bryophytes; 2) gently dipping strata dominated by phytoherm limestones of stems; and 3) horizontal strata dominated by bioclastic and oncoid limestones. The alternation of tufa growth and clastic input stages is interpreted as the result of climatic changes during Mid-Late Pleistocene.