Tufa associated with karst features in a fracture-system fed by meteoric water, Araripe basin, NE Brazil

Karstic tufa from the Brazilian semi-arid zone offers a key opportunity to study the depositional evolution, unveiling the past climate history with examples generated outside of the phreatic zone. Morpho-petrological identification, in addition to a structural survey, petrographic analysis, X-ray diffractometry (XRD), scanning electronic microscopy (SEM), and measurements of δ18O and δ13C, provided elements to explain the occurrence of tufa associated with a fracture system. Three sedimentary facies were identified, one allochthonous (detrital) and two autochthonous (in situ). Field recognition and structural analysis showed that tufa occurs only in the deformed portion of the Crato Formation, at the Triunfo Fault splay sector. Facies formed in an open fracture-controlled hydrological system during rainfall periods. The infiltration of meteoric water into the limestones of the Crato Formation facilitated the dissolution of the parent rock through time, with the percolation controlled by open fractures in tilted blocks. Breccia tufa developed in the early stage of the karstification by carbonate clast forming from the limestone, previously collapsed from the walls and later cemented by percolation of solutions enriched in calcium carbonate within fractures. The two autochthonous facies developed in later stages through continuous percolation of water, rock dissolution, bryophyte-microbial growth, soil development, and calcium carbonate precipitation in progressively more humid conditions. Thin section analyses and SEM imaging revealed calcite crystals closely related with organic structures. The association between tufa and microbial filamentous organisms (likely cyanobacteria) indicates an active biogenic role for the tufa origin. Calcite stable-isotopes analyses indicated tufa as a product of meteoric water as supported by δ18O isotope. Changes of the carbon ratios through time indicated influence of soil development and probably C3–C4 photosynthetic activity. An integrated structural, sedimentological, petrological, and geochemical approach provided elements to define these tufa deposits as formed in a fracture-system fed by meteoric water, as result of the post-Mesozoic denudation process that affected the Araripe Basin.