Fifth Meeting of Post-Graduation in Physics and Earth Sciences of University of Évora (PGUE2011) Perched springline tufa in Algibre flexure northern slopes Paulo Guerreiro (1,*) , Carlos Ribeiro (2) , Lúcio Cunha (1) , (1) Centro de Estudos de Geografia e Ordenamento do Território, U. Coimbra, (2) Centro de Geofísica de Évora, U. Évora * pauloguerreiro@gmail.com Abstract In Central and Eastern Algarve limestone country (known as “Barrocal”), tufas are quite common next to springs, some of those outcrops are extensive like at Alte, Cadouço, Rio Seco or Asseca Streams. Tufas are carbonate features associated to calcite supersaturated waters with a high pCO2 level, due to high concentrations on soil atmosphere that bonds to percolating water and enhance carbonates dissolution [1] . After springs, there are organic and inorganic processes which decreases this pCO2, the solution turns supersaturated and there is an intense CaCO3 sedimentation processes. Different sedimentation processes drive to an assemblage of facies that characterize each tufa system, not only by precipitation rates but also organism frameworks. The present case study focuses about perched springline [2] classification scheme) that exist in the Algibre flexure northern slopes, which drains to Mercês. Those springs recharge area is Algibre flexure, especially in the highest hills and low gradient areas. In the first, exist predominantly bare and soddy karst (e.g. Cabeça Gorda) and in the second some suffusion dolines on colluvium and sinkholes (e.g. Campina de Galegos doline field). The incrustating waters springs associated to the main outcrops are rather elevated than the main stream, because Triasic lithologies act as an impermeable barrier. This way, high gradient stream and rocky talweg enhance turbulence and CO2 degassing improve a fast calcium carbonate supersaturation and hence sedimentation [3] . Once those springs are located on low surface runoff country (even when stream’s topographical basin is quite large, because it’s a karst area), deposits development is easier. Perched springline deposits are developed in such conditions, characterized by proximal and distal deposits [1] . Proximal deposits are low gradient surfaces in springline downstream, where water runs in sluggish flow, often separated in many secondary streams, and some ponds are present. Olho de Paris spring (W of Amendoeira) is the biggest and better characterized, however it has long been occupied for agricultural purposes and drainage is somewhat organized by deteriorated anthropic levees. High slopes in the edges provide cascades and small pools where exist high sedimentation rates, probably enhanced for microbiological (algae and bacteria) and mosses, producing laminar biohermal tufa [4] . In the small pools between cascades, accumulates loose silt lime and the highest incrustation rates are in the last pool before Mercês stream. There exist distinctive environments on the proximal deposits, and we suggest considering cascade deposits as intermediary. Distal deposits don’t exist in this study area, because after cascades instead of a braided flow, there is Mercês stream which drain also a big area with low permeability on northern turbidites. Other important outcrop next to Penedos Altos, which incrusting waters are nowadays of diffuse discharge. Cascade deposits are the most important in this area with high verticality to NW with some cavities. Despite the absence of proximal and distal deposits, this outcrop has three plain levels stepped, two of them mainly composed by tufa. Nevertheless, the intermediate level tufas are more compact than the higher and probably earlier one. Tufas are also present in other valleys/slopes, however are poorly developed, just like some consolidated strata in the main valley floor. Different tufa and terraces positions in the valley geometry, provide palaeoenvironmental information about deposition cycles, which are climate controlled. These cycles are studied in Spain and other regions in throughout Europe, and its comprehension and articulation with other sedimentary deposits could be used to complete the stratigraphical Holocene province and regional event effects [5] . References [1] Pedley, M.: Classification and environmental models of cool freshwater tufas, Sedimentary Geology, 68 (1-2), 221-246, doi:10.1016/0037- 0738(90)90124-C, 1990. [2] Pedley, M.: Tufas and travertines of the Mediterranean region: a testing ground for freshwater carbonate concepts and developments, Sedimentology, 56, 221–246, doi: 10.1111/j.1365-3091.2008.01012.x, 2009. [3] Chaftez, H.; Folk, R.: Travertines: depositional morphology and the bacterially constructed constituents. J. Sed. Petro., 54 (1), 289-316, DOI: 10.1306/212F8404-2B24-11D7-8648000102C1865D [4] Guerreiro, P.; Cunha, L.; Ribeiro, C.; Candeias, A.: Tufos calcários das áreas de Estoi, Loulé e rib.ª das Mercês (Algarve, Portugal): caracterização e significado paleoambiental, e-Terra, 21 (7): http://e-terra.geopor.pt [5] Björk, S.; Walker, M.; Cwynar, L.; Johsen, S.; Knudsen, K-L.; Lowe, J.; Wohlfarth, B.; INTIMATE Members: An event stratigraphy for the Last Termination in the North Atlantic region based on Greenland ice-core record: a proposal by INITMATE group, J. Quat. Sci., 13 (4), 283-292, doi: 10.1002/(SICI)1099-1417(199807/08)13:4<283::AID-JQS386>3.0.CO;2-A