The potential of Ojo de Valjunquera cave (NE of Iberia) sediments for paleoflood reconstructions

Abstract

Speleothems and detrital deposits in caves are excellent archives of cave flood events but are still poorly exploited. In this study we evaluate, the potential of the Ojo de Valjunquera (Zaragoza, NE Spain) for the study of past floods based on geomorphological, topographical, hydrological, and chronological data. The cave comprises two subhorizontal levels. The lower level consists of a main horizontal conduit including six siphons. This level is connected to the upper one by shafts and ramps. That situation, together with the constrictions of the gallery in the lower level, favours the water rise during rainfall events. The upper level is characterized by a larger presence of speleothems and detrital sequences compared to the lower level. Current observations indicate that water can rise by ~9 m in some cave sectors during rainfall episodes >60 mm, although the hydraulic head rise is not homogeneous along the cave, depending on the section's morphology. The stalagmites and detrital sequences hosted in the upper gallery most likely contain evidence of extreme events of rainfall. However, geomorphological and sedimentological evidences also suggest that the cave outlet could have been blocked in the past by sediments, favouring the water rise to high places usually not affected by regular floods. The detrital sequences located in lower positions with respect to the cave entrance show a higher proportion of sandy sediments than those located in the higher sectors related to the water energy during the flood. Stalagmites show clean carbonate alternating with well-defined detrital layers. These detrital layers vary in thickness: the thinner ones are related to small floods, whereas the thicker ones are connected to large floods. Two important historical floods (1709 and 1755 CE) occurred in the area that coincide with distinct detrital layers recorded in the stalagmites. All these observations suggest that Ojo de Valjunquera cave contains an important paleoflood archive based on speleothems and detrital deposits during the Holocene.