Understanding Morphosedimentary Changes and Extreme Past Floods: The Case of Ojo De Valjunquera Cave (Iberian Range, Spain)

Abstract

During a flood inside a cave, sands and silts are transported through the cave system and deposited as (i) a coating of this detritus on speleothem surfaces and (ii) a flood bed accumulated on the cave passages which are protected from the main water stream. After the floodwater level drops, speleothem deposition is restarted and detrital coatings get trapped inside the carbonate laminae, while detrital flood bed sequences may contain clay laminae and water drops features at their contacts. When this process is unique and continuous (in the case of speleothems) record of paleofloods in a particular region is obtained. Typically, speleothems and detrital sequences close to usual groundwater table, record ordinary floods. On the contrary, sequences deposited in areas above the usual epiphreatic zone are expected to register extraordinary floods. Here, we present the geomorphosedimentary and chronological (U/Th, OSL) information derived from the Ojo de Val Junquera Cave (Iberian Range, Spain). The cave comprises two sub-horizontal levels connected by shafts and ramps. The lower level corresponds with an ephemeral spring and includes six siphons. Stalagmites from the upper level show single sand layers and brownish bands in relation to past low/high-frequency flood episodes, respectively. U/Th ages show that speleothems grew during Marine Isotope Stages (MIS) 9, 8, 7, 6, 3, 2 and 1. In the lower level, at the epiphreatic zone, a poorly consolidated detrital infill is formed by sands and gravels, indicating that the cave outlet was once blocked by these sediments, with an age (OSL) of 377 ± 39 ka (MIS 10–11). The study of detrital sediment facies and their chronology allow discerning between extreme and regular floods to provide high resolution records of extreme floods under climate variability up to a millennial scale in the Western Mediterranean.