AGE OF CUEVA DEL MUERTO AND AN INCISION RATE FOR STREAMS ON THE EASTERN MARGIN OF THE IBERIAN CHAIN, SPAIN

A minimum age for a limestone cave can be determined by dating materials that have been deposited within. Clastic cave sediments are often optimal for this purpose because they are frequently emplaced shortly after cave genesis. This is difficult, however, in hypogene caves due to limited clastic material accumulation. Cueva del Muerto is a 750 m long, 48 m deep branchwork cave found on the eastern margin of the Iberian Chain, within the Ebro River watershed (Ricla, Zaragoza Province). A 3-m thick deposit of fine-grained sediments is found near the lowest point in the cave. We analyzed the deposit for depositional context, sedimentary structures, paleo-environmental conditions, paleo-magnetism, organic content (via loss-on-ignition) and grain morphology/mineralogy (via SEM/EDX), to understand its origin and to constrain the age of the cave. The reddish-yellow sediment appears to be a slackwater deposit, These sediments range in size from clay to very-fine sand and show very fine laminations. Quartz and clay minerals, with iron oxides, make up the material. Desiccation and minor erosional contacts between layers show that the deposit formed through multiple events. The deposit has undergone some slumping, but position with respect to wall and ceiling shows that it is mostly in place. Speleothems (popcorn) related to evaporation and capillary seepage have locally formed on the face of the sediments. Organic content was low, ranging from 1.3 to 3.9%. Analysis by low/high-frequency susceptibility, and IRM/ARM acquisition & demagnetization indicate that the magnetic carrier is a high-coercivity mineral such as hematite or goethite, with a fine magnetic domain size. Magnetostratigraphy shows 3 reversed sample pairs over 5 normal sample pairs, indicating a change in magnetic polarity and a minimum age of deposition at 0.990 ka (C1r.1n; older end of Jaramillo). To our knowledge this is the first paleomagnetic date for a cave in the Iberian Chain. With a base elevation of approximately 512 m for the sediments, and an average local river channel elevation of 381.5 m, a maximum incision rate of 0.13 m/ka is derived.