Block alignments/talus flatiron stages as response to lithological factors and dynamic slope changes in the Central Ebro Basin, NE Spain

Abstract

The central sector of the Ebro Depression is characterized by an abundance of isolated buttes with relict talus flatiron morphologies and block slopes on the footslopes. This paper presents two unusual study cases where the block slopes are aligned in parallel with the retreating escarpment. The evolutionary process of both buttes started with the formation of talus flatirons and ended with aligned block slopes. This is because both buttes initially had a detrital caprock (Pleistocene pediment) which eroded away to expose a sandstone layer that functioned as a new caprock and so changed the slope dynamics. Seven stages were identified at La Cobeta butte and five at Puyalon butte. The relative ages obtained for stages II and IV enabled an estimation of the escarpment retreat rates. The chronologies of the other stages of block alignments were estimated using Schmidt-hammer rebound values. The estimated age of the stage II (ca. 6588-6222 cal yr BP) coincides with three radiocarbon datings obtained in the deposit. Stage III (ca. 4235-3684 cal yr BP) is contemporary with the 4.2 Bond event. Stage IV (ca. 2800-2500 cal yr BP) coincides with the 2.8 Bond event and is associated with ceramic potsherds and radiocarbon datings. Stages V and VI developed during the Warm Roman period and the Medieval Climatic Anomaly respectively. Lastly, stage VII was formed during the Little Ice Age. The information obtained regarding the morphological arrangement improves our knowledge of the regional evolutionary model where talus flat irons and block slopes develop in a semiarid environment.