Terrace staircase development in the Southern Pyrenees Foreland: Inferences from ~(10)terrace exposure ages at the Segre River

摘要

The fluvial network of the Southern Pyrenees is an example of transverse drainage systems in young (alpine) mountain belts and it features well preserved fluvial terrace records. Some of the major Southern Pyrenees tributary rivers have been studied previously and have some age controls on their fluvial terrace levels. We extend these records to one of the largest streams in the Southern Pyrenees, the Segre River. In this paper, we present new CIS data, field observations and exposure ages obtained from in situ produced terrestrial cosmogenic nuclides (~(10)Be). The focus of this paper is set to unravel the relative impact of climatic controls on the formation of the Segre terrace staircase in the Southern Pyrenees foreland (Ebro Basin). The Pleistocene terrace staircase of the Segre River is built up by seven major terrace levels, which range from 112 to 3 m above the present-day active channel. The prominent upper and middle terraces TQ1, TQ2, TQ3 and TQ4 have been sampled for ~(10)Be exposure dating. Exposure ages have been calculated using Monte Carlo parameter simulations. Additionally, we applied a Profile Rejuvenation method accounting for variable ~(10)Be inheritance in the samples, and which yielded comparable results. The Monte Carlo results show Middle to Late Pleistocene exposure ages for TQ1:202 ka(MIS7),TQ2:139 ka(MIS6),TQ3:100 ka(MIS5),and TQ4:62 ka(MIS4).involving denudation rates of 3-8 mm ka~'. The results indicate that the abandonment of Segre paleo-floodplains and the change from aggradation to predominant incision is related with Middle to Late Pleistocene cold-warm climate transitions (or early stages of interglacial periods). Major floodplain aggradation appears to be linked with intense deglaciation at the end of MIS 8, MIS 6, and MIS 4. The exposure of terrace TQ3 (MIS 5) indicates that terrace formation might also occur during relatively warm isotope stages with sufficient climate instability. On a regional scale, the staircase morphology at the Segre River shows similarities with adjacent fluvial systems in the Southern Pyrenees. In addition, previously published chronologies for these systems are consistent with the timing of aggradation and terrace abandonment Both, staircase morphology and chronology point to regionally synchronous phases of floodplain aggradation and river incision, which seem to be triggered by climate variability and Pleistocene glaciations in the Pyrenees since the Mid Pleistocene Climatic Revolution.