The recent fluvial fan of Lorca (Murcia) and its transition to the Guadalentín River: geological controls during the latest Pleistocene-Holocene

Abstract

The fluvial fan of Lorca (Murcia) (AFL acronym in Spanish), located in the Guadalentín depression to the SE of the city, is approximately 11.5 km long, 9 km wide and 19 m maximum thick, and is presently cut by the Guadalentín river. Data from 17 excavations in the city and its surroundings revealed the presence of silts and channelized sands and gravels in the proximal fan, and tabular silts and clays in both the mid and distal fan. The age of the unit is latest Pleistocene - Holocene and the age of its excavation early Holocene. Incision of the fan head stopped more than 5000 years ago, when the Guadalentín river reached approximately 13 m below the fan surface at the fan apex, and three main erosional terraces were formed. The fan was formed during the change from glacial to interglacial conditions by subsidence in the basin, enhanced by sedimentary loading and compaction of the pre-fan series. Transition from fan to river is attributed to downthrow of the Guadalentín depression to NE of Lorca. This is demonstrated by fan thickness distribution, fan-base and internal beds tilting, palaeocurrents shift, erosional meander formation, tectonic activity in both Alhama de Murcia fault and Guadalentín river blind fault, and headward erosion in Viznaga rambla. The end of the last endorheism in Lorca in early Holocene, and in the rest of Guadalentín depression during the late Holocene, suggests transpression and transtension effects increased by unloading processes in adjacent areas, and probably flexion by loading in the Mediterranean platform after Pleistocene ice melting.