High-resolution estimates of rates of depositional processes from an alluvial fan succession in the Miocene of the Ebro Basin, northern Spain

Obtaining estimates of the rates of sedimentation and frequency of depositional events at high resolution from pre-Quaternary successions is rarely possible, particularly in coarse-grained clastic successions of continental basin margins. In the Miocene strata of the Ebro Basin, northern Spain, basin-marginal deposits interpreted as the products of alluvial fans are part of the continuous aggradational fill of a basin of internal drainage. A palaeomagnetic reversal stratigraphy has been established in basin-centre sediments and correlation across to the basin margin makes it possible to estimate the rate of aggradation within the fan successions to be 90 mm ka−1. The deposits of one alluvial fan near the village of Linás de Marcuello have been analysed in the context of these rates to provide estimates of the frequency of depositional events. This analysis suggests that deposition on gravel bars approximately 700 mm thick in stream channel deposits occurred at intervals of 70-200 years over a 2.2 myr time period. This is the first time an estimate of the frequency of depositional events has been obtained at such high resolution from a pre-Quaternary alluvial fan succession.