The Oligocene‐Miocene Guadalope‐Matarranya Fan, Spain, as an Analog for Long‐Lived, Ridge‐Bearing Megafans on Mars

Numerous sedimentary fans on Mars have been studied to better understand the early martian hydrological system. Of particular interest is to estimate the duration of fluvial activity from alluvial fan size by dividing deposit volume by bankfull sediment flux. However, such a calculation requires an intermittency factor—a parameter relating sediment discharge from channel‐filling (bankfull) floods to the long‐term mean sediment discharge—which is poorly constrained. Here, we investigated fluvial fan deposits from the Oligocene–Miocene Caspe Formation, Spain, as an analog to fans on Mars because it has exhumed channel belts that create sinuous ridges in the modern topography, similar to those observed on Mars that are used for paleohydraulics. We made measurements of the thicknesses of dune and bar cross sets within exhumed channel belts at nine field sites to reconstruct bankfull channel depth (∼1.4 m) and bankfull sediment flux (∼0.48 m3/s). We estimated total bed‐material sediment volume of the fan (362 km3) from stratigraphic thickness, the area containing exhumed channel belts, porosity and sand fraction. Combined with previous constraints on depositional timespan (∼6 Myr), we calculated a sediment‐transport intermittency factor of 0.004 (range: 0.0004–0.04). Our approach can be applied to Mars by using remote sensing measurements of fluvial ridge morphology, which indicates the possibility of long depositional timespans exceeding millions of years for martian fans. Plain Language Summary We examined Oligocene‐Miocene river deposits in the Guadalope‐Mattaranya fan (Ebro Basin, Spain) as an analog for river deposits on Mars that potentially represent long durations of river activity. Similar to deposits on Mars, the Guadalope‐Mattaranya fan has been differentially eroded creating networks of branching, sinuous ridges that represent former river channel belts. Based on measurements of river bar and dune deposits, we estimated that the ancient rivers were sand‐bedded and 1.4 m deep and that they carried a bankfull sediment discharge of 0.48 m3/s. Using these data, we calculated that the entire fan could have been built in as little as 24,000 years if there was continuous bankfull flow in the rivers. However, in reality, the fan was built over 6 million years, highlighting the vast amount of time that sediment transport was inactive. Applying a similar fraction of intermittent sediment transport to case studies on Mars suggests that martian river systems co