ABSTRACT

Coal clasts associated with extreme floods are prone to survive and maintain their large size, contrary to the general belief that distance from the parent peat layer reduces the size of transported clasts. Contrary to apparent logic, moreover, a second flood event favors the preservation potential of such soft organic clasts, this being the minimal fragmentation. An Anthropocene example from an urban park in Spain demonstrates that peat clasts up to 1 m long can survive due to flotation for a distance of almost a hundred meters and are well preserved and stabilized thanks to a second flood. These peat blocks were generated by catastrophic flooding of urban peatlands along the Ebro River (city of Zaragoza) during exceptional rainfalls in Iberia. The water flow from the Ebro River flooded the peatland at the surface of the meander, ripping up peat clasts from a shear or detachment level formed by an indurated level characterized by rounded quartzite pebbles, which acted as a hydrological discontinuity surface. Extensive evidence of the paleoflow direction is provided by oriented crushed reeds and the widespread occurrence of imbricated and thrusted peat blocks on the eroded and exposed peatland and in the main urban accumulation areas. To be specific, peat blocks and minor clasts accumulated in four areas associated with different modes of transport and topographic steps. From proximal to distal these are as follows: i) a proximal rim including thrusted peat blocks on the eroded peatland, ii) two intermediate accumulation zones associated with topographic steps in the park, characterized by peat-clast imbrication, iii) gravity-fall peat clasts deposited in an artificial channel in the park, and iv) peat rafts of more than 1 m in diameter scattered over the surface of the park (at a distance of 90 m from the eroded peatland).

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