Generation, transport and preservation of armoured mudballs in an ephemeral gully system

Armoured mudballs have been reported from modern fluvial catchments and preserved in ancient (Quaternary and older) sedimentary sequences. Despite many descriptions of mudballs in the literature there is little systematic analysis of their physical properties, genesis, transport and deposition within alluvial sequences. This study uses two adjacent catchments developed on identical geology in southeast Spain that offer a unique opportunity to examine the key controls on armoured mudball development as one catchment produces armoured mudball whilst the second does not. Field survey using a Total Station together with laboratory tests for material composition and aggregate stability were used to examine the catchment and the armoured mudballs. The results show that the generation of the armoured mudballs is heavily dependent on the correct sediment supply (suitable aggregates to act as mudball ‘seeds’, and suitable fragments to source the armour) and good slope-channel coupling within the catchment. Coupling is facilitated by steep gully sides which deliver the weathered blocks of marl (mudball ‘seeds’) to the gully floor. The ‘seeds’ form the core of the mudballs in the ’mudball factory’ zone. If the ‘seeds’ are subjected to sealing by moisture from rainfall the mudball will hold together through negative pore pressures and can be transported by the ensuing gully flow. As the mudball rolls it picks up a surface armour which becomes embedded into the mudball. With longer transport paths the turbulent nature of the flow creates spherical mudballs which are size sorted downstream by the hydraulics of the flow rather than attrition. Armoured mudball deposition occurs within unit braid bars in areas of waning flow (due to transmission losses), within vegetated, point or mid-channel bars. The armoured mudballs are preferentially deposited within the coarser bar head of these depositional units, with smaller marl fragments forming the finer bar tail. Trenching of the bars reveals that the armoured mudballs are preserved in the subsurface although they may become increasingly flattened upon burial at depth, and may be difficult to recognise due to disaggregation by roots. This study suggests that armoured mudballs recognised in ancient alluvial sequences imply seasonality of climate and good slope-channel coupling within the catchment area, and that they are capable of transporting delicate flora undamaged, making them good targets for pollen recovery in ancient alluvial sequences. Where armoured mudballs were present but go unrecognised in the ancient record the mode and energy of transportation and deposition of the fine sediment may be misinterpreted as the fines are in fact being transported as bedload rather than suspended load. The armoured mudballs have also been noted to transport pristine microfauna such as ostracod shells from the source area material. This may provide challenges to palaeoenvironmental interpretations of the mud-dominated unit as environmentally significant fauna may not be contemporaneous with the unit.