Facies analysis, depositional activity and subsurface architecture of sieve lobe built alluvial fan (Planica Valley, NW Slovenia)

Sieve-lobe deposits are morphological features typical for gravel-rich and matrix-poor alluvial fans. Each sieve-lobe deposit is composed of open-framework, clast-supported, moderately-sorted gravels. Lobes form during sieve-deposition process where gravel is transported as bedload during short and intense rainfall events and abruptly deposited after the water quickly infiltrates into permeable underlying ground. Despite being a common morphological feature qualitatively described in several alluvial fan studies it lacks a detailed sedimentological description based on quantified data. The formative conditions and depositional activity of sieve lobes as well as subsurface architecture of a sieve-lobe built alluvial fan is largely unknown. In this study we present (i) a detailed sedimentary facies analysis based on sedimentary structure and texture of recent sieve lobes, (ii) a three-year aerial survey of sieve-lobe depositional activity, (iii) a direct linkage of depositional activity to specific rainfall triggers, and (iv) a depiction of subsurface architecture of intertwined sieve lobes.

The study was done in the Planica Valley (NW Slovenia), a typical post-glacial alpine valley, where several gravel-rich Holocene alluvial fans are located.  The study was performed on one of the more active alluvial fan, which is predominantly built of sieve-lobe deposits. The sedimentological analysis is based on granulometry, grain morphology, and grain fabric of 11 sieve lobes that differ in size and age. Sieve lobe generation was detected by aerial surveying using Small Unmanned Aircraft (SAM) and photogrammetric modelling of the surface of the deposits. Detected surface changes were temporally correlated with precipitation records from the nearby meteorological station. Ground-penetrating radar (GPR) technique was used to depict architecture and geometry of sieve-lobe built alluvial fan.

Sedimentary analysis reveals that majority of sieve lobes belong to textural group of gravel and some to sandy gravel, containing almost negligible percentage of mud fraction, which rarely exceeds 2.0%. Almost uniformly and regardless of age and size, all sampled sieve lobes exhibit downward coarsening, with distal parts of lobe being significantly coarser than proximal. Clasts are angular to subangular, predominantly moderately to highly spherical and not orientated.

Temporal surface changes are clearly visible on SAM-derived digital orthophoto and digital elevation models. Correlating surface changes to meteorological records shows that sieve lobes form with a subannual frequency, usually after rainfall events exceeding 50 mm of rainfall in 24-hours. During such events more than 1000 m³ of sediment was deposited.

GPR data from a radargram parallel to the direction of sediment transport displays stratified and progradational reflectors with sigmoid shapes that are continuous for 10s of metres and have 10° dip. Radargrams orientated perpendicular to the direction of sediment transport exhibit stratified hummocky and discontinuous reflections of up to 10 metres long. The reflection patterns are interpreted as series of stacked sieve-lobe deposits confirming formation of the studied alluvial fan predominantly by sieve-deposition process.

This study provides the first detailed quantified facies analysis of sieve deposits observed in nature. Monitoring of their deposition shows that they represent major building blocks of gravel-rich alluvial fans, and their deposition is directly linked to intense precipitation triggering events.