The formation of fan‐deltas in mountain lakes: Findings from Zelené Kežmarské Lake (Slovakia)

Abstract

Fan‐deltas are formed in mountain lakes, contributing to changes in shorelines' shapes and filling lake basins with sediments. Factors that condition sediment delivery to the lakes and the formation of fan‐deltas are not fully understood. This study aims to identify processes forming fan‐deltas in mountain lakes based on the sedimentary architecture of a fan‐delta filling Zelené Kežmarské Lake (Slovakia). Our study is based on ground‐penetrating radar and seismic refraction surveys conducted over the lake and debris‐flow fans in its vicinity, and grain‐size analyses of the surface deposits. The internal structure of the fan‐delta comprises foreset deposits representing the fan‐delta lobes. Mouth bars were identified in the near‐shore zone. The fan‐delta is built of sands and silty sands whereas an alluvial debris‐flow dominated fan west of the lake contains gravels. A general trend of downslope fining of the fan surface sediments, disturbed by zones of coarse gravels, was identified in the surficial sediments forming the fan. The fan‐delta was formed by depositional events in humid periods, alternating with dry periods featured with a small or no deposition. The lake‐level steadily rose in the humid periods and remained stable in dry periods. The contrast between the coarse sediments forming the alluvial debris‐flow dominated fan, and sands forming the fan‐delta was caused by a selective deposition. Coarse gravels in the fan were mobilized in humid periods but did not reach the lake‐shore. The reason was a basement rock ridge situated parallel to the lake shore. Sands were delivered to the fan‐delta in humid periods owing to frequent high‐energy flows. A deposition of silts took place in dry periods. The studied fan‐delta might preserve the sedimentary record of three humid periods from the last 200 years. However, further studies are needed to establish the timing of the fan‐delta formation.