Sediment fingerprinting and the mode of formation of singular and composite annual moraine ridges at two glacier margins, Jotunheimen, southern Norway.

Abstract

Sedimentary variation within and between annual moraine ridges formed at Storbreen and Styggedalsbreen, Jotunheimen, during the 1980s and 1990s is investigated with a focus on processes and mechanisms of moraine-ridge formation. Tops and bottoms of sediment slabs, deposited either as single annual moraines or incorporated into composite moraine ridges formed of several annual increments, were analysed and compared with possible source sediments with a view to sediment fingerprinting. Clast roundness and shape indicated mainly subglacial sediment origins while textural variability was mainly inherited from proglacial sediment sources, and macrofabrics and micromorphological characteristics revealed overprinting by various glaciofluvial, glaciolacustrine and periglacial processes. The evidence suggests one dominant mechanism of moraine-ridge formation involving the annual freeze-on and subsequent melt-out of overridden proglacial sediment slabs. This mechanism provides the basis of a simple model encompassing both singular and composite annual moraine-ridge formation, which appears appropriate for temperate glacier snouts characterized by deep seasonally frozen ground, close to the lower altitudinal limit of the zone of alpine permafrost.