Morphology of subglacial conduit deposits: control by bedrock topography, discharge flow variation, or both? A cautionary case study: Axelsberg, Nynäshamn, south central Sweden

The results of a case study of full-pipe conduit discharge into a Pleistocene proglacial lake provide improved knowledge for recognizing the sedimentology and genetic morphology of isolated glaciofluvial deposits. Most published studies have investigated the elongated deposits of esker systems within a stagnant or retreating ice sheets. Lack of genetic description for flow through full-pipe conduits where there are localized cross-sections of conduit has restricted recognition of changes in the dynamics of the flow in such situations. Axelsberg gravel pit, located about 40 km south of Stockholm, Sweden, shows a succession of glaciolacustrine deposits, both in the lee and on top of a Precambrian gneiss body that served as a local hill in the Baltic Ice Lake beyond the sediment-carrying conduit during deposition of the sediments in a sediment-carrying conduit. The succession is subdivided into five lithofacies, two of which are coarse gravel. These two facies are spatially separated from one another by an even coarser-grained gravel facies consisting of blocks up to 3 m in diameter, which occurs mainly on top of the gneiss hill. The lower gravel facies is interpreted as a remnant of the deposits formed during a phase of strong discharge into the lake through a subglacial conduit. The block-rich gravel facies formed at the phase of maximum velocity of the turbulent current, the concentration of blocks in this facies is explained by extremely strong turbulent flow conditions perhaps during a sudden, local discharge through the subglacial conduit. The presence of the gneiss hill just in front of the conduit may have created the exceptionally high-velocity turbulent current capable of transporting meters-large boulders. The upper, somewhat less coarse-grained but still boulder-rich facies was formed when the turbulent conditions had become less extreme. Boulders up to 1 m occur in this facies. The depositional features indicate that the succession in the Axelsberg gravel pit was formed within the stagnant frontal zone of the ice sheet. Lack of similar deposits farther to the north along the deglaciation path suggests a local, short-term event. As there is no evidence of such a rapid climate signal in the later part of the Younger Dryas documented by clay-varve chronology, the formation of this deposit as a response of the ice sheet to that would not be regarded as a probable alternative explanation. This study suggests that clastic sedimentary sections are among the reliable records of short-lived internally controlled glacial events, not necessarily climatically driven ones, in the past.