Younger Dryas glaciomarine sedimentation, push-moraine formation and ice-margin behavior in the Middle Swedish end-moraine zone west of Billingen, central Sweden

Abstract

New highway exposures and drilling reveal the stratigraphy and structure of the Middle Swedish end-moraine zone west of Billingen, Sweden. The material in the end moraines is primarily glaciomarine clay of Younger Dryas age that was deposited as varved clay in front of the retreating glacier and then pushed glaciotectonically to form push moraines during minor ice-margin oscillations during overall retreat during the Younger Dryas cold event. The moraines are composed of deformed and remobilized clay with some clayey diamicton and penecontemporaneously deposited and deformed sand. Between the moraines lie ‘intermoraine flats,’ composed of undeformed varved clay of Younger Dryas age and surface sands of Younger Dryas to early Holocene age. Based on estimations of moraine volume, sedimentation rate and ice-margin retreat rates, we calculate the overall ice-margin retreat and end-moraine construction to span 350–800 years within the Younger Dryas. Because the number of moraines in the Middle Swedish end-moraine zone varies across Sweden, we regard the individual oscillations west of Billingen to be driven by local physical and glaciologic factors rather than ice-sheet wide climate drivers. The study area is also the location of the early and final drainages of the Baltic Ice Lake. The final drainage of the Baltic Ice Lake took place several decades after the youngest moraine was formed. We consider it likely that the earlier, Allerød drainage of the Baltic Ice Lake (BIL) also took place at Billingen, despite the lack of clear local stratigraphic evidence. However, based on our model, a retreat driven solely by climate would not have exposed the outlet at Billingen, and we propose a dynamic break-up of the ice-margin likely centered on Valle Härad that was driven by the head difference between the BIL and the sea.