A sedimentological and stratigraphical study of Weichselian sediments in the Tvärkroken gravel pit, Idre, west-central Sweden

Abstract: A detailed sedimentological study was made on six section walls in the Tvärkroken gravel pit, Idre, west-central Sweden. Based on sediment characteristics, three sedimentary units, units 1-3, were identified. The lowest unit, unit 1, was formed by deposition from sediment-loaded underflows into a stagnant basin. A gradual decrease in water depth is indicated in the upper part of this unit. Post-depositional deformation in the form of an ice wedge cast, normal faults, water-escape structures and silt injections are identified in the unit. Unit 1 is overlain by unit 2 with an erosional contact. Unit 2 is characterized by proximal braided river sediments. A gradational contact separates unit 2 from the subglacially deposited sediments in unit 3. Unit 3 consists of a basal diamict, composed of deformed unit 1 and unit 2 sediments transformed into a subglacial traction till. On top of this is a stacked sequence of deformed sediments derived from unit 1 and 2, transformed into both massive diamicts and glaciotectonites, the latter characterized by boudins. Extensive fluvial erosion occurred in the area after the deposition of units 1-3. Channels were then cut into the sediment successions creating a landscape with flat plateaux, terraces and moraine hummocks, left as erosional remnants and separated by deep channels. Units 1 and 2 are suggested to be of a pre-Late Weichselian age, deposited at a stadial/interstadial transition, whereas unit 3 is suggested to be formed either at the beginning or the end of the last glacial phase of this area. The fluvial downcutting is presumably due to melt-water erosion from the last deglaciation.