Late-glacial and post-glacial deposition in a large, low relief, epicontinental basin: the northern Baltic Sea

摘要

This paper presents a model of late-glacial and post-glacial deposition for the late-Neogene sedimentary succession of the Archipelago Sea in the northern Baltic Sea. Four genetically related facies associations are described: (i) an ice-proximal, acoustically stratified draped unit of glaciolacustrine rhythmites; (ii) an onlapping basin-fill unit of rotated rhythmite clasts in an acoustically transparent to chaotic matrix interpreted as debris-flow deposits; (iii) an ice-distal, acoustically stratified to transparent, draped unit of post-glacial lacustrine, weakly laminated to homogeneous deposits; and (iv) an acoustically stratified to transparent unit of brackish-water, organic-rich sediment drifts. The debris-flow deposits of the unit 2 pass laterally into slide scars that truncate the unit 1; they are interpreted to result from a time interval of intense seismic activity due to bedrock stress release shortly after deglaciation of the area. Ice-berg scouring and gravitational failure of oversteepened depositional slopes may also have contributed to the debris-flow deposition. Comparisons to other late-Neogene glaciated basins, such as the Hudson Bay or glacial lakes formed along the Laurentide ice sheet, suggest that the Archipelago Sea succession may record development typical for the deglaciation phase of large, low relief, epicontinental basins. The Carboniferous-Permian glacigenic Dwyka Formation in South Africa may provide an ancient analogue for the studied succession. Chronological control for the studied sediments is provided by the independent palaeomagnetic and AMS-C-14 dating methods. In order to facilitate dating of the organic-poor early post-glacial deposits of the northern Baltic Sea, the 10 000 year long Lake Nautajarvi palaeomagnetic reference chronology (Ojala & Saarinen, 2002) is extended by 1200 years.