Marjamurto, an interstadial site in a till covered esker area of central Ostrobothnia, western Finland

Abstract

Till-covered glaciofluvial deposits, presumably eskers, are common in Central Ostrobothnia (Pohjanmaa) within a zone more than 50 km broad, in which no exposed eskers are present. The areal distribution, morphology and stratigraphy of these eskers are described with particular reference to the organic deposits found in many gravel pits within this zone. Some of the organic sediments, such as the Ollala site in Haapavesi, have been correlated with the Eemian Stage, whereas some sites such as Oulainen have been correlated with the Early Weichselian Brarup Interstadial. The Marjamurto site in Haapavesi, described here, probably belongs to the same esker chain as that at Oulainen. It is a freshwater sequence deposited in a kettle hole, and was later buried by littoral sands and till. The palaeovegetation represents an interstadial type of pine-dominated forest. On the basis of its lithostratigraphy and biostratigraphy, the Marjamurto sequence is correlated with that at Oulainen. A preliminary TL date for Marjamurto of 107 ka supports this correlation. The possibility of correlating the 'forested' interstadials of Ostrobothnia with the Early Weichselian Odderade Interstadial is also considered. The Early Weichselian deposits in Ostrobothnia are overlain by only one till unit, as indeed also are the deposits correlated with the Eemian Stage. This suggests that the Weichselian ice spread to southern Finland only once. Thus the tillcovered eskers of Ostrobothnia must themselves date from the late Saalian Stage.

OSL and TL dating of buried podsols and overlying sands in Ostrobothnia, western Finland

“…This conclusion is based on pollen stratigraphy of the organic beds and it is supported by a number of TL dates from fine sorted sediments below the covering till (e.g. Niemela and Tynni, 1979;Forsstrom, 1982Forsstrom, , 1988; Hirvas and Nenonen, 1987;Jungner, 1987;Donner, 1988; Gibbard et a/., 1988; Peltoniemi et a/., 1989; Kujansuu et a/., 1991; Niemela and Jungner, 1991 1.…”

Stratigraphy of the Koivusaarenneva exposure and its correlation across central Ostrobothnia, Finland

“…1 and 4), displays organic-bearing silt and sand and gyttja (a kettle-hole fill) on glaciofluvial sand and gravel overlain by fluvial sands and capped by a till unit. The pollen assemblage in the organic sediments at the Marjamurto site indicates that the sediments were deposited during an interstadial (Peltoniemi et al, 1989). Pollen evidence together with a thermoluminescence (TL) age of 105 ± 15 ka from the sand unit at the top of organic sediment (gyttja) show that the Marjamurto interstadial sediments were laid down during the Early Weichselian Brørup Interstadial (Peltoniemi et al 1989).…”

“…The pollen assemblage in the organic sediments at the Marjamurto site indicates that the sediments were deposited during an interstadial (Peltoniemi et al, 1989). Pollen evidence together with a thermoluminescence (TL) age of 105 ± 15 ka from the sand unit at the top of organic sediment (gyttja) show that the Marjamurto interstadial sediments were laid down during the Early Weichselian Brørup Interstadial (Peltoniemi et al 1989). An additional OSL sample taken at Marjamurto from cryoturbated fine sand within a fossil ice-wedge cast above was previously TL-dated fluvial sand.…”

“…4). The Marjamurto exposure in Haapavesi described by Peltoniemi et al (1989), located 85 kilometres NE from Koivusaarenneva ( Figs. 1 and 4), displays organic-bearing silt and sand and gyttja (a kettle-hole fill) on glaciofluvial sand and gravel overlain by fluvial sands and capped by a till unit.…”

The SE sector of the Middle Weichselian Eurasian Ice Sheet was much smaller than assumed

“…It is also thought that the Middle Weichselian maximum was preceded in S Finland by a warmer interstadial dominated by pine forest. This is known locally as the Oulainen (Donner, 1983) Marjamurto (Peltoniemi et al, 1989) Harrinkangas (Gibbard et al, 1989) or Horonkyl€ a (Nenonen, 1995) interstadial, and has been correlated with the Br€ orup interstadial (MIS 5c, 105e93 kyr BP) in Denmark.…”