Postglacial history and paleoecology of Wampum, Manitoba, a former lagoon in the Lake Agassiz basin

Abstract

A small paleolagoon along the margin of the glacial Lake Agassiz basin near Wampum, Manitoba, 130 km southeast of Winnipeg, contains an unusually thick and continuous sequence of late-glacial and Holocene organic sediment. Cores at 17 sites through this sequence provide insight into the fluctuations of the level of Lake Agassiz, the timing of the construction of the lake9s most extensive strandline (the Upper Campbell beach), and the paleoecology of this area during the past 10 000 14 C yr. The history of this area is established from 26 new AMS radiocarbon dates, siliceous microfossils, plant macrofossils, mineralogy, sediment characteristics, and stratigraphic relationships. Following deglaciation at Wampum about 11 14 C ka, deposition of largely unfossiliferous silty clay with occasional ice-rafted clasts represents relatively deep-water conditions in Lake Agassiz. Overlying this is a spatially and temporally variable mixture of silt, sand, peat, gyttja, and marl, containing an allochthonous assemblage of siliceous microfossils and plant macrofossils with a wide range of radiocarbon dates. These sediments were deposited in an embayment of Lake Agassiz, mainly in shallow, quiet waters, as the lake transgressed about 10 ka over the Moorhead low-water phase surface. Between about 9.7 and 9.3 ka, the Wampum embayment was progressively isolated from Lake Agassiz by growth of the Upper Campbell spit, resulting in the formation of a lagoon. Organic-rich deposits on the lakeward side of the lagoon were buried 9.3–9.4 ka by sand and gravel of the spit, and a thin overwash of sand marks this event throughout the lagoon. At about this time, Lake Agassiz fell below the Upper Campbell beach level. Subsequent accumulation at Wampum consists mainly of algal gyttja during the early Holocene and marl during the middle Holocene; only minor fluctuations in hydrological conditions occurred, except for slightly warmer and drier conditions between 6.2 and 4.0 14 C ka. These stable, shallow-water conditions may be due partly to persistent groundwater contributions to the site. Progressive encroachment of rooted aquatics led to present-day fen conditions; peat accumulation across the basin began by about 4.0 ka.