ABSTRACT: Postglacial history of the southeastern Assiniboine delta, Glacial Lake Agassiz basin: archaeological implications.

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Abstract

The Assiniboine Delta, located in south-central Manitoba, is the largest of several Pleistocene meltwater deltas in the glacial Lake Agassiz basin. Archaeological reconnaissance over the past 30 years has revealed a relatively dense concentration of Paleoindian (especially Plano) sites within the delta margins, suggesting that it was a 'central place' shortly after deglaciation. However, uncertainties in the history of water level fluctuations of glacial Lake Agassiz between ~10.8 and 9.4 ka BP, and the depth and complexity of sedimentary deposits on the Assiniboine Delta, have hindered interpretation of the early archaeological record in this region. In 2003, a new coring program was initiated in the southeastern portion of the delta (Rossendale area) in order to contextualize known Paleoindian sites within the larger history of glacial Lake Agassiz and late-glacial ecosystem dynamics. In this region, much of the upper 13+ m of sediment accumulation is characterized by multiple cycles of sandy rhythmites interbedded with massive to laminated silt. These sediments were deposited rapidly by traction or turbidity currents, and record the construction of the Assiniboine fan-delta during the deep-water Lockhart phase of glacial Lake Agassiz (>10.8 ka BP). Shortly before ~10 ka BP, fluvial incision into deltaic deposits occurred locally at the Rossendale gully site in response to the regression of glacial Lake Agassiz during the Moorhead phase. Plant macrofossils deposited in the gully by ~10 ka BP provide the first information on early postglacial plant colonization of the distal Assiniboine delta. These data suggest initial establishment of Scorpidium scorpioides, Potamogeton spp., Scirpus spp., and other wetland plants, followed by colonization of uplands by a Picea-Populus assemblage. Importantly, because the gully is located in a protected depression behind the Campbell beach, evidence of paludification from aquatic macrophytes suggests that glacial Lake Agassiz rose to a Campbell level during the early Emerson phase (~10 ka BP). Furthermore, no evidence exists for a post-Lockhart rise in Lake Agassiz above the Upper Campbell beach. If Agassiz stood at the Campbell level during the early Emerson phase, then drainage through the southern outlet may have been possible at this time. Several archaeological implications are suggested by these data: (1) much of the Assiniboine Delta was available for human occupation beginning shortly after 10.8 ka BP (early Moorhead phase); (2) vegetation colonization of the delta probably occurred between ~10.8 and 10 ka BP, resulting in a low ratio of fluted to Plano Paleoindian sites; (3) although Lake Agassiz was nearly abiotic, isolated wetlands on the margins of this proglacial lake were productive and probably able to sustain small prey populations; (4) Paleoindian sites above the Upper Campbell beach are probably in primary context and have not been redeposited by transgression of Lake Agassiz during the Emerson phase (~10 - 9.4 ka BP); and (5) because the bulk of sediment comprising the Assiniboine fan-delta was deposited during the Lockhart phase (>10.8 ka BP), Paleoindian sites should be visible in many upland settings

Bibliography of Canadian Geomorphology