MID-PLIOCENE CLIMATE AND ICE BUILD-UP IN THE JAMES BAY LOWLAND, CANADA

Glaciers expanded in the Northern Hemisphere during the middle Pliocene. However, whether or not they extended into the midlatitude lowlands remains unknown. A Pliocene sequence up to 170 m thick was obtained in a deep buried bedrock trench in Silurian carbonate bedrock near the Attawapiskat River, northern Ontario (52°49.5´N, 83°52.5´W). It consists of 50 m of till and 120 m of overlying lacustrine deposits. The till is a non-calcareous, clayey to sandy diamict rich in kaolinite. Facies and microtexture analyses indicate a subglacial environment under which the till was formed. The till contains detrital peat and organic-rich mud with sporadic glaciolacustrine silt and clay horizons. The predominance in pebble lithology of vein quartz, chert and quartzite suggests a major source from the Jurassic and Cretaceous fluvial sediments in the region. The overlying lacustrine sequence is also non-calcareous and kaolinite-rich. It contains laminated sand and silt with woody peat up to 30 m thick. The fining-upward lacustrine succession as a whole suggests rising lake level over time in a basin probably much larger than the current bedrock trench, and that present sequence is the erosional remnant of a much thicker lacustrine deposit. Pollen analysis indicates a Pliocene age for the deposits because of the presence in the pollen flora of Pterocarya and Sciadopitys that are believed to have become extirpated in North America by late Pliocene to early Pleistocene. Paleomagnetism study suggests a time span from 3.6 to 3.0 Ma for the Pliocene sequence. Till is deposited by glaciers. Its presence in the Pliocene deposits is direct evidence for continental glaciation in the James Bay Lowland at about 3.5 Ma (3.6 to 3.4 Ma). The ice that deposited the till would appear to have been part of a surging ice stream emanating from James Bay. After glaciation, rapid warming permitted thermophilic trees now exotic to this area to grow, which included Quercus, Carya, Liquidambar, Nyssa, and Taxodium. Furthermore, pollen analysis indicates alternating Carolinian deciduous and boreal evergreen forests under a climate that oscillated and cooled gradually over a prolonged postglacial period from 3.5 to 3.0 Ma.