Abstract:
The Dummer Complex extends 180 km along the Precambrian -
Paleozoic contact from Tamworth to Lake Simcoe. It is composed
of coarse, angular Paleozoic clasts in discontinuous, pitted, hummocky
deposits. Deposits are usually separated by bare or boulder
strewn bedrock, but have been found in the southern drumlinized
till sheet. Dummer Complex deposits show rough alignment with
ice-flow. Eskers cross-cut many of the deposits.
Dummer sediment subfacies are defined on the basis of dominant
coarse grain size and lithology, which relate directly to
the underlying Paleozoic formation. Three subglacial tills are
identified based on the degree of comminution and distance of
transport; the immature facies of the Dummer Complex; the mature
facies of the drumlinized till sheet and; the submature facies
which is transitional.
Carbonate geochemistry was used for till-bedrock correlation
in various grain sizes. Of the 3 Paleozoic formations underlying
the Dummer Complex, the Gull River Fm. is geochemically distinctive
from the Bobcaygeon and Verulam Formations using Ca, Mg, Sr, Cu,
Mn, Fe and Na. The Bobcaygeon Fm. and Verulam Fm. can be
differentiated using Ca and the Sr/Ca ratio. The immature facies
from 1.0 phi and finer is dominated by the non-carbonate, long distance
transported component which decreases slightly downice.
The submature till facies contains more long distance material than
the immature facies. Sr and Mn can be used to correlate the
Gull River immature till facies to the underlying bedrock the other
subfacies could not be distinguished from each other or their
respective source formation. This method proved to be ineffective
for sediments with greater than 35% non-carbonate component, due to
leaching of elements by the dissolving acid.The Dummer Complex is produced subglacially , as the compressional
ice encounters the permeable Paleozoic carbonates.
The increased shear strength of the ice and pore pressures in the
carbonates results in the basal ice zones becoming debris ladden.
Cleaner ice overrides the basal debris
.
laden dead ice which then
acts as the glacier bed. During retreat, the Simcoe lobe stagnates
as flow is cut-off by the Algonquin Highlands.
