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The Mesoproterozoic Frontenac terrane in the southern Grenville Province of Ontario is separated by shear zones from the Composite Arc Belt to the west and the Adirondack Lowlands to the east. The majority of the terrane is made up of granulite-facies metasedimentary rocks that are the oldest lithologies recognized in the terrane. Five samples were selected for U–Pb geochronology to constrain (1) metamorphic age, (2) depositional age of sedimentary protoliths, and (3) source regions of detrital zircon. Two pelitic gneisses and a marble are dominated by metamorphic zircon, whereas two quartzites contain a diverse population of detrital zircon that are surrounded by metamorphic overgrowths. Metamorphic zircon have 206Pb/207Pb ages of 1.19–1.16 Ga, and a small population is 1.22 Ga. These ages correspond to the Shawinigan and Elzevirian orogenies and provide minimum ages for deposition. The youngest detrital grains with ages of 1.25–1.24 Ga provide maximum depositional ages. Quartzites (and pelitic gneisses) have a wide range of detrital zircon ages that reflect local Mesoproterozoic Grenville sources and Paleoproterzoic and Archean sources in the northern Grenville Province and elsewhere in Laurentia. Most notable is a large population of 1.9–1.8 Ga zircon, which points towards derivation from the Penokean orogen in the midcontinent or Makkovikian–Ketilidian orogen of Labrador and Greenland, indicating long sedimentary transport distances. The similarities in depositional ages and detrital zircon ages between Frontenac terrane and Adirondack metasedimentary rocks suggest a shared sedimentary history, which we interpret as deposition in the same Trans-Adirondack backarc basin at ca. 1.25 Ga. [ABSTRACT FROM AUTHOR]

Several unconformities have been previously recognized in the Late Ordovician and early Silurian strata of southern Ontario. We examined the Georgian Bay, Queenston, Whirlpool, Manitoulin, Power Glen, and Cabot Head formations and associated unconformities. Detailed sequence stratigraphic and chemostratigraphic analysis of Late Ordovician and early Silurian outcrops between Niagara, New York, and Manitoulin Island, Ontario, reveals new insights on the timing of the erosional unconformities and the Ordovician–Silurian boundary. We recognize three significant lowstand unconformities in this interval, which are referred to as the Cherokee, S1B, and S2 unconformities. Additional small-scale surfaces are present but do not reflect any major change in sea level or pause in sedimentation. Using δ13Ccarb chemostratigraphy, biostratigraphy, and sequence stratigraphy, we tentatively correlate units and the mentioned unconformities from southern Ontario to other eastern North American sections of comparable age, showing how glacioeustasy had a widespread effect on the deposition and removal of strata in far-field, subtropical basins. The Cherokee unconformity appears to be a composite erosion surface found across eastern North America that formed during the two or more episodes of glacioeustatic sea level fall in the early to middle Hirnantian. The overlying S1B and S2 unconformities can also be found across eastern North America and appear to be the result of glacioeustatic sea level falls occurring during the early Silurian. These new insights on the timing of erosional unconformities help better constrain the placement of the Ordovician/Silurian boundary in Ontario. It appears that the boundary does not correspond to a lowstand-related disconformity as previously suggested but can be tentatively placed within the Power Glen or lower Cabot Head formations. [ABSTRACT FROM AUTHOR]

The Sandy Lake basin in northwestern Ontario is a potentially important area for insights into the late history of glacial Lake Agassiz because of its extensive glaciolacustrine deposits and well-preserved shoreline features of this geological episode. However, little information is available on its deglaciation history. Recent mapping shows the withdrawal of the ice from the basin center and subsequent deposition of extensive varved clay in the lake with an optically stimulated luminescence-dated maximum age at 11.4 ± 0.9 ka. With its further recession, the ice constructed the Opasquia moraine on the northern rim of the basin sometime before the development on the moraine of the first major shoreline of the lake (the The Pas, inferred at 10.1 ka). Lowering of the lake level formed many strandlines on the moraine and elsewhere in the basin, which can be correlated with those in the main Agassiz basin based on projected water planes (the The Pas to Ponton). Radiocarbon dating on basal wood remains of surface peat in a former strait defined by the Ponton shoreline and a nearby site on the former lake floor indicates the abandonment of this shoreline and hence the withdrawal of Lake Agassiz from the Sandy Lake basin by 8.3 ± 0.1 cal ka (UOC-7883). The date, although a minimum-limiting age, provides the hitherto best possible age constraint for the Ponton–Kinojévis shorelines, which many hypothesize represent one of the major lake levels during the final drainage of Lake Agassiz into Hudson Bay but have never been adequately dated before. [ABSTRACT FROM AUTHOR]