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Micropaleontologic studies were undertaken to establish the burial temperature of the Paleozoic sedimentary sequence in southern Ontario through investigation of color alteration of conodonts and palynomorphs. Over 500 samples were used from surface localities and several hundred from 20 wells that penetrated various units of Ordovician, Silurian, and Devonian age in the subsurface.
The results showed the existence of at least two thermal alteration zones defined along the surface and in the subsurface. The first identifiable thermal alteration zone extends from the top of the Paleozoic sedimentary sequence to depths coinciding with the base of the Silurian and possibly extending into uppermost Ordovician strata. In this zone, the conodont alteration index (CAI) is 1.5 and reflects a burial temperature of 50 to 90°C. The second zone includes the remainder of the Ordovician section in southwestern Ontario and part of the Ottawa Valley in eastern Ontario. The CAI values for this zone lie in range 2 to 2.5 and suggest burial temperatures of about 60 to 140°C.
Superimposed on this broad scale thermal alteration pattern that reflects burial depth, are several areas with higher alteration indices of 2.5 to 3 in the Ottawa Valley. These are interpreted as being the result of unusually high heat flow occurring after the main burial phase of alteration and probably related to Cretaceous rifting that produced the Ottawa-Bonnechere graben.
Study of palynomorphs (acritarchs) shows a change in color from light yellow to dark yellow in the first zone and to brown in the second zone. These paleontologic studies of thermal maturation are being integrated with studies of the isotopic composition of natural gases (by J. F. Barker and P. Fritz, Univ. Waterloo) and of the organic geochemistry of the oils (by T. G. Powell, Geol. Survey Canada) for southwestern Ontario hydrocarbon deposits.
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