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Progressive changes in organic matter maturation and clay mineral diagenesis have been studied in transverse and longitudinal profiles in the external domain of the Quebec Appalachians. Reflectance measurements (90 samples) on dispersed organic matter, probably asphaltic pyrobitumen, and illite crystallinity values reveal an increasing degree of diagenesis from northwest to southeast.
In the northwest, reflectance in oil ranges from 1.70 to 2.30 in the Cambrian Chaudiere nappe, from 1.53 to 1.90 in Cambrian to Ordovician Bacchus nappe, from 1.08 to 1.27 in Lower Ordovician Pointe de Levy nappe, and from 0.99 to 1.21 in Middle Ordovician Quebec Promontory nappe. Illite crystallinity ranges from 4 to 8 mm (0.53 to 1.05° 2^Thgr). In individual nappes, reflectance increases with stratigraphic depth but may not have been affected significantly by tectonic burial resulting from the superposition of nappes. Predeformational burial diagenesis is therefore suggested as the principal agent of thermal maturation.
In the southeast, the Middle Ordovician St. Henedine nappe, representing the anchimetamorphic zone, has reflectance values from 2.82 to 3.79% Ro and illite crystallinity (< 3 mm = 0.40°2^Thgr) lower than in the northwest. In this region, high reflectance in conjunction with low illite crystallinity may be related to elevated temperatures resulting from syndeformational or postdeformational regional metamorphism.
For hydrocarbon exploration in the Quebec Appalachians it appears significant that the trend of decreasing thermal maturation with increasing tectonic "burial" depth may continue into the little deformed platform sequence of the St. Lawrence Lowlands underlying the nappes of the external domain.
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