Pre-Orogenic and Synorogenic Diagenesis and Anchimetamorphism in Lower Paleozoic Continental Margin Sequences of the Northern Appalachians in and Around Quebec City, Canada.

Abstract

ABSTRACT Thermal maturation of organic matter in shales and illite crystallinity reveal a regional southeastward increase in grade from the late middle and late stages of diagenesis to epimetamorphism in early Paleozoic continental margin deposits of the Quebec Appalachians near Quebec City. Four diagenetic or low-grade metamorphic zones are distinguished: Zone I representing the late middle stage of diagenesis with a mean random reflectance (0%) of asphaltic pyrobitumen between 1.0 and 1.5 per cent and illite crystallinity (IC) between 5.5 and 8.5 mm (0.72 and 1.10°2) zone II (late diagenetic stage) with 0 varying between 1.5 and 2.8 per cent and IC between 3.2 and 5.5 mm (0.42 and 0.72°2) or up to 8.0 mm (1.04°2) for black shales, zone III (anchimetamorphic zone) with 0 varying between 2.8 and 3.8 per cent and IC between 1.8 and 3.2 mm (0.23 and 0.422) and zone IV (epizone) with 0 greater than 3.8 per cent and IC less than 1.8 mm (less than 0.23°2). In the northwest (zones I and IIA) reflectance and illite crystallinity increase, within individual nappes, as a function of age or depth of burial. Farther southeast (zones IIB to IV) no such dependence is observed; instead, diagenetic grade increases regionally in a southeastward direction. Diagenesis in zones I and IIA is either in part or entirely pre-orogenic and has been transported during nappe movement, because in the stack of nappes near Quebec City diagenetic grade decreases downward from the higher to the lower tectonic units. Depth of burial of Cambrian deep-water clastic rocks of the Chaudiere Nappe (highest tectonic unit) was estimated as 6 to 7 km on the basis of reflectance (1.71 to 2.30%, diagenetic zone IIA) and an assumed paleogeothermal gradient of 30°C/km. This burial depth may have been achieved entirely by syndepositional overburden. For the Middle Ordovician Quebec Promontory Nappe (lowest exposed tectonic unit) a reflectance of 1.01 to 1.15 per cent (diagenetic zone I) requires 4 to 5 km of overburden, part of which was probably provided by the higher tectonic nappes, because younger sedimentary rocks of sufficient thickness that could have once overlain these Middle Ordovician deposits are not known in the region. Diagenesis in zone IIA, therefore, was most likely formed entirely before orogenesis and nappe transport; in zone I it was probably raised after nappe emplacement. The grade of diagenesis and metamorphism reached in zones IIB to IV are interpreted as related to regional synorogenic heating in conjunction with the Taconic orogeny. Thermal maturation levels in zone I indicate that the rocks may not have passed the oil window, which is of interest for petroleum exploration in Quebec.