Thermal Maturity of Lower Paleozoic Sedimentary Successions in Arctic Canada1

Mean maximum graptolite reflectance values from numerous sections in Arctic Canada range from 0.6% in Cornwallis Island and northwestern Devon Island to 4.7% in Ellesmere Island. We attribute this great lateral reflectance variation to differing burial depths of the graptolite-bearing strata beneath thick synorogenic siliciclastic covers. We attribute the low maturity of rocks in northern Cornwallis and eastern Bathurst islands to a maximum burial of about 2 km. Elsewhere, we used the paleogeographic location and proximity to the Ellesmerian overthrust wedge to interpret the measured reflectance values.

In northern Ellesmere Island, where the highest graptolite reflectance values (4.7%) occur, as much as 9.6 km of synorogenic siliciclastics accumulated on a tectonically loaded carbonate shelf. Initial synorogenic siliciclastics encountered substantial submarine-to-basin relief, and thus about 2 km of sediment were deposited prior to the initiation of deposition on the adjacent drowned shelf. Also, the deep-water sequence probably was underlain by attenuated continental crust adjacent to the southeastward-advancing Ellesmerian overthrust wedge. Together, these factors caused deposition of as much as 7.6 km of sediment in western Melville Island and, as much as 9.6 km in northeastern Ellesmere Island. On the drowned shelf, synorogenic stratal thicknesses were less, a feature we attribute to a thicker, more rigid crustal sequence and greater distance from the Ellesmerian tectonic loading.

We expect liquid hydrocarbons to be generated from the organic matter in the shales in areas with a graptolite R_o maximum (GR_o max) of less than 1.7%. The presence of two types of solid bitumens having different reflectances, morphologies, and optical textures suggests that hydrocarbons were generated and migrated through the graptolite-bearing strata. We expect only gaseous hydrocarbons in areas where GR_o max exceeds 2.0% R_o.