Petrology of graptolites and their utility as indices of thermal maturity in Lower Paleozoic strata in northern Yukon, Canada

Abstract

The optical characteristics and utility of graptolites as a measure of thermal maturity have been evaluated by measuring maximum, minimum, and random reflectance of specimens from the Lower Paleozoic Road River Group in northern Yukon. The progressive changes in optical properties of graptolites produced by increasing depth of burial are corrected to the optical properties of other organic matter (conodonts and bitumen) in the same sample. Measurement of maximum reflectance ( R o max) is the most reliable method of assessing the level of thermal maturity of gratolites because they have biaxial optical properties. At high levels of maturation the reflectance of graptolites is greater than that of vitrinite, whereas at low levels of maturation the reflectance of vitrinite is greater. In the Road River Group, the mean maximum reflectance of graptolites varies laterally from 4.0% to 6.5% R o max· R rmo max f graptolites increasing depth of burial and the relationship between reflectance and depth fits a log (reflectance)-linear (depth) relationship. The vertical increase of maximum reflectance of graptolites through an 800-m section ranges from 4.2% R o max at the top to 6.5% R o max at the bottom yielding a maturation gradientof 0.39 log % R o max km −1. A correlation of graptolite reflectance calibrated by color changes of conodonts (CAI) indicates that Road River strata are overmature (CAI = 5,4.0–6.5% R o max, graptolite reflectance) with respect to hydrocarbon generation.