Acritarch and prasinophyte colours as thermal maturity indicators

Abstract

Measurement of changes in the reflectivity of vitrinite (fossil woody material) in incident light is the standard technique for assessing the organic maturity of sedimentary rocks. However, vitrinite grains are rare or absent in pre Late Silurian sediments so thermal histories of these units cannot be investigated by this method. Also, marine hydrocarbon source rocks of Late Silurian and younger age often contain little or no vitrinite. Acritarchs are enigmatic microfossils commonly found in Lower Palaeozoic sedimentary units and marine source rocks. Like miospores, the colour of acritarchs becomes darker with increasing thermal maturity. The colour of the long-ranging acritarch genus Veryhachium was calibrated as a thermal maturity indicator during this study. Rock samples were collected that contained vitrinite grains and Veryhachium specimens. The vitrinite reflectance of the sample was determined using standard techniques. Image analysis rather than traditional visual estimation was used to determine the colour of Veryhachium. Colour was described in terms of red, green and blue intensities (RGB). The RGB system is partially device dependent but ‘standards’ developed to correct for variations within and between systems were of only limited success. Mean blue and green Veryhachium intensities were found to be more sensitive to changes in thermal maturity than red intensity.