Graptolite reflectance and thermal maturity of lower Paleozoic sediments

Vascular land plants did not become widespread until Devonian times. Consequently, lower Paleozoic rocks contain very little vitrinite. Therefore, assessment of thermal maturity by organic petrographic methods must be made on structured marine-derived organic matter, for example, algae, chitinozoa, and graptolites. Systematic studies of the optical properties (% R{sub A}, %R{sub 0}, n, and k) of graptolite epiderm at 546 nm and dispersion in visible light of 400 to 700 nm show progressive changes similar to those that vitrinite undergoes with increased maturity of sediments. Also the sensitivity of the optical properties of the graptolites to the lithology of host rock, weathering, tectonism, and mineralization is further suggestive of a similarity to that of vitrinite. Maximum reflectance in oil and bireflectance are the best indicators of changes of carbon structure of graptolites with increasing maturity, i.e., depth of burial. These two parameters increase systematically with increase in conodont alteration index. Conodonts are found in sediments containing graptolite or vitrinite. Hence, an indirect correlation between the reflectances of graptolites and vitrinite is possible.