Relationship between statistical thermal alteration index and vitrinite reflectance for sedimentary rocks in northern Japan with reference to effects for unconformity, faulting, and contact metamorphism

Abstract

Two organic maturity indices, the statistical thermal alteration index (stTAI) and vitrinite reflectance (R O ), are used to gain insight into the geological histories of sequences of Tertiary and Upper Cretaceous sediments in northern Japan that contain an unconformity and which are affected by faulting and contact metamorphism. The stTAI is based on the brightness, or gray level, of fossil pollen of Pinus, Podocarpus, Abies, and Picea species. Pollen brightness is measured using a transmitted-light microscope equipped with a computer-driven digital image processor. The stTAI represents the mean value of the modes for the complete array of indigenous pollen in rock samples. The stTAI indicates the level of organic maturation for Neogene sedimentary rocks of Japan, from incipient diagenesis to early catagenesis (R O Յ1.0%). With the progressive diagenesis and catagenesis of sedimentary rocks, stTAI values generally show a progressive decrease, whereas R O values increase. The effects of an unconformity and faulting are more clearly recorded in stTAI trends than in R O trends. During early stages of organic maturation (R O Յ0.7%), stTAI shows a rapid decrease, while R O shows a rapid increase during the mature and post-mature stages (R O Ն0.8%). The occurrence of a range in R O values for a given level of organic maturity makes it difficult to determine the influence of the unconformity on the increasing R O trend. R O values show a progressive increase toward an igneous dyke, but this trend is not apparent in stTAI values. Measurements of Tertiary and Upper Cretaceous rocks in Japan reveal that stTAI is more sensitive to heating duration than R O , although R O is more sensitive to heating temperature.