Vitrinite reflectivity and illite-montmorillonite mixed layer minerals from borehole samples in the west of Sannohe district were studied in order to obtain information on thermal histry of the Miocene sediments in the Green Tuff region, northeast Honshu, Japan. The isoreflectivity-line profiles show an increase in reflectivity toward the west. The reflectivity gradient ranges from 0.05 to 0.08%R/100 m in the boreholes of the eastern part, and 0.3 to 1.4%R/100m in the 52MAHD-4 borehole of the western part. The unusually high gradient of the 52MAHD-4 borehole suggests anomalously high heat flow in the Miocene.
Highly expandable illite-montmorillonite mixed layer minerals (more than 80% expandable layers) begin to decrease rapidly the proportion of expandable layers at a vitrinite reflectivity of 0.7%. Based on BOSTICK's vitrinite reflectivity-temperature-time diagram, from the measured reflectivity on the "short burial" line of the diagram, the past temperature at the beginning of the conversion is estimated to be 100°C.
