Petrographic data from a stratigraphic interval containing a gamma-ray anomaly near the Theresa-Keeseville...

This dataset comprises optical and scanning electron microscopy images of sandstones from the Lower Ordovician Theresa Formation (Ontario, Canada). In several boreholes in the Ottawa area, the basal sandstone of the Theresa Formation is characterised by an elevated gamma-ray response in wireline geophysical logs. Subsurface drill core samples obtained from five boreholes intersecting this interval were found to contain an increase in low levels of uranium as measured using a portable X-ray fluorescence spectrometer. Samples from this interval were prepared as polished thin sections for examination. Microscopic analyses generated a petrographic dataset comprising transmitted light and reflected light photomicrographs along with backscattered electron micrographs. Images are organized in folders corresponding to individual boreholes and samples. Collectively, these images document a detrital grain assemblage dominated by quartz with subordinate potassium feldspar, and accessory heavy minerals comprising rutile and zircon. The detrital grains are cemented by intergranular calcite, dolomite, quartz, potassium feldspar, pyrite, chalcopyrite, rutile, and apatite. This petrographic dataset can be used to evaluate sediment texture, detrital mineralogy, and diagenetic processes associated with this interval in the Theresa Formation. The composition and textural characteristics of these sandstones, as illustrated by the dataset, also provide a basis for comparison with other sedimentary successions characterised by an elevated gamma-ray response in wireline logs.