Geology Underfoot - An Investigation of Jurassic Lacustrine Stratigraphy in the Lower Half of the Portland Formation on the Campus of Mount Holyoke College

Abstract

During the summer of 2020, Mount Holyoke College commissioned a geothermal company to drill a 6-inch diameter, 800-foot-deep borehole on campus grounds to measure the geothermal potential of the underlying geology. Professor Al Werner took advantage of this opportunity and collected cuttings during the well drilling operation. These samples were collected every 5 ft and document the detailed stratigraphy of the lower half of Portland formation, a Mesozoic lacustrine shale deposit of the Hartford basin. Geophysical data and imaging provide cm-scale stratigraphic changes and drill cuttings provide samples of the various units. These rocks record environmental conditions during the Early Jurassic. The record is made up of shales and siltstones that alternate in color (red, gray, and dark gray), suggesting changing environmental conditions. This has been confirmed by the existing literature of the Northeast American rift basins. By gathering physical and chemical proxies and finding significant relationships between them and Milankovitch cycles, this study supports previous interpretations that orbital changes explain the borehole stratigraphy. Using these proxies, I describe the changing, lacustrine paleoenvironment of the South Hadley area during the early Jurassic period.