PETROGRAPHY OF LIMESTONE OF THE EOCENE PINEY POINT FORMATION FROM THE BANBURY CROSS COREHOLE (USGS WELL 57G128), YORK COUNTY, VIRGINIA

In the summer of 2014, the Banbury Cross corehole in York County was drilled as part of a joint USGS and VA DEQ regional water resource investigation focused on the six geologic units comprising the Eocene Piney Point aquifer. Groundwater in the Virginia Coastal Plain is a heavily used resource, and improved information on the Piney Point aquifer is needed to plan more effectively for a sustainable water supply. The Banbury Cross corehole produced a continuous sediment core, and at a depth of -185 ft to -192 ft, moldic bioclastic grainstones and packstones of the Piney Point Formation were present, with very minor areas of patchy wackestones. To facilitate petrographic characterization, thin sections of seven samples obtained from that stratigraphic interval were made. Principal framework grains are pelecypod fragments (70 to 75 percent of the framework grain population), monocrystalline quartz (5 to 10 percent), glauconite and collophane (5 to 10 percent), ostacode fragments (1 to 3 percent), polycrystalline quartz including some that are extensively fractured (1 to 3 percent), and bioclastic debris including bryozoans, calcareous algae, echinoderms, gastropods, and forams (1 to 3 percent). The larger bioclasts reach or exceed 1 inch (long dimension). A few large bioclasts are broken in one or more places, probably as a result of reworking by currents subsequent to death of the organism and disarticulation. Smaller bioclasts include forams, gastropods, and bryozoans. Non-carbonate framework grains include subangular to rounded quartz, glauconite, and collophane, indicating moderate to extensive transport and reworking in the marine environment. Packing is minimal, and many grains are matrix supported. Porosity is predominantly biomoldic, consisting of partly to completely dissolved or destroyed pelecypod fragments. Some biomoldic pores are rimmed with crusts of isopachous non-ferroan calcite. Many glauconite grains also exhibit sparse to abundant grain moldic porosity. A few collophane grains have complex internal structure, including fractures that have been reduced—and in some grains completely filled—by glauconite. A few of the distinctive reticulate pores of the original stereomic microstructure in some echinoderm fragments are now reduced by glauconite. Minor fracture porosity is also present.