Lithofacies and Sedimentology of the Miocene-Pliocene Ringold Formation, Hanford Site, South-Central Washington

Abstract

The Miocene-Pliocene Rinsold Formation is one ofthe principal stratigraphic units vithin the suprabasah "unconfined" aquifer at fie Hanford Site in southcentral Washington.Consequently, undersunding the geology of the Ringold Fomation is necssary for th accurate characterization and nodeling of groundwater flow and contaminant transport st Hanford.The Ringold Fornation is a conplexly interstralified squence of unconsolidated to semiconsolidated clay, silt, sand, and granule to cobble gravel.Evaluation of borehole goloeical and geophysical data and outcrops on and adjacent to the Hanford Site suggests that the most accurate m$od for the study of Ringold sedinents is through the use of lithofacies.Six lilhofacies are defind using prinarily grain size data and borehol geophysical responses.These lirhofacies are: l) mud (M), 2) nud and snd (MS), 3) sand (SS), a) sand and granule to pebble gravel (SG), 5) sranule to pebble gravet (C), and 6) cobble to boulder gravel (CB).Analysis oflithofacies distributions and associarions reveal that: l) coarse-grained lithofacies (CB, G, SG, SS) tend to be concentrated along conplexly stacked, elongate fluvial channl tracts that displat a high degree of vertical interconnectedness, 2) fine-$ained lirhofacies(M, MS) tend to occupy relatively stable positions, and 3) individual lithofscies idenrified ar ih Hanfo.dSite tend to be discontinuous even over shon lateral distances.Based on the analysis of lithofacis distributions and associa, tions mo. accurare appraisals of the hydrogeolosy of the Hanford Site are possible.