Lower Miocene diatoms and other siliceous microfossils in the Cape Roberts Project drillcore (CRP-1), Roberts Ridge, western McMurdo Sound, are documented. Diatom biostratigraphy, along with 87Sr/86Sr ratios from mollusk shells and 40Ar/39Ar data from volcanic materials, constrain the interpretation of the magnetic polarity stratigraphy and allows construction of an age model for CRP-1. Together, ... [Show full abstract] these records indicate an age span from to ∼21.5 Ma at 147.69 mbsf (the bottom of the hole) to ∼17.5 Ma at 43.15 mbsf (top of the lower Miocene interval). Important diatom datums identified in this drillcore are the FAD and LAD of Thalassiosira praefraga and the LAD of Cavitatus rectus. Variation in diatom abundance and assemblage composition through the lower Miocene reflects changes in depositional environment. There are intervals of repeated ice cover (floating or grounded) in the lower Miocene section of CRP-1, where diatom production was limited, and intervals where diatoms were growing in open, although at times shallow (<35 m), marine seas. Relative numbers of planktic to benthic diatoms often parallel similar indices of water depth from sedimentologic and sequence stratigraphic interpretations. Approximately 70 diatom taxa, in addition to 2 silicoflagellate and 2 ebridian taxa, are reported. No radiolarians were encountered. Three tentative diatom biostratigraphic zones are introduced, though not formally named in this report. A Ross Sea diatom zonation for the lower Miocene to upper Eocene is under development, for future publication. The forthcoming zonation will incorporate diatom biostratigraphic information from CRP-1 and CRP-2, and earlier stratigraphic drilling in McMurdo Sound, as well as magnetostratigraphically calibrated datums from the Southern Ocean.