INVESTIGATING THE RELATIONSHIP BETWEEN LITHOFACIES AND DIATOM COMMUNITY COMPOSITION IN THE HUNTER CREEK FORMATION, NORTHWEST RENO, NEVADA

We present preliminary findings of diatoms from the Late Miocene to Pliocene age (4.75-2.5 Ma) Hunter Creek Formation in the Reno area. Geologic maps subdivide diatomite in the Hunter Creek into a lower ‘diatomaceous siltstone and sandstone’ member (Tnds) and an upper ‘lacustrine diatomite’ (Tnd). The purpose of this research project is to investigate whether a paleoenvironmental distinction between these units could be made through an analysis of their diatom assemblages. Samples were taken in a stratigraphic transect of Tnds-Tnd from Robb Drive to 4^th Street and additionally from isolated outcrops of each unit north of the transect. When just considering the stratigraphic transect, the lower samples from Tnds are dominated by colonial fragilariods (Staurosira, Staurosirella) commonly associated with littoral environments. The Aulacoseiraceae are also present and increase markedly at the top of Tnds, indicating turbid conditions in an expanding lacustrine environment. Transition to Tnd is marked by dominance in centric diatoms Aulacoseiraceae, Thalassiosiraceae (Lindavia, Discostella, Tertiarius, Mesodictyon), and Hemidiscaeae (Actinocyclus). While the ecology of all taxa are not precisely known, Actinocyclus, Lindavia and Discostella are all euplanktonic forms associated with lakes sufficiently deep to undergo seasonal stratification. A more complex picture between paleoenvironment and mapped units emerges when examining the localities to the north. Lithologic differences between localities mapped as Tnd vs Tnds are subtle and call into question whether it is appropriate to map them as separate units. Northern samples show no clear association of fragilarioid vs. centric-dominated floras between Tnds vs Tnd. High floral variability amongst Tnds samples in the north indicates rapid fluctuations between highly turbid and less turbid littoral benthic environments. Northern samples may be proximal to the lake margin, or in a shallower and well mixed part of the lake. Continued research on both the stratigraphic and spatial distribution of diatom assemblages relative to lithofacies within the Hunter Creek Formation will allow for more insight into the paleoenvironmental history of the Reno basin and the practicability of mapping Tnd and Tnds as separate stratigraphic entities.