RE-EVALUATION OF MIOCENE STRATIGRAPHY IN LIGHT OF NEW STRATIGRAPHIC, PETROGRAPHIC, AND AR39/AR40 DATA, NORTH OF LAKE MEAD, NEVADA

Complex structure caused by major strike-slip and normal fault motions north of the Lake Mead area make geologic interpretations difficult.  Petrographic, stratigraphic, and geochronologic data were used to shed light on the depositional history of one part of this area. Bohannon (1984) mapped a mixed siliciclastic-carbonate sequence immediately west of the White Basin Fault (WBF) as the Bitter Ridge Limestone Member (BRL) (13-13.5 Ma) of the Horse Spring Formation (HSF). Another mixed siliciclastic-carbonate sequence east of this area was mapped alternatively as the Rainbow Gardens Member (18-26 Ma) of the HSF by Bohannon (1984) and as the Thumb Member (14-16 Ma) by Beard et al. (2000). As part of a detailed basin analysis study, stratigraphic sections were measured through both sequences. Similar facies were found and we hypothesized a possible correlation. Subsequent petrographic analysis of ashes, including glass fragment size, shape, and mineral composition of phenocrysts, within these two units suggested a possible correlation.  Recent Ar³⁹/Ar⁴⁰ results strengthen this correlation and have significant implications for the timing of basin development in the larger area. Ashes from the east show ages of 16.19 ± 0.11 Ma and 16.15 ± 0.12 Ma, confirming that it is of Thumb age as mapped by Beard et al. (2000). An age of 16.3 ± 0.75 Ma obtained from an ash within the BRL is 2-3 Ma older than previous estimates, allowing a correlation between the areas. A definite correlation affects existing models of tectonic activity for the area. Additional information is needed to confirm our hypothesis.