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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5167

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Summary and Conclusions

The stratigraphy of sedimentary interbeds from 11 coreholes at the Idaho National Laboratory, Idaho, was interpreted using the best available lithologic and natural-gamma log information. This information was used to test hypotheses posed by earlier investigators regarding the spatial and temporal stationarity of aggregate interbed thickness distributions in the Quaternary basalt-dominated eastern Snake River Plain (ESRP) aquifer. A total of 122 sedimentary interbeds were observed in 11 subsurface borings using natural-gamma geophysical logs and lithologic core logs to identify the occurrence and thickness of interbeds beneath the southwestern part of the Idaho National Laboratory.

The statistical analyses conducted in this study tested the stationarity of individual interbed thickness distributions rather than aggregate thicknesses as conducted by earlier investigators. The following statistical tests were applied to the data (1) the Kolmogorov-Smirnov (K-S) two-sample test to compare distribution shape, (2) the Mann-Whitney (M-W) test for similarity of two medians, (3) the Kruskal-Wallis (K-W) test for similarity of multiple medians, and (4) Levene’s (L) test for the similarity of two variances. The results generally support the hypothesis that the frequency distribution of sedimentary interbed thicknesses in composite stratigraphic units designated 2 through 7 is temporally and spatially stationary (invariant). However, because sample size was small, the results should be considered preliminary until more high-quality corehole information becomes available. For example, the new data indicate that interbeds in composite units 2 and 3 may be thicker than those in units 4–7. The approach taken in this study (testing individual interbed thickness distributions) also might be applied to a data set developed for the INL by earlier investigators to explore more subtle aspects of statistical variability in a stratigraphic context, such as possible trends in the proportion of sediment relative to basalt in the uppermost part of the stratigraphic section.

The composite unit stratigraphy developed for the Idaho National Laboratory in an earlier study provides a useful framework for grouping stratigraphic intervals and correlating stratigraphy in new boreholes with that of nearby existing holes. However, unique, laterally persistent marker horizons are rare and it can be difficult to correlate stratigraphic intervals and assign composite unit stratigraphy when holes are separated spatially.

High quality lithologic logs greatly improve the ability to identify sedimentary interbeds in the ESRP aquifer, especially those that are relatively thin (less than 2–3 ft. thick). A combination of high-quality lithologic logs and natural-gamma logs provide a more confident characterization of aquifer lithology and stratigraphy. However, good quality stratigraphic data still can be obtained from geophysical logs alone when the presence of thin interbeds is not critical to the scope of the stratigraphic analysis.

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