PRELIMINARY RESULTS OF GEOSTATISTICAL MODELING OF EASTERN SNAKE RIVER PLAIN AQUIFER

We are applying 3-dimensional geostatistical analysis to correlate subsurface basalt and sedimentary strata in the Big Lost Trough depositional basin of the eastern Snake River Plain (ESRP) Idaho. This work is funded by the U.S. Geological Survey in support of their subregional-scale ground water flow modeling effort. The stratigraphic architecture and relative proportion of sedimentary interbeds in the basalt pile exerts significant controls on groundwater movement within the ESRP aquifer system. An improved understanding of interbed organization and depositional center development in this volcanic-sedimentary basin will lead to more accurate conceptual models of the aquifer and of the basin’s geologic evolution.

A database of lithologic and stratigraphic information compiled by Anderson et al. (1996) was analyzed geostatistically in terms of ‘composite units’ of multiple basalt and sedimentary components defined by Anderson and Liszewski (1997). Accumulated sediment within the basalt pile reflects the coupled evolution of volcanic and sedimentary deposition. Composite units provide a larger-scale view of stratigraphic architecture by consolidating multiple episodes of volcanism and sedimentation that control basin stratigraphy.

Indicator variograms of sediment thickness were analyzed in each of the youngest seven composite units (5-800 ka years). All composite units are characterized by similar autocorrelation characteristics (with ranges of about 6.1 to 15.2 km, relative nuggets of 10 to 60 %) suggesting that similar geologic processes have recurred over time.

Environmental Visualization System is being used to create a geostatistically-based stratigraphic model of the subsurface based on lithologic data from 333 coreholes. Three-dimensional kriging will be performed on both the interpreted stratigraphic units as well as on observed lithology to identify possible discrepancies in current stratigraphic interpretations and perhaps locate areas of uncertainty requiring additional stratigraphic control (from age dates, paleomagnetics, and/or geochemistry).

Future work will focus on constraining models of basin evolution and sediment sources possibly using cosmogenic dating and detrital zircon analysis.