HYDROSTRATIGRAPHIC CHARACTERIZATION AND QUATERNARY/NEOGENE HISTORY OF BOISE AND MERIDIAN, IDAHO USING DRILL-CUTTINGS ANALYSIS, BOREHOLE GEOPHYSICAL LOGS, AND HIGH RESOLUTION SEISMIC IMAGES

We integrated a 12 km long high-resolution seismic reflection profile from Boise to Meridian, Idaho with lithologic descriptions of well cuttings and borehole geophysical logs to characterize the hydrostratigraphy and define the Quaternary and Neogene history of a portion of the western Snake River Plain. Our study documents an irregular buried river-channel and terrace surface (unconformity) incised into gently dipping and faulted Miocene fluvio-lacustrine sediments. This gravel sub-aquifer unit is overlain by a 70-200 m thick sequence of courser-grained fluvial and lacustrine sediments. Borehole geophysical and lithologic logs show that a 5 to 15 m thick sequence of sandy cobble gravels associated with the floodplain terrace(s) produce nearly horizontal strong amplitude seismic reflections. The gravels overlie lacustrine sediments that appear to dip a few degrees to the southwest on the seismic profile. The fluvio-lacustrine geologic section above the gravel is sand-dominated with minor lenses of clay and silt within the mostly deltaic and fluvial nearshore sands. We believe the oolitic shoreline sands that appear in the overlying section, within 100 m of the land surface, represent the highest stand of the ancient lake system.

We associate the buried channel and terrace system with a fall in the level of Neogene Lake Idaho. The underlying lake sediments are correlative to the Chalk Hills Formation known from tephra-stratigraphy studies to have accumulated 8 to 6 Ma, followed by deformation, erosion, and river incision (recession). The lacustrine and fluvial section that lies on the buried gravel terrace indicates a refilling of the lake to a spill point.

The two gravel terrace layers, the deeply buried unconformity, and the shallow Pleistocene set of terrace gravels of the Recent Boise River that form the present-day land surface, appear to merge near the margin of the upper basin. Both gravel units serve as conduits for recharge of surface waters of the Boise River into the deeper sand aquifers relied upon for municipal water supply. This study shows that seismic imaging within an urban setting, when combined with borehole geophysical and lithologic information, can help unravel the geologic history and document aquifer conditions in a complex geologic setting.