REMARKABLE STRATIGRAPHIC COMPLEXITY IN A COLUMBIA RIVER BASALT AQUIFER: AN EXAMPLE FROM THE CLEARWATER CANYON, IDAHO

Aquifer characteristics on the Columbia River Plateau are strongly controlled by the nature of individual lava flows within the Miocene Columbia River Basalt Group (CRB). An ongoing source water assessment of a portion of the Clearwater Plateau, Nez Perce Reservation, north central Idaho, focuses on locating groundwater recharge areas to assess portions of the aquifer vulnerable to contamination. A conceptual model for the aquifer system suggests that permeable rubble basalt flow tops host stratified aquifers that are vertically isolated by aquitards consisting of columnar jointed basalt. In many instances the geometry of these units are complex; beds pinch and swell laterally, bedding surfaces display considerable topography, and strata are folded and cut by dikes and faults, which has resulted in compartmentalization of the aquifer that has not been previously documented. A spectacularly exposed example of some of the stratigraphic complexity that should be expected in CRB aquifer systems occurs in a Grand Ronde Basalt flow unit along the Clearwater River Canyon between Lewiston and Spalding, Idaho. This rubble flow top outcrops in a 500 m east – west exposure and represents the classic surficial expression of the aquifer skeleton that is normally only discerned through drilling and pumping tests. The deposit is clast supported and consists of scoria and ropy lava clasts that vary in size from 20 to 2 cm (avg. 5 cm). Unit thickness varies from 10.5 to 0.4 m (3 m avg.). Bedding surfaces locally display remarkable topography with a maximum apparent dip of 80° in strata that dip <10°. Other features in this zone that compound aquifer complexity include: 1) invasive dikes that appear to intrude both upwards and downwards into the rubble zone; 2) rubble mounds produced by bulldozing of flow top rubble by the subsequent flow unit; and 3) vertical contacts between rubble and lava flow rocks within the unit representing either preserved flow lobe fronts or lava tube collapse features. The variable thickness and internal complexity of this CRB flow top should directly affect aquifer transmissivity which is a function of aquifer thickness multiplied by the hydraulic conductivity. While these observations are site specific, they may help anticipate aquifer variability in similar CRB groundwater investigations.