Highway 397 Road Cut West of Nine Mile Canyon, Part I

 

Highway 397 is a spur highway off of Interstate 84 south of Kennewick that accesses the Finley area to the east of Kennewick. The route by-passes Kennewick, angling across the northern slopes of the Horse Heaven Hills south of Kennewick. A road cut just west of South Nine Canyon Road (46.07'47", 119.04'38") got my attention with a variety of geology units. This post is Part I and is focused on the west end of the cut slope where a sedimentary unit is exposed between Columbia River Basalt Group lava flows.

West end of the cut slope

Lake sediments from between lava flows. The sediment appears tuffaceous.

A mix of basalt rubble and sediment

Basalt rubble within sediment with some very 'cooked' sediment

Sediment at base of lava flow

Closer view of sediment at base of lava flow

Reidel and Fecht (1994) geology map covers this area at a 1:100,000 scale. The scale precludes some detail and the sediment interbeds are not shown. Furthermore the road cut post dates the geology mapping of the area. They note that these interbeds of sediment are generally best exposed in road cuts. Where the interbeds are mapped they are referred to as continental sedimentary deposits. In places where gravel is present, the gravel appears to be sourced from ancestral Columbia River or Salmon/Clearwater River flow paths as the grvael contains rocks with a continental affinity.

The map does indicate that the lower basalt below the sediment is the Umatilla Member and the upper basalt is the Pamona Member. Both of these members are part of the Saddle Mountains Formation of the Columbia River Basalt Group. 

The delineation of these basalt flows was accomplished via a combination of geochemistry, magnetic polarity and isotopic ages.  

The lava flows i the chart above between the Umatilla and Pamona are generally more localized lava flows or smaller flows restricted to ancestral river valleys. The lack of flows between the Umatilla and Pamona as well as the thinness of Continental deposits at this site suggests this site was elevated relative to the broader Columbia Basin similar to today.