STRUCTURAL CONTROLS FOR DEPOSITION OF MIOCENE LATAH FORMATION SEDIMENTS AND COLUMBIA RIVER BASALT IN THE SPOKANE, WASHINGTON AREA

The Latah Fm consists of a sequence of fresh-water deposited sand, silt, and clay containing abundant Lower and Middle Miocene flora. The Latah was deposited in small basins that now are found around the northeastern margin of the Columbia River Basalt (CRB). A popular theory for origin of the depositional basins for the Latah Fm has been damming of the ancestral drainage in the area by CRB; we suggest that a sequence of Latah Fm in the Spokane area of more than 350 m is inconsistent with damming by CRB. A conspicuous, 75-km-long linear trace that is now occupied by Hangman Creek and a segment of the Spokane River at the west edge of downtown Spokane is a fault along which some of the thickest Latah Fm occurs. The basin we describe extends north from downtown Spokane and east of the fault, which we call the Latah fault. The NW trend of the Latah fault is consistent with other NW trending faults in the Columbia Basin and the trend of CRB feeder dikes.

Stratigraphy on opposite sides of the fault is important in defining the nature and timing of movement on the fault. West of the fault, typical 30-m-thick flows of Grande Ronde Basalt (GR) of CRB contain relatively thin Latah Fm interbeds. East of the fault, the Latah Fm appears to be a continuous sequence that hosts irregularly distributed and discontinuous flows of GR. We propose that these discontinuous flows are channel fill and invasive bodies of GR. Both the Latah Fm. and GR are overlain by a CRB flow of the Priest Rapids Member (PR) of the Wanapum Basalt. There is no measurable offset of the PR, which overlies both units at essentially the same elevation on both sides of the fault indicating most of the movement on the Latah fault occurred prior to 14.5 ma when the PR was deposited.

We propose that initial movement on the Latah fault was down to the east, which allowed for deposition of the thick Latah Fm. This does not explain why flows of younger GR west of the fault occur juxtaposed against Latah Fm. Consequently, before GR reached the Spokane area movement reversed on the Latah fault; the east side became topographically higher. Latah Fm was eroded on the east side of the fault and redeposited as interbeds in basalt on the west side of the fault. The discontinuous flowsLewis and Clark line of GR in Latah Fm on the east side of the fault are channel fill and invasive basalt. GR did not cover this area of Latah Fm.