Stratigraphic relationships of subaerial, invasive, and intracanyon flows of Saddle Mountains Basalt in the Troy basin, Oregon and Washington

The Miocene Columbia River Basalt Group within the Troy basin and adjacent Blue Mountains of northeastern Oregon and southeastern Washington consists of Grande Ronde, Wanapum, and Saddle Mountains Basalts erupted during uplift and basin formation. By Wanapum and Saddle Mountains times (15.6 to 6.0 Ma) the Troy Basin was sufficiently developed to begin receiving thick accumulations of lacustrine and alluvial sediments along with a wide variety of relatively thick basalt flows. As subsidence, sedimentation, and volcanism proceeded, stratigraphic relationships became more complex, with some Saddle Mountains flows forming invasive and intracanyon phases and a possible sill, in addition to the subaerial sheet phases mapped by Ross (1978). This chapter reviews and revises the stratigraphy of the Saddle Mountains Basalt in view of recent mapping and additional major- and trace-element analyses. Principal revisions are: the Elephant Mountain Member consists of the Wenaha flow represented by subaerial sheet, intracanyon, and invasive phases; the Buford Member consists of 1 (Buford) or 2 (Buford and Mountain View) flows represented by subaerial sheet, intracanyon, invasive phases, and possibly a sill, interfingered with the Wenaha flow and sedimentary interbeds; the Eden flow consists of subaerial and invasive phases and is elevated to member status within the Saddle Mountains Basalt; the Umatilla Member consists of the Sillusi flow overlain by the Bear Creek flow, the latter being a new Umatilla Member flow. An andesite breccia that may pre-date the cessation of Columbia River Group volcanism in the area is also recognized. Also addressed is the problem of determining if the Wenaha and Buford Member phases repeated within the section represent discrete eruptions of identical lavas or invasive and subaerial phases of single lavas. The Wenaha phases are believed to have resulted from a single subaerial eruption that spread as a sheet flow until encountering thick, wet sediments, which it invaded. Locally the invasive phase extruded as intracanyon lavas into shallow valleys eroded into the invaded sediments. The subaerial sheet, intracanyon, and invasive phases of the Buford Member may also have resulted from 1 eruption (possibly with local sills to the east of the study area) or they may represent 2 discrete lavas.