MID-MIOCENE FLOOD BASALT VOLCANISM IN SOUTHEASTERN OREGON: NEW INSIGHTS ON AN OLD PROBLEM

Steens Basalt (SB) volcanism represents the initial manifestation of flood basalt volcanism on the Oregon Plateau.  While the ~16.6 Ma SB type section exposed at Steens Mountain is well characterized chemically and has been linked to other similar aged tholeiitic basalts exposed further north, the regional distribution and temporal nature of the entire package of mid-Miocene Oregon Plateau-wide basaltic volcanism remains ambiguous.  Carlson and Hart (1985) applied the term “Steens-type Basalts” to lavas exposed throughout the Oregon Plateau that are petrographically, chemically, and temporally similar to those exposed at Steens Mountain.  These “Steens-type Basalts” erupted from widespread loci throughout the Oregon Plateau and represent a greater range in age than just the Steens Mountain type section.  New age constraints and geochemical data substantiate this initial assertion and also link local “Steens-type Basalts” with the SB type section.  To illustrate this, we present major and trace element analyses from six stratigraphic sections of ~16.4 to 14.9 Ma tholeiitic lavas sampled near Steens Mountain.  Overlapping variations in major and trace element space are observed for all of these lavas including the SB exposed at Steens Mountain, regardless of age or location.  Additionally, all lavas from both the SB type section and our sampled stratigraphic sections exhibit the same variations in Zr/Nb (~10-19), K/Nb (~250-1300), K/P (2.5-11), and other trace element ratios. Unlike Columbia River Basalt Group lavas, differentiating tholeiitic lavas of the Oregon Plateau based solely on chemical differences is difficult.  We feel that these observed chemical similarities reflect magmatic processes that occurred throughout the region during this episode of flood basalt volcanism.  As a result, this entire suite of lavas should be treated primarily in a chronostratigraphic context while being compared with other mid-Miocene flood basalts throughout the Pacific Northwest.  Subsequently, temporal and spatial chemical variations can be clarified.