MAPPING NEWBERRY VOLCANO'S EXTENSIVE NORTH FLANK BASALTS

Paleomagnetism and geochemistry have been used to distinguish and map petrographically similar basaltic lava flows within previously lumped unit Qbn (Quaternary basalt of Newberry volcano, Sherrod and others, Bend 100k sheet, in press). The most extensive flows have been traced more than 50 km from known or presumed vents on the north flank of Newberry volcano, although they do not appear to be present as far north as Cove Palisades State Park where intracanyon basalts in both the Crooked River and Deschutes River drainages have reversed polarity and low-Ti composition. Preliminary argon dating indicates that the farthest-traveled of the Newberry flows was the earliest at about 700 ka. This "green" basalt (colors on map in progress) flowed through the area of Redmond airport and into both the Crooked and Deschutes Rivers. The poorly dated "blue", "brown", and "orange" basalts each erupted several hundred thousand years ago and are much more limited in their distribution. They stratigraphically underlie basalt that erupted at a NNW-trending chain of spatter cones 3 km south of Bend’s Knott Landfill. This as-yet-poorly-dated basalt of Stevens Cave erupted around 100-200 ka, flowing northward, filling the ancient channel of the Deschutes River (which was then located east of Pilot Butte) and diverting the river to the west. The Stevens Cave lava then flowed around to the east of Redmond and through Redmond Cave into Redmond Canyon. It stopped just short of the Crooked River, but it does form benches along the present Deschutes River north of Lower Bridge. The unit is easily traced because of its unique paleomagnetic direction, a steep normal polarity oriented to the SE. A similar remanent direction is found in the tube-fed basalt of Arnold Cave and the Badlands on the lower NNE flank of Newberry, but the preliminary age is not the same. The basalt of Bend is the most recent of the big northern basalts and has a 40Ar/39Ar plateau age of 78±9 ka. This basalt erupted from buried vents at least halfway up the north flank of the volcano and flowed through lava tubes west (through Lava River Cave) and north under downtown Bend to the southern edge of Redmond, diverting the former more easterly channel of the Deschutes River west to its present location.