EARLY RESULTS ON MIOCENE BASALTIC LAVAS EXPOSED IN THE INTERVENING AREA OF PICTURE GORGE AND STEENS BASALT OF THE CRBG, EASTERN OREGON

We sampled basaltic units that are abundantly exposed between the southern extent of what is traditionally been viewed as Picture Gorge Basalt including the Monument Dike Swarm and the northern extent of Steens Basalt in a broad corridor of the Malheur Forest between the towns of Burns and John Day, Oregon. Not much is known yet regarding composition, age, and chemical affinities of exposed basaltic units including dikes. These units have implications for correlation with main members of the Columbia River Basalt Group (CRBG) and/or defining transitional members coeval to them. An approximate mid-Miocene time window of sampled basaltic units is indicated by stratigraphic relationships to regional ignimbrites such as the Dinner Creek Tuff.

The Picture Gorge Basalt and the Steens Basalt largely represent geochemically distinct tholeiitic units of the CRBG. Lavas of Picture Gorge Basalt are relatively primitive (MgO 5-9 wt.%) while Steens Basalt ranges in MgO from >9 to 3 wt.% but both units are often coarsely porphyritic. Conversely, Steens Basalt compositions are on average more enriched in highly incompatible elements (e.g. Rb, Th) and comparably enriched in the lesser incompatible elements (e.g. Y, Yb) relative to the Picture Gorge Basalts. These compositional signatures produce inclined and flat patterns on mantle-normalized incompatible trace element plots but with similar troughs and spikes, respectively.

Searching our new data set for samples with chemical characteristics for either CRBG unit using trace elements (e.g. Ba/Ta, La/Ta, Th/Yb), indicate that numerous samples are indeed likely Picture Gorge Basalt thereby extending its distribution considerably south and southeastward. On the other hand, there are also samples that could be Steens Basalt and those that have a clearly different composition based on their incompatible trace element signatures.

Among sampled units, what were broadly mapped as basaltic on published maps, are platy phenocryst-poor andesitic lava flows similar to those of the Strawberry Volcanics; thereby also extending mid Miocene calc-alkaline compositions southwestward.