PICTURE GORGE BASALT

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       Waters (1961) recognized two units of the Columbia River Basalts (CRB): Yakima, covering much of eastern Washington and Picture Gorge, restricted to central Oregon near a type locality of the same name on the John Day River 7 miles west of Dayville. Swanson and others (1979) established the Picture Gorge Basalt (PGB) as one of seven formations of the Columbia River Basalt Group. In terms of extent of surface coverage, duration of activity, or total volume of magma, PGB is of modest significance when compared to the CRB.

PGB distributionmonument dikes

 

       PGB has been traced over approximately 4120 square miles of north central Oregon and is assumed by many to have all been erupted from a dike swarm near the town of Monument, although many other possible source dikes have been suggested.

big dikemultiple dike

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 Ages of PGB have been obtained from 16.7 to 15.2 Ma by isotopic analyses (CRB ages range from 16 Ma to 6 Ma). Total PGB magmatic volume has been estimated to be 576 cubic miles (estimates of CRB volume approximate 50,000 cubic miles).

       The number of distinct Picture Gorge lava flows has been estimated at 60, but only a few dozen are exposed in one locality.

PGB at Girds Crpgb at PG canyon

Viewed more regionally, it is clear that PGB lavas completely obscured all irregularities of the John Day surface and buried even the most prominent landforms under successive sheet flows of fluid basalt 

 

While the top of most units is capped by a red, oxidized scoriaceous rubble, there is little evidence of prolonged intervals of soil development between eruptions.

oxidized PGB soiltuff interbed

       No distinctive feature of texture or composition has been recognized that will allow construction of a detailed PGB stratigraphy applicable to the entire assemblage, but the highest level lavas appear to be paleo-magnetically reversed in contrast to units below. Also, the lower sequence contains more olivine and pyroxene phenocrysts than is common in later lavas. Between two of the lower flows, there is a thin layer of light-colored ash that contains a trace of organic material.

       One of the most interesting and unusual features of the PGB is a set of six lowermost flows. The basal unit (and presumably the first to be erupted) is a relatively thin and unremarkable scoriaceous rubble.

3-flow column

This was followed by a basaltic lava that is nearly 250 feet thick and surmounted by another lava of similar thickness!

2 mega flowsmega flow

Both are covered by a set of two lavas of normal thickness separated by a bed of light colored volcanic ash (as mentioned above). In turn, these are covered by yet another basalt flow approaching 450 feet in thickness and more widely expressed than any flows below.

    Each of these very thick flows is characterized by a basal third of coherent lava that often displays columnar jointing and stands out as a resistant cliff-former.

big columns

This is surmounted by a section of lava that contains remarkable coarse-grained horizontal pegmatoid segregation veins in the form of wedge-shaped lenses that vary from a few inches to several feet in length. They contain light-colored zeolite minerals (chiefly chabazite) and crystals of olivine, pyroxene, and plagioclase. Vertical cracks at this level are discolored by oxidation and this continues into the top third which becomes uniformly tan, yellow, and much less resistant to erosion.

 

       These very thick lava flows are best expressed in the canyon of the John Day River but can also be traced farther west and they appear to have covered an area extending at least to Sutton Mountain on the south and to Rowe Rim on the north.

Rowe rimsouth sutton

Farther downstream and to the west, they have been removed by erosion. It seems clear that the first PGB lavas emerged from the Monument dike swarm and followed down an ancestral John Day River. See my map of 6 early flows in the vicinity of the John Day Canyon:

mega flow map

 

       It is generally accepted that development of segregation veins in very thick lava flows is the result of hydrous volatile phases being enriched in SiO2, K2O, Na2O, P2O5, and TiO2 with decrease in MgO which is born out by chemical analyses of these lavas and their veins. It is not known if a higher volatile content was a part of the initial magma or if it was obtained during travel down a river valley. It is clear that these early PGB lavas moved westward down the John Day River valley and that no evidence of a topographic barrier has been found. But why should these lava flows have become so thick and of limited extent while other PGB lavas of the same composition spread out in thin sheets of wide distribution?

                                                                   

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