Correlation of Yakima Basalt flows across areas of poor exposure can be facilitated by use of TiO2 analyses combined with geophysical borehole logging procedures. Previous workers have noted a change in the TiO2 content between the lower and middle Yakima Basalt. This investigation shows that the TiO2 discontinuity is coincident with an erosional and nondepositional hiatus which is widespread in central and southeastern Washington. TiO2 values below the discontinuity characteristically range from 1.7 to 2.0 percent whereas those above range from 2.5 to >3.5 percent.

Gamma photon emission associated with the 40K content of certain flows and flow sequences can be used in correlating the basalts with natural gamma logs. Physical characteristics of individual flows and flow? units can generally be distinguished on the neutron-epithermal neutron log. By combining a knowledge of the position of the TiO2 discontinuity with the information afforded by the geophysical logs, it is frequently possible to correlate the section in a well or borehole with nearby described stratigraphic sections or similarly studied sections in other boreholes.

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