Columbia River Basalt Stratigraphy in the Pacific Northwest

In Cooperation with the Oregon Water Resources Department

Pomona basalt along the Columbia River (background), Umatilla Basin, Oregon (photograph by Terrence Conlon)

Sources of Information on CRBG Stratigraphy

Detailed geologic logs are available for wells penetrating the CRBG in Oregon . Many of the geologic logs were interpreted by Dr. Marvin Beeson, late professor at Portland State University, and Terry Tolan of Kennedy-Jenks, Consultants. Dr. Beeson interpreted much of the data from western Oregon, where he pioneered the application of geochemistry to understand the evolution, structure, and extent of the Columbia River Basalt Group. In memory of Dr. Beeson, geologists and hydrologists from Federal and State agencies and the private sector combined efforts to publish his and others’ geologic data of the Columbia River Basalt Group on this Website.

Well data

Geologic information for water wells is based on visual and geochemical analysis of carefully sampled borehole cuttings and interpretation of the driller’s water well report (available at the Oregon Water Resources Department's GRID Website: http://apps2.wrd.state.or.us/apps/gw/well_log/Default.aspx, and Washington State Department of Ecology Website, http://apps.ecy.wa.gov/welllog/).

. The driller’s report provides information on the hardness, water bearing properties, and color of the material penetrated by the well. Geochemical analysis of selected basalt cuttings provides oxide and trace element information to assist in classifying basalt into formation, members, and flow units.

Outcrop data

Geologic information for outcrop sites is based on visual, geochemical, and paleomagnetic analysis of samples of the outcrop. Geochemical analysis of samples provides data to assist in identifying the formation, member, and flow unit(s) from which the sample was obtained. Paleomagnetism indicates the magnetic polarity and magnetic direction of the flow, which varies because individual flows record the secular variation (change with time) of the magnetic field.  Paleomagnetic and geochemical data allow unique identification and correlation of flows over great distances.

Drill-Cuttings Sample Preparation and Analytical Methodology

The interpretive geologic logs were prepared by examining systematically collected drill cuttings and geochemical analysis of selected samples. Drill cuttings were usually bagged and labeled (well name, depth interval, date) by the drilling contractor.  Bagged drill-cuttings typically were not cleaned or processed in any manner by the drilling contractor. All bagged samples consisting of Columbia River basalt were cleaned (washed in tap water to remove any “mud” or powdered rock adhering to the cuttings) and dried.  Samples consisting of, or appearing to consist of, sedimentary cuttings were carefully examined to determine their “as sampled” condition and characteristics. Sedimentary drill cuttings were typically not “cleaned” because the washing process would have destroyed the sample. The cleaned samples were then placed in clean, labeled plastic bags.

The cleaned samples were then examined and logged by a geologist.  Lithologic and textural characteristics of the cleaned basalt drill cuttings were used to identify intraflow structures (e.g., flow top, dense interior, flow bottom, pillow complex, etc.). Accuracy in determining the depth of flow contacts and thicknesses of intraflow structures is usually limited by sampling interval. The accuracy of the depth of unit contacts and intraflow structure thicknesses is inferred to be plus or minus the sampling interval. Accuracy can be greatly improved if a downhole video log is available (accuracy typically + 1 foot) or from drilling advance rate records.  Identification of CRBG units penetrated in each well was based on a combination of criteria, including lithologic characteristics of the cleaned drill-cuttings, stratigraphic position, and geochemical analysis.

To confirm the identification of CRBG units penetrated in each well, representative samples from the different flows, or flow units, were selected for geochemical analysis.  Large, unaltered (fresh) cuttings from the dense interior portion of a flow were preferred.  From these cuttings approximately 20 to 50 grams were carefully hand picked to avoid altered or weathered chips, “foreign” chips and materials (e.g., chips from overlying flow, steel shavings from the drill bit, etc.) and further cleaned (washed in distilled water and ultrasonically cleaned) and dried to remove any potential surface contamination.  The ultrasonically cleaned samples were then re-examined for a finally time to confirm uniformity, freshness, and the presence of foreign materials.  The prepared geochemical samples were then submitted to either Activation Laboratories or Washington State University GeoAnalytical Laboratory.

Activation Laboratories is located in Ancaster, Ontario, Canada. Sample preparation at Activation Laboratories was done using their Code RX2 method which consists of crushing the sample in a mild steel mill (to prevent chromium contamination).  The prepared samples were analyzed under their exploration-grade lithogeochemistry whole-rock package (Code 4E-Expl). This package uses a combination of inductively coupled plasma emission (ICP) mass spectroscopy and Instrumental Neutron Activation Analysis (INAA) to determine major oxide, minor oxide, and trace and rare earth element concentrations within the whole-rock samples.  Major oxides were normalized to 100% on a volatile-free basis for comparison purposes. Arizona.

The GeoAnalytical Laboratory is located in at Washington State University in Pullman, Washington.  Sample preparation and X-ray fluorescence analysis methodology is described in detail in Johnson, D.M., Hooper, P.R., and Conrey, R.M, 1999, XRF analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead: JCPDS-International Centre for Diffraction Data, p. 843-867.