INTERPRETATION AND USE OF WELL DATA IN THE COLUMBIA RIVER BASALT GROUP, PULLMAN, WASHINGTON

Pullman is located on the western edge of the Moscow-Pullman basin along the eastern margin of the Columbia Plateau where topographic highs of pre-Tertiary rocks partly encircle flows and sediments of the Columbia River Basalt Group (CRBG).  The community is located on basalt in a narrow northwest-trending transition zone between the Moscow-Pullman basin on the east and the Palouse slope to the west.  The CRBG sequence determined from well chip analysis from base upward consists of the Imnaha (>92 m), Grande Ronde (569 m), and Wanapum (52 m) formations.  Correlations of the Wanapum-Grande Ronde contact in the transition zone delineate a northwest-plunging low-amplitude anticline.  In the 1970's workers documented differential basining during upper Grande Ronde time and the presence of a paleogeographic high during Wanapum time.  For purposes of ground-water modeling during the past two decades, the area has been portrayed as a sequence of undeformed, laterally consistent, horizontal flows that extend westward but thin eastward and become interlayered with sediments near Moscow, Idaho.  However, rock chip, downhole video, and basalt chemistry correlations between several wells within a 2-km radius show the subsurface to be complex with rapid lateral changes in thicknesses, interbed composition, vesicle content, fracture spacing, and internal basalt structures.  A >15-m thick basalt pebble to cobble gravel was identified between two Grande Ronde flows in only one well.  Periodic deformation in the transition zone caused the development of an intermittent paleogeographic high during basalt emplacement history.  This deformation is considered the primary reason for most of the lateral variations noted.  These changes between flows, flow groups, and sediments are considered to be responsible for compartmentalization of Grande Ronde and Wanapum aquifers in Pullman.  The well data from Pullman is being used to determine the evolution of paleogeographic events of the area.  These events will help determine the shape and extent of potential aquifers as well as assist in understanding the emplacement history of the CRBG.