GEOLOGY ALONG THE NORTHEAST MARGIN OF COLUMBIA RIVER BASALT GROUP ROCKS AT SPOKANE, WASHINGTON

CRBG units at Spokane Washington include R2 and N2 of Grande Ronde Basalt, and Priest Rapids Member of Wanapum Basalt. Lacustrine Latah Formation deposits containing locally abundant lower and middle Miocene flora underlie, are interbedded with, and overlie the various basalt flows; locally the Latah Formation may be up to 400 m thick. Collectively the lacustrine deposits were mapped as the Latah Formation because their age relative to different basalt flows cannot be confirmed. Many have speculated that a major north-northwest trending linear feature located just west of downtown Spokane was a fault; however, there was no demonstrated offset. Mapping of individual basalt units in the area has demonstrated that this linear, here called the Latah fault, was active during the Miocene prior to arrival of CRBG at Spokane. The west side of the fault, is down relative to the east side. N2 GRB was deposited only on the west side of the fault. It is not present on the east side of the fault where the PR Member, the final basalt deposited in the Spokane area, lies directly on R2 GRB. The Latah fault affected the nature and distribution of basalt in the Spokane area. R2 GRB was deposited as a sheet on the west side of the fault, but occurs only as channel fill and invasive basalt in Latah Formation east of the fault. Pillow basalts are common along and west of the fault suggesting water-filled depressions along the fault. Invasive basalt contains a variety of unique features considered indicative of the invasive nature. A quarry in north Spokane has exposed a body of invasive basalt more than 15 m thick and 80 m across. Overlying Latah Formation is baked at the contact with basalt. Some of the invasive basalt in this quarry has crude columnar jointing. In some areas of quarried basalt, vesicles can be as much as 5 cm across; they are usually flattened. Vesicles are commonly lined with botryoidal siderite, which is believed to be a key indicator of invasive basalt in the Spokane area. In other areas of invasive basalt, pillows and pillow breccias are common.