Chapter 2

Basalt flows of the Grande Ronde, Eckler Mountain, Wanapum, and Saddle Mountains Basalts of the Columbia River Basalt Group (CRBG) are the sole rock type present in the study area. Grande Ronde Basalt flows are the most abundant. The regional stratigraphic nomenclature used (Figure 5) is that of Swanson and others (1979) as modified by Tolan and others (1989) and Hooper and others (in press). The local flow-by-flow stratigraphic framework was defined by chemical analysis by XRF for twenty-seven major and trace elements of samples of each successive flow in five principal stratigraphic sections. The precision and estimated accuracy for each element analyzed by the XRF method employed here are described by Hooper and others (1993). Basalt flow descriptions and chemical data for the five major stratigraphic sections are provided in Appendices A and B. Chemical and sample location data for the remainder of the 347 samples collected are also provided in Appendix B.

Distinctions between the Grande Ronde, Dodge (Eckler Mountain), Wanapum, and Umatilla (Saddle Mountains) basalts are easily made on the basis of P2O5 and TiO2 content (Figure 6). The Umatilla and Wanapum basalts are particularly well separated from the Grande Ronde and Dodge basalts by the "titanium gap," (Seims and others, 1974). The R2 and N2 magnetostratigraphic units of the Grande Ronde basalts (Swanson and others, 1979) are compositionally distinguishable on the basis of P2O5 and TiO2 content only to a limited extent (Figure 6). Independent stratigraphic information is required to distinguish these flows. In ascending order the overall stratigraphy in the study area (Table 1) consists of Grande Ronde reversed magnetic polarity flows (Tgr2), Grande Ronde normal flows (Tgn2), a Dodge flow (Ted-U; Hooper and others, in press), the Lookingglass flow (Tlg), a Ginkgo flow (Tfgk), Sand Hollow flows (Tfsh), Sentinal Gap flows (Tfsg), and a Umatilla flow (Tu).

Grande Ronde Flows

The Grande Ronde Basalts may be divided into two reversed magnetic polarity and two normal magnetic polarity units (Figure 5). The four magnetostratigraphic units are designated Tgr1 (R1), Tgn1 (N1), Tgr2 (R2), and Tgn2 (N2; Swanson and others, 1979). Only the R2 and N2 flows occur in the study area and are distinguished in the field with a portable fluxgate magnetometer (for procedure see Appendix D). The maximum sampled thickness of R2 flows, 530 meters (1740 feet), occurs east of the Hite fault in the valley of the South Fork Walla Walla River (section "E" on Figure 3). The maximum sampled thickness of N2 flows, 290 meters (950 feet), occurs west of the Hite fault on the south side of Blalock Mountain (section "W" on Figure 3). In the study area, the top of the Grande Ronde is marked in most locations by a deeply weathered, highly oxidized flow top. In some locations an oxidized clay-rich soil baked to the texture of pottery occurs. The top of the Grande Ronde is particularly accessible and well exposed near the intersection of Spofford Road and Walla Walla River Road in the northeast quarter of section 20 in township 5 north, range 36 east (location 22 on Map 3) and along Highway 204 west of Tollgate in the northwest quarter of section 31 of township 4 north, range 38 east (location 41 on Map 4).

Forty-three informal stratigraphic units consisting of individual Grande Ronde flows or groups of a few successive compositionally similar flows have been distinguished in the five stratigraphic sections. To do this, side-by-side plots of stratigraphic sequence against individual elements were used (Figure 7 and Figure 8). No single element distinguishes all stratigraphic units; multiple plots using several elements are required. TiO2, P2O5, MgO, Cr, Ba, and Zr are among the more useful elements. The utility of the stratigraphic sequence in distinguishing units is illustrated by units 31, 32, and 34 of the Grande Ronde N2 magnetostratigraphic unit. Units 31 and 34 have similar P2O5, TiO2, and Zr contents and are indistinguishable on the basis of composition alone (Figure 8 and Figure 9). However, these two units must be distinct as they are separated in the stratigraphy by units with lower abundances of the same three elements including the lowest Zr (and highest Cr) Grande Ronde flow in the entire stratigraphic sequence, unit 32.

The stratigraphic units distinguished are confirmed by element-element plots on which each unit forms a cluster (Figure 10 and Figure 11). On these plots it is clear that few flows and compositional groups have sufficiently distinctive compositions to allow correlation on the basis of composition alone, hence the need for stratigraphic information. The most success in using chemical analysis to identify Grande Ronde flows as an aid to mapping was achieved by combining the sampling of a sequence of at least three to four flows at any one location with additional stratigraphic information from field location and magnetic polarity. Sampling a stratigraphic sequence of a few basalt flows as opposed to sampling a single flow at any one location increases the likelihood that a compositionally distinct flow will be sampled.

Post-Grande Ronde Flows

Post-Grande Ronde basalts in the study area belong to the Eckler Mountain, Wanapum, and Saddle Mountains formations. The particular units present are largely distinguishable on the basis of composition alone (Figure 6 and Figure 9). The single Dodge chemical type flow of the Eckler Mountain formation is distinguishable on the basis of low P2O5 and TiO2 (Figure 6) and by high Cr (Figure 9f). The Lookingglass flow is distinguishable from the Frenchman Springs flows of the Wanapum formation by higher P2O5 and lower MgO and Cr (Figure 12). While petrographically similar to the underlying Grande Ronde flows, the Lookingglass flow has much higher P2O5 and TiO2 (Figure 6). The Umatilla flow of the Saddle Mountains Basalt formation is distinguishable from all other flows by its uniquely high Ba and high Zr contents (Figures 9e and 9f). The Frenchman Springs member flows are distinguishable from other post-Grande Ronde flows by a unique range of P2O5, TiO2, MgO, SiO2, Cr, and Zr contents (Figure 12).

Separation of the individual Frenchman Springs units from each other is more difficult as their compositional ranges overlap (Figure 12). Sampled Frenchman Springs flows are distinguishable into three compositional groups on the basis of P2O5, TiO2, and Cr ( Figure 12). One group has significantly higher P2O5 than the other two (0.7 wt% versus 0.55 to 0.65 wt%; yellow on Figures 12 a-c). The two lower P2O5 groups are distinguished by TiO2 (2.98 to 3.10 wt% versus 2.82 to 2.94 wt %) and Cr (20 to 26 ppm versus 38 to 47 ppm). The high P2O5 group is sparsely phyric with individual plagioclase phenocrysts up to 10 cm in size. Flows of the lower P2O5 and TiO2, and higher Cr group (magenta on Figures 12 a-c) are sparsely to abundantly phyric with plagioclase phenocrysts and glomerocrysts up to 1.5 cm in size. The higher TiO2 and lower Cr group flows (green on Figures 12 a-c) are rarely to sparsely phyric typically with individual plagioclase phenocrysts less than 1 cm in size. On the basis of composition, petrography, and stratigraphic sequence, the three groups are tentatively labeled Ginkgo (Tfgk), Sand Hollow (Tfsh), and Sentinel Gap (Tfsg) respectively. No flows corresponding to Palouse Falls (Tfpf), Silver Falls (Tfsf), or Lyons Ferry (Tfly) were sampled. This part of the Columbia Plateau is the only area where largely aphyric Ginkgo type basalt occurs (Reidel, pers. comm. 1995).

In most locations in the study area, the Grande Ronde is directly overlain by the Lookingglass flow. One exception occurs on Blalock Mountain. In a series of samples collected in a road cut along the Boise Cascade access road in section 34 of township 5 north and range 37 east (location 37 on Map 4) no Lookingglass (or Ginkgo) is present and the Grande Ronde is overlain by a Frenchman Springs Member flow of Sand Hollow type. To the north on Klicker Mountain (sections 7 and 8 of township 6 north, range 38 east; Figure 1) the Grande Ronde is instead overlain by a flow of Dodge chemical type. No Lookingglass (or Ginkgo) flow is present at this location.

In the stratigraphic section on the south side of Blalock Mountain, the Lookingglass flow is overlain by a relatively thin flow of Ginkgo chemical type which was fed by locally occurring dikes. This Ginkgo flow does not occur far to the north or west of this location or to the east of the Hite Fault (Map 4). In the western part of the field area the Lookingglass is overlain by flows of Sand Hollow chemical type while on the east side of the Hite fault it is overlain by flows of Sentinel Gap chemical type. At the only location where both Sand Hollow and Sentinal Gap flows were observed, along the Boise Cascade access road on Blalock Mountain (location 37 on Map 4), a single Sand Hollow type flow is overlain by four Sentinal Gap type flows in agreement with the regional stratigraphy.

Feeder Dikes

Several Frenchman Springs feeder dikes were sampled in the study area. In the field, these dikes are distinguishable from the Grande Ronde flows, which they cut, by generally fresher appearance, horizontal columns in the case of vertical dikes and inclined columns in the case of low angle dikes, visible discordant margins, glassy chilled margins, and sometimes by the presence of large plagioclase phenocrysts. The dikes are divisible into four compositional groups. Three of these groups correlate clearly with flows identified above as Ginkgo, Sand Hollow, and Sentinal Gap (Figure 13). The fourth group has lower P2O5 and Zr contents than the other groups and is tentatively identified as Palouse Falls (Figure 13).

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