Intracanyon flows, Columbia River Basalt Group 259 / I .,,,,. / FRENCHMAN SPRINGS unconformity MEMBER ~ Figure I 1.-Contact relations of the Pomona Member intracanyon flow in the Viento Park area (shown in Fig. 10). View to the southwest from Interstate Highway 84. White lenses in the conglomerate are a quartzofeld­ spathic sandstone. ric, it commonly bas a greater (by a factor of 2 to 3) columns 5 to 15 cm wide. This flow outwardly resem­ concentration of phenocrysts than that in less phyric bles older Columbia River basalt flows in each of the areas. Where rich in phenocrysts, this unit takes on a areas discussed in this paper. It particularly resembles truly porphyritic character. Abundant phenocrysts and the Grande Ronde Basalt, which is by far the domi­ the presence of megascopic olivine led some early nant exposed unit within the Columbia River Gorge. mappers to misidentify the Pomona Member as an This resemblance bas contributed to past confusion olivine basalt of Cascadian origin, an understandable regarding the identification and recognition of intra­ error. canyon relations (Allen, 1932; Wells and Peck, 1961; The Pomona Member, like the Priest Rapids Sceva, 1966; Kienle, 1971; Waters, 1973; Beaulieu, Member, has reversed paleomagnetic polarity. How­ 1977). ever, the chemistry of these two units is quite different (Table I). The Pomona Member has relatively lower KLICKITAT RIVER AREA FeO, Ti02, P20 5, and K20 contents than other flows in the section and has higher concentrations of MgO and In the Klickitat River area (Fig. 10), a canyon CaO than the others. These differences are summa­ approximately l km wide and more than l 00 m deep rized in detail in Swanson and others {1979a). contains not only the Pomona Member (12 m.y. old) The jointing of the Pomona Member intracanyon but also the Elephant Mountain Member (10.5 m.y. flow (Fig. l l) is very similar to that observed at the old). These units are exposed where a bend in the type locality. It has well-developed basal columns and paleodrainage is transected by the Klickitat River can­ an entablature that appears to be hackly jointed but is yon. Both units are in contact with flows of the better described as being made up of narrow, irregular Frenchman Springs Member across steep buttress 260 Selected papers on the geology of Washington unconformities. The exposures of the paleocanyon sug- platy columns and normal natural remanence, whereas gest a width of approximately 1.25 km and a minimum the lower flow is described as having altered entabla­ depth of 100 m below the highest known paleotopog- ture with vesicle sheets and reversed natural remanent raphy at the top of the uppermost Frenchman Springs magnetic polarity (Myers and others, 1979). The Ele­ flow. It is probable that the top of this Frenchman phant Mountain Member is transitional when sub­ Springs flow closely approximates the paleocanyon rim jected to alternating field demagnetization in the labo­ because units normally within the intervening strati­ ratory (Swanson and others, 1979b). However, this has graphic interval are known to lap out 5 to 12 km to the not been confirmed for the intracanyon samples at the south, on the south side of the Horse Heaven Hills Klickitat River. The single Elephant Mountain flow in uplift (Anderson, 1987; Swanson and others, 1979a). the Klickitat River area is tentatively correlated here The depth of the paleocanyon is unknown due to with the lower of the two flows described in the Pasco insufficient depth of exposure. However, the basal basin on the basis of paleomagnetism. colonnade of the Pomona Member is exposed at river The presence of fluvial deposits between the level, suggesting that the flow base is not very far Pomona and Elephant Mountain Members clearly beneath the river level, assuming that relations are indicates that the river that occupied the Pomona similar to those in areas where the base is exposed, paleodrainage re-occupied the same channel after the such as at Mitchell Point. Whether gravel deposits are eruption. This conclusion is supported by the observa­ present beneath the Pomona at the Klickitat River tion that the Pomona Member flow top is located well area remains unconfirmed. However, such deposits below the minimum elevation of the paleocanyon rim, would be consistent with all other known localities. indicating that the flow underfilled the canyon. Preser­ Sedimentary deposits are exposed at the top of the vation of the top of the Pomona Member flow clearly Pomona Member and underlying the Elephant Moun­ reflects the establishment of a fluvial depositional envi­ tain Member. These deposits consist of a well­ ronment. Whether this depositional environment per­ cemented quartzofeldspathic micaceous sandstone unit sisted for the entire duration of the 1.5-m.y. hiatus overlain by pebble to cobble conglomerate with collec­ between eruptions is not clear, based upon present tive thickness of approximately 20 m. Significantly, evidence. The Elephant Mountain Member rests con­ the top of the underlying Pomona Member is pre­ formably on the interstratified sediment at the north served, and the basal sandstone is cemented with silica. side of the paleocanyon and is in contact with flows of This cementation is strongest near the underlying Java the Frenchman Springs Member across a steep but­ flow, suggesting that it resulted from ground-water tress unconformity. However, the picture is less clear flow along the relatively impermeable flow top. The on the south side of the canyon where the contact is composition of both the conglomerate clasts and the poorly exposed. sandstone suggests a source exotic to either the nearby A possibility that cannot be ruled out is that, after ancient Cascade Range or to the Columbia Plateau an initial period of deposition, the river incised its itself. Quartzite cobbles and pebbles in the conglomer­ channel along the south side of the canyon to some ate are less abundant than are basalt clasts. However, unknown depth. The conformable exposures could, quartzite and mica are major constituents of the under this interpretation, be viewed as overflow adja­ underlying sandstone. These sediments are present at cent to the main channel. The relative thinness of the the same stratigraphic horizon as the Rattlesnake Elephant Mountain member suggests that this inter­ Ridge Member of the Ellensburg Formation. pretation may be correct. Absence of this member to The Elephant Mountain Member, in intracanyon the west could mean that (I) the observed thinness exposure, has a prismatic to blocky basal colonnade 1 reflects lap-out resulting from volume attenuation due to 2 m thick and an entablature approximately 10 to to cessation of eruption, or (2) that the channels filled 15 m thick. It is an aphyric flow that appears to be by the two Saddle Mountains members separate at a relatively fine grained in hand sample. Geochemically, point downstream. The former interpretation would it is distinguishable from the underlying Pomona assume that no adjacent channel was present and that Member on the basis of its higher total Fe, Ti02, K20, deposition indeed characterized the 1.5-m.y. interim and P20 5 and lower CaO and Al20 8 contents (Table l). between eruptive episodes. The latter interpretation It can be distinguished from the underlying French­ implies that erosional downcutting occurred after a man Springs Member on the basis of its lower P20 5 period of deposition and that remnants of the Elephant concentration. Samples from this flow yielded reversed Mountain intracanyon flow may yet be discovered natural remanent polarity, as in the Columbia Plateau. within the Cascade Range. Two flows of this type have been reported in the Pasco In the Klickitat River area, the Saddle Mountains basin area. The upper flow was described as having paleocanyon appears to be filled above the Elephant Intracanyon flows, Columbia River Basalt Group 261 FACING DOWN RIVER (southeast) ELEPHANT MTN. MEMBER 10.5 m.y. interbed of Rattlesnake Ridge Member of the Ellensburg Formation Figure 12.-Contact relations of Pomona Member intracanyon flow in the Klickitat River area {location shown in Fig. 10). Sand Hollow and Gingko are informally named flows (Beeson and others, 1985). The labeled "interbed" is part of the Rattlesnake Ridge Member of the ELiensburg Formation. View to the southeast. Mountain Member by flows of the Simcoe volcanic pied a canyon that crosses the Saddle Mountains can­ field. Figure 12 shows the apparent contact relations yon. This younger canyon, of an ancestral Klickitat between the Columbia River Group flows and the River, crosses the Horse Heaven Hills uplift at younger overlying flows. The lowest and oldest of these Grayback Mountain and appears to be antecedent to flows is an olivine basalt flow with jointing very similar tha~ ridge. to that of the underlying Saddle Mountains Member The paleocanyon cut in Saddle Mountains time intracanyon flows. The paleomagnetic polarity of this does not cross the Horse Heaven Hills uplift at the flow, like that of the Pomona Member, is reversed. same point as the younger Simcoe paleocanyon. Field The chemistry and lithology, however, are distinctive; evidence suggests that the cross-over point is at least the flow is a high-alumina basalt (Kuno, 1966) con­ IO km to the west. The Saddle Mountains paleocanyon taining abundant olivine. The chemistry of this oldest probably predates the major part of the ridge deforma­ Simcoe Lava flow closely resembles that of the basaltic tion; no angular discordance is evident between flows suite overlying it. A K-Ar age obtained from the basal within the canyon and flows forming the canyon walls. colonnade of this flow yielded 4.8 ± 0.22 m.y., indicat­ The same is also true of the other localities discussed ing an approximate 5.7-m.y.