Physical Geology 101 Winter Quarter 2009 Eastern Washington Field Trip

Columbia River Basalts & the Yakima Fold Belt, and Missoula Flood features

Group Picture: Gingko Petrified Forest State Park. Thanks to Mike Brady for the fossil displays & the gingko trees!

The Winter 2009 physical geology classes toured south-central Eastern Washington to learn our local geology first-hand. Instructors Cassie Strickland and Signe Wurstner led 43 students to view flows of the Grande Ronde, Wanapum and Saddle Mountains Basalts of the Miocene age Columbia River Basalt Group, to understand how deformation has uplifted these basalts and their various sedimentary interlayers (collectively known as Ellensburg Fm.) into anticlinal ridges known as the Yakima Fold and Thrust Belt, to look at how the Yakima River has become a world-class example of a cross-axial drainage as it incised downward at the same time the Yakima Fold and Thrust belt was uplifting, and finally, to look at how recent catastrophic erosion (the Missoula Floods) has shaped the geomorphology of this region.

Breached Anticline, Benton City, WA. Pleistocene Missoula floodwaters undercut the cliff-forming Miocene age Columbia River Basalt, which is underlain here by Rattlesnake Ridge Fm., a sediment interlayer. Through differential erosion, the soft sediment was eroded by the force of the Missoula floods, and the overlying Saddle Mountain basalt was left without support, and collapsed. This area is known as the “Benton City Badlands.”

Zillah, WA, Pleistocene Touchet Beds. Great examples of Missoula Slackwater sediments form the poorly lithified ‘Touchet Fm’ beds here in Zillah. These fine grained sediments represent slack water deposits that formed as Missoula floodwaters backed up behind constrictions in the Yakima Fold and Thrust Belt, forming deep temporary lakes. A curious feature of the Touchet Beds here are clastic dikes (above, left-hand picture), where graded, vertical layering cross-cuts every Touchet layer. Clastic dike formations are poorly understood; one conjecture suggests that these are liquefaction features that may have formed due to the overlying pressure of subsequent flood waters, or perhaps due to earthquakes.

The Touchet beds underlie most of the Zillah region, and the effect of irrigation on these fertile soils is most obvious at these exposures adjacent to I-82. Irrigation of lawns and orchards has pushed water down through the sediments, reduced the internal friction within the layers, and has caused mass wasting. On the above right-hand picture, notice the crack under Joey Hornes foot. This fracture represents an incipient slide plane; the nearby waste treatment plant should expect to find a minor slump into the settling pond (directly to the west, out of sight on picture) in the near future!

Yakima Canyon, north of Selah, WA. The Yakima Canyon, a deep gorge created by the incised Yakima River, is a great place to see multiple flows of Columbia River Basalts. The oldest rocks in the Canyon are exposed in the core of Umtanum Ridge (picture above on right), an overturned anticline. The most rapid uplift of the Yakima Fold and Thrust belt occurred approximately 4-5 million years ago; not much has occurred since this time.

The Yakima Canyon was formed when the Yakima, a meandering river, became incised at the same time the Yakima Fold and Thrust belt began to uplift. The river was already flowing along this path, and rather than diverting its flow to another path, it instead cut its channel deeply downward. This is a world-class example of a cross-axial drainage. The Yakima Fold and Thrust belt did, however, divert another major river in this region, the Columbia River! Geologic evidence (quartzite-rich Columbia River gravels) suggests that the Columbia River once flowed through Sunnyside Gap (Sunnyside, WA) and down through the Satus Pass region towards Goldendale, to reach towards the ocean. Uplift of the Yakima Fold and Thrust belt apparently diverted the Columbia to its present day course through the Pasco Basin and Wallula Gap.

Picture above on left: Look, a big horn sheep! No, not at me, look OUT the window!

Yakima Canyon. See the white line behind T Holts head (above picture on right)? Don’t worry if you can’t see it….no one but Pearl Styron could at first, either (above picture on left.) This white line is actually a layer of volcanic ash deposited between fluvial deposits, whose provenance was the 7,700 year old eruption of Mt. Mazama in southern Oregon! Mt. Mazama was a composite volcano that had an enormous, caldera-forming eruption. The volcano collapsed into its magma chamber, and what we have left of Mt. Mazama is the lake that filled the caldera: Crater Lake. Crater Lake (1943 ft deep- NPS.gov) is the deepest lake in North America. Testimony to the size of the eruption is how widespread the ash fall was, as we can see here in the Yakima Canyon- ~360 miles north of Crater Lake!

Gingko Petrified Forest State Park, Vantage, WA. Gingko & Museum flows of the Columbia River Basalt Group are exposed here at the museum. The Gingko member is famous for its petrified wood, preserved mainly as various varieties of quartz (picture above on right- Olga K. next to a fossil tree trunk.) Based upon the geological evidence, the trees that would ultimately become fossils entombed in lava flow were NOT alive when it happened. The logs in the Gingko flow are mainly horizontal, indicating that they were downed at the time of burial. In addition, they were most likely part of a log ‘float’; in other words, they were in a pond or lake when the lava flow engulfed them. The process of petrification requires abundant water, and the presence of water cooled the lava flow so that instead of incinerating the trees, they were instead preserved. The petrification process replaces original cellular structure with silica; many of the original features of the wood, like growth rings, are readily visible. Some of the trees appear to have solid chalcedony cores; this may indicate a rotted, hollow core where massive mineral deposits formed.

Above, on left. Instructor Signe Wurstner tells students about petrified wood (Miocene in age) and the role that the Columbia River gorge (below, middle picture background) played during the Missoula Floods. Most of the Missoula floodwaters surged southward towards Wallula Gap through a terrain known as the ‘Drumheller Channels’, which is many tens of miles east of Gingko State Park. A large volume, however, also flowed down through Grand Coulee, Crab Creek Coulee and various other coulees to drain into the Columbia River to the north and south of Vantage, and flowed towards Sentinel Gap (picture below, on left.) Sentinel Gap is what we call the breach in the Saddle Mountain anticline (also part of the Yakima Fold and Thrust Belt) through which the Columbia River flows. This gap was another constriction to the Missoula floodwater, and was enlarged dramatically during the floods. In addition to floodwaters from glacial Lake Missoula, newer evidence supports an additional source of floodwater in the form of outburst floods from under the Okanogan lobe of the Cordilleran Ice Sheet, in the vicinity of what is now southern British Columbia, which also flowed down this channel, on its path to Wallula Gap (Dr. Robert Young, PNW NAGT Annual Conference, June 2008.)

Middle picture, below. When the Wanapum Dam was built directly south of Vantage, it created a reservoir which flooded not only the original town of Vantage (relocated to its present location) but also several important archaeological sites. Petroglyphs created by paleo-peoples were inundated; some of them were rescued and relocated to the Gingko museum. Walking along the path down to the petroglyphs, a sharp-eyed student spied the vesicle pipe in Columbia River basalt (the left-most picture below.)

Frenchman & Echo Coulees, Silica Rd. Top left & right photos, Frenchman Coulee (west is on the left of the photo.) A coulee is a steep-walled canyon cut by the Pleistocene floodwaters. Frenchman Coulee, and its parallel sister, Echo Coulee, are breath-taking examples of headward erosion by the floodwaters. Each coulee began as a cataract, or waterfall during the Missoula floods. This waterfall wouldn’t have looked like much at the time; the volume of floodwater flowing through these coulees would have filled them to t heir brim; the ‘waterfall’ would have looked like a drop of only a couple feet! ‘Headward erosion’ means that the coulee grew in an ‘upstream’ direction; lengthening of the coulee occurred as the waterfall undercut its cliff as churning, turbulent currents in its plunge pool eroded away the cliff base. When the waterfall precipice became too undercut, the cliff would collapse, thereby making the canyon longer, and pushing the cataract further eastward.

In the picture above on the right, notice the piles of debris, called talus, skirting the cliffs. These talus piles are created by a type of mass wasting called ‘rock fall.’ Yep, that’s the technical term. Finally, a geology term that isn’t 7 syllables long, and in a different language! Also evident is well-developed colonnade. Colonnade is what we call the well-defined polygonal columns that form in some of the basalt flows. As the bottom of the lava flows cools slowly (as compared to the top which cools quickly and creates ‘entablature’), the lava contracts and fractures during cooling, in a process called columnar jointing.

Photos Above: Top Left- Amy Hatch, Michelle Valenta & Laura Astudillo, Echo Coulee. Top Right- Corynn Miller, Echo Coulee Cataract. Middle left- Geology Class and professor Mike Brady on the cataract of Echo Coulee. Middle & Bottom Right- Chris Sanchez on Echo Coulee cataract- look, there is still a waterfall. Okay, this one is just irrigation run-off, but still pretty. Bottom left- Drew Foraker decides that Chris’s idea to take the ‘easy’ way down Echo Coulee is perhaps NOT the smartest route.

Diatomaceous Earth Mine, Silica Rd. These white sediments represent a paleo lake bed that had abundant critters known as diatoms. These critters had silica skeletons, and the sediment that contains their microscopic fossils is known as diatomaceous earth (DE.) DE has many commercial uses, which is why these open pit mines are here. In the picture on the left, students are looking for poorly preserved pieces of petrified wood that have turned into colorful pieces of chalcedony (green, red, brown) and some are semi-precious opal. In the picture on the right, Victoria Vargas examines the interesting alternating beds of basalt breccia and white diatomaceous material.

That’s all for our Trip, hope you had a good time! Come back for Historical in the Spring!

Picture to left: Pearl Styron, Cheyann Larsen, McKae Landon and Haylee Hansen at the Gingko Rock shop, Vantage, WA.

Back to Columbia Basin College Geology Dept, Pasco, WA