Geology 101, Fall 2007 UPPER YAKIMA RIVER VALLEY FIELD TRIP The evolution of Washington’s Southern Cascade Mountains over the last 55 million years can be divided into four distinct stages. 0. The base on which all of the following events were to occur are the original west edge of the North American craton, which consisted of Jurassic and Cretaceous age metamorphic rocks, similar to those in the North Cascades. 1. The coast of Washington 55 to 40 million years ago (my) was oriented northwest/southeast and, like today, was a convergent margin. This view is somewhat controversial, in that there is much rock evidence which does not fit the “classic” convergent margin model. Assuming the convergent story, a continental volcanic arc, called the Challis Arc, existed inland of the convergent margin’s subduction zone (east of the present Cascades). Numerous volcanic and sedimentary rocks were deposited during this time. Sedimentation occurred in numerous basins which probably were related to strike-slip faulting. Sometime around 52 my, movement along the Straight Creek Fault began and continued until, at the latest, 35 my. 2. After the accretion of the Olympic Mountains Block (and a coincident reconfiguration of plate motion), the subduction zone moved to its current location and north-south orientation off the coast of modern-day Washington. From 35 to 20 my, a continental volcanic arc existed where the Cascades are today. Evidence for this arc is preserved in the form of numerous volcanic and plutonic rocks. These rocks are referred to as the "West Cascades" because they are often but not always found to the west of the modern High Cascades. At the same time, sedimentary rocks were forming along the coast; these rocks would later make up the Blakeley and Renton Formations near Seattle (a formation is a group of age- or composition-related rocks found in a geographic location). 3. The eruption of the Columbia River Basalts, which began about 17 my, may have corresponded with a lull in Cascade volcanism. During this time, the Cascades were eroding, resulting in sedimentary rock such as that found in the Ellensburg Formation. The Cascades were probably lower in elevation, allowing more moisture to pass into eastern Washington. There is some evidence that some of the Cascadian volcanoes were active at this time. 4. Since 2 my, Cascade volcanism has again become active, resulting in the formation of the "High Cascades", such as Mt. Rainier, Mt. St. Helens, etc. Based on this history, we will examine various outcrops to show that there is evidence for some of the story. Though the geological history is not crucial for understanding and writing about the topics for the teaching module (see back of handout for the assignment), the maps and the history are included to flesh out an interesting time in Washington’s geological record.Take I-90 east to exit 80 (Roslyn-Salmon La Sac). Turn left under the highway and continue for about 4.5 miles and make a left onto State Route 903 towards Roslyn and Salmon La Sac. Follow the road about 12 miles and pull off to the left (along the lake) and examine the outcrops on the right. Stop 1: Swauk Formation/Teanaway Formation (52 to 47 my) Sketch the outcrop and identify and label the whitish rock. Include a scale! Identify and label the black rock surrounding it. What type of rock is it? How did this rock get here? What is the age relationship between the white rock and the black rock? What relative dating principle is invoked here? Is the white rock altered by the presence of the black rock (look at the white rock at stop 2, then look at the white rock near the contact)? So what type of rock is the white rock, technically? From the white rock, derive a rough paleocurrent (compass) direction of the black rock. Turn around and drive 1.7 miles, pulling off into a turnout on the right. The outcrop is across the road. Stop 2: Swauk Formation (55 – 51 my) How do the Swauk Formation rocks here differ from the Swauk Formation rocks in the previous stop? What is the rock type and name?Identify the clasts visible in the outcrop. Identify the fossils, if any, in the outcrop. Sketch examples of the fossils (include a scale). Have the rocks of the Swauk Formation been tectonically affected since they were deposited? If so, sketch evidence of this deformation. As you drive back down the road toward I-90, look at the overlying Teanaway Formation. Has it been tilted like the Swauk Formation? What kind of contact (conformable or unconformable) exists between the Teanaway and Swauk Formations? Return to Cle Elum and continue on SR 970 through town. At the intersection with SR 10, take SR 10 along the Yakima River. After about 5 miles, pull off the road at a turnout; the outcrop will be across the road. Stop 3: Manastash Ridge Member of the Ellensburg Formation (17 - 16 my) These rocks represent fluvial (river) and lahar (volcanic mudflow) deposits. What kind of rock(s) are these? Hint: it’s at least two different rocks (as indicated by the sources listed above). Sketch part of the outcrop that has these two rocks and label the sketch with the rock names. Note that the grain size of these rocks will be key in identifying the rocks.What was the source of the sediment in these deposits? In other words, from what geographical feature did the sediment originate? Hint: identify the bigger clasts! Does the variation in grain size between the rocks tell you anything about the energy of the transport agent that brought the sediment here? In other words, was there a variation in the stream (or whatever) flow rate? Are there any faults in this outcrop? Sketch an example (you may have to walk up or down the road to find one). Are these faults younger or older than the rocks? What dating principle is illustrated here? As usual, include a scale. What kind of tectonic stress did these rocks experience (hint: you can tell from what type of faults these are)? Besides the faulting, what evidence is there for the type of deformation seen in the Swauk Formation (a much older formation)? Continue east on SR 10 for several miles, until you see some yellow in the rocks on the roadcut to the left. Pull over at a convenient turnout overlooking the Yakima River. Stop 4: Grande Ronde Formation (17 my) What is the name of the dark rock here?The dark rock forms shapes. Sketch part of the outcrop,