So what parts of science have contributed to this understanding of geology? Petrology is the study of rocks and is obviously valuable. The identification of isotopes and their decay can help us determine radiometric ages. Strontium and Rubidium ratios have helped us to determine where the approximate coastline of the ancestral continent of
North America existed before the accretion of younger terranes. The identification of geologic structures such as faults along terrane boundaries, areas of deformation, or folding can help us decipher changes and the timing of changes that occurred in our area. Fossils can help date rocks to certain times, as can radiometric dating. The stratigraphy of rocks and their environments can help us understand the dates of events, as layers are deposited at different times. Just remember that the “top” may have once been at the bottom and that entire layers can be flipped or folded. Finally, uniformitarianism is an important concept in geology. This means that what we see happening in our world today can lead us to conclusions about what happened in the past, as we can assume that many of the same processes occurred to create similar features.
Here in the Methow one of the major events that occurred was the fore arc basin. The basin that occurred between island arcs allowed for deep deposits of sediment to form rocks. This was later uplifted as the area was squeezed between accreting terranes.
Next, in our general stratigraphy, came the Newby or Twisp formations in the late Jurassic. These argillites (or more informally shale and volcanic rocks are from early island arcs and the surrounding ocean rocks. These can be metamorphed later into other rocks and they can be a challenge to identify at times. Remember that green rocks often indicate this metamorphism. The red seen on some of these rocks usually points to iron oxide (rusting) on the rock surfaces and to get a good look break them open with a hammer (use safety glasses). Drawing their name from Newby Creek, these rocks have larger mineral formations speckled amid smaller particles. The sharp angles in the fragments in some of the volcanic breccias may form at the base of lava flows or as mud flows and the fragments have not had time to round as do river stones. Some of the more violent explosions formed tuffs, although the exact location of ancient eruptions in not clear. We are talking about millions of years ago after all! Many of these rocks can be sedimentary, with volcanic parentage. They become cemented together with silicas, iron, and calcite. They can contain fossils, so keep hunting out there, you may find a few. In fact, dinosaur fossils have not been found in
to date, but it seems likely that if found they would be in sedimentary rocks in the Methow area. Find one and you will be famous! Washington
|Conglomerates and shale on top of Virgina Ridge|
|Close-up of shale (and bitterroot) on Virginia Ridge|
range volcanoes deposited volcanic rocks on sedimentary rocks, shown in places such as Goat Wall. These are volcanic andesites. Midnight Peak Coast
The local Pipestone Formation, which are mainly conglomerates, not soapstone, are formed by erosion from
Okanogan batholith rocks to the east and were later cut out by glacial melt water. Plant fossils like the extinct Dawn redwood can be found in them. They weathered into pipe shapes and thus the name of the formations. Zircon crystals in sandstones formed in the cretaceous period about 70 million years ago, but the dating of these rocks could be just dating the older minerals which make them up. Any way you date them, they are still the youngest sediment stone found within the Methow.
Throughout time, intrusions of igneous rocks appear as granitic rocks, some from lava flows and explosive events and some as slower cooling subterranean magma pockets. The material to form these rocks was born in subduction zones, and the magma contained in these plutons can help to “stitch together” geologic terranes. During the
episode, many of these rocks built mountains. Along the margins of these rocks precious or valuable metals can be extruded from host rocks and concentrate. This can cause excitement for miners and not surprisingly many of the mines in the area are concentrated along intrusive margins. For example, the Alder Creek mine was a source of zinc and copper. Be sure to use extreme caution around mines because of unstable rocks, chemicals leeching, rotting timbers, and low oxygen levels within mines that can be difficult to detect until it is too late. Some other examples of intrusives are Coast Range tonalite, Oval Peak diorite, and Monument/Golden Horn granites, all from varying time periods. Fawn Peak
Next week we will cover activity to the West of us, notably the “Cascade Episode.” Stay tuned!
Answers to Questions from Above
1) A subduction zone at the margin is the usual event that creates an active margin.
2) The opening of the
Atlantic Ocean begins westward movement of the North American continent, creating the active margin on the Pacific west coast.
3) Subduction creates volcanic arcs.
4) The Eocene marks the arrival of the Crescent Basalts, about 50-40 million years ago.
5) The Pasayten fault separates the
Okanogan from the Methow.
Keith has spent his first summer working on the Beaver Project crew here in the Methow Valley and he's eager to learn all he can about this amazing place.