Armchair Geology: Notes from Class 2 with Eric Bard

By Keith Douville, student in the class

Hello again, another blog following the Geology of the Pacific Northwest class.  Our class this week was focused on “Methow Mysteries, Rocks, Geologic Events, and Connections.”  This time around we worked to draw correlations from the big picture to our corner of Washington and narrow the focus to our observations locally.  Students brought in rocks from around the Valley to observe and discuss.  This homework assignment of finding rocks was designed to get us comfortable looking at rocks and excited about local finds.  It did just that, with many folks bringing in great specimens to share.    

We began with a few questions to keep us thinking about the big picture.  I will list the questions here and follow them up with answers at the end.  Take the quiz and see how you do!

1)     What is the difference between a Passive and Active Margin?

2)     What major ocean opened up to change the Pacific coast from a passive to an active margin?

3)     What specific activity creates volcanic arcs?

4)     When did the eastern boundary crescent basalts arrive in geologic time:  Permian, Cretaceous, or Eocene?  (Hint: the times are listed from oldest to newest). 

5)     What fault divides the Okanogan and the Methow, running roughly from pipestone through 8 mile creek?

All of our sciences have roots in geology, as it is the foundation on which all natural events occur.  There are many good resources out there for geology students, and basic textbooks are a good place to start.  You don’t need to spend a lot of money, as often these texts can be found cheaply second hand at thrift stores or online.  One we discussed is Geology of the Pacific Northwest, by Orr and Orr.  Another more casual read that was discussed between students was The Restless Northwest: A Geological Story, by Williams.  When purchasing books keep in mind that many changes have occurred to our understanding of geology in the last few decades, so try to find one that discusses plate tectonics as this is relatively recent in our understandings of the Earth. 

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.   

Chert

The oldest rocks in our area are typically oceanic rocks from the Hozameen terrane.  These rocks were once part of the ocean floor and date from the Paleozoic to Jurassic time.  These basalts are often metamorphed, identified by a green color.  Radiolarians rich in silicas in deep water often form layers of chert (microcrystalline quartz) within these rocks.  These cherts provide evidence for a western source from the Hozameen Terrane for some strata in the Methow Basin fill (eg. Virginia Ridge formation).  

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 Washington 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!

Conglomerates and shale on top of Virgina Ridge

Close-up of shale (and bitterroot) on Virginia Ridge

Following the Newby rocks are the Fore arc basin fills.  These sedimentary sandstones, conglomerates, and shales were deposited over time between other formations and are of marine and stream origins.  They occur from different time periods ranging around the Cretaceous.  Some examples of these are in Virginia Ridge and Winthrop sandstones, and the Buck Mountain and Paterson Lake areas.  Some are comprised of more fine materials and are known as siltstones.  Many of the parent materials are of volcanic origin but as currents can move particulate great distances in the ocean it can vary.  Some argillites can be present, and we often see conglomerates in this formation which differ from breccias as the larger pieces contained within are rounded.  Remember that the conglomerates have rounded aggregate and indicate high energy rocks (think stones tumbling in a stream) and breccias have sharp aggregate and are from deposits that have not transported far or been eroded by water.  Many of the rocks in these formations are formed in submarine fans and under ocean mudslides.  Unique index fossils can be found in these rocks as well, which help us date them and learn about ancient extinct life such as Ammonites.  

The Midnight Peak Coast range volcanoes deposited volcanic rocks on sedimentary rocks, shown in places such as Goat Wall.  These are volcanic andesites. 

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 Coast Range 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 Oval Peak tonalite, Fawn Peak diorite, and Monument/Golden Horn granites, all from varying time periods. 

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.