G102/202                                                        Name_____________________________________           

Lab Eight                                                        Lab day   M   T    W

Geology of Oregon City

We begin our story during the Mesozoic Era.  This was a time when the coastline for Oregon was located near Idaho.  The place we call the Willamette Valley was under the ocean.  The oldest rocks in this region are known as the Waverly Heights Basalts and represent old ocean floor.  They were formed during the Eocene Epoch and accreted to the N. American continent due to plate tectonics.  Many episodes of accretion have occurred on the west coast of N. America and large parts of OR, WA and CA are made up of these accreted blocks.  Ok…back to the Willamette Valley (WV).  During the Paleocene and Eocene, the WV was a marine shelf environment and was accumulating sands, mud and marine sediments. As time progressed, the shoreline move to the west, and during the Oligocene, Miocene and Pliocene, the environment in the WV was a shoreline where rivers were emptying into an ocean.  We know this by the delta deposits found in the rock record for this time.  One of the reasons the shoreline moved westward was due to the uplift of the Coast Range.  Remember, before this time there was no Coast range or Cascade Mountains. It was during the late Oligocene  and early Miocene that the Coast range

began to uplift and the earliest Cascade Mountains (known as the Western Cascades) began to form.  The uplift of these two mountain ranges created a “trough” between them which we call the Willamette Valley. 

During the middle to late Miocene a series of large volcanic eruptions of basaltic lava began.  These lavas originated from fractures near OR-ID border and are called the Columbia River Flood Basalts (CRBs) (Fig 2).  These lava flows are called flood basalts because they were very fluid in nature and covered large areas with deep flows of lava.  These flows followed river valleys (often filling them in) and some of these flows managed to flow all the way out to the coast.  Today, they create the coastal headlands such as Tillamook Head, Cape Foulweather etc.  The CRB’s are found also throughout the Portland area.  The two major flows in this area are called the Grande Ronde Basalts and the Wanapum Basalts.  The CRB’s erupted over a time frame of several million years which means that after an eruption occurred there would be a time of no volcanic activity.  During these times, soils formed and trees/plants would grow.  One of these soil layers has been preserved by the Wanapum basalt flow that covered it.  It is known as the Vantage Horizon and is an important layer in that it has help scientists understand what the climate and area was like during the Miocene. 

Fig 2

Above:  Extent of CRBS showing origination fissures as dotted lines.  Right:  Abbreviated table of the different members of the CRBs

During the Miocene and into the Pliocene, the north end of the WV was a fairly large lake that accumulated sediments such as silts, mud, gravels etc from the Willamette and the Columbia River.  The sediments that were deposited at that time are known as the Sandy River Mudstone

and the Troutdale formation.  The Sandy River Mudstone is mostly lake deposits and is made primarily of mud and sands.  The Troutdale formation is made up of river cobbles and represents the ancestral Columbia River. 

During the end of the Pliocene the water receded from the north end of the valley due to continued uplift and warping of the coastal range.  At the end of the Pliocene (about 5 mil years ago) and into the Pleistocene a series of localized volcanic eruptions began in the Portland area.  This volcanic activity is called the Boring Lavas and the first vent originated near Boring OR. This activity created several small shield volcanoes (Larch Mountain, Mt. Sylvania, Highland Butte etc.) and a series of cinder cones (Mt. Tabor, Kelly Butte, etc.).  Most of the buttes found in the Portland area were created by this volcanic episode.  An area from Boring OR to just south of Wilsonville was affected by these eruptions.

The last major geologic event to affect the WV was the glacial floods that occurred during the Pleistocene.  These floods are known as the Missoula Floods due to where they originated.  During the Pleistocene, North America was experiencing a glacial period.  OR was much wetter overall and glaciers extended down from the north about 1/3 of the way into WA, ID and MT.  During this period, the WV was covered by a fine silt deposit known as loess that was created by the glaciers to the north and carried into the valley by wind.

About 15,000 years ago a glacier was blocking a valley in MT.  Water built up behind the glacier and as the water cut below the glacier, the glacial dam that was blocking the valley failed.  This sent 400 cubic miles of water flowing across MT, ID, WA and OR.  These floods, for the most part followed rivers and came through the gorge area and emptied into the WV.  When these waters emptied into the WV they formed a lake (lake Allison) that lasted for a couple of weeks until the water slowly drained out to the ocean.  However, many different types of deposits were

left behind by these flood waters.  One of the most common deposits left behind were the gravels.  These gravels were carried by the fast moving water and as the water slowed down in places the gravel would drop out to form gravel bars.  The town of Mollala is built on one of these bars.  Other deposits include old river channels (the floods changed the course of the Tualatin River) scoured rock surfaces (scablands), erratics and other large rock deposits.  The water depth in the WV was about 350-400 ft deep.  This is known because there are only loess deposits above this elevation.

These floods happened ~30-50 times from 15,000 to 12,000 years ago.

For today’s lab, you are going to use the information above and study the geologic history of Oregon City.  In your first exercise you will use a geologic map of Oregon City and list out the rock units or the lithology of the region, and describe what type of environment each unit was deposited in.  Read the information carefully and pick out the words you understand.  ASK QUESTIONS when you don’t understand something.

Using the geologic map of the Oregon City Quadrangle, give the lithology (rock types) and describe the environment that produced each of the formations.  Use the legend on the map for this information.