Baker Window

 

 

Nooksack Formation along Baker Lake Road

I ventured into the Baker Window last week. A window in geology terms is a structure where there is a hole in the overlying thrust sheet or sheets. The rocks under the thrust sheet are exposed in the window area.

Schematic overview of a thrust system. A hole in the nappe which exposes the underlying rocks below the thrust fault is called a window. A klippe is an island like block above the thrust likely due to erosion or additional faulting. Schematic from Wikipedia:  ( https://commons.wikimedia.org/wiki/File:Thrust_system_en.jpg#/media/File:Thrust_system_en.jpg)

Simplified geology map of Baker Window  (Tabor and others, 2003)

The Nookasck Formation (light blue and N) is exposed in the Window. Thrust nappes over the Nooksack Formation from base to top are the Excelsior Nappe (EX), Welker Peak Nappe (WP), Shuskan Nappe (S), and Gold Run Pass Nappe (GRP). Tabor and others (2003) suggest that the window is in part exposed due to a extensional fault noted on the west side of the Baker Window.

One can see that Mount Baker is sitting in the middle of the window. The rocks of the volcano are not part of the window. Mount Baker is a recent overprint that happens to be smack dab in the middle of the window significantly obscuring the exposures of Nooksack Formation posing a bit of a challenge correlating units across the area.

The Nooksack Formation is the autochthon and the nappes above it are all considered allochthonous. That is the Nooksack Formation was there first and the nappes all were moved via thrust faults to their present locations. At least three of the nappes are considered distinct separate terrains with uncertain origin, but likely came from very far away places. The Nooksack Formation itself also may originally have formed elsewhere, but relative to the above nappes it was there first and has not undergone as much metamorphism as the nappes that overlie it.

The Nooksack Formation rocks on the north side of Mount Baker are somewhat different than the units on the south. As Tabor and others (2003) noted, "Where fossils are absent, some of the more deformed clastic rocks of the Nooksack are difficult to distinguish from rocks of the Late Paleozoic Chilliwack Group". Indeed, earlier maps as recently as 1986 (Brown and others, 1986) put some of the Nooksack Group as Chilliwack.

I stopped at the cliff along the road shown above and looked for fossil belemnites, a definitive feature elsewhere in the Nooksack Formation. I found no fossils and found the rock to have multiple distinct slaty cleavage development. Those observations are consistent with Tabor and others (2003) as well as my own memory when I stopped along this road in 1983 with Chuck Ziegler during his time working with these rocks.

 

Samish Island Great Blue Heron Rookery

We had a sunny dry day last Saturday to take a walk about at the Samish Island heron rookery. The rookery is where great blue heron nest in trees on the island. The rookery is within a fairly mature second growth mixed species forest. The Skagit Land Trust was conducting a yearly nest count with volunteers.

 

Proud property owner of conservation easement

great blue heron nests

Tagged tree with nests 

Egg fragment on forest floor

Fire scarred mature Douglas fir with heron nests

Besides the heron nests I spotted a few other forest features

Douglas fir stump with bark having grown over the stump post cutting

Pacific yew

Glacial erratic

The forest is underlain by glacial drift

Upright Head Preserve Erratic and Forest

One last post on Upright Head (upright-head-wdot-property-walkabout and upright-head-conglomerate).

 

To the west of the Lopez Island ferry landing is Upright Head Preserve. The preserve was purchased by the San Juan Land Bank (sjclandbank.org/protected-lands).

 

A fair bit of the forest in the preserve is typical western Washington forest.

Western red cedar

Western hemlock

Trial through forest towards northern tip of Upright Head

The trail follows a former road route

The northern tip of Lopez at the north end of the preserve has another stand of junipers with madrones and balds as well as view to the north.

From left to right:

Orcas, Obstruction, Lummi and Blakely

For glacial minded folks there is a large glacial erratic in the forest east of the main trail. A bit of the British Columbia Coast Range brought to Lopez Island during the last glacial period.

Junipers, Sagebrush, Comedy and Terror

Plenty of computer screens have been covered with words and takes on antigovernment militant takeover of buildings at the Malheur Wildlife refuge in Oregon. I find it a fascinating mix of comic and terror.

For those that want to get into the details of the background story that led up to this current drama, its worth reading the BLM grazing permit denial for the allotments that the Hammonds had been using (wildfiretoday.com/dHammond_grazing_decision.pdf). It is worth considering what it must be like as a range or fire manager in dealing with folks like this. There is comic and terror in this document.

Some imagery of the Kumbo Butte burned area where one of the wildfires took place took place:

2006 image

Dark vegetation are western juniper trees

2006 image (USGS) of north edge of burn area

Note fire scars along road

2011 image

Note that there are now much fewer western junipers

The Hammond Ranch is within the Donner und Blitzen Valley north of Frenchglen at the west base of Steens Mountain. For those familiar with Oregon geology it is a bit north of the Diamond Craters area. The allotment area where the permit was denied is high elevation juniper and scrub steppe. Range land without western juniper and sage brush is more valuable for cattle - hence the use of matches during August 2006. The actions that took place in 2006 were comic but also terror with no regard to the risk those actions posed to fire fighters and other properties.

Snow Day

I generally do not like to do field work in the snow. It is not the cold. Snow limits my ability to see and read the landscape. But the snow cover was thin enough at this site that other than awkward footing on snow covered limbs and branches, visibility and access to the soils was adequate. The site was also located in an area that I had done a fair bit of previous work. This trip simply meant confirming a few things.

The snow was on the order of 8 inches with thinner patches in the forest so digging through it was not much of a chore. The soils were well drained glacial outwash and moraine material as expected from previous ventures.

I did get to see some bedrock. Metamorphosed sedimentary rocks with multiple cleavage surfaces and dissolution veins. A future post for that rock's story.

Upright Head WDOT Property Walkabout Notes

The east side of Upright Head on Lopez island is owned by the Washington State Department of Transportation. After checking out the conglomerate in the road cut down to the ferry (upright-head-conglomerate), a visit to the WDOT property to the east is a pleasant natural park setting with nice views, more conglomerate and mix of bedrock balds and unique forest.  

Looking north to the ferry dock and Orcas Island with bald in the foreground

One of my favorite Juniperus maritima trees is located at this site.

Besides the relatively tall juniper on the WDOT property, there are patches of small young junipers as well as some very photogenic specimens.

There is also a patch of Querus garryana (Oregon white oak or Garry Oak) and a few Pinus contorta (lodgepole pine)

patch of low growing oaks

lodgepole foliage and cone

Towards the south is a very nice stand of Arbitus menziesii (Pacific madrone).

Madrone and Douglas fir with salal understory

Up the slope through the madrone/fir forest back towards the road is a engineered wall built of gabbion baskets supporting the long term parking area above.

Upright Head Conglomerate

With the short winter days and three projects on three different San Juan Islands I had a bit of a lay over wait for the ferry from Lopez Island to San Juan Island.

The ferry landing at Upright Head has plenty of interesting sites on public access land. Hence, one can get in the ferry wait lineup early (not really an issue mid winter on a week day) and spend the time waiting by taking nearby short hikes.

The very first site to check out is the great road cut exposing pebble conglomerate adjacent to the road down to the ferry.

Conglomerates can have a very good story to tell, but despite the great exposure and easy access to this conglomerate deciphering this conglomerate's history has been a challenge. Whetten (1975) recognized that this conglomerate was younger and had a different history than conglomerates on the south part of Lopez. Brown (2012) has incorporated the information extracted from the pebble make up and age dates of zircons in the conglomerate to constrain the final assembly of the San Juan thrust nappe sequence.

Figuring out a story from conglomerates requires knowing a lot about potential source material. Where did the pebbles come from? This particular conglomerate has pebbles that resemble two thrust nappes to the north - Orcas Chert/Deadman volcanics and East Sound Group/Turtleback Complex. Pebbles associated with other thrust nappes are not present suggesting that those formations had not yet been thrust onto the San Juan nappe sequence when this conglomerate was deposited see (Brown, 2012) .

There are lots of other conglomerates in the San Juans. The older conglomerates are more metamorphosed. In this conglomerate the metamorphism is subtle. At the outcrop level there are a few hints. In most places, but not all the bedrock fractures at least partially through the pebbles (see above pictures). But in other places the fractures go around the pebbles suggesting some differential strain. But in the images below note the dissolution fractures in the rock including some that cut straight through pebbles.

While the best exposure is the fresh cut slope in the rock that the road, the same formation can be observed in its more native condition on bedrock outcrops along the shore and shore bluff to the east.

A walk to the shore bluffs is well worth while and includes great views as well as an interesting mix of trees.