2003 Geology Road Trip
MOUNTAINS AND VALLEYS
Date of trip: October 11, 2003
Time: 8:30 AM
Meeting place: Park Avenue Square Restaurant in Chewelah
We think of earthquakes in terms of destruction. Collapsing buildings and buckled
highways are disastrous, in human terms. From a geological perspective, any
earthquake is an ephemeral part of the work that is accomplished by a fault.
Folds set the architecture of the land in Pennsylvania. Sedimentation is the
foundation of Mississippi. A single fault shapes the wall of the Eastern Sierra
Nevada. The valleys and ranges of Northeast Washington are sculpted by a set
of fractures that worked, in concert for fifty million years. The southeastern
flank of the Colville Valley follows the Jump-Off-Joe Fault. The Newport Fault
is the design theme of the Pend Oreille Valley. Those two faults interacted
to form the Spokane Dome range of the Selkirk Mountains. The Kettle River, Sherman,
Bacon Creek and Okanogan Valley Faults repeat the patten, from North Idaho to
near the Cascade Mountains.
As I was originally writing this I perceived a problem that most of the chronology
was based on analyses done prior to 1974. Between 1976 and 1985, technologies
were pioneered that greatly improved radiometric dating. A few hours after finishing,
I opened the October issue of the Geological Society of America Bulletin to
find a paper by Gerald Ross and Mike Villeneuve of the Geological Survey of
Canada. "Provenance of the Mesoproterozoic Belt basin" is based on
a very large set of recent isotope analyses. Knowing the rate of sediment accumulation
and differing areas whence sediment was derived, clarifies the history of how
the basin was formed and evolved. The techniques are precise and selective that
individual zircon grains are dated. That lets tham be traced to their origin.
In this case the source started at the southeast and moved to the west. The
western land mass was cleaved away, during Cambrian time. The rocks with the
best fit as a source are now in eastern Australia. I'll leave the original text,
as a starting point for discussing the changing ideas.
Mileage:
0.0 Start south on Highway 395 from Park Avenue Square, Chewelah.
0.3 Turn left on Main Street to follow Flowery Trail Road to the east. The route
follows Flowery Trail Road all the way to Usk.
1.3 The small, rugged outcrops of rock are Edna Dolomite. As part of the Deer
Trail Group, it occurs in only the upper block (hanging wall) of the Jump-Off-Joe
Fault.
2.0 Turn left, into the access to the small quarry. Devonian to Missippian limestone
was altered by hot brine flowing through fractures of the Jump-Off-Joe Fault.
Near Springdale this rock contains many fossils and irregular nodules of chert.
There the fossils are re-crystallized, much of the CO2 is gone and the calcium
and silica have formed wollastonite. Continue the route to the east.
7.2 The Hecla Mine, about a quarter mile south of this turn was a minor producer
of copper, in the early Twentieth Century.
7.4 Colville National Forest boundary.
9.3Stop near the new road cut to look at the Flowery Trail Granodiorite. The
ease of weathering and erosion on the granodiorite has allowed Thomason Creek
to cut a large canyon. Granodiorite usually has abundant dark minerals, but
it is the abundance of
Geology Field Trip 2003
Mileage:
9.3 continued plagioclase and scarce quartz that define the rock. The K-Ar ages
of this rock are about 175 million years in hornblende and 48 million years
for biotite. So, the pluton was formed in the Jurassic Period. Then it was heated
enough for the mica to lose most of the radiogenic argon, in the Tertiary Period.
Continue east.
11.9 Flowery Trail Pass
14.4 Park in the intersection with the Nelson Creek Road. All of the bedrock,
in this area is Prichard Formation. The Prichard is the oldest part of the Belt
Super-group. The Belt underlies most of North Idaho and western Montana. In
the Purcell Mountains of British Columbia, it is known as the Purcell Group.
In 1975, Fred Miller correlated the Deer Trail Group with some of the younger
rocks of the Belt. He proposed that the Deer Trail sediment accumulated in a
basin near the Belt Seaway. After further study of the Belt, in the Couer D'
Alene District, he
proposed a different correlation. The Deer Trail rocks might be the thinner
equivalent, on the edge of the same basin, all of the Belt Formations that are
exposed in Washington. That relationship would require a displacement of tens
of miles on the Jump-Off-Joe Fault.
14.7 Park at the end of the guard rail, on the right. Sills of diabase characterize
the Prichard Formation. As the Belt Basin formed, possibly a setting similar
to the southern Gulf of California, basaltic magma intruded the sediment. For
the most part, the magma spread out, between the layers of soft rock, forming
horizontal layers. In a few places, such as near Kimberly, British Columbia
and Noxon, Montana. The lava and hot brine reached the sea floor. Silver, copper
and zinc minerals were leached out of the basalt and deposited on the sea bottom,
forming the Sullivan and Montanore ore bodies.
16.5 The White pine, on the left, were some of the first to be planted from
blister rust resistant stock.
17.1 The bedrock, here, is lighter colored than the granodiorite that we saw
before. The Phillips Lake Granodiorite contains less of the dark iron/magnesium
minerals than the Flowery Trail. It is about 100 million years younger.
19.6 Intersection with Barlette Road.
20.0 The scattered outcrops, on the left are schist. Much of the highly altered
rock, on this side of the Spokane Range, passes gradationally into Pritchard
Formation. Some of the gneiss and schist may be older than Pritchard. No dating
has been done, in this region, with the newest techniques.
21.6 Park near the intersection with Winchester Road. This road cut exposes
an angular unconformity. Glacial lake beds lie horizontally on steeply dipping
slate. The age of the slate is unknown, but we can make some guesses. The new
excavations provide much more information than when the maps were made.
22.6 Intersection with Danforth Road. Fractures rock of the Newport Fault is
exposed in the hill, just to the south.
26.4 Park near the intersection with Westside Calispell Road. The bedrock is
Tiger Formation conglomerate. This rock formed as the Pend Oreille Valley opened
Geology Field Trip 2003
Mileage
26.4 continued with the movement of the Newport Fault. Most of the Formation
is gravel sized conglomerate, but it includes sandstone, shale and coal near
Ione. Those sequences record movement on the fault. Coarse material accumulated,
after temblors, when stream gradients were steep. Shale and coal formed in the
quiet intervals, after filling episodes. Older parts of the Formation have only
clasts of rocks that were near the surface, before faulting. Later, when the
fault took the cover off of the Spokane Dome, gneiss is included.
Most of the view to the north and east is land lower than the level of the Box
Canyon reservoir. To drain the area, Calispell Creek is pumped over a dike.
Continue toward Usk.
28.7 Turn right on Highway 20.
34.4 Turn right on Turner Road.
35.4 Park near the Grange Hall. The Bracket Creek Riparian Restoration Project
was implemented in 1997. That many of the planting were not successful is common
in Eastern Washington riparian projects. Native shrubs are less robust than
cultivars. A large portion of the riparian and channel values have been restored,
compared to the un-treated segment, north of the road.
36.8 Return and continue south (right) on Highway 20.
37.9 Park along the right side of the of the Highway, near the rock knob between
the highway and railroad. This is the mylonite of the Newport Fault. This must
have been exhumed from a depth of more than 5,000 meters, by the fault displacement.
Most of the mica and hornblende have combined and re-crystallized to pyroxene.
Whatever meta-sedimentary, felsic intrusive, or gneissic rock this might have
been, the friction under high pressure converted into a completely new rock.
The large orthoclase porphyroblasts and quartz aggregates indicate a granitic
chemistry. The coarse grained trachylite was close to fluid, if not molten.
The finest textured ultra-mylonite was crushed in repeated, short movements.
44.4 The bluff on the right is underlain by Prichard Formation, in the upper
plate of the Newport Fault.
44.6 Park along the right side, near milepost 435. Carefully cross the highway
to the fractured outcrop. Some sedimentary structures are preserved in these
Prichard siltites and quartzite. The metamorphic alteration has been slight.
This rock could easily have been moved from directly over the top of the first
exposures that we saw.
You are welcome to return from here. If we have time and interest, some of the
group may return to Usk, by a different route, to visit the Manresa Grotto.
There full services in Newport, three miles south.
Thanks
for coming along.
Past Geology Road Trips:
Geology Trip - October 13, 2001
Geology Trip - October 12, 2002