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Fall 2001 Volume LV NO. 3
Synopsis:
Utah Field Trip 2001
News from the Board...
THE
DENHAM FORMATION-AN UNUSUAL ASSEMBLAGE
OF VOLCANIC AND SEDIMENTARY ROCKS
How Did Geology
Get Started? - Part III
CATACLYSMS
ON THE COLUMBIA
Announcements
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Synopsis:
Utah Field Trip 2001
By Marj Gangl
WOW!! (not a geological term.)
What a trip it was, thanks to our leaders, Rick Uthe and Walt
Blowers. Eastern Utah on the
Colorado Plateau is unique. The
desert tends to “grow” on you. A descriptive term like “scenic” is an under-statement.
On any given day, there were about 28 trekkers.
The thought crossed my mind that this trip could have been
sponsored by Kodak, as cameras were clicking continuously.
On the first two days we were around the Vernal, Utah area.
One could easily see the south side of the tall, snowcapped Uinta
Mountain Range, uplifted from the Colorado Plateau which moved northeast
during the Laramide Orogeny. Driving
through a series of flat floodplain terraces, we arrived at the Split
Mountain area where the Green River cuts
through the split. After
waving at a group of rafters, we had an excellent view of the lofty Weber
Sandstone Form-ation across the river.
Looking in the opposite direction, was our “old”
favorite, the red Moenkopi Formation, among others.
Our entire trip included many features such as anticlines, syn-clines,
monoclines, etc. In short
order, we viewed all the colorful formations in the Mesozoic Era. Some
were viewed from our open air shuttle bus ride UP to the Dinosaur quarry,
where the Morrison Formation (purple, grey, green and various shades
thereof) of shales and siltstones , holds the Dinosaurs' fossilized skeletons.
On Day two, as we passed Blue Mountain, a huge
anticline, we headed into rock formations of
the Cretaceous Period. On
a beautiful drive with stops along the way, we headed up to Harper's
Corner. At about 8000 ft.
elevation at the end of the road, we looked down and got a glimpse of the
Green and Yampa rivers joining. This was such an interesting day.
I cannot write about all the great things we observed.
It would take many pages. Find
someone who has a field trip guide and have that person copy pages 25
through 34, then read all about it, including Plug Hat Butte. The third
day was a lovely drive from Vernal, to Price.
Our altimeter registered 9000 ft. elevation at one point, in the
pines.
Day 4: Off
we went to to the San Rafael Swell area.
We were treated with a view of the Coconino Formation while standing on the Kaibab, which was
formed about 260 million years ago in the Permian Period. From
here we looked “up the ladder” into rocks of the Triassic Period.
At this point, we were at the western edge of the Colorado Plateau.
On through San Rafael Canyon, driving in a linear valley, called Joe’s
Valley, we came to a Turquoise colored reservoir.
Here we had a “test” which led to much discus-sion.
We reviewed four options as to the piracy of Cotton-wood Creek.
I think we all flunked, as there doesn’t seem to be a known
answer. I was satisfied in my
own mind that it is a “Niagara Falls “ syndrome.
Day 5 was
a study of the Mid-Cretaceous Seaway.
Incidentally, its eastern shoreline runs about north and south through the middle of Minnesota.
Several coal seams were visible in the area of Price.
Day 6 involved a trip up the Colorado National
Monument to view some ancestral Rockies near Grand Junction, Colorado.
From there, we drove along the Colorado River to Moab, Utah.
Here we learned about the Paradox Salt Basin.
This basin was formed southwest of the Uncompahgre uplift which we
viewed in western Colorado, one of the larger grabens of the ancestral
Rockies (320 Kilometers--200 miles-- long).
The average thickness of evaporite minerals in it is 4500 feet.
Day 7 was spent viewing Salt anticlines and Canyonlands National
Park. One spectacular view
was at the Green River overlook. Day 8 we learned how the arches and fins were formed in
Arches National Park. They
are very complex and beautiful.
On Day 9, our first stop was at an enormous
salmon-red Sandstone wall of the Comb Ridge.
The whitish outer Comb Ridge, mostly Navajo Sandstone, looked like
it could be Paul Bunyan’s comb (that is if he ever combed his hair). From here we ventured on to Natural Bridges National
Monument. We saw mature and
younger bridges formed by the White River. At
Owachomo Bridge, we could see the narrow span of about 9 feet in
thickness. It is 180 feet
wide but only 106 feet high. Its
demise is inevitable. From
here we drove “doowwwn” (Rick’s word) a short distance, a drop of
about 1100 ft., through a series of switchbacks.
It was breathtaking...then
on to Bluff, Utah. Day 10 was
a half day study of the Monument upwarp and taking in the scenery of the
monoliths and sand dunes in colorful Monument Valley. Then
we headed back to MN
This is an oversimplification of our trip.
For those of you that did not go on the trip,
plan ahead so you can go on the next one in two or three years (we
hope). Then you can get the
full explanation of what the rocks are made of, how old they are, and how
they got there. See you then!
~Marj
Gangl |
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News from the Board...
Three
of the four field trips scheduled for this summer will have already
occurred by the time you read this.
I would like to thank Rick Uthe and Walt Blowers for their very
successful field trip to eastern Utah, the northern Colorado plateau,
and the huge amount of effort that entailed.
I would also like to thank Gail Marshall, Margaret Rodina and
Rosie O’Donovan for their extensive work on the three short field
trips.
Call
Tom Schoenecker at 952-474-4600 to volunteer for staffing the
Geological Society of Minnesota (GSM) booth in the education building
at the state fair. He
wishes to fill 72 stints, each 4 hours long, from Aug 23 through Sept
3. You do not need
expertise in geology to do this. You
simply refer fair visitors to our lecture season, where they can hear
the experts.
GSM
needs a new video library chair and possibly a new video library
procedure. This procedure
might use phoning and computers to allow selection of the specific
tapes to bring to the lecture. If
interested in volunteering in this capacity or discussing the library
procedure, please call me.
Three
spots need to be filled on the GSM board of directors.
If you have an interest in becoming involved or in serving on
the nominations committee, please call Sylvia Huppler, 651-483-4796,
chair of the nominating committee.
Doug
Zbikowski has guided another very successful outreach program this last
school year. I
commend Doug on his design and implementation of this program, which
has now been taken to three other states by people who taught it here
in Minnesota. I also thank
him for all his work with this program.
The
Budget Committee has completed its work for the year. Marlys Lowe chaired the committee, and Steve Erickson and
Gail Marshall served on the committee.
I would like to thank them for
this effort.
If
you would like to bring cookies to any one lecture during the lecture
season, please call me.
~Bill
Robbins, President
651-733-9894 |
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THE
DENHAM FORMATION-AN UNUSUAL ASSEMBLAGE
OF VOLCANIC AND SEDIMENTARY ROCKS
Terrence
J. Boerboom, Minnesota Geological Survey
The
Denham Formation is the name assigned to a unique package of
metamorphosed sedimentary and volcanic rocks that outcrop in
northwestern Pine County, southeast of the town of Denham (Fig. 1, next
page). In
general, the glacial cover is quite thin in this area, and bedrock
outcrops are fairly abundant, although scattered.
The outcrops that define the Denham Formation are found in and
near a small valley formed by glacial meltwater.
This valley trends north-south, and cuts perpendicular to the
strike of bedding in the Denham Formation, thus providing a good
cross-section of the stratigraphy of the formation.
The Denham Formation is Paleoproterozoic in age, or about 2
billion years old. It lies
in depositional unconformity on top of the McGrath Gneiss, which is a
metamorphosed porphyritic granite of Neoarchean age.
The McGrath Gneiss has recently been dated at 2.55 billion years
old (Van Schmus and others, 2001), which is when the rock originally
cooled from a molten magma. Both
the Denham Formation and the McGrath Gneiss were deformed and
metamorphosed during the Penokean orogeny, a major collisional event
that occurred about 1.8 billion years ago.
The Denham Formation contains an unusual assemblage of
interbedded rock types that originally consisted of shale, siltstone,
arkosic sandstone, conglomerate, dolomite, and pillowed to fragmental
basaltic volcanic rocks. However,
these rocks have been metamorphosed to the amphibolite grade as a
result of crustal collision during the Penokean orogeny.
The rocks that started out as fine-grained sediments, such as
shale and siltstone, have retained the fewest primary bedding features
and are now completely recrystallized into coarse-grained staurolite-garnet-muscovite
schist and muscovite-biotite schist, respectively.
Also, the beds of dolomite are completely recrystallized to tan
or gray marble and retain no primary depositional features found
elsewhere in similar rocks of this age, such as algal stromatolites.
The primary sand grains in the arkosic rocks are well preserved
because the rock contains a high proportion of dolomite in the matrix,
which absorbed most of the strain associated with deformation.
The first of two deformation events that
affected these rocks was synchronous with metamorphism.
It produced an early foliation that typically is parallel to
bedding, and a locally strong, shallowly plunging, lineation.
The second deformation event folded both the early foliation and
bedding along steeply dipping axial fold surfaces.
In the Denham valley, the stratigraphic sequence dips variably
to the north, except for local overturned south-dipping limbs on second
generation folds. Both deformation events are the result of crustal compression
associated with the Penokean orogeny.
Despite deformation and metamorphism, many primary
depositional features can still be recognized in the rocks, and the
stratigraphy of the Denham Formation forms a coherent package that is
shown schematically on Figures 1 and 2.
The basal beds of the Denham Formation consist of siltstone that
is locally interbedded with cross-stratified pebble conglomerate.
This is overlain by coarse-grained and locally conglomeratic
arkose that apparently pinches out laterally.
The arkose is interbedded with amygdaloidal
basalt flows that grade stratigraphically upward (northward) from
massive, to pillowed, to fragmental.
The volcanic rocks are thickest at the eastern limit of outcrop,
where at least four flows of nearly 1,000 feet total thickness were
recognized; the rocks thin westward to two flows of 300 feet total
thickness. This
distribution implies that the eastern exposures are nearest to the vent
from which these volcanic flows emanated; however, there were likely
several other eruptive centers located along this same horizon to the
west. The volcanic rocks,
in particular, have retained primary features such as pillows,
amygdules, and fragmental textures in rocks erupted during explosive
underwater volcanism. The
overlying arkosic and pelitic strata (shale) apparently pinch out to
the east where the volcanic package thickens, and are not present in
drill holes to the north and east of the Denham valley.
The northern-most outcrops in the valley consist of very pure
dolomitic marble in which there are tightly folded quartz veins.
Drill cores show that the marble unit is at least 500 feet
thick, and is overlain by graywacke that is exposed discontinuously to
the north for some distance. Drill
cores indicate that the contact between marble and overlying graywacke
is fairly sharp, and marked by a thin layer of graphitic schist.
Field and petrographic observations imply that clastic
detritus (sand, silt, and clay) in the Denham Formation was derived
mostly from a weathering residuum developed on the underlying McGrath
Gneiss. Near the contact
with the base of the Denham Formation, the McGrath Gneiss grades
abruptly from granite gneiss containing quartz, orthoclase,
plagioclase, and biotite, to strongly foliated, quartz- and sericite-rich
schist that contains orthoclase, but no plagioclase.
The arkosic parts of the Denham Formation similarly lack
plagioclase and are composed of quartz and orthoclase grains, together
with scattered cobbles of granitic gneiss.
The basal Cretaceous strata
locally consist of reworked saprolite that includes beds of
cross-stratified sandstone and nearly pure kaolinitic shale.
Exposures of basal Cretaceous sediments locally contain
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coarse grains of orthoclase and quartz
derived by slight reworking of grus-textured, weathered granite.
The same process may have occurred in Paleoproterozoic time as
clay and grus were eroded from the weathered McGrath Gneiss and
reworked into beds of arkosic sandstone and kaolinitic shale.
These sediments were subsequently metamorphosed to quartz- and
orthoclase-bearing arkose and staurolite-garnet-sericite schist,
respectively, and the weathered residuum on top of the McGrath Gneiss
was metamorphosed to produce the muscovite-rich schist we see today.
The Denham Formation is interpreted
to represent an assemblage deposited on a developing rift-margin during
Paleoproterozoic time. In
this setting, the McGrath Gneiss was part of the continental margin
that was weathered and eroded to provide detritus to an evolving rift
basin undergoing simultaneous active, shallow water volcanism.
The lower portion of the Denham Formation, including the
siltstone, pillowed basalts, and arkose, was probably deposited into a
shallow marine environment. Interbedded
arkose and dolomite higher in the strati-graphic section represent
rapid subsidence of the conti-nental shelf and deepening water,
possibly within localized fault-bounded grabens.
The lack of arkose in the thick dolomite at the upper part of
the sequence indicates that deposition of coarse detritus was
restricted to the shallow, nearshore environment adjacent to the
McGrath Gneiss. The abrupt
upward change in sediment composi-tion from dolomite to graywacke
indicates rapid deepening of water and associated turbidite deposition.
Most of the observed deformation of the Denham Forma-tion is
inferred to be the product of basin closure during the Penokean orogeny
about 1.8 billion years ago. ■ |
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References:
Setterholm,
D.R., Morey, G.B., Boerboom, T.J., and Lamons, R.C., 1989, Minnesota
kaolin clay deposits: A
subsurface study in selected areas of southwestern and east-central
Minnesota: Minnesota
Geological Survey Information Circular 27, 99 p.
Van
Schmus, W.R., MacNeill, L.C., Holm, D.K., and Boerboom, T.J., 2001, New
U-Pb ages from Minnesota, Michigan, and Wisconsin:
Implications for Late Paleoproterozoic crustal stabilization
[abs.]: Institute on Lake
Superior Geology, 47th Annual Meeting, Madison, Wis.,
Program and Abstracts, v. 47, pt. 1, p. 100-101. |
Back in May we left the Earth in pretty shabby shape
for a future home of the human race. I promised you that it would get worse, and
here it is.
Another planet - this one about the size of Mars -
was somehow nudged out of its regular orbit around the sun. Perhaps its orbit
was chaotic, as Mercury’s orbit is today, and some tiny influence or other
bumped it into the path of the Earth. Now
this is an asteroid impact to end all impacts: two full sized iron-cored planets
in full collision. As a result,
both planets were completely destroyed. No
half measures - both mantles shattered and both crusts completely blown away by
the impact, reduced to random rocks of various sizes.
There’s only one comfort for the future. As a
result of the “conservation of mass” there’s no place for all this junk to
go, except to reform a completely new composite planet around the center of
mass. No doubt the two iron cores -
being the most massive pieces of the loose ends flying around - settled down
first, and the effect of their combined gravity helped assemble the lighter
stuff into a new mantle. No one
knows how long this re-assembly lasted. Experts' guesses range from a few days
to several centuries, and the general consensus is that it’s too bad there
wasn’t a video camera recording the whole she-bang.
But wait! We’re
not through yet - remember that we haven’t given the conservation of momentum
time enough to work yet. Such a
massive collision added several orders of magnitude to the total momentum of the
system (that is, the combined mass of the two planets).
The new planet can’t just eat all this extra energy, so it has to find
a way to use it. As the smashed up
rocks fall down to form the new mantle and crust, they find themselves moving
much faster in their orbits than ever before; little by little the newly-forming
Earth rotates faster. Finally,
instead of rotating once for every orbit around the sun, it now rotates almost
400 times - that soaks up plenty of momentum.
Some of the lighter rocks in the outer portion of the
debris cloud actually pick up enough momentum to permit them to set up a
consolidated orbit around the new planet, and (more or less) suddenly the new
planet has a satellite that requires about 10% of the momentum.
But
the most remarkable thing about the changing planetary circumstances is this: as
the new Earth forms and begins to rotate, it suddenly remembers that as a
rotating mass it can’t behave the way non-rotating masses do!
Normally, you would expect a large mass, impacted by another high-speed
mass, to tilt away from the impact. But
at the same time this is happening, the New Earth is beginning to rotate at an
increasing rate, and the old rules no longer apply.
Instead of the axis of spin responding to the axis of the torque, it
responds by moving at a right angle to both the axis of spin and axis of torque
- physicists call this “precession.” Why does it do this? I don’t know.
In the case of the New Earth, the axis of spin
finally settles down 23 degrees off the vertical. This means that the new planet
will not only have days and nights in regular progression, but will also have
seasons as it lopes around the Sun on its annual trip.
And a new moon thrown in to boot! What a deal!
But we’re still not through.
Remember those two iron cores that came together to form the New
Earth’s consolidated iron core? Keep
in mind that the term “iron” is a shortcut. Although iron is by all measures
the largest component of the cores, the term also includes every other metal
that is heavier than iron. It would surely be a remarkable coincidence if both
these “iron” cores had exactly the same composition. This means that core
today is subject to the same mixing and churning processes that the original was
- with the added angular momentum thrown in, and the additional possibility that
the two original cores had different magnetic signatures.
Hm!
I wonder if turmoil in the core could be responsible for the dramatic shifts in
the New Earth’s magnetic field down through the latest 3 billion years? What
will the future bring?
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A few
weeks ago a friend of
mine made an auto tour of the western Montana and central Idaho area,
where he had served in the Navy over fifty years ago. Since he was
covering essentially the same ground that we covered in our 1996 field
trip, I gave him my copy of the tour guide and suggested he check out
the Missoula Floods. To make a long story short, he was delighted with
the suggestion; although he didn’t cover the area as thoroughly as we
did, he has a perceptive eye and enjoyed having a professional insight
into the area. In return, he brought back a book, which he bought at
one of the interpretive centers along the way, describing the Missoula
Floods in great detail.
CATACLYSMS
ON THE COLUMBIA
John
E. Allen and Marjorie Burns, with Sam Sargent
Timber
Press, Portland, Oregon
194
pages, Appendices, Index - $14.95
The book covers the entire Columbia
River basin, from Missoulia to Astoria, and deals with many more sites
of interest than we could possibly have covered. However, it does not
have the detail that Dick Uthe’s tour guide has, and it lacks his
lively style. The language is pedestrian at best, and has an
unfortunate tendency to fall into a rather stiff pattern of using the
same descriptive terms to describe geological features. It has many
aerial photos illustrating the effects of the floods, but I would love
to get into them with a few labels and arrows to show which features
are which and how the dynamics worked to get the results demonstrated.
Nevertheless, it’s a book I should
like to have in my geology collection and I’m going to write off to
see what kind of a deal I can get. Their cover price is $14.95 -
perhaps if there are ten members who would like to have a copy we can
get them down to a flat $10. If you’re interested, let me know and
we’ll see how they respond!
Cheers! - - Chas Brennecke
(651) 222-6985
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Aug.
23 - Sept. 3
State
Fair, Saint Pau
(which day will YOU
be helping out a
the GSM Fair Booth?)
Sept.
14:
Fall Annual Meeting - 5 PM
Old Country Buffet - Maplewood
Sept.
30 Your GSM Membership Expires
Oct.
8 First Lecture of the 2001/2002
GSM
NEWS
Editor:
Reporter:
Tom Smalec
The purpose of this newsletter is to inform
members and friends of the activities of the Geological Society
of Minnesota. GSM NEWS
is published four times a year:
February 15, May 15, August 15, and November 15.
GSM NEWS
welcomes unsolicited
Geology and Earth
Science related articles and photographs.
Deadline for article submission is three weeks before
the date
of publication.
Send all material for GSM NEWS
to: GSM c/o Katy Paul, 6901 West 84th St., #351, Bloomington, MN
55438, phone/e-mail listed above.
Officers:
William Robbins, President;
(Vacant) Vice President;
Steve Erickson, Treasurer;
Judy Hamilton, Secretary.
Directors: In
addition to the officers listed above: David Christianson; Paul Lemke; Rose Mary O'Donovan; Gail
Marshall; Katy Paul.
Send all GSM membership dues, change of address
cards, and renewals to the GSM Membership Chair:
Gail Marshall, 12232 Allen Drive, Burnsville, MN
55337 phone 952-894-2961. Membership
levels are:
$10 Full-Time Students;
$20 Individuals, $30 Families
MEMBERSHIP
RENEWAL
Reminder...
your GSM membership expires September 30th. With
your support, GSM can continue to offer a fine lecture program, provide area
schools with an invaluable resource through the Outreach Program, and introduce
you to the pool of talented professionals in the field of geology.
Fill in the form below, and mail it with your check, to Gail Marshall,
Membership Chair.
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