======================================================================
Author: Joel Hanes (jjh00@eng.amdahl.com)
Title: Geology in Error?: The Lewis Overthrust
======================================================================
>In the rocky mountains, from Canada down to the state of Montana,
>there is large geological overshoot where the BOTTOM loayer is dated a
>hundred million years, and the TOP layer a billion years. The area is
>about 30 000 square kilometres.
In article jan@cs.umu.se (Jan T}ngring) writes:
>Is this correct?
Almost.
You refer to the Lewis Thrust, a region where a kilometers-thick
slice of Paleozoic sediments lies unconformably atop more
recent strata. The geology of the region has been beautifully
exposed by glacial action in Glacier National Park, in some of
the most magnificent scenery in the US. The Paleozoic strata
are tinted in striking pinks, reds, and greens, with white
and black intruding dikes and contact metamorphism.
>>How did the younger layer get on top of the older?
> TMakinen writes:
[ explanation by overturning plate segment deleted.
nice ASCII graphics, tho :-) ]
Well, overturning has occurred in various places around the world,
but it doesn't account for the Lewis Thrust.
The error in Jan T's statement of the Lewis stratigraphy is in
the implication that the ages of the strata look like this,
(with ages in Millions of years):
---------------
1000
---------------
750
--------------
500
--------------
250
--------------
100
--------------
undateable basement
which would be upside-down ordering, oldest on top and successively
younger layers downward; a situation that could be the result of
the overturn diagrammed by Teemu.
In fact, the Lewis stratigraphy looks more like this:
----------------
1000
----------------
1050
----------------
1100
=============== thrust fault
100
--------------
200
--------------
300
--------------
... many more layers
that is, the top layer is around a billion years old,
and it is above a 100-million-year-old layer, but the
100My layer is definitely not the "bottom".
What in fact we have is a big slab of normally-ordered old sediments laying
right-side-up on top of a big slab of normally-ordered newer sediments.
The plate-tectonic explanation goes like this:
Once upon a time, there was a passive continental margin that
collected sediments in deep layers:
west Montana east
-------------------------------------------------------------
300
-------------------------------------------------------------
500
--------------------------------------------------------------
750
--------------------------------------------------------------
1000
-------------------------------------------------------------
1050
...
etc
Then subduction started near the continental shelf, and other
landmasses began to collide with our continent, riding in
from the west on the moving ocean crust. The collision, which
occurs offstage to the left in my pictures, produced in its
earliest stages some high-angle
normal faulting and uplift to the west of present-day Montana,
thus:
Faulting
west Montana east
---------------------------|---------------------------------
200 |
---------------------------|---------------------------------
300 /
-------------------------/-----------------------------------
500 /
-----------------------/--------------------------------------
750 /
---------------------/----------------------------------------
1000 /
-------------------/-----------------------------------------
1050
...
etc
Uplift and Erosion in the west, sedimentation in the east
west Montana east
-------------------
300 \
---------------------
500 \
------------------------
750 \
-------------------------/-----------------------------------
1000 / 100
-----------------------/--------------------------------------
1050 / 200
---------------------/----------------------------------------
1100 / 300
-------------------/-----------------------------------------
1200 / 500
...
etc
Then, as the tectonic collision to the west intensified,
further uplift was coupled with crustal folding, tilting
the uplifted block, and accelerating the erosive removal
of the more recent sediments from the tops of the high
young mountains
Further uplift, crustal shortening, block tilts,
further erosion
west Montana east
/
/ \ / \
\/ \ / \ / \
\ \ \/ \/ \
\ 10 \ 1K \750 \ 500 \
\ 50 \ \ \ \
\ \ \ \ \
11 \ \ \ \ \
\ 00 \ \ \ \ \
\ \ \ \ \ /--------------------------------------
\ \ \ \ / 100
\ \ \ \ /----------------------------------------
\ \ \ \ \ / 200
\ \ \ \ /-----------------------------------------
\ \ \ \ / 300
Then, as the impacting plate smashed everything before it,
the compressive force of the collision detached an immense
block from the eastern face of the uplifted mountains, which
slid to the east on the Lewis Thrust fault
Overthrust, big block slides to east
west Montana east
/
/ \ / \ -----------
\/ \ / \ / \ / 750 \
\ \ \/ \/ \/ -------------------
\ Intensely crushed \ 1000 \
\and folded, too \ ----------------------
/ hard to draw well \ 1050 \
\ \ \ \ /----------------------------
\ / \ / \ \ \ / 1100
\ \ \ \ \ /================================== thrust fault
/ \ / \ \ \ / 100
\ \ \ \ /----------------------------------------
\ \ \ \ \ / 200
\ \ \ \ /-----------------------------------------
\ \ \ \ / 300
...
etc
Igneous dikes then intrude along faults, and between some
of the sedimentary layers.
After further erosion removes the top of the whole picture, and
glaciers carve big U-shaped valleys in the block that slid;
hey presto! Glacier National Park.
I was there three years ago - what a wonderful place to look
at rock! Deep time is spread out all over the landscape.
