Information on chalcedony veins
at Toadstool
Harmon D. Maher Jr., Dept. of Geography
and Geology, University of Nebraska at Omaha, 68182-0199
a) Introduction,
b) Field description of the veins at Toadstool,
c) Associated research questions,
d) Measurements we will take on the veins,
and e) Some links that discuss veins.
Introduction: Veins are
relatively planar cracks in the earth evident as mineral matter
which precipitated from aqueous fluids. These fluids can either
travel along a network of fractures and thus have traveled a significant
distance, or they could simply be sucked from the surrounding
rock as the crack grows. The mineral matter either fills a space
created by an opening crack (extensional veins), or they can replace
the wall material as fluids migrate along the crack (replacement
veins). The veins at Toadstool are extensional. Veins should form
perpendicular to the least compressive stress direction in the
rock (the direction of easiest opening), and thus they give an
indication of the internal forces at the time of deformation.
One can also measure the amount of extension associated with the
veins simply by adding up their cumulative thickness if they are
not replacement veins.
Field description of the veins at Toadstool:
- The veins are predominantly vertical, with
multiple preferred directions.
- The veins seem to be restricted to the Chadron
Formation, and pinch out with stratigraphic ascent before they
reach the overlying Brule Formation.
- Chalcedony, a cryptocrystalline form of quartz,
is the most common vein material. Because chalcedony is significantly
harder than the surrounding sediment the veins stand out as small
ridges.
- The veins are zoned, with darker chalcedony
at the margin, and lighter chalcedony towards the middle. The
larger veins also show a core of calcite.
- The veins are note evenly distributed. They
occur in distinct areas or patches. In addition, the patches
seem to be related to the faults. One patch occurs at the tip
of a larger fault. Others seem to truncate against faults.
- The larger veins often show evidence of slip
along the vein, suggesting a continuum between the two structures.
Interestingly, the slip often involves a vein shortening component
(they look like small thrusts). However, considering the vertical
orientation of the veins this is consistent with horizontal extension.
- The veins often come in stepped (en echelon)
geometries.
- Tips of overlapping adjacent, but parallel
veins can often be seen curving towards each other. These structures
are called tip curls.
Looking down on simple small vein
showing both tips. These veins have relatively low length to width
ratios.
Looking down on subvertical chalcedony
veining, showing zonation with a lighter interior and darker margins.
Also note the thin zone of green alteration in the brown adjacent
siltstones.
Looking down obliquely at stepped
(en echelon) veins showing a good tip curl as traced by red lines.
Looking down obliquely at stepped
(en echelon) veins showing a good tip curl as traced by red lines.
Note here that only one of the veins shows the tip curl.
The view is looking down. A more
complex array of veins showing two dominant directions at roughly
60 degrees to each other. Note the distinctive bend of the larger
vein adjacent to the smaller vein at the top.
The ridge that dips to the left
is a fault surface. The small red lines show some en echelon veins
that occur at its tip. Veining and faulting are related temporally
and mechanically.
Associated research questions: Even
though the veins are relatively simple structures there are plenty
of research questions to be explored.
- What time did the veins form?
- Why do the veins occur in distinct patches,
and why do the patches occur where they do?
- What is the significance of the various orientations
of veins?
- How far away did the fluids from which the
chalcedony and calcite grew derive?
- How hot were the fluids?
- Where the fluids ascending, descending, moving
sideways or did they have some more complex movement pattern?
- Why are the veins constrained in terms of
their vertical extent?
- How deep below the ground did they form?
- Are the veins antitaxial (with new material
added at the walls) or syntaxial (with new material added at
a median parting)? Related to this is the question as to whether
the chalcedony or the calcite formed first?
- What is the significance of the various colors
of chalcedony?
- Do they have a characteristic length-width
ratio, and if so what is it and why?
- How much local strain do they represent?
- Did these veins grow, propagate, quickly
or slowly?
- What were the internal forces in the earth
(the stresses) when these veins grew?
- What is the significance of the curved tips
some of the veins have?
Coming up with the questions is easy. Answering
some of the them will be the challenge.
Measurements we will take on the veins:
For vein fields we will measure the positions
and orientations of the veins. It will not be possible to measure
each vein. Instead, the idea will be to measure veins around
the periphery of the vein field and then, in traverses across
the vein field measure veins in the vein field every several
meters. These should plot up on a map as a patch of veins representative
of the patch that exists in real life.
Most veins within a patch have a dominant
direction. We will lay out measuring tapes perpendicular to that
dominant direction, and then vary carefully measure: a) the width
of each vein the tape intersects, b) the spacing between the
veins, c) the type of vein fill, and d) the width of any alteration
zone in the adjacent sediments. From this we can 1) get an estimate
of the amount of extension for the patch, 2) also look to see
whether the spacing is regular, or fractal, and 3) look to see
what the frequency distribution is of the vein width. For patches
with a significant secondary direction of veins, the same exercise
will be repeated.
For select veins, where a tip can be seen,
we will measure the width of the vein as a function of the distance
along the vein as measured from the tip. In addition, the interior
vein fill will be noted. Plots can then determine the characteristic
shape(s) of the veins, and see what type of consistency or variation
exists.
Pictures will be taken of vein curl tips
with a scale, so that these can be analyzed geometrically, later
in the lab.
Some links that discuss veins
( I encourage you to look at these also):
Don't hesitate to contact me with any
questions. Cheers!
