Introduction to chalcedony veins
in the Tertiary strata of western Nebraska and South Dakota
Harmon D. Maher Jr., Dept. of Geography
and Geology, University of Nebraska at Omaha, 68182-0199
This page was created for students
engaged in early undergraduate research (EUR) on chalcedony veins
in western Nebraska and South Dakota. The research is funded by
a NSF STEM grant to the University of Nebraska at Omaha.
The image above is looking down
on a simple and small chalcedony vein in the somewhat weathered
siltstones of Chadron Formation sediments in Toadstool Geologic
Park. It is only tens of centimeters long and perhaps 1 cm long.
Chalcedony is made up of very fine-grained intergrown quartz crystals
and is very hard. Being more difficult to erode, these chalcedony
veins become small scale fins and ridges that stand above the
more erodable siltstone.
b) Field description of the veins ,
c) Associated research questions,
d) Measurements we will take on the veins,
and e) Some links that discuss veins.
Introduction: Veins are cracks in the earth that filled with mineral
matter. The mineral matter 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 pulling apart or
easiest opening). More on this when we learn about stresses. Veins
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).
of the veins in the study area:
Looking down on subvertical chalcedony
vein within the brown siltstones of the Chadron Fm.. The chalcedony
vein shows zonation with a lighter interior and darker margins.
Also note the thin zone of green alteration in the brown adjacent
siltstones along the vein margins.
Looking down obliquely at stepped
(en echelon) veins (traced with red dashed lines) showing a good
tip curls. Such curl geometries can indicate relative timing (in
this case the two veins formed at the same time.
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. In this
particular case one might infer the vein with the tip curl came
The view is looking down. A more
complex array of veins showing two dominant directions at roughly
60 degrees to each other (average orientation indicated by red
lines). 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. However, this year we will be working mainly
with veins in places without faulting.
- The veins are predominantly vertical. In
some places they are run in the same direction, in other places
multiple directions exist, or even a random direction.
- The veins seem to be restricted to a geologic
unit known as 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
- 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)
- Tips of overlapping adjacent, but parallel
veins can often be seen curving towards each other. These structures
are called tip curls.
questions: Even though the veins are
relatively simple structures there are plenty of research questions
to be explored.
- In map view is the direction of the various
veins random or non random, and if non-random, in what direction?
This is the research question we will focus on.
- 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
- How far away did the fluids from which the
chalcedony and calcite vein fill 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
- 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
- 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
them is the challenge.
we will take on the veins:
For vein fields we will measure the GPS positions
and orientations of the veins.
For select veins, where both tips 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