chalcedony veins

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

2007

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.

a) Introduction, 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.

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.

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).

Field description of the veins in the study area:

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 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 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 later.

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.

Associated research 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 of veins?
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 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 them is the challenge.

Measurements 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):

Microstructures Online - lecture on veins.
Microstructures Online - lab on veins. Includes some good animations of different types of crack-vein growth.
Andy Heath's software model Cracker.

Don't hesitate to contact me with any questions. Cheers!