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:

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.

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!