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StraboTools: A Mobile App for Quantifying

Fabric in Geology

Allen F. Glazner, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA, afg@unc.edu;
J. Douglas Walker, Dept. of Geology, University of Kansas, Lawrence, Kansas 66045, USA

ABSTRACT “strong” in the field and bring an oriented in Figure 1D? These can be difficult ques-
Quantification of field observations is an sample back to the lab for further analysis tions to answer, and recently at the outcrop
essential step in making them reproducible if desired. in Figure 1D, several professional geolo-
and shareable, but field geologists have few Is the granodiorite in Figure 1B deformed? gists gave answers ranging from 5% to
tools for quantifying field observations of Most observers would say no, but this image, 30%. This illustrates how difficult this
important features such as foliation inten- of an originally isotropic granodiorite, was simple and important measurement can be
sity, crystal alignment, vesicle elongation, digitally distorted by flattening in one using the eye alone.
joint intensity, and mineral proportions. direction and stretching in the perpendicu- This sort of observation and measure-
Here we describe a mobile app, StraboTools, lar direction (pure shear). We venture that ment has occupied much of the field work-
which offers two ways to rapidly and objec- few would call this a measurable fabric in flow in structural geology, petrology, and
tively quantify these variables. The edge the field. Being able to detect and measure sedimentary geology for a century or more.
fabric tool examines grayscale gradients in such subtle features would greatly aid stud- Still, such work can be frustratingly quali-
a photograph and summarizes them with ies of deformation, flow alignment, and tative and incomplete. Quantitative and
the edge fabric ellipse. For deformation of a related fabrics. repeatable measurements are the backbone
homogeneous material with passive mark- Is there a pebble imbrication in Figure of much of scientific inquiry, yet field geol-
ers, this ellipse tracks the strain ellipse. 1C that gives the local direction of current ogists have few tools available for making
Edge fabric ellipses can be determined on flow? What is the proportion of dark minerals them on many types of features.
the outcrop and make quick work (5 sec-
onds) of formerly time-consuming and sub-
jective strain-analysis tasks (e.g., Fry and A B
R /f analysis). They are remarkably sensi-
f
tive to subtle deformations that are difficult
to see by eye. The color index tool deter-
mines the proportion of any component in
the photograph whose grayscale level can
be isolated (e.g., dark minerals in a granitic
rock, feldspar phenocrysts in a lava, or blue
epoxy in a thin section). Estimating propor-
tions by eye has poor precision and accu-
racy; the color index tool is both accurate C D
and precise if a suitable rock face is avail-
able. These tools can be used with photomi-
crographs and aerial photographs as well as
in the field.

INTRODUCTION
The granite outcrop in Figure 1A is
clearly deformed, with a nice shear fabric
running from lower left to upper right in
the photo, and a field geologist could easily
measure its orientation with a compass. Figure 1. Examples of difficult field problems that can be solved with StraboTools. Answers are in
Appendix 1. (A) Deformed granite. The foliation is obvious and easy to measure, but quantifying its
How strong is the fabric? That is harder to strength is difficult to do in the field. (B) Subtly deformed image of a granodiorite outcrop. Do you see a
quantify, and the geologist would likely fabric? What deformation was applied? (C) Shadowed outcrop photograph of a cliff in alluvial fan
deposits. Is there a pebble imbrication indicating the direction of stream flow? (D) What is the propor-
apply an adjective such as “moderate” or tion of dark minerals in this granodiorite?

4 GSA Today | August 2020

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