GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 72-8
Presentation Time: 3:30 PM


ALLMENDINGER, Richard W., Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY 14853 and KARABINOS, Paul, Dept. Geosciences, Williams College, Williamstown, MA 01267

Computers are faster at calculating and more accurate at plotting precise geometries than a human field geologist. If the field geologist can do a complex calculation in the field, the result of the calculation can be tested immediately. For example, we many want to calculate the complex trace of a planar contact across topography, fit a plane to the vertices of a digitized contact, calculate an orientation based on three points, rapidly calculate a map thickness of a unit, calculate a piercing point, or calculate mean vectors or cylindrical best fits. Most of these calculations require the X, Y, AND Z components of digitized points but, unfortunately, most mobile device mapping programs only record X and Y.

GMDE Mobile and desktop apps, written by the senior author, enable the field geologist to load a digital elevation model (DEM) for offline use and can carry out all of the above calculations. The Mesozoic stratigraphy of the Idaho Wyoming thrust belt presents an ideal test case because of the combination of good exposure and distinctive units. We have used these technologies for mapping and structural analysis in the Poker Peak Quadrangle. We have also used these programs with DEMs based on LiDAR data in high grade metasedimentary rocks in the Berkshires, to map fractures in Grenville basement of the Adirondacks, and map earthquake fault ruptures.

Allowing the computer to project a planar contact across topography not only results in more accurate contacts that bridge areas of no exposure but also aids significantly in identifying fault offsets by showing where a projections suddenly fails to match the geology. Map scale strike and dips from 3-point calculations are more consistent and display considerably less scatter than orientations measured by the geologist on the outcrop. An iterative approach, where the geologist first draws a contact segment by hand, best-fits a plane to that segment, and then has the iPad calculate the contact using the best-fitting orientation can help identify local geological anomalies.

More accurate contacts and more consistent orientations mean better cross sections and down-plunge projections. Outcrop scale noise, introduced either by poor measurement technique or by stochastic variations of the outcrop itself can be reduced by these computational field methods.