Lab 3 - part 2: Air photo map for Dougherty structure in the Arbuckle Mountains of Oklahoma.

The object is to produce as complete an air photo interpretation map of the geology of this area as possible using Adobe Illustrator or Inkscape. Both of these are vector graphics software packages, which are object oriented. Some of you are familiar with the software from the geodata course. You create and manipulate editable objects (lines, polygons, text, images) to produce the image you need. Adobe Illustrator is in many ways the industry standard, and is a very powerful tool. However, it is also expensive. Inkscape is freeware that can do many of the basic things that Illustrator can do. Both work with layers, an important aspect. Unfortunately it is not simple to share files between these two (but it can be done). Inkscape is free and you can easily download it to your own computer and work at home on projects. For the time being Adobe Illustrator is available in our computer labs. The various tools are in different places, have different appearances and sometimes work in different ways, but the basic set-up of the two programs is similar.

A template .svg or ai. file can be found at your class Blackboard site to start with. The first step is to download and open it. The second step is to study the air photo in detail, considering the information below, and try and understand the geology. Note you have been given a start on the interpretation. You should use Google Earth to aid your interpretation. Typing "Dougherty, Oklahoma" will get you to this area. The third step is to draft your geologic interpretation on top of the base map. This will require learning a suite of basic operations in Illustrator or Inkscape if you are not yet familiar with them.

Your final product should be a single page black and white geologic map (turn off the print/view option in the base air photo layer) showing the distribution of the units and features described below, complete with a legend, scale, north arrow, your name and date of construction. Submit your final product as a pdf file (you can urn your map file into a pdf in both software packages - either look for the export option, Save As, or print to a pdf file).

Each pixel of the large image is 2 m, so that the entire image is 7.5 km across. This scale will be important in recognizing the may units below (note that thicknesses are given).

Identify on your photo the following map units given in descending (lower is older) stratigraphic order:

Map the following features:

Suggestions: Start where you are most confident of your interpretation and work towards areas that are more confusing. As you make your 2-D map interpretation, think about the implied 3-D geometry of your map pattern (in essence you should have a conceptual 3-D model in your head for your map pattern).

Link to geodata course page that briefly describes how to construct geologic maps in Illustrator.

This is a smaller version of the image, which you can use for general reference purposes.

Interpretation of structural geology of topographic hogbacks: One of your challenges will be exactly where to place the stratigraphic contacts. While eroded and more vegetated than the classic hogback ridges seen in the western U.S., these ridges are basically due to the dip of stratigraphic units with different erodabilities. Below is a schematic depiction of a hogback formed by a more resistant limestone unit (unit b) between two less resistant units (could be shale). Note the more uniform dip slope (sub parallel to bedding) on the right side, and the 'striped' slope where bedding intersects the slope surface on the left. If the formation contact is right where there is a lithologic and erodability contrast then the map contact will tend to be lower down on the hogback slopes as depicted here, although on the strike slope (to the left here), it may be higher up on the slope as the more resistant cap rocks shelter more erodable rocks below. This is also where you can consider the relationship between dip and apparent thickness at the surface (shallower the dip the greater the apparent thickness). Remember to use dashed lines where you are less certain of the contact position.

Schematic image of a hogback (explained above).

Thickness of a tabular stratigraphic unit: A stratigraphic unit that is approximately tabular in 3D geometry can show up as a band of color on a geologic map, and multiple concordant stratigraphic units are sub-parallel bands of color that should continue unless they are faulted, intruded or otherwise covered. What controls the map width of these units? Three factors: 1) the true stratigraphic thickness (mesured perpendicular to bedding), 2) the dip, and the topographic slope. In a deformed area the dip is often a very significant factor and in general the more shallow (lower) the dip angle the wider the map expression. On some folds it is clear that the map width of the units on one limb is greater than the other limb, and usually that is because the fold is asymmetric with limbs tilted to different degrees.

Image showing how the map width of tabular geologic units is a function of different factors.