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Department of Geography, Geology, and the Environment

Rochford, SD

Purpose and Introduction

The purpose of this project is to introduce you to field mapping of metamorphic rocks. The project will help you develop the strategies and unique field techniques used when working in layered crystalline rocks that are often complexly deformed and difficult to distinguish from one another. In these types of rocks, a firm grasp and intuitive application of basic structural and stratigraphic principles is essential for success!

You are given four field days and one drafting day to complete a geologic map and cross-section of an area west of Rochford, South Dakota. The area is comprised of ~ 2 billion year old (Early Proterozoic), low to medium grade metasedimentary and metaigneous rocks that are deformed into upright, moderately- to steeply-plunging, tight to isoclinal folds. You will subdivide and name the exposed rocks into mappable units (i.e. Formations), providing detailed descriptions of these units in your map explanation. Depending on how finely you subdivide the section and where you happen to walk, you should have from 4-6 Formations on your map. Throughout the area, you will collect orientation data on tectonic foliations that are subparallel to the axial planes of the map-scale folds in the region, as well as lineations defined by the axes of mesoscopic folds throughout the area. These data are extremely important because they define the 3D orientation of the map-scale folds in the area and will be used to construct your geologic cross section.

Helpful Hints

  • The deformation occurred here at medium to high temperatures and pressures. Consequently, the rocks were fairly ductile and the folds likely formed by flexural flow or passive shear mechanisms. Folds that form by these mechanisms often exhibit a “similar” geometry in that layers tend to thicken in the fold hinges and thin in the fold limbs. In other words, layer thickness is not maintained throughout these structures! Constant layer thickness is a property of folds formed during lower temperature (sub-greenschist-grade, >250˚ C) deformation where substantial mechanical differences exist between different rock types.
  • Remember that a Formation is a mappable rock UNIT, not necessarily a single rock TYPE. You can lump several rock types into a single mappable Formation in the area. Draw contacts between your Formations at the most significant, easily recognizable, laterally continuous rock types.
  • Be wary of collecting your data on steep hillsides; weak rocks often slump out of place.
  • Deciphering the structure of the area is impossible without knowing the sequence of Formations in the area. Because these are mostly layered metasedimentary rocks, they abide by all basic stratigraphic principles. Formations that were not in original sedimentary contact with one another cannot now be in contact with one another without a fault being present. There are no mappable faults in this study area.

Project Components, Guidelines and Layout

  • Follow the Guidelines for Map and Cross Section construction when preparing your project.
  • The map, explanation and cross sections should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.
  • Construct your cross section using the grid method as presented in lecture.
  • Project the folds above the topographic surface, but color and label only those parts of the folds that are beneath the ground.
  • Contacts showing the folds above ground should be dashed.
  • Be sure to calculate the height of your cross section when designing the layout of your project; depending on your plunge data, the section may be twice as tall as the map is long!

At the end of this project you will turn in:

  1. A final colored and inked map,
  2. The map explanation with complete descriptions and formal names for each map unit,
  3. One geologic cross section assigned by the instructor(s),
  4. Your field notebook,
  5. Your field map,
  6. A stereoplot and data table showing all the foliations (at least 20) and lineations (at least 8) you measured throughout the map area. Show the average foliation and lineation orientation on your plot.