Directions to the folds: Follow I-5 south from Eugene to Sutherlin (about 60 miles). At Sutherlin, exit. Follow Hwy 138 about 1/4 mile; bear left on road to Umpqua. Follow that road about 10 mi to Umpqua. At the gas station, go left on road that follows the Umpqua River --so if you cross the river, you've gone too far. After about 4 miles, youíll see deformed gray rock in the roadcut on the left. Towards the south end of the roadcut, there is a big pull-out on the right.
Bring lunch, raingear, clipboard, paper, field book, protractor,
Make a simple map, make observations, collect strike and dip data, and draw cross-sectional sketches to illustrate the structural geology. Sketches should include directions and a scale.
First: Make a simple map to keep the spatial elements of your report on track. To do this, figure out how many of your paces = 50 m. Then, determine the length and trend of the roadcut (about 200 m). On paper, draw to scale the road and approximate position of the road cut. Then, define the roadcut into three sub-areas: a north, central, and southern area.
Second: Walk along the road cut, making observations and sketches of the folds within each area. Are the folds upright? Overturned? Do they exhibit constant bedding thicknesses or are the hinges or limbs thickened or thinned? Are there small-scale folds on the limbs of any of the larger folds? Is there any evidence for flexural slip?
Third: Collect data and continue making observations. For
collect abundant strikes and dips. You should have at least one
for every limb of every fold. Make sure you note if they came
the north, central, or southern areas. For folds, directly
fold hinge orientations if you can.
Determine orientations of axes for individual folds. To do this, plot the strike and dip for each limb on a stereonet and determine the line of intersection.
Make stereonet plots of 1) all bedding, to determine a general "fold axis" for the entire area; 2) bedding according to sub-area; 3) all fold axes. For #3, distinguish between the different sub-areas by using red for north, green for central, and blue for south.
Determine the amount of shortening across the photo posted on the
Click here to see the image.
Report: Due in class, Friday, June 2
Hand in your map, sketches, shortening estimate, stereonets, and a brief (<500 words) summary of the structural geology.
In the summary, include the following information:
* Location, Rock unit, rock types (Along Umpqua River, north of Roseburg; Eocene Umpqua Fm.; ....?)
* Fold geometries and orientations
* Percent shortening
* Style and mechanism of folding
* Consistency of fold orientations
* Associated Structures
Your summary, although brief, should explicitly refer to your map,
shortening estimate, and stereonets as figures 1, 2, 3, 4...