Maps of areas with folded rocks can be challenging because of the number of surfaces involved, and because of rapid changes of strike and dip. Often, even though layers may show complex folding, axial surfaces may be approximately planar. Under these circumstances it makes sense to separate the limbs of the fold by drawing axial traces, and even to draw contours on the fold axial surfaces. When doing this, it’s important to remember that a single fold will have multiple hinges (one for each folded surface), but that these hinges lie in a single fold axial surface.
Assignment
1.* The area you contoured last week (Great Cavern petroleum prospect) shows strike and dip symbols representing measurements of bedding orientation.
Plot poles to bedding for all the strike and dip measurements on an equal area projection, to make a pi-diagram.
Find and draw the best-fit great circle through the poles; this represents the profile plane. Mark the best estimate of the fold axis (pole to the profile plane) and determine its trend and plunge. Is it similar to the value you obtained by contouring last week, and to the values obtained by your other team members?
Lab 4 Map 2 Great Cavern Map
84 | More About Folds
2.* Look at the map of Somerset County in the Appalachians of Pennsylvania. The area has a long history of coal mining, and a number of coal seams are identified on the stratigraphic column that doubles as a legend.
a) The map has structure contours, shown in red, drawn on one surface. Which one surface? (Note: each formation on the map has a top and a bottom surface, so just a formation name is not a complete answer; your answer should be in the form ‘the boundary between Formation x and Formation y’.)
b) Use the spacing of the structure contours to determine the strike and dip, at its steepest point, on each limb of the most conspicuous fold. Note that the contours are in feet, and the scale of the map is 1:62500 (about 1 inch to 1 mile). Make sure that in your dip calculation you use the same units vertically and horizontally. Use the points where the contours cross the fold hinge to determine the average trend and plunge of the fold hinge.
c) Plot both limbs of the fold from part ‘b’ on a stereographic projection. Using the intersection and the angle between the planes, estimate the plunge and trend of the fold axis, and the interlimb angle of the fold.
d) Based on these observations, describe the orientation (plunge, tightness, overall orientation) of the fold in words (e.g. ‘tight, steeply plunging synformal anticline’)
e) If the fold is cylindrical, the fold axis orientation determined stereographically should coincide with the hinge orientation determined by contours. How close are they (in degrees)?
To find out more, take a look at: Flint 1965 Geology and mineral resources of southern Somerset County, Pennsylvania 3. Map 1 contains an angular fold in a more general orientation than the perfectly horizontal folds you dealt with
last week. In addition there are some unfolded rocks and an intrusion. An unconformity separates the more highly deformed folded rocks from the gently dipping younger rocks. To help you solve the map, note that in the west of the map, the bedding traces are somewhat parallel to the topographic contours. These regions correspond to a gentle fold limb. Elsewhere, the geological boundaries cut across the contours at a steeper angle; this is a steep fold limb.
Fold hinges can also be identified on the map from sharp swings in the trace of bedding that are not obviously related to valleys and ridges. Before you begin, try to use these hinges to sketch where there might be fold axial traces; these More About Folds | 85
should separate regions of steeper and more gently dipping beds. Do this very lightly – it is likely you will change your mind as you proceed.
a) Identify the various surfaces on the map. First, mark the unconformity surface in green or yellow. Then mark the boundary between marble and amphibolite in blue or violet. Mark the boundary between amphibolite and schist in red or orange. (Note: the colour scheme is provided for convenience. If you choose different colours, you must use them consistently throughout this exercise.)
b) Draw structure contours on the green surface.
c) Draw structure contours on the red surface. You will find that there are two parts to this surface: a steep limb and a gentle limb. These have separate sets of contours. The two sets of red contours intersect in a fold hinge.
Mark this on the map and trace over it with red.
d) Repeat for the blue surface and mark the blue hinge.
e) The red hinge and the blue hinge are both lines that lie in the axial surface of the fold. Join points of equivalent elevation on the red hinge and blue hinge with lines: these are structure contours on the axial surface. Number them in purple or violet.
f) Use the axial trace contours to predict the outcrop trace of the axial surface, and draw this trace on the map. The axial trace should separate the steep limb from the gentle limb of the fold.
g) Make a cross section along the line XY.
h) Plot both fold limbs, the axial surface, and the unconformity as great circles on an equal area projection. Mark the point corresponding to the orientation of the fold axis.
i) Describe the orientation of the fold in words as completely as you can
j) List the events in the geological history of the area for which you have evidence, starting with the oldest. In the case of the three folded units, where there is no direct evidence of age, you should assume that the fold is upward facing (i.e. synforms are also synclines; antiforms are also anticlines).
Lab5 Map of Folded Area Schmidt Net 15 cm
86 | More About Folds
Figure 1. Boudinage. Top: boudins; bottom: chocolate tablet structure.
F. Boudinage
Buckle folds are formed when strong (or ‘competent‘) layers of rock are shortened. What happens when strong layers are extended? Typically the layers start to thin at points of weakness (a process known in engineering as necking) producing a structure called pinch-and-swell.
As pinch and swell develops, the thin regions can separate, leaving a structure that looks like a string of sausages in cross-section. The remnants of the original layer are called boudins (a French word for a type of sausage), and the process is known as boudinage.
Although boudins are in many ways the extensional counterpart of folds, the terminology of boudinage is much less well developed than that of folds. In part, this is because layers undergoing boudinage do not affect adjacent layers in the same way, so that boudins are less likely to be harmonic than folds. Thus, although boudins do have axes, it is rarely possible to define an equivalent of an axial surface for boudins.
Sometimes layers undergo extension in all directions simultaneously, producing a more three-dimensional boudinage structure described as chocolate tablet structure. It is also possible to find examples of layers that have undergone both folding and boudinage during progressive deformation.
Figure 2. Boudins formed from quartz vein. Carmanville, Newfoundland.
F. Boudinage | 87
Figure 1. Four parts of deformation.
G. Kinematic Analysis and Strain
Everything we have done so far in this course has been about describing the ‘here and now’ of structural geology: where are structures and how are they oriented in the Earth’s crust at the present day.
To understand the origin of structures we need to know how things changed during the formation of those structures – how things moved. When we study how things moved over geologic time, we are studying kinematics.