Interpretation of Sedimentary Structures

    These exercises deal with  the application of sedimentary structures  to the solution of certain geologic problems such as determination of facing direction for a group of strata,  paleocurrent orientation and direction for both flat lying strata and inclined strata and determination of the amount of lateral displacement of a thrust fault. 

Getting Started

    Before you jump into the nuts and bolts of the laboratory you need to establish some familiarity with the character of sedimentary structures. The best way to do this is to get out into the world of depositional environments and watch what is happening, go to the beach! There are a number of published sources for pictures and diagrams of sedimentary structures. For example:

•   Collinson, J.D. and D.B Thompson:  Sedimentary Structures, 2nd ed. 

•   Conybeare, C.E.B and K.A.W Crook:  Manual of Sedimentary Structures. 

•   Pettijohn and Potter:  Sedimentary Structures:  plates galore!!!

•   Reineck and Singh, Depositional Sedimentary Environments

Exercise #1 Spend some time in the Randall Library looking at pictures and drawings of sedimentary structures in the above texts, in related texts and in the journals. I have placed some text books on Reserve for you but hit the stacks also A large section of your text book, Tucker, Sedimentary Petrology, is also devoted to the topic of sedimentary structures, particularly pages 21 to 41. How effective you will be in the following exercises will depend upon how diligent your are in this pre-lab study.

Facing Direction or Which way was up?

    You learned in your Freshman Geology course that sedimentary beds, when deposited, generally formed something like a horizontal sheet (principal of original horizontality). You also leaned that in an objective sequence of strata the bottom most was deposited and then the next was deposited upon it etc (principal of superposition).  But what if you have before you a sequence of strata inclined 80 degrees form the horizontal, which way was up when they were deposited? What is the correct sequence of beds? Are the beds overturned? What is the Facing Direction of the strata? (Stratigraphers call this the strat-up direction, structural geologist call it the facing direction.) The following is a listing of some of the rules of thumb, which can be  use to determine the up or facing direction of a suspected overturned bed.  The list is by no means complete but should help you make a reasonable interpretation in most situations.

Rules of Thumb for Determination of Facing Direction

For load casts, flame structures  and pillow structures it is 'sand deposited on mud'. The mud layer was deposited first followed by the sand layer. The load cast and pillows are found on the true bottom side of the sand layer.  Flame structures rise form the mud layer up into the sand layer. All three form because the sand is more dense than the mud hence it sinks and the mud rises.
Tools dig down into the substrate forming a tool mark but what is preserved may be a cast made by the overlying bed.  Marks are holes or divots dug into the substrate (real top of a bed) at deposition so marks are holes point down; casts appear as ridges and dimples that formed by a later bed filling in the marks (casts are situated on the real bottom side of the overlying bed) so these ridges and dimples point down.
Most graded bedding is coarse to fine.........in a sequence of graded beds the predominant scheme will be fining up though individual beds could coarsen up. Your interpretation should be made using as many beds as feasible.
Mud cracks open at the top surface and form a 'V' down into the substrate because water is drawn out from the bed top down. Look for the 'V' in a cross sectional view of the structure. This holds true for both desiccation cracks and syneresis cracks.
With ripple laminations and cross laminations of all types 'concave up' is the rule. The depositional surface is curvilinear and always concave facing up at deposition. Look at the cross section of the bed and place a straight edge (ruler, notebook edge) on one set of laminations and determine if it curves down or up.
If you have several layers of cross bedding, then look at the boundary between two layers. The overlying layer will frequently truncate the laminations of the underlying layer.  Think of a 'T' being formed.
The crest of a ripple points up unless the ridge you see is really a cast made by trough of a younger bed.  The true crest has a smaller radius of curvature than a trough when viewed in cross section.
Dead animals better known as body fossils help also. Cavities sometimes get partially filled with mud leaving a flat floored filling or geopetal structure with the flat floor being the upside.
Also more commonly seashells are found lying concave down than concave up (check this out next time you are at the beach).

Example from the Redwall Limestone (Mississippian) of Utah

Example from the Tillery Formation (Cambrian-Ordovician?) Caroling Slate Belt, Albemarle, North Carolina. This a turbidite sequence. Enlarged view

Exercise #2: You have been provided with a collection of rocks which have one or more sedimentary structures. Each rock has one side marked A and another marked B. You are to identify the structures/structures and you are to determine if the A side is up at deposition or the B side was up. Some samples may be indeterminate, if so indicate such. Record your answers for now in your lab notebooks. 

Paleocurrent Analysis

    Paleocurrent analysis involves using sedimentary structures to determine the direction of flow or orientation of flow like that of a river, a group of streams within a basin, the wind direction within a region, or the direction of oceanic currents.  Individual sedimentary structures tell you the flow direction at that geographic point and at that instant in time but in solving true regional scale problems we need to look statistically at populations of sedimentary structures.  These will give us a collective average of the current directions within a region over a period of time.  Not all sedimentary structures provide flow direction information and even those that do, do not provide the same kind of information.  For those that do provide flow information we have those that point down stream (down flow direction) like asymmetric ripples and trough cross-bedding and we have those like tool marks (skips, groves, and prods) and parting lineation that give us a current orientation, but we can not upstream from  downstream

    The way to ascertain a paleocurrent direction is first to go into the field and gather the data, that is make measurements with the Brunton Compass on the orientation of the structure.  For those that point down stream we record the down current direction while for those which only give us a stream orientation we record the bearing for that trend expressed as so many degrees east of north or west of north. We do this for all available structures within the area of interest (allowing for time constraints of course if the region has a generous supply of structures).  This is recorded in our field books.  A page in a field book may look like such:

    We make use of this data by plotting it on a rose diagram (see page 41 of Tucker).  To do this we first separate the apples from the oranges; the trough cross-beds give us a down current direction whereas the skip marks and groove casts give us an upstream-downstream line or just simply the current orientation.  These are plotted separately.  Build a table like the one below for down current data or trough cross bedding. Look through your field notes above and count up the number of trough cross bedding measurements that fell in the range of N0E to N14E and put that number in the frequency box. Do the same for each other class (N15E to N29E, N30E to N44E, etc. Sum up all of these frequencies and put that number in the total box. Next divide each frequency by the total and enter the result in the percent of total box for each class. This normalizes the data to 100% so that you can compare results of one set of measurements with another.

 Now to make the rose diagram, think of a rose, which has 24 petals each representing 15 degrees of arc and the length of each petal is proportional to the percent of total from the above table.  If say the rose diagram was to be 10 cm in radius then a petal representing 50% of the total would be 5 cm long whereas a petal which corresponds to a 15^o bearing range where there was no observed paleocurrent orientation would have 0% of the total and a petal length of 0.0 cm.  

For the groove cast and skip marks it is even easier since we have to deal with only 12 segments of data as S45W and N45E are the same.  We generally express it as simply N45E not S45W. When we plot the rose the petal length for the S45W to S59W petal it is the same as that for the N45E to N59E petal and we end up with a symmetrical rose.  Use the data set and do this now.

For the Web skilled of you go to the RockWare site at http://www.rockware.com and see if you can use their DEMO version of rose diagram plotting.

The paleocurrent direction for the population of measurements is the same as the direction of the longest petal of the rose; if things are simple.  This is called the mode of the data set.  Sometimes there are more than on current direction which then we say there is a bimodal current pattern (2 directions) or a polymodal pattern (several directions of which none is really dominant).  Keep in mind with the groove casts/skip mark rose the symmetrical pattern does not imply a bimodal pattern; to be bimodal such would look more like a “X”.  The accuracy of you paleocurrent determination depends upon two things (a) your skill in making the individual measurements and (b) the size of the population of measurements with the larger the size the more reliable the determination. Typical studies of paleocurents will involve hundreds of individual measurements. 

Exercise #3: What is the paleocurrent direction that is implied by the trough cross beds and what is the paleocurrent orientation that is implied by the tool marks and casts? Record this in your lab notebook.

Stereo Net Plotting of Paleocurrent Data*

If the beds from which you are making paleocurrent current measurements are not horizontal lying then you must correct for their structural orientation.  If the bed dips more than 15 degrees you need to do this, if not don’t bother.  Making the correction involves using a stereo net and a sheet of tracing paper or clear acetate film known usually as the plot sheet.  In the field, you must measure the strike and the dip of the bed containing the sedimentary structure and you must measure the orientation and down dip rotation of the sedimentary structure.  You are defining the orientation in space of the bed as a planar surface with a line of strike and a line of dip and you are defining in space the orientation of the sedimentary structures apparent current direction or orientation (direction of plunge) as a compass direction and a rotation (dip)  from the horizontal.

For lineations on the surface of a inclined bed (current orientation)

1. On the plot sheet at the trace of the rim of the stereonet.htm put tick marks at due North, due South, the bearing of the strike of the bed and the bearing of the plunge of the linear feature; label these N, S, B, and P respectively. 
2. Rotate the overlay plot so that the plunge (P) tick is at the East mark of the net and then count toward the center of the diagram, (the thumb tack), the number of degrees of dip of  the linear feature and place at that point a (L).
3. Rotate the plot so that the Bedding strike (B) is at the north of the net and then draw a line tracing the meridian that corresponds to the dip of the bedding.  This line should pass through point (L).
4. Keeping the plot in the same position as the above find the small circle that passes under (L) and follow it to the outer edge of the net and mark there point (L’)
5. Rotate the plot sheet to the original north position and read the bearing at (L’) this is the paleocurrent direction.

Give this a shot with the date below.  The direction in the right hand most column is the answered that you should get.

For inclined cross-bedding

1. Draw the arcs for bedding strike and dip and sets strike and dip
2. Along the bedding arc find the point 90 degrees from the cross of the two points (C)
3. Track the small circle to the outer rim and read the paleocurrent direction.

Below is a set of measurements and the resultant paleocurrent direction, try it.

*(Modified from Compton, Robert R., Geology in the Field)

Sedimentary Structures as Indictors of Thrust Fault Displacement

Thrust faults are going to telescope the strata and later erosion will expose the strata on opposite sides of the fault. If you were to map the character if particular sedimentary features such as 'average bed thickness' in the form of an isopleth map you would note at once discontinuities in the isopleth pattern. These discontinuities correspond to the position of the fault. Such mapping may even help reveal the trend of a fault. The example illustrated to the right is from a study of the turbidites of the Tillery Formation (Cambrian-Ordovician?) of the Carolina Slate Belt. Refer to Dockal, J.A. and Huntsman, J.R. 1990, Application of turbidite sedimentology to determination of thrust fault displacements in the Carolina Slate Belt. Journal of Structural Geology, vol. 12, no. 3, p. 285-296.
It is necessary to ascertain the direction of change of the feature that you are mapping. This will come from your paleocurrent analysis. Once you know the paleocurrent direction strike a line on your map at a right angle to that direction.  This is the Base Line. You will be evaluating the measured property of the feature relative to distance from this base line.
Cross plot the measurement of the feature using an appropriate scale versus the distance from the base line. It is helpful to discriminate by a symbol which block or thrust sheet various measurements came form. On the right (1) denotes the Ugly Creek Thrust Sheet, (2) the Uwharrie Thrust Sheet and (3) the Mt. Gilead Thrust Sheet. Note how each form distinctive sub populations of the data set.
Separate each sub population and shift its position parallel to the 'Distance from Base Line' scale until you reconstruct an unified curve the reflects how that particular feature should have changed latterly in an undeformed region. The amount of shift it took to do this is your 'shift vector'.  The direction of the shift vector is the down current direction that you determined earlier while the magnitude is the amount of shift that was necessary to reform the scale.
The thrust vector is determined from simple geometry. The thrust vector is the direction of lateral movement of the fault which is roughly normal to the direction of the trend of the fault. The magnitude of the thrust vector is the magnitude of the shift vector divided by the cosine of the angle between the shift vector and the thrust direction. If the two are parallel the math is simple and if the two are at 90o form one another the technique does not work.

Email to me the results of Exercise #2 and Exercise #3. Work it up into a real nice looking Microsoft Word file. Insert copies of your rose diagrams in the file .

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