Stratigraphic Cross-Section

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Bridget Nelson
6 years ago
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1 Stratigraphic cross-sections are used to show stratigraphic relationships of strata along some path. They differ from structural cross-sections in three ways. First, their goals differ. Stratigraphic cross-sections are meant to convey the depositional relationships of the units, as close to the time of deposition as possible. Structural crosssections are meant to portray present-day structural relationships, such as faults and folds. Second, they differ in their datum, the horizontal surface on which the columns are hung. Stratigraphic cross-sections use some surface thought to represent a moment in time as a datum. In contrast, structural cross-sections use present-day elevation as a datum. Because stratigraphic cross-sections use a surface that approximates a time line as a datum, rocks of equal age lie approximately along a horizontal line. This approximation weakens with distance from a datum, such that time lines far from a datum may be substantially inclined. Common surfaces used as a stratigraphic datum include volcanic ash beds (bentonites) and other event beds, as well as some types of disconformities, such as flooding surfaces, which are recognized as sharp contacts at which deeper water facies abruptly overlie shallower water facies. Third, most stratigraphic cross-sections have a substantial amount of vertical exaggeration because this allows detailed stratigraphic relationships to be seen over great distances. Structural cross-sections avoid vertical exaggeration, because it distorts structural relationships, such as perception of dip. Strike lines and dip lines To interpret a stratigraphic cross-section, it is essential to understand the orientation of the line of the cross-section at the time of deposition. On a dip line, the path of the cross-section is more or less parallel to depositional dip; that is, the cross-section passes from relatively landward at one end to relatively seaward at the opposite end. Dip lines are often the most useful in that they portray the greatest amount of regional variation in the stratigraphy. A strike line is nearly parallel to depositional strike, that is, parallel to the regional depositional coastline. Strike lines often display the least amount of stratigraphic variation laterally, because all points along the cross-section were at essentially the same water depth. Strike lines can be useful for displaying along-shore facies variations, such as transitions from deltas to strand plains. Strike lines are especially useful for detecting incised valleys, because the along-shore variations in the amount of erosion can be quite pronounced. An oblique line is any line that contains substantial components of both along-coast and across-coast facies variation. If possible, oblique lines should be avoided in favor of cross-sections that isolate facies variations along depositional strike and depositional dip. Sedimentary Geology

2 Types of contacts Three types of contacts can be found on a stratigraphic cross-section, and it is important to distinguish these in a key, as they are critical to the interpretation of the stratigraphic relationships. Sharp contacts represent abrupt contacts of unrelated facies, typically representing surfaces that separate facies that are not joined by Walther s Law. One of the most common types of sharp contact is a flooding surface, a sharp surface across which deeper-water facies abruptly overlie shallower-water facies. Sharp contacts may have local erosion, but generally do not display substantial erosional relief, that is laterally varying depths of erosion. Sharp contacts are typically drawn with a solid, straight line. Facies contacts are contacts between two facies joined through Walther s Law. Facies descriptions will commonly report such contacts by describing them as gradational. For example, if Facies 1 is described as passing gradationally upwards into Facies 2, then such a contact should be treated as a facies contact conforming to Walther s Law. Facies contacts are commonly drawn with a zig-zag line, often called a shazam line. Erosional contacts are contacts where a strongly erosional surface separates an overlying and underlying facies. Facies descriptions are often the best clue that the base of a par ticular surface is erosional. Erosional contacts are usually drawn with a wavy line. Sedimentary Geology 2

3 Presentation Stratigraphic cross-sections are presented in many different ways. One common approach (shown above) is to show each stratigraphic column as a narrow vertical strip, with contacts correlated between these strip. The advantage of this approach is that it emphasizes the data, that is, the succession of rocks observed in each exposure. The disadvantage is that the columns are much narrower in reality than the horizontal scale would suggest. This style is also difficult to display when there are many stratigraphic columns, unless the cross-section is made substantially wider. Note in the example below the features that should be in all cross-sections: a key to facies and surfaces, graphical vertical and horizontal scales, compass directions at the ends of the cross-section that describe its orientation, and the labeled direction of depositional dip (if the cross-section is a dip line). Another approach is to portray the individual columns as vertical lines, which has the advantage of not exaggerating their width (shown below). Color is used in this cross-section, and color is generally far better for portraying stratigraphic relationships than black and white symbols. Again, notice in this example the key, the labeling of column names across the top, horizontal and vertical graphical scales, and compass directions at the ends of the cross-sections. Sedimentary Geology 3

Common correlation examples Correlation is done by connecting equivalent contacts in adjacent columns, or by inferring those contacts between adjacent columns.

Several common scenarios typically arise: (1) In many cases, the same contact between facies is found at roughly the same stratigraphic position in adjacent columns.

4 Common correlation examples Correlation is done by connecting equivalent contacts in adjacent columns, or by inferring those contacts between adjacent columns. It is important to remember that correlation is a tracing of contacts between facies, not the tracing of the facies themselves. Several common scenarios typically arise: (1) In many cases, the same contact between facies is found at roughly the same stratigraphic position in adjacent columns. In most cases, correlating that contact will be the correct correlation. (2) In some cases, a sharp contact will be found in adjacent columns at roughly the same position, but the facies above (or below) that contact will differ in the two columns. A good starting point in this case is to correlate the sharp contact, and draw a facies contact between the two columns with different facies such that the facies contact intersects the sharp contact between the columns. In other words, one of the facies is said to pinch out between the columns. The pinch-out should not be drawn exactly at one of the columns, but should be made somewhere between the two columns. Sedimentary Geology 4

horizontal ly, where they can be truncated by the erosional surface. (4) Sometimes, a thin stratigraphic unit may be present in one column but not the adjacent column at the same position.

5 (3) Erosional contacts can strongly truncate underlying strata. If a series of strata in one column are at roughly the same elevation as strata overlying an erosional surface in an adjacent column, contacts from the first column should be extended roughly horizontal ly, where they can be truncated by the erosional surface. (4) Sometimes, a thin stratigraphic unit may be present in one column but not the adjacent column at the same position. It is possible that the unit is actually present in all columns, but simply overlooked in some columns. In that case, the contacts should be extended with dashed lines through the columns where the unit is absent. It is also possible that the unit pinches out between the columns and the contacts can be drawn this way. Sedimentary Geology 5

6 Correlation guidelines Because the datum is chosen at a surface that approximates a time line, other such time lines ought to be relatively flat. For example, sharp contacts and facies contacts should be relatively flat. For reasons we will discuss later, sharp contacts may dip slightly seaward, but they should not dip landwards or steeply seawards. Shallow-water facies should pinch out downdip and deeper-water facies should pinch out updip. This requires knowing your facies and first establishing whether the cross-section is a strike line or a dip line, and if it is a dip line, which way is landward and which way is seaward. The pinch-out of a facies should always be indicated between two columns, never immediately at or adjacent to a column, because it is unlikely that a facies would pinch out immediately next to where a column was measured. Correlation is a prediction of what is present between two columns. Be sure that what you predict is both possible and likely in a geological sense. This will require you to think about your units as depositional environments, not just bodies of rock. To avoid this trap, be sure to label your rock units with the name of the depositional environment that they represent and not with some arbitrary label like Facies A. Procedure The first step in constructing a cross-section is to build the framework. Begin by drawing a long horizontal line that will represent your datum. Next, start at one end and add tick marks to this datum that correspond to the scaled distances between the vertical columns. For each column, draw a vertical line perpendicular to the datum, and mark off properly scaled ticks that correspond to the thicknesses of the stratigraphic units. Adjacent to each interval, lightly label the facies name in pencil. The next step is correlating the stratigraphic contacts between the exposures. This is the hardest part, and it will require substantial thought and geological intuition. Contacts should be drawn according to whether they are sharp, facies, or erosional. Draw your contacts in pencil, as you will rethink and revise your correlations. To make revisions easier, many people photocopy their finished framework and work out their correlations in pencil on the photocopy, so that it is easy to erase any false starts. When you are satisfied with your cross-section, ink it in and erase any pencil lines or labels. Finally, color your facies. Add a key for the facies and a key for contact types. Add labels above the columns. Add compass labels to the ends of the cross-section; these should be from opposite ends of a compass, such as northwest and southeast. Add a graphical vertical scale and horizontal scale, with ticks in even increments. Add numerical scales next to your graphical scales, such as 1 cm = 5 km. If the cross-section is a dip line, write the words Depositional Dip at the top, with an arrow pointing in the downdip direction. Sedimentary Geology 6

7 What to do In this lab, you will construct a regional cross-section based on the facies descriptions presented in the environmental interpretation lab last week, and the thicknesses of those facies listed below. Vertical scale: 1 centimeter = 5 meters. Horizontal scale: 1 centimeter = 250 meters. Your cross-section should be neatly and professionally done. Although you can use computer drawing programs like Illustrator, I strongly recommend doing this lab by hand unless you are already proficient with these programs. Lines should be neatly drawn; in particular, do not make your facies contacts too exaggerated. Lines and text should be inked neatly in black, and the lettering should be neatly done. When coloring your facies, you will get the best results if you color lightly and in one direction with good colored pencils. Indicate on the top of the page whether this a strike line or a dip line, and if it is a dip line, indicate which direction is seaward. When you are finished, make a photocopy of your cross-section. Turn in the original, and keep the photocopy - you will need it for the next lab exercise. The data Listed below are the five measured columns that lie along the cross-section. The distance of each column from the first column is listed after its name. Below this, each line indicates the thickness of a facies given in the previous lab, and the descriptions are given in the order that they were measured. In other words, the first line corresponds to the lowest unit, the second line is the next highest unit, and so on. Make sure that you do not get your sections upside-down! Use the lowest contact of facies FH2 on FS2 as the datum. Column 1, southwest end of cross-section 7.6 meters of FS2 2.2 meters of FH2 2.5 meters of M 1.6 meters of FH2 2.5 meters of M 0.6 meters of FC 8.9 meters of M Sedimentary Geology 7

8 4.1 meters of FM 2.2 meters of FH2 4.4 meters of FM 3.8 meters of FH2 3.2 meters of FM 3.8 meters of FH2 2.5 meters of FS1 1.9 meters of FH1 8.6 meters of FM Column 2, 1.6 km northeast of Column meters of FS2 2.9 meters of FH2 1.3 meters of FM 1.3 meters of FH meters of FM 5.4 meters of FM 1.9 meters of FH2 4.4 meters of FM 2.9 meters of FH2 1.9 meters of FS meters of FH1 3.8 meters of FM Column 3, 3.6 km northeast of Column meters of FS2 2.9 meters of FH2 1.9 meters of FM 1.6 meters of FH2 3.2 meters of FM Sedimentary Geology 8

9 0.3 meters of FC 6.3 meters of FM 4.8 meters of FM 2.9 meters of FH2 7.9 meters of FS meters of FH meters of FM Column 4, 4.5 km northeast of Column meters of FS2 2.2 meters of FH2 2.2 meters of FM 1.0 meters of FH2 9.5 meters of FM 4.1 meters of FM 2.2 meters of FH2 3.5 meters of FM 4.4 meters of FH2 3.5 meters of FM 2.5 meters of FH2 6.0 meters of FH1 7.6 meters of FM Column 5, 5.8 km northeast of Column meters of FS2 2.2 meters of FH2 1.3 meters of FM 3.5 meters of FM Sedimentary Geology 9

10 0.3 meters of FC 6.0 meters of FM 4.1 meters of FM 1.6 meters of FH2 3.2 meters of FM 4.8 meters of FH2 4.1 meters of FM 2.9 meters of FH2 3.5 meters of FS1 1.9 meters of FH2 1.0 meters of FS1. The top of this unit is strongly stained red and has sparse carbonate nodules 5.1 meters of FM Sedimentary Geology 10

A. Refer to Appendix F in back of lab manual for list of commonly used geologic map symbols

Structural Geology Lab 2: Outcrop Patterns and Structure Contours I. Geologic Map Symbols A. Refer to Appendix F in back of lab manual for list of commonly used geologic map symbols 1. Emphasis: a. strike