This course is designed primarily to prepare students to Introductory Field Camp (course Geo 3911).  The Geo 3890 course has four main goals:
-  Develop a sense for the observation of sedimentary, igneous, and metamorphic rocks
-  Develop skills in the acquisition, recording, and representation of orientation data
-  Develop map-reading skills and familiarity with structure contours and cross sections
-  Develop a basic knowledge of the regional geology of Montana

The format of this class is very much lab-oriented.  I want the students to acquire as much practical experience as possible in preparation to field work.

EQUIPMENT

Please have pencils, a ruler, and basic drafting utilities.

Please acquire a hand-lens if you do not own one; it does not have to be an expensive one, but you will need to have a magnifying lens in the field, in order to identify minerals and textures.  In addition, you should have a geo-hammer and sturdy boots for field work.  In the first four days of Field Camp, we will be camping, so you will need a tent.  Because we will be camping for such a short time, it would be preferable if you could share tents; talk to your colleagues!!
 

GRADING POLICY

In order to receive credit for the course, students must turn in all assignments

Grades will be based on performance on assignments and also on attitude and participation in the class (this is to encourage you to participate!!)
 

CONTENT

Class time: Subjects covered:

  Simple map and cross section exercise, structure contours
  Map and cross section exercise
  Use of the compass- Orientation data:  Intro to stereographic projection
  Advanced stereographic projection:  Rotation, complex problems
  Sedimentary rocks
  Igneous and metamorphic rocks
  Regional geology of Montana

One optional field trip to Thomson Dam, MN, scheduled for April 28, 2002, in combination with the Petrology field trip.
 
 

This is the geologic map of Bundi-Bundi in eastern Australia.  You found the map but were not prepared to spend money on the accompanying explanatory notes.  You trust the geologic background you received at the University of Minnesota will allow you to reconstruct at least the first-order geologic history of the region.  To this end:

1)  Elucidate the geologic structure of the area and mark the attitude (dip and strike) of the beds on the map.

2)  Mark on the map any break in the stratigraphic succession

3)  Fill in the table of formations in the stratigraphic order, from oldest at the bottom to youngest at the top

4)  Draw a cross section along the line A-B

5)  Determine the thickness of beds and write their thickness in feet next to the stratigraphic boxes
 
 

To determine dip and strike of beds:

    -Look for a geologic contact that crosses the same topo contour (dashed lines) at two different points.  Because these two points have the same elevation, and are in the plane of contact, the line that joins the two points is the STRIKE line.  You can check at various localities that the lines determined from this method are ~ parallel to each other.  That means that all sedimentary units have the same strike.  The lines determined by this method are called STRUCTURE CONTOURS.

When structure contours are linear and parallel, then this means that the beds have a uniform strike (like a dipping set of books.  In contrast, for a perfect domal structure, the beds would have the shape of a part of a sphere, and the structure contours would form concentric circles.

    -Determining the dip is a little more involved:  In principle, the dip is determined from the structure contours.  If you can find two structure contours on the same geologic contact, then the diffrence in elevation of these two structure contours, and their spacing, will allow you to find the dip by simple trigonometry.

Example:  On this map, there is no place where the same contact crosses two topo contours, but there is one place (center of map, just west of dike) where the contact between the dotted unit and the unit shown by spaced lines trending N-S crosses the 700 feet contour twice (making a structure contour), and also just touches the 800 feet contour to the north.  This gives you three points in the plane of this contact, two at elevation 700 ft, and one at elevation 800 ft.  From the difference in altitude between these points and their spacing on the map, you can draw a triangle from which the dip of the contact is determined.

Cross section:

Then you can draw your cross section (you know the dip of beds), and from the cross section, you can determine the thickness of beds, and then complete the stratigraphic column (the boxes next to the map) that shows units from the oldest at the bottom to the youngest on top.

ONE IMPORTANT POINT:  Please draw your cross section with the vertical scale = horizontal scale.  Otherwise, the dips or the thicknesses of units are grossly exaggerated.

Good work!!
 

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Before going to field camp, I would like to introduce you to the principles of stereographic projection.  Stereographic projection is a way to represent in 2D structural data in a 3D geographic space.  Structural data include mostly planes and lines.  Bedding, foliation, fractures, veins, etc, are surfaces that can be represented locally by a plane.  Lineations, whether primary (paleocurrent directions, parting L) or tectonic (stretching or mineral L), are lines that can be represented locally by straight lines.  Note that the intersection of two planes is also a line.

Let's consider a simple example, a folded region of a sedimentary basin.  The sedimentary bedding is generally not horizontal but describes more or less complex surfaces that change orientation in a continuous manner.  To visualize this folded terrain, take a sheet of paper, hold it flat on the table, and fold it by sliding your hands toward each other.  The paper will buckle and form a fold.  Clearly, the orientation of bedding (the sheet of paper) changes continuously across the fold.

In the field, we record orientation data for planes and lines.  There are many ways by which you can record these data relative to north and the horizontal surface.  I will give you simply what I use:
-  to record the orientation of a plane, I take the strike of the plane and the dip and dip direction of the plane;  a plane oriented 123/46S is a plane striking at 143° from North and dipping 46° southward from the horizontal.  In this notation, strike varies from 0 to 180.  A strike of 0 or 180 is a plane striking N-S; a strike of 90 is for a plane oriented E-W.
A dip near zero represents a plane almost horizontal; a dip near 90 describes a nearly vertical plane.  Dip varies from 0-90.
-  to record a line, I use plunge an trend which I write as follows:  33 ----> 225.  The plunge of the line is the angle between the line and the horizontal line measured in the vertical plane that contains both lines (in this example, 33°).  The trend of the line is the azimuth relative to North, for example 225°.  In this example, the line plunges 33° from the horizontal in a direction 225° or SW.  The trend varies between 0-360, the plunge ranges 0-90.

Example:

Let's get back to our example of a folded terrain.  Bedding attitude was measured at various locations; plot these data on the stereonet
1.  025/66W         Note that strike is the same at all localities; this is because the folds have a
2.  025/31W         horizontal axis
3.  025/06W
4.  025/12E
5.  025/44E
6.  025/71E

Procedure:  Put tracing overlay on top of the stereonet and mark the North arrow and keep it on the north pole of the stereonet.  For the first bedding 025 W 66, find the strike 025 by counting 25° from the pole eastward, using the stereonet as a protractor.  Make a mark on the tracing overlay at 025.  Now rotate the overlay such that the mark you made coincides with the north pole.  Now you can use the stereonet as a template to draw all planes that strike 025 using the great circles (those lines convex outward).  Find the planes that dip 66W, 31 W, 06W, 12E, 44E, and 71E and draw them.  Note that they all intersect at the 025 strike point.  The axis of the fold is 0 ---> 025.

Determine the poles to planes.  Poles to planes are a point in the stereonet perpendicular to the plane; since there is only one point perpendicular to any plane, then the pole to the plane can represent the plane and does not crowd the stereonet as much as the great circles.  Draw the poles for the 6 planes above.  Note that the poles lie on a particular plane, the vertical plane striking 115, whose pole is the fold axis.

Now plot the poles to:  015/72E; 177/50E; 137/31N; 101/31N; 062/41N; 037/68W.

These poles fall on a great circle.  The pole to this great circle is 30 ---> 025 and represents the fold axis for this fold.
Now you realize that the second set of bedding planes above is exactly the same as the first set rotated 30° in a vertical plane striking 025.  Small circles in the stereonet allow rotations.

Rotations Exercise (please hand in, or put in my mailbox, by Tuesday April 30, 5:30pm) :

In a region of folded sedimentary strata, bedding contains the evidence of paleocurrent directions.  Paleocurrent is given by the orientation of sedimentary structures such as ripple marks and imbricated clasts in conglomerates.  At six localities, you measured attitude of bedding and also recorded the direction of paleocurrent as a line in the plane of bedding.

Bedding    Pitch or azimuth of line of paleocurrent

022/70W    (72 N)  pitch
045/46W    (84N) pitch
083/31N    (66 W) pitch
138/34E    (19 N)  pitch
169/56E    (166)  azimuth
002/76E    (179)  azimuth
 

Your project now is to determine what the direction of current was prior to folding.

You can  use your stereonet to rotate successfully the sedimentary strata and lines of paleocurrent directions back to horizontal, and read off the orientation of original paleocurrent.

(Please hand in your stereonet with data, and a brief paragraph explaining how you got your answer.)
 
 

1.  Classification of sedimentary rocks

Clastic and non clastic sedimentary rocks
Clastic sedimentary rocks are classified according to grain size.  The grains in a clastic sedimentary rock are called clasts.  From large to small, particles in conglomerates are called boulders, cobbles, and pebbles.  Finer clastic rocks are called sandstone, siltstone, and mudstone or shale in order of decreasing grain size.  In the sediment, gravel sizes range from pebble (2-64 mm) to cobble (64-256 mm) to boulder (>256 mm).  These three types of sediment lithify to form conclomerate (rounded fragments) or breccia (angular fragments); you may talk of a pebbly conglomerate to describe a fine-grained conglomerate.  Sand size is comprised between 2 mm and 1/16 mm, silt between 1/16 and 1/256 mm; and clay is finer than 1/256 mm; these lithify into sandstone, siltstone, and shale, respectively.  Strictly, this classification applies to size and not composition; however, most shale are made of tiny grains of phyllosilicates, sandstone is commonly made of quartz or other common minerals.
Non clastic sedimentary rocks include chemical sediments such as limestone (and more generally carbonates) and evaporites, and biogenic sediments such as coal and limestones and cherts made of calcareous or siliceous skeletons of marine organisms.  Sedimentary rocks contain fossils and trace fossils which tell us about the depositional environment, as long as they have not been transported.  The presence of fossils is not indicative of a non clastic environment.  Many clastic rocks contain fragments of shells that were transported.

Sedimentary depositional environments
Another important way to classify sedimentary rocks is to define the environment in which the sediment finally came to rest.  There are two major types of environment:  marine and non marine.
Marine environments characterize continental shelf sediments, continental slope sediments, and deep (abyssal) sediments.  The presence of marine fossils is indicative, of course, of a marine environment, as long as fossils have not been transported.
Shelf deposits are typically made of sandstone to mudstone, from the beach seaward (fining in grain size).  Further out, the continental slope is incised by large canyons which are channels in which turbitity currents develop.  Turbidity currents are due to a gravitational instability (sometimes triggered by earthquakes) that sends a mass of water and unconsolidated sediment down the slope.  These sediments are then dispersed over a great surface area in the deep ocean and settle slowly and selectively such that the coarse sediment is deposited first and the very fine particles are deposited last.  Over time, the sedimentary sequence is made of alternating layers of turbidite, which is very distinctive (called graded bedding).  The Indus River and Bengal River are a major source of sediments derived from the Himalayan mountains, which build turbidites in the Indus and Bengal fans.
Common non-marine depositional environments include glacial, fluvial (rivers), lacustrine (lakes), and eolian (wind) environments.  These environments also produce characteristic sedimentary structures that allows their identification.

2.  Sedimentary structures

Most sedimentary rocks are deposited very near horizontal.  The surface of deposition is called bedding or stratification.  Other structures in the plane of bedding, or at an angle to it, provide clues as to the conditions of deposition of the sediment.
Cross bedding indicates the formation of dunes or ripples, and paleocurrent direction can be derived from it.  Cross bedding can be produced by water or wind flow.  In water, sorting is such that the larger particles tend to fall at the base of the bed; in wind, the small particles at the crest of dunes tend to fly away, leaving a distinctive concentration of larger particles at the crest, or near the top of cross beds.
Ripple marks are wavy surfaces that indicate shallow water; ripple marks can be symmetrical with pointed ridges and broad troughs, in which case they can be used as way-up criteria, or asymmetric indicating the direction and sense of the current, if way-up is known independently.
Graded bedding may indicate a turbidity current environment, if it is repeated rythmically.  Varves indicate lacustrine deposits in cold regions.  Mudcracks indicate dry conditions, at least episodically.
Mud cracks are a polygonal pattern formed by drying of a thin layer of mud (mud is very water rich, therefore it contracts a lot upon drying).  The cracks are filled with the material covering the mud bed, leaving a distinctive imprint of the cracks, suggesting that the environment was one where dry conditions occurred, at least episodically.