1. Stratigraphy
Rocks, Fossils, and Time

2. Introduction Stratigraphy crucial to Historical
Helps us understand vertical and lateral changes in rocks/environments with time
Helps us understand paleogeograpy e.g. where the land was, where the ocean was, where the mountains were, etc

3. What is Stratigraphy?
A branch of geology concerned with composition, origin, age relationships, and geographic extent of layered or stratified rocks
Valley of the Gods, Utah

4. Stratigraphy Stratigraphic section
Tracing stratigraphic sections from outcrops leads to correlation of rock units
Sequence of sedimentary rocks on a bed-by-bed basis
Demonstrates equivalency across an area
To draw a stratigraphic section, you must have data from a sequence of rocks. You will need to have data on the thickness of each bed, and all of the physical, chemical, and biological characteristics of that bed, as well as the character of its contacts. Before you start, you need to examine your data to determine the total thickness of the section you plan to draw. Then determine a proper scale so that the entire section will fit on your paper (such as, 1" = 100'). Draw a vertical column in which you will plot your data, and then mark off the thickness of each bed or formation using the scale you established. Draw in the contacts between units; if the contacts are erosional, you should use a wavy line. Once you have drawn in contacts, draw in the lithologic symbols for each unit. Information on fossils and sedimentary structures, etc. may be placed within the unit, or beside it using a special symbol or small sketch. Color may be illustrated with a special symbol, or by coloring your section. There are standard symbols which have been established by oil companies and core logging companies. You may use theirs (see a reference book), or create your own. Your instructor may give you further instruction on this.
Once you have drawn several stratigraphic sections for an area, you may begin to correlate them

5. Sedimentary Facies
Body of sediment/rock characterized by physical, chemical, and biological attributes
Sediment/rock makes it recognizable and different from laterally adjacent rock
Could be textures
Could be fossils
Could be composition
Transgression – Sea Level rises, environments march landward
Regression– Sea Level drops, environments march seaward

6. Lateral Changes in Stratigraphy
Beds may change laterally in thickness or lithology
Differences due to changes in sedimentation rate or depositional environment

7. Lateral Changes
Pinchout
Intertonguing
Lateral Gradation

8. Walther’s Law
Those facies found in a conformable (no faults, unconformities, folds) lateral sequence will replace one another vertically

9. Conformable Vertical Successions
Transgression
Regression
Sea level ↑; shoreline moves inalnd
Sea level ↓; shoreline moves seaward
Facies are time transgressive
Ages of facies vary from place to place

10. Did sea level rise or fall?

11. Transgressive Sequence
Note time lines cut across units

12. Regressive Sequence

13. Transgressive and Regressive Sequence
V-shaped pattern produced by migrating facies during a transgression followed by a regression
Three stratigraphic sections illustrate facies generated in vertical succession at three different locations

14. Causes of Transgressions and Regressions
Cold and warm climates
Cold climate  glaciation
Warm climate  ice melt
Uplift and Subsidence
Uplift  regression
Subsidence  transgression
Seafloor spreading
Fast spreading  transgression
Slow spreading  regression

15. Fossils and Correlation
William Smith
Determined relative ages of rocks using fossils and assemblages of fossils
Established Principle of Faunal Succession

16. Principle of Fossil Succession
Fossils succeed one another through time in a regular and determinable order
Because of this, any time period can be recognized by its fossil content

17. Fossil Assemblages
Species have a time of origination and a time of extinction
Because of this, different times have different fauna and flora
If sedimentary rocks from different places contain the same fossils, they were deposited at the same time
If fossil A range = 40 – 20 my
If fossil B range = 25 – present
What is age of rocks containing last appearance of A & first appearance of B.

18. Establishing Correlation of Units
How do we establish correlation or equivalency of units?