Monday, January 3, 2011

Lecture 1: Sedimentology and Stratigraphy Concepts

The Course web site is the SmartSite for GEL109/109L and which is linked on the SmartSite.

I am responsible for:
1. Coming to lecture prepared to provide you with the opportunity to learn about sedimentology.
2. Preparing instructive homework assignments and tests that will help you learn the most important material. My philosophy is to make it clear what I think is important and provide you with tools to learn the material. I provide old tests and detailed study guides so that you can thoroughly learn the material and demonstrate your knowledge of it on the various assignments.
3. Grading your homework and tests in a fair and timely manner.

You are responsible for:
1. Coming prepared for lecture by reviewing your previous lecture notes and reading the text book. Reading the book is very important because it includes some information that I will cover only briefly in class, but I’ll expect you to know. For example, I will not discuss rock classification schemes in lecture because it is boring and the book explains the rock types well. However, I will use the terms sandstone, siltstone, shale, etc. because they are always used in geology, and you need to know them. In most cases, the material I cover in lecture and with the homework assignments are the concepts I think are most important, so the tests will focus on those.

2. Asking questions when you have them in class, through e-mail, during office hours, when you see me in the hall, etc. Asking questions is VERY important for two reasons. First, it will help you and your fellow students learn, which is the entire reason you are in this class. Second, it helps me gage how well I'm communicating information to you. The more I know about how much you are understanding and what you are thinking about, the better I can prepare explanations that are both clear and interesting.

Grades will be calculated using two formulas: 2 tests and 10 homework assignments with the midterm=33%, final=34%, and homework=33%; and 2 tests with the midterm=50% and final=50%. You will get the higher grade from the two formulas. I do not grade on a curve, so if you all thoroughly learn the material, you can all earn A’s.

My tests are hard, but fair. They are hard because they ask you to think about the material. I will give you detailed study guides which will contain all of the material that will be on the tests (plus some). The homework will include questions like those on the tests, and practice tests are available. The only people who have failed the tests are those that did not take advantage of the study tools.

The homework assignments are posted on SmartSite as pdf files. There are some web materials that supplement some of them. These materials are very useful and you should plan to use them. Homework is due by the end of the date listed.

For people not taking the lab:
The students who are also taking the lab (GEL109L) are spending an additional 6 hours per week working on sedimentology, so they will be more familiar with the material than you are. You can still do well, and it is particularly important that you ask questions. Also, there are two field trips required for the lab. I strongly recommend coming on them. You can learn much more in the field than in lecture or even lab!

Sediments and Strata
Sediments and sedimentary rocks cover most of earth, and weathering is occurring on the rest of it. The reshaping of the surface of the earth has had a huge influence on the planet, affecting everything from the evolution of life to the tectonics of mountain ranges. Sediments and sedimentary rocks record the events and processes that shaped the surface of earth – and other rocky planets. They provide the temporal framework that connects processes within the earth to those at the surface. They are important for:

1. Earth history. Sedimentary rocks contain features that allow us to interpret ancient depositional environments, including the evolution of organisms and the environments they lived in, how climate has changed throughout earth history, where and when faults were active, etc.

2. Economic resources. Petroleum reservoirs have organic-rich source rocks that produced the petroleum when heated, most oil and gas migrated through sedimentary rocks, and most of the reservoirs are hosted in sedimentary rocks. Water aquifers are dominantly found in sedimentary rocks (although some are in fractured metamorphic and igneous rocks). The composition of the rocks strongly influences water quality due to water-rock interactions. (Why does Davis water taste bad?) Sedimentary rocks also host economic minerals such as gold and diamonds, which are eroded from other rocks and concentrated to specific areas during sediment transport.

3. Environmental geology. Sediments cover 2/3 of the continents and all of the ocean floor, which totals 89% of the surface of earth. They host the biosphere, and they are most of the rocks we interact with directly and indirectly. Our actions as humans have an extremely strong effect on sedimentation and erosion. Understanding our impact on the environment must include a strong component of our impact on sediments and sediment transport.

Sedimentology and Stratigraphy
Sedimentology is the study of sedimentary rocks and sediment transport processes, and it is the focus of most of this course. If covers scales ranging from a single grain to entire planets, and focuses on processes. Stratigraphy is the study of the distribution of sediments and sedimentary rocks in space and time. We will place our sedimentological interpretations into a stratigraphic context with examples in lecture.

Sedimentology and stratigraphy are about as old as mineralogy as a field. Leonardo da Vinci provided one of the first environmental interpretations from sediments; he interpreted fossils in the Italian Apennines as evidence of an ancient ocean. He used the logic behind what we now call the Principle of Uniformitarianism: Similar organisms produce similar shells. The logic is, “If you see shells on the tops of mountains that look like those from organisms that live in the ocean today, the shells on the mountain tops were probably once in the ocean, too.” (Did the mountains go up or did the ocean go down? That is a question that was not thoroughly answered until we understood plate tectonics!) Here is a more formal statement of the Principle of Uniformitarianism:

Key Concept: The characteristics of sedimentary rocks can be used to determine the environmental conditions under which they were deposited, and the environmental conditions allow you to predict the characteristics of sediments that are likely to be deposited. This is the Principle of Uniformitarianism (formulated by James Hutton in the mid 1700’s) – the processes that formed ancient deposits are the same as those that form modern deposits.

Movies of sediment transport and photos of ripples and ripple cross-lamination:
See L1 Movie 1 and L1 Movie 2 in Resources/Videos on the SmartSite page
Modern Current Ripple Image:
Ancient Current Ripple Image:
Images of impact spherules not available online

In the much more recent past, for example in the 1970’s, the Principle of Uniformitarianism was interpreted by some as requiring continuous, incremental processes and as excluding dramatic, rapid events. For example, a meteorite impact was seen as a non-uniformitarian event. However, the view of uniformitarianism can encompass rare events. The basic idea is that catastrophic events also produce characteristic features. For example, a meteorite impact produces similar deposits no matter when it occurs in time. We can recognize impact spherules from Archean sedimentary rocks that formed and were deposited in essentially the same way as those from the Cretaceous impact that killed the dinosaurs. Or an impact on Mars will produce features similar to those produced by an impact on Earth. The key point is that similar processes produce similar products. All processes are not active at all times (large meteorites are not continuously bombarding earth!), and some, like burrowing by worms for example, did not occur at specific times, e.g. before the evolution of worms. However, if a feature is present that is characteristic of a specific process, e.g. a thin tube with a specific geometry, it is reasonable to interpret that process, e.g. burrowing by a worm, as having produced the feature. This is how we extract earth history from rocks, e.g. the absence of worm burrows before 540 Ma allows us to state with confidence that worms did not exist before 540 Ma. However, it is often challenging to identify which processes produce which features. There is rarely the nice, exact correlation that one would wish for. For example, a specific color variation in a rock could reflect a burrow or a water flow path. One needs to understand the uncertainties in the interpretations.

Summary of the Principle of Uniformitarianism:

There are two other really important concepts that were first articulated in the 1660’s: The Principle of Original Horizontality and the fact that younger sediments overlie older sediments. Nicolas Steno was the first to write down the idea that strata (or sedimentary rock layers) are deposited in a nearly horizontal position, an idea called the principle of original horizontality. Some layers are deposited exactly flat, but most layers follow the tilt of the depositional surface, which is not exactly horizontal. However, most sedimentary layers are close to horizontal for our purposes here. If layers are no longer horizontal, later deformation must have changed their orientation.

Images of the Grand Canyon and turbidites along Cache Creek, CA not available online

This idea is intimately associated with the idea of time in rocks.

Key Concept: Younger sediments overlie older sediments (if they are still approximately horizontal)– obviously, at least to us now.

Steno first wrote it down in 1667. The relative ages of sedimentary rocks gives us time. We can interpret changes in processes through time using the principle of uniformitarianism combined with the relative age (or stratigraphic succession) of rock layers. Steno’s work provided the intellectual framework for understanding relative time in a local areas. It was not until much later that the idea of “faunal succession” (articulated by William Smith, early 1880’s) provided a global time scale. Smith (and other geologists at the time) recognized that fossil organisms succeed one another in the stratigraphic record in an orderly, recognizable fashion. They recognized a key component of evolution, although they did not yet have the intellectual framework of evolution by natural selection. They formulated the basic ideas that if organisms evolve through time, rocks containing similar organisms are approximately the same age.

Discussion of the distribution of time in sedimentary rocks:

The observations leading to the definition of Cambrian, Ordovician, etc. were basically the same as those of da Vinci: Fossils can be used to interpret ancient rocks. In the case of da Vinci, he interpreted similar environments because the shells he saw were essentially identical to those in the modern oceans. In the case of Smith and other English geologists, similar fossils suggested the rocks were similar ages. These two ideas reflect the two key components of stratigraphy: rock types vary in both space and time, but the same type of rock can be deposited in different places at different times. These changes can be organized based on how different environments are distributed:

Key Concept: (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).

(*Conformably means that there is neither a break in sedimentation nor erosion between the two environments, e.g. there is no unconformity between them. Jumps in depositional environment can occur if the rocks do not provide a complete record of the environmental changes that occurred; rock types in a vertical succession separated by the unconformity do not necessarily represent neighboring environments.)

Images of environments and a Google Earth tour: Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.):
Environments from Davis to San Francisco:
Tidal Environments near Derby, Western Australia:

One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class.

Summary of Walther’s Law:

Next time - Sediment transport
Chapter 4 is the most important for Wednesday’s lecture.
Chapters 2 and 3 cover classification of sediments and sedimentary rocks. We are not going to cover them explicitly in class, but knowing the terminology will be critical. Thus, read these chapters soon! Chapter 2 will be very useful for those of you in lab.


  1. There was no distinctive difference specified between sedimentology and stratigraphy and also the laws are not explicitly stated.

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