Sedimentation & Stratigraphy

aka Sedimentary Rocks and Processes | Back to the GEOL 102 Homepage

Sedimentation & StratigraphyClass HandoutsAdditional ReadingsSummaryKey ThemesWeathering as observed in the fieldDetection of sea-level changes in the sedimentary and stratigraphic recordThe Western Interior SeawayA Wonderful Sedimentary RockCross-bedding imagesChertMiscellaneous Reference for FunSlide SetsSome Examples of Sedimentary Rocks in The Wild!

Class Handouts

"Virtual" handout on naming clastic sedimentary rocks
It's about time now to look at a geologic time scale.

Additional Readings

Messinian Evaporites
- Please read this one - questions about this material may show up on future exams.
Turbidites
Chalk

Summary

Introduction to the concepts of:

Weathering
Erosion
Transport
Deposition
Lithification

The concept of transport (water, wind) is important because it not only moves eroded material to new locations but it also sorts grain size and compositions.

Grain size is a primary way we name sedimentary rocks. Compare this to the way we named igneous rocks.

Resistance to weathering increases as you go from mafic to felsic minerals (i.e. in Bowen's Reaction Series, the later a mineral crystallizes the more resistant to weathering it is). As an example, quartz can become very well sorted and be deposited as "sand", lithifying to sandstone. Note that "sand" is an official term for a grain size designation - see your reading for official definitions of "grain sizes".

Deposition of very fine-grained material (forming rocks like mudstone and shale) requires a very calm environment, like a deep oceanic basin or a large calm lake. Any disturbance keeps fine-grained material in suspension.
We observe ripples in some of our sedimentary samples. What does the observation of symmetry or asymmetry in the ripples suggest about the depositional environment?

Key Themes

Transport: this process allows for sorting of grain size as well as sorting of composition as minerals break down. For example, quartz is very resistant to weathering, so transport allows quartz to be a more uniform composition that is deposited and lithified.
Formation of clays (e.g. Kaolinite clay from K-feldspar example in the slides)
Chemical weathering (e.g. the limestone weathering example in the slides)
Stratigraphy = sequence of sedimentary rocks. Application: understanding earth history such as changes in sea-level, or identifying the former existence of the Western Interior Seaway, and mapping out its changing shorelines over time (over about 40 million years)

Weathering as observed in the field

Speaking of weathering, sandstone (quartz!) should be more resistant to weathering than shale (just small grain sizes, but not a uniform and highly resistant material like quartz). The image below shows alternating layers of sandstone and shale, taken from what is called the Great Valley Formation. There is a local outcrop of it near Cayucos / Whale Rock Reservoir, and is a field trip site stop in GEOL 241 Physical Geology Laboratory.

Can you decide which color is the shale and which the sandstone?

shale-ss

Here is another example of a weathering pattern between a more resistant sandstone and a less resistant sedimentary rock: Weathering Example

Detection of sea-level changes in the sedimentary and stratigraphic record

The Western Interior Seaway

This is a super cool application of sedimentary rocks and stratigraphy!

From a fossils point of view. if you are interested in fossils and the history of life you can take GEOL 203 (Fossils and the History of Life) from Tony Garcia (every winter quarter).

This is a video of shorelines over time. In addition to using stratigraphy to map shorelines, you need to also think about how to date the age of the sedimentary rocks. More generally, determining dates for geologic events is called geochronology and we will have a brief chapter on that soon. The video is also embedded just below in this page:

Western Interior Seaway: Cretaceous Shorelines of Colorado from Interactive Geology Project on Vimeo.

A Wonderful Sedimentary Rock

This is one of my favorite sedimentary rock samples that we have in the geology classroom. In this sedimentary rock sample (from the Death Valley area) we can see several things (the scale is cm):

Changes in grain size, getting smaller from the bottom of the image upward. What might you name each of these three grain sizes? (e.g. conglomerate, sandstone, siltstone, mudstone, etc).
How do these grain sizes tell us about the change in the depositional environment here?
The middle layer shows cross-bedding - review that in your textbook reading. You can also see a diagram here of the cross-bedding process. Does this help you understand why the image is labeled with the "up" direction of the rock the way that it is?

sre