Sedimentary Rocks and Fossils

What percentage of sedimentary rocks cover the earth’s surface?

75%

Why study sedimentary rocks?

• contains all fossils and entire record of earth’s geological history

• evidence for environmental change

• source of economic minerals and materials

What are the kinds of sedimentary rocks?

How do you describe a clastic sediment?

• size

• shape

• composition

• bulk properties

• grain size distribution

• texture

• sedimentary structure

• porosity

• permeability

How do we measure particle/clast/grain size?

Sieving - standard for coarser grained sediment

Settling - standard for finer grained sediment, larger particles fall through water faster than smaller particles

What is the Udden-Wentworth grade scale?

How do you calculate phi?

Overview of classification of terrigenous clastic rocks?

How do you describe particle shape?

Roundness (angularity) - sharpness of edges and corners

Sphericity - spherical circular 

What determines size of particles, and shape and roundness?

What is packing or fabric?

The mutual arrangement of grains in the deposit

What is porosity and permeability?

Porosity: amount of void space, expressed as volume%

primary - formed at time of deposition

secondary - appeared in rock post depositional process

Permeability: describes the ease with which fluids can be forced through the porous sediment

What components are there to look at the geometry of the rock?

ppt slide of compositional components of sedimentary rocks

What is the origin of the clastic sediment?

Weathering of continental crust is primary source of terrigenous clastic material.

What are the major weathering processes?

How does weathering control erosion rates?

• chemical reactions quicker at high temps

• high rainfall provide water needed for chemical weathering

• humid tropics where chemical weathering is most rapid

• arid and temperate climates where weathering rates are lower

Relief controls time available for weathering.

High relief = rapid erosion and short time for weathering

Low relief = low erosion and long time for weathering

In the Goldich Weathering Series, of susceptability of minerals to weathering, what mineral is most resistant to weathering?

Tell me about quartz

• most common mineral in sediments 

• effectively inert, stable at surface T and P

• low solubility in water

• resistant to chemical weathering

• hard, and lack of cleavage

Gravel size quartz comes from vein quartz. Milky white

Sand size comes from crystalline source rocks e.g. granite, recycled quartz rich sandstone, or chert. Clear, glassy, slightly milky, NEVER DULL WHITE

Silt size quartz comes from fine grained metamorphic rocks, fine grained sediments, spalling of larger grains

What is meant by extinction?

Under crossed polars, extinction is where the mineral goes black every 90° rotation.

Tell me about feldspars

Subordinate to quartz in abundance.

• feldspar content much more variable than quartz

• mechanically weaker than quartz

• cleavage

• softer

• chemically weaker, easily hydrolysed

• unstable during weathering in soils

• chemical weathers, reacts readily with water and weak acids to form clay minerals

• white, cream, or pink but NEVER GLASSY, sometimes powdery if strongly weathered

Identifying feldspar and quarts

What are the different types of feldspars?

What are micas?

Silvery shiny catching light on bedding surface

What other constituents occur in sedimentary rocks?

What is responsible for the red/yellow/brown/purple colours?

Most sedimentary rocks contain some iron oxides.

Red = Hematite Fe2O3

Yellow, browns = Limonite/Goethite FeO.OH

What is the matrix often made from?

Clay minerals produced from chemical weathering of feldspars.

Often grey

What is cements?

Form during diagenesis

• precipitated into pore spaces after de

• position

• lithifying agent that turns sediment in sedimentary rock

What are two common cements in sandstones?

1) quartz

2) Calcite

Difference between diagenesis and lithification.

Diagenesis = All post-depositional changes before metamorphism (includes lithification)

Lithification = Sediment → Rock

How do you classify sandstones?

Proportion of matrix, and then consider proportion of quartz, feldspar, and rock fragments. 

Classification of arenite from quartz, feldspar, and rock fragments

With increasing distance and time of transportation ….

• grain size decreases

• grain size sorting increases

• angularity decreases/roundness increases rapidly

• sphericity increases slowly

• there is loss of feldspar, mica and other clasts in favour of clay matrix and quartz

The more matrix a rock has, tells us what about its transport?

The more matrix a rock has, the lower the energy, and the shorter the transport distance.

• High energy & long transport → fine particles removed → well-sorted, little matrix.

• Low energy & short transport → fine particles retained → poorly sorted, lots of matrix.

How does the name of the sandstone tell you about its maturity?