Origin and interpretation of sedimentary rocks

Today: Origin and Interpretation of Sedimentary RocksWed: Origin and Interpretation of Metamorphic Rocks... Sedimentary Rocks — Why Care?• They are the repository of Earth’s history, esp

Today: Origin and Interpretation of Sedimentary Rocks

Wed: Origin and

Interpretation of

Metamorphic Rocks

Sedimentary Rocks — Why Care?

• They are the repository of Earth’s history, especially the history of how the surface environment (including life)

changed over time To extract this history, we need to be able to “read” this rock record

• They hold all of the oil, gas,coal, and water — locating these materials requires an understanding of the

physical characteristics of different sedimentary rocks and how those characteristics change in 3-D

Coconino Sandstone

Coconino Sandstone up close

thin slice of Coconino sandstone seen

through microscope

un-cemented quartz

sand grains from a

sand dune

So how are sedimentary rocks made?

Physical & Chemical Weathering → Sediment & Dissolved Ions

hincks146-147

Sedimentary Structures

in the Coconino

This is the modern environment

where large cross-beds form

Wind Direction

Fig 5.21abc

W W Norton

So cross beds are tilted in the direction of

flow (downstream)

Fig 5.21d

Stephen Marshak

Stephen Marshak

Uniformitarianism — depositional environments of

today are physically the same as ancient environments,

so clues from the modern can be used to infer ancient environments from sedimentary rocks, or “the present is the key to the past”

Ancient ripples

Stephen Marshak

Mud cracks

Stephen Marhsak

Ancient mudcracks

Stephen Marhsak

Fig 5.24c

© Martin Miller

Modern Alluvial Fan, Death Valley

Ancient Alluvial Fan Deposits

Ooids with cement, seen in a thin section, through a microscope

So, the question is — how can we explain such a thick pile of shallow-water

Isostasy

Isostasy

Equilibrium level for wooden block

time 1 time 4

time 1 time 4

So, the question remains — how to get such a thick pile of shallow-water

As sea level goes down, previously deposited sediment will be eroded

In our case, crustal thinning makes sense because the sediments in question were deposited during and after continental rifting

• Make sediment

• Transport & deposit sediment

• Create space to allow sediment to accumulate

• Compact and cement sediment

Depositional environments are interpreted

based on studies of modern environments

where sediment is deposited

Summary on Making and Interpreting Sedimentary Rocks

Metamorphic rocks