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. NortonSo cross beds are tilted in the direction of flow (downstream)

Fig. 5.21d

Stephen Marshak

ripples

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 (made of calcite — CaCO₃)imply shallow (<3m), tropical marine depositional environment

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

800 meters of oolitic limestones

Monte Bove in Italy

2 mm

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

We need to think about isostasy and sea level change in our search for an explanation of how we can get such thick piles of sediment, all of which appear to be deposited in fairly 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 sediments?

Add a WeightIncrease DensityThin CrustRaise Sea Level

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