Today origin and interpretation of sedimentary rocks

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


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”

ripples
Stephen Marshak


Ancient ripples
Stephen Marshak


Mud cracks
Stephen Marhsak


Ancient mudcracks
Stephen Marhsak

Modern Alluvial Fan,
Death Valley

Fig. 5.24c
© Martin Miller


Ancient Alluvial Fan
Deposits



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


Monte Bove in Italy
800 meters
of oolitic
limestones

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