Even more sedimentary rocks

Today’s Lecture:
Chapter 7: Sedimentary Rocks

Sedimentary structures:
• Inferring depositional
processes from
sedimentary rocks
•Sea-level changes
& the facies concept


Sedimentary structures:
Features observed within a single bed.
Within sedimentary
beds, distinctive
structures can
usually be seen.
These include
systematic
variations in grain
size and sorting,
internal bedding
features, etc. that
are diagnostic of
particular
depositional
processes.


Sedimentary Structures

Graded beds:
Show a gradual
change in
particle size as
you move from
the bottom of a
bed to the top.

Bed 2

Bed 1


Fig. 7.26a
Stephen Marshak


Sedimentary Structures

Graded beds:
Show a gradual
change in
particle size as
you move from
the bottom of a
bed to the top.

Bed 2

Bed 1

Bottoms of beds: Coarser


Sedimentary Structures
Tops of beds: Finer

Graded beds:
Show a gradual
change in
particle size as
you move from
the bottom of a
bed to the top.

Bed 2

Bed 1


Graded Bedding

Finer

The bed to the
right shows a
change from
large grains at
the bottom, to
small at the

top. This is
called “normal”
grading.
Coarser


Graded Bedding

Lower Velocity

As transport
velocity declines,
coarser particles
settle out first
(see video
on turbidity
currents).

Higher Velocity


Graded Bedding

Lower Velocity

Thus, graded
beds tell us
how flow
velocity
changed

during
deposition!

Higher Velocity


Sedimentary structures: Cross-Bedding
Cross-bedding is
internal bedding
that is tilted at an
angle to the
primary bedding.
Cross beds are
formed by a scour
and fill transport
process involving
either wind or
water (see
ripple movie).


First we need to
distinguish between
primary bedding vs.
internal layering.


Primary bedding
vs. internal layering

Contacts between
sedimentary beds

Primary
Bed


Primary bedding
vs. internal layering

Internal,
inclined layers
Contacts between
sedimentary beds


Primary bedding
vs. internal layering

Internal
inclined layers
Contacts between
sedimentary beds


More cross-bedding

Bed contacts”

Fig. 7.25abc
W. W. Norton


More cross-bedding

Cross beds
Bed contacts”


Cross-bedding

Transport direction

Tilt-direction of cross beds indicates the
direction of transport (e.g., wind direction or
direction of water flow).


Large-scale cross-beds like these


What a geologist sees.

Which way did the wind blow?


What a geologist sees.

Paleowinds

Sedimentary Structures: Ripple Marks
Ripple marks form when moving wind or water
causes sedimentary grains to “hop” along the bottom.


Fig. 7.27a
Stephen Marshak
Ripple marks can be either symmetrical (formed by waves
sloshing back and forth), or symmetrical (formed by water
or wind flowing in one direction).