Ancient depositional environment of sedimentary rocks

1. Features of depositional
environments

2. Desert processes: sand dunes

3. Alluvial fans

4. Ephemeral lakes

5. River processes: channels,
floodplains

6. Braided streams are
characterized by
mid-channel bars..you
can think of a bar as a
variety of a current
ripple

7. What
channels
look like
in
outcrop
Downcutting
surface,
coarse
sediment

8. Meandering stream:
Single channel that
Is highly sinuous.
The bars in this
system are on the
inside of curves and
are called point bars

9. Channels are preserved, but
so are the bars: inclined
layers of sand that comprise
the point bar

10. The floodplain is also preserved: fine-grained muds that are
deposited from flooding, along with laterally discontinuous
layers of sand that form from breaches of the channel
banks (termed levees) that send sand out onto the floodplain

11. Glacial processes: till,moraines

12. Lateral moraine: the till
pushed to the sides by the
glacier; terminal moraine=
the material bulldozed
to the front
Layered muds in glacial lakes

13. Other evidence of the passage
of a glacier: the striated
surface of bedrock

14. DELTAS
It may be in a mud puddle, but this little delta has all the features
of a “real” delta.

15. What is different about these deltas are the processes that are
dominant on the delta top and front.
Waves rework delta front
Tides
rework
delta
top

16. Delta processes: delta
top channels; delta
front, pro-delta

17. The sediment plume that forms
where the river channel dumps
its load. This is why the delta
can build up and out: sediment
is always being added on the
delta front.

18. BEACHES AND BARRIER ISLANDS
A beach is a complex set of environments from the front (the
foreshore) to back (lagoon)

19. Beach processes: wave activity and wind
The backshore = dune
field
The shoreface

20. Wave-generated ripples
are symmetrical in
shape
The inclined surface of
the shoreface leaves
behind inclined layers
of sand

21. The backshore dune field
Beaches are often
separated from land by
a lagoon or bay
behind them. This is a
very low energy setting

22. TIDAL FLATS - form where there is
a large tidal ranges (>3m) and the
coastline is protected bays where
wave action is minimal.
The intertidal zone: tidal currents
produce ripples
Mudcracks form
from exposure
during low tide
Fluctuating tidal currents
deposit alternating layers of sand
and mud

23. Ocean processes
There’s vertical exaggeration
here, but offshore from the
shoreline is the almost-flat
continental shelf, and further
offshore the rise and slope
Where major rivers enter
the oceans the shelf is
cut by submarine canyons
that represent the old river
channels formed when sea
level was lower in the Pleistocene

24. The ocean floor is the
site of accumulation of
fine-grained sediment;
both clay and
carbonate mud
The deep ocean floor is
NOT devoid of life!

25. Ocean floor sediments are
relatively rare in the stratigraphic
record because ocean floor is
destroyed in subduction zones.
We DO find a record of deep-
water environments that form
in basins: turbidites.
A turbidity current is
like an underwater
avalance: sand
suspended in water
cascades down a slope
and comes to rest in
deep water muds. The
turbidite is the resulting
deposit.

26. Ocean processes; carbonate
platform
If you have warm
marine water and little
input of clastic
material, you can
form limestone
The lagoon with reef in
the distance.
Open ocean to the left,
the reef top, or crest.

27. A reef represents the optimal
condition for maximum
organic diversity and numbers
Fossil reef exposures
are common in the
geologic record. We
can recognize the
reef and lagoon
environments.