Turbidites form in the deep ocean near the continental shelf.
Classic turbidite sequences are made up of fining upward beds of
sediment. This means that the grains at the bottom of the sequence
are larger than the grains at the top of the sequence. In a
complete sequence the bottom sediments are pebble sized
conglomerates, followed by coarse then fine-grained cross-bedded
sandstone, then shale, and finally silt. This series is called the
Bouma cycle.
A turbidite is formed by a density flow. In this type of flow,
the fine grained sediments suspended in the water increases the
density of the water. The denser water is then able to suspend
larger particles than it normally would be able to. For example a
rock will sink quickly in pure water, but sink much more slowly in
thick mud.
Typically, the entire sequence is not seen
because each successive flow will erode off some of the top of the
last sequence, or if the deposit is at the end of the flow, only
the finer layers will be seen.
Usually turbidites form off of a convergent plate margin (where
two plates are colliding). A convergent margin provides the
mountains that are good sources of sediment, sufficiently steep
off-shore slopes and periodic earthquakes to trigger the undersea
landslides.
That is the case in the sequence of rocks at this
location. Only the fine-grained sand, shale and silt layers are
seen in these cliffs, suggesting that they formed at the far end of
the turbidite. Hundreds and possibly thousands of density flows
occurred to create the tall cliffs of repeating sand, shale, and
silt layers you see in these cliffs.
Turbidites continue to form just off the coast. A pair of deep
marine canyons extend out into the Santa Monica Bay from Mugu
Lagoon and Port Hueneme. These canyons transport loads of sediment
down into the Santa Monica Basin.
Logging requirements:
Send me a note with :
- The text "GCZJNF Point Mugu Turbidites" on the first line
- The number of people in your group.
- In a 3-foot section, how many density flows occurred? Include
how you determined the start and end of each flow.
- Based on the thickness of each of the layers was each event the
same size? Include how you came to that conclusion.
The above information was compiled from the
following sources:
- Wikipedia,
http://en.wikipedia.org/wiki/Turbidite
- SUZANNE REYNOLDS (1987) A recent turbidity
current event, Hueneme Fan, California: reconstruction of flow
properties Sedimentology 34 (1), 129–137.
doi:10.1111/j.1365-3091.1987.tb00565.x
- Field trip guide to the Marin Headlands (Golden
Gate National Recreation Area) and the Point Reyes National
Seashore; Geology of the National Parks, San Francisco State
University, April 20, 2002
