Friday, July 13, 2012

Turbidites! Hey! Watch your mouth!

Turbidite Field trip with Arnold Bouma, the Turbidite Man

            Several years ago I found myself on a field trip in southern Missouri, northern Arkansas, and eastern Oklahoma with none other than Arnold Bouma the turbidite man.  The Bouma sequence is named after him.  Here are a few photos of turbidites that can be viewed on road-cuts, quarries, river bluffs, and dams.  You can't see the rocks anywhere else in the Midwest!  Too many dang-blamed (beautiful) trees!

Turbidites and Turbidity Currents:

Turbidity currents are a type of hyperpycnal density current that contain high amounts of suspended sediment.  The density difference between turbidity currents (higher density) and the surrounding water allows the current to move sediments great distances over short periods of time.  The gradient does not necessarily need to be very steep.  The resulting deposit is termed a turbidite.  Coarser particles settle out of the current before finer particles, which results in a normally graded (coarse at the base, finer grained at the top) deposit.  The Bouma “sequence” (1962) shows this deposit as representing five phases of settling, labeled a-e.  The “a” interval is a unit with commonly occurring scour marks and flute casts on the base.  The flow is extremely turbulent in this phase.  Coarser particles tend to settle out before finer particles.  This results in a fining upward of grain size.  Interval “b” consists of parallel lamination from the upper flow regime.  Interval “c” contains new bedforms commonly showing ripple lamination.  Interval “d” displays a second phase of parallel lamination from the lower flow regime.  Finally, interval “e” consists of the finer particles that settle out only after the current has passed.  Some layers (commonly d and e) may be missing due to erosion by subsequent turbidity currents.

As turbidity currents pass a given point, turbulence and mixing involved in the head of the flow is responsible for a large amount of scour.  Coarser particles settle out and are deposited as the head of the current passes the point.  Then finer particles settle out followed by clay size particles.  (Bouma, 2007)

A complete and ideal Bouma sequence consists of intervals labeled a through e, coarse to fine respectively.  (Bouma, 2007)

An Arkansas road cut.  Distal deposits, very thin.

Quarry wall.  Soil movement has folded the tops of these turbidites over.
Delta lobes, splays, and channels.

Load features at the base of a turbidite.  These form as a new turbidity current deposits sand on top of wet mud. The sand sinks in the mud as the mud oozes upward.         

Quarry wall.

Notice the "rythmic" turbidites, sand-mud-sand-mud-etc.

Road cut.  Vertical turbidites.  Here you can see that the actual thickness of one Bouma Sequence can vary quite substantially.

And just because Arkansas is beautiful...

Dr. Loch taking notes...I think he was the only one.  Shame on the rest of us!

Me an Arnold posing in front of some folded turbidites.

The crew.  I don't remember everyone but I am far right.  Two over is Mitch then Dr. Bouma.  DIIIAAAAANNNNNAAAAAAAAA!!!!!!! is trying to hide and the white hard hat is held by Dr. Zellers.  Kevin is in the very back.  

               Deep Sea Canyon Fill. Deep sea canyons are a typical path for a turbidity flow. Shock caused by tectonic movement often starts the flow. Flows usually travel through a canyon very quickly because of the steep gradient. Intra-canyon deposits typically spread laterally from wall-to-wall. The characteristic lobes that are so common in submarine fans are not present in the canyon itself. Due to the frequency and intensity of the flows inside the canyon, the finer layers of sediment are commonly stripped away by subsequent turbidity currents. This leaves only a small amount of mud preserved in the rock record.

A representation of the Campos Basin.  Turbidite flows exit the canyons as a deep sea fan.
(Guardado et al 1990)

               Deep Sea Fans. Submarine fans spread out from the mouths of the canyons that feed them. Turbidity flows continue forward and begin to spread laterally because the restrictive canyon walls are no longer present. The current will follow the present channel until it is filled up with sediment. As one channel fills with turbidites, the current follows the path of least resistance and abandons the old channel to begin a period of deposition in a new channel. This is called channel switching (Walker, 1984). Channel switching results in the formation of several finger-like extensions called lobes.

               ***Geologists care about this type of system because much of the world's oil and gas reserves are contained in the sandstones of a turbidite succession.***

P.S.  Why are there Turbidites exposed in southern MO, northern AR, and eastern OK? Well, the Ozark-Ouachita Highlands were, at one time, deep in the ocean between the N. American Plate and the S. American Plate.  As these plates collided, the rocks in my pictures were folded and uplifted to become a giant mountain chain.  Erosion has since reduced their size drastically.  Know you know.  See here.

Bouma, A. H., 1962. Sedimentology of some Flysch deposits; A graphic approach to facies          interpretation. Amsterdam: Elsevier Pub.
Bouma, A. H., 2007. Active and Passive Margins Have Different Types of Depositional   Locations. Texas A&M University, Department of Geology and Geophysics. p. 6-7.
Guardado, L. R.,Gamboa, L.A.P., and Lucchesi, C.F., 1990, Petroleum Geology of the Campos   Basin, Brazil, a model for producing Atlantic type Basin. AAPG memoir 48, p. 3-79.
Walker, R. G., 1984. Turbidites and Associated Coarse Clastic Deposits. Geoscience Canada,      Reprint Series 1, Facies Models, Second Edition. p.178-181.

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