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Accretionary Wedge #58: Signs!

July 23, 2013

After a long hiatus due to general craziness at work and at home, I am starting up the blog again with this call for a cool geology sign.  The Ross Sandstone is a upper Carboniferous (Pennsylvanian) formation along the coast in western Ireland that is famous for its excellent turbidite channel and lobe exposures.  See this page for more details.  If you haven’t got the chance to go see those rocks and the beautiful countryside of the Emerald Isle, I highly suggest a trip.

In fact I will be there next week teaching a field trip, so maybe I will do a little day-by-day blogging about the rocks there.  Stay tuned!

ross

The Bouma sequence and turbidite deposits

November 7, 2012

For a while now, the most popular page on my site has been this one, a photo of a Halloween pumpkin I carved to look like the Bouma sequence.  It is the most popular because people are looking for information about the Bouma sequence, so it is time to do a real post on the Bouma sequence, with more detail about turbidite deposits and the turbidity currents that produce them.

Turbidity currents are a type of sediment gravity flow where turbulence is the dominant mechanism for grain support.  A turbidity current that is more familiar to most people is a snow avalanche.  A turbidity current is structured like the image below, with a head, body, and tail.  In (A), grains are represented by the black dots – note that the coarser grains are located near the bed and towards the front of the flow.  In (B) is a turbidity current produced in a laboratory experiment that shows the downslope evolution of the flow.

Turbidity currents in the world’s oceans produce spectacular seafloor architectures like canyons, channels, and lobes/fans, depending on the amount of erosion or deposition taking place at a particular location. The sedimentary architecture is influenced by many factors, including grain size and distribution, slope gradient, sediment supply, etc etc etc.

Turbidites are the products of deposition from a turbidity current.   The simplest case is a current that is slowing down (waning) and entirely depositional (e.g. on a lobe).  , a turbidity current produces the classical turbidite, which was famously described by Arnold Bouma in 1962 and interpreted by Roger Walker in 1965.  The Bouma sequence, as it has become known, is the idealized sequence of sedimentary structures that represents the waning of a turbidity current as it passes over a single point.  The five Bouma divisions are (in stratigraphic order):

top

Te – pelagic mud

Td – planar laminated mud produced from suspension settling

Tc – ripple or climbing ripple cross lamination

Tb – high velocity planar lamination

Ta – structureless (aka massive) division

base

An important concept reflected by these structures is that the energy (bed shear stress) is decreasing upwards as the current passes by, and this is also manifested in the normal grading of the bed – coarser at the base, finer at the top.  This photo from the Mt Messenger Formation in New Zealand says a thousand words (this was published in a cool paper in Nature Geoscience):

Note the nice normal grading in the deposit (coarser stuff is slightly tannish in the Ta-Tb, then going to grey in the Tc, and finally to mud in the Td-Te).  The squiggly yellow line in the photo is caused by the denser sand loading into the soft mud at the start of deposition.  Notice that this loading repeats in the bed above, suggesting fairly high sedimentation rates.

The Bouma sequence can also be expressed in a cartoon fashion:

Many variations on the Bouna sequence are possible: it is common to lose the Ta in distal environments where there is not enough energy in the current, and in proximal settings, amalgamated Ta beds are common, where the rest of the sequence was either never deposited or eroded away.

The next post will focus on the processes of deposition of the Ta division and how the Bouma sequence relates to the ‘Lowe’ sequence, which is typically used to describe much coarser grained turbidites…

Geology on the Wing Wednesdays #11 – Mississippi River meander bend at New Madrid, MO

November 7, 2012

The blog has been quiet for a while, but here is one I had to share.  This photo was taken about 35,000 feet above the Mississippi river near New Madrid, MO.  New Madrid is famous for earthquakes in the early 1800s that altered the course of the river (see this ppt for an overview).

The reason I took this photo was not about the earthquakes, but about the large meander bend that is nearly at cutoff.  Flow is from lower left to upper right, and this bend is only 1 river width away from becoming an oxbow lake.  For a nice time lapse view of how this happens, click here.  Given current channel migration rates (~50 m per year for undisturbed portions), this cutoff will occur within the next few years (unless the Amry Corps of Engineers chooses to fight the river and reinforce the banks).  I suspect they have already done so (an intrepid reader could check the Google Earth time slider bar…)

Video of a turbidity current!!

July 21, 2012

This is the coolest thing I have seen in a long time – thanks to Dave Petley for the link.  This event occurred in Cabo San Lucas, on the southern tip of the Baja California peninsula in Mexico.  I imagine that these divers were reef-diving (as evidenced by the angelfish in the first few seconds of the video).  This turbidity current starts out as a small flow on a very steep slope (more than 30 degrees) and is quickly overtaken by the main part of the flow.  The real action starts then, as this current is moving very fast (more on speeds below), and it much thicker and turbulent.  This flow quickly overtakes the scuba divers and keeps flowing downslope.  You can see in the video that the flow keeps thickening with time, entraining the surrounding seawater (here is a paper about how hard it is to model entrainment).

I ballpark this flow to be moving about 5 m/s, or about 10 mph (from my rough estimates of distance in the video), which is similar to turbidity currents measured in nature.  Jingping Xu measured currents with instruments in Monterey canyon at 2.8 m/s, and numerous papers measuring submarine cable breaks estimate speeds from 5 to 25 m/s.  It is important to note that velocity and speed are two different things, and that shear stress is more important than either when discussing sediment movement (i.e. erosion and deposition) associated with turbidity currents.  More on that in another post.  For now, just enjoy the turbidity current – click this image to see the video – I cant figure out how to embed the video…

Here is the link in case you need it – http://www.liveleak.com/view?i=c45_1342620679.  This is awesome.

A photo tribute to Boris Avdeev

May 5, 2012

Boris Avdeev, a brilliant young geologist, and a friend, has passed away.  He went up into the High Sierra on April 19th, and was found by a recovery team on May 3.  He was 31 years old, and a recent PhD graduate of the University of Michigan, specializing in low temperature thermochronology and its application to tectonic evolution (see his recent paper).  He was due to start a post-doc at UC Berkeley soon, and will be sorely missed by all who knew him.

I first met Boris in 2003, when he came to the US to pursue a masters degree at the University of Texas at Arlington (UTA).  I was an undergraduate there, and I recall thinking, “damn, this guy is really smart!”  Although he spoke little english at the time, my buddy Walt and I prided ourselves in teaching him english.  His accent was very heavy then, and I remember a time when he wasplanning a hiking trip, and he was practicing with me how to say “Howdy” and “Good morning” and “Afternoon!” so that people wouldn’t think he had an accent when he passed them on the trail!

Throughout the next few years, Boris and I spent quite a bit of time together in classes, on research projects, and playing in the outdoors.  We spent a memorable summer mapping abandoned river meanders along the Missouri River, which resulted in 4 published quadrangle maps (here is Boris’ map).  We both moved on from UTA in 2005, Boris going to Michigan and me to California.  We kept up, though, and saw each other when we could – the last time was at AGU in 2011, where we reminisced about our days in Arlington.

I will tell one funny story about Boris, and then Ill let the photos do the rest of the talking.  In 2003, we went to a bar with Boris and he wasnt yet acquainted to American bar traditions.  We ordered beers, and in typical Boris fashion, he said, “I would like a glass of vodka.” The waitress came back with a shot glass of vodka and he scoffed, and pointed at a cocktail glass, and repeated his order.  She poured the shot into a glass, thinking he wanted it ‘neat.’  But Boris had other plans and proceeded to tell the waitress that he wanted a full glass of vodka.  She complied, and he slugged it down, and said, “One more!”  It brought the house down, to say the least!

So, here is a photo tribute of sorts, based on a few photos that I have of Boris from his early days in the US (2003-2006).  Unfortunately, I don’t have a lot of photos from that time, due to the fact that I though digital cameras were a fad, but here are a few:

Boris, Walt, Lance, and myself went on a 3 day hiking trip in Big Bend in the winter of 2004, and then Boris came home for Christmas with me – we had a blast that winter hiking, and it is one of my favorite memories of Boris.

Before our big hike – Boris (third from left) looking stoic as ever.

During the hike – Boris again looking unconcerned.  Just after this photo was taken, Boris was making fun of the instructions on an MRE that we packed for food, and got a huge kick out of the fact that the instructions said to prop the heater packet on a “rock or something.”  The photo in this link shows the instructions, and maybe you had to be there to get the joke…

Now he is excited about something!

Boris with friends at Christmas, 2004.

The next group of photos shows one of several canoe trips that we took during our time at UTA, and Boris and I shared a canoe on this particular trip.  He was, as always, full of energy and a blast to be around.

Boris and Tatiana enjoying the canoe trip and passing the bag-o-cheap-wine that we brought…

Boris ‘wielding’ a canoe paddle to get the wine back.

This last photo is from 2012 from his Google+ page.  Boris was a great scientist, a mentor to Walt and I, and a great friend.  His enthusiasm and energy was unparalleled, and this photo typifies him – doing what he loved in the outdoors with his friends.

{UPDATE, May 8, 2012} – please also visit this website www.borisavdeev.ru, or the english version www.borisavdeev.com to see more photos, or to submit your own.

 

{UPDATE, January 11, 2013} – Jim Lewis from Richardson, TX has sent me his tribute to Boris – download the word document here.  Also, Jim alerted me to this link about Boris’ experience with the law while at UT Arlington – I had forgot about that, but it made me smile – that was Boris!

A geological pilgrimage to the Cave of Crystals, Chihuahua, Mexico

April 30, 2012

This month’s Accretionary Wedge, hosted by  life as a geologist, asks bloggers to discuss a geologically cool place that they would love to go or have been to, a sort of  ‘geological pilgrimage.’  This place should be:

“geologically unique,  relatively remote, and requires some difficulty to get to. If you have already done your geological pilgrimage, please share with us your experience. If you are still planning your pilgrimage, then let us know where your sacred geological spot is and why.”

Almost exactly three years ago (in April 2009), I had the rare opportunity to go visit the ‘Cueva de los Cristales‘ in a remote part of the state of Chihuahua, Mexico.  It was a once in a lifetime opportunity and one that I will always remember, for the fascinating geology, the trip to get there, and the temperature inside the cave, which was an experience in itself.  Let me preface this whole story by saying that I liberally use facts from the fascinating paper published in Geology on this area by Garcia-Ruiz et al in 2007.

The cave was discovered in the Naica Mine, which produces lead, silver, and zinc from hydrothermal veins in a Cretaceous limestone sequence.  The hydothermal activity is associated with Tertiary volcanic activity, and there is still an active magma chambers about 5 km away from the mine.  Hence, the mine, and cave, is very warm (more on that later).  The cave was discovered in 2000 during mine exploration, and had to be drained to be accessed.  Once the miners went inside, they realized that this was something special – many caves with large crystals had been found due to the unique geological conditions in the mine, but nothing even close to this.  Single crystals of selenite (a colorless, crystalline form of gypsum) up to 1.5 m in diameter and ~15 m in length littered the cave, growing from floor to ceiling, leading the miners to call them vigas, or ‘beams’.  National Geographic came in, and using advanced photography, captured some great images, including the one below:

The article on Nat Geo is worth a read, as it discusses the crystals, and the hardships of the explorers.  Needless to say, we didn’t take as high quality photos as them, but we enjoyed it nonetheless.  Now before you say you want to go too, the mine is private and does not offer tours of the mine or the cave.  A friend of one of the people in our group had some connections, and we were just lucky to get to go.

Of course we jumped at the chance, and on only 3 days notice, 6 of us flew into Chihuahua city and met the rest of our group.  We then took a few suburbans on a 2 hour drive through farmland and orchards and then up into the Naica mountains where the mine is.  Upon arriving, we had an orientation and were issued safety gear.  We then jumped into a big van and headed down the main mineshaft.  The cave is located about 300 m vertically under the ground, but it is a 20 minute drive down due to the angle of the mineshaft.  It got hotter as we went down, and was about 95 degrees F and humid in the mineshaft outside the cave.  Our guide then opened a steel door, and we walked into the anteroom of the cave. It was even hotter in here, maybe 110 degrees F.  We walked upslope to the cave entrance, which is only about 2 m across, and is sealed by a glass door.

When the door was opened and I stepped into the cave, it was like hitting a brick wall.  It was so hot and humid that you immediately start sweating, and it is hard to breathe.  I didn’t have a thermometer, but our guide said the temperature was about 130 degrees F and 99% humidity, and Nat Geo confirms that.  However, I soon forgot about the heat and tried to pick my jaw up off the floor when I saw the crystals.  They are so large and stunningly translucent that it seems impossible that they are real, and natural.  So here it is, proof that I went to the Cueva de los Cristales in Naica, Mexico:

That crap-eating grin is me enjoying the crystals, before I started sweating.  Here is what another of our group (who just happens to be my dad) looks like after being in the cave for a few minutes:

Sweating profusely, but in wonderment of the crystals.  We were only allowed to stay in the cave for about 15 minutes, then we had to take a 30 minute break and drink water before going in for another round.  After two rounds in the cave walking around and looking at the crystals, this is what our group looked like:

I am in the lower right corner, drenched in sweat, but still have that same grin on my face.  It was an unreal experience, and one that I have a hard time relating on paper. It was simply one of the coolest things I have ever done, or ever will do.

When I got home I found the Geology paper mentioned above, and learned the rarity of the formation.  According to Garcia-Ruiz et al (2007), it seems that these crystals grew in briny waters of ~ 54 degrees C.  These geochemical conditions had to remain constant for a very long period of time in order to grow such large crystals without nucleating lots of small crystals – no temperature fluctuations, no large earthquakes, no changes in salinity.  They estimate that the growth of the crystals took 10^6 years – yes, that’s millions of years!

So this truly is probably the only place on earth where such a geological wonderland exists.  I encourage you to read the Nat Geo article and watch the associated videos, and also to read the Geology paper – it is fascinating, and goes into much more detail about the local geology and the formation of the crystals.

That was my geological pilgrimmage – a once in a lifetime visit to a Cave of Crystals that only a few hundred people have ever been to, and when it is flooded after mining operations cease, I hope the crystals keep on growing…

Submarine slide captured off Gabon in 2005

April 19, 2012

“Submarine slide initiation and evolution offshore Pointe Odden, Gabon — Analysis from annual bathymetric data 2004–2009”.

Check out this paper from the coast of Gabon, where the authors use repeat multibeam surveys to capture a large-scale slope failure very near the coast.  They estimate that more than 2 million cubic meters of sediment were evacuated during 2005 (probably in an event that lasted less than a few days)!  The authors also estimate that due to the dynamic nature of the seafloor in the area, that all evidence of this submarine slide scar will disappear from the seafloor in about 20 years. Spectacular!