I tweeted an image that got a lot of attention the other day and wanted to follow it up with a quick post describing the deposit. The back story is this: Lesli Wood, a submarine landslide expert, showed an image at a recent conference that is a spectacular example of a mass transport deposit (MTD), or more simply, a submarine landslide deposit. Landslides occur on land (example video), causing plenty of infrastructure damage and other problems. While they are difficult to view and visualize, landslides also occur on the seafloor, causing massive reorganization of the seafloor that can generate tsunamis. In fact, the Storegga slide that occurred offshore Norway about 6000 B.C. likely killed many Europeans.
Due to their volume and size, submarine landslides are usually characterized best with seismic reflection data – the map and cross section below from this paper by T.M. Alves. The map shows large blocks of rock that have been broken apart and transported downslope (from left to right) and the cross section shows what the internal character of those blocks are. Note the discordant nature of the blocks, very similar to the image above of the outcrop, with rocks in all directions.
Outcrops usually are too small or not well enough exposed to view these types of features, but the outcrops created by road making on the Boso Peninsula in Japan are definitely good enough. This paper by Yamamoto et al (download the paper here)describing these outcrops has a very nicely drawn diagram that demonstrates the deposit. There are probably two landslide deposits that are stacked here, with a turbidite separating them (grey layer in the middle of the diagram).
Here is another nice photo from a field symposium website showing a closeup of the deposit.
Callan Bentley over at Mountain Beltway just posted about Aden Crater, and I though i would share a few photos from Kilbourne Hole, a nearby maar volcanic crater. It’s only about 15 miles from my childhood home, and I went there quite a bit in high school, both for geology and just to get out into the desert (mainly to drink beer).
Geologically speaking, it is the crater that resulted from a magmato-phreatic explosion, or when, as Wikipedia says:
rising magma super-heats water-saturated earth, far enough below the surface that a high pressure can be contained. At some point, the pressure is too much, and a steam explosion occurs, throwing the earth out in a catastrophic event. Country rocks are fragmented and expelled in the atmosphere (together with fragments of the magma), eventually creating a deep crater, the bottom of which sits below the pre-eruptive ground surface.
For some nice images of other maars in action, go here. The main reason I like these features so much is that the ash cloud that the eruption produces (called a base surge) is a sediment gravity flow (just like a snow avalanche and a turbidity current). So, these are just really hot turbidity currents on land!
Looking on Google Earth at Kilbourne Hole and nearby Hunt Hole, you see one peculiar thing – there is a prominent ridge on the eastern side of the crater, but not the western side. This is thought to be due to westerly winds (i.e., winds out of the west blowing eastward) during the eruption that pushed most of the ash to the east. The wind in west Texas and southeastern New Mexico usually blows out of the west, and since this eruption only occurred ~25,000 years ago, I think that explanation makes good sense.
In the above photo, you can see the ridges on the eastern side of the crater, and I made a simple cross section showing the formation of the ridges and dunes from the explosion. Type in these coordinates in Google Earth to get you there – take a look at it yourself! 31°58’19.35″N, 106°57’45.23″W.
Looking north, you can see the right hand side (eastern) ridge is much higher than the western ridge. This is due to the wind blowing the exploded ash and bombs eastward. The crater is large – 2 x 3 km and 12 m deep – looks impressive from the ground:
One of the biggest attractions at Kilbourne Hole are the xenoliths full of beautiful green olivine – Callan has some nice photos here. However, we came to look at the base surge deposits on the rim of the crater, which formed when ash was falling after the explosion and piling up into big dunes and ridges on the eastern side of the crater. They are impressive, and quite thick:
In the above photo you can see many dune forms, which are all made of accretionary lapilli, which are basically sand-size ash clumps. If you want to learn more about these kinds of deposits, take a look at this thesis. A detailed photo of the lapilli is below.
Note that these deposits are normally graded (biggest grains on the bottom), as is common in many places where sediment gravity flows deposit sediment. Almost all turbidites are normally graded – see here and here for more info. And if you ever drive through the area, do yourself a favor and take a trip out to Kilbourne Hole (only takes 1 hour from downtown El Paso) – you’ll be glad you did.
Geologists are always taking multi-picture panoramas of outcrops and other geologically interesting phenomena, but then have to go back to the office and use photoediting software to stitch them together into a seamless image. The problem lies that the stitching is often imperfect due to photo overlap and the resultant images are hard to view on the computer because they are so large.
Gigapan (http://gigapan.com/) found a way to make this process better – you stick your camera into a small tripod mounted robot and tell it to take a large panorama (aka a gigapan), and then use Gigapan software to stitch it together and upload it onto the internet. The online viewer is pretty slick, and as you zoom in the resolution improves. In short, it is definitely the way to view large photographs interactively. You can even tag parts of a photo with a description of what is there.
Zoltan over at Hindered Settling introduced me to this whole Gigapan process (check out his Gigapan page too) , and we have taken many gigapans together. However, I wanted to try it on my own, so I took the robot out to the Guadalupe Mountains to test it out. My camera skills arent spectacular, so my first gigapan has a few vignetting issues, but it is still really cool. Since I was using a telephoto lens, this image is made up of 351 photos, resulting in almost a 3 gigapixel image!
This subject of the photo is a place called Slaughter Canyon, a prograding and aggrading Permian carbonate shelf margin. You can clearly see the progressively younger reef fronts moving from lower left to upper right. You can also see the very steep forereef slopes exposed just to the right of the cliffy, massive reef fronts – the one at far right is the best and longest slope, and gives an indication of the relief on this margin ( about 300 m). Here is a diagram showing the general morphology of that carbonate reef – if you could have walked around here in the Permian, this area may have looked somewhat similar to the modern coast near Oman, with dry desert on land and a carbonate reef in the shallow ocean.
The image below should link to the actual gigapan, but here is a link too. Be sure to push the full screen button and scroll around to see the full resolution. Enjoy!
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!
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):
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
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…
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…)
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.