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Climbing-ripple successions in turbidite systems: depositional environments, sedimentation rates and accumulation times

March 28, 2012

Wow, that is a title right there! The third and final paper (PDF) of my thesis is finally published – I submitted this paper in 2010, and it was published online in the fall of 2011. Now however, it is finally out in print, and it feels real now. It was my most ‘academic’ project, but one that I enjoyed immensely, and I hope has an impact. It is an interesting extension of some research into the deposition rate and accumulation time of climbing ripples that was started in the late 1960s by JRL Allen, but was largely forgot about until the early 2000s, when Jaco Baas published a nice paper on the deposition rates of turbidity currents. I was also inspired by a colleage who wrote a blog post and a mathematical model about these deposits.

The project started when I was a young grad student. I had taken a sedimentology class and learned about climbing ripples and thought, ‘wow, that is really cool!’. Then Bill Morris from ConocoPhillips came to with a problem and an idea. He had a core from a reservoir in the Gulf of Mexico (GOM) that was puzzling. It was a turbidite core that was about 80 m long, and was almost exclusively climbing rippled. Bill had done a lot of work in the Tanqua Karoo of South Africa and had also noted that there was a lot of turbidite deposits there with climbing ripples too, and suggested that I look into any similarities between the two locales. So, I did, and also incorporated another field locale (New Zealand), and I hope you like the resulting paper.

The basic jist is this. If a sediment gravity flow is depositing sand as ripples, and there is also sediment falling out of suspension, turbulence is dampened near the bed, thus decreasing the erosion of one ripple as it migrates over the next ripple downstream. The result is that the ripple climbs over the back of the downstream ripple, giving the name ‘climbing ripples.’ As the rate of sedimentation increases, the angle of climb increases (ok, its not that simple, read the paper for more details). A diagram of a climbing ripple is shown below.

Now here is a photo of a real turbidite that is predominantly climbing rippled, from New Zealand. Note the thickness of this bed, about 90 cm, or about 3 feet.

So lets get to the point, you may think – quit rambling on about these things, and tell me what is important. So, here we are (drumroll). The bed above is NZ-S-2, which in the diagram below has a sedimentation rate of about 0.2 mm/s (thats about 1 cm per minute). Since this bed is about 90 cm thick, it takes 78 minutes to deposit this bed! See the diagram below for all of the beds I studied in the various places:

So, you can see that the vast majority of the measurements were from the New Zealand outcrops, mostly due to the wonderful exposure there (see these posts). Although it turns out to be difficult to quantitatively compare sedimentation rates and accumulation times between enironments with any statistical certainty, there do seem to be qualitative differences in the depositional environment (see the paper). Thanks for reading!

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