I went for a walk yesterday (Thanksgiving holiday) with my brother-in-law and niece to get a bit of fresh air and, as we strolled past a small playground, we saw this:
Here’s a shot zoomed in a bit more:
The next image has my interpretation:
As you can see, the layer of snow has been deformed into a little fold belt at the base of the slide. The slide itself acted as an effective detachment surface (sometimes called a décollement). I suspect that as the temperature went above freezing and a bit of snow melted, a threshold was reached where water between the snow layer and the metal slide acted to ‘detach’ the snow from the slide. The relatively coherent slab of snow then deformed into the fold belt at the down-slope termination of the detachment surface (the slide).
This is a very loose analogy to gravity-driven deformation observed on sediment-rich continental margins. As kilometers of sediment piles up and loads the margin, shale or salt horizons act as effective detachment surfaces for large-scale ‘gravitational tectonics’ characterized by extensional faults in up-slope positions balanced by compressional fold-thrust belts in down-slope positions. The amount of throw on these normal and reverse faults can be significant (100s to 1,000s of meters).
Here’s just one of numerous schematic representations of this phenomenon.
After seeing my post the other day of a turbidity current caught on video, a good friend of mine sent me the link to this wonderful little film called Beach: A River of Sand, which was produced in 1965.
It’s 20 minutes long and well worth watching when you have a bit of idle time in your day. It focuses on the origin, transport, and fate of beach sand in southern California (from Santa Barbara to San Diego). It combines information from observations and scaled-down models to tell the story of how sand makes it way southward along the coast and, ultimately, into submarine canyons. There’s even footage of sand cascading down into the head of La Jolla submarine canyon!
Although this video is almost 50 years old I think it’s just as good, if not better, than documentary material produced today. Sure, we might have fancier graphics/animations today, but they tell the story in such a simple and clear way. I love it.
This video posted by MBARI (Monterey Bay Aquarium Research Institute) is quite amazing. This past August, a submarine ROV (remotely operated vehicle) was in the head of Mendocino submarine canyon (~400 m water depth), offshore northern California, when a sediment density flow (turbidity current) occurred. The ROV captured the event on video and made measurements of sediment concentration.
These observations indicate that this flow had a lower layer that was more concentrated (but still very dilute, <0.1% sediment by volume) and an upper, less concentrated layer. In total, the turbidity current is estimated to have been >100 meters tall. Approximately 1:25 into the video there’s not much to see because the ROV is caught in the more concentrated layer — experiencing a sediment ‘black out’. Check out the GRL paper by Sumner and Paull that accompanies this footage.
This is really exciting because basic observations of these important processes are exceedingly rare. To my knowledge, this is the best video footage of an event like this.
I recently submitted a review paper along with four co-authors on the topic of signal propagation in sedimentary systems across timescales. The idea that landscapes contain within them information about controls such as tectonics and climate has been a part of our science for a very long time. But, recent advances in the measurement/calculation of rates of processes (for example, with cosmogenic radionuclides) as well as theory and modeling related to how such ‘signals’ generate sediment and propagate across the Earth’s surface to be, potentially, encoded into stratigraphy motivated us to write a review. I’ll post more about the paper once it’s gone through the review-and-revise process, but wanted to write a brief post here on the topic.
Let’s start simple. Consider a sedimentary source-to-sink system with erosional uplands (sediment production) connected to depositional lowlands and/or marine basin (sediment accumulation). A tectonic or climatic change can change the rate of sediment production in the uplands that is potentially recorded down-system as a change in deposition. The morphology and length-scales of the system play a huge role in the behavior, which, in turn affects how (or if) that up-system signal is ‘preserved.’
As analogy, consider human-made debris basins. These structures, common in steep and tectonically active mountains such as the west coast of North America, are designed to mitigate debris-flow hazards on communities built on slopes that are prone to mass failure, especially during precipitation events. Debris basins are positioned on failure-prone slopes above concentrated population and/or infrastructure and designed to capture newly liberated sediment as it flows down slope, preventing that sediment from being transferred further down slope where potential damage and/or injury could occur.
Essentially, these basins are localized sinks that store sediment, thus preventing the signal (in this case, a rain storm) from propagating down system as a mass-wasting event. However, if the magnitude of the event exceeds the storage capacity of the sink, part of the signal will propagate down system anyway. For example, if the volume of liberated material exceeds the volume the debris basin can hold, the excess mass would bypass the basin after it fills to capacity. For debris basins to be effective they must be emptied following an event such that the storage capacity is returned to its maximum. So, in addition, time and the accumulation of multiple events plays a critical role in system behavior. For example, the sediment volume released from a single rain storm may only be enough to fill a debris basin to 10% its capacity. But, material from >10 storms of similar magnitude, if not removed, would effectively erase the signal-stopping action of the basin, which would allow future events (signals) to propagate down system.
What is exciting (and quite daunting) is applying these concepts to much larger length-scales and much longer timescales. Over longer and longer time periods the only evidence remaining of these mass-transfer dynamics is the stratigraphic record.
See this post from FOP about debris basins. And, if you haven’t already, read John McPhee’s “The Control of Nature,” which has a section about debris flows in the San Gabriel Mtns of southern California.
You’ve probably seen the fantastic How Science Works interactive diagram and website developed by the University of California Museum of Paleontology. If not, I encourage you to check it out. The main message is that the scientific method is not a simple, linear process. This is an important aspect for both budding scientists and the public to appreciate.
A new video on the YouTube channel for the Consortium for Ocean Leadership discusses the nonlinear, iterative, collective, and highly creative endeavor of doing science in the context of Earth science and ocean drilling. The video is only 10 minutes long and includes footage from IODP Expedition 342 that I sailed on in summer 2012. (If the video isn’t embedded below, here’s the YouTube link.)
Hey look at this, I’m posting on this blog again! The academic year has ended and I feel like I can take a breath. When people told me that being a tenure-track junior prof was going to keep me very busy they weren’t kidding, holey moley. But, it’s all exciting, fun, exhausting, challenging, and rewarding.
In an attempt to resuscitate this blog, I wanted to post about a few papers in sedimentary geoscience that I’m currently reading (or will read) this summer:
- From gullies to mountain belts: A review of sediment budgets at various scales — Matthias Hinderer, Sedimentary Geology, 2012 — This is a review paper taking a look at the importance of considering mass balance in Earth surface systems with an emphasis on methods for determining sediment budgets at geologic timescales. This is not about calculating sediment budgets in a hydrology/geomorphology sense — where flux can be monitored and measured in real time along different reaches of a stream, for example — this is about what we can (and, importantly, cannot) estimate or infer regarding paleo sediment budgets. For example, what tools are being used and what critical uncertainties exist for determining sediment budget in Quaternary (<2 million years old) systems? What about further back in time? The paper includes several case study examples that span a range of spatial and temporal scales.
- Scaling laws for aggradation, denudation, and progradation rates: The case for timescale invariance at sediment sources and sinks — Peter Sadler and Douglas Jerolmack, online-early contribution to upcoming GSL Special Publication “Strata and Time: Probing the Gaps in Our Understanding” — I predict this paper will be cited a lot by those of us interested in Earth surface dynamics, it’s a good one. I’ve written about Sadler’s contributions on this blog before (nearly seven years ago, time flies!), which deal with how unsteady depositional processes create a potentially misleading ‘artifact’ when comparing accumulation rates measured over different durations. Sadler & Jerolmack make the point that denudation (mass removal) rates rarely have this problem because methods like catchment-wide cosmogenics or thermochronology (for exhumation information) already sample the entire area of interest. Depositional (mass accumulation) rates, on the other hand, are typically measured in single locations, e.g., a borehole or outcrop section, which rarely are representative of the entire depositional area. This is perhaps an obvious point to make, but sometimes really important points are simple, and I think they articulate it very well. They then go on to make the case that cross-sectional area (a 2-D slice through a depositional volume) is potentially sufficient to approximate the true 3-D mass balance, which is difficult to impossible to constrain in ancient records.
- Mid-Cretaceous to Paleocene North American drainage reorganization from detrital zircons — Mike Blum and Mark Pecha, Geology, 2014 — I have not read this yet, I’m looking forward to diving in soon. This paper uses a database of >5,000 detrital zircon ages to reconstruct continent-scale drainage basin evolution of North American over 10s of millions of years of geologic history. The punchline is that during the Cretaceous much of North American drained north, toward the Arctic, and then in the Paleocene it switched towards the Gulf of Mexico (i.e., what has become the modern Mississippi drainage system).