Clastic Detritus

This month’s installment of the geoscience blog carnival, The Accretionary Wedge, which you can find here on the Geology Happens blog, asked potential participants these questions:

This AW is to share your latest discovery with all of us. Please let us in on your thoughts about your current work. What you are finding, what you are looking for. Any problems? Anything working out well?

Because I work in the private sector I do not (and will never) blog about the work I do on a daily basis in any detail. In fact, Clastic Detritus is designed to be completely disconnected just so there is no confusion. Read the disclaimer page for more.

What I do blog about, however, is my published research — this is the stuff that is out there for the scientific community to read and evaluate so I feel that this blog can be an additional venue for sharing it. And, although this work was submitted many months ago and maybe not the very latest, it is a good representation of my current research interests and activities.

As my research interests page states, in the most general sense I’m interested in utilizing the sedimentary record to investigate and understand past and present Earth conditions. The sedimentary record is a representation of Earth surface processes through time and, as such, can be analyzed to reconstruct ancient environments as they relate to tectonic activity, climatic and sea-level fluctuations, oceanic conditions, and intrinsic dynamics of depositional systems.

I specialize in the study of submarine channel and fan systems, which are some of the largest accumulations of detrital material on Earth (e.g., Bengal submarine fan). Sand, silt, and mud that is eroded from the continent and the continental margin is deposited out into the deep sea by submarine sediment gravity flows called turbidity currents. If you’ve been reading this blog for a while you are well aware of my love of all things turbidite related!

I’m mostly interested in using these deep-sea sedimentary records to answer questions about the volumes, rates, and distribution of detritus that is transferred across continental margins. Essentially, I want to use the patterns we observe and measure in the stratigraphic record and relate it to fundamental processes such as plate tectonics or paleo-environmental conditions. A very interesting debate in the stratigraphic community is how to (or even if we can) unravel the extrinsic vs. intrinsic controls on the observed patterns — more on that another time. To try and address these questions my colleagues and I are analyzing both ancient and modern systems.

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Ancient Deep-Sea Sedimentary Systems

In some cases the history of sedimentary basin development is followed by mountain-building processes, which results in the preservation of sediments deposited 10s to 100s of millions of years ago in outcrops now exposed on the Earth’s surface. The value of investigating outcrops is that the details of the system at the scale of individual beds (a few centimeters) to packaging of those beds (i.e., stratigraphy) at many kilometers is on full display. One such system that I spent several years working on (and am still working on) is the Magallanes sedimentary basin in southern Chile. These rocks were deposited in the Late Cretaceous (~70-80 million years ago) in a deep foreland basin that developed adjacent to the paleo-Andes.

Because I’m interested in understanding these systems at multiple scales — that is, what controls the character of the beds? what controls how multiple beds stack? what controls the distribution of detritus at the basin scale? — I looked at these strata at different scales. To get at some of the sedimentological details and stacking patterns at the outcrop scale I investigated a particularly well-exposed outcrop at a mountain called Cerro Divisadero. I’ve already blogged about this study, which was published in Sedimentology, so I won’t repeat it here.

So, that study looked at patterns from the centimeter- to kilometer-scale, but what about much larger patterns — for example, at the scale of the entire basin (10s to 100s of kilometers)? What’s cool about that question is that, in addition to integrating the mapping and characterization of the entire region, we looked at the microscopic level for answers. I published a paper in Basin Research (currently in the ‘early view’ here) that summarizes the results of analyzing the ages of zircon grains extracted from these sedimentary rocks. I’m still working on a blog post that discusses this work in more detail, but to make a long story short, the detrital zircon record help constrain the timing of significant tectonic activity in the adjacent mountain belt. This tectonic activity (in this case, the emplacement of thrust sheets) had a profound effect on the basin-scale stratigraphic patterns.

But not all ancient sedimentary basins are now uplifted into mountains — in fact, a lot are technically still sedimentary basins. That is, the ancient sediments are buried very deeply (sometimes up to several kilometers!) by younger deposits. Investigating the subsurface utilizes an entirely different suite of tools (e.g., seismic-reflection data), but the fundamental questions are the same.

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Modern Deep-Sea Sedimentary Systems

When discussing sedimentary systems from a stratigraphic point of view, ‘modern’ refers to those that are still actively receiving and distributing sediment and continuing to evolve. This commonly equates to systems active during the past glacial and current interglacial climate cycles — or, the past 10,000 to 30,000 years. In other words, even if a turbidity current hasn’t occurred for a several hundred years we will still refer to the system as ‘modern’ because the depositional patterns from geologically recent activity is still in place.

Because one of my goals is to understand how sedimentary systems respond to the myriad interacting factors that control them, investigating modern systems offers the chance to establish relationships that are impossible with ancient systems. For example, although geochronometric approaches continue to improve, I highly doubt it that we’ll be able to constrain the timing of individual depositional events at the scale of few hundred years for strata that are 100 million years old. Let’s put it another way — I challenge you geochronologists out there to accomplish that! :)

Context is also critical. Ancient sedimentary systems are only partially preserved — we have to make inferences and interpretations about the nature of source areas that have long since eroded. By comparison, modern sediment dispersal systems are laid out in all their glory for us to study. For example, when characterizing ancient deep-sea sediments we commonly make an interpretation that the deposits represent a submarine channel. Depending on data the degree of confidence of such an interpretation can be high or low. Well, with modern systems, we don’t have to interpret some of those fundamental aspects — we simply know if we are looking at a channel (as long as the bathymetric mapping is good enough, of course). Given this context we can then ask some more specific questions about the system.

I published a paper last fall in GSA Bulletin about the depositional history of a modern submarine fan system offshore southern California. We integrated high-resolution mapping of the fan with a sediment core from which we obtained radiocarbon dates. We were able to constrain the timing of sedimentation for the past 7,000 years very well and saw a nice relationship to independent records of paleoclimate in southern California. This is another study that I have a blog post in draft stage that I hope to finish soon. Stay tuned.

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Integration: Characterizing Sedimentary Systems from ‘Source to Sink’

In recent years researchers have been emphasizing ways to integrate observations and measurements from different segments of sedimentary systems — from erosion-dominated mountainous areas to deposition in rivers and floodplains and, ultimately, to sediment ‘sinks’ in coastal deltas or deep-sea basins. I wrote a post about this approach a couple years ago after reading a very nice essay by Allen in Nature, which I highly recommend.

Read that post and Allen’s essay for more, but what I’ll comment on here is some work that I am doing right now that fits nicely within the source-to-sink framework. One way to investigate how sediment source areas relate to the sediment sink areas is to look at rates of erosion and rates of deposition. Some relatively new methods of dating cosmogenic nuclides of exposed bedrock surfaces and/or sediments is helping geomorphologists constrain erosion rates. How this exactly works is something I am still learning myself (check out this article that explains it better than I could).

Some colleagues of mine did some preliminary work using cosmogenic-nuclide-derived erosion rates and compared them to deposition rates in offshore segments of the dispersal system. The preliminary results were promising so we decided to expand the dataset and we are currently awaiting the results.

I could go on but this post is already way longer than I planned and is mostly my stream-of-consciousness ramblings. I’m going to cut myself off here! I hope it’s not too confusing and if you’ve read this far you are either really interested in what I’m working on and/or are really bored!