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Provenance and plate tectonics

This post is part of the How To…series – Provenance, sedimentary basins and plate tectonics

Deciphering the history of sedimentary basins is one of the more exciting tasks geologists can undertake; the provenance of sandstones plays an important part in this adventure.

Sedimentary basins are crustal structures. They are regions of long-term subsidence, responding to tectonic and sediment loads, cooling in the crust and upper mantle, and tectonic dislocation along crustal structures like transform faults. The processes that create sedimentary basins and the sediments that fill them are inextricably linked to plate tectonics.

The idea that the composition of sedimentary rocks was related to large-scale crustal processes was acknowledged by Charles Darwin, Charles Lyell, Henry Sorby and others, but it wasn’t until the 1950s – 60s that sedimentologists began to develop empirical models of sandstone provenance. Robert Folk, Francis Pettijohn, Robert Dott and contemporaries observed links between sediment composition and tectonic domains (or provinces), such as stable cratons and geosynclines – this was the era before plate tectonics.

For geoscientists, the discovery and development of plate tectonic theory changed everything. This is where William R. Dickinson comes into the picture. Dickinson recognized the fundamental link between sedimentary basins and plate tectonics, particularly at plate boundaries. He developed models that relate the modal composition of sandstones to plate tectonic provinces such as collision orogens, magmatic arcs, forearc and foreland basins, and stable cratons. It is important to remember that these models are based on empirical evidence – analysis of 1000s of thin sections that he and many others had recorded from diverse locations.

The models are based on ternary plots like those used by Dott to classify sandstones. Dickinson and his co-workers used different combinations of the quartz, feldspar and lithics end-members to emphasize certain characteristics of the sediment and the source rocks. Both models shown here use the full suite of minerals. Other plots used only the lithic components, or polycrystalline quartz and lithics.

The Qt-F-L plot combines all varieties of quartz (mono- and polycrystalline quartz, including chert) as a single category and as such emphasizes the maturity of the sediment. Deposits with greater volumes of quartz are generally considered more mature, where mechanical and chemical weathering during sediment transport and deposition have removed less- stable components like feldspar and lithics.

In the Qm-F-L plot, polycrystalline quartz is shifted to the lithic field and in so doing emphasizes the source rocks and production of rock fragments (the quartz component consists only of monocrystalline varieties). Lithics here are key indicators of reworked orogenic provinces along continent-continent and continent-magmatic arc collision provinces. Here, erosion of sedimentary cover and volcanic rocks tends to produce greater proportions of rock fragments.

The second set of diagrams shows typical plate tectonic configurations that correspond to the various QFL fields. The diagrams are highly simplified. In addition to pigeonholing sandstone compositions, the plots provide a useful means of documenting systematic changes as uplift and erosion expose deeper crustal rocks.

For example, unroofing a fold and thrust belt along a collision margin will yield an initial rush of lithics derived from the deformed sedimentary cover. Gradual exposure of a metamorphic core will yield increasing volumes of quartz (mostly polycrystalline) and a new suite of heavy minerals. Likewise, unroofing a magmatic arc complex (the “dissected arc” field in these plots) will provide abundant volcanic lithics followed by more felsic sediment from the deeper intrusive rocks. This is shown schematically in the cartoon below.

The Dickinson plots, like any scientific model, are highly simplified versions of the real world. No two collisional orogens are alike, no two magmatic arcs are exact duplicates. Finding exceptions to any of the models does not indicate their failure – quite the opposite. Their value lies in providing a direction for investigation.

Dickinson’s models have been through several iterations, but their basic structure has survived 40 years of intense scrutiny by geoscientists. They are still useful starting points for unravelling the links among sediment composition, sedimentary basins and plate tectonics.

Check out the companion article – Provenance of sandstones

Some useful texts and papers:

Petrology of Sedimentary Rocks, Sam Boggs Jr. 2012

Dickinson, W. R. and C. A. Suczek, 1979, Plate tectonics and sandstone compositions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2164–2182.

W. R. Dickinson, 1988. Provenance and Sediment Dispersal in Relation to Paleotectonics and Paleogeography of Sedimentary Basins. In New Perspectives in Basin Analysis, Editors, Karen L. Kleinspehn & Chris Paola, Springer-Verlag, pp 3-25.

R.V. Ingersoll, T. F. Lawton, and S.A. Graham, 2018. Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson. Geological Society of America, Special Paper v.540, 757 pages

Polar bears do not live in the Antarctic, there are no Penguins in the Arctic. The asymmetry of the poles

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This post is about asymmetry – the Arctic and Antarctic polar regions. They are the most frigid places on Earth, but that is about all they have in common; with one other exception – they are both stunningly beautiful. I can attest to this for the Arctic, or at least the Canadian Arctic Islands where I spent several summers; but I’ve never been to Antarctica. Visual treats everywhere. And silence – above the wind and the hum of a few insects – silence.

There is an intriguing asymmetry in their respective geographies, the timing of ice accumulation, present climates, the flora and fauna. What follows are a few comparisons and contrasts. Continue reading →

Extreme living conditions; the origin of life and other adventures

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Extremophiles – life forms that live in really hazardous conditions.

Extreme events are fascinating. Extreme sports may give us a vicarious thrill, at least until something goes awry at which point we might comment about the foolishness of the act. Extremes in the natural world are the stuff of movies; asteroids, tsunamis, tornadoes, plagues. Perhaps our morbid fascination with such events derives from the realization that they can be real.

Over the last 2-3 decades, science too has developed a fascination for extreme living, for creatures that happily thrive in conditions that most other life forms, including us, would find inclement. They are extremophiles, life forms like bacteria, algae and small critters that can endure extremes of temperature, pressure (e.g. deep sea black smokers), radioactivity, darkness, low levels of oxygen, high acidity or alkalinity, and even lack of water. The variety of extreme environments in which these life forms have evolved is, from a scientific perspective, quite stunning in that it provides us with many different analogues for our quest to understand the origin of life on earth, and whether life can exist on other planets. A few examples are noted below. Continue reading →

The crosses I bear

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I have had occasion recently to bear the brunt of criticism about some of the decisions one makes in life. These comments, made during a discussion on climate change, were a polite but thinly veiled criticism of some of my choices about where and for whom I had worked over the past 4 decades. I was given to understand that working for oil companies (mainly offshore Taranaki) and mining companies (NZ, Chile) was a blight on my character; how could I talk sensibly about climate change science when I was one of the main contributors to the world’s problems. That I had assuaged my conscience by also working on groundwater and geothermal projects (renewable), CO₂ sequestration, teaching and research seemed irrelevant. I had given in to the dark side.

I’ve had discussions like this on several occasions and as with them, little was resolved during this particular discourse. But I did feel annoyed. The phrase “but you had choices” and something about selling of souls had been trotted out two or three times during the chat, and it was this insistence that grated most. Choice, it seems is a convenient crutch on which to foist an opinion.

There are plenty of everyday choices which are obvious: what to have for dinner, who not to vote for. These kinds of choices are explicit and for the most part reasoned. But there are also choices that, while not necessarily unconscious are hidden, or if not hidden then conveniently tucked away lest they become uncomfortable. Discussions about renewable resources, climate change and environmental issues are good examples where our efforts to be responsible citizens (and scientists) can begin to unravel if we take too much notice of these hidden choices.

Consider the following – you are going to a conference. Here is a list of a few inconvenient but inevitable choices you will make:

You will need to travel (car, plane, train). Every step of this journey will require the use of fuels, plastics, metals, food, clothing, communications (the list goes on). Hydrocarbons will be front and centre of virtually everything that gets you from A to B. Would you choose not to attend the conference because of the carbon footprint? Most will feel some guilt but attend anyway, either ignoring the issue or designing some arcane explanation involving necessity.
Accommodation (same kind of list)
Most people at the conference will have at least one form of communication – phone, tablet, laptop (plastics, metals, including rare earths). Tweeting directly from a conference is now common place. But will any thought be given to the atrocious working conditions under which some foreign governments and mining companies extract the rare earth metals used in these devices?

There may of course be arguments that the improvements to society and science that someone makes by going to the conference (or to the office, the lab, the field, home) is worth the sacrifice of a few dark choices. Or that some choices are worse than others. But this is a pretty self-serving and unnecessary position to take. I doubt there would be many who would begrudge the scientist traveling to Antarctica to collect data that improves the veracity of climate models.

I now feel vaguely vindicated. My soul is largely intact. I have played my part in providing the wherewithal for conference goers and field trippers. In the end whatever tasks I might have undertaken for the dark side, were no better or worse than the crosses that our conference goer and field scientist have to bear. As Jane Austin’s Mr Bennett said “I will get over it and probably more quickly than I should.”

I also have faith in science’s and society’s ability to find solutions to many of these unfortunate choices. I expect it will be a gradual process. Yes, we can learn to recycle, locally, our old cell phones, and purchase electric vehicles when they are reasonably priced and when there is sufficient infrastructure. But in the meantime most of us will still need to fill the petrol tank or hop on that plane. If climate change is a reality (and there seems to be a consensus that it is), then so too is the length of time it will take to make the necessary structural changes to the way we live. In the end, moral indignation at these darker circumstances seems neither fair or useful.

The Bubbles That Changed our Perspective on the World’s Climate

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One of my geology field seasons in the Canadian Arctic worked out of a base-camp on Axel Heiberg Island (west of and snuggled against Ellesmere Island). It was the spring thaw and all rivers and streams were muddy. Our only source of clean water turned out to be a small melt-water pond atop an iceberg in Strand Fiord, a few hundred metres offshore. The helicopter would make daily trips with a 45-gallon drum to collect the water. The ice and its water were crystal clear and probably a few thousand years old. It was a treat. Perhaps the only thing missing was the occasional Scotch or G&T. Continue reading →

Sea-Level Change; Busting a Few Myths

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So you think sea level is the same everywhere!

Canon Fiord, Arctic Canada

Climate change predicts that sea levels will rise at an increasingly rapid rate. Some of NASA’s new satellite altimetry data hints that this is already happening. There is a multitude of voices crying out for government planners to prepare for inundation of vulnerable coasts. Small island states are particularly at risk. Forward planning would certainly be a wise move. If average sea level rises say a metre in the next 100-200 years many coasts will be inundated and storm surges will push farther inland. Forward planning does make sense. Continue reading →

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Tag Archives: Antarctic

Provenance and plate tectonics

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Polar bears do not live in the Antarctic, there are no Penguins in the Arctic. The asymmetry of the poles

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Extreme living conditions; the origin of life and other adventures

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The crosses I bear

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The Bubbles That Changed our Perspective on the World’s Climate

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Sea-Level Change; Busting a Few Myths

So you think sea level is the same everywhere!

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