Rugby distraction

I've been distracted for the last two months, partly because I was away for 5.5 weeks, and partly because the Rugby World Cup was on, and I'm a huge (All Blacks) fan.  In fact, my overseas trip was so that I could catch a bit of the rugby in New Zealand, which is like home to me.  I did mix in some geology with my travels: I was on a volcano, I visited some ancient clastics and some modern carbonates. I even did some work on my thesis and a manuscript my supervisor and I are working on. And I watched a lot of rugby, on TV, and in the stadium. I went to five games: three pool matches and two quarterfinals.

When I came back to Canada, I struggled to get over the jet lag, because the rugby was still on, and I was up until 4 or 5am watching the semifinals, and then the bronze final, and then the final. SPOILER: A week later, and I'm still pretty elated about the results.

Now that the RWC is over, I need to get my nose back to the grindstone and finish off my thesis and that manuscript. They're getting there. I'm struggling a bit though, because I'm in rugby withdrawal. So last night I went through some of my photos from Australia, and printed off a few from my outback trip, so that I could look at the really cool geology and be motivated to get some writing done. It also got me thinking about the things I saw that I want to blog about, so now I have a list, and a schedule. Hopefully it will be a nice distraction from writing and figure-making. Anne Jefferson's writing challenge couldn't have come at a better time.

In the meantime, here are a few teaser pictures from my time away (and if anyone knows how I can save them so that the colour doesn't sometimes get changed when they are being viewed, I'd be grateful):

Beside a fumarole on White Island

Fumaroles!

Uluru

Valley of the Winds, Kata Tjuta

King's Canyon

Standing beside the Webb Ellis Trophy, two weeks before New Zealand won it

Foreland basins in a nutshell: Part III - Retroarc and peripheral foreland basins

Part I
Part II

The Western Canada Foreland Basin (WCFB) is a retroarc foreland basin. There are two main types of foreland basins, retroarc and peripheral. Both form during collisional tectonics.

A peripheral foreland basin, also known as an Alpine-style foreland basin, forms on the subducting plate (usually during continent-continent collision).

A retroarc foreland basin, also known as an Andean-style foreland basin, forms on the upper plate (usually during continent-oceanic plate collision).

Note: nothing is implied in the scale of the two figures above: there is definitely some vertical exaggeration going on.

I've often wondered if there is a difference in the sediment deposition patterns in the two classes of foreland basins. A reading of Hugh Sinclair's 1997 paper, Tectonostratigraphic model for underfilled peripheral foreland basins: an Alpine perspective, in which he describes the "underfilled trinity," (a favourite term of mine, by the way) felt a lot like reading a description of the Jurassic succession in the WCFB, except that he was describing a peripheral foreland basin. I suspect, therefore, that the sedimentological record alone would not be enough to discriminate between these two types of foreland basin.

In fact, this is what has been found by other authors, such as Dickinson: "Designation of a given foreland basin as either a retroarc basin or a peripheral basin thus depends upon a knowledge of the sequence and timing of tectonic events in the adjacent orogen" (Dickinson, 1974, p21).

Why does this matter? Well, having an "underfilled trinity" in the WCFB means that the foreland basin depositional sequences began appearing sooner than everyone thought, and that fits with what I'm proposing with my research. It also means that although foreland basins contain a record of the adjacent orogeny, we cannot use the basin fill alone to say if it is a retroarc or a peripheral foreland basin. It may seem strange that we wouldn't know this, but recent, and controversial, hypotheses about the evolution of the western margin of North America imply that the western foreland basins are peripheral, not retroarc (and by mentioning this, I'm not saying that I agree with those hypotheses).

In part IV, I'll talk about foreland basin fill: flysche, molasse, backbulges, underfilling...

References:

Dickinson, W. (1974). Plate Tectonics and Sedimentation. In W. Dickinson (Ed.), Tectonics and Sedimentation (pp. 1-27). Society of Economic Paleontologists and Mineralogists.

Sinclair, H. H. D. (1997). Tectonostratigraphic model for underfilled peripheral foreland basins : An Alpine perspective. Geological Society of America Bulletin, 109(3), 324-346. Geological Soc America.

Childhood memories

A recent twitter discussion that began with me wishing there was a "Tectonics for Toddlers" I could give my nephew got me thinking about one of my favourite toys as a preschooler. It was a Fisher-Price hiking set, and I used to hike around the neighbourhood with it. I always dreamed about going to the coulees behind our yard, but that was forbidden territory to little me (and rightly so--I was only about five years old at the time).

My hiking set is long gone, and a recent Google search turned up an picture of the set in an "old toys" website. But when I go out in the field these days, I still carry some of the same tools, just more grown-up versions of them.

Missing from this picture is the compass that had a mirror on the other side

Accretionary Wedge #37: Sexy Geology

My preference is for big picture geology, especially studying how tectonics influences sedimentation. For this, you need to understand a little bit of everything (or at least be willing to contemplate it). My undergraduate background in geophysics is helpful for understanding geodynamics, and living and studying in Alberta has meant that I've had ample exposure to sedimentary geology in both my degrees. That being said, there are a couple of sedimentary structures that make me swoon. I love a good cross-bedded sandstone. But the one that really gets me is ripples.

I know ripples are pretty common, and I know they aren't great paleocurrent indicators. They are, however, good environmental indicators and for me, seeing a big rippled surface in the field is pretty exciting, especially when it is far away from anywhere it could have formed. What excites me is that they are evidence of tectonics. They've been buried, they've moved,  they've been uplifted and they've been exposed.

There's just something about being up on a ridge, far away from any water and even farther away from a beach, and seeing ripples. It's sexy geology.

Ripples on Dinosaur Ridge, Colorado.
Embiggen to see the scale bar in bottom left.

Ripples in the Puchuni Valley, on the Bolivian Altiplano.
No scale, because this was above me on a narrow path on the edge of a steep hill.

Foreland basins in a nutshell: Part II - Basic definition of a foreland basin

Part 1

Prior to the development of plate tectonic theory, sedimentary basins were discussed in terms of the geosynclinal system, in which basins were described and classified according to their geometry and fill. This was not a simple classification system. For example, in just a two-page Science paper in 1944, Kay described miogeosynclines, eugeosynclines, deltageosynclines and autogeosynclines. He recognized that the original definition, credited to Dana in 1873, had been misused and misinterpreted in the intervening seventy-odd years (Kay, 1951). In its simplest form, Kay defined a geosyncline as "the surface developed at the base of extensive surficial rocks that subsided deeply during their deposition or accumulation," (Kay, 1951, p3).

Kay used the terms eugeosyncline and miogeosyncline to distinguish between geosynclines that either have or lack active volcanism respectively. When discussing orogenies and foreland basins, these are the two historic terms that are most relevant (miogeosynclines are associated with the stable platform and eugeosynclines are associated with volcanic arcs). All of the other named geosynclines are really variations of these two types. For a good overview of geosynclines, and the problems with the theory, I recommend The Story of Geosynclines.

An idealized cross-section through a geosyncline.
Image source: http://www.geology.ohio-state.edu/~vonfrese/gs100/lect21/index.html

The mechanisms behind the formation of geosynclines were not always understood, and I think this is a good example of just how unifying plate tectonic theory really is. This was recognized at least as early as 1974, when Dickinson wrote that “the geosynclinal terminology used prior to the advent of plate tectonics is inadequate to describe fully the plate-tectonic settings of sedimentary basins” (p 1).

In fact, it was Dickinson who wrote that despite its emphasis on horizontal movements, plate tectonic theory provided the best mechanism for the large vertical displacements required in order for the thick accumulations of sediment found in some parts of the continent to occur (1974).

Foredeep trough is a term used to describe "an isostatically induced peripheral depression that developed in response to the load imposed on the lithosphere by the ... flow of supracrustal rocks up on to the flank of the craton" (Price, 1973, p 498). Price referred to supracrustal rock flow because he was describing deformation in the southern Canadian Rockies at a Rocky-sized scale of observation.

Even before plate tectonics, the term foreland was being used to describe regions adjacent to orogenic belts (Kay, 1954), but Dickinson (1974) is often credited with first defining foreland basins as such. He redefined eugeosynclines and miogeosynclines to fit within a plate tectonic framework, but, in his own rather eloquent words (p8):

It does not allow for the considerable sophistication of geosynclinal theory in full flower, and results in the unnecessary lumping of things that the full geosynclinal terminology accords different status. Nor does it meet the need to relate various types of sedimentary basins to different kinds of plate interactions, rather than just to the two main kinds of substratum.

Dickinson (1974) designated foreland basins as the retroarc basins on the cratonal or platformal interior of the continent in arc-trench systems, while recognizing that foreland basins can form during continental collisions as well. (This publication, Tectonics and Sedimentation, is also a good read).

The problem I personally have with these definitions is that they tie the formation of a foreland basin specifically to a developing fold-thrust belt. In Western Canada, the Cordilleran Orogeny involved more than just the fold-thrust belt, and I suspect the terrane collisions and subsequent deformation would have caused a flexural response prior to initiation of the thrust faults.

Leckie and Smith (1992) defined a foreland basin as follows: "A succession of sedimentary rocks deposited in a cratonic region adjacent to an active orogenic belt. Sediments are derived mainly from the orogenic belt and thicken toward it." I like this one because it doesn't limit the orogenic belt to the thrust-faults.

Regardless of how it is defined, what a foreland basin is fundamentally the same. It is the basin adjacent and parallel to an active orogeny. The subtle variations in the definitions, do however, impact how the timing of the initiation of the foreland basin is determined; this is one of the major components of my thesis.

The difference between the two types of foreland basins, retroarc and peripheral, will be the subject of part III.

References:
Dickinson, W. (1974), Plate Tectonics and Sedimentation, in Tectonics and Sedimentation, edited by W. Dickinson, pp. 1-27, Society of Economic Paleontologists and Mineralogists.

Kay, M. (1944), Geosynclines in continental development, Science, 99(2580), 461. 

Kay, M. (1954), North American geosynclines, The Geological Society of America, New York.

Leckie, D. A., and D. G. Smith (1992), Regional setting, evolution, and depositional cycles of the Western Canada Foreland Basin, American Association of Petroleum Geologists Memoir, 55, 9–46.

Price, R. (1973), Large-scale gravitational flow of supracrustal rocks, southern Canadian Rockies, Gravity and tectonics, 491–502. 

Foreland basins in a nutshell: Part I - Foreland Basin vs. Sedimentary Basin

I study foreland basins. Specifically, I'm looking at the initiation of the western Canada foreland basin and challenging the traditional view of when this initiation took place. In order to do this, I need to use a different definition/model for foreland basins than what has previously been used in western Canada.

Different model, you ask? As I have learned over the last two years, there are different types of foreland basins, and different models for explaining deposition within them. Because I want to have a clear picture of all these different models in my head for my thesis, I'm going to attempt to give an overview of them here.

Today, part I will focus on clearing up a common misconception, at least here in western Canada. The western Canada foreland basin (WCFB) is not the same as the western Canada sedimentary basin (WCSB). The WCSB is the entire sedimentary record in western Canada, beginning with the Belt-Purcell and Windermere deposits in the Proterozoic and continuing through to present times. The WCFB is the sediments deposited during the Cordilleran Orogeny through the Mesozoic and early Paleogene. In the following figure, from the Geological Atlas of Western Canada, you can see the extent of the WCSB. The WCFB is approximately coincident with what is labeled here as the Alberta Basin. However, the foreland basin deposits do actually extend into the Cordillera, like the WCSB does.

Many maps often erroneously depict the western edge of the WCSB and/or the WCFB as being coincident with the eastern edge of Cordilleran deformation (as is the case with the Alberta Basin shown here). Most of the fold-thrust belt contains sedimentary rocks, therefore they are deposited in the WCSB, and in the case of the Mesozoic and Paleogene rocks, the WCFB.

Image source: www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch03/ch_03.html

In part II, I will look at the most basic definition of what a foreland basin is, and how plate tectonic theory influenced our understanding of foreland basins.

Favourite Geology Word

I've been thinking about the current Accretionary Wedge topic intermittently, because there are a lot of geology words I love, for either phonetic or geological reasons. But I've been preoccupied with my sister's wedding of late, and those words have buried themselves somewhere in the recesses of my mind. Now that I'm back home, I can start thinking about rocks again, and perhaps those words will come back to me.

In the meantime, there is a word that represents a lot of what my master's thesis is about and if used literally, speaks to why I became an earth scientist. Without further adieu, I give you,

geohistory
Geohistory, or as I more often call it, subsidence analysis, analyzes a sediment column to reveal information about the tectonic conditions it was deposited in. You literally peel back the layers and try to restore the sediments to their depositional conditions. In the case of the master's thesis I am writing write now, I'm looking at subsurface wells in the Western Canada Foreland Basin to show the link between the Cordillera and the foreland basin.

I love this work because it is big picture geology. It ties together the mountains and the basin, and reinforces the fact that you have to understand the regional tectonic framework to study a sedimentary basin.

What does this have to do with why I became a geoscientist? From a very young age, I knew that whatever career I ended up having, I wanted to be a storyteller. I never imagined that I would be reading the rocks to understand and retell the story of the earth, but that's what we all do. In a very literal sense of the term, what we all do is geohistory.