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.