The etiology of rivers

The Ordovician was a primitive time. No mammals. No birds. No flowers. Most geologists know this, right? How about this: No meandering rivers.

Recently several geo-bloggers wrote about geological surprises. This was on my shortlist. 

A couple of weeks ago, Evan posted the story of scale-free gravity deformation we heard from Adrian Park and his collaborators at the Atlantic Geological Society's annual Colloquium. My own favourite from the conference was Neil Davies' account of the evolution of river systems:

Davies, Neil & Martin Gibling (2011). Pennsylvanian emergence of anabranching fluvial deposits: the parallel rise of arborescent vegetation and fixed-channel floodplains.

Neil, a post-doctoral researcher at Dalhousie University in Nova Scotia, Canada, started with a literature review. He read dozens of case studies of fluvial geology from all over the world, noting the interpretation of river morphology (fluvotype?). What he found was, to me at least, surprising: there were no reported meandering rivers before the Devonian, and no anabranching rivers before the Carboniferous. 

The idea that rivers have evolved over time, becoming more diverse and complex, is fascinating. At first glance, rivers might seem to be independent of life and other manifestly time-bound phenomena. But if we have learned only one thing in the last couple of decades, it is that the earth's systems are much more intimately related than this, and that life leaves its fingerprint on everything on earth's surface. 

A little terminology: anastomosing, a term I was more familiar with, is not strictly the correct term for these many-branched, fixed-channel rivers. Sedimentologists prefers anabranching. Braided and meandering river types are perhaps more familiar. The fluviotypes I'm showing here might be thought of as end members — most rivers show all of these characteristics through time and space.

What is the cause of this evolution? Davies and Gibling discussed two parallel effects: bank stabilization by soil and roots, and river diversion, technically called avulsion, by fallen trees. The first idea is straightforward: plants colonize river banks and floodplains, thus changing their susceptibility to erosion. The second idea was new to me, but is also simple: as trees got taller, it became more and more likely that fallen trunks would, with time, make avulsion more likely. 

There is another river type we are familiar with in Canada: the string of beaver dams (like this example from near Fort McMurray, Alberta). I don't know for sure, but I bet these first appeared in the Eocene. I have heard that the beaver is second only to man in terms of the magnitude of its effect on the environment. As usual, I suspect that microbes were not considered in this assertion.

All of this makes me wonder: are there other examples of evolution expressing itself in geomorphology like this?

Many thanks to Neil and Martin for allowing us to share this story. Please forgive my deliberate vagueness with some of the details — this work is not yet published; I will post a link to their forthcoming paper when it is published. The science and the data are theirs, any errors or inconsistencies are mine alone. 

Update on 2011-08-23 11:40 by Matt Hall

Davies and Gibling went on to publish this research in Nature Geoscience in August 2011.

The BBC covered the story in their science pages. 

Shale vs tight

A couple of weeks ago, we looked at definitions of unconventional resources. Two of the most important play types are shale gas and tight gas. They are volumetrically important, technologically important, and therefore economically important. Just last week, for example, Chevron bought an unconventional gas company for over $4B.

The best-known examples of shale gas plays might be the Barnett in Texas, the Marcellus in eastern US, and the Duvernay in Alberta. Tight gas plays arguably had their hyper-popular exploration boom five or so years ago, but are still experiencing huge investment in areas where they are well-understood (and have nice reservoir properties!). Prolific examples include the Bakken of northern US and the Montney of Alberta.

So if we were to generalize, perhaps over-generalize: what's the difference between shale gas plays and tight gas plays?

	Shale gas	Tight gas
Grain-size	Mostly mud	Substantially silt or fine sand
Porosity	up to 6%	up to 8%
TOC	up to 10%	up to 7%
Permeability	up to 0.001 mD	up to 1 mD
Source	Mostly self-sourced	Mostly extra-formation
Trap	None	Facies and hydrodynamic
Gas	Substantially adsorbed	Almost all in pore space
Silica	Biogenic, crypto-crystalline	Detrital quartz
Brittleness	From silica	From carbonate cement

 

Over-generalization is a problem. Have I gone too far? I have tried to indicate where the average is, but there is a space in the middle which is distinctly grey: a muddy siltstone with high TOC might have many of the characteristics in both columns; the most distal facies in the Montney are like this.

Why does this matter? Broadly speaking, the plays are developed in the same way: horizontal wells and fracture stimulation. The difference is really in how you explore for them.

Accretionary Wedge #31

This is my first contribution to the Accretionary Wedge; the theme this time is 'What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)?' Like most of the entries I've read so far, I could think of quite a few things fitting this description. I find lots of geological concepts surprising or counterintuitive. But in the end, I chose to write about the thing that obsessed me as an undergraduate, right at the beginning of my career:

The Devonian day was 22 hours long

In November I moved to the Atlantic coast of Canada. It's the first time I've lived right at the seaside, but I am originally from the tiny island of Great Britain so never lived too far from the edge. There is a deeply maritime feel to this part of the continent, even in the sheltered Bay of Fundy. The famously macrotidal regime there permeates the culture: artists paint the tidal landscapes; musicians sing about the eerie currents; geologists crawl around on the mud-flats and cliffs. The profound consequences of a 17-metre tidal range and its heartbeat, regular as clockwork.

← Tidal forces shape a bar-built estuary, Pamlico Sound, USA.

It's easy to see the effects of the tide in the geological record. Tidal successions are recognizable from some combination of pin-stripe lamination, mud-drapes, bi-directional ripples, proximity to shore, diagnostic fossils, brackish trace fossil assemblages, and other marvellous sedimentological tools. Less intuitively perhaps, at least for a non-biologist like me, marine animals also express these tidal frequencies in their growth patterns. So a coral, for example, might have a lunar breeding cycle. This periodicity results in growth rings just like a tree, only they record not the seasons but the bi-monthly beat of spring and neap tides. The tides are driven by the relative positions of the sun and moon relative to earth. Celestial bodies created banded coral.

From Scutton (1963): diurnal rings and and monthly bandsColin Scrutton, one of my professors at the University of Durham in the northeast of England, measured the growth ridges of rugose corals of Middle Devonian successions in Michigan, Ontario and Belgium (Scrutton 1964). He was testing the result of a similar experiment by John Wells (1963). The conclusion: the Devonian year contained 13 lunar months, each lunar month contained 30.6 days, so the year was 399 days long. According to what we know about planetary dynamics in the solar system, the year was approximately the same length so Devonian days were shorter by a couple of hours. The reason: the tides themselves, as they move westward around the eastward-spinning earth, are a simple frictional brake. The earth's rotation slows over time as the earth-moon system loses energy to heat, the ultimate entropy. Even more fascinatingly, the torque exerted by the sun is counteractive, introducing further cyclicities as these signals interfere. Day length, therefore, has probably not slowed monotonically though time.

For me, this realization was bound up with an obsession with cyclicity. I could not read enough about Milankovitch cycles: wobbles and ellipticity in the earth's dance through space scratching their pulse into the groove of the stratigraphic record and even influencing sea-floor spreading rates, perhaps even mass extinctions. The implications are profound: terametre-scale mechanics of the universe control the timing of cellular neurochemical functions.

Why anyone needs astrology to connect with this awesome fact is beyond me. 

References

Panella, G, et al (1968). Palaeontological evidence for variation in length of synodic month since late Cambrian. Science 15 (3855), p 792–796, doi: 10.1126/science.162.3855.792.
Scrutton, C (1964). Periodicity in Devonian coral growth. Palaeontology 7 (4), p 552–558, pl 86–87.
Wells, J (1963). Coral growth and geochronometry. Nature 197, p 948–950. doi: 10.1038/197948a0.

How to make a strat column

A few weeks ago I posted about the brilliant TSCreator, a Java application for creating custom geological timescales. One of the nicest features of this tool is that you can create your own lithostratigraphic columns, stick charts, transgression-regression plots, isotope curves, etc. It's a slightly fiddly process, so I wanted to try to give some pointers; this post is about how to make a simple lithostrat column. The other column types are built in a similar way; the full details are described in the Manual (starting on page 20). 

The example I'm showing is the Western Cape Breton column, as given by the Nova Scotia Geological Highway Map. I can't vouch for its accuracy as I've never worked this section; I built it purely to show the method. You can see the result here >

You build the data file, which TSCreator calls a Datapack, in a spreadsheet. I use Google Docs, but you can use any tool you like (OpenOffice.org, Microsoft Excel etc), as long as it will save a tab-delimited text file. The spreadsheet has a header and a data section; here's what the header looks like in my example:

format version: 1.4
date: 10/02/2011
Chart Title: Western Cape Breton
age units: Ma

You can see my example file here (opens in Google Docs). To use it, first save it as a text file: Google Docs > File > Download as > Text. Give it a .txt extension when you get the chance. Then launch TSCreator and select File > Add Datapack. If you get an error it's probably because you have violated one of the formatting rules. It may take some back and forth to get it how you want it.

Finally, I just made the unhappy discovery that you cannot save your chart after you load a custom datapack. Apparently to export an image or SVG file (my preference), you need TS-Creator Pro. Or you get very clever with screen grabs!

If you have your own tips, please leave them in the comments!

Note, TimeScale Creator is a trademark of the Geologic TimeScale Foundation. I am not connected with the software or its creators in any way. Microsoft Excel is a trademark of Microsoft Corporation. Java is a trademark of Oracle Corporation.

Potash mine photo tour

On Friday, Matt and I went on a tour of the PCS potash mine in Penobsquis, New Brunswick, as a precursor to the 2011 Atlantic Geoscience Society Colloquium in Fredericton.

The evaporites of the Early Carboniferous Windsor Group were formed as a result of two marine incursions into an otherwise clastic red bed sequence within the Moncton sub-basin. The evaporites containing the potash ore have been folded into a NE-SW trending anticline as shown in the diagram below.

Brian Roulston hosted 24 visitors into the mine. We were lowered about 400 m down to the main workings then driven approximately 10 km underground to three main attractions: a cavern stope in the Basal Halite; an active stope in the halite; finally an active stope in the potash ore (sylvinite).

Thanks to Brian and his team at PCS for putting this tour together for us, it was so much fun.

Signage

People

Gear

Salts, rusts, colors, and textures 

Darkness

Mining the ore

Measuring value

Often in upstream oil and gas we are challenged with a simple question: what's the value? What's the value of that 3D seismic? What's the NPV of this study? How does your professional network affect our bottom line?

Sometimes, at least for me, the first reaction is indignation. Let's take the seismic example; it goes like this:

Finance guy - So, this $28M... 3D seismic. What's the value of this data set?
Geoscientist - What's the value? Of that 3D? That state-of-the-art, high-fold, wide-azimuth, long-offset, high-bandwidth, eco-friendly, ultra-safe 3D seismic survey I just spent four months designing and soliciting bids on?
Finance guy - Yeah
Geoscientist - We have four wells on twenty square kilometres of land. We want to drill forty more. The wells cost $10M each. The seismic will allow us to pick the best locations. It's 3D seismic, the best quality. We always do it. Everyone does it. We can't do subsurface science without it, not very well anyway.
Finance guy - Yeah... Sorry, what's the value of the seismic? Dollars will do.

Finance guy just wants a number. The value is clear to everyone involved. But maybe money is tight this year and finance would like to defer some costs to next year. Maybe we can lower the cost by making the survey smaller, or reducing the fold. Before too long someone utters the unspeakable: 'Value of Information'. The next month of your life becomes a frustrating spreadsheet nightmare of trying to get the process to yield the answer your team wants so you can get on with finding oil and gas.

I think there is a better way. What do you think?

Where on Google Earth #266

Brian nailed Where on Google Earth #265. He doesn't have a blog of his own so he asked me to host it for him. So, over to Brian...

Much thanks go to Matt here for hosting this WoGE for me since I do not yet have a blog of my own. I'm already looking into options. This is just too much fun for a Google Earth addict like me.

Although this image is zoomed in pretty good I'll invoke the Schott Rule just to give newcomers like myself a chance. For those unaware, this means you must wait one hour for each previous WoGE win before you can post your answer. [Here are the previous winners in Ron Schott's KML file — Matt].

I've also hidden the orientation compass so you can safely assume North isn't necessarily at top. Can't make it too easy now, can we?

This one isn't just about the geology, but also the historical significance.

Please post responses in the comments. Posted at 0800 Atlantic, 1200 GMT.

Where on Google Earth #259

I got WoGE #258 by the skin of my teeth, as I found the location but failed to fully identify the feature. I got the country rock right, but the igneous one wrong. As a soft rock chap, I consider this to be a technicality. Luckily, so did Metageologist Simon, the host. So I humbly accept my failings as a geoscientist and offer you the next instalment: number 259, and hereby post it at 1300 AST, 1700 GMT. 

Where on Google Earth is the best use of your lunch-break since Worms Reinforcements (the only computer game I ever wanted to play twice). If you are new to the game, it is easy to play. The winner is the first person to examine the picture below, find the location (name, link, or lat-long), and give a brief explanation of its geological interest. Please post your answer in the comments below. And thanks to the Schott Rule, which I am invoking, newbies have a slight edge: previous winners must wait one hour for each previous win before playing.

So: where and what on Google earth is this? (There are quite a few interesting things here, both geomorphologic and geologic; see how many you can get!)