Adjacent Analogues

Sedimentary geologists, with our uniformitarian approach to the world, often look for modern environmental analogues that might help us to better understand the ancient deposits we study. Where they occur in close physical conjunction, the modern can inspire scientists analyzing the ancient. Although these sorts of things may well occur often, I can’t seem to find an existing term that describes this phenomenon. I propose that we call them adjacent analogues.

Across the river, Christ Church Cathedral is framed by dead roots on the north side Green.

I started thinking about these things a couple of weeks ago, when I was visiting Fredericton. My brother and I were walking along the north side Green, and I got taken with photographing the abundant driftwood left along the shore by recent high water. The Saint John is a large river, and in full flow it can move substantial objects. In some places, there were even clumps of worn and abraded trees, still attached by their roots.

Turning from the shore, my attention was drawn by large blocks of stone that had been placed to separate the boat launch area from the lawns. Many of these consisted of relatively monotonous sandstone, but one of the boulders closest to the water appeared to have more varied features. Crouching to examine it closely, I could see large chunks of well-preserved ancient wood, much of it still consisting of organic material, surrounded by debris and rusty stains. The sandstone blocks appeared to be consistent with Carboniferous (Mississippian to Pennsylvanian) deposits that make up much of the bedrock in the Fredericton area, so this fossil wood was in the range of 320 million years old (see Whitehead, 2001, for an outline of Fredericton area geology; a pdf can be found here).

The boulder (above) and a detail of the wood (below). The brilliant white circle is a Canadian quarter (diameter 24 mm) reflecting the sun.

Then it struck me: here I was, surrounded by driftwood on the bank of a large river, and the rock sitting on the bank just happened to contain ancient driftwood that had been deposited under similar conditions hundreds of millions of years ago. The ancient and modern in immediate conjunction! Worlds in worlds, wheels in wheels!

Of course, if we compare them in more detail, there are significant differences. The Saint John at Fredericton flows out of gentle hills and meanders broadly; most sediment along its banks is well-weathered and silty. The Carboniferous sandstones, on the other hand, were apparently deposited in alluvial fan conditions and contain quartz grains mixed with rock fragments suggesting that the material had not been transported a huge distance. Still, as an illustration the comparison was striking.

The bank by the footbridge is covered with a thick layer of dead wood.

In this quarry at the top of the hill on the south side of Fredericton, cross-bedded rust-stained sandstones can be observed in place.

The sandstone is a mixture of quartz grains and darker minerals and rock fragments.

Such adjacent analogues must occur in many  places. There are huge areas where modern rivers cut through ancient floodplain sediments; excellent examples can be seen in various places in western Canada. Similarly, in the Churchill area of Manitoba the famous Ordovician rocky boulder shoreline occurs in conjunction with a modern rocky shore, and deposits representing an ancient tropical cove occur within a modern subarctic one.

Perhaps one of the strangest examples I can think of readily is at Kuppen in Gotland, Sweden, where Silurian sea stacks and other rocky shore features occur close to sea stacks along a modern shore (see Fig. 3 in this pdf; note that it is a large file).

My brother’s cellphone photo suggests that I may appear a bit eccentric when I get interested in looking at rocks. Note the huge quantity of driftwood beside the boulder. (photo by Chris Young)

The scientists working at such field locations cannot help but be inspired by the modern environments that surround them. Of course we must be cautious in applying modern observations to long-gone paleoenvironments, but they can certainly provide us with valuable insights and give us ideas that we might never have if we only saw the two in separate places and at separate times.

Adjacent analogues also serve as wonderful examples for field trips by students, professionals, and interested amateurs. It would be great to compile a list of such places; if you know of other good examples, please send them to me and I will try to do another post on this topic!

Willows surrounded by receding floodwater

© Graham Young, 2012