Historical Geology/Paleocurrents

From Wikibooks, open books for an open world
Jump to: navigation, search

In this article we shall look at how we can use sedimentology to determine the direction of paleocurrents: former currents of wind or water.

The reader may find it helpful at this point to go back and look at the main article on rivers and on deserts to place the following article in its proper perspective.

Cross-bedding[edit]

Cross-bedding in sandstone, Zion National Park, Utah.

It is possible to determine the past direction of currents of wind or water by examination of sedimentary structures in rocks.

In ripples and dunes formed by wind or water, the cross-beds consist of the remnants of beds formed by sediment cascading down the lee face of the dune. Where the stoss side of a ripple is preserved, it has a shallower slope than the lee side. These considerations allow us to deduce the former direction of currents by studying the arrangement of sediment laid down by them. For example in the photograph to the right we can see that the current (of wind, in this case) that formed the cross-beds was traveling roughly from left to right.

In the case of streams and rivers, the current direction is necessarily downstream. In the case of sand, you might not suppose that there is a direction associated with the current: the wind is, after all, proverbially capricious; but it is not quite so capricious as a layperson might think. Pretty much any spot on the face of the earth has a prevailing, though not constant, direction in which the wind blows: for example in the Sahara the prevailing wind is north-easterly, and this is reflected by the motion of the dunes and the sets of cross-beds that they deposit.

Clast size[edit]

On a larger scale, the direction in which a river flows is indicated by the average clast size on its bed: there is a tendency for smaller clasts to be found further downstream.

The reason for this is that if at any point in its course the river slacks off due to becoming broader and/or deeper, this will diminish its capacity to carry the larger of the clasts that it has carried up to that point, and so they will be deposited. The river may well regain speed after that point, so that it could carry clasts of that size once more; but by that time they have already been deposited further upstream.

How do we know?[edit]

We can verify that these relationships between currents, sedimentary structures, and clast size hold good in the present.

Now in this case the principle of actualism is if anything even more compelling than usual, because we are dealing here with really basic laws of physics. Is it even conceivable that in the past the laws of fluid dynamics could have been so different that (for example) the stoss side of a ripple would be steeper than the lee?

The agreement between current direction as inferred from ripples and as inferred from clast size is another reason for confidence in these methods.

Sediments and climate · Biogeography and climate