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Time and motion

The one topic that distinguishes geology from all other sciences from which we regularly steal and borrow, is time. It’s the metric that enables much of what we see and postulate as geologists to be reality, but which we tend to take for granted. Indeed, it is commonly hidden from the casual observer. There are of course catastrophic events that take place geologically instantaneously and leave their mark in the geological record, but time is the differentiator that enables most geology to happen.

Why is time so important?

We know the age of the earth, with reasonable certainty and accuracy. It’s around 4.7 billion years old, as determined from the decay of radioactive elements. We’ve come a long way since Bishop Ussher, in 1654, declared with naive precision that the earth was created on 22 October 4004 bce, according to his analysis of the scriptures back through the generations of prophets. But it was Arthur Holmes, measuring the half-life of radioactive elements, who turned dating into a precise science and gave us quantifiable deep time. In 1913 he published The Age of the Earth and calculated its age to be remarkably close to the value accepted today. It is this time that allows geologically slow cumulative processes to have enormous effects and cover global distances. Without deep time, evolution, plate tectonics, continents morphed beyond recognition, oceans vanished, species lost in mass extinctions, hot house, ice house, and sea-level change would not be possible.

Include time in your thinking

Since the dawn of our understanding of plate tectonics in the 1970s, we have become accepting of large-scale lateral tectonic movements. Recognition of vertical movements of the crust have taken something of a back seat, despite well-accepted notions of plate collision, orogeny, and thermal relaxation subsidence. When we work in sedimentary basins, well data and seismic usually provide unequivocal evidence of both geological time and vertical movements. Take a look at the geoseismic section opposite. It doesn’t matter where this section originates, suffice to say it’s a tilted and partially inverted rift basin in which sequences and horizons are exceptionally well dated. The fault blocks at the northern end of the section are also present in the deeper southern part of the basin, but are here buried beneath a series of sequences that unconformably overlies the fault blocks and thickens to the north, the direction from which they were derived. The large inversion structure to the south is still active today, and sediments within hanging walls of the main fault blocks are thicker than in corresponding foot walls. Dating of the sequences indicates that significant amounts of time are represented by the interpreted bounding surfaces.

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Visualize time and motion

A powerful visual way of highlighting changes through time in sedimentary basins is through one-dimensional burial history plots. These can be hand drawn, although there are many commercial packages that use sediment thickness data, sediment and surface ages, and knowledge (or guestimates) of uplift amounts. The resulting models trace the movement of sedimentary units or events in basins through geological time. Two models are located on the line. Such burial models illustrate the remarkable variation in sediment thickness, erosion, and uplift across the section — in a quantitative time framework. Note the varying amount of uplift , and consequential time gaps in the section as illustrated by the two models. Next time you present a geological interpretation, supported by maps and sections in space, ask yourself how you can also traverse geological time and what vertical movements have taken place. Don’t take time or motion for granted.


References

Aubry, M P (2009). Thinking of deep time. Stratigraphy 6, 93–99.

Holmes, A (1913). The Age of the Earth. Harper, London. 228 p.

Lewis, C (2000). The Dating Game: One Man’s Search for the Age of the Earth, Cambridge University Press. 216 p.

Where did the data come from?

What I learned as a geophysicist wannabe

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