Precambrian To Devonian

The youngest strata of the Precambrian sequence in the Canyon are dramatic, often red-coloured coarse boulder conglomerates, breccias and alluvial sediments (called the Sixty-mile Formation). Among the jumble of boulders, some individual blocks are tens of metres in size and are comprised of huge chunks of strata from older parts of the succession. Evidently something dramatic went on in late Precambrian time when these rocks were being formed. Geologists who have studied these rocks in detail reckon that earthquakes must have frequently shaken the region as repeated fault movements uplifted landscapes a metre or so at a time over thousands of years. The landscape was broken up as fault cliffs rose up with steep and unstable slopes. Every now and again a cliff would collapse, with huge blocks crashing and sliding downslope. Slower but equally powerful changes were made as whole masses of rock debris, mobilised by frequent earth tremors, crept and slumped downslope over distances of kilometres.

The dating of these events is not well constrained, but it was around 700 million years ago. Since these inhospitable landscapes did not support any life at the time, there are no fossils to help us match deposits across the region or give them a relative age.

Above the Sixty-mile Formation lies a widespread sequence of marine sandstones (the Tapeats Sandstone) and shales (the Bright Angel Shale). In places, these do

Map Counties

A drawing from a photograph illustrating the unconformity between the Cambrian age Tapeats sandstone and the underlying tilted Precambrian strata.

contain fossils that signify an early Cambrian age, around some 520 million years old. As we have seen, the timing of the beginning of Cambrian times is known now to be close to 542 million years ago. So there is a very large time gap in the Grand Canyon succession between the last 700-million-year-old Precambrian deposits to be preserved and the earliest Cambrian ones, which were laid down on top of them some 180 million years later. Why should this be and what was going on in the interval?

John Wesley Powell was struck by the way the strata in the bottom of the Canyon are tilted and truncated, as if planed off by some immense invisible force before having Palaeozoic strata 'spread over their upturned edges'. This striking feature is a magnificent example of what geologists call an angular unconformity. Powell was aware that such structures have great geological significance and he had a pretty good idea of what that significance was.

Over 80 years before Powell first clapped eyes on the Canyon, the sight of a similar rock formation at Siccar Point on the east coast of Scotland provided a kind of epiphany for the mathematician John Playfair. In 1788, the sight of these wave-swept rocks produced such an effect on Playfair and his geological guide James Hutton that Playfair wrote how 'the impression made will not easily be forgotten' with 'the palpable evidence presented to us, of one of the most extraordinary and important facts in the natural history of the earth'. What was it about these rocks that so astonished them?

The site is certainly attractive in a bleak sort of way on a wild stormy day, but by no stretch of the imagination does it compare with the Grand Canyon. And yet the influence of Siccar Point on our understanding of the Earth's history has arguably been just as great, if not more so. Playfair explains:

we felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited ... from the waters of a superincumbent ocean. An epocha still more remote presented itself, when the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immeasurable force which has burst asunder the solid pavement of the globe.

They were looking at an angular unconformity that stimulated Playfair to further reflect that 'the mind seemed to grow giddy by looking so far into the abyss of time'. It was Hutton's exploration and explanation of the rocks that generated these famous words. And it was Hutton who was instrumental in unshackling our view of Earth history from biblical constraints.

By the time Powell encountered the same phenomenon in the depths of the Canyon, the received wisdom of the time suggested that such an unconformity implied that the Precambrian strata were first folded by immense compressive tectonic pressures. The large-scale compression also elevated the rock strata to form landscapes that were subject to long-term weathering and erosion. Over many millions of years, the rocks were worn away and reduced to form new sedimentary debris. This in turn was carried away by wind and water and dumped in the encroaching ocean, which lay not too far away to the west. The hills and valleys of the landscape were gradually reduced to sea level and eventually the sea returned and lay down a new succession of marine deposits. However, we now know that there are important differences in the developing story.

Critically, the main tectonic event here was not compressive but extensional; that is, stretching of the crust on a regional scale that is linked to the break-up of the supercontinent of Rodinia between 750 and 550 million years ago. It was this stretching motion that led to extensive faulting, as most rocks are brittle and do not easily stretch; although it has to be said that rocks that have been heated within the Earth do become plastic enough to stretch. Some sediments that are still soft and have not been lithified into brittle rocks can also be stretched. By around 550 million years ago a new seaway was established between Laurentia and the ancestral Pacific Ocean, a seaway formed as global sea levels rose and flooded onto the deeply eroded landscapes of the Precambrian.

The original recognition of Cambrian Earth Time, as we have seen, had a troubled history in mid-nineteenth-century Britain, but by the time Walcott arrived in the Grand Canyon, the Cambrian System was firmly re-established and acknowledged as this first period of the Palaeozoic Era by geologists around the world.

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