The great Cambrian unconformity

My first field trip from the Geology Department at the University of Birmingham in autumn 1964 was located within hooter distance of the giant British Leyland car plant at Longbridge. It involved a rubbish-filled linear quarry behind a row of shops on the main road through south Birmingham. Not very prepossessing but it clearly exposed a white quartzite, which we were told was a beach deposit laid down by a massive marine transgression at the start of the Cambrian. An hour later we were shown an equally grim exposure of weathered volcanic rocks in the Lickey Hills; they were a sort of purple brown, and said to be Precambrian in age. Not an excellent beginning to a career, but from time to time other Cambrian quartzites sitting unconformably on Precambrian rocks entered our field curriculum: in the West Midlands, Welsh Borders and much further afield in NW Scotland, as it transpired on what had been two separate continental masses of Avalonia and Laurentia. This had possibly been a global marine transgression.

In North America, then the Laurentian continent, what John Wesley Powell dubbed the Great Unconformity in the Grand Canyon has as its counterpart to the Lickey Quartzite the thrillingly named Tonto Group of the Lower Cambrian resting on the Vishnu Schists that are more than a billion years older. Part of the Sauk Sequence, the Tonto Group is, sadly, not accompanied by the Lone Ranger Group, but the Cambrian marine transgression crops out across the continent. In fact it was a phenomenon common to all the modern continents. Global sea level rose relative to the freeboard of the continents then existing. A recent study has established the timing for the Great Unconformity in the Grand Canyon by dating detrital zircons above and below the unconformity (Karlstrom, K, et al. 2018. Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience, v. 11, p. 438-443; doi:10.1038/s41561-018-0131-7). Rather than starting at the outset of the Cambria at 542 Ma, the marine transgression was a protracted affair that began around 527 Ma with flooding reaching a maximum at the end of the Cambrian.

Extensive flooding of the continents at the end of the Cambrian (credit: Ron Blakey , Colorado Plateau Geosystems)

It seems most likely that the associated global rise in sea level relative to the continents was a response to the break-up of the Rodinia supercontinent by considerable sea-floor spreading. The young ocean floor, having yet to cool to an equilibrium temperature, would have had reduced density so that the average depth of the ocean basins decreased, thereby flooding the continents. The creation of vast shallow seas across the continents has been suggested to have been a major factor in the explosive evolution of Cambrian shelly faunas, partly by expanding the range of ecological niches and partly due to increased release of calcium ions to to seawater as a result of chemical weathering.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

New Feature: Picture of the month

Having belatedly discovered The Earth Science Picture of the Day website (it has been going since September 2000; as long as Earth Pages!) I thought readers of EPN might like the aesthetic boost that it provides. So, on the last day of the month I intend to insert a link to what I think is the best of those contributed to EPOD over the previous 4 weeks or so.

The Great Unconformity of the Grand Canyon (credit: Stan Celestian
The Great Unconformity of the Grand Canyon (credit: Stan Celestian)

EPOD has a vast archive of contributions and each one has a brief description and links to other visual resources.

The Grand Greenland Canyon

One of the properties of radar is that it can pass through hundreds of metres of ice to be scattered by the bedrock beneath and return to the surface with sufficient remaining power to allow measurement of ice depth from the time between transmission of a pulse and that when the scattered energy returns to the antenna. Liquid water simply absorbs the radar energy preventing any return from the subsurface. As far as rocks and soils are concerned, any water in them and the structure of minerals from which they are composed limit penetration and energy return to at most only a few metres. While radar images that result from scattering by the Earth’s solid surface are highly informative about landforms and variations in the surface’s small-scale texture, outside of seismic reflection profiling, only ice-penetrating radar (IPR) approaches the ‘holy grail’ of mapping what lies beneath the surface in 3-D. Unlike seismic surveys it can be achieved from aircraft and is far cheaper to conduct.

English: Topographic map of Greenland bedrock,...
Greenland’s topography without the ice sheet. (Photo credit: Wikipedia)

It was IPR that revealed the scattering of large lakes at the base of the Antarctic ice cap, but a survey of Greenland has revealed something even more astonishing: major drainage systems. These include a vast canyon that meanders beneath the thickest part of the ice towards the island’s north coast (Bamber, J.L. et al. 2013. Palaeofluvial mega-canyon beneath the central Greenland ice sheet. Science, v. 341, p. 997-999). At 750 km long and a maximum depth of 800 m it is comparable with active canyon systems along the Colorado and Nile rivers in the western US and Ethiopia respectively. A less-well publicised feature is ancient leaf-shaped system of buried valleys further south that emerges in a great embayment on West Greenland’s coast near Uummannaq, which may be the catchment of another former river system. In fact much of the data that revealed what appears to be pre-glacial topography dates back to the 1970s, though most was acquired since 2000. The coverage by flight lines varies a great deal, and as more flights are conducted, yet more detail will emerge.

The British, Canadian and Italian discoverers consider that glacial meltwater sinking to the base of the ice cap continues to follow the canyon, perhaps lubricating ice movement. The flatter topography beneath the Antarctic ice cap is not so easy to drain, which probably accounts for the many sub-glacial lakes there whereas none of any significance have been detected in Greenland. The earliest time when Greenland became ice-bound was about 5 Ma ago, so that is the minimum age for the river erosion that carved the canyon

Grand Canyon now the Grand Old Canyon?

Grand Canyon in Winter
Grand Canyon in Winter (credit: Wikipedia)

Among the best known and certainly the most visited topographic feature on the planet, the Grand Canyon resulted from erosion by the Colorado River keeping pace with uplift of the south-central United States. It is the archetype for what is known as antecedent drainage. Since that uplift is still going on, albeit slowly, the Grand Canyon has been assumed to be a relative young landform. By dating the first appearance of debris from the eastern end of the canyon in sediments at its western limit geomorphologists estimated that incision began around 6 Ma ago. Yet a range of other observations present puzzling contradictions. One means of settling the issue is to somehow to date the uplift radiometrically.

A long-used technique is to determine ‘cooling ages’ of crustal rocks exposed by uplift and erosion, exploiting the way in which rock temperature determines whether or not products of radioactive decay cab be preserved intact. One method uses the tracks of defects produced by electrons or helium nuclei from radioactive decay as they pass through various minerals that incorporate high amounts of elements such as uranium. Above a certain temperature the fission tracks anneal and disappear quickly, while below it they accumulate over time. Quantifying that build-up allows the date of cooling below the threshold temperature to be estimated. Similarly, gases produced by radioactive decay of some radioactive isotopes, such as argon from the decay of 40K or helium from uranium and thorium isotopes, can only stay in their host mineral if it remains cooler than a narrow range of temperatures. As rock rises towards the Earth’s surface, it starts out hot at depth but cools by conduction as it get closer to the surface. For the 1.8 km of uplift of the Grand Canyon and the relatively cool nature of the underlying crust, neither the fission-track nor the  40Ar/39Ar cooling-age methods give meaningful results. However, minerals lose helium at temperatures above about 70°C, so a method based on helium accumulation from uranium and thorium isotope decay is a possible means of assessing uplift timing. But there have been plenty of snags to overcome to make this approach reliable. In the case of the Grand Canyon analytical quality and careful sample collection has given a credible result (Flowers, R.M. & Farley, K.A. 2012. Apatite 4He/3He and (U-Th)He evidence for an ancient Grand Canyon. Science , doi 10.1126/science.1229390)

Flowers and Farley from the University of Colorado at Boulder and the California Institute of Technology, Pasadena, respectively, produced a result that completely overturns previous conceptions. The western end of the Canyon had been incised to within a few hundred metres of modern depths by 70 Ma ago; more than ten times earlier than previously thought. The eastern end has a more complex history that reveals cooling events in the Neogene as well as an end-Cretaceous initiation of uplift and erosion. Their data are consistent with early incision of the Grand Canyon by a Cretaceous river flowing eastward from the Western Cordillera, with a reversal of flow in the late-Tertiary as uplift of the Colorado Plateau began and western mountains subsided. Whether or not this fits with Cretaceous and later geological history of the SW US, is beyond my ken, but you can bet there will be a storm of comment from US geomorphologists once the paper appears in the print issue of Science.