Another gap in the Canyons record

We now know that there is a huge time gap between the top of the Chuar Group (the upper part of the Grand Canyon Supergroup) and the overlying sediments that were laid down in Cambrian times. Like the underlying and older Unkar sediment, the Chuar strata were also laid down in shallow seas, with sediments ranging from muds and sandstones to stromatolitic carbonates (originally calcium carbonate limestone, but subsequently altered to a magnesium carbonate rock called dolostone). There are frequent signs of exposure of the sediment surface to the air such as mudcracks, but red beds are less common than in the Unkar strata. For continued shallow-water sedimentation like this, the whole basin of deposition has to have been slowly sinking at roughly the same rate as it was being filled with sediment.

Several other features, such as the presence of pull-apart faults that developed during sedimentation, show that the basin of sedimentation was actively sagging and stretching. This can only happen when the underlying, basement rocks are being pulled apart on a regional or larger scale. Furthermore, the uppermost 60 m of Grand Canyon Supergroup sediments are made up of a dramatic sequence of red beds with angular pebble breccias (called the Sixty-mile Formation). They reflect exposure at the surface and terrestrial conditions, with some substantial uplift and erosion related to block faulting, which is also typical of extensional basins.

Some experts argue that this sedimentary record of deposition on a newly formed western continental margin to Laurentia is the handiwork of the big extensional event that signalled the break-up of Rodinia. The possibility is supported by paleomagnetic evidence showing that palaeopoles for Laurentia began to diverge from those of East Gondwana (Australia-Antarctica-India) after about 720 million years ago.

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