Grand Canyon Pulses of Sedimentation (1):

Figure 1 provides a commonly accepted summary of the sedimentary sequences exposed within the Grand Canyon^10,11

                                                      (a)                                                                                                              (b)

Fig 1 Typical chronology of sedimentary sequences in (a) the Grand Canyon area [comprising Visnu Schists (dark green), Grand Canyon Super Group (red), Tonto Group (blue), Grand Stair Group Paleozoic (yellow)] and the contiguous (b) Bryce Canyon region [Grand Stair Group Paleozoic (yellow), Grand Stair Group early Mesozoic (brown), Grand Stair Group late Mesozoic (green)], with indicative elevations above current mean sea level.

As observed by Sloss^5,6,7 and others these sequences are separated by unconformities at which in some cases very substantial time gaps exist between adjacent sedimentary layers (here existing at locations of colour changes). For example, the youngest rocks at the top of the underlying Vishnu Schists (dark green) date from around -1600 Ma. These are overlaid by the "Grand Canyon Super Group" (red) which commenced deposition at around -1100 Ma with the extent of any further cycles of uplift, erosion and burial over the intervening 500 Ma being unknown. As a brief summary of what must have occurred over the period from around -1600 Ma to -1100 Ma, Fig 2 shows a sequence of inferred time snapshots of a typical vertical column of the upper lithosphere - these show:

(a Fig 2a   the ancient Visnu Schists (yellow), which must have once been very deeply buried beneath average mean sea level (amsl) to have experienced the sort of metamorphic changes they exhibit. Having experienced a regional uplift to well above amsl and then subjected to an unknown amount of sub-aerial erosion to expose what are now its youngest top rocks having an age of -1600 Ma, at some time before -1100 Ma then experience a regional subsidence to below amsl, followed by

(b Fig 2b   the start of a new spurt of deposition at -1100 Ma of the sedimentary beds now referred to as the Grand Canyon Super Group (GCSG) (with the dark red indicating that part of the GCSG up to -740 Ma that still exist today). The 500 Ma of missing time between the Visnu Schists and the GCSG constitutes what is now termed the Great Unconfomity. But what we can also infer from the evidence is that   

(c Fig 2c  continuing very deep sedimentary beds must have been laid down while regional subsidence of the sea bed continued (with the light red indicating what must have been very deep additional sedimentary beds) to result in the

 F(Fig 2d  the extant GCSG (dark red) being buried to a depth sufficient for the geothermal heat to reach levels required to produce the forms of buckling distortions so visible today in the GCSG at the lower reaches of the Grand Canyon. These now missing very deep sediments (light red) could have continued deposition for possibly another 215 Ma. What then happened is of course largely unknown but at sometime before -525 Ma it is clear that

(e Fig2e  another massive regional uplift occurred, thrusting the sedimentary beds well above amsl with subsequent sub-aerial erosion of the GCSG continuing until it reached

(f) Fig 2f the topmost, youngest surviving rocks of the GCSG sediments which have an age of -740 Ma.

What we do not know is the extent of these missing sediments post -740 Ma. It is even possible, and indeed likely, given the possible causes for these epeirogenic burials and exhumations, there may have been further cycles of emergence and subsidence before the start of the deposition of the Tonto Group at around -525 Ma. What it perhaps a little clearer from the now distorted form of the GCSG, suggested in Fig 2d, is there must have been continuous sedimentation for the next 215 Ma. This would have been necessary to allow the GCSG to be buried sufficiently deep, prior to -525 Ma, that a combination of extreme geothermal heat and associated massive pressure would result in the stress levels required to produce the buckles and deformations now evident in the exposures of the GCSG at the base of the Grand Canyon. 

     (a)                   (b)               (c)               (d)               (e)                (f)

Fig 2 From left to right shows (a) an eroded Visnu basement subsiding beneath average mean sea level and having from -1100 Ma sediments of the Grand Canyon Supergroup deposited until (b) at least -740 Ma and (c) some unknown time prior to -525 Ma of depth sufficient to cause (d) tectonic distortion before (e) uplift and (f) erosion back to a peneplain with youngest exposed sediments -740 Ma.

I have laboured the above description of what must have taken place so long ago to produce the rock structures at the base of the Grand Canyon to make the cycle of "burial and exhumation" of the rocks, referred to at the 2017 meeting at the Geological Society, very clear. However, I understand it is quite a lot to take on board, so I will leave it to future posts to continue the forensic analysis of what the geology of the Colorado Plateau really does tell us and why it is so important if we are to be able to fully explain how it all happened.