Grand Canyon Pulses of Sedimentation (2):

It is almost impossible to comprehend the enormity of the Earth processes required to have created the geology left within the traces of the Grand Canyon Super Group at the base of the canyon. And it is difficult to fully contemplate what might have happened in the intervening 215 Ma before the events that led to the deposition of the Tonto Group from around -525 Ma. It is though clear, that at some time prior to -525 Ma this region had experienced further sub-aerial erosion, as picked up in Fig 3a from the Fig 2f of the previous post, and witnessed the Grand Canyon Super Group ground down to the essentially horizontal, peneplain, surface indicated in Fig 3a.

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

Fig 3: From left to right shows (a) an eroded Grand Canyon Supergroup (b) subsiding beneath amsl and having from -525 Ma sediments of the Tonto group deposited until (b) at least -505 Ma and (c) some unknown time prior to -385 Ma before (d) uplift and (e) erosion continuing (f) to a peneplain having youngest exposed sediments -505 Ma.

At around -525 Ma the region must have experienced a further subsidence to below amsl to see the Tonto Group commencing deposition at -525 Ma, Fig 3b. Having explained the interpretation of these representations of the sedimentary columns in the previous post, I will try to be a little more succinct in the description of subsequent pulses in deposition. However, it is important to recognise that as the sedimentary deposits built up as shown in Fig 3b-c the oceanic lithosphere must have been experiencing a continuation of subsidence to accommodate the accumulating depth of sediments. Since without continuing subsidence the sediments would eventually have built-up to a level where they would no longer be beneath amsl - and further sedimentation would have been impossible. How could this continuing subsidence be possible? Clearly, any model purporting to explain these cycles of vertical tectonic motions would also need to be capable of accounting for this behaviour.

But to continue with the analysis of the Tonto Group, another pulse of substantial uplift, as shown in Fig 3d, and a long period erosion Fig 3e would see much of the previous sediment build up reduced to the level shown in Fig 3f  in which the upper most rocks of the Tonto Group would be those deposited at -505 Ma, just prior (in geological speak) to yet another cycle of subsidence.