Bryce Canyon Pulses of Sedimentation

Outcrops of sediments younger than -270 Ma have been largely eroded from the area of the Grand Canyon but fortunately are still abundantly exposed in the adjacent Grand Staircase leading up to Bryce Canyon¹². The clear geological link between the Grand and Bryce Canyon regions is shown in Figure 4 of the previous post (repeated here for convenience). These exposed sediments of the Grand Staircase are not just a spectacular tourist attraction for their rich, colourful, terraces but provide a continuing valuable record of geological activity for a further 230 Ma. 

Fig 4. Cross section illustrating the link between the sedimentary deposits of the Grand Canyon (right side) and the Grand Staircase leading up to Bryce Canyon (left side). It graphically illustrates the km scale uplifts that have occurred in the past -270Ma and the erosion of the post -270Ma sediments from the Grand Canyon area.

Exhumation and any erosion at circa -270 Ma was clearly short lived and submergence at around -260 Ma saw the start of deposition of early Mesozoic mega-sequences until sometime between -165 Ma and -140 Ma as shown in Fig 6. From -165 Ma there is a gap of 25 Ma during which both sedimentation and sub- aerial erosion will likely have occurred – the extent and timing of each being uncertain. But what is very clear is that from around -140 Ma the region commenced another extended period of subsidence coupled with the deposition of deep sedimentary sequences of late Mesozoic age that now make up the top 1 km of the exposed Grand Staircase. These additional cycles of subsidence, sedimentation, uplift and erosion are summarised in Fig 7 and 8. 

     

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

Fig 7 From left to right shows (a) an eroded Supai Group subsided beneath average mean sea level and having from -265 Ma sediments of the Grand Stair Group (early Mesozoic) deposited until (b) at least -165 Ma and (c) some unknown time prior to -140 Ma before (d) uplift and (e) erosion continuing until a peneplain was formed having youngest exposed sediments -165 Ma (f).

                 

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

Fig 8 From left to right shows (a) an eroded early Mesozoic Grand Stair Group subsided beneath average mean sea level and having from -140 Ma further sediments of the Grand Stair Group deposited (b) until at least -40 Ma and some unknown time prior to present (c) before (d) uplift and  (e) erosion continuing to form the highest outcrops of the Grand Staircase (f) with youngest exposed sediments -40 Ma. 

At some indeterminate time during the past 40 Ma the whole region was uplifted by at least 3.2 km in the area of Bryce Canyon and very possibly more in the region of the Grand Canyon where the matching -270 Ma strata are currently some 2.2 km above the equivalent strata beneath Bryce Canyon. That marine sediments laid down within the past 40 Ma in the Grand Canyon area have risen by perhaps as much as 5.4 km and had possibly in excess of 2.2km of post -270 Ma sediments ground away seems highly likely given the stratigraphic record evidence exhibited in Fig 4.  

These massive cycles of subsidence, sedimentation, uplift and erosion within the Bryce Canyon are summarised in the schematic sedimentary columns of Fig  7 and 8.

It is worth stressing that to explain what occurred between each of the unconformities exposed within the rocks of the Colorado Plateau, namely kilometre scale, cyclic, rises and falls of continental and oceanic crust, is exactly the challenge laid down to the attendees of the Geological Society meeting in 2017 - referred to in the recent post of 1st August 2024. But the challenge is even greater than that made to this meeting. Since what is so clear from the above description of the rocks exposed within the Colorado Plateau is the need to also provide plausible explanations for these largely unresolved observations of this same behaviour having occurred in a cyclic fashion over at least the past 1.6 Ba - and likely even longer. Moreover, with very similar cycles recorded to have taken place synchronously over very widely dispersed spatial domains the challenge to this meeting is even more formidible. 

Further comments and suggestions relating to these challenges will be addressed in the next few posts. These will also attempt to demonstrate that very long term climate cycles, driven by the solar systems interaction with our galaxy, and the influence of these climate cycles on the global distribution of ice and water could possibly provided part of an explanation for this recorded behaviour.