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.   

Grand Canyon Pulses of Sedimentation (3):

 Deposition of the Tonto Group ceased sometime between -505 Ma and -385 Ma. At some unknown time prior to -385 Ma subsidence and oceanic transgression occurred prior to the start of another pulse of deposition commencing at -385 Ma. This deposition of the Paleozoic sequences referred to as the Supie Group continued for at least another 115 Ma, to have produced the sediments of age -270 Ma corresponding to the upper most sediments at the lip of the Grand Canyon. What then happened must have followed a similar pattern to that of the underlying Tonto Group, which would imply the behaviour depicted in Fig 5a-f. However, the situation with the upper most group in the area of the Grand Canyon is a little more complex. Moreover, to understand this complexity it is necessary to follow the uppermost strata of the Grand Canyon, known as the Kaibab Formation, across into the adjacent area leading up via the Grand Staircase to the Bryce Canyon.

Fig 4 reproduces a schematic cross-section of the surface morphology and the underlying sedimentary patterns from the Bryce Canyon, shown up at the top left, down through the long wavelength folding of the Grand Staircase and back up to the Grand Canyon at the right. The brown layer shown at the top of the Grand Canyon is the Kaibab Formation, which above the Grand Canyon is some 2.5km higher than the same strata beneath the Bryce Canyon area. Given that this sedimentary strata, and all the others, would have been horizontal when laid down beneath the primordial ocean, it is very clear that this region has undergone serious differential vertical tectonic uplift over the past 270 Ma - in all probability much more recently. As shown at the North (left side) boundary there has been some considerable vertical faulting associated with this uplift. And on the reasonable assumption that the deposition of the strata above the Kaibab Formation had also occurred above what is now the Grand Canyon it is clear there has been substantial erosion of at least 2.5km of these same sediments above the Grand Canyon. In all probability this post 270 Ma uplift of the Grand Canyon area relative to that of the Bryce Canyon area resulted from the massive erosion of these post -270 Ma sediments. The layer cake type of erosion that has been responsible for the development of this surface morphology over the Grand Staircase, showing a series of discrete cliff formations (and hence the designation staircase) are reminiscent of what would be produced by time separated periods of massive glacial ice-sheet erosion. However, the doming between the Chocolate cliffs up to the rim of the Grand Canyon must post-date the last pulse of glacial erosion, if indeed that is what caused the 2.5km exhumation of the early Mesozoic Formation that would have been above the Grand Canyon. Indeed, it is almost certain that this doming occured within the past 40 Ma - the age of the youngest rock strata at the top of the Grand Staircase - and can be explained by the isostatic uplift resulting from the erosion of the post -270 Ma sequences above the Grand Canyon area. Some of this doming may also have resulted from the isostatic rebound following the loss of the final overlying ice sheets during an earlier glacial period, reaching down as far as the latitude of the Colorado Plateau, or an isostatic adjustment associated with the much later carving out of the Canyon by the fluvial erosion from the Colorado River – perhaps a bit of all these factors. But whatever the cause there is no doubt that the geology of this region is quite spectacular which makes one in awe of the processes that shaped it and of course the rest of our Earth – especially when it is recognised that what has been exhumed in this region has been happening pretty much everywhere else albeit perhaps not quite so dramatically exposed.    

Fig 4 From left to right shows the surface topology of the roughly 200 mile region between the Bryce Canyon at the top left down through the Grand Staircase and back up to the Grand Canyon at the right. The brown sediment layer at the rim of the Grand Canyon represents the Kaibab Formation dating from-270 Ma. Note the exposed post deposition long wave distortions of the Kaibab Formation, which above the Grand Canyon lies at around 2.7km amsl dipping down to around 0.3km amsl beneath the Brian Head and Bryce Canyon.     

The patterns of the Tonto Group emerging and suffering subaerial erosion back to the youngest upper sediments of age -505 Ma were covered in the previous post. What is clear from the existing sedimentary record is that at -385 Ma a regional subidence to below amsl had accured to see the commencement of the deposition of the Paleozoic Supai Group, Fig 5a, continuing deposition accompanying continuing subsidence, Fig 5c, followed by uplift, Fig 5d, erosion, Fig 5e, back to what would have been a peneplain at the top of the extant Tonto Group dating from -525Ma shown at Fig 5f.  At some time prior to -385 Ma the region underwent a further subsidence to beneath sea level to commence deposition of the late Paleozoic Supai Group, Fig 5a-b, which with continuing subsidence and deposition lasted until at least -270 Ma and possibly some unknown time prior to -265 Ma, Fig 5c,  before again experiencing regional uplift, and possibly erosion back to -270 Ma. These cycles of subsidence, sedimentation, uplift and erosion within the Grand Canyon are summarised in the schematic sedimentary columns of Fig 5.

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

 Fig 5 From left to right shows (a) an eroded Tonto group subsiding beneath average mean sea level (amsl) and (b) having from -385 Ma sediments of the Grand Stair Group (yellow) deposited until at least -270 Ma and (c) some unknown time prior to -260 Ma (light hyellow) before  (d) uplift and erosion (e) continuing to produce a peneplain (f) with youngest exposed sediments -270 Ma in the area of the Grand Canyon.

To continue our analysis of the geological processes that have occurred after -270 Ma, it is necessary to turn our attention to the nature of the sequences below Brian Head and up to the Bryce Canyon. This will be the aim of the next few posts.

 

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.    

 

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. 

Why choose the Colorado Plateau:

 

Evidence of kilometre scale uplift and subsidence at locations remote from any recognised plate boundaries, the existence of mega-sequences of post-rift marine sediments over widespread intra-cratonic areas, and the consideration that pulses of deposition display a clear periodicity and synchronicity over widely dispersed spatial domains, remain largely unresolved issues within current geological theory. One of the finest demonstrations of the repetitive cycles of deposition, uplift, erosion, and again subsidence can be found within the Colorado Plateau. Due to massive uplift experienced over the past 30 Ma or so, followed by the deep incision of the Colorado River and its tributaries over the past 10Ma, rich layers of sedimentary formations going back at least 1,500 Ma have been revealed. Since these deep sediment formations exposed within the cliffs of the Grand and adjacent canyons display clear cycles of continuous deposition separated by at times massive periods of missing time, during which there was either no deposition or whatever deposition might have occurred has been ground away, they provide an important repository of geological evidence against which any theory purporting to explain these cycles of uplift and subsidence should be gauged.   

 

Grand Canyon showing clearly the remarkably horizontal strata including 

the peneplaination at the upper surface 

With this in mind, the following will reappraise the sedimentary records exposed within the Grand Canyon which, as Sloss^5,6,7 repeatedly pointed out, display clear cycles of deposition and non-deposition reflecting kilometre scale cycles of crustal rise and fall. These cycles of deposition and non-deposition and their associated vertical motions still require explanation. This reappraisal of the phasing of the cycles of subsidence and upheaval, burial and exhumation, will in later posts be related in time with the ice- and hot-house cycles of climate that have occurred at periodicities of circa 130 Ma over at least the Phanerozoic (-540 Ma to present)^8,9. Because any sedimentary records within the Grand Canyon^10,11 have been lost from the early Mesozoic (-270 Ma to present) the evidence from the contiguous region of the Grand Staircase leading up to Bryce Canyon will be used to complete the analysis up to at least -40 Ma.

Paria Badlands Western Grand Staircase (photo by Tim Peterson)

A further reason for choosing the Colorado Plateau is that the extensive studies reported by Sloss^5,6,7 have shown the phasing of the periods of deposition and non-deposition at this location, show strong correlations with the records over widely dispersed regions of the USA. And it might be added with extensive regions on other continents.   

 Cross section illustrating the link between the sedimentary deposits of the Grand Canyon and the Grand Staircase leading up to Bryce Canyon. It graphically illustrates the km scale uplifts that have occurred in the past 40Ma and the erosion of the post -270Ma sediments from the Grand Canyon area.

 

A Recent Challenge for Plate Tectonics

Over October 2017 a meeting of very distinguished geologists, all having strong associations with the development of plate tectonics (PT), was held at the Geological Society London. Its purpose was to celebrate 50 years of PT achievement¹. Not unexpectedly, this meeting spent most of its time in a form of congratulatory mode highlighting the considerable achievements that had been made over this 50 years - in many cases by the attendees in the room. However, one of the attendees was brave enough to raise what appears to be a very serious challenge for PT, and one which was not wildly out of line with the issues that this blog was addressing back in 2016.

The nature of this challenge was to ask the attendees at this meeting to provide plausible explanations for the largely unresolved observations of kilometre scale, cyclic, rises and falls of continental and oceanic crust². The meeting was reminded that these kilometre scale cycles of burial and exhumation exhibit synchronicity over wide spatial domains and often occur in regions where PT models suggest passive tectonic activity³. That such motions occur and so often result in major unconformities in the sedimentary sequences were the cornerstones of James Hutton’s awakening of the field of geology⁴ in the closing decades of the 18^th C. And yet, even with the undoubted advances in the field of geological science over the past 50 years, the noticeable silence on this challenge from delegates suggested there remains no satisfactory explanation for these fundamental processes.

Having come across this meeting report sometime in 2019 encouraged me to return to my own concerns relating to this very issue, with the result that a short paper was written and just happened to be ready for submission when UCL launched a new open environment publication venture. The submission was entitled “Phanerozoic Climate and Vertical Tectonic Cycles” and as it transpired turned out to be the very first paper to be submitted to this new publishing outlet. The next few blog posts will be concerned with providing a slightly amended version of this partially published paper - I say partly published due to the referees not being happy with aspects of the paper, so although it appears somewhere in the online files it was not actually put forward for final publication*. This paper took as an important test of the suggested mechanism for vertical tectonics, outlined in earlier posts, the evidence so explicitly revealed in the geological records of the Colorado Plateau. So the next few postings will first summarising the observational evidence from the Grand and Bryce Canyon areas that demonstrate so clearly the cyclic nature of the periods when this massive area of the N American craton was beneath the ocean having deep sediments buried and other periods when it was back above sea level and subject to erosive actions. This evidence will allow identification of the unconformities in the sedimentary sequences where there is considerable missing time in the sedimentary and paleontological records. It will be indicated how, although the exact timing of uplift and erosion associated with these major unconformities are difficult to assess, the age of sediments immediately above provide vital temporal markers for the onset of subsidence and/or associated sea level rise. By reconsidering the much studied sedimentary sequences of the Grand and Bryce Canyon areas the next few posts will attempt to show that the at least over the Phanerozoic eon (the past 540Ma) the initiation of new pulses of deposition seem to occur at times when earth climate is emerging from ice-house to hot-house** conditions. Furthermore, it will be indicated that the recorded periods in which global occurrences of epeirogeny have occurred appear to correlate closely with the end of hot-house periods and the onset of ice-house global climate conditions. Finally, the tentative thermo-geodynamic explanations for this apparent causal link between global climate and vertical tectonics, briefly covered in earlier posts, will be further elaborated. While it might reasonably be claimed that any correspondence between vertical tectonics and long term climate on the Colorado Plateau is hardly sufficient to claim a generic such relationship, it is worth keeping in mind, and evidence will be provided for this, that such kilometre scale cycles of burial and exhumation at this particular location of recognised^6,7 passive tectonic activity are well documented as exhibiting synchronicity over very wide spatial domains across N America. 

 * https://journals.uclpress.co.uk/ucloe/plugins/isolinear/article/1979/version/1/

** In earlier posts I used the term "ice-age" to indicate the very long periods (many 10's Ma) during which other climate drivers produce alternating periods of "glacials" (measured in 10's kA) when ice and permafrost are features of Earth's the mid- to high- latitudes and other periods "interglacials" (also measured in 10's kA) when there are ice sheets and permafrost in only very high latitudes. Because the term "ice-age" is commonly used to refer to "glacials" I will in the future try to avoid the confusion and use the term "ice-house" in reference to these very long term ice prevalent periods. And to preserve some symmetry in terminology, the very long periods when Earth is pretty much free of ice and permafrost (again lasting for many 10's Ma) I will adopt the commonly used label "hot-house". We are currently living in an interglacial within an ice-house period.    

 

It's been a long break!

 It is now 8 years since my last burst of blogging energy during a relaxing sojourn spent in Tuscany. In the meantime I have had occasion to return to one of the little problems that exercised much of my research while at UCL - the buckling of thin shells and especially those used for the construction of space launch vehicles. But I will for the moment, spare details of this area since they represent a fairly major break from the themes I have been developing in this blog.

Sadly, having been inactive for so long, I was unable to find out how to add new posts - partly because I could not recall my password! I have now rectified this and will attempt to report some more recent brushes against the joys of peer review in relation to aspects of the work hinted at in earlier postings.

This relates to the ideas being advanced in the posts of 2016 where it was suggested that the ups and downs of the Earth's lithosphere over geological timescales might relate to the very long term cycles of climate (those having 10s to 100 million year periodicities). A study of the evidence on display in the Grand and Bryce Canyons seemed to provide some substance to these ideas. But alas, for reasons that I will recount in the next few posts, full publication of the analysis reported in a paper entitled  “Phanerozoic Climate and Vertical Tectonic Cycles” (UCL Open Environment 2019) was eventually blocked. But in getting involved in this study with fascinating research on climate cycles over the past 600Ma has brought me face to face with one of the science problems currently exercising the globe - climate change and whether CO2 is such a guilty culprit to require the current global efforts to reach net zero carbon emissions. Hopefully, later posts will try to address some of the very legitimate concerns about the state of the science being used to inform important global and local policy decisions in this area.