References in Analysis of Colorado Plateau Geology

1. William Smith Meeting 2017: Plate Tectonics at 50, Geological Society London, 3 October, 2017. https://www.geolsoc.org.uk/wsmith17 
2. Green, P et alia (2017) Kilometre-scale burial and exhumation of passive margins and continental interiors: an overlooked consequence of plate tectonics? Paper presented in session 8, William Smith Meeting 2017: Plate Tectonics at 50, Geological Society London, 3 October, 2017
3. Green, P et alia. 2018 Post-breakup burial and exhumation of passive continental margins:Seven propositions to inform geodynamic models, Gondwana Research, 53 (2018), 58–81.

4. Hutton, J (1795) Theory of the Earth, vol. 1, Edinburgh.
5. Sloss, LL, Krumbein, WC, Dapples, EC (1949) Integrated facies analysis, appearing in Longwell, CR, Sedimentary facies in geological history, GSA Bulletin, 39, 91-124.
6. Sloss, LL (1963) Sequences in the cratonic interior of North America, GSA Bulletin, 74, 93-114.
7. Sloss, LL (1964) Tectonic Cycles of the North American Craton, in Symposium on cyclic sedimentation: Kansas Geological Survey, ed. Merriam, D. F., Bulletin 169, 449-459.
8. Shaviv, N. J. and Veizer, J. (2003): Celestial driver of Phanerozoic climate? GSA Today, July, 4-10.
9. Shaviv, N. J. (2002): The spiral structure of the milky way, cosmic rays, and ice age epochs on Earth, New Astronomy, 8, 39-77.
10. Beus, S. S. and Morales, M. (ed) (2003) Grand Canyon geology, NY: Oxford University Press, 2^nd Edition, see contributions from Beus, S. S., Blakey, R. C. et alia.
11. Timmins, J. M. and Karlstrom, K. E. (ed) (2012) Grand Canyon geology: Two biion years of Earth’s history, GSA Special Paper 489, see contributions from Blakey, R. C., Middleton, L. T., Timmons, J. M., Karlstrom, K. E. et alia.   
12. Sprinkel, D. A., Chidsey, D. C., Anderson, P. B. (ed) (2003) Geology of Utah’s parks and monuments. Utah Geological Association Publication 28, 2^nd Ed., see paper by Sprinkel, D. A.    
13. Croll, J. G. A. (2007): A new hypothesis for Earth lithosphere evolution, New Concepts in Global tectonics, Newsletter, 45, December 34-51.
14. Croll, J G A (2011) Some Comments on Lithosphere Evolution, paper presented at Frontiers Meeting, Geological Society London, 14 November, 2011.
15. Illis, B. (2009): Searching the Paleo Climate record for estimated correlations: temperature CO2 and sea level, see Kalenda et al..  
16. Kalenda, P, Neumann et al. (2011) Tilts, global tectonics and earthquake prediction, research monograph, in press.

And Some Concluding Remarks from Colorado Plateau Geology

A reconstruction of the sedimentation records of the Grand Canyon region reveals a strong correlation between the timing of the recommencement of deposition cycles above major unconformities and the transition from global ice-house climates to hot-house. This has been explained in terms of sea level rises accompanying the melting of surface ice. The massive thermal capacity of oceans, their currents, convective mixing and tidal flows provide a more effective heat transfer mechanism for the outflow of geothermal heat energy. An increased geothermal gradient resulting from the rising geothermal heat flux has the effect of thinning the lithosphere, possibly through a process of phase change, re-magmafication, at the lower lithosphere boundary. Increased average density of the lithosphere column will then result in continuing subsidence as sediments are added from above and lower lithosphere is removed. Furthermore, this circa 130 Ma cycle of pulses in sedimentation, in phase with the onset of hot-house climate conditions, appears to be related to similarly timed pulses of uplift and mountain building observed to coincide with the onset of ice-house periods. I will return to this latter process in subsequent posts. 

With hot- and ice-house conditions known to have had global reach also starts to explain the often-noted synchronicity of burial and exhumation events over widely dispersed geographic domains that are often remote from any plate tectonic recognised active tectonic zones. Additionally, the high levels of temperature change at a given crustal depth, associated with the periodic thickening and thinning of crust, will induce massive cycles of horizontal thermal strain and deformations. Restraint, of these thermally induced strains could in turn account for many of the observed fracture patterns during tension cycles or folding and metamorphism during compression cycles.

While the evidence from the Colorado Plateau cannot be regarded as conclusive, it does appear to support a model in which very long-term climate cycles could be providing an important contribution to the clear geological evidence of ups and downs of both continental and oceanic lithosphere. Could this be at least a partial answer to the challenge laid down at the 2017, William Smith Meeting at the Geological Society London?  

Some Further Comments on what the Colorado Plateau Tells Us

The most recent posts have been based upon the rejected paper submitted to what was a new publication venture by UCL in 2019. This paper attempted to use the fascinating geology exposed within the canyons and the geomorphology of the Colorado Plateau and contiguous areas to assess whether a hypothesised mechanism for explaining the well recognised and often massive rises and falls of continental and oceanic lithosphere might have any relevance. This afterall is an area of geology that seems to be very poorly covered within the current prevailing paradigm of plate tectonics. And as noted at the start of this new burst of blogging activity, these posts, like the paper on which they have been derived, have concentrated on the sedimentary sequences of the Grand and Bryce Canyon areas simply because they are so clearly exposed and thoroughly researched. Furthermore, as the stratigraphic geometry shown in Fig 4 of the 8^th August post seems to demonstrate, the Grand Canyon area in particular has recently, and certainly post hypothesised "breakup", been subject to major uplift and what on the face of it seem to be ice sheet erosions. Earlier sequence stratigraphy also suggests that this area has been subject to repeated cycles of ice- and hot-house climate cycles and consequently what appears to be a direct correlation of tectonic activity and climate. But clearly the massive erosion occurring during the ice-house phases must produce in contiguous areas equally massive accumulations of ice erosion deposits, suggesting a counter-phasing of deposition in areas contiguous with those directly affected by the growth of ice-sheets – and of course vice versa during the erosion of possibly uplifted areas during hot-house climatic conditions.   

From the direct evidence of the Grand Canyon, along with the reported synchronicity over craton-wide and even continent-wide areas noted by Sloss and others, there appears to be strong evidence of a causal link between cycles of uplift and erosion during ice-house global climate, and subsidence and deposition during hot-house climatic conditions. It is therefore surprising that the thermo-geodynamics underpinning these links have not been more thoroughly investigated. If as suggested there is a causal link between at least vertical tectonics and global climate cycles this could significantly add to our ability to explain other important aspects of global tectonics. As opined at the 2017 meeting at the Geological Society of London the current version of the mobilist concepts do not adequately explain the all too clear evidence within the Colorado Plateau of cyclic ups and downs of the lithosphere. Nor do these concepts account for the seemingly strong evidence that these ups and downs seem to occur in a regular cyclic pattern. Changing climatic domains exposed within the sedimentary record are currently accounted for by invoking major latitudinal movements of continents. By looking at Earth climate records it seems clear that these records can be equally accounted for without the need to move continents around quite so much!

Fig 12. Crossection covering the area between Brian Head and the Grand Canyon. 

Horizontal scale 20 times verttical scale to emphasise vertical tectonic motions.  

Fig 12 is a redrawn cross-section linking the relatively high point at Brian Head down through the Grand Staircase to the incision of the Grand Canyon created by the recent (geologically speaking) erosion by the Colorado River. The vertical scale is exaggerated by around 20 fold relative to the horizontal scale to emphasise some of the important tectonic features. First, is the massive regional uplift of the sedimentary sequences covering the 500 Ma from the Cambrian sequences of the Tonto Group (-525 Ma) through to the residual late Mesozoic sequences of the Grand Staircase (-40 Ma). Not shown clearly at the left of the Brian Head is a steeply sloping fault within which the sedimentary sequences beneath Brian Head have been uplifted by around 2 km relative to the matching sequences to the left (name?). Furthermore, at some time after -40 Ma the post Cambrian sequences in the Grand Canyon area have been domed-up by as much as 2.2 km relative to the area beneath Brian Head. Whatever induced these post -40 Ma massive relative motions clearly involved tremendous tectonic forces. It is worth noting that this 4 km depth of sediments would have been laid down horizontally and remained so up to some time after -40 Ma when a process of erosion kicked-in.

Geothermal Mechanism for Epeirogenic Uplift

Previous studies have noted the close link between periods of mountain building with climate cycles^14,15,16. Recorded periods of mountain building¹³ are summarised by the bar charts at the top of Fig 9. The 4 recorded periods of widespread mountain building during the Phanerozoic, often occurring synchronously over widely dispersed geographical domains, are shown to correspond closely with the periods of ice-house climate conditions. Tentative explanations for how ice-house conditions could result in spurts of uplift required for mountain building and subsequent erosion have again focused on how changes in surface disposition of ice and water could influence the rate of geothermal heat loss as expressed by the geothermal flux^13,14. Reductions in the geothermal heat flux caused by the insulating effects of the development of deep surface ice sheets and permafrost would result in a lowering of the geothermal gradient and a concomitant increase in lithosphere thickness brought about by aggradation, due to phase change at the lower lithosphere-mantle boundary. Associated reductions in average density within the lithosphere and mantle could then be expected to result in regional uplift of the crust. This is summarised in Fig 11.

                   (a)                                  (b)                                  (c)      

Fig 11 Mechanism for epeirogenic uplift and sediment erosion based upon (a) the high geothermal gradient due to high heat flux into the overlying ocean during hot-house conditions which with (b) the development of thick ice-sheets and permafrost after transition to ice-house climate reduces the geothermal heat flux which over a few Ma (c) readjusts to a lower geothermal gradient and increased crustal thickness through aggradation of lower crust while crust continues to rise during ice erosion. 

 A variant and perhaps more prevalent situation than that of the above model for epeirogenic uplift and sediment erosion would be as follows. Again starting with the situation of Fig 11a which depicts a high geothermal gradient due to high heat flux into the overlying ocean during hot-house conditions, a lowering of mean sea level (msl) at the onset of an ice-house period might expose the previous sea bed to sub-aerial conditions - the situation of Figure 11b but without the overlying ice sheet. This would be  followed by a long term thickening of the crust due to a lowering of the geothermal heat flux as indicated in Fig 11c - again without the overlying ice sheet but as a result of the reduced heat flow into the overlying atmosphere. This lowered geothermal gradient and associated thickening of the lithosphere over a few Ma Fig 11c would result from phase changes and consequent aggradation of lower crust. 

It might be observed that such a mechanism for the raising of a previous, largely horizontal, seabed helps to explain why so frequently vertical tectonics produces uplifted peneplains. Frequently, as I hope later posts will describe, mountain building results from erosion of such uplifted peneplains.  

 

 

 

Geothermal Mechanism for Epeirogenic Subsidence

 It is possible the following will be a little repetitive of earlier posts. But it is perhaps worth reiterating in the current context.

An explanation as to how the transition from ice-house to hot-house conditions could trigger an extended period of sedimentation has focused on the adjustments to the geothermal flux occurring when low lying continental crust is inundated by rising sea levels as global ice sheets and extensive permafrost melts^13,14. This is summarised in Fig 10 for a situation that might arise at high latitudes. An icesheet over the very long average ice cover during an ice-house period will have resulted in a thickening of the lithosphere associated with a low geothermal flux as depicted in Fig 10a. After ice and permafrost melt, sea level rise and oceanic inundation, Fig 10b, will eventually lead to an increased geothermal heat flux caused by the greater heat transfer capacities of sea water, enhanced by mixing due to tides and currents. Over time this will result in steeper geothermal gradients and a concomitant decrease in crustal thickness brought about by phase change, re-magmafication, at the lower lithosphere-mantle boundary, Fig 10c. This crustal thinning will in turn see an increase in average density within the lithosphere and upper mantle which in turn would be expected to result in regional subsidence. Such a model, involving as it does wasting of crust at the lower lithosphere-mantle boundary, starts to account for how km scale sedimentary sequences can be continuously added from above. As suggested, this requires maintenance of a consistent equilibrium thermal gradient even as sediments are added – a process much more likely given the relatively thin nature of oceanic crust.

To aid visualisation of isostacy these crustal columns are drawn relative to the mean magmatic level (mml) – the free surface of an idealised magmatic fluid.  

                                                 (a)                                                    (b)                                                   (c)

Fig 10 Mechanism for epeirogenic subsidence and sediment accumulation based upon (a) the low geothermal gradient due to insulation effects of overlying ice-sheets and permafrost during ice-house conditions which with (b) rise in mean sea level after transition to hot-house climate floods upper surface of the crust which over a few Ma (c) readjusts to a higher geothermal gradient and decreased crustal thickness through degradation of lower crust allowing accumulation of sediments from above. 

A slight variant of the above postulated mechanism for subsidence, followed by initiation and continuation of sedimentation would be as follows. A very low-lying continental area resulting from extensive sub-aerial erosion, at the tail end of an ice-house period but at a low enough latitude to be not covered with ice sheets, would after the melting of the ice and permafrost at the start of a hot-house period find itself being inundated by sea level rise as in Fig 10b. In this case the low geothermal gradient prior to the inundation would have been the consequence of the low heat loss into the overlying atmosphere. But once started the sedimentation process would again continue while the lithosphere experiences ongoing phase change at the lower lithosphere boundary, as suggested in Fig 10c.         

 

Relating Phases of Deposition to Climate Cycles

Earlier posts, particularly those from 2012 to 2016, have suggested that a possible explanation for the periodic rises and falls of the Earth’s crust (here referred to as the lithosphere to include the gradually more liquid like upper mantle below the more solid upper rock) could relate to the very long-term cycles of Earth climate. This hypothesised relationship is predicated upon the effects of the changes in the disposition of ice, water and the troposphere overlying the lithosphere, and how they affect the rate at which the heat within the Earth’s core and mantle are escaping to the surface – referred to as the geothermal flux. If for example, the lithosphere is overlain with sea water this has the effect of increasing the geothermal flux due to the oceans capacity to suck out and absorb the Earth’s heat. In contrast, an overlying thick ice sheet with its interstitial pockets of water will reduce the geothermal flux by acting a little like the insulating effects provided by a tea cosy keeping the tea in a teapot hot. Or if the lithosphere is overlain with the atmospheric gases, it will experience a low heat flux due to the low thermal capacity of the atmosphere to suck out the Earth’s heat energy.

Because the heat flux is directly proportional to the thermal gradient, and because the temperature of both the mantle and the Earth’s surface are pretty much fixed, a high heat flux will relate to a thin lithosphere while a low heat flux will be associated with a thick lithosphere. In this model these thickness changes are postulated as being made possible through very long terms phase changes at the underside of the lithosphere. And since the more solid phases of lithosphere rock have a lower density than the mantle, a thick lithosphere will have its upper surface floating higher than a thin one. Hence, if very long-term climate cycles have the effect of altering the surface disposition of air, water and ice they may well help to explain how over wide regions the crust can rise to form mountains while being eroded and sink beneath the oceans to have mega sequences of sediments continuously laid down.  

All very interesting, but where is the evidence that this model has any relevance?

As previously suggested the Colorado Plateau provides an incredibly rich record of Earth’s vertical tectonics over at least the last 1.6 Ba and especially over the Phanerozoic eon - the last 540 Ma. As the previous few posts have demonstrated the exposed geological record tells us that this area has been subjected to periodic rises and erosion as well as periodic subsidence and sedimentation. But do these periodicities have any relationship with Earth climate cycles?  

Fig 9 Cycles of average earth surface temperatures over the Phanerozoic showing the correlation between: 1. the onset of ice-house periods and pulses of epeirogenic uplift and mountain building (upper bar charts); and 2. the onset of hot-house periods and pulses of deposition.      

 Fig 9 summarises the pulses in deposition for which the most robust temporal signals of vertical position, relating to the vertical movements of the Earth’s lithosphere, are the commencement of deposition caused by subsidence beneath average mean sea level (amsl) following a hiatus, marked by the existence of unconformities, in which there was either no sedimentation occurring or erosion after uplift has removed the evidence of any sedimentation that had occurred. At the intra-cratonic location of the Grand Canyon, subsidence below sea level, likely combined with indeterminate moderate to large rises in sea level, saw the start of new pulses in sedimentation occurring over the Phanerozoic at -525 Ma, -385 Ma, -265 Ma, and -140 Ma. These ages at which renewed pulses of sedimentation started, immediately above unconformities, are well defined and consequently marked with strong black bars on the lower bar chart of Fig 9. The black bars indicate the periods known to have produced continuous deposition. However, when sedimentation ceased or when erosion started are rather less well defined and so the ending of the deposition is marked with alternating black and yellow pulses indicating either non-deposition or uplift and erosion. 

Also plotted in Fig 9 are the geological reconstructions of the average surface temperatures over this same time period⁸ along with predictions of surface temperatures based upon analysis of the variations in cosmic ray flux⁹ experienced by the solar system. What is noteworthy in these plots are: the close, and possibly causal^8,9, relationships between the intensity of cosmic rays and climate cycles, and; the correlation between the onset of deposition as recorded by strata immediately above recorded major unconformities and their consistent phasing within the climate cycles. In each case, deposition is seen to commence soon (in geological terms) after earth climate emerges from an ice-house period, shown by the black sections of the lower of the upper bars in Fig 9, and enters into a period of hot house (shown perhaps confusingly as blue sections). After a long period of glacial and inter-glacial cycles during the ice-house period, it might be anticipated that ice erosion will have reduced continental land surface elevations in the vicinity of ice sheets to near sea level. This means that moderate rises in average sea levels, due to the full melting of ice sheet and permafrost accompanying the transition from average ice-house climate to hot-house, might be expected to inundate the low continental land surfaces – a clear precondition for the onset of marine sedimentation.

Geophysical Implications for Plate Tectonic Models

 The previous posts have highlighted some rather awkward questions for the current paradigm of Plate Tectonics (PT). The clearly cyclic nature of the regionally extensive ups and downs, experienced well beyond what is now the Colorado Plateau over at least the past 540Ma, does not appear to have an explanation within the current PT modelling. This is especially a problem given the well documented observations that these vertical movements have also been experienced and are largely synchronised with similar vertical movements across much of the N American craton.

Perhaps the most credible PT model for regionally experienced vertical motions of the lithosphere is that of “dynamic topography”. This relies upon temporal and spatial changes in the nature of the convective motions within the underlying lower mantel. To explain just one of the up and down cycles observed over the Colorado Plateau would require a convective motion to be reversed so that a hot up-welling zone, considered in the dynamic topography model to be the cause of uplift, would need to be transformed into a cold down-welling zone within the mantle’s convective dynamics. But this would not seem to be compatible with the observations that whatever is happening in the region of the Colorado Plateau has also been experienced on a craton wide basis. In addition, the consideration that similar ups and downs have been recorded to have occurred synchronously over other continental areas makes it even less likely that a dynamic topography explanation could be the answer. It does not seem feasible within any conceivable convective geometry that an upward welling convective motion could be simultaneously occurring over such widespread areas. It also seems most unlikely that in what appears to be a regular periodicity of the ups and downs the convective motion would experience phased reversals to allow the upwelling to be transformed into a cold down-welling zone – as required by the reversal of the lithosphere motion.

Some Geophysical Observations from the Colorado Plateau

In anticipation of future more general discussions of some issues relating to geophysics, especially in connection with mountain building, it is worth pausing to observe some of the more important features on display within the rocks of the Colorado Plateau. In relation to the incredible unconformity above the Grand Canyon Super Group, in which there is around 225 Ma missing time, it is clear, from the massively folded strata beneath, that this extant portion of the super group was eventually buried by deep sediments. It is also clear that after being exhumed most of these overlying sediments were sub-aerially eroded to create the planated surface upon which the horizontal strata of the overlying Tonto Group were laid down. I say they must have been deeply buried to have allowed the geothermal heat to reach levels required to produce the ductile folding, of otherwise brittle rock, that is so clearly in evidence. Furthermore, the folding indicates that these strata were subjected to massive horizontal compressive strains sufficient to account for the shortening required to produce these ductile folds. All of which suggests the folding may have resulted from the restraint offered to the high levels of horizontal expansion that would have otherwise occurred in these strata when at depth they were subject to such high levels of increased geothermal heating. Prior to the deposition of the Tonto Group it is conceivable there would have been at least one cycle of massive uplift, erosion and subsidence, no longer in evidence, to allow these deeply buried strata to be brought to the surface.

It is perhaps also worth digressing to reflect on similar processes in evidence within so many rocks exposed over major areas of the British Isles. A walk along shorelines in the SW of England, the W coast of Ireland, … will often reveal cliff faces in which ancient rocks display incredibly deformed, sometimes even recumbent, folds that could only have been formed when these rock strata were deeply embedded within the lithosphere. This inference is again possible from the nature of the folds. As shown in the photos below, of a typical cliff faces to the East of Bude, on the N coast of Cornwall, the very tight folds display virtually no sign of brittle fracture. In areas where the high curvatures of these folds would at low temperatures exhibite massive brittle fracture patterns, there is virtually no signs of any brittle failures. Instead, the folds display all the hallmarks of ductile folding. In other words, these folds must have been generated when the rock strata were at extremely high temperatures associated with very deep burial. To continue the analogy with the Great Unconformity of the Grand Canyon, a walk over the uplands around Bude towards Exmoor reveals very low-lying relief typical of a peneplain in the making – fairly flat undulating terrane whose geomorphology is typical of that formed by the action of great ice-sheets – or very mature fluvial erosion. Should this area of SW England experience subsidence, inundation and extensive burial with sediments, it is interesting to speculate whether a geologist, some 100's Ma plus years in the future, might uncover an unconformity not unlike that of the Grand Canyon’s Great Unconformity - or indeed, Hutton's historically important unconformity at Siccar Point in Scotland.      

Some examples of highly folded rocks on display within the eroded cliffs along the shoreline East of Bude, Cornwall. 

Folded rocks underlying overlain sloping strata at Sicca Point, Scotland. This was the geological evidence that awakened Hutton to the enormity of geological time and set the scene for the evolution of geological science. 

The angular unconformity of the Great Unconformity at the base of the Grand Canyon is in contrast with those occurring at later periods over the next 500Ma. At each of the later unconformities there is a remarkable correspondence between the angular disposition, basically horizontal, of the strata above and below each of the subsequent unconformities. This implies that during the post emergence uplift the strata having been laid down horizontally experienced a remarkably uniform, epeirogenic, uplift during which there was no significant tilting. Furthermore, the subsequent erosion must have removed the strata uniformly to create a further peneplain, implying that there was either very little erosion or more likely the erosion of deposits below these unconformities was of a nature that it resulted in peneplaination down to essentially horizontal surfaces – indicating either very long term and mature fluvial erosion or the possibility of erosion by overlying ice sheets. It is another remarkable feature of these Phanerozoic unconformities that the strata overlying the unconformities are generally parallel with those below and were the result of a form of uniform subsidence in what might be labelled epeisubsidence. It is also noteworthy that despite the -270 Ma age of the surface strata away from the incision made by the Colorado River, and the consideration that this area has relatively recently risen by some 2.2km relative to the contiguous area leading up to the Bryce Canyon, the terrain around the Grand Canyon still largely preserves this planated form – characteristic of a peneplain - albeit in a now domed onfiguration.

But perhaps the most remarkable feature, to be focused upon in the next few posts, is the existence of the 5 well defined unconformities within the sedimentary sequences on display beneath the Colorado Plateau. Four of these unconformities have occurred within the Phanerozoic eon (the last 540 Ma). The past few posts have indicated that the most robust temporal signal at these unconformities is the commencement of deposition following a hiatus in which there was either no sedimentation occurring or uplift and sub-aerial erosion has removed the evidence of any sedimentation that may have occurred. And of course these renewed pulses of sedimentation represent the moment in time when subsidence has resulted in the lithosphere once again sinking beneath sea level, or possibly being flooded by rising sea level, but, significantly, continuing to sink as sediments were added. Over the Phanerozoic these moments at which new sequences of sediments began to be deposited above the most significant unconformites appear to have occurred at -525 Ma, -385 Ma, -265 Ma, and -140 Ma. Although there are a few perhaps less significant time hiatuses within the record, it remains to be explained what it is that can possibly account for this apparent cyclic behaviour having an average periodicity of around 130 Ma? 

 

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.