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?