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.
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)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.
No comments:
Post a Comment